Reduced IFNL1 and/or IFNL2, but not IFNL3 is associated with worse outcome in patients with COVID-19.

The recent pandemic was caused by the emergence of a new human pathogen, SARS-CoV-2. While the rapid development of many vaccines provided an end to the immediate crisis, there remains an urgent need to understand more about this new virus and what constitutes a beneficial immune response in terms of successful resolution of infection. Indeed, this is key for development of vaccines that provide long lasting protective immunity. The interferon lambda (IFNL) family of cytokines are produced early in response to infection and are generally considered anti-viral and beneficial. However, data regarding production of IFNL cytokines in COVID-19 patients is highly variable, and generally from underpowered studies. In this study, we measured all three IFNL1, IFNL2 and IFNL3 cytokines in plasma from a well characterised, large COVID-19 cohort (n=399) that included good representation from patients with a more indolent disease progression, and hence a beneficial immune response. While all three cytokines were produced, they differed in both the frequency of expression in patients, and the levels produced. IFNL3 was produced in almost all patients but neither protein level nor IFNL3/IFNL4 SNPs were associated with clinical outcome. In contrast, both IFNL1 and IFNL2 levels were significantly lower, or absent, in plasma of patients that had a more severe disease outcome. These data are consistent with the concept that early IFNL1 and IFNL2 cytokine production is protective against SARS-CoV-2 infection.