Toll-like receptor 2 controls expansion and function of regulatory T cells

TLDR

Tregs suppress immune reactions and prevent autoimmunity, yet during acute infection they can impede effector T cell activity, and pathogen‑recognition TLRs on innate cells are essential for mounting effective immunity. In TLR2‑deficient mice, CD4⁺CD25⁺ Tregs are markedly reduced, and stimulation of TLR2 with Pam3Cys directly activates Tregs via MyD88, enhancing their proliferation and transiently abrogating their suppressive phenotype; moreover, systemic TLR2 ligand administration during acute Candida infection neutralizes adoptively transferred WT Tregs, reducing fungal outgrowth by 100‑fold, thereby demonstrating that TLR2 directly regulates Treg function and links TLR signaling to immune response control.

Abstract

Tregs play a central role in the suppression of immune reactions and prevention of autoimmune responses harmful to the host. During acute infection, however, Tregs might hinder effector T cell activity directed toward the elimination of the pathogenic challenge. Pathogen recognition receptors from the TLR family expressed by innate immune cells are crucial for the generation of effective immunity. We have recently shown the CD4CD25 Treg subset in TLR2 mice to be significantly reduced in number compared with WT littermate control mice, indicating a link between Tregs and TLR2. Here, we report that the TLR2 ligand Pam3Cys, but not LPS (TLR4) or CpG (TLR9), directly acts on purified Tregs in a MyD88-dependent fashion. Moreover, when combined with TCR stimulation, TLR2 triggering augmented Treg proliferation in vitro and in vivo and resulted in a temporal loss of the suppressive Treg phenotype in vitro by directly affecting Tregs. Importantly, WT Tregs adoptively transferred into TLR2 mice were neutralized by systemic administration of TLR2 ligand during the acute phase of a Candida albicans infection, resulting in a 100-fold reduced C. albicans outgrowth. This demonstrates that in vivo TLR2 also controls the function of Tregs and establishes a direct link between TLRs and the control of immune responses through Tregs.