Natural killer cell proliferation requires canonical IRE1 function during viral infection (CITE-Seq)

The unfolded protein response (UPR) aims to restore ER homeostasis under conditions of high protein folding load, a function primarily serving secretory cells. Additional, non-canonical UPR functions have recently been unraveled in immune cells. We addressed the function of the inositol-requiring-enzyme 1 (IRE1) signaling branch of the UPR in NK cells in homeostasis and microbial challenge. Cell-intrinsic compound deficiency (DKO) of IRE1 and its downstream transcription factor XBP1 in NKp46 + NK cells, did not affect basal NK cell homeostasis, or overall outcome of viral MCMV infection. However, mixed bone marrow chimeras revealed a competitive advantage in the proliferation of IRE1 sufficient Ly49H + NK cells after viral infection. CITE-Seq analysis confirmed strong induction of IRE1 early upon infection, concomitant with the activation of a canonical UPR signature. Therefore, we conclude that cell-intrinsic IRE1/XBP1 activation is required for NK cell proliferation early upon viral infection, as part of a canonical UPR response. Mixed bone marrow chimeras were generated as follows: CD45.1 animals were sublethally irradiated, followed by replenishment of the hematopoietic system with a 50/50 mixture of bone marrow cells isolated from CD45.1.2 (wild type) and CD45.2 (NK-DKO-/-) animals. At least 8 weeks after successful reconstitution, animals were infected with MCMV Smith strain or left untreated (i.e. mock infected with DMEM). On days 1,5 and 4,5 post-infection, spleens were isolated and distinct congenically marked NK cells were sorted for CITE-Seq analysis. Uninfected animals were included on day 1,5 post-infection.