ZAP-70 AND P72(SYK) ARE SIGNALING RESPONSE ELEMENTS THROUGH MHC CLASS-II MOLECULES

Ligation of major histocompatibility complex (MHC) class II antigens expressed on antigen-activated human CD4(+) T-lymphocytes induces early signal transduction events including the activation of tyrosine kinases, the tyrosine phosphorylation of phospholipase-C gamma l and the mobilization of intracellular calcium. Similar responses have been observed in B-cells following stimulation of MHC class II molecules, including the increased production of intracellular cAMP: In this report, we demonstrate that the ZAP-70 tyrosine kinase is a responsive signaling element following cross-linking of HLA-DR in class II+ T-cells, and that the homologous tyrosine kinase p72(syk) is stimulated in B-cells following ligation of class II antigens. Antibody mediated co-ligation of the T-cell antigen receptor (TCR/CD3) with class II molecules resulted in augmented tyrosine phosphorylation of ZAP-70. Comparable to antibody induced receptor ligation, bacterial superantigen (SEA and SEB) treatment of HLA-DR(+) T-cells stimulated ZAP-70 tyrosine phosphorylation, consistent with class II transmembrane signaling by ligation of HLA-DR and V(b)eta in cis. Modulation of the TCR/CD3 led to abrogation of class II induced ZAP-70 tyrosine phosphorylation, but did not result in sequestering of ZAP-70 from the cellular cytoplasm. Hyperphosphorylated ZAP-70 was associated with TCR/CD3 zeta-chain following cross-linking of HLA-DR, suggesting a mechanism for the TCR/CD3-dependence of class II induced signals in alloantigen-activated human T-cells. In both tonsillar B-lymphocytes and B-cell leukemia lines, p72(syk) was rapidly phosphorylated on tyrosine residues following HLA-DR cross-linking. Tyrosine phosphorylation of p72(syk) induced through ligation of either the B-cell antigen receptor or class II molecules was potently inhibited by herbimycin A. MHC class II ligation on B-lymphocytes resulted in cell death, which was both qualitatively distinct from Fas-induced apoptosis and partially protected by herbimycin A pretreatment. Thus, ligation of MHC class II molecules expressed on human lymphocytes stimulates the ZAP-70/p72(syk) family of tyrosine kinases, leading functionally to a tyrosine kinase-dependent pathway of receptor-induced cell death.