The question

Tuberculosis (TB) is primarily a lung infection caused by the intracellular pathogen Mycobacterium tuberculosis (Mtb). TB exists as a spectrum of disease, from latent TB infection and subclinical disease to active TB, with variable clinical outcomes potentially linked with the lung immune landscape1 and granulomas, a hallmark of TB. A granuloma is a collection of immune cells (including B cells and T cells) recruited to the site of infection to contain Mtb proliferation; an organized granuloma-associated lymphoid tissue (GrALT) comprises well-defined lymphoid follicles localized near or within TB granulomas, orchestrates optimal interactions between lymphocytes and is associated with Mtb control2,3 and latent TB infection4. However, the molecular interactions that generate efficient pulmonary immunity during TB are unknown. We designed this study to delineate the specific T cell and B cell protective functions within the GrALT that mediate Mtb control.

The discovery

IRF4, which encodes the crucial transcription factor interferon regulatory factor 4 that is required for B cell and T cell differentiation, is downregulated during progression to active TB in humans and animal models of TB1. We used mouse models with conditional deletion of Irf4 in T cells and B cells (Cd4creIrf4fl/fl mice that lacked T helper 1 (TH1), TH17 and TFH-like cells, and Cd19creIrf4fl/fl mice that lacked germinal center B cells, respectively), and we show that Irf4 expression in T cells is essential to support GrALT formation and Mtb control (measured as lung bacterial load). IRF4+ T cells co-express B cell lymphoma 6 protein (BCL6) during Mtb infection. Bcl6 deficiency in T cells (Cd4creBcl6fl/fl mice lacking TFH-like cells) but not in B cells (Cd19creBcl6fl/fl mice lacking germinal center B cells) prevents GrALT formation and Mtb control. In addition, Mtb-specific B cells are needed for Mtb control and GrALT formation in mouse and macaque models. In mice, we show that important B cell effector mechanisms, such as antibody production (as shown in Cd19creBlimp1fl/fl mice, which lack the BLIMP1 transcription factor required for plasma cell differentiation), antigen presentation (as observed in Cd19creiABfl/fl mice, which lack the beta component of the major histocompatibility complex class 2), or germinal center B cells (in Cd19creBcl6fl/fl mice) are not required for either control of Mtb or GrALT formation. Upon Mtb infection, levels of programmed cell death 1 ligand 1 (PD-L1) were increased in B cells, and the interaction of PD-L1 with its receptor programmed cell death protein 1 (PD-1) expressed on lung TFH-like cells was required to enhance the differentiation from pre-TFH-like cells to mature TFH-like cells and localize TFH cells within GrALT to mediate Mtb control (Fig. 1). Mice that lack Mtb-specific B cells (IghelMD4 mice) failed to do so. Thus, our results reveal a crucial protective function for TFH-like cells localized within GrALT, and support the contribution of B cells in the strategic localization of cytokine-producing TFH-like cells within the GrALT for Mtb control.

Fig. 1: TFH-like cells localize within GrALT for Mtb control.
figure 1

Mice were infected with Mtb, and their lungs were collected at 50 or 128 days post infection (dpi). Lung sections were immune-labelled and analyzed for the percentage of PD-1+CD3+ TFH-like cells within B cell areas. Data are mean ± s.d. B6, wild-type mice strain. *P ≤ 0.05; **P ≤ 0.005; ***P ≤ 0.0005. © 2023, Swanson, R. V. et al.

The implications

We show a crucial contribution of the transcription factors IRF4 and BCL6 in CD4+ T cells in the generation of cytokine-producing TFH-like cells, which localize within GrALT to mediate Mtb control. Our results suggest that Mtb-specific B cells are needed to orchestrate TFH-like cell differentiation and accumulation in the GrALT, induce cytokine production via PD-L1–PD-1 engagement, and influence GrALT organization. Altogether, these results answer long-standing questions about the contribution of TFH-like cells and B cells in the generation of a protective GrALT involved in the generation of local immunity to control Mtb infection.

Although our study does not show a direct protective role for antibody-producing or antigen-presenting B cells in lung Mtb control, it is possible that these B cell effector mechanisms function to control TB dissemination. We hope that this research will provide targets and immune pathways to improve the design of TB vaccines.

Shabaana A. Khader and Ananya Gupta

University of Chicago, Chicago, IL, USA.

Expert Opinion

“This is an exciting, well-developed, and important study for the field of TB immunology. Understanding the intricate relationships between protective and detrimental immune responses in the lung is of critical importance for understanding Mycobacterium tuberculosis pathogenesis and TB vaccine development. More broadly, the mechanistic relationships between B cells, TFH-like cells and GrALT formation described in this study could have implications for other diseases.” Andreas Kupz, James Cook University, Cairns, Australia.

Behind the paper

For the past decade, we have been interested in understanding the protective function of T cells and B cells within GrALT. To mechanistically address this question, we generated the various mouse models described in this paper and systematically addressed the specific role of TFH-like cells and B cell function in mediating Mtb control. An important but surprising turning point for the paper came from the experiments that showed that it was not the size of GrALT, but instead a certain threshold abundance of TFH-like cells localized within GrALT that correlated with protection against Mtb. This study took about five years from conception to completion, and one additional year of work will hopefully enable us to refine the targets of the T cell–B cell axis for TB vaccine design. S.A.K.

From the editor

“Virulent Mycobacterium tuberculosis infection induces the formation of follicular-like lymphoid structures within lung granulomas. Here, the authors find that B cells in these structures are needed to recruit TFH cells, but the other functional activities of B cells (including antigen presentation) are dispensable. Instead, it is the TFH cells’ ‘help’ that is crucial for the formation of the granuloma structure and control of the Mtb bacilli.” Laurie Dempsey, Senior Editor, Nature Immunology.