Differential immunogenicity of HLA mismatches in clinical transplantation

Why can sensitization by an HLA-DR2 mismatch lead to antibodies that react also with HLA-DR1?

HLAMatchmaker is a matching algorithm that can be used to characterize antibodies specific for structurally defined epitopes. Under the auspices of the 15(th) International Histocompatibility Workshop, we are conducting a multilaboratory collaborative project to characterize these epitopes and also to determine how often they induce specific antibodies in patients with rejected kidney transplants. This report addresses the reactivity of post-allograft nephrectomy sera tested for DRB antibodies with Luminex assays using single alleles. This analysis was performed for 19 informative kidney transplant cases contributed by 13 laboratories worldwide. There were 11 cases with a single DR2 mismatch (DR15 or DR16), and nine of them (82%) showed antibodies with both DR2 and DR1. Although these antigens might share an epitope recognized by these antibodies, this interpretation is incorrect. The HLAMatchmaker analysis offers a clearly different explanation that involves antibodies induced by DR51, which commonly associates with DR2. DR51 has an epitope defined by the 96EV eplet, which is also present on DR1 but no other DR antigen. This means that the reactivity with DR51 and DR1 reflects the presence of 96EV-specific antibodies. Conversely, we analyzed eight patients sensitized by a single DR1 mismatch that has no associated DR51. All of these patients reacted also with DR51, and this could be explained only by antibodies against the shared 96EV eplet. These findings demonstrate that 96EV represents a highly immunogenic epitope that can induce cross-sensitization between antigens encoded by the different DRB loci, and also that DR51 is important in determining DRB mismatch acceptability of potential donors. This analysis has also demonstrated that antibody responses are restricted to a few epitopes on these immunizing DR antigens. For DR2 they are 142M3 (unique for DR2), 71QAA (shared with DB5*02), and 96QV (shared with DR10). DR51 mismatches appear to have three immunogenic eplets: 96EV (shared with DR1), 108T3 (unique for DR51), and 40HFD (shared with DR9). Immunogenic eplets on DR1 are 12LKF2 (unique for DR1), 14FEH (shared with DR9 and DR10), and 25HRL (shared with DR10).

Differentially modulated dendritic cells induce regulatory T cells with different characteristics

Dexamethason (DEX) treated DC display several features that establish them as candidates for specific allogeneic tolerance induction. We report the results of in vitro studies of polarization of the alloimmune T cell response with two types of differentially modulated human DC. Both DEX treated DC triggered by LPS for 6 h (DEX6-DC) and DEX treated DC triggered by LPS for 48 h (DEX48-DC) acquired low levels of costimulatory, adhesion, and MHC class II molecules compared with mature DC (mDC). In contrast to mDC, both DEX6-DC and DEX48-DC did not produce any IL-12. DEX6-DC were able to produce significant amounts of IL-10 whereas DEX48-DC did not actively produce IL-10. Conversely, the induction of IL-10 producing cells was significantly increased when PBL were stimulated with DEX48-DC compared with DEX6-DC. Both stimulation of PBL with DEX6-DC and DEX48-DC led to the induction of cell populations able to suppress the proliferative alloimmune response of primed T cells in a cell-cell contact independent and antigen-nonspecific manner. Tregs obtained after stimulation with DEX48-DC were also able to inhibit the IFN-gamma production of the effector cells and this effect could be blocked by anti-IL-10. Tregs induced by DEX6-DC produced similar amounts of IL-10, yet were not able to inhibit IFN-gamma production of the effector T cells, indicating a different mechanism. In summary, we show that differential modulation of DC results in the induction of different populations of regulatory T cells.

Structural epitope matching for HLA-alloimmunized thrombocytopenic patients: a new strategy to provide more effective platelet transfusion support?

BACKGROUND

During the past three decades, HLA matching for platelet (PLT) transfusion of refractory thrombocytopenic patients has been based on serologic cross-reactivity between HLA-A and HLA-B antigens. Although many blood banks are using this matching strategy, the general experience is that such matched PLT transfusions are often ineffective.

STUDY DESIGN AND METHODS

This report describes a new HLA matching algorithm that considers structurally defined epitopes recognized by antibodies. HLAMatchmaker is a computer program that determines histocompatibility at the amino acid level initially designed as triplets (i.e., linear sequences of three residues in molecular surface-exposed positions) but now updated as eplets representing patches of antibody-accessible polymorphic residues surrounded by residues within a 3-A radius. The eplet version of HLAMatchmaker is also useful in the analysis of HLA antibody reactivity patterns of alloimmunized patients so that acceptable mismatches can be identified.

CONCLUSION

An HLA epitope-based matching protocol is proposed that may permit a more effective PLT transfusion management of refractory patients. This protocol includes high-resolution HLA-A, -B, and -C typing of patients and donors, serum screening to identify acceptable mismatches, and the identification of suitable donors in a donor database that incorporates HLAMatchmaker as a search engine. HLAMatchmaker programs can be downloaded from the Web site http://tpis.upmc.edu/tpis/HLAMatchmaker/.

Severe acute graft-vs-host disease in a patient with acute monocytic leukemia having a recombination event between HLA-A/B loci from a multiple recombinant family

Human leukocyte antigen (HLA) recombination, particularly multiple recombinations can produce novel haplotypes, thereby complicating donor-recipient selection and possibly inducing severe graft-vs-host disease (GVHD) after nonfully matched allogeneic hematopoietic stem cell transplantation. Here, we report for the first time that a 30-year-old female acute monocytic leukemia patient with an HLA-A/B recombinant haplotype, who has three unmatched and one HLA-B/DRB1 recombinant haplotype siblings, presented grade IV acute GVHD (aGVHD) after transplantation from a sibling with a single allele only mismatch at the classical HLA-A locus. Furthermore, using a new three-dimensional structure modeling application, we inferred that the structural differences in peptide-binding and T-cell receptor interaction sites can significantly change the immunogenicity of mismatched HLA molecules, potentially one of the main causes of aGVHD.

Evaluation of alloreactivity in responder-stimulator pairs by determination of gamma interferon-producing cells and cytotoxic-T-lymphocyte precursor frequencies

We used the enzyme-linked immunospot (ELISPOT) assay and the cytotoxic-T-lymphocyte precursor frequency assay to evaluate alloreactivity in responder-stimulator pairs. High frequencies of responder cells among cells from HLA-mismatched pairs and low frequencies among cells from pairs of siblings with identical HLA types were detected by both assays. The ELISPOT assay thus illustrated the helper and cytotoxic-T-cell response to allogeneic HLA antigens.

Intravenous immunoglobulin, HLA allele typing and HLAMatchmaker facilitate successful transplantation in highly sensitized pediatric renal allograft recipients

The use of intravenous immunoglobulin (IVIG) in sensitized transplant candidates has resulted in reduced HLA antibody levels and shorter transplant wait times. In addition, the HLAMatchmaker program has been used to identify acceptable mismatches to permit transplantation in highly sensitized patients. We used IVIG desensitization in conjunction with high resolution HLA allele typing and HLAMatchmaker grading of donor offers to facilitate successful transplantation in two highly sensitized children who were awaiting second renal transplants. Both patients lost their initial transplant in <10 days to accelerated acute rejection, and were on dialysis for an average of 50 months with high panel reactive antibody (PRA) levels. They were started on monthly IVIG infusions (2 g/kg/dose). Within one wk following their third and fifth IVIG doses, both patients received a crossmatch compatible, deceased donor renal transplant selected by HLAMatchmaker as a suitable donor offer. Both patients remain rejection free with excellent renal function 19 and 15 months post-transplant, respectively. In conclusion, combining IVIG therapy and donor selection by HLA humoral epitope matching permitted successful transplantation of two highly sensitized children. Further studies in larger numbers of patients with longer follow-up are needed to determine the individual role played by, and relative importance of each component of this combined strategy.

HLA mismatching within or outside of cross-reactive groups (CREGs) is associated with similar outcomes after unrelated hematopoietic stem cell transplantation

The National Marrow Donor Program maintains a registry of volunteer donors for patients in need of a hematopoietic stem cell transplantation. Strategies for selecting a partially HLA-mismatched donor vary when a full match cannot be identified. Some transplantation centers limit the selection of mismatched donors to those sharing mismatched antigens within HLA-A and HLA-B cross-reactive groups (CREGs). To assess whether an HLA mismatch within a CREG group ("minor") may result in better outcome than a mismatch outside CREG groups ("major"), we analyzed validated outcomes data from 2709 bone marrow and peripheral blood stem cell transplantations. Three-hundred and ninety-six pairs (15%) were HLA-DRB1 allele matched but had an antigen-level mismatch at HLA-A or HLA-B. Univariate and multivariate analyses of engraftment, graft-versus-host disease, and survival showed that outcome is not significantly different between minor and major mismatches (P = .47, from the log-rank test for Kaplan-Meier survival). However, HLA-A, HLA-B, and HLA-DRB1 allele-matched cases had significantly better outcome than mismatched cases (P < .001). For patients without an HLA match, the selection of a CREG-compatible donor as tested does not improve outcome.