Autophagy is an immediate macrophage response to Legionella pneumophila

Abstract

SummaryAfter ingestion by macrophages, Legionella pneumophila enter spacious vacuoles that are quickly enveloped by endoplasmic reticulum (ER), then slowly transferred to lysosomes. Here we demonstrate that the macrophage autophagy machinery recognizes the pathogen phagosome as cargo for lysosome delivery. The autophagy conjugation enzyme Atg7 immediately translocated to phagosomes harbouring virulent Legionella . Subsequently, Atg8, a second autophagy enzyme, and monodansyl-cadaverine (MDC), a dye that accumulates in acidic autophagosomes, decorated the pathogen vacuoles. The autophagy machinery responded to 10-30 kDa species released into culture supernatants by Type IV secretion-competent Legionella , as judged by the macrophages' processing of Atg8 and formation of vacuoles that sequentially acquired Atg7, Atg8 and MDC. When compared with autophagosomes stimulated by rapamycin, Legionella vacuoles acquired Atg7, Atg8 and MDC more slowly, and Atg8 processing was also delayed. Moreover, compared with autophagosomes of Legionella -permissive naip5 mutant A/J macrophages, those of resistant C57BL/6 J macrophages matured quickly, preventing efficient Legionella replication. Accordingly, we discuss a model in which macrophages elevate autophagy as a barrier to infection, a decision influenced by regulatory interactions between Naip proteins and caspases.

CASP4/caspase-11 promotes autophagosome formation in response to bacterial infection

“…CASP4 restricts the growth of L. pneumophila , an intracellular pathogen that resides in autophagosomes in WT macrophages [3,16,25]. Notably, B. cenocepacia is also cleared by autophagosomes in healthy macrophages [5], but the role of CASP4 in intracellular trafficking of this bacterium is not known.…”

“…The breakdown of damaged organelles and proteins via autophagy leads to the recycling of nutrients and sustains cellular energy levels. In addition, autophagy targets various intracellular pathogens such as Mycobacterium tuberculosis [1], Salmonella enterica serovar Typhimurium [2], Legionella pneumophila [3], Streptococcus pyogenes [4], and Burkholderia cenocepacia [5]. This bacteria-specific autophagic process, designated xenophagy, promotes bacterial clearance and prevents cellular escape.…”

Pathogens and autophagy: subverting to survive

“…19 Likewise, immediately after L. pneumophila internalization, the autophagic markers Atg7 and Atg8 are recruited to the Legionella phagosomal compartment. 17 These results clearly indicate that similarities exist between these two pathogenic bacteria.…”

“…An interesting new finding is that autophagy is activated by molecules secreted via the Legionella Type IV system, as indicated by the processing of Atg8 and the formation of autophagic vacuoles. 17 Similarly, we have observed that numerous LC3/Atg8 decorated vesicles are formed in Coxiella-infected CHO cells 19 and we have proposed that Type IV secretion of bacterial molecules activates autophagy in the host cell. A protein secreted by the Type III system in Salmonella also induced autophagy in infected macrophages.…”

“…Indeed, it has been shown that vacuoles that harbor Legionella, as well as the autophagic vacuoles stimulated by the Legionellareleased factors, mature more slowly than normal cellular autophagosomes. 17 It has been proposed that cells are also protected against viral infection by autophagy. 10,12 However, in the case of mouse hepatitis coronavirus, replication complexes colocalize with the autophagy proteins LC3 and Atg12.…”