Interferon-λ inhibits immunopathology and boosts T cell-mediated immunity in a mouse model of mucosal viral infection.

Interferon lambda (IFN-λ) has emerged as a critical inducer of immune regulation in addition to its mucosal antiviral properties. However, the cellular and molecular mechanisms that govern its dual role during viral infections remain unclear. Here, we discovered that mucosal delivery of IFN-λ abrogates the herpes simplex virus type 2-induced (HSV-2-induced) vaginal tissue pathology observed in the absence of type I IFN signaling, through drastically inhibiting IL-6 production. At the same time, we demonstrated that IFN-λ rescued local production of CD4+ T cell-dependent IFN-γ, which is essential for protection against the development of toxic megacolon following genital HSV-2 infection. Mechanistically, we found that IFN-λ enhances the differentiation of CCR2+Ly6Chi cells into monocyte-derived antigen-presenting cells, thereby boosting T cell responses. In vivo depletion of CCR2+ cells abrogated this phenomenon. Together, these findings suggest that IFN-λ orchestrates a potent antiviral response, inhibiting hyperinflammatory responses while selectively augmenting effector T cell function. This study positions IFN-λ as a promising therapeutic candidate for enhancing local antiviral immunity and offers new insights into the regulation of immune responses in the context of mucosal viral infections.