HLA polymorphism : genomic typing and impact on unrelated stem cell transplantation

Abstract

The HLA (Human Leukocyte Antigen) region is the most polymorphic in the human genome. The extended polymorphism contributes to the ability both of individuals and populations to combat diversity in the pathogens to which they are exposed. The region was first discovered through its influence on transplantation rejection and on antigen-specific immune responses. Today, we know that HLA molecules are receptors for peptides derived from self and foreign antigens, which are captured within the cleft of the HLA molecule and presented to T cells. The interaction between the T cell receptor (TCR) and the MHCpeptide complex is a first step in the activation of the T cell. In clinical transplantation HLA incompatibilities between patient and donor leads to an immune response directed against the transplanted cells (host versus graft reaction) or in stem cell transplantation, against the cells of the recipient in immunodeficient patients (graft versus host reaction, GVHD).

Initially, the HLA antigens were defined using serological typing techniques. However, human alloantisera are of limited value when detecting allelic variation located deep within the cleft of the HLA molecules which is inaccessible to antibodies. Consequently, many polymorphisms in HLA molecules can be serologically silent or undetectable by allotypic antisera. During the last few years DNA based typing techniques have begun to replace the serological techniques in clinical applications. Initially, the DNA methods were applied to class 11 typing, but more recently they have been used to determine class I alleles as well. We have developed a polymerase chain reaction-based technique, using sequence-specific primers (PCR-SSP) for HLA class I and class 11 typing. In this thesis PCR- SSP typing is described for the DQB 1, DPA 1 and DPB 1 loci. The recent use of DNA based typing techniques has resulted in many new HLA alleles being identified. One new allele, DQB 1*0609, which was found during the development of DQB1-typing by PCR-SSP, is described here.

Finally, we have, retrospectively, studied the correlation between the grade of HLA matching and the outcome of the transplantation, in patients receiving stem cells from unrelated donors. We found a high frequency of severe GVHD and a high mortality rate in patients receiving HLA-B allele level mismatched stem cells. We also found that patients that received stem cells from donors mismatched for HLA-C, as well as for the NK cell KIR ligand motif had a decreased survival and decreased disease-free survival. Finally, we observed that patients receiving DPA1 mismatched stem cells had decreased survival and relapse-free survival (RFS), which indicates that DPA1 polymorphism is immunologically relevant in stem cell transplantation. These patients also had an increased frequency of relapses.

In conclusion, genomic HLA class I and II typing adds substantially to the success of transplantation of hematopoetic stem cells from unrelated donors. The awareness of HLA class I and II mismatches in a recipient-donor pair form the basis for the appropriate preand post-transplantation treatment, which will improve the outcome of the transplantation.