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Summary A zebrafish genetic screen for determinants of susceptibility to Mycobacterium marinum identified a hypersusceptible mutant deficient in lysosomal cysteine cathepsins that manifests hallmarks of human lysosomal storage diseases. Under homeostatic conditions, mutant macrophages accumulate undigested lysosomal material, which disrupts endocytic recycling and impairs their migration to, and thus engulfment of, dying cells. This causes a buildup of unengulfed cell debris. During mycobacterial infection, macrophages with lysosomal storage cannot migrate toward infected macrophages undergoing apoptosis in the tuberculous granuloma. The unengulfed apoptotic macrophages undergo secondary necrosis, causing granuloma breakdown and increased mycobacterial growth. Macrophage lysosomal storage similarly impairs migration to newly infecting mycobacteria. This phenotype is recapitulated in human smokers, who are at increased risk for tuberculosis. A majority of their alveolar macrophages exhibit lysosomal accumulations of tobacco smoke particulates and do not migrate to Mycobacterium tuberculosis. The incapacitation of highly microbicidal first-responding macrophages may contribute to smokers’ susceptibility to tuberculosis., Graphical Abstract, Highlights • Lysosomal storage diseases reduce macrophage endocytic recycling and migration • Reduced macrophage migration increases tuberculosis severity via granuloma breakdown • Tobacco smoke particles accumulate in lysosomes of smokers’ alveolar macrophages • Lysosomal particles reduce smokers’ macrophage migration to infecting mycobacteria, Lysosomal dysfunction caused by genetic mutations or accumulation of tobacco particulates in the lysosomal compartment impairs macrophage function and formation of stable tuberculous granulomas, providing a possible explanation for the increased susceptibility to tuberculosis in smokers.

Schistosome eggs provoke formation of granulomas around them. For the host, the granulomatous response can be both protective and pathological. Granulomas are also postulated to facilitate parasite egg extrusion through the gut lumen, a necessary step for transmission. Using zebrafish larvae, we find that mature Schistosoma mansoni eggs recruit macrophages, which form granulomas within days. Inert egg-sized beads also rapidly induce granulomas through a foreign body response. Strikingly, immature S. mansoni eggs do not recruit macrophages. We distinguish mature and immature eggs via size analysis and find that only mature S. mansoni eggs are shed into the feces of infected mice. Importantly, analysis of egg sizes from previously reported data shows that humans also shed mature eggs only. Our findings support the model that the immunologically inert eggshell inhibits granuloma formation long enough for the parasite to mature. Then parasite antigens secreted through the eggshell trigger granulomas, facilitating egg extrusion.

Schistosome eggs provoke the formation of granulomas, organized immune aggregates, around them. For the host, the granulomatous response can be both protective and pathological. Granulomas are also postulated to facilitate egg extrusion through the gut lumen, a necessary step for parasite transmission. We used zebrafish larvae to visualize the granulomatous response to Schistosomamansoni eggs and inert egg-sized beads. Mature eggs rapidly recruit macrophages, which form granulomas within days. Beads also induce granulomas rapidly, through a foreign body response. Strikingly, immature eggs do not recruit macrophages, revealing that the eggshell is immunologically inert. Our findings suggest that the eggshell inhibits foreign body granuloma formation long enough for the miracidium to mature. Then parasite antigens secreted through the eggshell trigger granulomas that facilitate egg extrusion into the environment. In support of this model, we find that only mature S. mansoni eggs are shed into the feces of mice and humans.

Infections by schistosomes result in granulomatous lesions around parasite eggs entrapped within the host tissues. The host and parasite determinants of the Schistosoma mansoni egg-induced granulomatous response are areas of active investigation. Some studies in mice implicate Tumor Necrosis Factor (TNF) produced in response to the infection whereas others fail to find a role for it. In addition, in the mouse model, the S. mansoni secreted egg antigen omega-1 is found to induce granulomas but the underlying mechanism remains unknown. We have recently developed the zebrafish larva as a model to study macrophage recruitment and granuloma formation in response to Schistosoma mansoni eggs. Here we use this model to investigate the mechanisms by which TNF and omega-1 shape the early granulomatous response. We find that TNF, specifically signaling through TNF receptor 1, is not required for macrophage recruitment to the egg and granuloma initiation but does mediate granuloma enlargement. In contrast, omega-1 mediates initial macrophage recruitment, with this chemotactic activity being dependent on its RNase activity. Our findings further the understanding of the role of these host- and parasite-derived factors and show that they impact distinct facets of the granulomatous response to the schistosome egg., Author summary Schistosomiasis is a disease caused by parasitic flatworms which lay eggs within the veins of their human host. Upon sensing the parasite egg, macrophages, the first line defense cells, aggregate tightly around the egg to encapsulate it within an immune structure known as a granuloma. These granulomas are the key pathological structures which determine both host disease outcome and parasite transmission. Studies in mice have implicated omega-1, a secreted parasite protein. Omega-1 is an RNase, an enzyme that degrades host RNA. Mouse studies have also suggested that a host defense protein, Tumor Necrosis Factor (TNF), is required to form granulomas around the egg. We used the small and transparent zebrafish larva to examine the requirement of omega-1 and TNF for granuloma formation. We find that omega-1 induces rapid macrophage migration and that its RNase activity is required for this. In contrast, TNF is not involved in the initial recruitment of macrophages. Rather, it enlarges granulomas after they are initiated. These findings improve our understanding of the role of omega-1 and TNF, and show that they impact distinct facets of granuloma formation around Schistosoma eggs.