CD4 and colitis: To determine whether the B cell-driven augmentation of peripheral Treg cell frequencies relies on a numerical increase of either B or T lymphocyte populations in the inoculum, Rag−/− hosts injected with a given number of CD4+CD25− T cells (either 3 or 6 × 106) also received an amount of B cells corresponding to the numbers present either in the colitogenic low dose (10 × 106) or in the colitis-protective high dose (25 × 106) of CD25− splenocytes (Supplementary Figure 4A,B).