Several subsets of Foxp3+ regulatory T (Treg) cells work in concert to maintain immune homeostasis. However, the molecular bases underlying the phenotypic and functional diversity of Treg cells remain obscure. We show that in response to interferon-γ, Foxp3+ Treg cells upregulated the T helper 1 (TH1)-specifying transcription factor T-bet. T-bet promoted expression of the chemokine receptor CXCR3 on Treg cells, and T-bet+ Treg cells accumulated at sites of TH1-mediated inflammation. Furthermore, T-bet expression was required for the homeostasis and function of Treg cells during type-1 inflammation. Thus, within a subset of CD4+ T cells, the activities of Foxp3 and T-bet are overlaid, resulting in Treg cells with unique homeostatic and migratory properties optimized for suppression of TH1 responses in vivo.

The evolutionary survival of Mycobacterium tuberculosis, the cause of human tuberculosis (TB), depends on its ability to invade the host, replicate, and transmit infection. At its initial peripheral infection site in the distal lung airways, M. tuberculosis infects macrophages which transport it to deeper tissues1. How mycobacteria survive in these broadly microbicidal cells is an important question. Here we show that M. tuberculosis, and its close pathogenic relative Mycobacterium marinum, preferentially recruit and infect permissive macrophages while evading microbicidal ones. This immune evasion is accomplished by using cell surface associated phthiocerol dimycoceroserate (PDIM) lipids2 to mask underlying pathogen-associated molecular patterns (PAMPs). In the absence of PDIM, these PAMPs signal a toll-like receptor (TLR)-dependent recruitment of macrophages that produce microbicidal reactive nitrogen species. Concordantly, the related phenolic glycolipids (PGL)2, promote recruitment of permissive macrophages via a host chemokine receptor 2 (CCR2)-mediated pathway. Thus, we have identified coordinated roles for PDIM, known to be essential for mycobacterial virulence3 and PGL, which (along with CCR2) is known to be associated with human TB4,5. Our findings also suggest an explanation for the longstanding observation that M. tuberculosis initiates infection in the relatively sterile environment of the lower respiratory tract, rather than in the upper respiratory tract, where resident microflora and inhaled environmental microbes may continually recruit microbicidal macrophages through TLR-dependent signaling.

Mycobacterium tuberculosis (Mtb) infection is initiated in the distal airways, but the bacteria ultimately disseminate to the lung interstitium. Although various cell types, including alveolar macrophages (AM), neutrophils, and permissive monocytes, are known to be infected with Mtb, the initially infected cells as well as those that mediate dissemination from the alveoli to the lung interstitium are unknown. In this study, using a murine infection model, we reveal that early, productive Mtb infection occurs almost exclusively within airway-resident AM. Thereafter Mtb-infected, but not uninfected, AM localize to the lung interstitium through mechanisms requiring an intact Mtb ESX-1 secretion system. Relocalization of infected AM precedes Mtb uptake by recruited monocyte-derived macrophages and neutrophils. This dissemination process is driven by non-hematopoietic host MyD88/interleukin-1 receptor inflammasome signaling. Thus, interleukin-1-mediated crosstalk between Mtb-infected AM and non-hematopoietic cells promotes pulmonary Mtb infection by enabling infected cells to disseminate from the alveoli to the lung interstitium.