IFNL1 and viral infectious disease: Intestinal epithelial cells upregulate IFNλ1 and IFNλ2/3 upon virus infection. The importance of IFNλs in controlling viral infection in intestinal epithelial cells has been well described using cells depleted of the IFNλ receptor [6,37,39–45], studies in mice have further demonstrated that loss of the cytokines Ifnl2 and Ifnl3 phenocopies the absence of Ifnlr signaling, underscoring their essential role in mucosal antiviral defense [44].