Subtype-specific functions of basal IFNλs.

Type III interferons (IFNλs) represent the newest interferon family, comprising four human subtypes (IFNλ1-4) that signal through the tissue-restricted IFNLR1 receptor. While traditionally characterized as antiviral cytokines protecting mucosal barriers, accumulating evidence suggests that IFNλs possess broader physiological roles that remain incompletely understood. A critical gap in our knowledge concerns whether the four IFNλ subtypes function redundantly, or serve distinct biological purposes. Recent discoveries challenge the assumption of functional redundancy among IFNλ subtypes. Emerging data indicate that different subtypes exhibit distinct signaling kinetics, potencies, and downstream effects on epithelial biology. Beyond their established antiviral functions, IFNλs appear to regulate fundamental aspects of epithelial homeostasis, including barrier integrity, cellular differentiation, and tissue architecture. The discovery of constitutive basal IFNλ expression in healthy epithelia-driven by microbiota and endogenous danger signals-suggests that these cytokines continuously shape tissue physiology rather than functioning solely as inducible defense molecules. Understanding subtype-specific IFNλ functions has become increasingly urgent as these cytokines enter clinical development. The tissue-restricted expression of IFNLR1 offers therapeutic advantages over broadly-acting type I interferons, but optimal clinical application requires comprehensive knowledge of how individual subtypes influence both pathogen control and tissue homeostasis. Critical questions remain: Do specific subtypes preferentially regulate barrier function versus antiviral immunity? Can imbalanced subtype expression contribute to epithelial pathology? How do pathogens differentially induce IFNλ subtypes to evade immunity or promote tissue damage? Addressing these questions will illuminate fundamental principles of mucosal immunity and guide rational design of IFNλ-based therapeutics that maximize protection while preserving epithelial health.