CD5 costimulation up-regulates the signaling to extracellular signal- regulated kinase activation in CD4⁺CD8⁺ thymocytes and supports their differentiation to the CD4 lineage

CD5 positively costimulates TCR-stimulated mature T cells, whereas this molecule has been suggested to negatively regulate the activation of TCR- triggered thymocytes. We investigated the effect of CD5 costimulation on the differentiation of CD4⁺CD8⁺ thymocytes. Coligation of thymocytes with anti- CD3 and anti-CD5 induced enhanced tyrosine phosphorylation of LAT (linker for activation of T cells) and phospholipase C-γ (PLC-γ) compared with ligation with anti-CD3 alone. Despite increased phosphorylation of PLC-γ, this treatment down-regulated Ca²⁺ influx. In contrast, the phosphorylation of LAT and enhanced association with Grb2 led to activation of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase. When CD3 and CD5 on CD4⁺CD8⁺ thymocytes in culture were coligated, they lost CD8, down- regulated CD4 expression, and induced CD69 expression, yielding a CD4⁺((dull))CD8^-CD69⁺ population. An ERK inhibitor, PD98059, inhibited the generation of this population. The reduction of generation of CD4⁺CD8^- cells resulted from decreased survival of these differentiating thymocytes. Consistent with this, PD98059 inhibited the anti-CD3/CD5-mediated Bcl-2 induction. These results indicate that CD5 down-regulates a branch of TCR signaling, whereas this molecule functions to support the differentiation of CD4⁺CD8⁺ thymocytes by up-regulating another branch of TCR signaling that leads to ERK activation.