Autophagic cell death and innate immunity
Different experimental evidences have revealed the presence of autophagosomes and the rearrangement of the cytoskeleton in dying cells, but wherever this is a strategy of cell survival rather than an active death mechanism, has been a controversy. Conclusive data, using apoptosis inhibitors and Bax/Bak-deficient mouse embryonic fibroblasts have showed that autophagic cell death is an alternative to apoptosis in which cell dies in response to chemical stressors or cytotoxic substances like staurosporine or etoposide, through a process that involve the formation of multiple vesicular organelles and can be reversed using classical autophagy inhibitors such as 3-methyladenine(3-MA) or silencing ATG genes (reviewed in (Tsujimoto & Shimizu, 2005)). It seems that like regulated necrosis, autophagic death is activated under certain stimulus including infection when apoptosis is blocked or inhibited, playing a protective role in host defense. In line with this, different in vivo studies have demonstrated the importance of theATG genes against bacterial and viral infections such asListeria monocytogenes, Toxoplasma gondii , herpes simplex virus 1 and Sindbis virus in mice (Orvedahl et al., 2007; Orvedahl et al., 2010; Zhao et al., 2008).
As occurs with apoptosis and regulated necrosis, autophagic cell death can be activated by PRRs as result of direct contact with pathogens, inducing an alternative defense that helps cells to deal with viruses or intracellular microorganisms, a key element of innate immune response. The activation of the autophagic death by TLRs such as TLR1/2, TLR3, TLR4 and TLR7 has been corroborated in mice-derived macrophages and dendritic cells, in response to agonists like bacterial lipopolysaccharide (LPS) or under Mycobacterium tuberculosisinfection. In the same way, peptidoglycan has demonstrated to induce autophagy through NOD proteins, mainly Nod1 and Nod2 in murine myeloid and epithelial cells (reviewed in (Sumpter Jr & Levine, 2010)). Interestingly, muramyldipeptide a peptidoglycan derived product, also activates autophagy in human dendritic cells, in a process that requires NOD2, RIP2 , and the ATG genes,ATG5, ATG7 , and ATG16L1 , increasing MHC Class II antigen expression and CD4+ T lymphocytes proliferation (Cooney et al., 2010). Not only bacterial but viral infections also induce an autophagic defensive response in host cells. Viral single strand RNAs (ssRNAs) are recognized by TLR7 and NOD2 stimulating antiviral defense in RAW 264.7 macrophages and type I IFN production in dendritic cells. The ssRNAs can additionally activate other intracellular sensors like RNA-dependent protein kinase (PKR) which activate autophagy via Beclin-1 in mouse embryonic fibroblasts and primary neurons infected with herpes simplex virus 1 (Gu, Wang, & Yang, 2014).
The process of autophagy by itself is intrinsically connected to innate immune clearance of intracellular pathogens. Thus, when a pathogen interacts with a host cell it triggers an autophagic process that could lead to its own phagocytosis or eventually to the death of the infected cell, exposing this microbe to other effector cells of the immune system. Experimental results with intracellular Salmonella , enteropathogenic Echerichia coli , Streptococcus pyogenesand L. monocytogenes show that engulfed bacteria are targeted to autophagosomes and subsequently to autolysosomes by a coat of ubiquitin that interacts with the cytoskeleton protein LC3. This was corroborated by colocalization and confocal microscopy studies, together with the use of Atg5 or p62 deficient cells and genetically modified microbial strains insensitive to ubiquitin recognition (Sumpter Jr & Levine, 2010). In the context of infection, different autophagic signalling proteins such as p62, CALCOCO 2 (calcium binding and coiled-coil domain containing protein 2) and optineurin can act as adaptor proteins in the inflammatory pathway ultimately activating NF-κB and cytokine secretion. Additionally, p62 is also involved in the recruitment of cytosolic proteins such as ubiquitin or ribosomal proteins to autolysosomes, to be converted by internal proteolysis in antimicrobial peptides, highly effective to intracellular pathogens like M. tuberculosis(reviewed in (Deretic, 2012).
The processes of autophagy and autophagic cell death have been also revised due to its controversial role in cancer. Some studies reveal that autophagy actually contribute with survival, metabolic reprogramming and tumor development. Interestingly, the same process can lead to the elimination of cancer cells by autophagic cell death (Yang et al., 2015). Autophagy is also required for immune cell activation, antigen presentation, thymus selection and cytokine release in different cell types such as NK cells, Dendritic cells (DC) and T lymphocytes, but represents a “double edge sword” that can also regulate these cell populations by autophagic cell death. Current research is focus on the understanding of this dual role and its potential application in cancer immunotherapy (Jiang et al., 2019).