Rescue of cytotoxic function in the CD8α knockout mouse by removal of MHC class II

CD8 plays an important role in the activity of cytolytic T cells (CTL). However, whether or not CD8 is required for the development of CTL has not been clearly determined. Cytotoxic activity in the CD8α knockout mouse is difficult to induce, and has only been demonstrated against allogenic MHC targets. The lack of cytotoxicity may result from impaired lineage commitment of CTL in the absence of CD8, or diminished competitiveness during selection against (unimpaired) development of CD4⁺ T cells on MHC class II (MHC II). To differentiate between these possibilities, we have generated a double-knockout mouse (MHC II^-/- CD8α^-/-). In MHC II^-/-CD8α^-/- mice, developing MHC class I (MHC I)-reactive thymocytes cannot rely upon CD8 for selection, but they also cannot be overwhelmed by efficient selection of MHC II-reactive thymocytes. In this mouse, a large, heterogeneous population of peripheral coreceptor double-negative (DN) and CD4⁺ T cells develops. Peripheral DN T cells are fully functional CTL. They display cytolytic activity against allogeneic MHC, and against syngeneic MHC following lymphocytic choriomeningitis virus (LCMV) infection. Cells from LCMV-infected mice bind more MHCI tetramer at lower concentrations than their wild-type CTL counterparts. These results demonstrate unequivocally that CD8 is not required for commitment of thymocytes to the CTL lineage.