CD4/Immunoglobulin Interaction: Implications for Immune Physiology and Autoimmunity

Abstract
CD4 has an important role in T cell activation events that depend on its binding to non-polymorphic MHC class II determinants on antigen-presenting cells. Here, we provide evidence that CD4 also interacts with immunoglobulins (Ig). The Ig-binding region lies within residues 21-49 of VI domain of CD4. Immunochemical studies suggest that this property of CD4 does not depend on the three-dimensional folding of the CD4 molecule. Synthetic peptides (/)) encompassing amino acid residues 16-49 and 21-49 of CD4 bind immunoglobulins in comparable way to the intact molecule. In vitro p 16-49 enhances significantly idiotype/anti-idiotype and some weak antigen-antibody interactions. Antigen antibody complexes formed in antigen excess bind CD4 peptides with much higher avidity then non-complexed antibodies. The possible role of the CD4/Ig interaction in T-B cell cooperation is discussed.