Feline morbillivirus elicits robust neutralizing antibodies in domestic cats without cross-neutralizing activities against canonical morbilliviruses

Feline morbillivirus (FeMV), first identified in 2012, is a member of the genus <i>Morbillivirus</i>. Like other morbilliviruses, FeMV utilizes signalling lymphocytic activation molecule (CD150 or SLAMF1) as a cellular receptor; however, it is genetically distinct and notable for its unique tissue tropism, particularly its ability to establish persistent infections in the feline kidney. In this study, we established a plaque assay system using Vero cells stably expressing feline SLAMF1 and quantitatively evaluated neutralizing antibodies against FeMV in the sera of domestic cats. The results showed that FeMV-infected feline sera contained exceptionally high titres of FeMV-specific neutralizing antibodies, often far exceeding those observed in animals infected with other morbilliviruses. Importantly, sera with high neutralizing activity against FeMV exhibited undetectable cross-neutralizing activity against other morbilliviruses. In contrast, significant cross-neutralization was observed among non-FeMV morbilliviruses. These findings indicate that FeMV surface glycoproteins are antigenically distinct, underscoring the immunological uniqueness of FeMV. Moreover, while feline SLAMF1 functions as a receptor for various morbilliviruses, FeMV utilized only feline SLAMF1. These data suggest that the receptor-binding domain of FeMV is uniquely adapted to the feline SLAMF1. This study offers valuable tools and insights for FeMV research, advancing our understanding of its antigenicity and host receptor usage.