Differential but direct abolishment of human regulatory T cell suppressive capacity by various TLR2 ligands

CD4(+)CD25(high) regulatory T cells (Tregs) control cellular immune responses and maintain peripheral tolerance. We investigated whether TLR2 ligands are able to abrogate Treg-induced suppression in humans based on different reports about effects of triacylated lipopeptide Pam(3)CSK4 in mice. Pretreatment of human Tregs with a mixture of TLR2 ligands Pam(2)CSK4, FSL-1, and Pam(3)CSK4 reduced the Treg-mediated suppression of CD4(+)CD25(-) responder T cells in the majority of the analyzed donors. Differential effects of individual TLR2 ligands are explained by usage of different TLR2 heterodimers in the recognition of Pam(2)CSK4, FSL-1, and Pam(3)CSK4. In contrast to the murine system, TLR2 ligand-mediated abrogation of human Treg function was not associated with a downregulation of FoxP3 transcription factor. Furthermore, our results excluded an effect of TLR2 ligands on granzyme A/B release by human Tregs as a potential mechanism to abolish Treg-mediated suppression. Our data suggest that a downregulation of p27(Kip1) and restoration of Akt phosphorylation in human Tregs pretreated with TLR2 ligands result in a reversal of suppression on responder T cells. Moreover, our data indicate that a mixture of TLR2 ligands can be used to modulate human Treg activity.