CLINICAL IMPORTANCE OF VARIATION WITHIN THE Bw4 COMPLEX A

CLINICAL IMPORTANCE OF VARIATION WITHIN THE Bw4 COMPLEX A.W. Harmer, R. Collins, A.J. Heads, E. Kondeatis, G. Page, R.W. Vaughan South Thames Tissue Typing, Guy's Hospital, London, UK

Routine antibody screening of a patient with a failed renal transplant showed panel reactive antibodies of 70 - 85% over a two year period. The antibody specificity determined by both cytotoxic screening and ELISA (PRA-STAT) showed positive reactions with most HLA-Bw4 associated B locus antigens, with the exception of B27, B37 and some B44s. The original serological typing had identified the class I type of the patient as A2,30 B14,47 Bw4,6 Cw7,8. The class I type of the live donor of the failed graft was A2,30 B47,51 Bw4. On returning to the transplant waiting list the patient was retyped by PCR-SSP. The molecular type confirmed the serological type with the exception of Bw4. Sequence based typing (SBT) was then performed on recipient and donor. The recipient was identified as having the allele B*4703. The amino acid sequence of B*4703 at positions 77-83 (the Bw4/Bw6 region) shows the allele to be identical to the Bw6 motif at positions 77-81 whilst the amino acids at positions 82-83 are those common to the 5 different Bw4 motifs. Further analysis of the antibody reactivity was performed using the SOFT-STAT II program's pepito function (Sangstat) which identifies particular epitopes that account for the reactions seen. This identified amino acids at positions 80-81 which correspond to 2 of the Bw4 sequence motifs. SBT confirmed that the recipient had been mismatched for these 2 amino acids which are present on the B*51011 allele of the donor. This motif is present in the majority of Bw4 associated alleles including B*2702 and B*4406 but not in the other B27 or B44 alleles and not in B13 or B37 which is in complete agreement with the cytotoxic and ELISA screening results. The use of SBT to confirm the mismatches at the sequence level has provided the information necessary to identify the precise specificity of the patient's antibody and to determine future unacceptable mismatches based on allele sequences.