CD4⁺ T cells can acquire MHC II/peptide complexes from antigen-presenting cells.

(A) CD4⁺ T cells purified from DO11.10 transgenic mice were cocultured with bone-marrow–derived BALB/c DCs pulsed with different concentration of OVA_323-339 peptide. After overnight coculture, the cells were harvested and stained with PE-conjugated anti–H2-A^d and FITC-conjugated anti-CD4 (solid line). The level of H2-A^d expression on DO11.10 CD4⁺ T cells was measured by flow cytometry. Only the CD4⁺ population was gated for analysis. (DO11.10 CD4⁺ T cells in medium control [shaded area]: mean fluorescence intensity [MFI] = 4.08; % in M1 gate = 3.04%.) (B) CD4⁺ T cells from DO11.10 transgenic mice were cocultured with BALB/c DCs pulsed with 10 μg/mL OVA peptide (Table 1). Cells were harvested at different time points. The level of H2-A^d on DO11.10 CD4⁺cells is presented by histograms. The shaded portion indicates the level at 0 h; the solid line, 3 h; and the dotted line, 20 h. (C) 3A9 CD4⁺ T-cell hybridoma were cocultured with DAP.3-H2-A^k transfectant prepulsed with or without 10 μg/mL HEL_46-61 peptide. The level of H2-A^k expression and H2-A^k:HEL_46-61complex on peptide-pulsed DAP.3 transfectant was detected by staining with FITC-conjugated anti–H2-A^k or C4H3 monoclonal antibodies specific for H2-A^k:HEL_46-61 complex followed by FITC-conjugated antimouse IgG (shaded area). After overnight coculture, 3A9 cells were harvested and stained for H2-A^k and complex expression as mentioned, finally with PE-conjugated anti-CD4. The levels of H2-A^k expression and C4H3 epitope on 3A9 cells (solid line) were detected by flow cytometry. Only CD4⁺ populations were gated for analysis. (DAP.3 control [solid line] for C4H3 staining: MFI = 4.41, % in M1 gate = 1.29%; for H2-A^k staining: MFI = 3.44, % in M1 gate = 0.35%; 3A9 cells in medium control [shaded area] for C4H3 staining: MFI = 2.96; % in M1 gate = 0.66%; for H2-A^kstaining: MFI = 3.23; % in M1 gate = 0.81%.)