TNFRSF9 and viral infectious disease: It has been confirmed in several studies that some CD4+T cells with cytotoxic potential were specifically induced at the site of infection during virus infection and implicated in the killing of virus-infected cells [32–33,44–46], and the IL-2 signaling, low antigen dose, as well as OX40 and 4-1BB stimulation may promote the development of CD4+T cells with cytotoxic potential [47–49].