Differential and site specific impact of B cells in the protective immune response to Mycobacterium tuberculosis in the mouse.

Cell-mediated immune responses are known to be critical for control of mycobacterial infections whereas the role of B cells and humoral immunity is unclear. B cells can modulate immune responses by secretion of immunoglobulin, production of cytokines and antigen-presentation. To define the impact of B cells in the absence of secreted immunoglobulin, we analyzed the progression of Mycobacterium tuberculosis (Mtb) infection in mice that have B cells but which lack secretory immunoglobulin (AID(-/-)µS(-/-)mice). AID(-/-)µS(-/-) mice accumulated a population of activated B cells in the lungs when infected and were more susceptible to aerosol Mtb when compared to wild type (C57BL/6) mice or indeed mice that totally lack B cells. The enhanced susceptibility of AID(-/-)µS(-/-) mice was not associated with defective T cell activation or expression of a type 1 immune response. While delivery of normal serum to AID(-/-)µS(-/-) mice did not reverse susceptibility, susceptibility in the spleen was dependent upon the presence of B cells and susceptibility in the lungs of AID(-/-)µS(-/-)mice was associated with elevated expression of the cytokines IL-6, GM-CSF, IL-10 and molecules made by alternatively activated macrophages. Blocking of IL-10 signaling resulted in reversal of susceptibility in the spleens and lungs of AID(-/-)µS(-/-) mice. These data support the hypothesis that B cells can modulate immunity to Mtb in an organ specific manner via the modulation of cytokine production and macrophage activation.

Mycobacterium tuberculosis infection induces il12rb1 splicing to generate a novel IL-12Rbeta1 isoform that enhances DC migration.

RNA splicing is an increasingly recognized regulator of immunity. Here, we demonstrate that after Mycobacterium tuberculosis infection (mRNA) il12rb1 is spliced by dendritic cells (DCs) to form an alternative (mRNA) il12rb1Deltatm that encodes the protein IL-12Rbeta1DeltaTM. Compared with IL-12Rbeta1, IL-12Rbeta1DeltaTM contains an altered C-terminal sequence and lacks a transmembrane domain. Expression of IL-12Rbeta1DeltaTM occurs in CD11c(+) cells in the lungs during M. tuberculosis infection. Selective reconstitution of il12rb1(-/-) DCs with (mRNA) il12rb1 and/or (mRNA) il12rb1Deltatm demonstrates that IL-12Rbeta1DeltaTM augments IL-12Rbeta1-dependent DC migration and activation of M. tuberculosis-specific T cells. It cannot mediate these activities independently of IL12Rbeta1. We hypothesize that M. tuberculosis-exposed DCs express IL-12Rbeta1DeltaTM to enhance IL-12Rbeta1-dependent migration and promote M. tuberculosis-specific T cell activation. IL-12Rbeta1DeltaTM thus represents a novel positive-regulator of IL12Rbeta1-dependent DC function and of the immune response to M. tuberculosis.

In a murine tuberculosis model, the absence of homeostatic chemokines delays granuloma formation and protective immunity.

Mycobacterium tuberculosis infection (Mtb) results in the generation of protective cellular immunity and formation of granulomatous structures in the lung. CXCL13, CCL21, and CCL19 are constitutively expressed in the secondary lymphoid organs and play a dominant role in the homing of lymphocytes and dendritic cells. Although it is known that dendritic cell transport of Mtb from the lung to the draining lymph node is dependent on CCL19/CCL21, we show in this study that CCL19/CCL21 is also important for the accumulation of Ag-specific IFN-gamma-producing T cells in the lung, development of the granuloma, and control of mycobacteria. Importantly, we also show that CXCL13 is not required for generation of IFN-gamma responses, but is essential for the spatial arrangement of lymphocytes within granulomas, optimal activation of phagocytes, and subsequent control of mycobacterial growth. Furthermore, we show that these chemokines are also induced in the lung during the early immune responses following pulmonary Mtb infection. These results demonstrate that homeostatic chemokines perform distinct functions that cooperate to mediate effective expression of immunity against Mtb infection.

ESAT-6-specific CD4 T cell responses to aerosol Mycobacterium tuberculosis infection are initiated in the mediastinal lymph nodes.

CD4(+) T cell responses to aerosol Mycobacterium tuberculosis (Mtb) infection are characterized by the relatively delayed appearance of effector T cells in the lungs. This delay in the adaptive response is likely critical in allowing the bacteria to establish persistent infection. Because of limitations associated with the detection of low frequencies of naïve T cells, it had not been possible to precisely determine when and where naïve antigen-specific T cells are first activated. We have addressed this problem by using early secreted antigenic target 6 (ESAT-6)-specific transgenic CD4 T cells to monitor early T cell activation in vivo. By using an adoptive transfer approach, we directly show that T cell priming to ESAT-6 occurs only after 10 days of infection, is initially restricted to the mediastinal lymph nodes, and does not involve other lymph nodes or the lungs. Primed CD4 T cells rapidly differentiated into proliferating effector cells and ultimately acquired the ability to produce IFN-gamma and TNF-alpha ex vivo. Initiation of T cell priming was enhanced by two full days depending on the magnitude of the challenge inoculum, which suggests that antigen availability is a factor limiting the early CD4 T cell response. These data define a key period in the adaptive immune response to Mtb infection.