OVA cross-presentation by MSCs induces the proliferation of OT-I CD8⁺ T cells in vitro and in vivo. (A) Flow cytometry analysis of surface expression of H2-K^b on untreated or IFN-γ–stimulated peritoneal macrophage collected form WT and β2-microglobulin^−/− mice. Dotted line, solid line, and bold line represent the isotype, untreated cells, and IFN-γ–treated cells, respectively. (B) β2-microglobulin^−/− mice were injected with 3 × 10⁶ CFSE⁺-OT-1 CD8⁺ T cells intravenously followed 3 hours later by an intraperitoneal injection of 4 × 10⁶ OVA-primed IFN-γ–stimulated MSCs, 4 × 10⁶ OVA-pulsed LPS-stimulated DCs, or PBS intraperitoneally. At 5 days later, lymph nodes were retrieved, and CD8⁺ T-cell proliferation was assayed by flow cytometry analysis of the CFSE level.