Presence of a 16/6 related human anti-DNA common idiotype (SA1) in the anticardiolipin antibodies of patients with primary antiphospholipid syndrome.

Patients with primary antiphospholipid syndrome (PAPS) have few or no autoantibodies, other than the antiphospholipid antibodies (aPL) that could be natural autoantibodies encoded by germline genes. Some of the autoantibodies marked by the human anti-DNA common idiotype 16/6 have been found to be encoded by unmutated germline genes. Hence, we tested the sera of 19 patients with PAPS for the presence of the 16/6 idiotype which has also been found to be expressed on antibodies that bind cardiolipin. For this we used an ELISA method with antiserum against the SA1 idiotype which recognizes the 16/6. Five of our patients had the idiotype in at least one serum. Among the patients there was one with a variant of PAPS with hemolytic anemia and an IgM antibody to phosphatidylcholine that is akin to the natural autoantibody of normal mice encoded by germline genes VH11 and VH12. Inhibition studies with ssDNA, dsDNA and cardiolipin revealed that all 3 antigens decreased the serum levels of the SA1 idiotype despite absence of detectable anti-DNA antibodies by other methods. Our findings suggest that within the B cell clones that produce aPL in patients with PAPS there are some that produce immunoglobulins bearing 16/6 related idiotypes. This could indicate that some of the aPL present in patients with PAPS derive from natural autoantibody producing cell clones.