Conditional NKT Cell Depletion in Mice Reveals a Negative Feedback Loop That Regulates CTL Cross-Priming

NKT cells are unconventional T cells whose biological role is incompletely understood. Similar to TH cells, activated NKT cells can cause dendritic cell (DC) maturation, which is required for effective CTL responses. However, it is unclear whether and how NKT cells affect CTLs downstream of the DC maturation phase. This is partially due to the lack of techniques to conditionally deplete NKT cells in vivo. To overcome this problem, we have developed two approaches for this purpose in mice: the first is based on mixed bone marrow chimeras where Jα18 knockout and depletable CD90 congenic bone marrow is combined, and the second used PLZFCre × iDTR bone marrow chimeras, which target innate-like T cells. Using these tools, we found that NKT cell depletion at 20 h, that is, after initial DC activation, did not render CTLs helpless, as CD40L signaling by non-NKT cells sufficed. Instead, NKT cell depletion even augmented CD8 T cell expansion and cytotoxicity by mechanisms distinct from reduced STAT6 signaling. These findings revealed a negative feedback loop by which NKT cells control CTL cross-priming downstream of DC maturation. The techniques described in this study expand the toolbox to study NKT cells and other unconventional T cell subsets in vivo and uncovered a hidden immunoregulatory mechanism.