Stem cell transplants, whether related or unrelated, require precise HLA matching between donor and patient. Because HLA antigens are inherited, patients are more likely to find a matching donor within their own racial or ethnic communities. To provide patients of every ethnic community a better chance at finding a matched donor, the NMDP has ongoing recruitment programs to bring more African-American, American Indian/Alaska Native, Asian/Pacific Islander, and Hispanic donors to the NMDP Registry. Currently, approximately 31% of the volunteer donors listed in the NMDP Registry are from racial and ethnic minority groups. NMDP Cord Blood Program
The NMDP currently lists more than 25,000 units of donated umbilical cord blood from a growing partnership with cord blood banks. The addition of cord blood banks to the NMDP Network was vital to provide more transplant options for patients. All cord blood units at NMDP cord blood banks are listed in the NMDP Registry and are automatically included in every patient search. The NMDP's cord blood program is a clinical trial developed in 1998 under an Investigational New Drug application (IND) with the US Food and Drug Administration (FDA). The NMDP continues to work with existing cord blood banks to establish new contacts and list more cord blood units in the NMDP Registry. NMDP Office of Patient Advocacy
The NMDP's Office of Patient Advocacy (OPA) works with patients to remove barriers to obtaining an unrelated donor transplant. The OPA connects patients to transplant-related resources, helps patients find a transplant center, and assists them with financial and insurance matters. In addition, the OPA assists patients, their families, and physicians with any concerns or questions they may have regarding an NMDP-facilitated search and stem cell transplant. NMDP Research Program
The NMDP collects detailed medical data on patients who receive a transplant from an NMDP donor. These data are part of a comprehensive research database the NMDP maintains to assist medical researchers in the field of unrelated stem cell transplantation. The NMDP research program develops and promotes research aimed at increasing opportunities for and improving outcomes of unrelated donor stem cell transplants. NMDP resources available to researchers include:
- The largest database of HLAtyped individuals in the world (more than 4.9 million volunteer donors).
- Outcome, histocompatibility data, donor search, and donation sideeffects data on approximately 90% of the more than 16,000 unrelated donor stem cell transplants the NMDP has coordinated since 1987.
- The largest unrelated stem cell donor and recipient HLA database in the world (more than 7,600 paired samples).
Lymphocytes differentiate between self and nonself cells by examining the HLA antigens expressed on the surface of cells. To prevent graft rejection and other posttransplant complications, stem cell donors and recipients must be closely HLA matched. The HLA antigens are encoded on the short arm of human chromosome 6, on a segment called the major histo- compatibility complex (MHC). There are 3.5 million bases in the MHC, but only a small number are currently matched in stem cell transplantation. The relevant portions of the MHC are further divided into two regions- class I and class II. Class I antigens (HLA-A, -B, and -C) and class II antigens (HLA-DRB1, -DP, and -DQ) are the antigens most frequently examined when matching potential stem cell donors and transplant recipients. The NMDP requires that donors and patients have no more than a one-antigen mismatch at the HLA-A, -B or -DRB1 locations. Because there are two HLA antigens at each of these three locations, a perfect match is referred to as a 6 of 6 match, and a one-antigen mismatch is termed a 5 of 6 match. Using this terminology, the NMDP will allow a 5/6 match or a 6/6 match for marrow and peripheral blood stem cell transplants, but not a 4/6 match or less. The NMDP will allow a 4/6 match for cord blood transplants, provided the mismatched antigens are not at the same loci. This less stringent HLA matching for cord blood is permitted because of the lowered immunologic competence of cord blood T cells.[7-9] Some transplant centers require additional matching at the HLA-C, -DP, and/or DQ loci. Newer DNA-based methods of HLA typing have proven to be more accurate than serologic methods and have largely superseded them. Impact of HLA Match on Transplant Outcome
In allogeneic stem cell transplantation, the degree of donor/recipient HLA match is an important factor in engraftment, the development of graftvs- host disease (GVHD), and overall survival.[11,12] The association of HLA class I allele disparity with graft failure was examined by the Seattle group, who retrospectively analyzed data from 21 patients experiencing graft failure and 42 case-matched controls. Complete allele- level matching for class I was identified in 45% of controls and 10% of graft failure cases. The effect of the number of HLA disparities was subsequently studied in unrelated- donor stem cell transplant for CML patients. Among allele-matched donors/recipients and donors/recipients mismatched for a single class I allele, the graft failure rate was 2%. When two or more class I disparities at HLA-A, -B, and/or -C were present, the graft failure rate increased to 29%. In unrelated-donor stem cell transplantation, the risk of clinically significant GVHD is also influenced by the extent of HLA disparity between the donor and recipient. GVHD is a potentially life-threatening complication involving an immunologic reaction in recipients that is mediated by the transplanted T cells.