A Role for CD99 in T Cell Activation

The function of the T cell surface protein CD99 was investigated in human CD4+peripheral T cells. Crosslinking of the CD99 molecule using anti-CD99 mAbs in the presence of anti-CD3 Ab resulted in a marked enhancement of proliferation. CD99 coligation also enhanced CD25 expression and early markers of T cell activation, CD69 and CD40L. Ligation of CD99 resulted in the pronounced tyrosine phosphorylation of an ∼29-kDa protein suggesting that a specific CD99-induced signal transduction pathway may exist. Simultaneous costimulation with anti-CD99 and anti-CD28 Abs appeared to have additive effects on CD40L expression while CD99 ligation had no effect on CD2-mediated T cell induction of CD40L expression. These results demonstrate that CD99 signal transduction can deliver effective costimulatory signals to T cells.