The negative regulation of T-or B-cell antigen receptor signaling by CD5 was proposed based on studies of thymocytes and peritoneal B-1a cells from CD5-deficient mice. Here, we show that CD5 is constitutively associated with phosphotyrosine phosphatase activity in Jurkat T cells. CD5 was found associated with the Src homology 2 (SH2) domain containing hematopoietic phosphotyrosine phosphatase SHP-1 in both Jurkat cells and normal phytohemagglutinin-expanded T lymphoblasts. This interaction was increased upon T-cell receptor (TCR)-CD3 cell stimulation. CD5 co-cross-linking with the TCR-CD3 complex down-regulated the TCR-CD3-increased Ca 2؉ mobilization in Jurkat cells. In addition, stimulation of Jurkat cells or normal phytohemagglutinin-expanded T lymphoblasts through TCR-CD3 induced rapid tyrosine phosphorylation of several protein substrates, which was substantially diminished after CD5 cross-linking. The CD5-regulated substrates included CD3, ZAP-70, Syk, and phospholipase C␥l but not the Src family tyrosine kinase p56 lck . By mutation of all four CD5 intracellular tyrosine residues to phenylalanine, we found the membrane-proximal tyrosine at position 378, which is located in an immunoreceptor tyrosine-based inhibitory (ITIM)-like motif, crucial for SHP-1 association. The F378 point mutation ablated both SHP-1 binding and the down-regulating activity of CD5 during TCR-CD3 stimulation. These results suggest a critical role of the CD5 ITIM-like motif, which by binding to SHP-1 mediates the down-regulatory activity of this receptor.CD5 is a 67-kDa cell surface glycoprotein expressed on thymocytes, mature peripheral T cells, and a subpopulation of peritoneal B cells (B-1a cells) which are increased in some autoimmune diseases and are associated with the production of autoantibodies (7). Molecular cloning of mouse and human CD5 (mCD5 and hCD5) (16, 17) revealed that it belongs to the scavenger receptor cysteine-rich (SRCR) family group B, which comprises a group of leukocyte membrane or soluble proteins with one or more domains homologous to the aminoterminal domain of type I macrophage SRCR domain (21). Thus far, 10 members of this group of proteins have been identified: CD5, CD6, WC1, M130, Sp␣, Pema-SREG, Ebnerin, CPR-ductin, hensin, and gallbladder mucin (2).Biochemical studies suggest that CD5 is associated with CD3 in the T-cell receptor (TCR)-CD3 complex and with the B-cell receptor (BCR) complex (6,24,32). Two different ligands for CD5 have been reported: CD72, a 42-kDa type II constitutively expressed glycoprotein on B cells (28, 51); and CD5L, an activation antigen expressed on splenocytes (3). The physiologic roles of CD5-CD72 and CD5-CD5L interactions are not known but would be consistent with a potential T-cell-B-cell cooperation during antibody-mediated immune responses (8).Early in vitro studies of T lymphocytes and thymocytes demonstrated that monoclonal antibodies (MAbs) to CD5 were costimulatory for T-cell proliferation (9, 18, 42). However, in vivo studies showed that CD5 down-modulation by spe...

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Background: A number of fluorescent caspase substrates and FRET-based indicators have been developed to study the in vivo activation of caspases, a conserved family of proteases critical in inflammatory, and apoptosis signaling pathways. To date, all substrates have measured only one caspase activity. Here, we describe a FRET-based probe for simultaneously measuring two distinct caspase activities in living cells. Methods: This probe consists of a CFP-YFP-mRFP fusion protein containing a caspase-3-cleavage motif, DEVD, between CFP and YFP, and a caspase-6-cleavage site, VEID, between YFP and mRFP. DEVDase and VEIDase activities could be assessed simultaneously by monitoring diminished FRET mediated by cleavage of either or both of these protease cleavage sites using flow cytometry. Results: DEVDase and VEIDase activities were completely inhibited by the pan-caspase inhibitor z-VAD-fmk and enhanced by DNA-damaging drugs or by anti-Fas stimula-

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