IMMUNOGLOBULIN-V REGION SEQUENCES OF 2 HUMAN ANTIPLATELET MONOCLONAL AUTOANTIBODIES DERIVED FROM B-CELLS OF NORMAL ORIGIN

Autoimmune thrombocytopenia has been attributed to the presence of ant iplatelet autoantibodies which mediate platelet destruction. The deriv ation of these autoantibodies is presently unknown. While normal B cel ls do not produce these autoantibodies in vivo, it has been demonstrat ed in vitro by somatic cell hybridization that the B lymphocytes of no nthrombocytopenic individuals have the potential to produce antiplatel et autoantibodies. Antigen specificities of these antibodies are simil ar to those seen in autoimmune thrombocytopenic purpura and the lupus anticoagulant syndrome. The immunoglobulin V region genes encoding two such human monoclonal antiplatelet antibodies, an anti-GP IIb (STO 17 1) and an anti-phospholipid antibody (STO 103) derived from tonsillar lymphocytes of a non-thrombocytopenic male, have now been sequenced. T hese antiplatelet antibodies were found to be encoded by unmutated ger mline VH and VK genes. The third complementarity determining region (C DR3) of the genes encoding both of these antibodies have unique D regi ons with evidence of N-nucleotide additions, and the light chain genes show VK-JK junctional diversity. STO 103 is encoded by the VH4 V71-2 germline gene and a truncated JH4 gene. The light chain gene showed cl osest homology with the VK4 Humk18 gene and JK2 gene. STO 171 showed c losest homology with the VH4.18 germline gene and had a complete germl ine JH6 gene. The light chain of STO 171 is encoded by the VK3 Humkv32 5 germline gene, which is also used by some rheumatoid factors and col d agglutinins, and a JK4 gene. Although these antibodies were not deri ved from circulating B cells or found to be actively producing antibod y at the time they were harvested, it is possible that naturally occur ring antibody producing B cells, similar to those represented here, ar e recruited for the development of pathogenic autoantibodies in immune thrombocytopenia.