Engagement of TLR2 Does not Reverse the Suppressor Function of Mouse Regulatory T Cells, but Promotes Their Survival

Abstract

TLRs are a class of conserved pattern recognition receptors that are used by cells of the innate immune system. Recent studies have demonstrated the expression of TLRs on both human and mouse T cells raising the possibility that TLRs play a direct role in adaptive immunity. TLR2 is activated primarily by bacterial wall components including peptidoglycan and lipoproteins. Several studies have shown that mouse regulatory T (Treg) cells express TLR2 and claimed that engagement of TLR2 by synthetic ligands reversed their suppressive function. In contrary, enhancement of Treg function was observed following engagement of TLR2 on human Treg. We have reexamined the expression and function of TLR2 on mouse Treg purified from Foxp3-GFP knock-in mice. TLR2 ligation by TLR2 agonist, the synthetic bacterial lipoprotein Pam3CSK4, enhanced the proliferative responses of both conventional T cells and Treg in response to TLR stimulation in the absence of APC. Treatment of Foxp3+ Treg with Pam3CSK4 did not alter their suppressive function in vitro or in vivo and did not reduce their level of Foxp3 expression. An additional effect of TLR2 stimulation of Treg was induction of Bcl-xL resulting in enhanced survival in vitro. Treatment of mice with the TLR2 agonist enhanced the Ag-driven proliferation of Treg in vivo, but did not abolish their ability to suppress the development of experimental autoimmune encephalomyelitis. Development of methods to selectively stimulate TLR2 on Treg may lead to a novel approaches for the treatment of autoimmune diseases.

Differential but Direct Abolishment of Human Regulatory T Cell Suppressive Capacity by Various TLR2 Ligands

“…The inability of Tregs to produce IL-2, IFN-g, RANTES, and IP-10 is caused by the constitutive expression of FoxP3, which can bind to the forkhead-binding sites located within promoter regions of cytokine genes and thus suppress cytokine production and probably chemokine production (40). In accordance with data in the mouse model (30), we observed that the treatment of human Tregs with TLR2 ligands did not downregulate FoxP3 protein expression. In contrast to the murine counterpart (28-30), TLR2 treatment and TCR activation of human Tregs induced a further upregulation of intracellular FoxP3, explaining the sustained suppression of cytokine production in human Tregs after activation.…”

“…These data are in contrast to the murine counterpart in which TLR2 ligand treatment of Tregs is linked to a concomitant proliferative activity of these cells in the absence of IL-2 (28). Others, however, have reported that the addition of exogenous IL-2 is necessary for proliferation of murine Tregs upon TCR/TLR2 ligand stimulation in vitro (29,30). In contrast to the experiments with murine Tregs by Chen et al (30), we observed that pretreatment of human Tregs with TLR2 ligands resulted in an abrogation of their suppressive activity on cytokine production by responder T cells.…”

“…Others, however, have reported that the addition of exogenous IL-2 is necessary for proliferation of murine Tregs upon TCR/TLR2 ligand stimulation in vitro (29,30). In contrast to the experiments with murine Tregs by Chen et al (30), we observed that pretreatment of human Tregs with TLR2 ligands resulted in an abrogation of their suppressive activity on cytokine production by responder T cells. The reason for this discrepancy might be explained by the usage of different TLR2 ligands.…”

“…4A). There are controversial reports on the effects of TLR2 ligands on FoxP3 expression in murine Tregs (28,30). However, we observed that TLR2 ligand preincubation did not have any significant effect on intracellular FoxP3 expression in human Tregs as measured by flow cytometry.…”

“…It is possible that there are speciesspecific differences in the control of Treg activity by TLR ligands. (28)(29)(30)(31). In mice, triacylated lipopeptides, such as Pam 3 CSK4, are recognized by a TLR1/2 heterodimer (32) or independently of TLR1 (33).…”