Figure 9.

Model of autophagic response to pneumococci within zebrafish macrophages. After internalization by phagocytes, pneumococci are sequestered within Lc3-decorated vacuoles which are formed by three independent mechanisms: ROS-dependent LAP or two ROS-independent Ply-mediated pathways including STIL. LAP occurs as a result of osmotic imbalance within bacteria-containing phagosome caused by ROS production by NADPH oxidase. In contrast, STIL is induced upon membrane perturbation caused by secretion of bacterial pneumolysin manifested by sphingomyelin presence on the outside leaflet of the phagosome. STIL is dependent on Tecpr1a which recognizes sphingomyelin and mediates Lc3 lipidation. Last, the Atg16l1- and Ply-dependent pathway contributes to the smallest portion of Lc3 lipidation. However, the specific mechanisms controlling this process remain to be revealed. Following Lc3-assisted vacuolar sequestration, pneumococci are more efficiently targeted for lysosomal degradation and the Lc3 decoration is removed. The classical xenophagy receptors (sqstm1 and optineurin) as well as the canonical autophagy initiation molecule (atg13) are dispensable in this model. Despite the absence of these xenophagy-mediating factors, the bacteria are efficiently degraded. Figure created with BioRender.