Bidirectional T:T presentation by the T cells that acquired the MHC II/peptide complexes can induce T-cell hyporesponsiveness and apoptosis.

DO11.10 CD4⁺ T cells after coculture with BALB/c DCs either pulsed with or without OVA_323-339 peptide were purified after 4 hours' coculture. The purified CD4⁺ T cells that had acquired H2-A^d (CD4^II) or H2-A^d:OVA_323-339 complexes (CD4^Iip) were cultured for 24 hours to allow them to engage in T:T interaction. (A) Some cells were harvested and stained with annexin V and propidium iodide to test for apoptosis. DO11.10 CD4⁺ T cells cultured before in medium only (CD4) and DO11.10 CD4⁺ T cells cultured with OVA_323-339 peptide-pulsed BALB/c DCs for 24 hours (CD4-DC/p) were used as controls. The remaining cells were then rested in the presence of blocking anti–H2-A^d antibody for 5 days. (B) The proliferative response to antigen rechallenge with BALB/c DCs was measured by T-cell proliferation. (Background cpm: CD4 = 208.7, CD4^II = 269.7, CD4^IIP = 5619.2, CD4-DC/P = 6027.1.) (C) IL-2 production by the DO11.10 CD4⁺ T cells upon antigen rechallenge was measured by CTLL2 bioassay. (Background cpm: CD4 = 726.3, CD4^II = 1191.9, CD4^IIP = 7203.1, CD4-DC/P = 2986.8.) (D) The response of the DO11.10 CD4⁺ T cells to exogenous IL-2 was measured by T-cell proliferation assay. Error bars indicate SD of triplicates in the same experiment.