Review 1: "CD4 T Cells and CD8α+ Lymphocytes are Necessary for Intravenous BCG-Induced Protection Against Tuberculosis in Macaques"

RR:C19 Evidence Scale rating by reviewer:

• Strong. The main study claims are very well-justified by the data and analytic methods used. There is little room for doubt that the study produced has very similar results and conclusions as compared with the hypothetical ideal study. The study’s main claims should be considered conclusive and actionable without reservation.

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Review: This study is a follow-up of previously published work showing that intravenous BCG vaccination provided significant protection against tuberculosis in nonhuman primates (NHP). Those studies indicated that antigen-specific CD4 T cells producing cytokines (IL-2, TNF, IFNg, IL-17) and natural killer (NK) cell numbers in the airways were likely the major immune cells that contributed to protective immunity. This study uses antibodies that selectively deplete either CD4 T cells, all CD8 T cells (innate and adaptive) or only classical MHCI-restricted CD8 T cells in vaccinated NHPs to investigate each immune cell subset’s contribution more directly. The experimental design is rigorous, statistically well powered and the data are robust, thorough and presented in a clear and straightforward manner. The main findings related to the importance of CD4 T cells are not surprising but are important, and the potential role of innate CD8 T cells is worth further study. The role of classical CD8 T cells in this model appears to be limited; while this is consistent with the relatively low CD8 response elicited by BCG, it remains likely that CD8 T cells do play an important role in conferring durable immunity. Longer term follow up of animals to assess bacterial burden, immune responses and survival would help to address these gaps. Inclusion of more detailed characterization of the phenotype and functional capacity of CD4 and CD8 T cell subsets, and NK cells in the different depletion groups using in vitro assays (eg proliferation, mycobacterial growth inhibition assays etc) would also provide functional context to their observations that would inform other vaccine candidates in addition to BCG.

The most important takeaway from this study is that increasing the numbers of vaccine-induced antigen-specific CD4 T cells—and potentially innate CD8 T cells within lung compartments is important for intravenous BCG vaccine-induced immunity against TB in NHP models. While the iv route of BCG is highly unlikely to be administered to newborns and children (who currently receive BCG intradermally), this study and their previously published work highlight the importance of generating and maintaining high levels of memory T cell pools in the lung where M. tuberculosis infection occurs and provides insights relevant to designing more efficacious vaccines for TB.