Peptidoglycan (PG), a heteropolysaccharide component of the mycobacterial cell wall can be shed during tuberculosis infection with immunomodulatory consequences. As such, changes in PG structure are expected to have important implications on disease progression and host responses during infection with Mycobacterium tuberculosis. Mycobacterial amidases have important roles in remodeling of PG during cell division and are implicated in susceptibility to antibiotics. However, their role in modulating host immunity remains unknown. We assessed the bacterial burden and host immune responses to M. tuberculosis mutants defective for either one of two PG N-acetylmuramyl-L-alanine amidases, Ami1 and Ami4, in bone marrow-derived macrophages (BMDM) and C57BL/6 mice. In infected BMDM, the single deletion of both genes resulted in increased proinflammatory cytokine responses. In mice, infection with the Δami1 mutant led to differential induction of pro-inflammatory cytokines and chemokines, decreased cellular recruitment and reduced lung pathology during the acute phase of the infection. While increased proinflammatory cytokines production was observed in BMDM infected with the Δami4 mutant, these effects did not prevail in mice. Infection using the Δami1 and Δami4 Mtb mutants showed that these genes are dispensable for intracellular mycobacterial growth in macrophages and mycobacterial burden in mice. These findings suggest that both Ami1 and Ami4 in M. tuberculosis are not essential for mycobacterial growth within the host. In summary, we show that amidases are important for modulating host immunity during Mtb infection in murine macrophages and mice.

Background Interleukin-4-induced gene 1 (IL-4i1) encodes L-phenylalanine oxidase that catabolizes phenylalanine into phenylpyruvate. IL-4i1 is mainly expressed by antigen-presenting cells, inhibits T-cell proliferation, regulates B-cell activation, drives macrophage polarization and modulates Th1 inflammatory immune responses, but its role in bacterial infections is understudied. Methods Herein, we evaluated IL-4i1 deletion in macrophages and mice upon infection with virulent H37Rv and W-Beijing lineage hypervirulent HN878 Mycobacterium tuberculosis (Mtb) strains. The bacterial growth and pro-inflammatory responses were measured in vitro and in vivo. Histopathological analysis, lung immune cell recruitment and macrophage activation were assessed at the early and chronic stages of Mtb infection. Results IL-4i1 -/- mice displayed increased protection against acute H37Rv and HN878 and chronic HN878 Mtb infections; with reduced lung bacterial burdens and altered antigen-presenting cell (APC) responses when compared to wild-type mice. Moreover, “M1-like” interstitial macrophage numbers, nitrite and interferon production were significantly increased in IL-4i1 -/- mice when compared to wild-type mice during acute Mtb HN878 infection. Conclusions Together, these data suggest that IL-4i1 regulates APC-mediated inflammatory responses during acute and chronic Mtb infection. Hence IL-4i1 targeting has the potential as an immunomodulatory target for host-directed therapy.

Lymphoblastic leukemia 1 (Lyl1) is a well-studied transcription factor known to exhibit oncogenic potential in various forms of leukemia with pivotal roles in hematopoietic stem cell biology. While its role in early hematopoiesis is well established, its function in mature innate cells is less explored. Here, we identified Lyl1 as a drastically perturbed gene in the Mycobacterium tuberculosis (Mtb) infected mouse macrophage transcriptome. We report that Lyl1 downregulation upon immune stimulation is a host-driven process regulated by NFκB and MAP kinase pathways. Interestingly, Lyl1-deficient macrophages have decreased bacterial killing potential with reduced nitric oxide (NO) levels while expressing increased levels of pro-inflammatory interleukin-1 and CXCL1. Lyl1-deficient mice show reduced survival to Mtb HN878 infection with increased bacterial burden and exacerbated inflammatory responses in chronic stages. We observed that increased susceptibility to infection was accompanied by increased neutrophil recruitment and IL-1, CXCL1, and CXCL5 levels in the lung homogenates. Collectively, these results suggest that Lyl1 controls Mtb growth, reduces neutrophilic inflammation and reveals an underappreciated role for Lyl1 in innate immune responses.