CD4 and viral infectious disease: However, more evidence claims that CD4+ CTLs are a separate Th subset in regards to its differentiation trajectory, effector function and regulatory networks.255,276,277 Furthermore, heterogeneous populations within CD4+ CTLs have been identified in viral infection.277,278 In general, CD4+ CTLs are highly associated with antiviral immunity, however, aberrant CD4+ CTL activity has also been linked with immunopathology in some settings.279–281 For example, CD4+ CTLs contribute to the disease severity during SARS-CoV-2 infection267,282 and lung fibrosis.267,283