Non-immune VH-binding specificity of human protein Fv

The specificity of human F(ab)-binding Protein Fv (previously called Protein F), a sialoprotein released into the digestive tract mainly during hepatitis, was investigated with fragments or chains of monoclonal immunoglobulins. Protein Fv bound an unreduced H-chain dimer of a monoclonal human IgA2m(1) molecule but neither the corresponding L-chain dimer, nor several Bence-Jones molecules. Using enzymatic subfragments of F(ab)mu, or F(ab')2 gamma, a significant binding was observed with Fv fragments (VH + VL), while Fb fragments (CH1 + CL) were inactive. Taken altogether, these results prove that the structure recognized by Protein Fv is located in the VH domain. This structure probably involves discontinuous epitopes linked by a disulphide bond, which are destroyed by combined reduction and dissociation. Protein Fv does not interfere with the antigen-binding site, since there was no reciprocal inhibition with the antigen-antibody reaction.