Toll like Receptor 2 engagement on CD4⁺ T cells promotes TH9 differentiation and function

We have recently demonstrated that mycobacterial ligands engage Toll like receptor 2 (TLR2) on CD4⁺ T cells and up-regulate T-cell receptor (TCR) triggered Th1 responses in vitro and in vivo. To better understand the role of T-cell expressed TLR2 on CD4⁺ T-cell differentiation and function, we conducted a gene expression analysis of murine naïve CD4⁺ T-cells stimulated in the presence or absence of TLR2 co-stimulation. Unexpectedly, naïve CD4⁺ T-cells co-stimulated via TLR2 showed a significant up-regulation of Il9 mRNA compared to cells co-stimulated via CD28. Under TH9 differentiation, we observed up-regulation of TH9 differentiation, evidenced by increases in both percent of IL-9 secreting cells and IL-9 in culture supernatants in the presence of TLR2 agonist both in polyclonal and Ag85B cognate peptide specific stimulations. Under non-polarizing conditions, TLR2 engagement on CD4⁺ T-cells had minimal effect on IL-9 secretion and TH9 differentiation, likely due to a prominent effect of TLR2 signaling on IFN-γ secretion and TH1 differentiation. We also report that, TLR2 signaling in CD4⁺ T cells increased expression of transcription factors BATF and PU.1, known to positively regulate TH9 differentiation. These results reveal a novel role of T-cell expressed TLR2 in enhancing the differentiation and function of TH9 T cells.