STRUCTURAL-ANALYSIS OF V(H)4-21 ENCODED HUMAN-IGM ALLOANTIBODIES AND AUTOANTIBODIES AGAINST RED-BLOOD-CELLS

We have sequenced the variable heavy chain regions of a number of V(H) 4-21 encoded monoclonal IgM anti-Rh(D) antibodies produced in response to deliberate immunization. These were compared with the sequences of similarly encoded IgM anti-I cold agglutinins (CA) derived from patie nts with lymphoproliferative diseases. The anti-Rh(D) antibodies show evidence of clonal expansion and somatic diversification. Even though they are produced in response to an antigenic stimulus, they demonstra te limited hypermutation in the variable heavy chain (V-H) segments an d there is no evidence of selective pressure acting on the complementa rity determining regions (CDRs). The CA demonstrate a higher rate of m utation and yet this results in a lower ratio of replacement to silent mutations (R:S) in the CDRs than seen in the anti-Rh(D) antibodies. I t is not clear whether the different pattern of mutations seen in the CA is related to their auto-reactivity or their tumour origin. In both groups of antibodies the region encoded by the V(H)4-21 segment can b e found in germline configuration at the amino-acid level indicating t hat other V-gene structures, i.e. light chains or CDR(H)3s, are crucia l to the generation of either specificity. A role of the CDR(H)3 is in dicated by the identification of a motif shared by four CAs and one Rh (D) antibody which also demonstrates CA activity independent of its an ti-Rh(D) specificity. Amongst the anti-Rh(D) antibodies there seems to be an obligatory combination with V-L having closest homology to the DPL16 germline segment indicating this as particularly important in ge neration anti-Rh(D) specificity.