Modulation of Extracellular ISG15 Signaling by Pathogens and Viral Effector Proteins

ISG15 is a ubiquitin-like modifier that also functions extracellularly, signaling through the LFA-1 integrin to promote interferon (IFN)-γ release from natural killer (NK) and T cells. The signals that lead to the production of extracellular ISG15 and the relationship between its two core functions remain unclear. We show that both epithelial cells and lymphocytes can secrete ISG15, which then signals in either an autocrine or paracrine manner to LFA-1-expressing cells. Microbial pathogens and Toll-like receptor (TLR) agonists result in both IFN-β-dependent and -independent secretion of ISG15, and residues required for ISG15 secretion are mapped. Intracellular ISGylation inhibits secretion, and viral effector proteins, influenza B NS1, and viral de-ISGylases, including SARS-CoV-2 PLpro, have opposing effects on secretion of ISG15. These results establish extracellular ISG15 as a cytokine-like protein that bridges early innate and IFN-γ-dependent immune responses, and indicate that pathogens have evolved to differentially inhibit the intracellular and extracellular functions of ISG15. [Display omitted] •ISG15 is released from multiple cell types to signal to LFA-1-expressing lymphocytes•Mutational analysis separates ISG15 secretion from LFA-1 binding and ISGylation•Intracellular conjugation of ISG15 negatively modulates its secretion•Viral de-ISGylases, including SARS-CoV-2 PLpro, positively modulate ISG15 secretion Swaim et al. characterize cell types and immune agonists that stimulate ISG15 secretion and signaling to lymphocytes. Intracellular conjugation of ISG15 is shown to inhibit secretion, whereas viral de-ISGylating enzymes, including SARS-CoV-2 PLpro, enhance ISG15 secretion, suggesting a potential role for ISG15 in pro-inflammatory responses associated with viral infections.