Figure 1.

Two-step priming model. During the first step of T cell priming (left), CD8⁺ T cells and CD4⁺ T cells are initially activated independently by different DC subtypes that present antigen on MHC class I and class II, respectively. In the second step of priming (right), recently activated CD4⁺ and CD8⁺ T cells interact with the same lymph node-resident cDC1 co-expressing MHC class I and MHC class II epitopes. Helped CD8⁺ T cells undergo optimal priming by signaling via various costimulatory and cytokine signals that emerge from the helped cDC1, resulting in an optimal CTL effector program (11).