FIG 7.

Three suggested mechanisms for CD4⁺ T-cell-mediated CTL priming. (1) Three-cell interaction, where an antigen-specific CD4⁺ T cell and CD8⁺ T cell are in close proximity, interacting through the same DC. The CD4⁺ T cell directly influences CTL priming by cytokine release. (2) Sequential two-cell interaction, where the DC is licensed by an antigen-specific CD4⁺ T cell. This licensing happens by a combination of signals 1, 2, and 3; our data support this model best. (3) Sequential; CD4⁺ as antigen-presenting cell (APC), where the CD4⁺ T cell acquires antigen-presenting cell capacities by receiving peptide-MHC complexes. One proposed mechanism is via exosomes.