Th1 cells are critical for containment of Mycobacterium tuberculosis infection, but little else is known about the properties of protective CD4 T cell responses. In this study, we show that the pulmonary Th1 response against M. tuberculosis is composed of two populations that are either CXCR3hi and localize to lung parenchyma or are CX3CR1hiKLRG1hi and are retained within lung blood vasculature. M. tuberculosis–specific parenchymal CD4 T cells migrate rapidly back into the lung parenchyma upon adoptive transfer, whereas the intravascular effectors produce the highest levels of IFN-γ in vivo. Importantly, parenchymal T cells displayed greater control of infection compared with the intravascular counterparts upon transfer into susceptible T cell–deficient hosts. Thus, we identified a subset of naturally generated M. tuberculosis–specific CD4 T cells with enhanced protective capacity and showed that control of M. tuberculosis correlates with the ability of CD4 T cells to efficiently enter the lung parenchyma rather than produce high levels of IFN-γ.

IFN-γ–producing CD4 T cells are required for protection against Mycobacterium tuberculosis (Mtb) infection, but the extent to which IFN-γ contributes to overall CD4 T cell-mediated protection remains unclear. Furthermore, it is not known if increasing IFN-γ production by CD4 T cells is desirable in Mtb infection. Here we show that IFN-γ accounts for only ~30% of CD4 T cell-dependent cumulative bacterial control in the lungs over the first six weeks of infection, but >80% of control in the spleen. Moreover, increasing the IFN-γ–producing capacity of CD4 T cells by ~2 fold exacerbates lung infection and leads to the early death of the host, despite enhancing control in the spleen. In addition, we show that the inhibitory receptor PD-1 facilitates host resistance to Mtb by preventing the detrimental over-production of IFN-γ by CD4 T cells. Specifically, PD-1 suppressed the parenchymal accumulation of and pathogenic IFN-γ production by the CXCR3+KLRG1-CX3CR1- subset of lung-homing CD4 T cells that otherwise mediates control of Mtb infection. Therefore, the primary role for T cell-derived IFN-γ in Mtb infection is at extra-pulmonary sites, and the host-protective subset of CD4 T cells requires negative regulation of IFN-γ production by PD-1 to prevent lethal immune-mediated pathology.

Streptococcus pneumoniae is a Gram-positive, extracellular bacterium that is responsible for significant mortality and morbidity worldwide. Pneumolysin (PLY), a cytolysin produced by all clinical isolates of the pneumococcus, is one of the most important virulence factors of this pathogen. We have previously reported that PLY is an essential factor for activation of caspase-1 and consequent secretion of IL-1β and IL-18 in macrophages infected with S. pneumoniae. However, the host molecular factors involved in caspase-1 activation are still unclear. To further elucidate the mechanism of caspase-1 activation in macrophages infected with S. pneumoniae, we examined the involvement of inflammasomes in inducing this cellular response. Our study revealed that apoptosis-associated speck like protein containing a caspase recruitment domain (ASC), an adaptor protein for inflammasome receptors such as NLR family, pyrin domain containing 3 (NLRP3) and absent in melanoma 2 (AIM2), is essentially required for the induction of caspase-1 activation by S. pneumoniae. Caspase-1 activation was partially impaired in NLRP3−/− macrophages, while knockdown and knockout of AIM2 resulted in a clear decrease in caspase-1 activation in response to S. pneumoniae. These results suggest that ASC inflammasomes, including AIM2 and NLRP3, are critical for caspase-1 activation induced by S. pneumoniae. Furthermore, ASC−/− mice were more susceptible than wild-type mice to S. pneumoniae, with impaired secretion of IL-1β and IL-18 into the bronchoalveolar lavage after intranasal infection, suggesting that ASC inflammasomes contribute to the protection of host from infection with PLY-producing S. pneumoniae.

Although the NLRP3 inflammasome plays a pivotal role in host defense, its uncontrolled activation is associated with inflammatory disorders, suggesting that regulation of the inflammasome is important to prevent detrimental effects. Type I IFNs and long-term LPS stimulation were shown to negatively regulate NLRP3 activation. In this study, we found that endogenous NO is involved in the regulation of NLRP3 inflammasome activation by either IFN-β pretreatment or long-term LPS stimulation. Furthermore, S-nitroso-N-acetylpenicillamine (SNAP), an NO donor, markedly inhibited NLRP3 inflammasome activation, whereas the AIM2 and NLRC4 inflammasomes were only partially inhibited by SNAP. An increase in mitochondrial reactive oxygen species induced by ATP was only modestly affected by SNAP treatment. Interestingly, S-nitrosylation of NLRP3 was detected in macrophages treated with SNAP, and this modification may account for the NO-mediated mechanism controlling inflammasome activation. Taken together, these results revealed a novel role for NO in regulating the NLRP3 inflammasome.

SUMMARY Recent data indicate that the differentiation state of Th1 cells determines their protective capacity against tuberculosis. Therefore, we examined the role of Th1 polarizing factors in the generation of protective and non-protective subsets of Mtb-specific Th1 cells. We find IL-12/23p40 promotes Th1 cell expansion and maturation beyond the CD73+CXCR3+T-betdim stage, and T-bet prevents deviation of Th1 cells into Th17 cells. Nevertheless, IL-12/23p40 and T-bet are also essential for the production of a prominent subset of intravascular CX3CR1+KLRG1+ Th1 cells that persists poorly and can neither migrate into the lung parenchyma nor control Mtb growth. Furthermore, T-bet suppresses development of CD69+CD103+ tissue resident phenotype effectors in lung. In contrast, Th1 cell-derived IFNγ inhibits the accumulation of intravascular CX3CR1+KLRG1+ Th1 cells. Thus, although IL-12 and T-bet are essential host survival factors, they simultaneously oppose lung CD4 T cell responses at several levels, demonstrating the dual nature of Th1 polarization in tuberculosis.

Protection against Mycobacterium tuberculosis (Mtb) infection requires CD4 T cells to migrate into the lung and interact with infected macrophages. In mice, less-differentiated CXCR3+ CD4 T cells migrate into the lung and suppress growth of Mtb, while CX3CR1+ terminally-differentiated Th1 cells accumulate in the blood vasculature and do not control pulmonary infection. Here we examine CD4 T cell differentiation and lung homing during primary Mtb infection of rhesus macaques. Mtb-specific CD4 T cells simultaneously appeared in the airways and blood ~21–28 days post-exposure, indicating that recently primed effectors are quickly recruited into the lungs after entering circulation. Mtb-specific CD4 T cells in granulomas display a tissue-parenchymal CXCR3+CX3CR1−PD-1hiCTLA-4+ phenotype. However, most granuloma CD4 T cells are found within the outer lymphocyte cuff, and few localize to the myeloid cell core containing the bacilli. Using the intravascular stain approach, we find essentially all Mtb-specific CD4 T cells in granulomas have extravasated across the vascular endothelium into the parenchyma. Therefore, it is unlikely that lung-homing defects introduced by terminal differentiation limit the migration of CD4 T cells into granulomas following primary Mtb infection of macaques. However, intralesional positioning defects within the granuloma may pose a major barrier to T cell-mediated immunity during tuberculosis.

Mucosal-associated invariant T (MAIT) cells are potential targets of vaccination and host-directed therapeutics for tuberculosis, but the role of MAIT cells during Mycobacterium tuberculosis (Mtb) infection in vivo is not well understood. Here we find that following Mtb infection MAIT cells mount minimal responses, and MAIT cell-deficient MR1 −/− mice display normal survival. Preinfection expansion of MAIT cells through 5-OP-RU vaccination fails to protect against subsequent Mtb challenge. In fact, 5-OP-RU vaccination delays Mtb-specific CD4 T cell priming in lung-draining lymph nodes, and conversely MR1 deficiency or blockade accelerates T cell priming. The MAIT cell-mediated delay in T cell priming is partly dependent on TGF-β. Surprisingly, 5-OP-RU treatment during chronic infection drives MAIT cell expansion and an IL-17A-dependent reduction in bacterial loads. Thus, during early infection MAIT cells directly contribute to the notoriously slow priming of CD4 T cells, but later during infection MAIT cell stimulation may be an effective host-directed therapy for tuberculosis.

CD4 T cells are critical for control of Mycobacterium tuberculosis (Mtb) infection and represent the best hope for vaccine-elicited protection. However, little is understood about the properties of Mtb-specific CD4 T cells that mediate control, and the lack of correlates of protection present a significant barrier to the rational development of new vaccination and therapeutic strategies which are sorely needed. Here we discuss the features of protective CD4 T cells including recent evidence for IFN-γ dependent and independent mechanisms of protection, poor protection by terminally differentiated cells and the importance of T cell migratory capacity for the control of Mtb infection.