Evaluation of the flow cytometric crossmatch. Preliminary results of a multicenter study

Biomarkers in transplantation: Prospective, blinded measurement of predictive value for the flow cytometry crossmatch after negative antiglobulin crossmatch in kidney transplantation

This prospective, blinded observational study was conducted to measure the predictive value the of flow cytometric crossmatch for biopsy-proven acute rejection, graft loss, or death following kidney transplantation. Patients were selected for renal transplantation on the basis of a conventional antihuman globulin cytotoxic T-cell crossmatch. Flow crossmatch was performed simultaneously, but the results were not disclosed to the transplant team. A total of 257 kidney transplant recipients were enrolled in the study; 78 patients experienced biopsy-proven rejection in the first post-transplant year, and 41 patients lost their graft or died during the period of follow-up (mean: 2046 days). Kaplan-Meier estimates of rejection, graft loss, or patient death did not differ between subjects with a positive or negative flow crossmatch. Cox analyses showed no influence of the flow crossmatch on the risk of biopsy-proven acute rejection (P = 0.987). The sensitivity and specificity of the flow crossmatch for prediction of biopsy-proven rejection were 0.128 and 0.883, and the positive and negative post-test probabilities were 0.323 and 0.301, respectively. The magnitude of the channel shift did not influence the multivariate Cox regression model. The area under the receiver operating characteristic curve of the flow crossmatch was 0.483 (P = 0.71) and 0.572 (P = 0.38), respectively for the living and cadaver transplant recipients, indicating no discriminative value in this study population. Flow crossmatch appears to have no significant incremental value in predicting biopsy-proven acute rejection, graft loss, or death following kidney transplantation in patients who have a negative antihuman globulin cytotoxic T-cell crossmatch against their donor.

Improved flow cytometric detection of HLA alloantibodies using pronase: potential implications in renal transplantation

BACKGROUND

Flow cytomeric crossmatch (FCXM) has grown in popularity and has become the "standard of practice" in many programs. Although FCXM is the most sensitive method for detecting alloantibody, the B cell FCXM has been problematic. Difficulties with the B cell FCXMs have been centered around high nonspecific fluorescence background owing to Fc-receptors present on the B cells and autoantibodies. To improve the specificity and sensitivity of the B cell FCXM, we utilized the proteolytic enzyme pronase to remove Fc receptors from lymphocytes before their use in FCXM.

METHODS

Lymphocytes isolated from peripheral blood, spleen, or lymph nodes were treated with pronase and then used in a three-color FCXM. A total of 167 T- and B cell FCXMs using pronase-treated and untreated cells were performed. Testing used serial dilutions of HLA allosera (22 class I and 6 class II), with the titer of each antibody at one dilution past the titer at which the complement-mediated cytotoxicity anti-human globulin crossmatch became negative.

RESULTS

After pronase treatment, the actual channel values of the negative control in both T cell and B cell FCXMs declined from 78+/-10 to 57+/-4 (P<0.05) and 107+/-11 to 49+/-3 (P<0.00001), respectively. Pronase treatment resulted in improved sensitivity of the T and B cell FCXM in detecting class I antibody by 20% and 80%, respectively. In no instance was a false-positive reaction observed. In this study, pronase treatment improved the specificity of B cell FCXM for detecting class II antibodies from 75% to 100% (P=0.03). In no instance was a false-negative reaction recorded. Lastly, on the basis of these observations we re-evaluated three primary transplant recipients who lost their allografts because of accelerated rejection. One of the patients was transplanted across negative T and B cell FCXM, whereas the other two patients were transplanted across a positive T cell, but negative B cell, FCXM. After pronase treatment, T and B cell FCXMs of each patient became strongly positive, and donor-specific anti-HLA class I antibody was identi. fied in each case.

CONCLUSION

Utilization of pronase-treated lymphocytes improves both the sensitivity and specificity of the FCXM.

The application of flow cytometry to histocompatibility testing

Flow cytometry is a powerful technique that enables the sensitive and quantitative detection of both cellular antigens and bound biological moieties. This article reviews how flow cytometry is increasingly being used as histocompatibility laboratories for the analysis of antibody specificity and HLA antigen expression. A basic description of flow cytometry principles and standardisation is given, together with an outline of clinical application in the areas of pre-transplant cross-matching, antibody screening, post-transplant antibody monitoring and HLA-B27 detection. It is concluded that flow cytometry is a useful multi-parametric analytical tool, yielding clinical benefit especially in the identification of patients at risk of early transplant rejection.

Utilization of crossmatch techniques for renal transplantation

The pre-transplant cytotoxic crossmatch test is a necessary requirement for renal transplantation to proceed. However, there is no universally accepted policy on the clinical significance of a positive flow-cytometric crossmatch despite the increasing use of this technique. Differences in clinical significance found by different groups may be due to the outcome measures used and differences in the technique. The definition of a positive crossmatch can affect the results and investigators may gain information of clinical importance by considering changes in antibody levels over a series of samples from individual patients. The use of sensitive screening methods may assist in the interpretation of crossmatch results. Pre- and post-transplant crossmatching may provide data which can be used to modify immunosuppression in patients at risk of graft dysfunction.

Multicenter evaluation of the flow cytometry T-cell crossmatch: results from the American Society of Histocompatibility and Immunogenetics-College of American Pathologists proficiency testing program

BACKGROUND

The performance characteristics and interlaboratory comparisons of the T-cell flow cytometry crossmatch remain largely unknown.

METHODS

This study was performed using data from the ASHI-CAP proficiency testing program. Four unknown sera and two unknown cells were sent to participating laboratories twice a year for 4 years.

RESULTS

In one survey in which different crossmatch techniques were compared, flow cytometry was slightly more sensitive than the antiglobulin method and considerably more sensitive than direct cytotoxicity. However, the proportion of participants in any given survey detecting antibodies in all sera expected to be positive was 50-60% and has not changed over the years. Failure to detect antibodies correlated with low antibody concentration, diluting the unknown serum by the testing laboratory, and with the instrument used. False positive results with normal sera were infrequent. Fluorescence intensity values were not standardized and were highly variable, but when fluorescence units reported by individual laboratories were divided by their own positive-negative cutoff values, results from different centers were more comparable. In general, fluorescence-to-cutoff ratios >5 correlated with complement binding activity, whereas values <5 denoted concentrations below those required to fix complement.

CONCLUSIONS

Flow cytometry, as used by most centers, is highly sensitive and allows relative antibody quantitation. Furthermore, the data define objective parameters that may help to standardize the test and improve its predictive value in clinical transplantation.

A future for HLA matching in clinical transplantation

The arguments for and against minimizing HLA antigen mismatching in clinical transplantation have been debated for the past 30 years (Terasaki & Cecka, 1994), but at least two major kidney exchange programmes (EURO- and UK-Transplant), with waiting lists totalling 17 thousand patients, prioritize the least HLA mismatched recipients. In the USA, in the United Network for Organ Sharing (UNOS) zero HLA-A,-B, DR mismatch for exchange of kidneys is mandatory. There is now a clear understanding of the in vivo function of HLA molecules, which have a central role in antigen presentation to T cells and thus initiate the immune response both in the normal individual and in transplant recipients. In the latter case, direct recognition of foreign HLA molecules by recipient T cells is also possible (Shoskes & Wood, 1994). With such a clear understanding of the central functional role for HLA in the immune response, it is not surprising that reports of increased kidney transplant survival correlate with reduced HLA mismatching, and it is of concern that some centres choose to allocate kidneys for transplant ignoring HLA. Reports of benefits of minimizing HLA mismatching in heart (Smith et al., 1995) and lung (Iwaki et al., 1993) transplantation are limited to retrospective studies, and no centre has claimed to allocate these organs taking HLA mismatch into account. Consequently, HLA mismatched thoracic organ transplants predominate, but nevertheless recipients of completely mismatched organs have decreased survivals (Opelz & Wujciak, 1994) and more rejection (Baan et al., 1991; Jarcho et al., 1994). Since transplantation of hearts and lungs is essentially only effective on one occasion, all effort should be made to maximize survival and incorporation of HLA mismatches in allocation criteria for these organs is now overdue (Disesa et al., 1990; Morris, 1994). In this brief review we choose not to cover the extensive literature on the role of HLA mismatching in organ transplantation, but consider the field at the present time with particular emphasis on realistic, practical application of HLA typing to the clinical situation and, in particular, the development of allocation procedures that will best suit the needs of individual patients.