Notch Ligand Delta-like 4 Promotes Regulatory T Cell Identity in Pulmonary Viral Infection

Regulatory T (T reg ) cells establish tolerance, prevent inflammation at mucosal surfaces, and regulate immunopathology during infectious responses. Recent studies have shown that Delta-like ligand 4 (Dll4) was upregulated on APC after respiratory syncytial virus (RSV) infection, and its inhibition leads to exaggerated immunopathology. In the present study, we outline the role of Dll4 in T reg cell differentiation, stability, and function in RSV infection. We found that Dll4 was expressed on CD11b + pulmonary dendritic cells in the lung and draining lymph nodes in wild-type BALB/c mice after RSV infection. Dll4 neutralization exacerbated RSV-induced disease pathology, mucus production, group 2 innate lymphoid cell infiltration, IL-5 and IL-13 production, as well as IL-17A + CD4 T cells. Dll4 inhibition decreased the abundance of CD62L hi CD44 lo Foxp3 + central T reg cells in draining lymph nodes. The RSV-induced disease was accompanied by an increase in Th17-like effector phenotype in Foxp3 + T reg cells and a decrease in granzyme B expression after Dll4 blockade. Finally, Dll4-exposed induced T reg cells maintained the CD62L hi CD44 lo central T reg cell phenotype, had increased Foxp3 expression, became more suppressive, and were resistant to Th17 skewing in vitro. These results suggest that Dll4 activation during differentiation sustained T reg cell phenotype and function to control RSV infection.