Toll-like receptors in control of immunological autophagy

Abstract

Autophagy is a cell biological process, enabling cells to autodigest their own cytosol when starved, remove cytoplasmic protein aggregates too large for proteasomal degradation, eliminate aberrant or over-proliferated organelles, and sanitize the cytoplasm by killing intracellular microbes. The role of autophagy has been expanded in recent years to include diverse immunological effector and regulatory functions. In this review, we summarize the multiple immunological roles of autophagy uncovered to date and focus primarily on details of induction of autophagy by pattern recognition receptors, as a newly established Toll-like receptor output. Taken together with other links between autophagy and innate and adaptive immunity processes, this cell-autonomous antimicrobial defense may be evolutionarily positioned at the root of immunity with the multiple innate and adaptive immunity connections uncovered to date reflecting a co-evolution of this ancient cell-defense mechanism and more advanced immunological systems in metazoans.

Figure 1

Autophagy – morphological stages and regulation. (a) Initiation: formation of phagophore crescents poised to capture cytoplasmic targets (cytosol, organelles such as mitochondria, pathogens). Elongation: phagophore enlarges assisted by two systems (i) Atg5–Atg12/Atg16 protein complex and (ii) LC3-II, which is a lipidated protein (Atg8) with phosphatidylethanolamine added to its C-terminus. It warps around its cytoplasmic target and closes to form double membrane autophagosome. Maturation: Autophagosome received lysosomal components by vesicular trafficking or fusion with lysosomes (Lys) and converts into an acidified, hydrolytic organelle termed autolysosome. Growth factor and nutritional signals through Tor and Atg1 control the downstream Atg factors. (b) Autophagy is additionally controlled by the Class III PI3 kinase, hVPS34 in a protein complex with the key autophagy regulator Beclin 1 (Atg6). When Beclin 1 is associated through its BH3-like domain with Bcl-2 family members, autophagy is inhibited. When Bcl-2–Beclin 1 complex is disrupted, this is compatible with autophagy activation. Different factors (above the arrow) can effect dissociation of Bcl-2–Beclin 1 complexes, and assist or lead to autophagy activation

Figure 2

Pattern recognition receptor (PRR) agonists, signaling modules, and immunological outputs. Red, input microbial products (PAMPs) acting as PRR agonists: DAP, diaminopimelic acid, LipoProt, lipoprotein; MDP, muramyl dipeptide, PG, peptidoglycan, β-gluc, β-glucan; ss, single stranded; ds, double stranded. PRRs (blue): CLR, C-type lectin receptors; NLR, NOD-like receptors; TLR, toll-like receptors; Cytosolic, endosomal or plasma membrane (PM) localization is indicated; RLR, RIG-I-like receptors; CARD, caspase recruitment domain; PYR, pyrin domain; BIR, baculovirus inhibitor repeat domain. Green, adapters interacting with PRRs: ASC, apoptosis-associated speck-like protein containing a CARD. Gray (boxed, center) signaling pathways engaged by PRR and downstream adapters. Gray (bottom), proinflammatory cytokines output. Autophagy is shown as a new, earlier unappreciated output of PRR signaling that can (i) be directly microbicidal, (ii) fuel further PRR activation, or (iii) participate in adaptive immunity processes. A colour version of this figure is available online

Figure 3

Signaling and regulation of PRR-induced autophagy. 1. PAMP agonists stimulate TLRs (TLR4 and TLR7/TLR8 are depicted) leading to signaling through adaptors (TRAM–TRIF or MAL–MyD88) and downstream kinases (not shown – see text). 2. One molecular mechanism linking TLR signaling and autophagy induction is the association of Beclin 1 (a key regulator of autophagy) and MyD88-containing protein complexes, affecting Bcl-2–Beclin 1 interactions: when Bcl-2 is in a complex with Beclin-1 this inhibits autophagy; when Bcl-2 dissociates from Beclin 1 (as shown to be the case downstream of TLR4 signaling), Beclin-1 (along with other Atg factors and type III PI3K hVPS34, not shown) is free to initiate autophagy. 3. Autophagy can act as a topological inversion device delivering PAMP molecules to endosomal TLRs. Note that the topological inversion occurs by sequestration of cytosolic PAMPs (e.g. from a replicating virus) into the autophagosome, in which they now are in organellar lumen, which puts them topologically on the same side of the membrane as the receptor domain of endosomal TLRs. 4. PGRP-LE, a Drosophila PRR, reacts to bacterial PAMPs and induces autophagy as an innate immunity output protecting the fly from infection in vivo. 5. Inhibitory action of PAMP through NF-κB on autophagy. Inhibition of autophagy by NF-κB has been earlier described in the context of TNF-α signaling. A balance between activating/amplifying pathways 1,2, and 3, and inhibitory signaling through pathway 5 may determine the net outcome in terms of induction or inhibition of autophagy. These relationships have not been explored, but need to be delineated. 6–8, immunological outputs of PAMP–PRR–autophagy cascade: 6. Autophagy induced by PAMPs may result in direct elimination of offending microbes. 7. Autophagy assists cytosolic antigen delivery to MHC II processing and loading compartments, akin to the delivery of cytosolic PAMPs to the lumenal domains of endosomal TLRs. It is not known whether PRR-induced autophagy assists endogenous antigen MHC II presentation, but this can be predicted from the depicted circuitry. 8. Autophagy may inhibit IL-1β activation or secretion; it is not known whether autophagy acts on inflammasome, an apparatus that processes inactive pro-IL-1β and secretes it as active IL-1β, normally activated by PAMPs or danger-associated molecular patterns (DAPMs) – body’s own molecules also known as ‘alarmins’ capable of inducing inflammation and defenses. 9. When individual PAMP–PRR pairs do not activate autophagy, it is possible that a combinatorial engagement of multiple PRRs may be needed to activate autophagy (as shown for TLR2/6 plus Dectin). Green arrows and boxes, positive regulatory pathways and molecules; Red arrows and letters, negative regulatory pathways; Gray arrows, immunological outputs. A colour version of this figure is available online

Figure 4

Proposed evolutionary relationships between autophagy, apoptosis, intracellular pathogens, and mitochondria. See text section ‘Is there a connection between innate immunity roles of autophagy and cell survival/cell death pathways?’ for details.