CD5 IS ASSOCIATED WITH THE HUMAN B-CELL ANTIGEN RECEPTOR COMPLEX

On human B cells the antigen receptor complex is composed of the membr ane form of the immunoglobulin molecule and the non-covalently associa ted Ig alpha/beta heterodimer. A small subpopulation of normal B cells and chronic lymphocytic leukemia B cells express (analogous to T cell s) the transmembrane molecule CD5, a counterstructure of B cell-specif ic CD72. Numbers of CD5(+) B cells are increased in several physiologi cal and pathological conditions. Moreover, CD5(+) B cells are being he ld responsible for the production of autoreactive antibodies and seem to have signaling characteristics distinct from conventional B cells. On T cells, CD5 associates with the T cell receptor CD3 complex and li gation of CD5 leads to the generation of co-stimulatory signals, that act on T cell activation. We here demonstrate that CD5 is associated w ith the B cell receptor (BCR) complex and serves as substrate for BCR- induced tyrosine kinase activity. Hence, CD5(+) B cells have a unique potential to modulate BCR signals.