PROJECT SUMMARY Recent levels of the bacterial disease pertussis are at their highest in 60 years. While pertussis in adolescents and adults is characterized by a persistent debilitating cough, pertussis in infants can progress from respiratory symptoms to more severe and complicated disease. This often requires hospitalization and admission to a pediatric intensive care unit, and pertussis still causes an alarming number of deaths in infants. However, no effective therapies exist for treatment of severe pertussis and we still have a relatively poor understanding of the pathogenesis of this disease and the nature of protective immune responses. Through RNAseq transcriptomics analysis we identified the type III interferon (IFNλ) receptor subunit, IFNLR1, as one of the most significant upstream activators of gene expression in the lungs of B. pertussis-infected adult mice during the inflammatory phase. Type III IFNs are key cytokines in immune responses and antiviral defense, but they also have diverse effects on inflammation and pathogenesis in models of infection and disease. We found that IFNλ signaling plays an important role in promoting lung inflammatory pathology in B. pertussis-infected adult mice. However, we have found that pertussis pathogenesis is markedly different in infant mice from that in adult mice, reflecting the age-dependent outcomes of infection in humans. Infected infant wild type mice (inoculated at 7 days of age) do not upregulate IFNλ expression and suffer a fatal disseminating infection with leukocytosis and pulmonary hypertension, features also seen in fatal pertussis cases in human infants. Infant IFNLR1 KO mice (in which IFNλ signaling is abrogated) inoculated at 7 days of age also suffered fatal pertussis infection, with deaths occurring in the same timeframe as those in wild type mice. However, pertussis lethality is age-dependent in young mice, since wild type mice inoculated at 10 days of age survive infection with the same dose. In striking contrast, 80% of IFNLR1 KO mice inoculated at 10 days of age suffered fatal pertussis infection. We hypothesize that age-dependent IFNλ signaling plays an important role in protecting infants against severe and fatal pertussis, and that infants younger than a certain age fail to induce this protective IFNλ response. Therefore, the aims of this exploratory proposal are to (i) investigate age-dependent effects of the IFNλ signaling pathway on outcomes in B. pertussis-infected infant mice, and (ii) determine whether treatment with purified IFNλ provides protection against lethal B. pertussis infection in infant mice. These studies will increase our understanding of age-dependent IFNλ biology, reveal an important immune deficiency in infants that renders them susceptible to lethal pertussis infection and identify a potential novel host-targeted treatment for severe pertussis that will help save the lives of infected infants.