Cross-talk between 4-1BB and TLR1–TLR2 Signaling in CD8+ T Cells Regulates TLR2's Costimulatory Effects

The activation of TLR-MyD88 (Toll-like receptor- Myeloid differentiation factor 88) signaling within T cells functions as a potent costimulatory signal that boosts antitumor and antiviral responses. However, the molecular mechanisms underlying the costimulatory processes are poorly understood. We compared microarray gene analysis data between TLR1-TLR2 stimulated and unstimulated T-cell receptor transgenic 'pmel' and MyD88-/- pmel CD8+ T cells and identified changes in the expression of several TNF family members. In particular, TLR-stimulation increased 4-1BB levels in pmel but not in MyD88-/-pmel T cells. A link between 4-1BB and TLR1-TLR2 signaling in CD8+ T cells was highlighted by the suboptimal responses of 4-1BB-/- T cells to TLR1-TLR2 agonist, but their normal response to CD28 or OX40 costimulation. Blocking 4-1BB signaling with antibodies also hindered the costimulatory effects of the TLR1-TLR2 agonist. The elevated levels of 4-1BB transcripts in TLR1-TLR2-stimulated cells were not due to increased mRNA stability nor increased histone activation, but instead were associated with increased binding of p65 and c-Jun to two distinct 4-1BB promoter sites. Combining TLR1-TLR2 ligand with an agonistic antibody to 4-1BB enhanced the antitumor activity in mice with established melanoma tumors. These studies reveal that the costimulatory effects of TLR1-TLR2 signaling in CD8+ T cells are in part mediated by 4-1BB and are important for mounting an effective antitumor immune response.