A practical view of the role of HLA matching in unrelated donor haematopoietic stem cell transplantation
Abstract
The therapeutic effect of unrelated donor stem cell transplantation (SCT) is predominantly determined by genetic nonidentity – HLA-mismatch – between recipient and donor. This facilitates both the desirable graft versus leukaemia (GVL) effect, which reduces the risk of relapse in malignancies as well as the graft-versus-host disease (GVHD), which increases mortality.
This paper attempts to summarize the current view on the overall significance of HLA match and to interpret the qualitative and quantitative effect of mismatches in individual HLA genes on the outcome of SCT from an unrelated adult donor, particularly in malignant diseases. The current standard involves an effort to find an allele-level matched donor at least in HLA-A,-B,-C,-DRB1, because isolated mismatch in each of these genes increases mortality by approximately 10% and multiple mismatches actually have a negative synergistic effect.
However, the consequences of incompatibility are significantly influenced by disease stage, as in high-risk patients these are much less significant or even negligible because of the accentuated GVL response. With the possible exception of the HLA-C locus, mismatches either on allelic or antigenic level seem to be comparably tolerated in peripheral blood stem cell transplantation.
If it is necessary to accept a mismatched donor, then in the case of bone marrow it is best to avoid mismatches in HLA-A and DRB1, while in the case of peripheral blood stem cells the worst tolerated “antigenic” mismatch involves HLA-C. Apart from HLA match, there are many factors on the donor side that affect SCT outcome.
These especially include timely transplantation, as the speed of finding an unrelated donor and SCT within the shortest possible time are almost as important as the degree of HLA-compatibility.