Cellular Mechanisms and Immunological Contexts of Lysosomal Damage Protection in Macrophages

Abstract

As professional phagocytes, macrophages are susceptible to endolysosomal membrane damage inflicted by the pathogens and noxious particles they ingest. Whether macrophages have mechanisms for limiting such damage is a fundamental question of macrophage biology that is poorly understood. Previously we reported a phenomenon, termed “inducible renitence,” in which LPS activation of macrophages protected their endolysosomes from membrane damage initiated by the phagocytosis of silica beads. In this thesis, I uncover mechanistic details of the underlying process and more broadly define the immune contexts in which renitence can be induced.

Applying a quantitative, fluorescence microscopy assay for measuring lysosomal damage, I determined that mechanistically renitence limits the release of small but not large molecules from lysosomes by restricting the time window of release. Morphological analysis of a large imaging data set derived from these studies led to the discovery of a novel, structural correlate of renitence: large, damage-resistant vacuoles that form adjacent to bead-containing phagosomes in LPS-activated but not resting macrophages. These structures, which we term “renitence vacuoles” (RVs), formed coincident with silica bead uptake in a process associated with macropinocytosis, and persisted around bead-containing phagosomes. RVs fused with lysosomes, whereas phagosomes associated with RVs did not, consistent with a model in which RVs act as structural barriers to prevent fusion between damaged phagosomes and intact lysosomes.

Complementing these studies of cellular mechanism, several molecular candidates – namely, cholesterol, synaptotagmin-7, and the autophagy component Atg7 - were evaluated for their contribution to renitence based on their established roles in resistance against or repair of organellar damage in other contexts. However, none of these factors were found to definitively underlie renitence.

A growing body of literature finds evidence for the existence of several functionally distinct macrophage populations in vivo. To expand our understanding of the inflammatory contexts in which renitence acts, I examined whether macrophages of these well-defined subtypes exhibit renitence. These studies revealed that classically activated and regulatory macrophages, but not alternatively activated macrophages, exhibit renitence. Furthermore, stimulation of a subset of Toll-like receptors was sufficient to induce renitence. Of the macrophage subtypes examined, those harboring the greatest capacity for renitence shared similarities in their cytokine secretion profile. Likewise, macrophages subtypes less capable of inducing renitence possessed a common profile of cytokine secretion distinct from that associated with renitent macrophages. Thus, the polarization state of macrophages influences their susceptibility to lysosomal damage.

Taken together, this work establishes renitence as an activity of macrophages specialized in host defense or immune regulation and identifies a novel mechanism by which endocytic processes contribute to the reinforcement of lysosomal integrity.