Positive T cell co-stimulation by TLR7/8 ligands is dependent on the cellular environment

Toll-like receptors (TLRs) are mediators of innate immune responses detecting conserved pathogen-associated molecules. Whereas most TLRs are expressed on the cell surface, TLR3, 7, 8 and 9 are predominantly localized in endosomal compartments. Recent studies reported that TLRs are also expressed by T lymphocytes, resulting in direct co-stimulation of isolated CD4(+) T cells for example by Pam3CSK4 (TLR2 ligand) or flagellin (TLR5 ligand). We here describe enhanced IFN-γ production and T cell proliferation by anti-CD3 T cell receptor (TCR) or antigenic stimulation of purified human CD4(+) T cells upon co-culture with TLR7/8 specific single-stranded oligoribonucleotides or small molecule ligands. Surprisingly, TLR7/8 stimulation of CD4(+) T cells within a whole peripheral mononuclear cell (PBMC) environment did not result in enhanced T cell proliferation, but in a lack of proliferation that was cell-cell contact dependent. Immune cell depletion assays pointed towards a monocyte-mediated effect. Different TLR ligands influenced T cell proliferation differently. The effect of inhibition of T cell proliferation was most prominently seen for TLR7 ligands whereas the effects were minimal for TLR8 and TLR9 ligands indicating that the suppressive phenotype is unique only for certain TLRs. Our results strongly suggest that co-stimulation of T cell proliferation by TLR7/8 agonists is dependent on the specific cellular context.