Immunology - Do MHC I ligands occur on CD4+ and CD8+ T-cells? (Oct/05/2009 )

The APCs (antigen-presenting cells) present (display) foreign peptides via MHC (I or II) that leads to activation of naive T-cells. MHC I found on all nucleated cells provide these cells the ability to display foreign peptides (e.g., viral peptide) for recognition by experienced (previously activated) CD8+ T-cells (the killer/cytotoxic T-cells) in the event, for example, a virus has infected. This website www.roitt.com might give some additional insights.

Got it. As a rule, at least two signals are required to activate naive T-cells. One signal is via MHC-I that interacts with T-cell receptors found on T-cells. Another is via the costimulatory molecule, one being "B7." B7 is found mainly on professional APCs (antigen-presenting cells) and interacts with CD28 found on T-cells. Thus, APCs possess both MHC-I and MHC-II along with co-stimulatory molecules such as B7 and have the capacity to activate naive CD8+ T-cells and CD4+ T-cells. Since T-cells do not have the qualities of APCs (in particular the aforementioned co-stimulatory molecules), they cannot activate naive T-cells.