Innate and adaptive immunity through autophagy

Abstract

The two main proteolytic machineries of eukaryotic cells, lysosomes and proteasomes, receive substrates by different routes. Polyubiquitination targets proteins for proteasomal degradation, whereas autophagy delivers intracellular material for lysosomal hydrolysis. The importance of autophagy for cell survival has long been appreciated, but more recently, its essential role in both innate and adaptive immunity has been characterized. Autophagy is now recognized to restrict viral infections and replication of intracellular bacteria and parasites. Additionally, this pathway delivers cytoplasmic antigens for MHC class II presentation to the adaptive immune system, which then in turn is able to regulate autophagy. At the same time, autophagy plays a role in the survival and the cell death of T cells. Thus, the immune system utilizes autophagic degradation of cytoplasmic material, to both restrict intracellular pathogens and regulate adaptive immunity.

Figure 1

Molecular Machinery of Macroautophagy Atg6 (Beclin-1) is part of the type III PI3K complex that initiates autophagosome formation. Two ubiquitin-like systems are required for formation of the isolation membrane and couple Atg8 (LC3) and Atg12 to phosphatidylethanolamine (PE) and Atg5, respectively. The five C-terminal amino acids of Atg8 (LC3) are cleaved of by Atg4 to reveal glycine 120 (G₁₂₀), which is required to link the protein after activation by Atg7 and ligation by Atg3 to PE in the autophagosomal membrane (green circles). Similarly, glycine 140 (G₁₄₀) is used by Atg7 and Atg10 to couple Atg12 to Atg5. This complex then localizes to the outer membrane of the forming autophagosome (blue squares). Upon autophagosome completion, the Atg12-Atg5 complex recycles from the outer membrane, and only Atg8 (LC3) remains associated with the completed autophagosome. Autophagosomes then fuse with late endosomes and lysosomes for degradation of their cargo and their intravesicular membranes.

Figure 2

Innate Immune Control by Macroautophagy Viruses, bacteria, and parasites get cleared or interfere with their destruction by macroautophagy. Herpesviruses encode inhibitors of autophagosome formation, whereas picornaviruses and coronaviruses inhibit fusion of autophagosmes with lysosomes in order to replicate on autophagosomal membranes. Rickettsia conorii and group A Streptococcus, examples of bacterial pathogens that escape the endosome, are degraded by macroautophagy, but Shigella flexneri has developed an escape strategy that avoids its import into autophagosomes. Brucella abortus and Legionella pneumophila block autophagosome-lysosome fusion and replicate in autophagic vesicles. Pathogens such as Toxoplasma gondii and Mycobacterium tuberculosis condition phagosomes as replication niches, which can be cleared by macroautophagy after immune activation of the host cell.

Figure 3

Role of Macroautophagy in Adaptive Immune Responses Autophagic pathways can deliver antigens for MHC class II presentation. Autophagosomes and LAMP-2a, the transporter associated with chaperone-mediated autophagy, can transport antigens into the MHC-class-II-loading compartment (MIIC). In MIICs, the antigen is processed and loaded onto MHC class II molecules for CD4⁺ T cell stimulation. Activated CD4⁺ T cells can then in turn enhance macroautophagy and autophagosome-lysosome fusion via type II IFN and TNF family members (IFN-γ, TNF, TRAIL, and CD40L). In addition, Th2-polarized CD4⁺ T cells are susceptible to cell death by macroautophagy.