Re-expression of OX40 on sorted CD4⁺ T cells by allospecific stimulation. OX40 sorted splenic T cells were cultured in vitro with low-dose IL-2 and assessed for OX-40 expression on the days indicated. After the addition of IL-2 on day 20, the population increased to greater than 95% positive in 3 days and OX40 expression decreased to 16% by day 27, at which time they were stimulated with F1 stimulator cells. After 4 days, greater than 95% of the cells were again OX40⁺. Control cultures had 13% positive cells.