In the present study aortic murine smooth muscle cell (SMC) antigen presentation capacity was evaluated using the E alpha-GFP/Y-Ae system to visualize antigen uptake through a GFP tag and tracking of E.. peptide/MHCII presentation using the Y-Ae Ab. Stimulation with IFN-gamma (100 ng/mL) for 72 h caused a significant (P < 0.01) increase in the percentage of MHC class II positive SMCs, compared with unstimulated cells. Treatment with E alpha-GFP (100 mu g/mL) for 48 h induced a significant (P < 0.05) increase in the percentage of GFP positive SMCs while it did not affect the percentage of Y-Ae positive cells, being indicative of antigen uptake without its presentation in the context ofMHC class II. After IFN-gamma-stimulation, ovalbumin-(OVA, 1 mg/mL) or OVA(323-339) peptide-(0.5 mu g/mL) treated SMCs failed to induce OT-IICD4(+) T cell activation/proliferation; this was also accompanied by a lack of expression of key costimulatory molecules (OX40L, CD40, CD70, and CD86) on SMCs. Finally, OVA-treated SMCs failed to induce DO11.10-GFP hybridoma activation, a process independent of costimulation. Our results demonstrate that while murine primary aortic SMCs express MHC class II and can acquire exogenous antigens, they fail to activate T cells through a failure in antigen presentation and a lack of costimulatory molecule expression.