CCL21 costimulation is effective during early T-cell activation. DO11.10 T cells were cocultured with OVA_323-339-loaded congenic splenocytes in the presence or absence of CCL21 (100 nM). (A) CCL21 was added at indicated times, and proliferation of T cells was analyzed after 48 hours. Statistical significance was determined using analysis of variance comparison of peptide-stimulated T-cell proliferation without or with CCL21 added at indicated times. *P < .05 compared with “no chemokine.” (B) Up-regulation of early activation markers CD69 and CD25 on CD4⁺ KJ1-26⁺ DO11.10 T cells 8 and 24 hours after Ag-specific T-cell activation in the presence or absence of CCL21 (100 nM). One representative experiment of 3 is shown. (C) IL-2 production of DO11.10 T cells after 24 hours as determined by intracellular staining. One representative experiment of 2 is shown. In panels B and C, numbers indicate percentage of positive cells and mean fluorescence intensity (MFI), respectively.