Immunologically restricted and inhibitory anti-Ro/SSA in monozygotic twins.

A pair of monozygotic twins with Sjögren's syndrome are described who both have large amounts of anti-Ro/SSA in their sera by ELISA, but no precipitin in double immunodiffusion. Unique among previously encountered specimens, sera from the twins inhibit the formation of Ro/SSA-precipitins in double immunodiffusion by known anti-Ro/SSA positive SLE patients. The twins have near identical, clonally restricted anti-Ro/SSA autoantibodies as evaluated by isoelectric focusing and bind the same 60 kD Ro/SSA peptides. Thus, this pair of monozygotic twins has an identical fine specificity in their immune response to 60 kD Ro/SSA, despite potential differences in their immune response generated by random processes in the formation of immunoglobulin molecules and T cell receptors. These data imply that the anti-Ro/SSA found in these sera binds less than three epitopes such that soluble antigen/antibody complexes are formed instead of an insoluble complex that precipitates.