The tuberculosis (TB) vaccine bacille Calmette-Guérin (BCG) prevents disseminated childhood TB; however, it fails to protect against the more prevalent pulmonary TB. Limited understanding of the immune response to Mycobacterium tuberculosis, the causative agent of TB, has hindered development of improved vaccines. Although memory CD4 T cells are considered the main mediators of protection against TB, recent studies suggest there are other key subsets that contribute to antimycobacterial immunity. To that end, innate cells may be involved in the protective response. In this study, we investigated the primary response of innate lymphoid cells (ILCs) to BCG exposure. Using a murine model, we showed that ILCs increased in number in the lungs and lymph nodes in response to BCG vaccination. Additionally, there was significant production of the antimycobacterial cytokine IFN-γ by ILCs. As ILCs are located at mucosal sites, it was investigated whether mucosal vaccination (intranasal) stimulated an enhanced response compared to the traditional vaccination approach (intradermal or subcutaneous). Indeed, in response to intranasal vaccination, the number of ILCs, and IFN-γ production in NK cells and ILC1s in the lungs and lymph nodes, were higher than that provoked through intradermal or subcutaneous vaccination. This work provides the first evidence that BCG vaccination activates ILCs, paving the way for future research to elucidate the protective potential of ILCs against mycobacterial infection. Additionally, the finding that lung ILCs respond rigorously to mucosal vaccination may have implications for the delivery of novel TB vaccines.

Tuberculosis (TB) is a serious infectious disease caused by infection with Mycobacterium tuberculosis, and kills more people annually than any other single infectious agent. Although a vaccine is available, it is only moderately effective and an improved vaccine is urgently needed. The ability to develop a more effective vaccine has been thwarted by a lack of understanding of the mechanism of vaccine‐induced immune protection. Over recent decades, many novel TB vaccines have been developed and almost all have aimed to generate memory CD4 T cells. In this review, we critically evaluate evidence in the literature that supports the contention that memory CD4 T cells are the prime mediators of vaccine‐induced protection against TB. Because of the lack of robust evidence supporting memory CD4 T cells in this role, the potential for B‐cell antibody and “trained” innate cells as alternative mediators of protective immunity is explored.

Reversion of immune sensitization tests for Mycobacterium tuberculosis (M.tb) infection, such as interferon-gamma release assays or tuberculin skin test, has been reported in multiple studies. We hypothesized that QuantiFERON-TB Gold (QFT) reversion is associated with a decline of M.tb-specific functional T cell responses, and a distinct pattern of T cell and innate responses compared to persistent QFT+ and QFT- individuals. We compared groups of healthy adolescents (n=~30 each), defined by four, 6-monthly QFT tests: reverters (QFT+/+/-/-), non-converters (QFT-/-/-/-) and persistent positives (QFT+/+/+/+). We stimulated peripheral blood mononuclear cells with M.tb antigens (M.tb lysate; CFP-10/ESAT-6 and EspC/EspF/Rv2348 peptide pools) and measured M.tb-specific adaptive T cell memory, activation, and functional profiles; as well as functional innate (monocytes, natural killer cells), donor-unrestricted T cells (DURT: γδ T cells, mucosal-associated invariant T and natural killer T-like cells) and B cells by flow cytometry. Projection to latent space discriminant analysis was applied to determine features that best distinguished between QFT reverters, non-converters and persistent positives. No longitudinal changes in immune responses to M.tb were observed upon QFT reversion. M.tb-specific Th1 responses detected in reverters were of intermediate magnitude, higher than responses in QFT non-converters and lower than responses in persistent positives. About one third of reverters had a robust response to CFP-10/ESAT-6. Among those with measurable responses, lower proportions of TSCM (CD45RA+CCR7+CD27+) and early differentiated (CD45RA-) IFN-γ-TNF+IL-2- M.tb lysate-specific CD4+ cells were observed in reverters compared with non-converters. Conversely, higher proportions of early differentiated and lower proportions of effector (CD45RA-CCR7-) CFP10/ESAT6-specific Th1 cells were observed in reverters compared to persistent-positives. No differences in M.tb-specific innate, DURT or B cell functional responses were observed between the groups. Statistical modelling misclassified the majority of reverters as non-converters more frequently than they were correctly classified as reverters or misclassified as persistent positives. These findings suggest that QFT reversion occurs in a heterogeneous group of individuals with low M.tb-specific T cell responses. In some individuals QFT reversion may result from assay variability, while in others the magnitude and differentiation status of M.tb-specific Th1 cells are consistent with well-controlled M.tb infection.