Immune Regulation in Mycobacterial and Fungal Infections

National Institute Of Allergy And Infectious Diseases

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Abstract

Mycobacterium tuberculosis (Mtb) is a leading cause of global mortality due to infectious disease. The current vaccine for Mtb infection, Bacillus Calmette-Gurin (BCG), is administered neonatally and provides protection against severe forms of tuberculosis (TB) in children, but it does not provide sufficient protection in adolescents and adults to drastically reduce the global burden TB. Over the past few years, clinical vaccine trials using BCG re-vaccination or an adjuvanted fusion protein have demonstrated 4050% protection, and several vaccine studies in (NHPs) have afforded extraordinary levels of protection, showing that a highly effective vaccination for TB is achievable. A better understanding of the mechanisms of host protection against Mtb infection would improve our ability to develop novel vaccination as well as host-directed therapeutic strategies for TB. Control of Mtb infection requires the generation of T cells that can traffic to pulmonary granulomas, but little is known about the basic biology of granuloma T cells (i.e., the mechanisms regulating the generation, differentiation, homing, and effector functions of protective granuloma T cells). CD4 T cell responses to mycobacterial infections in humans, NHPs, and mice are primarily comprised of IFN-producing effector cells. However, there are differences in the quality of CD4 T cell differentiation observed in response to mycobacterial infection in C57Bl/6 mice compared to macaques and humans. In mice, Mtb-specific CD4 T cells develop into highly polarized Th1 cells, with some cells even differentiating into terminally differentiated Th1 effector cells. In contrast, these terminally differentiated cells are not observed in NHPs or humans. Instead, in primate hosts, Mtb-specific T cells mostly differentiate into atypical IFN-producing cells referred to as Th1* cells that are characterized by a mixture of Th1 and Th17-like features. Another key difference between the most common mouse models and primates is in the pulmonary lesions that develop at sites of bacterial infection. Specifically, mice do not develop typical granulomatous lesions characterized by a well-organized macrophage core circumscribed by lymphocyte cuff. While efforts are underway to improve on the classic mouse model by changing the dose or strain of bacteria or strain of mouse, NHPs are currently the most powerful animal model for investigating granuloma T cell responses in a setting that closely mirrors that seen in humans. In this reporting period, we identified unique properties of Mtb granuloma T cells. We compare gene expression profiles of FACS-purified CD4 and CD8 T cells from the blood, bronchoalveolar lavage (BAL), and individual granulomas of Mtb-infected rhesus macaques and identify homing receptors, transcription factors, cytokines, and co-stimulatory molecules that distinguish granuloma T cells from those appearing in circulation and non-granulomatous tissue. To identify a core granuloma T cell signature, we focused on granuloma-enriched genes that were shared between CD4 and CD8 T cells and whose expression occurred independent of granuloma bacterial loads. This analysis identified the TNF superfamily CD30 as a key molecule of interest in granuloma T cells. Importantly, we show in mice that expression of CD30 on CD4 T cells drives their differentiation and expression of multiple effector molecules and is required for host survival of Mtb infection. Together with our previous findings demonstrating that CD30 ligand, CD153, is also required for CD4 T cellmediated control of Mtb infection, these data point to a central role for the CD30/CD153 co-stimulatory axis in CD4 T cellmediated control of Mtb infection. The only available vaccine for Mtb is Bacillus Calmette-Gurin (BCG), which is given as an intradermal injection within the first 6 months of age. While a single administration of ID BCG is protective in children, it does little to prevent TB in adolescents and adults, and novel highly effective vaccine strategies for TB are sorely needed. One approach to enhance vaccine-mediated protection against Mtb has been to develop alternate vaccine platforms, such as adjuvanted proteins, viral vectors, or other attenuated mycobacteria. Another has been to improve the use of BCG itself. It was recently shown that homologous boosting in adolescents, referred to as BCG re-vaccination, displayed protection based on the novel metric of sustained quantiferon conversion. There has also been interest in the ability of other routes of BCG administration to enhance protection against Mtb. This is perhaps best illustrated by recent reports that intravenous BCG vaccination induces extraordinarily high levels of protection against Mtb challenge in nonhuman primates. Mucosal vaccination routes also display great promise. Administration of BCG into the lungs induces high levels of protection against subsequent challenge in nonhuman primates, and is being explored in humans. Oral vaccination is another potentially important route of BCG vaccination. In fact, BCG was first deployed a century ago as an oral vaccine. Several studies in mice and guinea pigs have found that oral BCG vaccination induces immune responses similar to ID BCG vaccination, and is at least as protective against Mtb infection. One study even showed that rectal BCG vaccination of macaques induced IFN-spot forming cell responses in the spleen of similar magnitude as ID vaccination. A recent clinical trial testing several different combinations of oral and ID BCG vaccination showed that oral BCG vaccination not only generates Ag-specific T cells in the blood, but also induces higher responses in the airways compared to ID BCG. To better understand the generation of Mtb-specific T cell responses in the airways after oral vaccination, here we compare the magnitude, function, and phenotype of mycobacteria-specific CD4 T cells generated after either intradermal or gavage BCG vaccination in rhesus macaques.