Mycobacterium tuberculosis (M.tb) survives in macrophages in part by limiting phagosome–lysosome (P-L) fusion. M.tb mannose-capped lipoarabinomannan (ManLAM) blocks phagosome maturation. The pattern recognition mannose receptor (MR) binds to the ManLAM mannose caps and mediates phagocytosis of bacilli by human macrophages. Using quantitative electron and confocal microscopy, we report that engagement of the MR by ManLAM during the phagocytic process is a key step in limiting P-L fusion. P-L fusion of ManLAM microspheres was significantly reduced in human macrophages and an MR-expressing cell line but not in monocytes that lack the receptor. Moreover, reversal of P-L fusion inhibition occurred with MR blockade. Inhibition of P-L fusion did not occur with entry via Fcγ receptors or dendritic cell–specific intracellular adhesion molecule 3 grabbing nonintegrin, or with phosphatidylinositol-capped lipoarabinomannan. The ManLAM mannose cap structures were necessary in limiting P-L fusion, and the intact molecule was required to maintain this phenotype. Finally, MR blockade during phagocytosis of virulent M.tb led to a reversal of P-L fusion inhibition in human macrophages (84.0 ± 5.1% vs. 38.6 ± 0.6%). Thus, engagement of the MR by ManLAM during the phagocytic process directs M.tb to its initial phagosomal niche, thereby enhancing survival in human macrophages.

Vaccine and antiviral development against SARS-CoV-2 infection or COVID-19 disease would benefit from validated small animal models. Here, we show that transgenic mice expressing human angiotensin-converting enzyme 2 (hACE2) by the human cytokeratin 18 promoter (K18 hACE2) represent a susceptible rodent model. K18 hACE2 transgenic mice succumbed to SARS-CoV-2 infection by day 6, with virus detected in lung airway epithelium and brain. K18 ACE2 transgenic mice produced a modest TH1/2/17 cytokine storm in the lung and spleen that peaked by day 2, and an extended chemokine storm that was detected in both lungs and brain. This chemokine storm was also detected in the brain at day 6. K18 hACE2 transgenic mice are, therefore, highly susceptible to SARS-CoV-2 infection and represent a suitable animal model for the study of viral pathogenesis, and for identification and characterization of vaccines (prophylactic) and antivirals (therapeutics) for SARS-CoV-2 infection and associated severe COVID-19 disease.

Mycobacterium tuberculosis enhances its survival in macrophages by suppressing immune responses in part through its complex cell wall structures. Peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor superfamily member, is a transcriptional factor that regulates inflammation and has high expression in alternatively activated alveolar macrophages and macrophage-derived foam cells, both cell types relevant to tuberculosis pathogenesis. In this study, we show that virulent M. tuberculosis and its cell wall mannose-capped lipoarabinomannan induce PPARγ expression through a macrophage mannose receptor-dependent pathway. When activated, PPARγ promotes IL-8 and cyclooxygenase 2 expression, a process modulated by a PPARγ agonist or antagonist. Upstream, MAPK-p38 mediates cytosolic phospholipase A2 activation, which is required for PPARγ ligand production. The induced IL-8 response mediated by mannose-capped lipoarabinomannan and the mannose receptor is independent of TLR2 and NF-κB activation. In contrast, the attenuated Mycobacterium bovis bacillus Calmette-Guérin induces less PPARγ and preferentially uses the NF-κB–mediated pathway to induce IL-8 production. Finally, PPARγ knockdown in human macrophages enhances TNF production and controls the intracellular growth of M. tuberculosis. These data identify a new molecular pathway that links engagement of the mannose receptor, an important pattern recognition receptor for M. tuberculosis, with PPARγ activation, which regulates the macrophage inflammatory response, thereby playing a role in tuberculosis pathogenesis.