Serologic subtyping of HLA-DR8 by means of the cytotoxic human monoclonal antibody 5643

EBV-transformed B cells from a multiparous woman with anti-HLA antibodies were hybridized with CB-F7 heteromyeloma cells. The resulting hybridoma produced the cytotoxic human IgM(κ) m Ab 5643. Testing of the hybridoma supernatant against HLA homozygous cell lines demonstrated positive reactions (titer range 128–512) only with three cell lines that carried the DR (α,β1∗0801) subtype of DR8. There was no reaction with cells expressing other variants of DR8; i.e., carrying the DRβ1∗0802, 0803, or 0804 chains. The only difference between the DRβ1∗0801 and DRβ1∗0802/0804 chains and between the DRβ1∗0801 and 0803 chains is a single aa substitution, α Ser→Asp substitution at residue 57 and a Phe→Ile substitution at residue 67, respectively. Residues 57 and 67 are both situated on the α-helix of the DR β chain. The distance between residues 57 and 67 is two full turns of the α-helix; i.e., about 1 nm. It is possible that both 57 Ser and 67 Phe are directly involved in the epitope recognition by mAb 5643.