Mycobacterium tuberculosis is a global health problem in part as a result of extensive cytotoxicity caused by the infection. Here, we show how M. tuberculosis causes caspase-1/NLRP3/gasdermin D-mediated pyroptosis of human monocytes and macrophages. A type VII secretion system (ESX-1) mediated, contact-induced plasma membrane damage response occurs during phagocytosis of bacteria. Alternatively, this can occur from the cytosolic side of the plasma membrane after phagosomal rupture in infected macrophages. This damage causes K+ efflux and activation of NLRP3-dependent IL-1β release and pyroptosis, facilitating the spread of bacteria to neighbouring cells. A dynamic interplay of pyroptosis with ESCRT-mediated plasma membrane repair also occurs. This dual plasma membrane damage seems to be a common mechanism for NLRP3 activators that function through lysosomal damage.

Specific serum antibodies could be helpful in defining the status of Pseudomonas aeruginosa infection as well as the response to early intervention treatment in patients with cystic fibrosis (CF). We used 1,791 serum samples from 375 European CF patients with known respiratory microbiology status to define titers of P. aeruginosa antibodies directed against alkaline protease (AP), elastase (ELA), and exotoxin A (ExoA). Pseudomonas antibody titers were also measured in a separate cohort of 56 patients undergoing antibiotic treatment for eradication of P. aeruginosa. At a specificity of 97.5%, the sensitivity was highest for antibodies against AP (85.4%), followed by ELA (76.2%) and ExoA (72.0%). AP, ELA, or ExoA antibody titers were significantly higher (P < 0.001) in patients chronically infected with P. aeruginosa compared to patients with negative cultures. The sensitivity of the combined three ELISAs was higher than that for any single ELISA alone. Based on the newly defined cut-off levels, positive serum antibody titers against at least one of the three antigens were present in 43% of patients with new onset of P. aeruginosa infection. Longitudinal assessment of antibody titers assessed before and after inhaled antibiotic therapy in patients with first P. aeruginosa isolation showed a significant decrease in antibody titers against AP and ExoA in patients clearing P. aeruginosa infection, whereas titers increased in patients in whom antibiotic therapy failed to eradicate the organism. Antibody testing against AP, ELA, and ExoA offers high sensitivity and specificity for the presence of P. aeruginosa in respiratory cultures of CF patients. Although serum antibody titers are on average low at the time of first P. aeruginosa isolation from respiratory specimens, they may be useful to monitor response to therapy. However, because variability between patients is considerable, treatment decisions should not be based on P. aeruginosa antibody levels alone.

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Iron is an essential nutrient for microbes and many pathogenic bacteria depend on siderophores to obtain iron. The mammalian innate immunity protein lipocalin 2 (Lcn2, NGAL, 24p3, Siderocalin) binds the siderophore carboxymycobactin, an essential component of the iron acquisition apparatus of mycobacteria. Here we show that Lcn2 suppressed growth of Mycobacterium avium in culture, and M. avium induced Lcn2 production from mouse macrophages. Lcn2 was also elevated and initially limited the growth of M. avium in the blood of infected mice, but did not impede growth in tissues and during long-term infections. M. avium is an intracellular pathogen. Subcellular imaging of infected macrophages revealed that Lcn2 trafficked to lysosomes separate from M. avium, whereas transferrin was efficiently transported to the mycobacteria. Thus mycobacteria seem to reside in the Rab11+ endocytic recycling pathway, thereby retaining access to nutrition and avoiding endocytosed immunoproteins like Lcn2.