Babirye, J. P. | Search bioRxiv

bioRxiv2022

A hierarchy of cell death pathways confers layered resistance to shigellosis in mice

Bacteria of the genus Shigella cause shigellosis, a severe gastrointestinal disease driven by bacterial colonization of colonic intestinal epithelial cells. Vertebrates have evolved programmed cell death pathways that sense invasive enteric pathogens and eliminate their intracellular niche. Previously we reported that genetic removal of one such pathway, the NAIP-NLRC4 inflammasome, is sufficient to convert mice from resistant to susceptible to oral Shigella flexneri challenge (Mitchell, Roncaioli et al., 2020). Here, we investigate the protective role of additional cell death pathways during oral mouse Shigella infection. We find that the Caspase-11 inflammasome, which senses Shigella LPS, restricts Shigella colonization of the intestinal epithelium in the absence of NAIP-NLRC4. However, this protection is limited when Shigella expresses OspC3, an effector that antagonizes Caspase-11 activity. TNF, a cytokine that activates Caspase-8-dependent apoptosis, also provides protection from Shigella colonization of the intestinal epithelium, but only in the absence of both NAIP- NLRC4 and Caspase-11. The combined genetic removal of Caspases-1,-11, and -8 renders mice hyper-susceptible to oral Shigella infection. Our findings uncover a layered hierarchy of cell death pathways that limit the ability of an invasive gastrointestinal pathogen to cause disease.

immunology↗

bioRxiv2021

IFN-γ-independent control of M. tuberculosis requires CD4 T cell-derived GM-CSF and activation of HIF-1α

The prevailing model of protective immunity to tuberculosis is that CD4 T cells produce the cytokine IFN-{gamma} to activate bactericidal mechanisms in infected macrophages. Recent evidence has expanded this model, and it is now clear that CD4 T cells can control M. tuberculosis infection in the absence of IFN-{gamma} production. To identify factors and pathways involved in IFN-{gamma}-independent control, we developed a co-culture model using CD4 T cells isolated from the lungs of infected mice and M. tuberculosis-infected murine bone marrow-derived macrophages (BMDMs). We show that IFN-{gamma}-independent control is primarily mediated by CD4 T cell production of the cytokine GM-CSF and requires activation of the macrophage transcription factor HIF-1. HIF-1 activation drives a metabolic shift toward aerobic glycolysis and leads to the production of lipid droplets, both of which support host defense against infection. Surprisingly, recombinant GM-CSF is insufficient to rescue the absence of control by GM-CSF-deficient CD4 T cells during co-culture with BMDMs. In peritoneal macrophages, GM-CSF is sufficient to control growth, induces lipid droplet biogenesis, and requires HIF-1 expression for control. While HIF-1-mediated control following IFN-{gamma} stimulation requires nitric oxide, we find that HIF-1 activation by CD4 T cells and recombinant GM-CSF is nitric oxide-independent, implying a distinct pathway of activation. In addition to GM-CSF, CD4 T cells produce a factor that helps maintain phagosome membrane integrity during infection and blocks bacterial access to host lipids, a primary nutrient source. These results advance our understanding of CD4 T cell-mediated immunity to M. tuberculosis, clarify the role of nitric oxide as primarily immunomodulatory during M. tuberculosis infection, and reveal a novel mechanism for the activation of HIF-1. Furthermore, we establish a previously unknown functional link between GM-CSF and HIF-1 and provide evidence that CD4 T cell-derived GM-CSF is a potent bactericidal effector.

immunology↗