We previously reported that V beta 3(+) CD4(+) T cells maintained a protracted expansion, with the phenotypes of memory Th2 cells, for 30 days in C57BL/6 (B6) mice implanted with SEA-containing mini-osmotic pumps. In the present Study, we followed the fate of V beta 3(+) CD4(+) T cells in CD28(-/-) mice. V beta 3(+) CD4(+) T cells increased to a degree similar to that of B6 V beta 3(+) CD4(+) T cells until day 10 after implantation, then declined rapidly reaching the control level by 28 days. Remaining V beta 3(+) CD4(+) T cells at that time did not exhibit memory phenotypes nor Th2-deviated responses. The rapid drop in VP3(+) CD4(+) T cells in CD28(-/-) mice was attributable to upregulated induction of apoptosis owing to marginal inductions of Bcl-2 and Bcl-x(L). Collectively, these data indicate CD28 to play critical roles in the generation and maintenance of SEA-reactive CD4(+) T cells in vivo. (c) 2006 Elsevier Inc. All rights reserved.