METHOD AND MEANS FOR DETERMINATION OF CELLS IN A SAMPLE

Abstract: Methods and compositions are provided for concentrat­ ing particles in a minute area on a solid surface. The method permits the detection of small amounts of analytes by providing for an observable signal in relation to the concen­ tration of particles in the area.

Description: BACKGROUND OF THE INVENTION The clinical laboratory has become an increasingly important adjunct to medicine, both in diagnosis and therapy. As the variety of situations in which determinations are desired have expanded, there has been an increasing variety of approaches for measuring the substance of interest. There are many considerations involved in the development of the assay. One consideration is the simplicity of the protocol. The more measurements and steps that are required, the greater the likelihood for error. A second consideration is the concentration range and absolute amount to be measured. A third consideration is the nature of the sample involved. A fourth consideration is the nature of pretreatments which may be required. A fifth consideration is the nature of interfering substances in the samples. A sixth consideration is the intended environment in which the assay is performed and the technical skill of the persons who will perform the assay. This will also involve whether an instrument is to be used or only a visual determination. Thus, each new development provides advantages which find particular applications as to analytes, preparation of reagents, nature of the users, and manner of performance. Description of the Prior Art Anderson, Anal. Biochem. (1970) 38:175-189 describes the use of cellulose wicks to monitor agglutination reactions. FR-A-2379072 describes a method for determining a ligand, typically an antigen, in a liquid medium using immunoassay technology, typically radioimmunoassay technology, with the improvement that the labelled antigen-antibody complex is separated from free labelled antigen with a column comprising an adsorbent material which is selective for binding either the free or complexed species. It is directed to the analysis of low molecular weight analytes and not the detection of cells. US-A-4264766 discloses a water-insoluble immunological reagent comprised of latex particles to which a water soluble polyhydroxy compound is covalently bound having covalently bound there to an immunologically active material. The reagent is described as useful for diagnostic immunological methods such as the determination of human placental lactogen by agglutination. It indicates that cells can be linked to latex beads for use as a reagent for determining the presence of an analyte, but not as the analyte itself. US-A-4098876 discloses a reverse solid phase sandwich immunoassay for the detection of an analyte in a sample, said assay comprising the following steps: (1) incubation of the sample suspected of containing the analyte with a labelled antibody, (2) a second incubation with an immobilized antibody, (3) a separation step, and (4) a detection step. It recognizes that it is advantageous to add the labelled antibody to the sample before adding the immobilized antibody. It does not indicate that the method described therein is applicable to the detection of cells and only mentions the use of sedimentation, centrifugation or magnetism for separating bound and unbound species. The present invention recognizes that immunoassay techniques, employed in a method that separates bound from unbound label, can be applied to the determination of analytes that are orders of magnitude greater in size than previous analytes described in the art, i.e. cells. According to one aspect of the present invention there is provided a method for detecting the presence of cells in a sample, said method comprising: contacting an area of a bibulous member with an aqueous assay medium containing said cells, whereby said assay medium is wicked away from said area, said cells either being within a predetermined size range or having been converted to a predetermined size range, said size range being predetermined in relation to the bibulous member such that the cells are thereby concentrated in a localized site on said bibulous member in the region of the air/liquid interface, and detecting a signal as a result of a signal-producing system, wherein said signal is related to the amount of said cells in said localized site. According to another aspect of the present invention there is provided a device for conducting an assay, said device comprising a non-bibulous member having an orifice and serving as a support for a bibulous member, said bibulous member having an orifice that extends through said bibulous member, and is in register with the orifice in said support. The invention also provides diagnostic kits for use in the methods of the present invention. DESCRIPTION OF THE SPECIFIC EMBODIMENTS The subject invention is based on the concentration of particles at a small site, generally having one dimension less than one mm wide, on a bibulous solid support in relation to the presence or absence of an analyte in a sample. The site may be a point, a straight or curved band. The concentration of particles at the predetermined site can be used to provide a detectable signal at a site. The sample may influence the concentration of the particles at the site and/or the production of a detectable signal. The detectable signal will be determined in a detection zone, which may or may not be the concentration site. The particles which are involved in the assay may be present in the sample, may be added as reagents or formed in situ . The nature of the particle may vary widely, being naturally occurring or synthetic, being a single material, a few materials, or a combination of a wide variety of materials. Naturally occurring particles include nuclei, mycoplasma, plastids, mammalian cells and unicellular microorganisms, e.g., bacteria. Synthetic particles may be prepared from synthetic or naturally occurring materials, such as metal colloids or latex particles made from polystyrene polyacrylates or naturally occurring materials, such as polysaccharides, e.g., agarose. Non-naturally occurring particles may be varied depending upon the particular assay, the protocol for the assay, or other considerations. The size of the particles will vary widely, generally ranging from 0.05 to 100 µm, more usually from 0.1 to 75 µm. The particles may be charged, either positively or negatively, may be amphoteric or lack any charge, being neutral. The presence or absence of charge will affect other parameters involved in the assay. The means for detecting the detectable signal at or away from the concentration site may or may not be an intrinsic property of the particles. The particles may be labeled with a wide variety of materials which allow for detection, such as radionuclides, dyes, fluorescers, enzymes, or other convenient label providing for a detectable signal, either visually observable or detectable by instrumentation. The various labels would normally be covalently bonded to the particle, using linking arms as appropriate. The labels may be bound to the surface or, when feasible, extend throughout the particle. Any convenient bibulous absorbent solid material may be employed which allows for capillary transport of a liquid away from the interface between the air and liquid. Various materials include paper, cellulose particles, silica gel, cellulosic beads, glass fiber and filter paper. The surface should be relatively smooth, so as to allow for the formation of a concentrated particle site, for example, in the form of a sharp band or point. The size and shape of the bibulous material may be varied widely considering the purpose of the material. Namely, one wishes to form a fine band, curve, circle, line, or point by concentrating small particles which are relatively uniformly dispersed in a liquid medium. The concentrating is best achieved by transporting a relatively large volume through a relatively narrow area. The bibulous material may be shaped, therefore, as a narrow strip of from one to five millimeters in width, a triangular shaped structure having a rounded tip or a circular disc having a central small orifice or other structure. The size of the orifice may vary depending on whether the orifice extends solely through a non-bibulous support, leaving the bibulous member underneath the orifice intact, or the orifice extends through the bibulous member underneath. In the first situation, the orifice will generally be 0.10 to 2 mm, usually 0.25 to 1 mm. In the second case, the orifice will generally be less than one millimeter, ranging from 0.1 to 0.5 mm. In each case, the bibulous material will usually have a support which provides structural strength. The non-bibulous material may be a water impermeable layer or coating. The liquid medium will normally be an aqueous medium, which may have from 0-40 volume percent of a miscible solvent such as alkanols, ethers, sulfoxides, or amides, generally ranging from 1 to 6 carbon atoms. In performing the subject invention, one wishes to concentrate or collect cells at the air-liquid interface. By appropriate choice of conditions in the aqueous medium, one can modulate the size of cells which will concentrate at the air-liquid interface as contrasted with following the solvent front. The choice of conditions will vary, with the nature of the cell, as to size, charge, polarity or other property which affects the repulsion or attraction of the cells to each other. The conditions will be chosen so that cells of above a certain size will be retained at the air-liquid interface, while cells smaller than that size will travel away from the air-liquid interface. Factors that affect the size of the cells which will migrate involve repulsive and attractive forces, which can be influenced by pH and ionic strength. Where the cells are heavily charged with the same charge, at a relatively high ionic strength, the charge will be neutralized which will allow the cells to aggregate or band together. Where the cells have acidic or basic groups, a pH can be chosen to reduce the number of charges present on the cell. Conditions can be chosen so that the cells will not aggregate unless a binding means is provided. The binding means will be a member of a specific binding pair, which is polyvalent and is, therefore, capable of binding to at least two cells. The pH will vary with the nature of the cell. For basic cells, one may induce repulsive charges by lowering the pH, while for acidic cells, one may induce repulsive charges by raising the pH. The pH will be chosen so as to maintain mild repulsion between cells, so as to encourage the transport of the cells, except where a plurality of cells are joined together. Ionic strength may also be used in a similar fashion to modulate repulsion. By reducing or raising the ionic strength, one may modulate the repulsive effect between cells, so that at low ionic strength, one enhances the repulsive effect, while at high ionic strength, one reduces the repulsive effect between charged cells. Therefore, depending on the nature of the cells, one will modify the conditions of the medium. The various conditions will be optimized depending on the nature of the system. The pH which is employed will generally be in the range of 2 to 12, with the range varying from 3 to 7 for positively charged cells and from 6 to 11 for negatively charged cells. The ionic strength will generally vary from 10⁻¹ to 10⁻⁴. One may optimize these two parameters empirically depending upon the size and nature of the cells involved. A further factor is the inclusion of surfactants in the assay medium. The surfactants aid in the migration of the cells through the bibulous support. By modifying the surface tension, migration of the cells may be enhanced or diminished. The surfactants may be anionic, cationic or non-ionic, preferably non-ionic or combinations of non-ionic and anionic. Surfactants will be used in minor amount, generally being present in from 0 to 2 vol% of the assay medium, more usually from 0.005 - 1.5 vol%, and preferably from 0.01 to 1 vol%. Various surfactants may be used, such as Tween® 20 (ICI Americas Inc., Wilmington Del 19897., Polysorbate 20, polyoxyethylene 20 sorbital monolaurate, C₅₈ H₁₁₄ O₂₆), Triton® QS44 (Rohm and Haas registered trademark, a polyoxyethylene ether surface active agent. Sigma Chemical Co., 3050 Spruce Street, St. Louis, MO 63103.), PEG® 1500 (polyethylene glycol 1500: copolymer H(OCH₂CH₂) n OH, n = 29-36, M.W. = 1300-1600. Sigma Chemical Co., 3050 Spruce Street, St. Louis, MO 63103.), etc. Other factors may also be employed to affect properties of the assay medium. Chaotropic or antichaotropic agents may be employed. Illustrative chaotropic agents include fluoride ion and polyethylene glycol. Illustrative antichaotropic agents include trichloroacetate, thiocyanate and dextran. Agents to modify the viscosity of the medium may be employed. Elevated or reduced temperatures may also find application. Method As indicated previously, the subject method is predicated on concentrating cells over a predetermined size at a localized site at the air-liquid interface. For example, a clinical sample which has a heterogeneous population of cells, where one wishes to determine the presence of a particular species or strain. By employing a labeled antibody in the medium, one could rapidly tag any cells of interest. The presence of the tag at the localized site would be diagnostic of the presence of the particular cells. Any unbound label would follow the solvent front, minimizing any background. The absence of an observable band would indicate the absence of the cells of interest in the sample. One can relate the presence of a detectable signal at a localized site on a bibulous surface to the presence of the cell of interest in the assay medium. The concentration of cells at the localized site is achieved by providing bridges between cells of specific binding members. Specific binding members can be broken up into two primary groups: (1) ligands and receptors; and (2) complementary polynucleotides. The ligands and receptors involve organic molecules, where the ligand is any molecule for which a receptor is available or can be made. The ligand is characterized by having a polar and spatial organization which binds to a reciprocal or homologous receptor. The receptor is conventionally a macromolecule which has a structural organization complementary to the ligand so as to have a high avidity for the particular structure of the ligand to provide for a specific binding complex. Conventional receptors include antibodies and fragments thereof, enzymes and naturally occurring receptors. In the subject invention, the ligands may be haptens or antigens, but where the ligand is monovalent and has to serve as a bridge, it will be provided in a polyvalent form. The polynucleotides may be DNA or RNA, where the bridge will have a sufficiently extended complementary sequence, for example, by repeating the same sequence to allow binding to two different fragments of complementary polynucleotide sequences. In carrying out the method, one combines the sample, the assay medium having the appropriate conditions for the cells, particles, if particles are to be added, and the bridging system for bridging the cells. Depending upon the manner in which the localized site is to be detected, a signal producing system may also be involved, where one or more labels are provided bound to members of the bridging system or to binding members which bind to the analyte. By having two different binding members involved in the production of a detectable signal, one can provide for detection of an analyte having two different reciprocal binding members. After combining the sample with the bridging members and any signal producing members, as appropriate, in an appropriate assay medium, one then contacts a small portion of the bibulous member with the assay medium, where a major portion of the bibulous member does not contact the assay medium and may act as a wick or well for absorbing liquid, for example, by capillary or wicking action, so as to draw liquid through the area at the air-liquid interface. After sufficient time to allow for a sufficient proportion of the assay medium to be absorbed by the bibulous member, so that concentrations of the cells at the localized site can form, as appropriate, contact with the assay medium may be terminated. Depending upon the signal producing system, the presence of the site may be determined or additional members of the signal producing system may be added to the area at which the site may have formed to provide a detectable signal. If a quantitative result is desired, the detectable signal may be measured by any convenient means. An illustrative example is where one is interested in a particular bacterial strain. In this method one might choose a bibulous member circle mounted on a non-bibulous plastic support having a centrally located orifice of about 0.5 mm dia. A swab is taken of a clinical sample and dispersed in PBS-0.05% Tween® 20. To the dispersion is added a monoclonal antibody conjugated to an enzyme and the mixture is incubated. A few drops of the sample are placed over a device comprising a lower bibulous disc bonded to an upper hydrophobic plastic disc having a central orifice of about 0.5 mm dia. The liquid passes through the orifice and spreads evenly outward from the orifice with the organisms concentrated to the spot on the bibulous member underneath the orifice. The monoclonal antibody-enzyme conjugate will bind to any organisms which have the reciprocal epitopic site. Excess monoclonal antibody-enzyme will wick away so that only specifically bound enzyme will remain as the spot. To ensure removal of residual enzyme which is not specifically bound, a few drops of the PBS-Tween® 20 solution may be placed over the orifice and allowed to wick. About 0.05 ml of a developer solution is then added, e.g., H₂O₂+4-chloronaphthol with HRP as the enzyme and the solution allowed to wick. Color will indicate the presence of the organism. A large number of patents have been issued which describe a wide variety of labels which have found use in diagnostic assays. Various protocols can be developed where these labels may be used with advantage. Illustrative of such patents are US-A-3,850,752; 4,255,329; 4,233,402; and 4,208,479. For performing the method, kits can be provided where the various reagents are combined in predetermined amounts in combination with various ancillary materials for combination with the sample. In view of the wide spectrum of protocols and reagents, a wide variety of kits may be prepared. For the most part, where the method involves the addition of particles, the kits will involve particles which have a member of a specific binding pair substantially irreversibly bound to the particle, either covalently or non-covalently. Also, there may be a label bound to the surface of the particle or dispersed therein, particularly a dye, which may be colored in the visible range or fluorescent. In some instances, the particle may also be labelled with an enzyme. Where particles are not to be included, the reagents will normally involve labelled receptors or ligands, where the labels provide for a detectable signal and may provide for the inhibition of migration of the cells present in the assay medium. In addition to the labelled reagents, there will be ancillary reagents such as buffers, stabilizers, detergents, and as appropriate substrates for enzymes and bulking agents. Also included in such kits would be the wicking material--prepared as strips or discs--with precut orifices. The following examples are offered by way of illustration and not by way of limitation. EXPERIMENTAL In the following experiment, a variety of beads of different colors and sizes were combined to demonstrate that one could achieve bands of different colors depending upon the sizes of the beads and their color and the effect of the combination of the beads in a band. The protocol was to combine 0.05 ml of each 2% bead preparation plus 20 ml of a 1% solution of normal sheep serum in PBS containing 0.05% QS44 and 0.05% Tweeen® 20. The paper which was employed was triangular-shaped Millipore membrane paper with a rounded tip, where the tip was inserted into the assay medium. This shape provides for enhanced concentration of the bead particles, while providing a large wicking reservoir. The following table indicates the beads that were employed, the expected color and the observed color. The colors were predicted based on the 0.5 µm particles being retained on the surface adjacent the interface, while the small particle migrated with the solvent front; thus, the complementary color of the larger beads is seen. In the next experiment, an assay was performed to detect the presence of Streptococcus pyogenes (strain: Lancefield Group A, 19615; reference: American Type Culture Collection, Bacteria Catalog, Library of Congress No. 76-64127, lSSN: 0363-2970) in a mixture of Group A and Group B. The assay medium was prepared by removing cells of the two organisms from agar growth by loop and suspending them in 1 ml of PBS plus 0.05% Tween® 20. A mouse monoclonal antibody against Group A (anti-A) at about 5 mg/ml was employed. Also employed were goat antibodies against mouse IgG (G-anti-IgG) which was conjugated to horseradish peroxidase. As a developing solution, a solution was employed containing 200 µg/ml of sodium 4-chloro-2-naphthol, 50 mM glucose, 2 mg/ml bovine serum albumin and excess hydrogen peroxide. The protocol was to combine 100 ml of the organism solution (about 10⁷ cells/ml), 20 µl of the antibody-horseradish peroxidase conjugate and 20 µl of the anti-A, incubate the mixture and then wick 50 ml on a cellulose strip (cellulose strips used for tlc). The strip was then placed in the substrate solution and a dark band formed indicating the presence of Group A organisms. A similar experiment was carried out, where enzyme channeling was involved, demonstrating a rapid antibiotic sensitivity assay. Group A strep. pyogenes (0.2 ml of 10⁸ cells/ml) were incubated for 2 3/4 hrs. at 37°C in the presence or absence of penicillin at 2 units/ml. This solution was then combined with 0.8 ml of phosphate buffered saline (0.02 ml phosphate plus 0.05% Tween® 20 - pH 7.2) followed by the addition of 0.01 ml of a monoclonal anti-Group A antibody conjugated to horseradish peroxidase (HRP). 0.2 ml of this mixture was wicked for 7 min. with a cellulose TLC paper strip. The wick was then transferred into developer (0.2 ml of the above-described 4-chloronaphthyl substrate solution plus 0.5 µl glucose oxidase. The wick was developed for 30 min. Results . Bacterial cells exposed to the penicillin formed a less intense blue-purple band than did the unexposed cells. The difference was easily detectable by the eye. It is evident from the above results, that a simple rapid method is provided for detecting the presence of a cell. The method can be qualitative or quantitative and can be used with a variety of protocols which can be adapted to particular samples. Furthermore, simple equipment is employed and the results can be obtained by visual observation.

Claims: A method for detecting the presence of cells in a sample, said method comprising: contacting an area of a bibulous member with an aqueous assay medium containing said cells, whereby said assay medium is wicked away from said area, said cells either being within a predetermined size range or having been converted to a predetermined size range, said size range being predetermined in relation to the bibulous member such that the cells are thereby concentrated in a localized site on said bibulous member in the region of the air/liquid interface, and detecting a signal as a result of a signal-producing system, wherein said signal is related to the amount of said cells in said localized site. A method according to claim 1, which comprises adding a specific binding pair member ("sbp member") reciprocal to a sbp member on said cells and capable of converting said cells to a predetermined size range. A method according to claim 2, wherein said reciprocal sbp member is bound to a particle. A method according to claims 1, 2 or 3, wherein said assay medium, or said reciprocal sbp member, or said particle contains a member of a signal producing system. A method according to claim 1, wherein said signal is detected by contacting said site with members of a signal producing system and then examining said site for the presence of a signal to determine said cells. A method according to claim 2, wherein one of the members of the signal producing system is a labelled sbp member reciprocal to said sbp member on said cells. A method according to claim 1, wherein said cells are bacterial cells, fungal cells, protozoan cells or other parasitic or infectious agent. A method according to claim 3 or claim 4, wherein said label is an enzyme. A method according to claim 1, wherein said bibulous member has a small orifice surrounded on one side of said bibulous member with a water impermeable layer, and said assay medium is contacted with said bibulous member at said orifice. A method according to claim 1, wherein said bibulous member is supported by a plastic support having a centrally located orifice. A device for conducting an assay, said device comprising a non-bibulous member having an orifice and serving as a support for a bibulous member, said bibulous member having an orifice that extends through said bibulous member, and is in register with the orifice in said support. A diagnostic kit suitable for use in the method of claim 1, said kit comprising in packaged combination in predetermined ratios for combination with said cells according to said method, (a) the device of claim 11, (b) a signal producing system, and (c) ancillary materials. A diagnostic kit according to claim 12, which further comprises, in a separate container, particles to which are bound at least one member of a specific binding pair reciprocal to a sbp member on said cells.

Inventors: GOULD, DENNIS RICHARD, ZUK, ROBERT F.
Applicants: ABBOTT LABORATORIES
Application year: 1984
Main classification: G01N33
IPC codes: G01N33, C12Q1