Tuberculosis (TB) is an infectious disease caused by bacteria belonging to M.tuberculosis complex.

Extensive pneumonia with necrotic areas and granulocyticinfiltrates characterizes some of the aggressive forms of TB that affects not onlychildren and immunocompromised individuals, but also individuals infected with highvirulencestrains. Although granulocytes contribute to the early immune responseagainst TB, their role in progression of severe disease is still controversial. In addition,an important mechanism involved in regulating the granulocyte recruitment in TBoccurs through Irg1 expression and itaconate production. Therefore, understandingthe molecular mechanisms that induce Irg1 expression is an important step tounderstand the etiology of severe forms of TB. Considering these aspects, this studywas subdivided in two main branches: a) to assess the role of myeloid-derivedsuppressor cells (MDSCs) in the progression of severe TB caused by hypervirulentMP287/03 (Mycobacterium bovis) in C57BL/6 mice; and b) to investigate the molecularpathways by which Mycobacterium tuberculosis (Mtb) triggers Irg1 gene expression inmurine macrophages. Regarding TB caused by MP287/03 mycobacteria, a decreasein body weight, as well as a worsening of the pulmonary pathology, was observed fromday 21 to 28 post-infection (p.i.). During this period, CD11b&#43GR1int cells progressivelyaccumulated in the bone marrow, blood and lungs, which suggests that they originatein the bone marrow and migrate into the lungs through blood stream. While lungCD11b+GR1high and CD11b&#43 GR1int cells in controls were respectively the neutrophils(Ly6G&#43Ly6Clow) and monocytes (Ly6G-Ly6Chigh), the CD11b&#43GR1int population inMP287/03-infected mice showed intermediate Ly6G and Ly6C expression andgranularity similar to neutrophils. These cells also expressed the immature myeloid cellmarkers CD117 (c-kit), CD124 (IL-4R) and CD135 (Flt-3). Moreover, bone marrowCD11b&#43GR1&#43 cells from hypervirulent strain infected mice suppressed CD4&#43 and CD8&#43 T cell proliferation and IFN-&#947 production. T cell suppression in vivo was evidenced bydecreased IFN-&#947 and IL-17 production in the lungs at day 28 p.i. compared to day 21p.i., while IL-10 production was increased. The high levels of PD-L1 expression onMDSCs surface seem to be one of the mechanisms involved in theimmunosuppression. To eliminate MDSCs, MP287/03-infected mice already showingweight loss were treated with anti-GR1 monoclonal antibodies. A reduction in thepulmonary bacterial load and inflammation, as well as increased IFN-&#947 production andprolonged mouse survival, was observed in anti-GR1 treated mice. These results showthat MDSCs induce immunosuppression and consequently aggravate the diseasecaused by hypervirulent mycobacteria. Regarding the mechanism involved in theinduction of Irg1 gene expression by M. tuberculosis, we observed a partial role for theTLR2-MyD88-NFkB signaling pathway. In addition, assays using inhibitory drugsrevealed a major requirement for phagocytosis and endosomal acidification in theresponse to mycobacteria, but not to LPS or PAM3CSK4. Importantly, we found thatthe Irg1 response induced by mycobacteria is also highly dependent on bacterialESAT-6 and the macrophage signaling pathways mediated by STING and Type 1interferon (IFN-I). Based on these findings, we hypothesize that mycobacteria induceIrg1 expression in macrophages via two interacting triggers: 1) TLR2-MyD88dependent NFkB induction presumably at the host cell plasma membrane and 2) acritical amplifying signal stimulated by phagocytosis and dependent of ESAT-6-mediated release into the cytosol of bacterial products with consequent STINGactivation and IFN-I production. Together, the data obtained in this study contribute toa better understanding of the immunopathology of TB and opens perspectives for thedevelopment of new therapeutic approaches that aim to mitigate the severity of thedisease. (AU)