Autophagy Adaptor Protein p62/SQSTM1 and Autophagy-Related Gene Atg5 Mediate Autophagosome Formation in Response to Mycobacterium tuberculosis Infection in Dendritic Cells

Abstract

Mycobacterium tuberculosis is an intracellular pathogen that can survive within phagocytic cells by inhibiting phagolysosome biogenesis. However, host cells can control the intracellular M. tuberculosis burden by the induction of autophagy. The mechanism of autophagosome formation to M. tuberculosis has been well studied in macrophages, but remains unclear in dendritic cells. We therefore characterized autophagosome formation in response to M. tuberculosis infection in dendritic cells. Autophagy marker protein LC3, autophagy adaptor protein p62/SQSTM1 (p62) and ubiquitin co-localized to M. tuberculosis in dendritic cells. Mycobacterial autophagosomes fused with lysosomes during infection, and major histcompatibility complex class II molecules (MHC II) also localized to mycobacterial autophagosomes. The proteins p62 and Atg5 function in the initiation and progression of autophagosome formation to M. tuberculosis, respectively; p62 mediates ubiquitination of M. tuberculosis and Atg5 is involved in the trafficking of degradative vesicles and MHC II to mycobacterial autophagosomes. These results imply that the autophagosome formation to M. tuberculosis in dendritic cells promotes the antigen presentation of mycobacterial peptides to CD4+ T lymphocytes via MHC II.

Cell death and autophagy in tuberculosis

“…Since Mtb inhibits conventional phagosome biogenesis, its capture and degradation in autolysosomes provides an opportunity to generate peptides for loading onto MHC class II molecules. This outcome was demonstrated by Seto et al [159] in murine bone marrow derived DC and DC2.4 cells infected with Mtb Erdman. The recruitment of MHC II followed the capture of ubiquitinated bacilli in autophagosomes and fusion with lysosomes.…”

“…By virtue of its ubiquitin-associated domain and LC3 interaction region, p62 serves as an adaptor bridging ubiquitinated bacilli and LC3 on the preautophagosome [146]. A role of p62 in recruiting an E3 ligase to ubiquitinate Mtb has also been proposed but the precise targets for ubiquitination on bacterial cells and the full spectrum of mediators for this conjugation reactions are incompletely defined [159]. …”

Unique role for ATG5 in neutrophil-mediated immunopathology during M. tuberculosis infection

“…In cultured cells, Atg5 , p62/ SQSTM1, and Ulk1 have similar roles in controlling Mtb survival and replication 1,4,5,22 . We therefore explored the role of these and other genes involved in autophagy in vivo , by infecting mice with germline deletions of Ulk1 , Ulk2 (autophagy induction), Atg4B (isolation membrane elongation), or p62 /SQSTM1 (substrate targeting to autophagosome).…”

Immunologic manifestations of autophagy

“…In DCs, which have the general capacity to utilize autophagic machinery for prolonged MHC class II presentation (81), a measurable fraction of the intracellular M. tuberculosis bacilli is spontaneously susceptible to basal autophagy (139). However, unless autophagy is activated by exogenous stimuli, only a very small fraction of the intracellular bacilli can be eliminated by basal autophagy in macrophages, the cell type readily parasitized by M. tuberculosis (4,50,138,139).…”

“…However, unless autophagy is activated by exogenous stimuli, only a very small fraction of the intracellular bacilli can be eliminated by basal autophagy in macrophages, the cell type readily parasitized by M. tuberculosis (4,50,138,139). This minor susceptible fraction represents the bacilli that escape from conventional phagosomes (142) or otherwise come in contact with the cytosol (138), where they can be subjected to spontaneous elimination by xenophagy.…”