IL21 and cancer: IL-21 also regulates CTL cytotoxic activity and several preclinical studies of cancer immunotherapy showed that IL-21 sustains antitumor CTL functions [40]. Il21r−/− mice have normal numbers of CD8+ T cells; however, they showed reduced CTL responses to a vaccinia-virus encoded antigen, further suggesting a role of IL-21 in CTL responses [39].