The unfolded protein response triggers the ancestral immune deficiency pathway

The insect immune deficiency (IMD) pathway is a defense mechanism that senses and responds to Gram negative bacteria. Ticks lack genes encoding upstream components that initiate the IMD pathway. Despite this deficiency, core signaling molecules are present and functionally restrict tick-borne pathogens. The molecular events preceding activation remain undefined. Here, we show that the Unfolded Protein Response (UPR) initiates the IMD network in Ixodes scapularis ticks. The endoplasmic reticulum (ER) stress receptor, IRE1, is phosphorylated in response to tick-borne bacteria, but does not splice the mRNA encoding XBP1. Instead, through protein modeling and reciprocal pulldowns, we show that Ixodes IRE1 complexes with TRAF2. Disrupting IRE1-TRAF2 signaling blocks IMD pathway activation and diminishes the production of reactive oxygen species. Through in vitro, in vivo, and ex vivo techniques we demonstrate that the UPR-IMD pathway circuitry limits the Lyme disease-causing spirochete Borrelia burgdorferi and the rickettsial agents Anaplasma phagocytophilum and A. marginale (anaplasmosis). Altogether, our study uncovers a novel linkage between the UPR and the IMD pathway in ticks.