Signaling pathway of autophagy associated with innate immunity.

Autophagy has recently been shown to be an important component of the innate immune response. The signaling pathways leading to activation of autophagy in innate immunity are not well studied. Our recent study shows that Toll-like receptor 4 (TLR 4) serves as an environmental sensor for autophagy. We define a new molecular pathway in which lipopolysaccharide (LPS) induces autophagy in human and murine macrophages by a pathway regulated through Toll-interleukin 1 receptor domain-containing adaptor-inducing interferon-beta (TRIF)-dependent, myeloid differentiation factor 88 (MyD88)-independent TLR4 signaling. Receptor-interacting protein (RIP1) and p38 mitogen-activated protein-kinase (MAPK) are downstream components of this pathway. This signaling pathway does not affect cell viability, indicating that it is distinct from an autophagic death signaling pathway. We further show that LPS-induced autophagy can enhance mycobacterial co-localization with the autophagosomes. The above study raises important questions. (1) What is the complete signaling pathway for LPS-induced autophagy? (2) Does TLR3 mediate autophagy? (3) What are the mechanisms that determine whether autophagy acts as a pro-death or pro-survival pathway? (4) What are the physiological functions of LPS-induced autophagosomes? Future studies examining the above questions should provide us with important clues as to how autophagy is regulated in innate immunity, and how autophagy can be utilized in pathogen clearance.