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The replication of  Tropheryma whipplei  (the agent of Whipple's disease) within human macrophages is associated with the expression of IL-16, a cytokine known for its chemotactic and inflammatory properties. In this study, we asked whether IL-16 acts on  T. whipplei  replication by interfering with the endocytic pathway. We observed that in macrophages,  T. whipplei  was located within late phagosomes that were unable to fuse with lysosomes; in monocytes,  T. whipplei  was eliminated in phagolysosomes. Moreover, adding IL-16 to monocytes induced bacterial replication and inhibited phagolysosome formation. On the other hand, blocking IL-16 activity, either with anti-IL-16 antibodies in human macrophages or by using murine IL-16 −/−  bone marrow-derived macrophages, inhibited  T. whipplei  replication and rescued phagolysosome biogenesis. Furthermore, we propose that IL-16-mediated interference with the endocytic pathway is likely related to macrophage activation. First, IFNγ induced  T. whipplei  elimination and phagolysosome formation and inhibited IL-16 production by macrophages. Second, the full transcriptional response of murine macrophages to  T. whipplei  showed that  T. whipplei  specifically modulated the expression of 231 probes in IL-16 −/−  macrophages. Gene Ontology analysis revealed that 10 of 13 over-represented terms were linked to immune responses, including proinflammatory transcriptional factors of the NF-κB family. Our results demonstrated a previously unreported function for IL-16 in promoting bacterial replication through inhibited phagolysosome biogenesis and modulated macrophage activation program.

IL-16 Promotes T. whipplei Replication by Inhibiting Phagosome Conversion and Modulating Macrophage Activation