Cross-linking of 4-1BB activates TCR-signaling pathways in CD8+ T lymphocytes

Cross-linking of 4-1BB, a member of the TNFR family, increased tyrosine phosphorylation of TCR-signaling molecules such as CD3epsilon, CD3zeta, Lck, the linker for activation of T cells, and SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76). In addition, incubation of activated CD8(+) T cells with p815 cells expressing 4-1BBL led to redistribution of the lipid raft domains and Lck, protein kinase C-theta, SLP-76, and phospholipase C-gamma1 (PLC-gamma1) on the T cell membranes to the areas of contact with the p815 cells and recruitment of 4-1BB, TNFR-associated factor 2, and phospho-tyrosine proteins to the raft domains. 4-1BB ligation also caused translocation of TNFR-associated factor 2, protein kinase C-theta, PLC-gamma1, and SLP-76 to detergent-insoluble compartments in the CD8+ T cells, and cross-linking of 4-IBB increased intracellular Ca 21 levels apparently by activating PLC-gamma1. The redistribution of lipid rafts and Lck, as well as translocation of PLC-gamma1, and degradation of IkappaB-alpha in response to 4-1BB were inhibited by disrupting the formation of lipid rafts with methyl-o-cyclodextrin. These findings demonstrate that 4-IBB is a T cell costimuiatory receptor that activates TCR-signaling pathways in CD8(+) T cells.