Cell adhesion mediated by CD4 and MHC class II proteins requires active cellular processes.

Human CD4 is an accessory molecule found on the cell surface of a subset of T lymphocytes. We have previously shown by infection of simian fibroblasts with an SV40-CD4 recombinant virus that CD4 acts as an adhesion molecule by binding to human MHC class II Ag expressed on the surface of human B lymphocytes. This report confirms that human B lymphoblastoid cell lines expressing class II molecules at the cell surface can bind to Chinese hamster ovary cells that have been stably transfected with human CD4. This cellular adhesion is a late event, which is first detected after 2 h, but remains stable for up to 16 h. The association between CD4 and class II is energy dependent, as it is detected at 37 degrees C, but not at 4 degrees C or if either cell type is fixed with paraformaldehyde. ATP is required for the establishment and maintenance of stable CD4/class II-mediated cell conjugates. Cytoskeletal interactions also regulate CD4/class II adhesion as treatment with the microtubule and microfilament inhibitors colchicine, cytochalasin-D, and nocodazole rapidly dissociates even preformed cell conjugates. Our observations also indicate that the Ag-independent engagement of class II by CD4 induces homotypic clustering of human B cells. This effect is blocked by reagents directed against lymphocyte function-associated Ag-1 and may result from the induction of a high affinity phenotype of lymphocyte function-associated Ag-1 induced by class II signaling. Finally, we discuss the implications of CD4/class II-mediated adhesion and the role of CD4 in the regulation of T cell adhesion.