The Maastricht Transplant Center: Clinical setting and epitope searches in HLA class II molecules: Does the structural localization of a polymorphic site contribute to its immunogenicity?

The role of HLA-DP mismatches and donor specific HLA-DP antibodies in kidney transplantation: a case series

BACKGROUND

The impact of HLA-DP mismatches on renal allograft outcome is still poorly understood and is suggested to be less than that of the other HLA loci. The common association of HLA-DP donor-specific antibodies (DSA) with other DSA obviates the evaluation of the actual effect of HLA-DP DSA.

METHODS

From a large multicenter data collection, we retrospectively evaluated the significance of HLA-DP DSA on transplant outcome and the immunogenicity of HLA-DP eplet mismatches with respect to the induction of HLA-DP DSA. Furthermore, we evaluated the association between the MFI of HLA-DP antibodies detected in Luminex assays and the outcome of flowcytometric/complement-dependent cytotoxicity (CDC) crossmatches.

RESULTS

In patients with isolated pretransplant HLA-DP antibodies (N = 13), 6 experienced antibody-mediated rejection (AMR) and 3 patients lost their graft. In HLAMatchmaker analysis of HLA-DP mismatches (N = 72), HLA-DP DSA developed after cessation of immunosuppression in all cases with 84DEAV (N = 14), in 86% of cases with 85GPM (N = 6/7), in 50% of cases with 56E (N = 6/12) and in 40% of cases with 56A mismatch (N = 2/5). Correlation analysis between isolated HLA-DP DSA MFI and crossmatches (N = 90) showed negative crossmatch results with HLA-DP DSA MFI <2000 (N = 14). Below an MFI of 10,000 CDC crossmatches were also negative (N = 33). Above these MFI values both positive (N = 35) and negative (N = 16) crossmatch results were generated.

CONCLUSIONS

Isolated HLA-DP DSA are rare, yet constitute a significant risk for AMR. We identified high-risk eplet mismatches that can lead to HLA-DP DSA formation. We therefore recommend HLA-DP typing to perform HLA-DP DSA analysis before transplantation. HLA-DP DSA with high MFI were not always correlated with positive crossmatch results.

Are We Ready for Epitope-Based HLA Matching in Clinical Organ Transplantation?

This overview describes recent developments demonstrating the significance of epitopes in HLA antibody responses and matching for organ transplantation. HLA epitopes are defined by molecular modeling and amino acid comparisons between HLA alleles and the HLAMatchmaker algorithm considers eplets as essential components. Each allele represents a distinct string of eplets and matching is done by aligning donor and recipient strings. Evidence is summarized how mismatched eplet loads affect antibody responses and transplant outcomes. Epitope-based matching has been applied not only to identify acceptable mismatches for sensitized transplant candidates but also to identify more suitably mismatched donors for nonsensitized patients. Three recently proposed theories will further our understanding of the immunogenicity of individual HLA eplets.It has become apparent that epitope-based matching is superior to antigen matching; we should be ready soon to apply this principle in the clinical transplant setting very soon.

Allelic and Epitopic Characterization of Intra-Kidney Allograft Anti-HLA Antibodies at Allograft Nephrectomy

The reasons for the increased incidence of de novo anti-human leukocyte antibody (HLA) donor-specific antibodies (DSAs) observed after kidney allograft nephrectomy are not fully understood. One advocated mechanism suggests that at graft loss, DSAs are not detected in the serum because they are fixed on the nonfunctional transplant; removal of the kidney allows DSAs to then appear in the blood circulation. The aim of our study was to compare anti-HLA antibodies present in the serum and in the graft at the time of an allograft nephrectomy. Using solid-phase assays, anti-HLA antibodies were searched for in the sera of 17 kidney transplant patients undergoing allograft nephrectomy. No anti-HLA antibodies were detected in the graft if they were not also detected in the serum. Eleven of the 12 patients who had DSAs detected in their sera also had DSAs detected in the grafts. Epitopic analysis revealed that most anti-HLA antibodies detected in removed grafts were directed against the donor. In summary, our data show that all anti-HLA antibodies that were detected in grafts were also detected in the sera. These intragraft anti-HLA antibodies are mostly directed against the donor at an epitopic level but not always at an antigenic level.