NRP1 and susceptibility to pneumonia measurement: Recently, a study using a murine model of pneumonia virus of mice (PVM), a mouse-specific pneumovirus related to RSV, revealed that surface expression of Semaphorin 4a (Sema 4a), the ligand for Nrp-1, by pDCs contributes to the expansion of Nrp-1+ Tregs that prevent the development of PVM-induced bronchiolitis in early life and subsequent asthma following reinfection [130].