Host Defense and Recruitment of Foxp3 ⁺ T Regulatory Cells to the Lungs in Chronic Mycobacterium tuberculosis Infection Requires Toll-like Receptor 2

Abstract

Acute resistance to low dose M. tuberculosis (Mtb) infection is not dependent on Toll-like receptor (TLR) 2. However, whether TLR2 contributes to resistance in chronic Mtb infection has remained uncertain. Here we report that, following low dose aerosol infection with Mtb, mice lacking TLR2 (TLR2KO), in comparison with wild type (WT) mice, exhibit enhanced cellular infiltration and inflammation in the lungs, and fail to stably control bacterial burden during chronic infection. IFNγ and IL-17 was expressed at equivalent levels in the two groups; however, the characteristic accumulation of Foxp3 ⁺ T regulatory cells (Tregs) in pulmonary granulomas was significantly reduced in TLR2KO mice. Nonetheless, this reduction in Tregs was independent of whether Tregs expressed TLR2 or not. To directly link the reduced number of Tregs to the increased inflammation present in the TLR2KO mice, we used a macrophage adoptive transfer model. At seven weeks post-Mtb infection, TLR2KO mice, which were adoptively transferred with WT macrophages, displayed enhanced accumulation of Tregs in the lungs and a concomitant reduction in inflammation in contrast with control mice that received TLR2KO macrophages. However, the pulmonary bacterial burden between the two groups remained similar indicating that TLR2's role in modulating immunopathology is functionally distinct from its role in restricting Mtb growth in chronic infection. Together, these findings unequivocally demonstrate that TLR2 contributes to host resistance against chronic Mtb infection and reveal a novel role for TLR2 in mediating the recruitment of Foxp3 ⁺ Tregs to the lungs to control inflammation.

Author Summary

Tuberculosis (TB) is an important cause of mortality in many parts of the world. Infection with Mycobacterium tuberculosis (Mtb), the causative agent of TB, is usually acquired via inhalation of airborne droplets containing the bacteria. Following inhalation, Mtb interacts with specialized receptors, called Toll-like receptors (TLRs), on phagocytic cells present in the lung. In this study, we examine the contribution of TLR2 in activating the body's natural defenses against Mtb. Wild type mice infected with Mtb by the aerosol route are able to control bacterial replication in the lung and maintain it at a steady level during chronic infection. However, in genetically modified mice that do not express TLR2 (TLR2KO), Mtb infection leads to increased inflammation in the lung and inability to control Mtb growth. Here, we identify that the increased inflammation present in the lungs of Mtb-infected TLR2KO mice is due to the diminished ability of a type of regulatory cell (Foxp3 ⁺ Tregs) to accumulate in the lungs. The ability to recruit Tregs to the lungs is restored in TLR2KO mice if they are adoptively transferred with macrophages from wild type mice. In summary, we demonstrate that TLR2 functions in protection against chronic Mtb infection by controlling Treg accumulation in the lung to limit inflammation and tissue damage.