Molecular mechanisms underlying the promotion of antibacterial immunity by LC3-associated phagocytosis

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Phagocytosis of invading bacteria by neutrophils and macrophages is a key component of antibacterial immunity. In the phagosome, the bacteria are inactivated, killed and degraded. Some pathogens, such as Mycobacterium tuberculosis or Listeria monocytogenes, however, have evolved mechanisms that prevent their killing in conventional phagosomes. We have recently identified a non-canonical autophagy pathway called LC3-associated phagocytosis (LAP) that allows macrophages to effectively kill phagocytosed L. monocytogenes (Gluschko et al., Cell Host & Microbe 2018). LAP also is crucial for elimination of other microbial pathogens, such as M. tuberculosis or Aspergillus fumigatus, indicating that it generally represents a particularly microbicidal pathway. During LAP, phagosomes are decorated by the protein LC3, which enhances the fusion with lysosomes and thus the killing of the phagocytosed pathogen. However, only a subpopulation of pathogen-containing phagosomes is targeted by LAP and decorated with LC3. Hence, the high antimicrobial potential of LAP is not fully retrieved. Therapeutic approaches based on increasing the targeting of phagocytosed pathogens by LAP would therefore potentially markedly enhance cell-mediated antibacterial defense.The objective of this project is to further elucidate the molecular mechanisms of LAP induction and thereby identify potential starting-points for using the superior microbicidal activity of this cellular mechanism in the treatment of microbial infections.Illustrating that promoting LAP may be a promising strategy for enhancing cell-mediated antibacterial defense, our preliminary data show that the targeting and killing of L. monocytogenes by LAP is substantially increased by the proinflammatory cytokine tumor necrosis factor (TNF). Thus, TNF is an endogenous promotor of LAP and, thereby, antibacterial defense. The first specific aim of this project is to decipher the molecular mechanisms through which TNF promotes LAP induction.Regarding the induction of LAP, we and others have shown that reactive oxygen species (ROS) produced by the NADPH oxidase Nox2 are critically required. In addition to their well-known direct function in killing of phagocytosed pathogens, ROS also exert regulatory functions. How the ROS induce the recruitment of LC3 to phagosomes by LAP, however, remains completely elusive. The second specific aim of this project therefore is to unravel the molecular mechanism through which ROS induce LAP.The insights gained by this project into how macrophages bring to use the particularly microbicidal mechanism of LAP will substantially further our understanding of cell-mediated antimicrobial immunity and identify potential starting-points for novel therapeutic approaches based on increasing the targeting of phagocytosed pathogens by LAP.