Antigen-specific T-cell activation by B cells leads to generation of T cells with aberrant phenotype. Naive splenic CD4⁺ T cells were cocultured in vitro with mature DC or B cells, both loaded with specific antigenic peptide. After 72 hours, proliferation and expression of surface markers were determined by fluorescence-assisted cell sorting (FACS). (A) Specific antigenic peptide loaded B cells efficiently induce proliferation and up-regulation of CD25 in CD4⁺ T cells. In contrast to DC, however, B cells fail to down-regulate CD62L. (B) B cells remain unable to down-regulate CD62L even at high antigen doses. (C) The failure to downmodulate CD62L levels by B cells is not restricted to a two-dimensional environment as it is also seen in a three-dimensional environment using a collagen matrix. (D) CD62L is down-regulated in cultures in which DC were added (10% of corresponding B-cell number). The data shown are representative of 3 to 5 independent experiments.