The pseudo-immunoreceptor tyrosine-based activation motif of CD5 mediates its inhibitory action on B-cell receptor signaling

Genetic studies revealed that CD5 could be a negative regulator of the B-ce ll antigen receptor (BCR). We explore here the effect of human CD5 on BCR-t riggered responses. B cells were obtained expressing a chimera composed of extracellular and transmembrane domains of Fc gamma type IIB receptor fused to CD5 cytoplasmic domain (CD5cyt). Coligation of the chimera with the BCR induces CD5cyt tyrosine phosphorylation. A rapid inhibition of BCR-induced calcium response is observed, as well as a partial but delayed inhibition of phospholipase C gamma-1 phosphorylation. Activation of extracellular reg ulated kinase-2 is also severely impaired. Moreover, at the functional leve l, interleukin-2 production is abolished. Src homology 2 domain-bearing tyr osine phosphatase SHP-1 and Src homology 2 domain-bearing inositol 5'-phosp hatase SHIP usually participate in negative regulation of the BCR. We show that they do not associate with the phosphorylated CD5 chimera. We finally demonstrate that the pseudo-immunoreceptor tyrosine based activation motif present in CD5cyt is involved because its deletion eliminates the inhibitor y effect of the chimera, both at biochemical and functional levels. These r esults demonstrate the inhibitory role of CD5 pseudo-immunoreceptor tyrosin e based activation motif tyrosine phosphorylation on BCR signaling. They fu rther support the idea that CD5 uses mechanisms different from those alread y described to negatively regulate the BCR pathway.