Abstract Rationale Healthy children are at higher risk of developing severe influenza A virus (IAV) pneumonia compared to healthy adults. We previously demonstrated in a mouse model of IAV pneumonia that inflammation persists in juvenile mice despite adequate viral clearance. This suggests differences in the immune response contribute to susceptibility to IAV pneumonia. In adult mice, CD4+CD25+FoxP3+ Regulatory T (Treg) cells promote the resolution of inflammation and tissue repair during lung injury, however their role in juveniles is less clear. Adult Tregs reduce inflammation by suppressing proliferation of conventional T (Tcon) cells and modulating innate immune cell function. Depletion of adult Tregs during severe IAV pneumonia results in increased bronchoalveolar lavage fluid (BALF) IL-6, lung neutrophils, and lung injury. To assess the suppressive capacity of juvenile Treg cells, we used the Rag1-/- model of T cell-induced colitis and depleted Tregs in juvenile mice infected with IAV. Methods Juvenile (24-28 days) and adult mice (8-14 weeks) were infected intratracheally with a sublethal dose of IAV (A/WSN/33 H1N1). Juvenile FoxP3DTR mice were treated with intraperitoneal diphtheria toxin beginning 5 days post influenza infection to deplete Treg cells. Cytokines in BALF were measured by enzyme-linked immunosorbent assay, immune cells were quantified with flow cytometry, and lungs were assessed by standard H&E staining. Murine splenic Treg cells from adult and juvenile mice were used in standard lymphocyte suppression assays. Rag1-/- mice were injected with adult Tcon cells to induce colitis and rescued with juvenile or adult Treg cells. Results Juvenile mice infected with IAV exhibited increased lung neutrophils, monocyte derived alveolar macrophages, and lung injury despite increased numbers of lung Treg cells compared to adult mice. Depletion of Tregs during IAV infection in juvenile mice did not alter BALF levels of IL-6, quantities of lung neutrophils, and degree of lung injury. Juvenile Treg cells displayed reduced capacity to suppress proliferation of Tcon cells in vitro compared to adult Treg cells. Rag1-/- mice with Tcon cell induced colitis rescued with juvenile Treg cells exhibited increased tissue injury and cell infiltrates compared to mice rescued with adult Treg cells. Conclusions These data suggest that juvenile Treg cells have reduced suppressive capacity, which delays inflammation resolution and impairs recovery from influenza pneumonia. Further understanding of the suppressive function of juvenile Treg cells may provide insights into strategies to mitigate lung injury in children with severe viral pneumonia. Funding Supported by the Parker B. Francis Foundation and NHLBI 1K08HL176208. This abstract is funded by: Parker B. Francis Foundation and NHLBI 1K08HL176208
Prigge et al. (Fri,) studied this question.