Direct stimulation of human T cells via TLR5 and TLR7/8:: Flagellin and R-848 up-regulate proliferation and IFN-γ production by memory CD4+ T cells

TLRs are involved in innate cell activation by conserved structures expressed by microorganisms. Human T cells express the mRNA encoding most of TLRs. Therefore, we tested whether some TLR ligands may modulate the function of highly purified human CD4(+) T lymphocytes. We report that, in the absence of APCs, fiagellin (a TLR5 ligand) and R-848 (a TLR7/8 ligand) synergized with suboptimal concentrations of TCR-dependent (anti-CD3 mAb) or -independent stimuli (anti-CD2 mAbs or IL-2) to up-regulate proliferation and IFN-gamma, IL-8, and IL-10 but not IL-4 production by human CD4(+) T cells. No effect of poly(I:C) and LPS, ligands for TLR3 and TLR4, respectively, was detected. We also observed that CD4(+)CD45RO(+) memory T cell responses to TLR ligands were more potent than those observed with CD4(+)CD45RA(+) naive T cells. Moreover, among the memory T cells, CCR7(-) effector cells were more sensitive to TLR ligands than CCR7(+) central memory cells. These data demonstrate for the first time a direct effect of TLR5 and TLR7/8 ligands on human T cells, and highlight an innate arm in T cell functions. They also suggest that some components from invading microorganisms may directly stimulate effector memory T cells located in tissues by up-regulating cytokine and chernokine production.