Localized conformational changes induced in a class I major histocompatibility antigen by the binding of monoclonal antibodies.

We describe two monoclonal antibodies, R3/47 and YR1/1, directed against different epitopes of the expressed rat class I major transplantation antigen RT1Aa, that interact with each other so that the binding of one antibody, YR1/1, is greatly enhanced by the binding of the other. The positive interaction between R3/47 and YR1/1 also occurs when using RT1Aa molecules solubilized from cell membranes in detergent. It is therefore unlikely that the molecular environment of the membrane contributes to the interaction. The ability of R3/47 to modify the YR1/1 determinant on the RT1Aa molecule is mediated without any significant loss of potency by highly purified monomeric Fab fragments. This result suggests that the binding of R3/47 to the RT1Aa molecule alters the YR1/1 determinant by initiating a propagated conformational change in the antigen.