Permissible and immunogenic HLA-A mismatches: cytotoxic T-cell precursor frequencies reflect graft survival data.

Analysis of the in vivo immunogenicity of single HLA mismatches, in the context of a patient's own human leukocyte antigen (HLA) phenotype, has been used to define permissible and immunogenic HLA mismatches. Kidney graft survival in the case of permissible mismatches was similar to that of completely HLA matched combinations, whereas immunogenic mismatches lead to a significantly poorer graft survival. The present study tested whether such permissible and immunogenic HLA mismatches are reflected in the in vitro cytotoxic T-lymphocyte (CTL) allorepertoire. Limiting dilution experiments were performed to analyze the number of precursor CTL directed against individual HLA class I antigens. In general, the frequency of CTLp directed against permissible HLA-A antigens (n = 70, mean frequency 27 CTLp per million peripheral blood lymphocytes [PBL]) was found to be significantly lower compared with the CTLp directed against immunogenic HLA-A antigens (n = 73, mean frequency 59 CTLp per million PBL). The difference was found both in healthy individuals and a population of renal transplant candidates. These results were confirmed by a retrospective analysis of CTLp frequencies performed between partly mismatched unrelated bone marrow donors and their potential recipients. In conclusion, on the population level the permissible and immunogenic HLA-A mismatches are indeed reflected in the CTL allorepertoire. However, due to the big overlap of the CTLp frequencies in these populations, the permissible or immunogenic nature of a mismatch for a particular patient should be determined on an individual basis.

In vitro CTL precursor frequencies do not reflect a beneficial effect of cross-reactive group (CREG) matching.

Adjustment of histocompatibility-based allocation criteria in kidney transplantation from HLA matching to matching on the basis of cross-reactive groups (CREG), was recently suggested to be a good alternative to transplant with more "well-matched" kidneys, without negatively influencing graft survival. Because graft rejection is often mediated by cytotoxic T cells (CTLs), we investigated whether a beneficial effect of CREG matching is reflected in vitro by lower CTL precursor frequencies (CTLpf). Therefore, CTLpf were determined in a group of healthy individuals and analyzed with respect to the number of HLA and CREG mismatches. A clear correlation was found between the number of HLA mismatches and the CTLpf, that is, the lowest mean frequency in case of 0 HLA-A, B mismatches (66 CTL precursors per 10(6) cells) and the highest in combinations with 4 HLA mismatches (mean = 303 CTLp/10(6) cells). The situation was different in the case of CREG mismatches. Although the highest frequency was found in the group of 4 CREG mismatches, no significant differences were observed between 0, 1, and 2 CREG mismatches. High CTLpf, up to 430/10(6), were even seen in the case of 0 CREG mismatches. Also within a well-defined group of single HLA-A or HLA-B mismatches no difference in CTLpf were observed between the subgroups with 0 vs. 1 CREG mismatches. The present study showed that in vitro the CTLpf correlates better with HLA than with CREG matching. These data are consistent with findings reported by several groups that matching for the CREG does not benefit transplant outcome.

Cross-reactive group matching does not lead to a better allocation and survival of donor kidneys.

BACKGROUND: In cadaveric renal transplantation HLA-A, -B, -DR matching of donor and recipient is beneficial for graft survival. However, allocation based on HLA matching seems to favor recipients with more frequently occurring HLA antigens. In this study we investigated whether matching on the basis of cross-reactive groups (CREGs), defined according to the United Network for Organ Sharing (UNOS), would be a good alternative for the allocation of kidneys without negatively influencing graft survival. Theoretically, this approach would provide more recipients with an immunologically well-matched donor organ. METHODS: The influence of CREG matching on graft survival was studied in univariate analyses using the Eurotransplant database. RESULTS: No beneficial effect of CREG matching was observed, whereas a significant HLA matching effect was observed in the 0 CREG mismatched donor/ recipient combinations. Only in the small subgroup with 1 MM for HLA-A, -B and 0 MM for HLA-DR, a significantly better survival was observed, when this mismatch belonged to the 0 or 1 MM CREG group versus two or more MM CREG group. However, this subgroup concerns only 8% of the transplants performed. CONCLUSIONS: In contrast to other reports, our study showed that HLA matching is by far more beneficial than CREG matching. In the homogenous Eurotransplant population, adjusting the matching criteria toward CREG matching would not lead to an improved graft survival.

Beneficial and detrimental human leucocyte antigen (HLA)-DR mismatches are not reflected by a differential effect of immunosuppressive drugs on the in vitro alloimmune response.

The introduction of molecular tissue typing techniques has led to an enormous increase in the number of human leucocyte antigen (HLA) alleles. This increasing polymorphism of the HLA antigens makes the selection of a well-matched unrelated donor a difficult task. Recent data suggest that some HLA mismatches are more immunogenic than others. This has led to the introduction of terms like beneficial or acceptable versus detrimental or taboo mismatches. The study considered whether the differential immunogenicity as reflected by graft survival studies can be detected in vitro as well. Mixed lymphocyte reaction (MLR) and primed lymphocyte tests (PLT) were performed with different HLA-DR mismatched combinations in the presence and absence of cyclosporine A and prednisolone. Differential effects of these immunosuppressive drugs were observed. Some reactions could easily be blocked by cyclosporine alone, whereas others need the addition of high doses of prednisolone as well before a significant inhibition was found. These differences were not only found between individual responders but also within one individual dependent on the stimulatory HLA-antigen involved. When the group of beneficial mismatches was compared with the group of detrimental mismatches, no differences were observed. Our data show that immunosuppressive drugs have a differential effect on in vitro alloimmune responses but these do not differentiate between beneficial and detrimental mismatches as defined by kidney graft survival.

Auto- and alloimmune reactivity to human islet allografts transplanted into type 1 diabetic patients.

Allogeneic islet transplantation can restore an insulin-independent state in C-peptide-negative type 1 diabetic patients. We recently reported three cases of surviving islet allografts that were implanted in type 1 diabetic patients under maintenance immune suppression for a previous kidney graft. The present study compares islet graft-specific cellular auto- and alloreactivity in peripheral blood from those three recipients and from four patients with failing islet allografts measured over a period of 6 months after portal islet implantation. The three cases that remained C-peptide-positive for >1 year exhibited no signs of alloreactivity, and their autoreactivity to islet autoantigens was only marginally increased. In contrast, rapid failure (<3 weeks) in three other cases was accompanied by increases in precursor frequencies of graft-specific alloreactive T-cells; in one of them, the alloreactivity was preceded by a sharply increased autoreactivity to several islet autoantigens. One recipient had a delayed loss of islet graft function (33 weeks); he did not exhibit signs of graft-specific alloimmunity, but developed a delayed increase in autoreactivity. The parallel between metabolic outcome of human beta-cell allografts and cellular auto- and alloreactivity in peripheral blood suggests a causal relationship. The present study therefore demonstrates that T-cell reactivities in peripheral blood can be used to monitor immune mechanisms, which influence survival of beta-cell allografts in diabetic patients.

In vitro reactivity of allospecific cytotoxic T lymphocytes does not explain the taboo phenomenon.

Matching for human leucocyte antigens (HLA) is important for graft survival in kidney transplantation. Nevertheless, most patients receive a kidney graft with multiple HLA mismatches. Some of these mismatches seem to be more harmful than others. By studying the effect of single HLA mismatches in the context of the patients' own HLA, we have previously identified donor/recipient combinations with a significantly higher incidence of early graft failure, the so-called taboo combinations. In the present study we investigated whether a higher cytotoxic T lymphocyte (CTL) response towards taboo mismatches may be involved in this phenomenon. CTL reactivity was determined both in taboo and control combinations by in vitro CTL precursor assays, using peripheral blood mononuclear cells and proximal tubular epithelial cells as target cells. Inhibition studies with CD8-antibody as well as Cyclosporin A were performed to identify high avidity and primed CTLs. Furthermore, in committed CTLp assays indirect recognition of the taboo mismatch was tested using synthetic peptides. The CTL precursor frequencies in taboo combinations were always lower than the CTL precursor frequencies in control combinations. No difference in avidity and activation status of the CTLs could be detected when taboo combinations were compared with the controls. In the committed CTLp assays no reactivity towards any of the synthetic peptides was observed. The significantly poorer graft survival of taboo combinations cannot be explained by a higher number of donor-specific CTLs. Furthermore, the avidity or activation status of these CTLs does not provide a clue to the taboo phenomenon.