Abstract:
A saliva enhancement reagent for use in an ELISA wherein a horseradish peroxidase (HRP) enzyme conjugate system is disclosed.

Description:
This application is a continuation of application Ser. No. 618,400 filed Nov. 27, 1990, abandoned, continuation-in-part of application Ser. No. 321,415 filed March 17, 1988, abandoned which is a continuation-in-part of application Serial No. 844,098 filed Mar. 26, 1986 which issued as U.S. Pat. No. 4,853,325 on Aug. 1, 1989. The disclosure of application Serial No. 321,415 and of U.S. Pat. No. 4,853,325 are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to an improved ELISA for the detection of FeLV group specific antigens in cat saliva. More particularly the invention relates to a saliva enhancement reagent for use in such an ELISA wherein a horseradish peroxidase (HRP) enzyme conjugate system is employed. The invention also relates to kits for detecting the presence of feline leukemia virus FeLV antigens in saliva samples. 
     BACKGROUND OF THE INVENTION 
     Pat. No. 4,853,325 describes a saliva test for FeLV. That test employs a probe having an immunochemically sensitive member for collecting saliva from the oral cavity of a cat and employs ELISA reagents for the incubation of the probe and the development of color reactions to indicate the presence or absence of FeLV within the saliva sample collected onto the probe. 
     The probe, wetted with cat saliva, is then incubated in an incubation vessel containing an enzyme conjugate of anti-FeLV antibodies in a solvent or diluent. Typical conjugate diluents include buffers such as phosphate, borate or carbonate; protein stabilizers such as bovine serum albumin (BSA), casein or gelatin; surfactants such as polyoxyethylene alcohols, polyoxyethylene fatty acid esters or alkylaryl polyether alcohols. The concentration of the conjugate in the diluent may lie from about 0.1 μg/mL (microgram per milliliter) to about 10.0 μg/mL. Unbound conjugate is removed. The bound conjugate which remains on the probe allows the immunologically sensitive member to be color developed. Color development indicates that FeLV and FeLV antigens were present in the saliva sample. 
     The specificity of this test is sometimes compromised by excessive nonspecific background and by false positives. 
     SUMMARY OF THE INVENTION 
     This invention provides a saliva enhancement reagent (SER) useful to reduce nonspecific background and false positives in saliva tests for FeLV of the type described in U.S. Pat. No. 4,853,325. 
     Pursuant to the invention the novel SER is added to the enzyme-antibody conjugate prior to use. It has been discovered that the presence in the conjugate diluent of an aqueous solution of saponin in a high salt buffer significantly reduces nonspecific background color in dipstick/saliva based assays which utilize HRP enzyme conjugate systems. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a perspective view of a FeLV test probe inserted into a cat&#39;s oral cavity towards the buccal crease illustrating the process of rotating the probe. 
     FIG. 2 is a perspective view of the probe of FIG. 1 inserted into an incubation vessel illustrating the submersion of the immunochemically sensitive member of the probe within an incubation reagent. The nipple shaped control submember is shown to support the test submember slightly above the bottom of the incubation vessel, the shape of which minimizes the volume of incubation reagent required to surround the test submember. 
     FIG. 3 is a perspective view of the probe of FIG. 2 inserted into a development vessel and submerged in a chromogenic substrate. 
     FIG. 4 is a perspective view of the probe of FIG. 3 illustrating a negative color development on the test submember, indicating an absence of FeLV and/or FeLV antigens in the saliva sample, and a positive color development on the control submember, indicating that the incubation and development reagents are active. 
     FIG. 5 is a perspective view of the probe of FIG. 3 illustrating a positive color development on the test submember, indicating the presence of FeLV and/or FeLV antigens in the saliva sample, and a positive color development on the control submember. 
     FIG. 6 is an enlarged fragment of the probe of FIG. 5 illustrating the frosted texture of the immunochemically sensitive member with positive color development. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     U.S. Pat. No. 4,853,325, column 5, line 13 to column 8, line 2 describes a FeLV saliva ELISA test of the kind in which this invention is useful. 
     The SER of this invention is added to the enzyme, preferably HRP, conjugate solution in which the cat saliva bearing probe is incubated. 
     Typical enzyme conjugate solutions include from about 0.1 μg/mL to about 10.0 μg/mL of the conjugate in a diluent such as: 
     Phosphate Buffered Saline (0.01M phosphate) 
     1% BSA 
     0.1% phenol (0.0106 M) 
     0.5% Tween 20 (polyoxyethylene 20 sorbitan monolaurate) 
     Phosphate Buffered Saline 
     6% BSA 
     0.1% phenol 
     0.5% Tween 20 
     0.1 mg/mL mouse IgG (mouse immunoglobulin) 
     The SER of this invention is added to the conjugate diluent. The preferred amount of the SER is from about 0.1% w/v to about 1% w/v in a conjugate diluent as above. 
     The SER is a solution of saponin in a buffered concentrated salt solution. The salt is preferably sodium chloride NaC1 but may also be potassium chloride or other non-interfering salt. The SER is buffered, preferably with HEPES (hydroxy-ethyl piperazine ethane sulfonic acid) to a pH from about 7 to 8, preferably about 7.5. Other buffers may be used, e.g., phosphate, borate or carbonate. 
     A preferred SER composition contains 1M NaCl, 1% (v/v) saponin 10 MM Hepes and, if desired 0.2% (v/v) cosmocil CQ (preservative) in the solvent or diluent medium. Saponin concentrations in the ranges of 0.1-2% v/v and NaCl concentrations from about 0.5 M to about 2.0 M may be utilized. 
     The kits of the invention include an enzyme antibody conjugate diluent, i.e., the SER described herein. More specifically, the kits include a probe for acquiring the saliva sample, the probe comprising a handle and a member coated with immobilized FeLV, e.g., anti-p27 antibody, an incubation solution including an enzyme conjugate of the anti-FeLV antibody, an incubation vessel, a developing solution containing a chromogenic substrate for generation of color in the presence of the enzyme conjugate, the SER of this invention and a container sized to contain the probe, the incubation solution, the developing solution, the incubation vessel and the SER solution. 
     EXAMPLE 1 
     This comparative example reports the results of FeLV testing on FeLV negative cats. The test procedure entailed the use of a kit of the kind described generally in U.S. Pat. No. 4,853,325. In this procedure, the test reagents were added at 100 ul per tube of conjugate prior to dipstick addition. After sampling the cat saliva the sticks were added to the various conjugates and the assay proceeded according to the stated protocol. At the end of the assay the substrate reaction was quenched with sulfuric acid and the optical densities were measured at 450 nm. 
     
         ______________________________________Additive            Avg Absorption A450 nm______________________________________None (control)      .07510 mM Hepes Buffer  .0661M NaCl/10 mM Hepes .0241% Saponin/10 mM Hepes               .0251M NaCl/1% Saponin/10 mM Hepes               .014______________________________________ 
    
     As shown above, the presence of Saponin or high salt decreases nonspecific background from the buffer control. Together the reagents reduce background by another two fold to 4.75 times less than the buffer control. 
     EXAMPLE 2 
     A group of FeLV negative cats were subjected to a saliva test for FeLV. In each case the test was carried out using a kit of the kind generally described in U.S. Pat. No. 4,853,325. 
     The results of these tests are set forth in Table I. 
     
                       TABLE I______________________________________  Abs 450 nmCAT ID#  SER     control CAT ID#  SER   control______________________________________36453    0.014   0.028   36830b   0.015 0.04536423a   0.013   0.025   36830c   0.019 0.04636423b   0.007   0.022   281148f  0.022 0.0436423c   0.01    0.063   281148g  0.013 0.02336296    0.033   0.03    281148h  0.009 0.01736340a   0.017   0.044   36825    0.008 0.02936340b   0.004   0.031   36825a   0.026 0.02536340c   0.008   0.037   36825b   0.012 0.03736365    0.008   0.024   36816    0.016 0.02336386    0.01    0.046   281148i  0.009 0.03436222    0.006   0.028   281148j  0.016 0.02736343    0.022   0.087   36827a   0.031 0.02336188    0.006   0.081   36827b   0.01  0.01536364    0.005   0.042   36827c   0.015 0.02636217    0.005   0.053   36827d   0.01  0.01936346a   0.01    0.042   281148k  0.013 0.05936346b   0.01    0.05    80413    0.01  0.03136346c   0.007   0.039   76022    0.012 0.02636417a   0.009   0.049   76021    0.005 0.02736417b   0.003   0.035   76036    0.007 0.04836417c   0.013   0.07    80830    0.007 0.0436191    0.011   0.047   80405    0.021 0.08936264    0.008   0.034   80832    0.021 0.07936418    0.014   0.035   76037    0.011 0.05836416    0.022   0.059   76038    0.01  0.02936366    0.008   0.052   80428a   0.011 0.02636812a   0.027   0.032   80428b   0.015 0.04336812b   0.007   0.047   80428c   0.009 0.032281148a  0.01    0.034   36946    0.012 0.084281148b  0.024   0.03    36936    0.027 0.022281148c  0.016   0.045   76027a   0.012 0.048281148d  0.009   0.032   76027b   0.026 0.035281148e  0.023   0.05    76051a   0.01  0.034366151   0.011   0.026   76051b   0.012 0.05136830a   0.01    0.042______________________________________           SER         control______________________________________Mean            0.013       0.04Std Dev         0.006       0.017______________________________________ 
    
     All of these cats are FeLV negative. Table I shows that the signal obtained with the SER is approximately 30% of that obtained with the standard formulation. 
     EXAMPLE 3 
     This example reports the result of a clinical evaluation of the SER of this invention. 
     FeLV infected cats were tested, in each case, in the manner described in Example 2. 
     The results of this clinical evaluation are set forth in Table II. 
     
                       TABLE II______________________________________         SER            control#     Cat Identification               Visual  A450   Vis. A450______________________________________ 1    Whitey        +       .238   +    .578 2    Spazz         +       .214   +    .745 3    Midnight      +       .460   +    .634 4    Nave          +       .564   +    .604 5    Smudge        +       .478   +    .466 6    Marble        +       .100   +    .265 7    Pazuzu        +       .461   +    .471 8    Winky         +       .444   +    .706 9    Drop Off      +       .400   +    .46210    Easter        +       .289   +    .64811    Slots         +       .140   +    .39312    Blackjack     +       .167   +    .27613    Roulette      +       .467   +    .72914    Bingo         +       .397   +    .61615    Vegas         +       .695   +    .86316    Lovely Rita   +       .402   +    .57217    Barbie        +       .117   +    .62018    Rhoda         +       .428   +    .77319    Samba         +       .177   +    .43420    Tomascena     +       .132   +    .27321    Petite Pearl  -       .012   -    .04122    Sveetie       -       .012   -    .05823    Josie         -       .015   -    .04924    Blackstone    -       .014   -    .045______________________________________ 
    
     Cats 21-24 had tested positive for FeLV in the past, but the disease is latent in these animals, and they were not shedding virus at the time of the test. They are currently negative in both formats. All other FeLV positive cats yielded positive test results in both formats. 
     Positive results are those samples which are distinctly blue after the substrate incubation times. This correlates to A450&gt;.075. A direct comparison of Absorbances between the two test formats is difficult because of antigen sampling variation inherent in the saliva assay. This is deemed largely irrelevant due to the qualitative nature of the test.