Selection of unrelated donors for bone marrow transplantation is improved by HLA class II genotyping with oligonucleotide hybridization.

As the demand for donors for bone marrow transplantation increases, the use of HLA-matched, genetically unrelated donors represents a promising strategy. It is well documented that the clinical outcome of bone marrow transplantation is directly dependent on optimal matching for HLA class I and class II specificities. Molecular studies have revealed the existence of a much larger number of HLA class II alleles than was anticipated, many of which cannot be recognized by routine serological typing. Currently this "hidden" polymorphism represents a major limitation to the generalized use of unrelated donors for bone marrow transplantation. It has recently become possible, however, to identify HLA allelic polymorphism directly at the DNA level by hybridization with sequence-specific oligonucleotide probes ("HLA oligotyping") after amplification of DNA by polymerase chain reaction. In this study, we have investigated whether donor-recipient pairs that are fully matched for HLA by serology are truly HLA-DR, -DQ, and -DP identical and to what extent class II differences influence the primary mixed lymphocyte culture. We show that HLA oligotyping, performed on 50 pairs of HLA class I and II serologically matched individuals, can indeed reveal phenotypically relevant allelic differences at either DRB or DQB loci in 56% of these pairs and can therefore improve HLA class II typing and the choice of bone marrow donors quite significantly. Oligotyping for DRB/DQB/DPB polymorphism also allows prediction of a positive mixed lymphocyte culture, as established in 31 donor/recipient combinations, and even detection of polymorphic differences that were not revealed by this test. This approach is well suited for accurate HLA typing of large pools of bone marrow donors and was successfully applied to select fully matched donors for bone marrow transplantation.