CD4 is a cell surface glycoprotein that identifies the subset of human T lymphocytes that induces sIg+ B lymphocytes to differentiate and secrete Ig after intimate T-B cell contact. In the course of studying a recombinant, truncated form of CD4 (rT4) we noticed that goat antibodies of apparently irrelevant specificities bound to immobilized rT4. To directly study whether rT4 interacts with Ig molecules, purified human IgG was added to rT4-coated wells and a dose-dependent interaction between IgG and rT4 was observed by ELISA. Purified myeloma IgG proteins bound to immobilized rT4 with the same avidity as polyclonal IgG that suggests that rT4-IgG binding was not due to the presence of anti-rT4 antibodies in the IgG fraction. IgG from 6 sera bound to rT4 in concentration dependent manner similar to purified IgG. Immobilized rT4 specifically bound IgG, and not IgM, IgA, IgD, or beta 2-microglobulin. The specific interaction of rT4 and IgG was also observed when IgG or IgM were immobilized, demonstrating that IgG binding was not a unique property of immobilized rT4. As with low affinity receptors for IgG, rT4 bound heat aggregated IgG with increased avidity. Neither anti-CD4 mAb nor dextran sulfate inhibited rT4-IgG binding. rT4 bound Fc but not F(ab)2 fragments. Each of the purified IgG subclasses; IgG1, 2, 3, and 4 bound to rT4 with similar avidity. Taken together, these data suggest that rT4 specifically interacts with a public structure on IgG Fc.
