Patent Description:
IHC represents an important tool for both research and clinical applications. Among the many techniques used to characterize protein expression, IHC is one of the few that provides information on expression level as well as localization, e.g., at the cellular and/or tissue levels. IHC is therefore a critical tool used in research to characterize a protein of interest. IHC has also become an important diagnostic tool in the clinic, e.g., to categorize patients for a variety of personalized medicine applications. As one example, the HercepTest™ (Dako Denmark A/S) semi-quantitative HER2 IHC assay has been approved by the FDA for use in assessing HER2 protein status for almost <NUM> years. This test allows clinicians to identify patients whose tumors overexpress HER2. While HER2 is overexpressed in many types of cancer, <NUM>-<NUM>% of breast cancers have been shown to overexpress HER2, and this marker is correlated with shortened disease-free and overall survival. Therapies that target HER2 in these patients (e.g., anti-HER2 antibody treatment) provide significantly improved overall survival, response rate, duration of response, and time to disease progression. See, e.g., <NPL>.

Establishing a reliable IHC assay for a particular target presents multiple challenges. First, one must identify an antibody that is specific to the target of interest and does not cross-react with other targets. This requires appropriate positive and negative controls that express the target at known levels, which are difficult to identify when the target's expression is uncharacterized.

Second, even if an antibody is available, it can be difficult to identify positive and negative controls that are reliable, readily available, and easy to mass-produce. Tissue samples and cell lines (e.g., cell pellets) have both been used as IHC controls; however, both have significant limitations. For many targets, appropriate tissues are not available. For others, expression in tissues may be variable, uncharacterized, or too weak to detect. Cell lines are easier to obtain than tissue samples (although growing certain cell lines on an industrial scale can be difficult), but like expression in tissue samples, expression in cell lines can be variable, uncharacterized, or too weak to detect. One can engineer a cell line to overexpress a target of interest, but overexpression can be significantly higher than expression in actual tissues and can lead to artefactual subcellular localization. Overexpression can also be heterogeneous within a population of cultured cells. Cell lines can be produced in batches, but these can be quickly exhausted, requiring the production of new batches that can have different characteristics.

A variety of approaches aimed at establishing a standardized approach to generate IHC controls have been attempted. Over <NUM> years ago, Brandtzaeg described a method to create "artificial tissue" samples: millimeter-sized blocks of glutaraldehyde-fixed rabbit serum, into which human immunoglobulin fractions or whole serum were allowed to diffuse (<NPL>). This general technique was revisited and extended with apparent success over the next dozen years (<NPL>; <NPL>; <NPL>; <NPL>), but has also been described as prone to inhomogeneous and nonspecific staining (<NPL>), and has seen little use in recent practice. More widely used are clonal cell lines with variably well-characterized abundance of target proteins (<NPL>). These are invaluable in many settings, but cell line controls can show remarkably heterogeneous expression of specific targets in separate subclones, different passages of one clone, or even within one culture population, frustrating the goal of creating a homogeneous and reproducible standard.

<NPL> describe spotting peptides directly to glass slides, or coupling peptides to glass beads (see also <NPL>). However, implementation of this approach led to technical challenges. For example, since it was difficult for technicians to see the spots on which the peptides were applied to the glass (they are invisible prior to staining), many slides were stained with insufficient amounts of reagent to cover all of the controls, thereby leading to artifacts in staining (see <NPL>). These peptide spots were also much thinner than actual tissue sections, and therefore intense positive control staining was difficult to achieve. Other groups have tried to mix a target of interest in a lysozyme solution, which can be prepared like a formalin-fixed paraffin-embedded tissue section (see <NPL>). They noted that gel formation depended on protein concentration and isoelectric point (<NPL>. However, this approach did not work for many targets, such as peptides, which can leak out of the lysozyme gelatin. Agarose was also tested as a potential tissue surrogate. However, the peptides dispersed in agarose were not homogeneous. In addition, subjecting these agarose-based peptide gels to antigen retrieval, which typically includes boiling, melted the agarose and caused it to separate on the glass slide.

Therefore, a need exists for an approach that provides reliable positive and negative IHC controls that are sensitive, specific, and can be adapted to a wide range of targets, including targets for which no suitable biological control exists. Such an approach would also provide a useful assay for determining antibody specificity, which is particularly advantageous when screening a large number of antibodies to identify and validate a new antibody specific for a target of interest, as well as for optimizing IHC staining protocols.

<CIT> describes mixing a sample comprising a biological molecule with an embedding material (gel) and preparing an array, for detection of the biological molecule. The embedding material is agarose and the gel is contacted with a cross-linking agent after formation of the gel.

<NPL>) describe the use of a collagen matrix gel culture to study the role of growth factors in thyroid tissue regeneration, by examining their effects on thyroid folliculogenesis and angiogenesis. The collagen was contacted with a cross-linking agent after formation of the gel.

<NPL>) describe the use of collagen as a carrier in the formation of a solid antigen/carrier protein gel, where the collagen was contacted with a cross-linking agent after formation of the gel.

<NPL>) describe gels to be used as drug delivery vehicles which are formed from BSA through a redox mechanism and contact with a cross-linking agent after gel formation.

To meet these and other demands, provided herein are methods for generating a solid antigen/carrier protein gel. These solid gels can be processed like tissue samples or other biological samples according to standard IHC or electron microscopy (EM) processing methods (including fixation, sectioning, antigen retrieval, and so forth). Since the gels contain a known amount of an antigen of interest, they can be used, e.g., to create a series of gels having known concentrations of antigen to standardize staining with a particular antibody, or to screen for antibodies that specifically recognize an antigen of interest and are suitable for IHC/EM analyses. These methods are thought to provide a general platform that allows for control staining of any antigen of interest, simulating multiple levels of expression using known concentrations of antigen.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs.

The term "polypeptide" or "protein" are used interchangeably herein to refer to polymers of amino acids of any length. The polymer may be linear or branched, it may comprise modified amino acids, and it may be interrupted by non-amino acids. The terms also encompass an amino acid polymer that has been modified naturally or by intervention; for example, disulfide bond formation, glycosylation, lipidation, acetylation, phosphorylation, or any other manipulation or modification, such as conjugation with a labeling component or toxin. Also included within the definition are, for example, polypeptides containing one or more analogs of an amino acid (including, for example, unnatural amino acids, etc.), as well as other modifications known in the art. The terms "polypeptide" and "protein" as used herein specifically encompass antibodies.

The term "antigen" herein is used in the broadest sense and encompasses various forms of both polypeptide and non-polypeptide antigens, including, without limitation, small peptide antigens, full-length protein antigens, carbohydrate antigens, lipid antigens, and nucleic acid antigens.

The term "antibody" herein is used in the broadest sense and encompasses various antibody structures, including but not limited to monoclonal antibodies, polyclonal antibodies, multispecific antibodies (e.g., bispecific antibodies), and antibody fragments so long as they exhibit the desired antigen-binding activity. The term "immunoglobulin" (Ig) is used interchangeable with antibody herein.

A "purified" antigen refers to an antigen has been increased in purity, such that it exists in a form that is more pure than it exists in its natural environment and/or when initially produced and/or synthesized and/or amplified under laboratory conditions. Purity is a relative term and does not necessarily mean absolute purity. In some embodiments, the antigen is purified to at least <NUM>%, at least <NUM>%, or at least <NUM>% purity.

The present disclosure relates methods for generating a solid antigen/carrier protein gel.

The present disclosure provides a solid antigen/carrier protein gel comprising a purified antigen and a carrier protein which is a serum albumin protein) The purified antigen is cross-linked to the carrier protein. As described below, these gels may find use, e.g., as a control for IHC staining or EM imaging analyses.

In some embodiments, the serum albumin protein is a mammalian serum albumin protein. Examples of serum albumin proteins include, without limitation, mouse, rat, Guinea pig, rabbit, porcine, bovine, goat, sheep, horse, and human serum albumin.

In some embodiments, the solid antigen/carrier protein gel comprises the carrier protein at a concentration of greater than or equal to <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v). In some embodiments, the solid antigen/carrier protein gel comprises the carrier protein at a concentration of less than or equal to <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/ v). That is, the solid antigen/carrier protein gel can comprise the carrier protein at any concentration of a range of concentrations having an upper limit of <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v) and an independently selected lower limit of <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v), wherein the lower limit is less than the upper limit. In some embodiments, the carrier protein comprises an albumin protein (e.g., a serum albumin protein), and the solid antigen/carrier protein gel comprises the carrier protein at greater than <NUM>% (w/v), e.g., between <NUM>% and <NUM>% (w/v). In some embodiments, the carrier protein comprises an egg white protein or mixture of egg white proteins, and the solid antigen/carrier protein gel comprises the carrier protein at greater than <NUM>% (w/v), e.g., between <NUM>% and <NUM>% (w/v). In some embodiments, the carrier protein comprises gelatin, and the solid antigen/carrier protein gel comprises the carrier protein at greater than <NUM>% (w/v), e.g., between <NUM>% and <NUM>% (w/v) or about <NUM>% (w/v).

In some embodiments, the solid antigen/carrier protein gel contains the antigen at a concentration of at least about <NUM>. In another embodiment, the solid antigen/carrier protein gel contains the antigen at a concentration of about <NUM>. In yet another embodiment, the solid antigen/carrier protein gel contains the antigen at a concentration of about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, or more. In another embodiment, the solid antigen/carrier protein gel contains the antigen at a concentration of at least about: <NUM>, <NUM>, <NUM>, or <NUM> to about <NUM>. As described herein, a solid antigen/carrier protein gel of the present disclosure can comprise the antigen at a wide range of concentrations, depending, e.g., upon the sensitivity of the detection method, desired application of the gel, and so forth.

In one embodiment, the methods include two or more antigen/carrier protein gels. In one embodiment, two gels are used where each gel optionally has a different concentration of antigen. In another embodiment, three gels are used where each gel optionally has a different concentration of antigen. Indeed, multiple gels comprising an antigen at different concentrations may be useful, e.g., as a control for IHC staining to represent a range of antigen concentrations.

In some embodiments, the antigen comprises a polypeptide antigen (e.g., a peptide antigen or full-length protein antigen). In some embodiments, the polypeptide antigen comprises an N-terminal tyrosine, a C-terminal cysteine, or both (e.g., for chemical cross-linking the antigen to a carrier protein). In some embodiments, the N-terminal tyrosine and/or C-terminal cysteine is cross-linked to the carrier protein. For example, a C-terminal cysteine may be used to cross-link the antigen with the carrier protein. A variety of cysteine-reactive reagents are known in the art and include, without limitation, sulfhydryl-reactive crosslinker reactive groups such as haloacetyls, maleimides (e.g., sulfo-SMCC and its analogs), aziridines, acryloyls, arylating agents, vinylsulfones, pyridyl disulfides, TNB-thiols, and disulfide reducing agents.

In some embodiments, the solid antigen/carrier protein gel has been fixed with a fixative, e.g., as described below. In some embodiments, the fixative is a cross-linking fixative (e.g., an aldehyde-based fixative). In some embodiments, the fixative is not a cross-linking fixative (e.g., a precipitating fixative, such as Carnoy's). As described herein, a solid gel containing a carrier protein and an antigen may be prepared by denaturing and precipitating the carrier protein and/or the antigen by heating, or by adding a precipitating fixative to the mixture, causing the antigen to be immobilized in the protein gel. For some antigens (particularly those of small molecular size), cross-linking the antigen to the carrier protein (either during or after the process of forming a gel by denaturing the carrier protein) is thought to help to retain the antigen in the gel and in the resulting sections, e.g., during the embedding, sectioning, and/or staining processes.

In some embodiments, the antigen comprises a non-polypeptide antigen. Examples of non-polypeptide antigens include, without limitation, carbohydrates, lipids, and nucleic acids.

In some embodiments, the solid antigen/carrier protein gel has a thickness of between about <NUM> and about <NUM>. A suitable thickness for the solid antigen/carrier protein gel may depend, e.g., upon the application. In some embodiments, the solid antigen/carrier protein gel has a thickness of greater than or equal to about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. In some embodiments, the solid antigen/carrier protein gel has a thickness of less than or equal to about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. That is, the solid antigen/carrier protein gel can have a thickness having an upper limit of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> and an independently selected lower limit of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, wherein the lower limit is less than the upper limit. For example, a solid antigen/carrier protein gel used for IHC staining may have a thickness, e.g., of between about <NUM> and about <NUM>, whereas a solid antigen/carrier protein gel used for EM may have a thickness, e.g., of between about <NUM> and about <NUM> or between about <NUM> and about <NUM>. Methods for sectioning a solid antigen/carrier protein gel of the present disclosure are described below.

In some embodiments, the solid antigen/carrier protein gel has been subjected to antigen retrieval. Exemplary methods for antigen retrieval are described in greater detail below.

In some embodiments, the solid antigen/carrier protein gel is embedded in a medium of the present disclosure, such as paraffin (e.g., embedding in a paraffin block); epoxy, acrylic, or plastic resin (e.g., EPON™ resins, methacrylate, LR White resin, Araldite®, Spurr plastic, LOWICRYL®, and the like); synthetic wax; blends of paraffin wax and plastic polymer or co-polymer alloys; polyethylene glycol; or GACH embedding medium (Glutaraldehyde-Carbohydrazide). In some embodiments, the solid antigen/carrier protein gel is affixed to a solid substrate, including without limitation a glass slide (e.g., for IHC), support, grid, or stub (e.g., for EM).

In the invention, the method for generating a solid antigen/carrier protein gel comprises mixing a purified antigen with a liquid solution comprising a carrier protein to produce an antigen/carrier protein liquid solution and heating the antigen/carrier protein liquid solution to form the solid antigen/carrier protein gel.

In some embodiments, the antigen/carrier protein liquid solution comprises the carrier protein at a concentration of greater than or equal to <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v). In some embodiments, the antigen/carrier protein liquid solution comprises the carrier protein at a concentration of less than or equal to <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v). That is, the antigen/carrier protein liquid solution can comprise the carrier protein at any concentration of a range of concentrations having an upper limit of <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v) and an independently selected lower limit of <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% (w/v), wherein the lower limit is less than the upper limit. In some embodiments, the antigen/carrier protein liquid solution comprises the carrier protein at greater than <NUM>% (w/v), e.g., between <NUM>% and <NUM>% (w/v).

In some embodiments, the methods of the present disclosure include (e.g., prior to heating the antigen/carrier protein liquid solution) cross-linking the antigen with the carrier protein using a cysteine-reactive reagent. A variety of cysteine-reactive reagents are known in the art and include, without limitation, sulfhydryl-reactive crosslinker reactive groups such as haloacetyls, maleimides (e.g., sulfo-SMCC and its analogs), aziridines, acryloyls, arylating agents, vinylsulfones, pyridyl disulfides, TNB-thiols, and disulfide reducing agents.

In some embodiments, the methods of the present disclosure include fixing the antigen with the carrier protein. In some embodiments, a fixative is included in the antigen/carrier protein liquid solution. In some embodiments, the fixative is a cross-linking fixative. In some embodiments, the fixative is a non-cross-linking fixative. Examples of suitable fixatives include, without limitation, formaldehyde, formalin, paraformaldehyde (PFA), glutaraldehyde, Davidson's fixative, Bouin's fixative, Karnovski's fixative (e.g., ½ strength Karnovski's fixative), Zenker's solution, Helly's solution, Carnoy's solution, Zinc formalin, neutral-buffered formalin (NBF), Periodate-Lysine-paraformaldehyde (PLP), Zamboni's fixative, dimethyl suberimidate (DMS), acetone, alcohols (e.g., methanol or ethanol), and zinc salts (e.g., zinc acetate, zinc chloride, zinc trifluoroactetate, etc.). In certain embodiments, the fixative comprises formaldehyde (e.g., at a concentration of at least about <NUM>% or at least about <NUM>%).

In some embodiments, the antigen is cross-linked with the carrier protein using selective chemistry, including without limitation azide-alkyne addition. In some embodiments, selective chemistry is used to cross-link a non-protein antigen to the carrier protein, e.g., a carbohydrate or lipid antigen.

In some embodiments, the methods of the present disclosure include dehydrating the solid antigen/carrier protein gel. Compounds suitable for dehydrating are known in the art and can include, e.g., alcohols such as ethanol or methanol. In some embodiments, the methods of the present disclosure include embedding the solid antigen/carrier protein gel. In some embodiments, the solid antigen/carrier protein gel is embedded after dehydration. For example, in some embodiments, the solid antigen/carrier protein gel is dehydrated by exposure to a sequence of increasing or graded alcohols (e.g., a sequence of increasing ethanol concentrations), followed by exchange of alcohols with xylene, and exchange of xylene with paraffin. A variety of media are known in the art and can be used for embedding, including without limitation paraffin (e.g., embedding in a paraffin block); epoxy, acrylic, or plastic resin (e.g., EPON™ resins, methacrylate, LR White resin, Araldite®, Spurr plastic, LOWICRYL®, and the like); synthetic wax; blends of paraffin wax and plastic polymer or co-polymer alloys; polyethylene glycol; and GACH embedding medium (Glutaraldehyde-Carbohydrazide).

In some embodiments, the methods of the present disclosure include incubating the solid antigen/carrier protein gel in an embedding medium (e.g., a liquid embedding medium). A variety of embedding media are known in the art and can include, without limitation, Optimum Cutting Temperature (OCT) Compound (e.g., Tissue-Tek® O. Compound or Tissue-plus® O. T Compound), PELCO® Cryo-Embedding Compound, PolarStat™ or PolarStat Plus™ Embedding Medium, and Tissue Freezing Medium or TFM™. These embedding media can contain, e.g., water-soluble glycols and resins; the exemplary embedding media OCT Compound includes <NUM>-<NUM>% polyvinyl alcohol and <NUM>-<NUM>% polyethylene glycol. In some embodiments, the methods of the present disclosure include freezing the solid antigen/ carrier protein gel (e.g., after incubating the solid antigen/carrier protein gel in an embedding medium). In some embodiments, the methods of the present disclosure include freezing a solid antigen/carrier protein gel that has not been fixed (e.g., that does not include a fixative), similar to the preparation of frozen, unfixed fresh tissue samples.

In some embodiments, the methods of the present disclosure include sectioning the solid antigen/carrier protein gel into one or more solid antigen/carrier protein gel sections. In some embodiments, the one or more solid antigen/carrier protein gel sections have a thickness of between about <NUM> and about <NUM>. In some embodiments, the one or more solid antigen/carrier protein gel sections have a thickness of greater than or equal to about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. In some embodiments, the one or more solid antigen/ carrier protein gel sections have a thickness of less than or equal to about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>. That is, the one or more solid antigen/carrier protein gel sections can have a thickness having an upper limit of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM> and an independently selected lower limit of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or <NUM>, wherein the lower limit is less than the upper limit. For example, a solid antigen/carrier protein gel section used for IHC staining may have a thickness, e.g., of between about <NUM> and about <NUM>, whereas a solid antigen/carrier protein gel section used for EM may have a thickness, e.g., of between about <NUM> and about <NUM> or between about <NUM> and about <NUM>. Instruments for sectioning a solid antigen/carrier protein gel of the present disclosure are known in the art and can include, without limitation, a microtome, cryostat (for frozen gels), knife (e.g., diamond, glass, or sapphire), and the like. In some embodiments, the solid antigen/ carrier protein gel is dehydrated, embedded, fixed, frozen, or a combination thereof prior to sectioning.

In some embodiments, the methods of the present disclosure include subjecting the solid antigen/carrier protein gel to antigen retrieval. A variety of antigen retrieval methods are known in the art. In some embodiments, subjecting the solid antigen/carrier protein gel to antigen retrieval comprises heating the solid antigen/carrier protein gel in a liquid solution (e.g., heat-induced epitope retrieval), such as by boiling. In some embodiments, the liquid solution comprises a buffer, such as Tris/EDTA or sodium citrate buffer. In some embodiments, subjecting the solid antigen/carrier protein gel to antigen retrieval comprises treatment with one or more proteolytic enzymes (e.g., trypsin, proteinase K, pepsin, ficin, or pronase) or antigen retrieval reagents (e.g., hydrochloric acid, formic acid, sodium dodecyl sulfate (SDS), citrate buffer, EDTA, citrate-EDTA, Tris, Tris-EDTA, Tris-HCl, Tris-buffered saline, or citraconic anhydride). For additional descriptions of antigen retrieval, see, e.g., <NPL>.

In some embodiments, the methods of the present disclosure include blocking a solid antigen/carrier protein gel. As is known in the art, blocking (e.g., prior to IHC staining) reduces non-specific interactions with antigens by preventing binding to sites not related to specific antigen: antibody interactions. A variety of suitable blocking reagents are known in the art and can include, without limitation, serum or a protein solution (e.g., comprising a serum albumin protein, gelatin, non-fat dry milk, or the like).

In some embodiments, the methods of the present disclosure include fixing a solid antigen/carrier protein gel (e.g., by including a fixative in the antigen/carrier protein liquid solution), dehydrating the solid antigen/carrier protein gel, embedding the dehydrated solid antigen/carrier protein gel, sectioning the paraffin block with the embedded solid antigen/carrier protein gel into one or more sections, subjecting the one or more sections to antigen retrieval, and blocking the one or more sections after antigen retrieval. In other embodiments, the methods of the present disclosure include incubating the solid antigen/carrier protein gel in a liquid embedding medium, freezing the solid antigen/carrier protein gel in the embedding medium, sectioning the frozen antigen/carrier protein gel into one or more sections, and blocking the one or more sections.

Other aspects of the present disclosure relate to tissue microarrays (TMAs) comprising one, two, or more solid antigen/carrier protein gel(s) of the present disclosure. For example, a TMA of the present disclosure may be prepared by preparing a solid antigen/carrier protein gel of the present disclosure (e.g., embedded in a paraffin block as described above), punching a core comprising the solid antigen/carrier protein gel, and transferring the core to a recipient TMA. In some embodiments, the core has a diameter of about <NUM>. An exemplary method for preparing a TMA is described infra, e.g., by transferring core(s) from a donor paraffin block to a recipient TMA block, then heating (e.g., at 37C overnight, then 70C for <NUM> minutes), cooling, and sectioning the recipient TMA block. In some embodiments, the TMAs include two or more solid antigen/carrier protein gels of the present disclosure with different antigens. In some embodiments, the TMAs include two or more solid antigen/carrier protein gels of the present disclosure with the same antigen at different concentrations. In some embodiments, the TMA further comprises one or more orientation reference(s), e.g., comprising a colored pigment for identification. In some embodiments, the TMA is a TMA section on a histology slide. Without wishing to be bound to theory, it is thought that solid antigen/carrier protein gels of the present disclosure are particularly advantageous for use in TMAs, e.g., for providing a range of epitope concentrations and/or types. This range can also be provided adjacent to a tissue section, thereby allowing a convenient reference for quantitative analyses.

Solid antigen/carrier protein gels (e.g., as described in section II above and/or exemplified in the Examples below) may find use in a variety of applications, including but not limited to controls for IHC or EM analysis.

In some embodiments, a method for control immunohistochemical (IHC) staining of an antigen comprises providing multiple solid antigen/carrier protein gels of the present disclosure (e.g., two or more, three or more, four or more, five or more, etc.) representing different concentrations of an antigen of interest. For example, the methods can include providing two solid antigen/carrier protein gels made with different concentrations of a purified antigen. In some embodiments, the methods include contacting the solid antigen/carrier protein gels with a primary antibody that specifically binds the antigen and is coupled to a detectable moiety. In other embodiments, the methods include contacting the solid antigen/ carrier protein gels with a primary antibody that specifically binds the antigen, then contacting the solid antigen/carrier protein gels with a secondary antibody that specifically binds the primary antigen and is coupled to a detectable moiety. In some embodiments, the methods further include detecting signal(s) of the detectable moiety from one or more of the multiple solid antigen/carrier protein gels. Amount of signal detected from the multiple solid antigen/carrier protein gels can then be detected and, optionally, compared against amount or concentration of antigen present in each of the multiple solid antigen/carrier protein gels to provide control IHC staining of the antigen at various levels.

In some embodiments, one of the antigen/carrier protein gels contains no antigen (e.g., <NUM>), and lack of a detectable signal from IHC staining of this gel indicates negative control or background IHC staining of the sample. In some embodiments, the methods further include contacting a sample with the primary antibody that specifically binds the antigen and is coupled to a detectable moiety, or with the primary antibody that specifically binds the antigen and the secondary antibody that specifically binds the primary antigen and is coupled to a detectable moiety, and detecting a signal of the detectable moiety from the sample. The amount of signal detected from the sample can then be compared against the amount of signal (s) detected from the solid antigen/carrier protein gel(s), e.g., to compare the amount of antigen present in the sample with known concentrations or amounts of antigen present in the solid antigen/carrier protein gel(s).

In some embodiments, a method for control immunohistochemical (IHC) staining with a secondary antibody comprises providing multiple solid antigen/carrier protein gels of the present disclosure (e.g., two or more, three or more, four or more, five or more, etc.) representing different antibody isotypes. For example, the methods can include providing two solid antigen/ carrier protein gels made with antibodies representing different antibody isotypes. In some embodiments, the methods include contacting the solid antigen/carrier protein gels with a secondary antibody that specifically binds one of the antibody isotypes and is conjugated to a detectable moiety. For control staining of the secondary antibody, signal can be detected from the multiple solid antigen/carrier protein gels. In this example, the secondary antibody specifically binds the cognate antibody isotype, and any signal detected from a gel lacking the cognate antibody isotype indicates background staining. Thus, detection of the signal associated with the solid antigen/carrier protein gel containing the cognate antibody isotype and the lack of the signal associated with the solid antigen/carrier protein gel lacking the cognate antibody isotype indicates control staining with the secondary antibody.

In some embodiments, a method for immunohistochemical (IHC) staining of an antigen comprises providing one or more solid antigen/carrier protein gels containing the antigen and a sample; contacting the solid antigen/carrier protein gel and the sample with a primary antibody that specifically binds the antigen and is coupled to a detectable moiety, or contacting the solid antigen/carrier protein gel and the sample with a primary antibody that specifically binds the antigen and contacting the solid antigen/carrier protein gel and the sample with a secondary antibody that specifically binds the primary antibody and is coupled to a detectable moiety; detecting a signal of the detectable moiety from the solid antigen/carrier protein gel; and detecting a signal of the detectable moiety from the sample. Thus, the one or more solid antigen/ carrier protein gels can be used as a positive control for IHC staining of the sample.

An exemplary flow diagram for process <NUM> for IHC staining of an antigen, such as an antigen in a sample and in a solid antigen/carrier protein gel of the present disclosure (used as a control for staining of the sample for the antigen) is shown in <FIG>. At block <NUM>, tissue is collected to provide a sample for analysis, and/or a solid antigen/carrier protein gel of the present disclosure is produced (e.g., as described in section II above). In some embodiments, the solid antigen/carrier protein gel contains an antigen (e.g., a purified antigen) of interest, which may or may not also be present in the sample. Optionally, at block <NUM>, the sample and/or solid antigen/carrier protein gel is fixed (e.g., as described in section II above). In some embodiments, the solid antigen/carrier protein gel is incubated with a fixative. In other embodiments, the antigen/carrier protein liquid solution used to make the solid gel comprises a fixative. Optionally, at block <NUM>, the sample and/or solid antigen/carrier protein gel are embedded (e.g., as described in section II above). As an optional alternative to block <NUM>, at block <NUM>, the sample and/or solid antigen/carrier protein gel can be frozen in an embedding medium. Optionally, at block <NUM>, the sample and/or solid antigen/carrier protein gel are sectioned (e.g., as described in section II above). Optionally, at block <NUM>, the sample and/or solid antigen/carrier protein gel are subjected to antigen retrieval and blocking (e.g., as described in section II above).

In some embodiments, as shown at block <NUM>, the sample and the solid antigen/carrier protein gel are contacted with a primary antibody that specifically binds the antigen. In some embodiments, the primary antibody comprises a detectable moiety. In other embodiments, as shown at block <NUM>, the sample and the solid antigen/carrier protein gel are contacted with a secondary antibody that specifically binds the primary antibody. In some embodiments, the secondary antibody comprises a detectable moiety conjugated to the secondary antibody (in this example, biotin).

In some embodiments, a signal of the detectable moiety is detected from the solid antigen/carrier protein gel and/or sample. As described below, a variety of detectable moieties are contemplated. In this example, a chromogenic detection method is used. At block <NUM>, the sample and the solid antigen/carrier protein gel are contacted with an enzyme complex that binds to the secondary antibody (e.g., a streptavidin-conjugated enzyme complex that binds to a biotinylated secondary antibody). At block <NUM>, the sample and the solid antigen/carrier protein gel are contacted with a chromogenic substrate or chromogen solution that, upon incubation with the enzyme complex, leads to formation of a colored precipitate indicating presence of the antigen in the solid antigen/carrier protein gel and, if the antigen is present, the sample.

In some embodiments, presence of an antigen in a solid antigen/carrier protein gel of the present disclosure is detected by direct detection, e.g., by incubation of the gel with an antibody or other antigen-binding moiety that specifically binds the antigen and is coupled to a detectable moiety. In other embodiments, presence of an antigen in a solid antigen/carrier protein gel of the present disclosure is detected by indirect detection, e.g., by incubation of the gel with a primary antibody or other antigen-binding moiety that specifically binds the antigen and a secondary antibody or other antigen-binding moiety that specifically binds the primary antibody or other antigen-binding moiety, where the secondary antibody or other antigen-binding moiety is coupled to a detectable moiety.

A variety of detectable moieties are contemplated for use with a solid antigen/carrier protein gel of the present disclosure. In some embodiments, the detectable moiety comprises an enzyme, e.g., horseradish peroxidase (HRP) or alkaline phosphatase (AP). Signal is then detected by exposing the detectable moiety to a chromogenic substrate of the enzyme and detecting a signal from the chromogenic substrate upon reaction with the enzyme (e.g., as described above for blocks <NUM> and <NUM>). Examples of chromogenic substrates include, without limitation, <NUM>,<NUM>'-diaminobenzidine (DAB), which is converted to a brown product by HRP; and <NUM>-amino-<NUM>-ethylcarbazole (AEC), which is converted to a red product by AP. Alternatively, a peroxidase can chemically activate a tyramide moiety conjugated to one of several reporters (for example a fluorescent molecule); the activated tyramide conjugate can then covalently couple to nearby molecules in the sample, creating a detectable signal in the location of the peroxidase. Indirect detection can also be used with chromogenic staining. For example, a biotinylated secondary antibody can be incubated with an avidin- or streptavidin-labeled enzyme complex, a polymer such as a dextran can be coupled with one or more secondary antibodies and one or more enzyme complexes, or an enzyme complex can be polymerized directly onto a secondary antibody. In some embodiments, the detectable moiety comprises a fluorophore, such as Fluorescein (FITC), Rhodamine or its derivatives, TRITC, Cyanine (Cy3), Phycoerythrin (R-PE), and CF™, and so forth. For example, a fluorophore can be conjugated to the primary or secondary antibody, or conjugated to a tyramide moiety that can covalently couple to the sample after being activated by a peroxidase. In some embodiments, the detectable moiety comprises a metal particle, such as gold. For example, a gold particle can be conjugated to the primary or secondary antibody and imaged, e.g., by immuno-EM. In some embodiments, the detectable moiety comprises a radioisotope, such as <NUM>S, <NUM>I, or <NUM>I and imaged, e.g., by radioimmunodetection. For example, a radioisotope can be conjugated to the primary or secondary antibody. In some embodiments, the detectable moiety comprises a nucleic acid, such as DNA or RNA. For example, a nucleic acid can be conjugated to the primary or secondary antibody. A primary or secondary antibody can be coupled to a metal ion detectable with a mass spectrometer or mass cytometer [a la CYTOF and MIBI; see <NPL> and <NPL>]. A primary or secondary antibody can be coupled to a solid state, semiconductor, or carbon-based nanoparticles (e. g "quantum dots") detectable by fluorescence imaging [see <NPL>]. A primary or secondary antibody can be coupled to a peroxidase such as horseradish peroxidase which is then detected by incubation with a chemiluminescent substrate. A primary or secondary antibody can be coupled to an electrochemiluminescent reporter (for example, ruthenium; see <NPL>).

<FIG> shows exemplary workflow <NUM> for IHC staining in tissue samples. Importantly, IHC staining demonstrates not only the expression level of a target, but also its subcellular and tissue distributions.

The following Example describes a new platform approach for generating IHC controls that can be adapted to a wide range of targets. This approach was validated using human BCL2, a target for which <NUM>+ positive control tissues can be identified (for example, human chronic lymphocytic leukemia tumor samples often score as <NUM>+ in this assay), but are difficult to procure, or are limited by ethical considerations, for routine use as staining controls. The generality of the approach was also demonstrated using mouse and rat IgG and a human protein of interest.

Approximately <NUM>-<NUM> of antigen containing approximately <NUM> to <NUM>-<NUM> M of the target epitope was mixed with <NUM> <NUM>% BSA in PBS in a <NUM> microcentrifuge tube. <NUM> <NUM>% formaldehyde was added to the tube. The tube was heated for <NUM> minutes at <NUM>, then left overnight at room temperature to solidify if containing a fixative (e.g., formalin). The resulting gel was removed from the tube and put into <NUM>% neutral buffered formalin (NBF). Subsequently, the gel was dehydrated through graded alcohols to xylene, infiltrated with warm paraffin wax, embedded in a paraffin block, sectioned on a microtome, and stained using the primary mouse monoclonal anti-BCL2 antibody <NUM> and the horseradish peroxidase (HRP)/ <NUM>,<NUM>'-diaminobenzidine (DAB) chromogenic enzyme/substrate system according to standard IHC methods.

The IF detection assay was developed on the Ventana Discovery Ultra automated staining instrument. Four micron sections containing the target BSA/peptide samples were cut, deparaffinized and pretreated with CC1 cell conditioning solution. The primary mouse monoclonal anti-BCL2 antibody <NUM> was incubated for <NUM> minutes at <NUM> and detected with the OmniMAP anti-mouse HRP Detection System for <NUM> minutes, followed by the Ventana Discovery Red <NUM> detection system for <NUM> minutes.

Whole-slide brightfield and immunofluorescent images were acquired at a scanning resolution of <NUM> microns/pixel using the Hamamatsu Nanozoomer-XR digital slide scanner equipped with a 20X <NUM> NA objective lens and fluorescent module. Brightfield imaging was performed and used for focusing. When applicable, immunofluorescence signal was acquired using a TRITC filter at 1x exposure (<NUM> photon collection) and 1x gain. Autofluorescence was captured using a DAPI (2x exposure, <NUM> photon collection, 2x gain) and CFP filter (4x exposure, <NUM> photon collection, 2x gain). Illumination power was set at <NUM>% for all immunofluorescence acquisition. Image analysis was performed using Matlab version <NUM>. Regions of interest (ROI) were manually marked up (brightfield scan only) or generated using thresholding and morphological filtering on the autofluorescence channels and transferred onto the TRITC channel for intensity measurement. Average grayscale intensity was calculated in <NUM>-bit depth. Average optical absorbance (brightfield DAB-labeled) or emittance (fluorescently labeled) was computed by summing all pixel absorbance or emittance values within a ROI (as calculated by the respective natural log formula below) and normalized to the total ROI pixel count. All zero pixel grayscale intensity values were approximated using a value of <NUM>. Brightfield pixel absorbance = log(<NUM>/pixel grayscale intensity). Fluorescent pixel emittance = log(pixel grayscale intensity).

For validation experiments, the BSA gel samples were generated in microcentrifuge tubes as described above, embedded in paraffin blocks, then used to create a tissue microarray of <NUM> micron-diameter cylinders cut from the paraffin blocks and re-embedded into the array using standard tissue microarray techniques.

For BCL2 experiments, a peptide comprising the sequence Ac-YGSGGAAPAPGIFSSQPGGSGC -amide (SEQ ID NO:<NUM>) was used. Underlined amino acids correspond to the wild-type (unmutated) amino acids <NUM>-<NUM> of the human BCL2 sequence as set forth in UniProt Accession No. P10415. The peptide also includes an amino-terminal linker sequence (Ac-YGSG) (SEQ ID NO:<NUM>) with acetyl-Y (tyrosine) that allows coupling with an aldehyde (e.g., formaldehyde) and a carboxy-terminal linker sequence (GSGC-amide) (SEQ ID NO:<NUM>) with C (cysteine)-amide that is reactive with a variety of cross-linking reagents, e.g. maleimide-, haloacetyl- or pyridyl disulfide-containing compounds.

BSA/peptide gels were prepared as described supra. Mixing the antigen (e.g., a peptide) with BSA in a liquid solution before producing a gel allowed the peptide to be mixed uniformly. The resulting gel (<FIG>) can be constructed in any desired shape (in this example, a gel was produced in the shape of a <NUM> microcentrifuge tube). The gel can also be sectioned (<FIG>) and subjected to typical IHC processing steps such as antigen retrieval, blocking, primary/secondary antibody incubation, and detection (e.g., using an enzyme/substrate approach). For example, <FIG> shows a sliced gel portion prepared for dehydration and paraffin processing, and <FIG> shows a sliced gel portion embedded in a paraffin donor block. <FIG> shows a TMA created from various paraffin gel donor blocks, along with cores containing black and green pigment for orientation references. Peptide antigens are able to cross-link to the BSA more efficiently than to lysozyme, thereby retaining the antigen in the BSA gel during IHC processing and detection.

To examine the feasibility of peptide/BSA gels as IHC controls, varying concentrations of BCL2 peptide were mixed into BSA gels, then sectioned and prepared according to standard IHC techniques. BCL2 was detected using a BCL2 primary antibody directed against the human BCL2 protein (the "CONFIRM anti-bcl-<NUM> (<NUM>) Mouse Monoclonal Primary Antibody" BCL2 from Ventana Medical Systems), a biotinylated secondary antibody, avidin-conjugated HRP as a chromogenic reporter, and DAB as a chromogenic substrate. As shown in <FIG>, serially increasing BCL2 levels from <NUM> to <NUM>/mL (<NUM> * <NUM>-<NUM> M) led to a graded increase in IHC staining intensity. A negative control lacking peptide caused no staining, whereas BCL2 peptide was resolvable through the entire tested range of <NUM>×<NUM>-<NUM> mg/mL (corresponding to <NUM> peptide concentration) to <NUM>/mL (corresponding to <NUM> * <NUM>-<NUM> M peptide concentration). Importantly, <NUM>+, <NUM>+, and <NUM>+ BCL2 staining levels were each achieved at different peptide concentrations. As consistent and uniform <NUM>+ BCL2 staining is typically difficult to find in readily available tissue samples, this alternative approach provides an easier and more standardized way to obtain a gradient of BCL2 expression controls than assaying different types of tissues.

To examine the consistency of staining, six independent experiments were conducted by staining the BCL2 peptide/BSA gels as described above and analyzing the resultant images using MATLAB. Analyses of both optical density (OD; <FIG>) and image intensity (<FIG>) as a function of peptide concentration indicated that staining was consistent between experiments.

BCL2 peptide/BSA gels were also subjected to quantitative immunofluorescence staining. Like the results obtained using chromogenic staining, IF analyses showed a graded increase in staining intensity as a function of peptide concentration (<FIG>). Analyses of optical emittance (<FIG>) and signal intensity (<FIG>) again demonstrated consistent, graded increases in staining. Negative control staining with a naive control primary antibody (<FIG>, <FIG>, & <FIG>) or autofluorescence of the gels (<FIG> & <FIG>) showed no increase in intensity, as expected.

Secondary antibody controls were also examined. Using a donkey anti-mouse secondary antibody, mouse IgG embedded in a BSA gel was readily detected, whereas rat IgG gave no detectable signal, as shown in <FIG>. Similarly, using a donkey anti-rat secondary antibody, rat IgG embedded in a BSA gel was readily detected, whereas mouse IgG gave no detectable signal (<FIG>). This confirms the specificity of the BSA gel approach, demonstrating that secondary antibody does not non-specifically bind to peptide/BSA gels lacking the target of the antibody.

To examine the specificity of staining, six peptides with clinically relevant single amino acid substitutions in the anti-BCL2 clone <NUM> antibody epitope were synthesized, embedded in BSA as described above, and stained. As shown in <FIG> (and seen in <FIG>, <FIG>, and <FIG>), these mutations showed a clear effect on IHC staining (i.e., a decrease of varying degree in the intensity of signal, depending on the specific amino acid substitution), demonstrating the practicality of quantifying the effect of specific amino acid substitutions on staining intensity.

Next, the generality of the peptide/BSA gel platform was examined by screening several uncharacterized antibodies for specific detection of a target of interest. Twenty-seven (<NUM>) antibody clones isolated from mice immunized with human KSR2 were screened by IHC for detection of positive and negative controls (<FIG>). For the negative control, BSA gel with no other added protein or peptide was used. For the positive control, BSA gels were generated with the human KSR2 protein that had been used to immunize mice. As expected because the candidate antibody clones had been selected in a separate process for binding the unfixed human target protein, most antibody clones were able to detect the human protein in the protein/BSA gels (as evidenced by DAB signal of some intensity above background). However, antibodies most suitable for use in IHC assays should show little to no detectable signal for the negative control, while at the same time showing strong staining for the positive control. Among the antibodies tested, clones <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM> were the most promising, as these gave the highest ratio of specific staining of the target antigen to non-specific staining in the negative control sample. In contrast, clones <NUM>, <NUM>, <NUM>, and <NUM> showed ineffective staining of the target antigen, while antibodies <NUM> and <NUM> showed weak but significant non-specific staining of the negative control sample. These results serve to illustrate the problem that not all antibodies able to bind a target of interest in its unfixed state are suitable to use in IHC staining. The peptide control reagents and processes described here can help to identify the most suitable antibodies for use in IHC applications.

Taken together, these results demonstrate that the antigen/BSA gel approach provides a robust platform for generating IHC controls that is applicable to many target antigens. The approach yields controls representing a gradient of IHC staining that correlates with a range of target concentration, thus demonstrating the quantitative potential of target/BSA gels. Unlike IHC controls from tissues or cell lines, peptide/BSA gels are readily created in the laboratory as need requires, consistent, reproducible, inexpensive, versatile, and antigen-specific. Secondary antibody controls demonstrated the specificity of the approach. Moreover, the approach is ideally suited for identifying antibodies suitable for IHC analysis of a target of interest, a process in which hundreds to thousands of candidate antibodies are typically screened in order to identify a small handful (e.g., <NUM>-<NUM>) of leads for further characterization.

The following Example describes testing a range of conditions for their effect on gel formation.

For evaluation of BSA gel formation, <NUM> of a BSA solution in PBS was heated at a range of temperatures from <NUM>-<NUM>. No formaldehyde was included in the BSA/PBS solution. The solution was heated continuously and observed under heated conditions at various time points for liquid/solid phase. Solutions were tested at the following BSA concentrations: <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and <NUM>%.

For evaluation of other protein sources for making gels, <NUM>% solutions of the following were generated in PBS: casein, lact-albumin, soybean flour, and non-fat dry milk. 500µL <NUM>% formaldehyde was mixed with 500µL of the respective <NUM>% protein/PBS solution and incubated for <NUM> minutes at <NUM>.

For evaluation of fixatives, 500µL fixative was mixed with 500µL <NUM>% BSA solution in PBS and incubated for <NUM> minutes at <NUM>. Fixatives tested included: <NUM>% neutral buffered formalin (NBF), ½ strength Karnovski's (glutaraldehyde/paraformaldehyde), Methacarn, Carnoy's fluid, and Bouin's.

For evaluation of formaldehyde concentration, 500µL formaldehyde solution was mixed with 500µL <NUM>% BSA solution in PBS and incubated for <NUM> minutes at <NUM>. Formaldehyde solutions were tested at the following concentrations (concentration refers to the original concentration prior to diluting with BSA solution): <NUM>%, <NUM>%, <NUM>%, <NUM>%, and <NUM>%. Specifically, stock formaldehyde solution at <NUM>% was mixed with distilled water to make a final volume of <NUM> microliters containing <NUM>%, <NUM>%, <NUM>%, <NUM>%, or <NUM>% formaldehyde; these solutions were then mixed (each in separate microfuge tubes) with an equal volume (<NUM> microliters) of <NUM>% BSA solution and heated as described above.

<FIG> illustrates the effects of heating temperature and time on <NUM>% BSA gel formation. Heating at <NUM> or <NUM> did not result in any gel formation, even after heating overnight. At <NUM>, gel formation was only observed after heating overnight. However, after heating at <NUM> or <NUM>, solid gel formation was observed within <NUM> and <NUM> minutes, respectively. These results demonstrate the effects of heating temperature and time on BSA gel formation.

Next, the effect of BSA concentration on gel formation was tested. BSA was dissolved in PBS at the following concentrations: <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and <NUM>%. As shown in <FIG> (the results for <NUM>% BSA are shown in <FIG>), gel formation was observed for all concentrations upon heating at <NUM>. For BSA concentrations at <NUM>% and above, gel formation was observed within <NUM> minutes, whereas the <NUM>% BSA solution formed a gel after <NUM> minutes. No gel formation was observed for any time after heating at <NUM> or <NUM>. These results demonstrate that a wide range of BSA concentrations can generate a BSA gel under suitable heating conditions.

Next, casein, lact-albumin, soybean flour, and non-fat dry milk were tested for their ability to form gels. Each was tested at a final concentration of <NUM>% in PBS after mixing with formaldehyde (final concentration: <NUM>%), as described above. Under these conditions, only soybean flour was observed to form a gel, but the resulting solid was not homogeneous. These data demonstrate the unique properties of serum albumin in promoting gel formation.

Next, various fixatives were tested for their ability to form a BSA gel, including <NUM>% NBF, ½ strength Karnovski's (glutaraldehyde/paraformaldehyde), Methacarn, Carnoy's fluid, and Bouin's. Of these, <NUM>% NBF and Methacarn failed to form a solid. Carnoy's formed a solid before it could be mixed. ½ Karnovski's and Bouin's fixatives formed a clear solid.

The effect of formaldehyde concentration on gel formation was also tested. All concentrations of formaldehyde were able to promote solid gel formation except for <NUM>%. These results show the effect of fixative on gel formation.

The following Example describes evaluating additional proteins for their ability to form a gel suitable for IHC staining.

BSA (used at <NUM>% final concentration), egg white protein or a mixture of egg white proteins (used at <NUM>% final concentration), gelatin (used at <NUM>% final concentration), lact-albumin (used at <NUM>% final concentration), liver protein powder, soy flour (used at <NUM>% final concentration), casein (used at <NUM>% final concentration), and non-fat dry milk (used at <NUM>% final concentration) were evaluated for the ability to generate solid gels suitable for IHC staining. Protein solutions were mixed with <NUM>% formaldehyde and heated to <NUM> for <NUM> minutes.

Each type of gel was produced with or without <NUM>/mL normal rabbit IgG (DA1E). Sections were stained with donkey anti-rabbit biotinylated secondary antibody at 5µg/mL, followed by ABC-HRP detection.

For gelatin gels, <NUM>% molten gelatin was mixed with <NUM>/mL normal rabbit IgG (DA1E), then cooled for <NUM> hour at <NUM>, transferred to <NUM>% NBF overnight, transferred to <NUM>% ethanol for <NUM> days, then processed for IHC staining like tissue. For lact-albumin and liver protein powder gels, Histogel™ was added to solidify the gels. For casein gels, <NUM> NaOH was added dropwise to bring the pH to <NUM>, with agitation as required to form solution.

For imaging gels, slides were scanned on a Hamamatsu Nanozoomer, and the digital images were captured using image viewing software.

Additional substrate proteins were tested for their ability to form gels suitable for IHC staining, e.g., by forming a homogeneous gel that allows for IHC staining and adheres to a glass IHC slide. As shown in <FIG>, in addition to BSA, egg white protein or a mixture of egg white proteins and gelatin formed gels that exhibit specific IHC staining (in this case, after the gels containing a rabbit IgG antibody sample were stained with an anti-rabbit secondary antibody) with low non-specific staining (as seen after staining a gel lacking rabbit IgG with an anti-rabbit secondary antibody). In contrast, gels generated from lact-albumin, liver protein powder, or soy flour failed to generate a homogenous gel (<FIG>). Gels generated from casein or non-fat dry milk failed to adhere to IHC slides (<FIG>). A description of each gel substrate and the material resulting from attempted gel formation is provided in Table A.

In summary, BSA, egg white protein(s), and gelatin provided gels amenable for specific IHC staining. Other protein substrates either failed to form a homogeneous gel or failed to adhere to slides during IHC processing.

The synthetic IHC control concept described above was further analyzed using proof of concept applications relevant to research and clinical use, including IHC assay calibration and quality control.

Peptides were synthesized at ><NUM>% purity by New England Peptide (Gardner, MA). Amino acids <NUM>-<NUM> of the human BCL2 protein (UniProt P10415) were extended at the N-terminus by four amino acids including acetylated tyrosine to facilitate cross-linking with formaldehyde, and a spacer sequence, GSG. The C-terminus included a GSG spacer sequence followed by cysteine-amide to facilitate cross-linking with formaldehyde (<NPL>); <NPL>) or sulfhydryl-reactive reagents. The entire <NUM> amino acid peptide sequence is Ac-YGSGGAAPAPGIFSSQPGGSGC-amide (SEQ ID NO:<NUM>). Additional peptides were synthesized containing sequences from the human MYC protein (UniProt P01106): Ac-YGSGNRNYDLDYDSVQPYFYGSGC-amide (amino acids <NUM>-<NUM>; SEQ ID NO:<NUM>); Ac-YGSGDSVQPYFYCDEEENFYGSGC-amide (amino acids <NUM>-<NUM>; SEQ ID NO:<NUM>); Ac-YGSGQQQSELQPPAPSEDIWGSGC-amide (amino acids <NUM>-<NUM>; SEQ ID NO:<NUM>); Ac-YGSGFELLPTPPLSPSRRSGGSGC-amide (amino acids <NUM>-<NUM>: SEQ ID NO:<NUM>). A negative control peptide containing <NUM> amino acids from the first exon of human MCL1 (UniProt Q07820) was synthesized with the same N- and C-terminal sequence extensions described above. For some experiments, arginine, serine or tyrosine replaced the N- and C-terminal amino acids in the peptides described above. Lyophilized peptides were dissolved in a minimal volume of distilled water, DMSO or dimethylformamide. The C-terminal <NUM> amino acids (amino acids <NUM> to <NUM>) of the Kinase Suppressor of RAS <NUM> (KSR2; Uniprot Q6VAB6) protein was expressed as a <NUM> amino acid N-term [His]<NUM> tagged, maltose binding protein (amino acids <NUM>-<NUM>) fusion construct. The fusion protein was expressed as a baculovirus construct in Trichoplusia ni Tni Pro insect cells (Expression Systems; Davis, CA), then purified by sequential nickel-nitrilotriacetic acid affinity, amylose affinity and Sepharose S200 size exclusion chromatography. Purified mouse IgG1 clone MOPC-31C (BD Pharmingen, San Jose, CA), rat IgG1 clone R3-<NUM> (BD Pharmingen, San Jose, CA) and rabbit IgG clone DA1E (Cell Signalling Technology, Danvers, MA) were obtained commercially. Bovine serum albumin (BSA; Ultra Pure) was purchased from Cell Signaling Technology (Danvers, MA). Food-grade gelatin and dried egg whites were from Knox (Oakbrook, IL) and Judees Gluten Free (Columbus, OH), respectively.

Unless otherwise noted, <NUM> of solution containing the desired antigen in <NUM>% (w/v) BSA / phosphate-buffered saline (PBS) was mixed in a <NUM> microfuge tube with an equal volume of <NUM>% formaldehyde (Electron Microscopy Sciences; Hatfield, PA), heated for <NUM> minutes at 85C to coagulate the solution, then fixed overnight at room temperature. The final reagent concentrations were <NUM>% (<NUM>) BSA and <NUM>% (<NUM>) formaldehyde. Peptide antigens had final concentrations of <NUM> x <NUM>-<NUM> M to <NUM> x <NUM>-<NUM> M in the fixed gels. Gels containing naive mouse, rat and rabbit IgG had a final concentration of <NUM>/ml (<NUM> × <NUM>-<NUM> M) IgG. The human [His]<NUM>-MBP-KSR2 fusion protein had a final gel concentration of <NUM>/ml (<NUM> × <NUM>-<NUM> M).

In tests of alternative fixation protocols, formalin-free zinc fixative (Cat. number <NUM>, BD Pharmingen; San Jose, CA) was used in place of <NUM>% formaldehyde and NBF in the protocols above. In other experiments, antigen in <NUM>% BSA in PBS was solidified by heating for <NUM> minutes at 85C in the absence of formaldehyde. The solidified gel was then transferred to <NUM>% neutral buffered formalin (NBF; VWR International, LLC, Radnor, PA), <NUM>% paraformaldehyde (PFA; VWR International, LLC, Radnor, PA), or zinc fixative for overnight fixation at room temperature.

Tissue microarrays (TMA) were constructed using a TMA Grand Master tissue microarrayer (3DHISTECH, Budapest, Hungary). Duplicate <NUM> diameter cores were punched from donor paraffin blocks containing the desired protein gels, then transferred to recipient paraffin blocks. Completed recipient TMA blocks were heated at 37C overnight, then at 70C for <NUM> minutes, before being cooled and sectioned.

Primary antibodies used were mouse anti-human BCL2 clone <NUM> (Ventana Medical Systems; Tucson, AZ), rabbit anti-human BCL2 clone EPR17509 (Abcam; Cambridge, MA), rabbit anti-human BCL2 clone SP66 (Ventana Medical Systems, Tucson, AZ), rabbit anti-human BCL2 clone E17 (Abcam, Cambridge, MA), rabbit anti-human MYC clone Y69 (Ventana Medical Systems, Tucson, AZ) and rabbit anti-human MCL1 clone SP143 (Ventana Medical Systems, Tucson, AZ). The Fluidigm EPR17509-<NUM>Nd antibody was purchased from Fluidigm (South San Francisco, CA). A panel of <NUM> mouse hybridoma antibodies to the [His]<NUM>-MBP-human fusion protein was generated at Chempartner (Shanghai, China). Biotinylated donkey anti-rabbit, rat and mouse secondary antibodies were purchased from Jackson Laboratories. Staining protocol details are summarized in Table <NUM>.

Four micron paraffin sections were deparaffinized and rehydrated in xylene and graded alcohols. Staining was performed on the Ventana Benchmark XT, Ventana Discovery XT, Ventana Benchmark Ultra XT instruments (Ventana Medical Systems, Tucson, AZ) or the Dako Universal Autostainer (Agilent, Santa Clara, CA). Sections were pretreated with Cell Conditioning Solution <NUM> (CC1) (Ventana Medical Systems, Tucson, AZ) or Target Retrieval Solution, pH6 (Ready-To-Use) (Dako - Agilent Technologies, Santa Clara, CA) depending on the optimized antibody protocol. Slides stained on the Ventana instruments were counterstained with Ventana hematoxylin and bluing reagents (Ventana Medical Systems, Tucson, AZ) for <NUM> minutes each. Slides stained on the Dako Universal autostainer were counterstained in Mayer's hematoxylin (Rowley Biochemical, Danvers, MA) and Richard-Allen Scientific Bluing Reagent (Thermo Fisher Scientific, Waltham, MA) for <NUM> minute each. Stained sections were dehydrated in graded alcohols to xylene before coverslipping. Immunofluorescent slides were coverslipped with Prolong Gold mounting media (Life Technologies, Carlsbad, CA).

Sections were baked at 70C for <NUM>, deparaffinized, rehydrated in descending EtOH series and pretreated with Target Retrieval Solution, pH6 (Dako - Agilent Technologies, Santa Clara, CA), blocked for <NUM> minutes in <NUM>% donkey serum, <NUM>% BSA in PBS, then incubated with <NUM>Nd-EPR17509 in blocking buffer. Slides were incubated without coverslip at <NUM>° C overnight in a humidified closed container, then rinsed <NUM> times in PBS, post-fixed in <NUM>% Glutaraldehyde/PBS for <NUM> at 20C slides, rinsed in ddH<NUM><NUM>, dehydrated in increasing EtOH series, air dried and stored at 20C until imaging.

TMA cores stained with <NUM>Nd-labeled EPR17509 were analyzed in the Fluidigm Hyperion imaging mass spectrometer (South San Francisco, CA) by defining ablation regions of interest (ROIs) of <NUM> microns square in each TMA core. The integrated ion counts for each ROI were converted to antibody mass using antibody standard data as described below.

Control aliquots of <NUM>, <NUM> and <NUM> micrograms/ml <NUM>Nd-labeled EPR17509 were prepared in <NUM>% donkey serum in <NUM>% BSA in PBS. One microliter of each antibody sample was spotted on a glass slide and air dried before being ablated in the Hyperion machine (UV laser intensity = <NUM>). The integrated ion count for each antibody spot was used to calibrate the ion counts measured in ROIs from stained TMA cores.

Whole-slide brightfield images were acquired at a scanning resolution of <NUM> microns/pixel using the Hamamatsu (Bridgewater, NJ) Nanozoomer-XR digital slide scanner equipped with a 20X <NUM> NA objective lens. Brightfield imaging was performed in semiautomatic batch mode. The scan area and focus points were manually created for each slide prior to automated high resolution whole-slide imaging. Immunofluorescence whole-slide images were acquired using the Nanozoomer-XR or 3D Histech Pannoramic <NUM> scanner (using a 20X <NUM> NA objective lens with a resolution of <NUM> microns/pixel). On the Nanozoomer-XR system, illumination power of the fluorescent module was set at <NUM>% and immunofluorescence signal was captured using a TRITC (antibody signal at 1x exposure, <NUM> photon collection, 1x gain), DAPI (autofluorescence at 2x exposure, <NUM> photon collection, 2x gain) and CFP filter (autofluorescence at 4x exposure, <NUM> photon collection, 2x gain). Acquisition on the Pannoramic <NUM> system was performed using a CY5 (antibody signal at <NUM> exposure), FITC (antibody signal at <NUM> exposure), DAPI (autofluorescence at <NUM> exposure) and CFP filter (autofluorescence at <NUM> exposure). Image analysis was performed using Matlab version <NUM>. Regions of interest (ROI) on brightfield images were manually created and edited to exclude areas of the gel that had artifacts or were torn. Small holes or tears within the ROI were excluded using manual color thresholds. ROI for immunofluorescent images were either manually created or automatically generated when sufficient signal above background of the gel is available from the autofluorescence image by thresholding (on either the DAPI or CFP channel) and morphological filtering. ROIs were transferred onto images acquired in the antibody-fluorochrome channel for intensity measurement. Average grayscale intensity was calculated in <NUM>-bit depth for both brightfield and fluorescent images. Plotted Y-axis values for average brightfield pixel intensity represent <NUM> minus average pixel grayscale intensity. Digital slide scan images are presented without alteration of the original intensity or contrast. Staining intensity profile quantification on lines drawn across donor block sections were assessed using the Analyze/ Plot Profile function in ImageJ (version <NUM>. 52a; Wayne Rasband, imagej.

Graphing was done with Prism GraphPad (version <NUM>). Signal intensity data, corrected for glass slide background, were plotted vs. the logic of the formulated antigen concentration. Curve fitting used the variable slope <NUM>-parameter model constrained so that the bottom of the fitted curve was equal to the mean intensity of the no-antigen cores for each assay, reported in the tables associated with each graph as "Bottom". The other parameters reported in the tables [signal maximum, span, antigen concentration at half-maximum signal (ACHM), and Hill slope], were calculated by the software.

As demonstrated in Examples <NUM>-<NUM>, when incorporated into protein gels, synthetic peptides encoding an antibody target epitope can be detected using routine immunohistochemical and immunofluorescent procedures. Donor blocks containing target peptides had relatively homogenous antigen distribution when assessed by chromogenic assays (<FIG>). A tissue microarray (TMA) can be constructed containing the desired antigens in a range of antigen concentrations. <FIG>show a TMA composed of duplicate cores containing either no added peptide, serial dilutions of a negative control peptide from the human MCL1 protein, or dilutions of peptide encoding amino acids <NUM>-<NUM> of the BCL2 protein. Parallel sections of this TMA were stained with four anti-BCL2 antibodies: clone <NUM>, raised against the same peptide sequence used in the target peptide; SP66 and E17, both raised against peptide antigens C-terminal to the sequence in the reagent (<NPL>); and EPR17509, raised against an undisclosed BCL2 peptide antigen (www. com/BCL2-antibody-epr17509-hrp-ab209039. Separate slides were stained using chromogenic (all antibodies) and immunofluorescent (for clone <NUM> only) methods.

As expected, SP66 and E17 did not react detectably with any core in the sections from this TMA (<FIG>). With both clone <NUM> and EPR17509, the signal in the TMA cores increased with increasing concentration of BCL2 peptide (<FIG>), consistent with a specific interaction between the antibody and antigen in the cores.

Digital image quantification allowed a more precise evaluation of the data (<FIG>). The fitted curves and the associated parameters reported in <FIG> quantify the minimum and maximum signal intensity, the dynamic range, the antigen concentration at which the signal is half-maximal (ACHM), and the steepness of the antigen concentration vs. signal intensity curve in this range (HillSlope). With the conditions tested here, non-specific signal in cores containing no added peptide was <NUM>% of the maximum detectable signal in the clone <NUM> chromogenic assay, <NUM>-fold lower than this in the fluorescent clone <NUM> assay, and <NUM>% in the EPR17509 assay. On the other hand, for EPR17509, the ACHM value, which reflects the relative sensitivity of the assay, was approximately <NUM>-fold lower (i.e., more sensitive) than the ACHM for the chromogenic clone <NUM> assay, and more than <NUM>-fold lower than the fluorescent clone <NUM> assay. The immunofluorescent clone <NUM> assay had a slope <NUM>-<NUM>% steeper than either chromogenic assay, reflecting the narrower range of antigen concentration between the threshold of detection and maximum signal.

To assess the reproducibility of the peptide controls in repeated assays, a second BCL2 peptide TMA was constructed using new donor paraffin blocks formulated to have the same target BCL2 peptide concentrations as those used to build the TMA in <FIG>. Replicate sections of this second TMA were stained by two operators, on six separate days in a <NUM> month interval using the same anti-BCL2 clone <NUM> chromogenic IHC protocol used in <FIG>. Quantitative digital image analysis of the stained sections showed the signal intensity for each core was highly reproducible (<FIG>; see also <FIG> & <FIG>).

In parallel experiments, clone <NUM> was used to stain TMAs containing BCL2 peptide with or without prior antigen retrieval. The results showed that antigen retrieval improved signal strength approximately <NUM>-fold, but was not required for staining (<FIG>).

Without wishing to be bound to theory, it is thought that the concentration of peptide that is available to bind antibody is reduced from the formulated value by three parameters: the efficiency of crosslinking the peptide to the protein matrix, the biochemical integrity of the peptide and accessibility of the cross-linked peptide to antibody. Formaldehyde was intended to cross-link BSA sidechains to the target peptides by reaction with the N-terminal tyrosine and C-terminal cysteine included in the peptide sequence. Of the amino acids internal to the BCL2 peptide (A, F, G, I, P, Q, S), only glutamine has been reported to react with formaldehyde (<NPL>).

To assess the effect of alternative N- and C-terminal amino acids on signal intensity, four variants of the BCL2 peptide were tested. The N-terminal tyrosine was replaced with serine, expected to be minimally reactive with formalin, or with arginine, reported to be <NUM>% more reactive than tyrosine (<NPL>). Other variants included tyrosine or arginine at both the N- and C-termini. TMA cores containing serial dilutions of each peptide were prepared and stained as described above.

The results showed a significantly higher signal for the peptide with N-terminal arginine ("R-C", <FIG>). The ACHM for this variant was <NUM> x <NUM>-<NUM> M peptide, <NUM>-fold lower than the corresponding value for the original Y-C variant. The other variants showed a range of intensities similar to (S-C) or weaker than (R-R, Y-Y) the original Y-C variant. Notably, variants containing arginine or tyrosine at the N-terminus with cysteine at the C-terminus reacted more strongly than peptides with arginine or tyrosine at both ends.

Next, experiments were undertaken to establish whether peptides containing epitopes for proteins other than BCL2 would react in a similar fashion. The anti-MYC antibody Y69 is reported to bind an epitope in the N-terminal <NUM> amino acids of the human MYC protein (www. com/c-MYC-antibody-y69-ab32072. Candidate epitopes from this region were incorporated into BSA gels as described above and tested for Y69 binding.

A peptide containing MYC amino acids <NUM>-<NUM> reacted strongly with the antibody, whereas other peptides reacted only weakly (aa <NUM>-<NUM>) or not at all (aa <NUM>-<NUM> and aa <NUM>-<NUM>) (<FIG>). A TMA was constructed containing duplicate cores, in the range of concentrations described earlier, of both the MYC aa <NUM>-<NUM> peptide and the BCL2 peptide. Chromogenic detection using anti-BCL2 clone <NUM> and anti-MYC clone Y69 showed a range of signal intensity, with no cross-reactivity to the non-target peptide (<FIG>). Quantification of the resulting data (<FIG>) showed the MYC protocol had a <NUM>-fold lower ACHM than does the BCL2 protocol, with similar dynamic range and HillSlope parameters (<FIG>).

Dual immunofluorescent detection on the same TMA used in <FIG> was done using serial incubation with both anti-BCL2 and anti-MYC primary antibodies appropriate detection reagents (<FIG>). Qualitative results (<FIG>) showed the expected specificity with no cross-reactivity between either antibody and the non-target peptide. Isotype controls used in place of antigen-specific primary antibodies resulted in no signal. Quantification of the resulting fluorescent data shows increased HillSlope parameters and increased replicate variability under the conditions tested, relative to the chromogenic protocol.

These results confirm the broadly applicable utility of peptide antigens as IHC controls using diverse epitopes and detection protocols.

The quantitative data obtained allowed a determination of the limit of detection (LOD) and reproducibility of the clone <NUM> BCL2 IHC assay.

The experiments illustrated in <FIG>, <FIG>, <FIG>, & <FIG> represent <NUM> independent analyses, each with duplicate TMA cores containing no target peptide (blank) and <NUM> concentrations of BCL2 peptide. <FIG> and Table C summarize the data. For each experiment, the means of duplicate TMA cores at each peptide concentration were used in the calculation. Values are pixel intensities of the TMA cores corrected for the background intensity of the glass slide (<NUM> +/- <NUM> units).

According to an accepted clinical laboratory convention (Armbruster and Pry, <NUM>), the limit of blank (LOB; = meanblank + <NUM> x SDblank) for these data is <NUM> units, and the limit of detection ((LOD; = LOB +<NUM> x SDlow-positive sample) is <NUM> units, corresponding to <NUM> × <NUM>-<NUM> M peptide. This concentration equates to a formulated antigen density of approximately <NUM> molecules per cubic micron of gel.

Consistent with these calculations, results of two-tailed t-test comparisons of data for cores with increasing BCL2 peptide concentrations (Table C) showed that cores with <NUM> x <NUM>-<NUM> M peptide (below the calculated LOD) are not significantly different from cores lacking peptide, whereas cores with <NUM> × <NUM>-<NUM> M peptide and higher (above the calculated LOD) are statistically different adjacent cores. Notably, signal in cores containing the two highest peptide concentrations are statistically different, despite the fact that the subjective intensity of these cores is similar.

As described in Example <NUM> above (see <FIG>), another protein was incorporated into a BSA gel and used to evaluate antibodies generated from hybridoma clones. In this case, a peptide including the C-terminal <NUM> amino acids (i.e., amino acids <NUM>-<NUM>) of human KSR2 was used. The results described above demonstrate that the synthetic IHC controls approach provides a useful method of identifying antibodies suitable for IHC analysis of a target of interest, a process in which hundreds to thousands of candidate antibodies are typically screened in order to identify a small handful (e.g., <NUM>-<NUM>) of leads for further characterization.

Candidate antibodies were further tested for reactivity against the antigen without prior antigen retrieval. For each antibody, antigen retrieval was required for detectable reactivity (<FIG> & <FIG>). This contrasts with the results with BCL2 peptides described in Example <NUM>, for which antigen retrieval improved signal strength approximately <NUM>-fold, but was not required for detectable reactivity (<FIG>).

As a test of full length proteins, rabbit, rat and mouse full-length immunoglobulins (IgG; (<NUM>/ml; <NUM> x <NUM>-<NUM> M) incorporated into BSA gels were prepared and used as technical controls for IHC assays employing biotinylated donkey anti-rabbit, anti-rat and anti-mouse secondary antibodies in detection steps. The results show the expected signal and specificity of the anti-rabbit, anti-mouse and anti-rat secondary antibodies (<FIG>). As shown in <FIG>, incorporated rabbit IgG detected with donkey anti-rabbit IgG secondary antibody shows <NUM> units of pixel intensity. Assuming quantitative retention of the added rabbit IgG, <NUM> cubic micron (<NUM>-<NUM> L) of this sample contains <NUM> × <NUM>-<NUM> moles, or about <NUM> detectable molecules of rabbit IgG.

Because some epitopes are rendered non-reactive by formalin-containing fixatives, a commercially available formalin-free zinc fixative was tested as an alternative to the <NUM>% formaldehyde used in previous experiments.

Donor blocks containing BCL2 and MYC peptides in BSA gels were fixed with either formalin or zinc-based fixatives with heating to 85C (<FIG>). For both BCL2 and MYC peptides, the signal was approximately <NUM>-fold stronger with formaldehyde fixation than with zinc fixation during heating. The loss of signal strength correlated with heating in the presence of zinc.

Alternative procedures in which BSA - antigen mixtures were heated to 85C in the absence of fixative, followed by fixation at room temperature in a variety of fixatives (<NUM>% paraformaldehyde, neutral-buffered formalin, zinc-containing formalin-free fixative) resulted in comparable signal to the standard protocol described above (<FIG>). This demonstrates that the technique can accommodate a variety of fixatives, potentially broadening the range of epitopes and antibodies that could be used.

In the experiments reported above, the number of chromogen or fluorochrome molecules deposited for each molecule of antibody bound is unknown, so the absolute concentration of epitope detected cannot be calculated. To avoid this limitation, a more quantitative direct detection procedure using the Hyperion mass spectrometry-based imager was tested.

Sections of the BCL2 peptide TMA were stained with <NUM>Nd-labeled anti-BCL2 antibody EPR <NUM>, then analyzed by ultraviolet laser ablation and quantitative mass spectrometry. The TMA sample quantified was typically an area <NUM> microns square by <NUM> microns thick, containing <NUM> × <NUM><NUM> cubic microns. The results showed a graded signal that increases as the BCL2 peptide concentration in the target increases (<FIG>). The EPR17509 antibody signal in cores containing no peptide or in cores containing the negative control MCL1 peptide was less than <NUM>% of the maximum signal.

The correlation between measured ion counts and antibody concentration was determined by analyzing known amounts of <NUM>Nd-labeled antibody spotted directly onto glass slides. The resulting calibration data (Table D) showed a ratio of approximately <NUM> antibody molecules per <NUM>Nd ion detected.

By comparing the amount of BCL2 peptide formulated in each TMA core sample with the amount of <NUM>Nd-labeled anti-BCL2 antibody measured, the fraction of BCL2 peptide that was detectable in the TMA cores was determined. Results (Table E) showed that the amount of detectable BCL2 peptide increased with increasing peptide concentration, as expected.

The results also showed that the proportion of added BCL2 peptide that is detectable decreased as the antigen concentration increased; <NUM>% of the added peptide was detectable at <NUM> x <NUM>-<NUM> M, whereas ~<NUM>% of added peptide was detected at <NUM> x <NUM>-<NUM> M.

Taken together, the results of Examples <NUM>-<NUM> demonstrate that antigen-containing gels can be created using materials and methods available in any histology laboratory, and can be embedded and sectioned to produce uniformly stained samples. These examples demonstrate that the method is compatible with a variety of fixatives and with detection using chromogenic, immunofluorescent and mass spectrometry-based methods. The choice of possible antigens is limited only by the availability of the target protein or knowledge of the linear epitope sequence. The ability to create synthetic controls of known composition offers the opportunity to more precisely characterize and control routinely used immunohistochemistry protocols, independent of complicating factors inherent in heterogeneous tissue samples and subjective human interpretations.

These results provide proof of concept detection of linear peptide epitopes from BCL2, MYC and MCL1 using antibodies specific to each protein, and further demonstrate detection of full-length IgG and a <NUM>-amino acid human protein. The target epitope concentrations formulated and tested here span four orders of magnitude, from <NUM> × <NUM>-<NUM> M to <NUM> x <NUM>-<NUM> M, extending to the upper end of the range of protein concentrations found in tissue. At the high end of this range, average intermolecular distance is less than <NUM>, approaching the distance between the two arms of a full-length IgG molecule (~<NUM>). Only very abundant proteins reach this density. The protein concentration used in this procedure is in the range (<NUM>-<NUM>%) found in many tissues (<NPL>), and so reproduces some mechanical and biochemical tissue properties.

These results further demonstrate that quantitative parameters relevant to IHC assay performance in tissues - non-specific background, limit of detection, dynamic range, antigen concentration at half-maximum signal and Hill Slope - can be assessed with objectively definable precision in any laboratory with access to a digital slide scanner and basic image analysis capabilities. The results demonstrate that these parameters can vary with different experimental conditions when using one antigen-antibody pair, with different antibodies detecting the same antigen and with different antibody/antigen pairs. For instance, measured ACHM values differed by more than <NUM>-fold in the three assays shown in <FIG>. In a series of <NUM> replicate experiments with the clone <NUM> BCL2 assay, the calculated data parameters for maximum signal, dynamic range, log(ACHM) and Hill Slope have coefficients of variation of less than <NUM>%, but higher precision may be possible. These experiments used serial <NUM>-fold dilutions of antigens across a physiologic range (<NUM> x <NUM>-<NUM> to <NUM> x <NUM>-<NUM> M), but the precision of ACHM and slope quantification could potentially be improved by including more samples between <NUM>% and <NUM>% of the dynamic range of the assay. Because assay performance can be assessed more objectively and precisely than is possible by subjective human evaluation of tissues or cell pellets, the performance of assays can be tailored more precisely to the clinical need, and more rigorously controlled.

Synthetic controls with a range of antigen concentrations allow IHC staining protocols to be optimized, quantified and controlled over time using a reproducible standard. The concept described here allows quality control of an IHC assay at a level intermediate between the two extremes of assessing the interaction between purified antibodies and antigens under controlled in vitro conditions and assessing antibody reactivity in tissue samples by empirical optimization.

Application of this method to ongoing quantitative immunohistochemical analyses is contemplated. A synthetic antigen gel sample is homogeneous, has a uniform thickness, and contains a known number of epitope molecules, allowing correlation of signal intensity to antigen concentration. An on-slide TMA section including a useful range of epitope concentrations can easily fit adjacent to a diagnostic tissue section, permitting assay technical adequacy, or quantitative image analysis to be assessed on any slide. Because the components and procedures used in this method are completely defined, reagents created in different laboratories should, in principle, be functionally similar. This approach can allow investigators to compare and calibrate protocols used in different laboratories, to communicate more clearly when describing qualitative staining endpoints, and ultimately, to more precisely control of IHC assays used both in research and patient-care decision-making.

The above Examples (see, e.g., Example <NUM>) describe substrate proteins tested for their ability to form gels suitable for IHC staining, e.g., by forming a homogeneous gel that allows for IHC staining and adheres to a glass IHC slide. This Example describes testing poly-lysine for antigen/gel matrix formation.

Poly-lysine provides multiple formaldehyde-reactive side chains and is commercially available in a variety of molecular weights ranging from <NUM>-<NUM> kDa up to greater than <NUM> kDa. A poly-lysine monomer has a formula weight of <NUM> Da (assuming no salt counterion), so the commercially available poly-lysine molecules include polymers in a range of <NUM>-<NUM> lysine monomers to greater than <NUM>,<NUM> lysine monomers.

As demonstrated above, BSA at <NUM>% (final concentration), upon heat denaturation and reaction with formaldehyde, produces a useful matrix in which target antigens can be embedded and cross-linked. BSA at <NUM>% is at a concentration of <NUM>, assuming a formula weight of <NUM>,<NUM> Da. Each BSA molecule contains <NUM> lysine side chains. In addition to lysines, each molecule of BSA also contains <NUM> arginine and <NUM> tyrosine side chains, each of which can react with some efficiency with formaldehyde. Thus, in a <NUM>% BSA gel, there are <NUM> x <NUM>, or <NUM>, lysine side chains.

A sample of poly-lysine gel matrix based on similar calculations therefore contains approximately <NUM> lysine side chains in the final gel. Assuming a formula weight for a single lysine side chain in a poly-lysine polymer of <NUM> Da, the final concentration of poly-lysine in a gel is approximately <NUM>/mL. A higher weight per volume is used if the commercial poly-lysine contains a counterion salt. For example, poly-lysine * HBr salt has a formula weight of <NUM>/mol. The desired final concentration of poly-lysine is achieved by mixing equal volumes of <NUM> poly-lysine solution (approximately <NUM>/mL pure, <NUM>/mL as HBr salt) with concentrated formaldehyde stock (<NUM>%). Without wishing to be bound to theory, it is thought that lysine side chains in the sample cross-link with each other, within and/or between poly-lysine strands, to form a gel.

Claim 1:
A method for generating a solid antigen/carrier protein gel for immunohistochemical (IHC) staining, the method comprising:
(a) mixing a purified antigen with a liquid solution comprising a carrier protein to produce an antigen/carrier protein liquid solution; and
(b) heating the antigen/carrier protein liquid solution to form the solid antigen/carrier protein gel;
wherein the carrier protein consists of a serum albumin protein and the antigen/carrier protein liquid solution includes a cross-linking fixative.