Salmonella Typhi asparaginase-dependent activation of GCN2 promotes bacterial killing in murine macrophages

Many intracellular pathogens stimulate host cell stress by directly or indirectly causing an imbalance in host nutrients. Depletion of amino acid pools, in particular, can act as a danger signal to infected cells. Using a restrictive host model of Salmonella enterica serovar Typhi (S. Typhi) infection, we identify early induction of the integrated stress response (ISR) by viable bacteria, but not by heat-killed bacteria. Genetic deletion of the amino acid-sensing ISR kinase GCN2 (also known as EIF2AK4) prevented early ISR activation during S. Typhi infection and murine macrophages lacking GCN2 show impaired bacterial clearance and decreased cytokine output. Supplementation of wild-type C57BL/6 murine macrophages with only the non-essential amino acid asparagine was sufficient to suppress S. Typhi-induced ISR activation, and deletion of S. Typhi ansB, encoding an asparaginase, prevented ISR activation during infection. Pharmacological inhibition of mammalian target of rapamycin (mTOR), the other major amino acid-sensing pathway in eukaryotic cells, prevented GCN2 activation and ISR induction in murine macrophages, indicating an upstream role for mTOR in signaling to GCN2. These findings suggest a role for the ISR in macrophage innate immune responses to S. Typhi infection and highlight a potential difference in nutrient-dependent signaling between the S. Typhi-susceptible human host and the restrictive murine host centered around asparagine, mTOR, and GCN2.