CD40 associates with the MHC class II molecules on human B cells

The MHC class II and CD40 molecules are two major components of the immune system that are involved in cell-cell interactions and signal transduction. Data obtained in the course of the present investigation show that these t wo molecules are physically associated on the surface of various human B ce ll lines and on normal tonsilar B cells. The CD40/MHC class II complexes we re not detected on the germinal center B cell line Ramos. However, stimulat ion of these cells via CD40 or MHC class II triggered their association, su ggesting that the formation of the complex is related to the activation sta tus of the cells. The formation of these complexes did not alter the intera ction of MHC class II molecules with one of their natural ligands, the stap hylococcal enterotoxin A (SEA), as evidenced by the ability of SEA to bind MHC class II/CD40 complexes. Cross-linking of MHC class II or CD40 molecule s leads to the association as well as the co-association of both molecules to the NP-49-insoluble cellular matrix. Such association allowed us to demo nstrate that only a fraction of these molecules can be physically associate d on the cell surface. Based on previous observations and those presented h ere, it is highly possible that the CD40/MHC class II complexes may have an important role in signal(s) induced via both molecules and during T/B cell s interactions.