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-cell antigen receptor (BCR). We explore here the effect of human CD5 on BCR-triggered 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 regulated kinase-2 is also severely impaired. Moreover, at the functional level, interleukin-2 production is abolished. Src homology 2 domain-bearing tyrosine phosphatase SHP-1 and Src homology 2 domain-bearing inositol 5'-phosphatase 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 inhibitory effect of the chimera, both at biochemical and functional levels. These results demonstrate the inhibitory role of CD5 pseudo-immunoreceptor tyrosine based activation motif tyrosine phosphorylation on BCR signaling. They further support the idea that CD5 uses mechanisms different from those already described to negatively regulate the BCR pathway.