CD4(+) T cell division in irradiated mice requires peptides distinct from those responsible for thymic selection

We investigated the mechanism by which alpha/beta T cells expand upon trans fer to T cell-deficient host mice by injecting carboxyfluorescein diacetate succinimidyl ester-labeled T cells into mice depleted of T cells by sublet hal irradiation. We found that CD4(+) T cells divided when transferred to i rradiated hosts and that the division of more than half of these cells requ ired class II expression. However, division of transferred CD4(+) T cells d id not occur in irradiated hosts that expressed class II molecules occupied solely by the peptide responsible for thymic selection, indicating that pe ptides distinct from those involved in thymic selection cause the division of CD4(+) T cells in irradiated mice. These data establish that class II-bo und peptides control the expansion of CD4(+) T cells transferred to T cell- deficient hosts and suggest that the same peptides contribute to the mainte nance of T cell numbers in normal mice.