Harnessing ER stress responses and trained innate immunity for novel host-directed antiviral strategies

Abstract

Six years after the emergence of SARS-CoV-2, emerging and re-emerging viruses continue to pose a growing global health threat, underscoring the need for host-directed antiviral strategies that exploit conserved cellular defense pathways. One such approach is the modulation of host stress responses to enhance cellular resistance to infection. Thapsigargin (Tg), a natural compound and classical inducer of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR), is one example. Consistent with previous findings, I show that Tg potently restricts replication of the human coronavirus HCoV-229E. Tg inhibits the accumulation of double-stranded (ds)RNA replication intermediates at early time points, indicating suppression of early stages of viral RNA replication. The antiviral effect of Tg is not dependent on individual expression of any single UPR sensor. However, selective activation of the PERK arm of the UPR is sufficient to inhibit HCoV-229E replication, phenocopying the effect of Tg. In addition, Tg primes cells to mount amplified antiviral responses upon secondary stimulation, particularly through interferon regulatory factor 1 (IRF1) and IRF3/7-dependent signaling, resulting in enhanced induction of interferon-stimulated genes (ISGs), including MX1. Notably, this potentiation occurs independently of canonical JAK/STAT signaling and type I or III interferon responses. While all three UPR branches are required for Tg-mediated ISG induction, selective activation of the inositol-requiring enzyme 1 (IRE1) pathway alone is sufficient to prime downstream ISG expression, identifying IRE1 as a central immunomodulator of ER stress responses. Finally, I extend the concept of antiviral priming by examining cross-protection mediated by trained innate immunity and the downstream cytokine interferon-γ (IFNγ). I show that Bacillus Calmette-Guérin (BCG) vaccine-induced training, as well as IFNγ stimulation, preconditions cells to resist subsequent coronavirus infection. These findings demonstrate a novel antiviral effect of trained innate immune cells independent of adaptive immunity. Furthermore, the antiviral activity of IFNγ is partially dependent on guanylate binding protein 2 (GBP2), although GBP2 alone is insufficient to inhibit infection. Together, these studies define ER stress-mediated UPR activation, trained immunity, and IFNγ signaling as complementary mechanisms of host antiviral conditioning, establishing host cell pre-conditioning as a unifying principle for the development of broad-spectrum host-centric antiviral strategies.