Immunoassay utilizing two incubations with labelled antigen

Abstract: The present invention relates to an immunoassay for antibodies against an infectious agent comprising incubating a sample suspected of containing antibodies with immobilised and free labelled antigen of the infectious agent, separating the immobilised components and incubating them with further free labelled antigen, and determining amount of labelled antigen immobilised. The present invention also relates to the use of the immunoassay and a kit for performing the immunoassay.

Description: The present invention relates to an immunoassay. The present invention also relates to the use of the immunoassay and a kit for performing the immunoassay of the present invention. Immunoassays employ antibodies as analytical reagents for the detection of analytes. Immunoassays are used to detect the presence of an infectious agent by assaying either for the infectious agent or for antibodies raised by the infected host against the infectious agent. The present invention relates to an immunoassay for detecting antibodies against an infectious agent. Immunoassays for detecting antibodies can involve two incubation steps. In the first incubation, a sample to be tested is incubated with antigens of an infectious agent which have been immobilised on a surface, for instance the surface of a microwell. Any antibodies in the sample against the infectious agent become bound to the antigens on the surface. After a wash step, the second incubation is performed using antigens of the infectious agent labelled with a detectable substance. Labelled antigen will become bound to the antibodies immobilised on the surface. After a further wash step, the amount of label bound to the surface is determined. A high signal indicates the presence of antibodies against the infectious agent. Figure 1 gives a schematic representation of such a prior art immunoassay. Such a prior art immunoassay is also described in EP-A-0 313 986. A commercially available example of such a prior art immunoassay is the ORTHO™ HIV-1/HIV-2 Ab-capture ELISA test system. The accurate detection of HIV and other infections is of considerable importance. Early diagnosis of HIV infection enables medical treatment to be started and precautions to be taken in order to limit transmission of the virus. The avoidance of false positives in detecting HIV infection is also of considerable importance. The prior art immunoassays have a number of limitations, including limited sensitivity and difficulty in discriminating between weak positive and negative samples. The limitations of the prior art immunoassays are discussed in EP-A-0174652. Accordingly, there is a need for an immunoassay with increased sensitivity and specificity. The present invention provides an immunoassay for antibodies against an infectious agent comprising: i. incubating a sample suspected of containing said antibodies with immobilised antigen of the infectious agent and free labelled antigen of the infectious agent; ii. separating immobilised components from non-immobilised components; iii. incubating the immobilised components with further free labelled antigen of the infectious agent and then separating immobilised components from non-immobilised components; and iv. determining the amount of labelled antigen immobilised, wherein the amount of label is indicative of the amount of said antibodies present in the sample. The presence of free labelled antigen in the first incubation of the immunoassay has been found to increase weak positive signals while negative signals remain unchanged. The presence of free labelled antigen in the second incubation in addition to the first incubation is required, especially if the sample is a strongly positive sample or suspected of being a strongly positive sample. A strongly positive sample is a sample wherein the amount of antibody against the infectious agent is sufficiently high to saturate the immobilised antigen and the free labelled antigen. For example, a strongly positive sample is a sample which is still positive at a dilution of greater than 1/5000. The term "infectious agent" as used herein means any organism or particle that can infect a patient. Infectious agents include bacteria and viruses. Preferably, the immunoassay of the present invention is for use in detecting antibodies against one or more of HIV-1, HIV-2, hepatitis B virus (HBV) and hepatitis C virus (HCV). The immunoassay of the present invention can be used to detect the presence of antibodies which are against different infectious agents. Accordingly, the immunoassay of the present invention can be used to detect the presence of more than one infectious agent. It will be apparent to one skilled in the art that when the immunoassay is used to detect the presence of antibodies against different infectious agents, antigens of each different infectious agent must be used in the immunoassay. The immunoassay of the present invention must use one and preferably uses two or more antigens for each infectious agent being detected. Preferred antigens are those having an epitope which is easily recognisable and strongly bound by an antibody. It is further preferred that the antigen has an epitope that is stable and not prone to mutation thereby reducing the risk that a mutated form of the infectious agent will not be detected. Preferably, when the infectious agent is HIV, the antigen is selected from gp120, p24, gp41, gp160, Env10 and Env13 A/L. Preferably, when the infectious agent is HBV, the antigen is selected from HBs (hepatitis B surface antigen) and HBc (hepatitis B core antigen). In the immunoassay of the present invention, the antigen is preferably immobilised on a surface. The surface may be a wall of a microtitre well or other receptacle for receiving the sample and/or free labelled antigen, a dipstick or beads. Surfaces suitable for immobilising an antigen are described in "Immunoassays" (Diamandis, E.P. and Christopoulos T.K. Eds., Academic Press, London (1996)), especially pages 205 to 216. The antigen may be immobilised via a number of standard techniques known to those skilled in the art. For example, by physical adsorption of the antigen itself or the antigen coupled to a carrier protein or macromolecule (see "Immunoassays" Diamandis, E.P. and Christopoulos T.K. Eds., Academic Press, London (1996), especially pages 216 to 222 and 229). The free labelled antigen may be labelled with any type of detectable label provided the label does not interfere substantially with binding of the antigen to the antibody. Suitable labels include: enzymes, such as horseradish peroxidase (HRP) and chloramphenicol acetyl transferase (CAT); digoxygenin (DIG); fluorescein; and radioisotopes such as 125 I, 3 H and 14 C. Preferably, the antigen is labelled with HRP. Depending on the label used, the amount of labelled antigen immobilised is determined using standard methods known to one skilled in the art. For example, if the label is HRP, the degradation of luminol by the enzyme and the associated emission of chemiluminescence can be measured. However, if a radioactive label is used, the presence of the label is measured by detecting the emitted radiation. If antibodies against different infectious agents are being detected, the free labelled antigens of each different infectious agent may be labelled differently so that it is possible to distinguish between the antibodies against each infectious agent. The sample can be any fluid or tissue which contains antibodies, such as blood, serum, bone marrow, saliva or urine. However, if the sample is a tissue sample, it may be necessary to break up the tissue in a suitable solution, such as saline, so that the antibodies are present in solution. Preferably, the sample is blood. In some circumstances, it may be necessary to remove certain components from the blood sample, such as red and white blood cells, before the sample is used. Most preferably, the sample is blood plasma. Preferably, the immunoassay of the present invention additionally comprises the use of an assay buffer in order to provide a suitable biological environment for performing the immunoassay. Suitable assay buffers are known to those skilled in the art and include phosphate buffers and may comprise NaCl to increase ionic strength and proteinaceous material to reduce non-specific binding and interferences. The present invention also provides the use of the immunoassay of the present invention in the diagnosis of the presence of an infectious agent. Preferably, the infectious agent is HIV-1 or HIV-2. The present invention further provides a kit adapted for performing the immunoassay of the present invention, comprising: i) a surface on which an antigen of the infectious agent has been immobilised or a surface on which an antigen of the infectious agent can be immobilised in combination with means for immobilising an antigen of the infectious agent; ii) free labelled antigen; iii) signal reagent and/or apparatus for detecting the presence of the labelled antigen; and iv) a receptacle for incubating the surface with a sample and/or the free labelled antigen, wherein the receptacle may optionally comprise the surface, wherein the kit is adapted so that: (a) free labelled antigen is incubated with the sample and the immobilised antigen; (b) the immobilised components are separated from non-immobilised components; and (c) further free labelled antigen is incubated with the immobilised components. The surface on which an antigen has been immobilised may be a wall of a microtitre well or other receptacle for receiving the sample and/or free labelled antigen, a dipstick or beads. Surfaces suitable for immobilising an antigen are described in "Immunoassays" (Diamandis, E.P. and Christopoulos T.K. Eds., Academic Press, London (1996)), especially pages 205 to 216. The signal reagent and/or apparatus will depend on the label used in the kit. For example, if the label is HRP, the signal reagent may comprise luminol. Preferably, the kit of the present invention also comprises an assay buffer in order to provide a suitable biological environment for performing the immunoassay. The present invention is now described further by way of example only with reference to the accompanying figures in which: Figure 1 is a schematic diagram showing a prior art immunoassay. A patient's sample containing antibody is added to immobilised antigen in the first incubation of the immunoassay. Any unbound antibody is washed off during the wash step. Antigen conjugated to HRP is then added in the second incubation of the immunoassay and binds to any free binding sites of the bound antibody. The amount of bound HRP and consequently bound patient's antibody is measured by the addition of a signal reagent; and Figure 2 is a schematic diagram showing the immunoassay of the present invention. A patient's sample containing antibody and antigen conjugated to HRP are added to immobilised antigen in the first incubation of the immunoassay. Any unbound components are washed off in the wash step. Antigen conjugated to HRP is then added in the second incubation of the immunoassay and binds to any free binding sites of the bound antibody. The amount of bound HRP and consequently bound patient's antibody is measured by the addition of a signal reagent. Comparative Example Prior art, two step Protocol (see Figure 1) Add to an HIV antigen-coated microwell, prepared by passive adsorption of Env10, Env 13, Env A/L and p24 (supplied commercially from Chiron) by overnight coating in a borate buffer (pH 9.0, 90mM), 80 µl of sample and 20 µl of assay buffer. Incubate for 25 minutes. Wash with wash buffer. Add 100 µl HRP-labelled antigens (i.e. HRP-labelled Env10, Env13, Env A/L and p24 (supplied commercially from Chiron)) and incubate for 5 minutes. Wash with wash buffer (supplied commercially from Ortho Clinical Diagnostics (O.C.D.), Amersham). Add 200 µl signal reagent (supplied commercially from O.C.D, Amersham) and detect the signal using enhanced chemiluminescence. Example 1 Double HRP labelled antigen Incubation Protocol (see Figure 2) Add to an HIV antigen-coated microwell, prepared as above, 80 µl of sample, 20 µl of assay buffer and 20 µl HRP-labelled antigens (as defined above). Incubate for 25 minutes. Wash with wash buffer. Add 100 µl HRP labelled antigen (as defined above) and incubate for 5 minutes. Wash with wash buffer (supplied commercially from O.C.D, Amersham). Add 200 µl signal reagent and detect (supplied commercially from O.C.D, Amersham) the signal using enhanced chemiluminescence. Reagent Formulations Assay Buffer HRP-Labelled Antigen Reagent Deionised water 1000g Deionised water 580g Disodium hydrogen Disodium hydrogen orthophosphate 0.31g orthophosphate 0.63g Potassium di-hydrogen Potassium di-hydrogen orthophosphate 1.09g orthophosphate 0.20g Sodium Chloride 8.2g Sodium Chloride 4.83g Kathon 5.0g Kathon 10.0g EDTA 0.35g Foetal Calf Serum 410g Antifoam 0.01g Potassium ferricyanide 0.34g Tween 20 0.5g HRP-labelled HIV antigens 17g Antifoam 0.01g Results Using the double HRP-labelled antigen incubation protocol of the present invention, increased differentiation between negative signals and weak positive signals is achieved (see Table 1). This improved differentiation can be used to increase both the sensitivity and the specificity of the immunoassay by positioning the cut-off appropriately. Seroconversion panel sensitivity is also improved by using the immunoassay of the present invention. From the results shown in Table 2 it can be seen that sample AB2 gives a negative result with the standard assay format but a clear positive result when HRP-labelled antigen is added to the first step of the immunoassay. Sample R1 also gives a stronger positive result with the extra HRP labelled antigen. Both of these samples are key seroconversion samples that are not detected by the majority of commercially available immunoassays. Effect on signal by addition of HRP labelled antigen into the first step of a 2 step immunoassay. Signal (light units) Sample labelled Standard Assay Double HRP Antigen Weak Positives Calibrator 0.68 4.08 QCA 0.12 4.74 QCB 4.54 20.56 QCC 9.80 50.36 QCD 13.27 32.13 Negatives 1 0.07 0.10 2 0.02 0.02 3 0.02 0.03 4 0.02 0.02 5 0.03 0.03 6 0.02 0.03 Mean negative result 0.03 0.04 Positive signals increased by between 3 and 40 fold. No significant increase in negative results. Improved detection of Seroconversion Samples by addition of HRP labelled antigen into the first step of the reaction. Normalised Results (>1=positive) Seroconversion labelled Samples Assay Standard Assay Double HRP antigen R1 1.70 2.32 R2 14.81 15.41 E8 0.13 0.12 E9 7.31 7.85 E10 53.50 40.92 AB2 0.85 1.28 AB3 89.06 72.25 W8 0.11 0.09 W9 5.86 5.83 W10 26.13 21.17 Accordingly, the invention offers the following advantages to the performance of immunoassays: 1. Improved seroconversion sensitivity; 2. Improved dilutional sensitivity (i.e. an increased ability to detect smaller quantities of antibodies); and 3. Improved specificity. Other embodiments will be evident to those of skill in the art. It should be understood that the example is provided for clarity only and is merely exemplary. The spirit and scope of the present invention are not limited by the above example, but are encompassed by the following claims.

Claims: An immunoassay for antibodies against an infectious agent comprising: i. incubating a sample suspected of containing said antibodies with immobilised antigen of the infectious agent and free labelled antigen of the infectious agent; ii. separating immobilised components from non-immobilised components; iii. incubating the immobilised components with further free labelled antigen of the infectious agent and removing non-immobilised components; and iv. determining the amount of labelled antigen immobilised, wherein the amount of label is indicative of the amount of said antibodies present in the sample. The immunoassay of claim 1 wherein the antibodies are against two or more different infectious agents. The immunoassay of claim 1 or claim 2 wherein two or more antigens of each infectious agent are used. The immunoassay of any one of claims 1 to 3 wherein the infectious agent is one or more of HIV-1, HIV-2, HBV or HCV. The immunoassay of any one of the previous claims wherein the infectious agent is HIV-1 and/or HIV-2. The immunoassay of claim 5 wherein the antigen is one or more of gp120, p24, gp41, gp160, Env10 and Env13 A/L. The immunoassay of any one of claims 1 to 4 wherein the infectious agent is HBV. The immunoassay of claim 7 wherein the antigen is one or more of HBs and HBc. The immunoassay of any one of the previous claims wherein the labelled antigen is labelled with horseradish peroxidase. The use of the immunoassay of any one of the previous claims in the diagnosis of the presence of an infectious agent. The use of claim 10 wherein the infectious agent is HIV-1 and/or HIV-2.

Inventors: GLOVER JUSTIN MARK, GLOVER, JUSTIN MARK
Applicants: ORTHO CLINICAL DIAGNOSTICS, ORTHO-CLINICAL DIAGNOSTICS
Application year: 1998
Main classification: G01N33
IPC codes: G01N33