Competition controls the rate of transition between the peripheral pools of CD4+CD25- and CD4+CD25+ T cells

Recent reports have hinted that it is possible to regenerate CD4(+)CD25(+) regulatory T cells (Treg) from CD4(+)CD25(-) cells, a phenomenon termed conversion. We evaluated the relative contribution of this process to the Treg pool by transferring purified populations of CD4(+) T cells into T cell-deficient mice. We report that conversion of CD25(-) cells into the CD4(+)CD25(+)Treg pool is minor if other bona fide CD25(+) Tregs are present. Moreover, in the same hosts, the loss of CD25 expression by a population of Tregs also decreases in the presence of co-injected CD4(+)CD25(-) cells. Thus, the rate of exchange between CD25(-) and CD25(+) T-cell populations is determined by the presence or absence of T-cell competitors. Our results attest for the role of competition in the contribution of different T-cell subsets for the regeneration of the peripheral CD4(+) T-cell pool during lymphopenia.