Adoptive transfer of CD4⁺ T cells restored oral tolerance in invariant chain–deficient mice. Ii^°/° mice were either left untreated or transferred intravenously on day –8 with (A) 10 × 10⁶ CD4⁺ T cells or CD4-depleted cells from naive C57BL/6 mice or (B) graded numbers of CD4⁺ T cells. All mice were fed either vehicle or DNFB one day later, sensitized epicutaneously with 0.5% DNFB on day 0, and ear challenged with 0.25% DNFB on day +5. The CHS response was determined by ear swelling at various times (A) or 48 hours (B) after hapten challenge. Standard errors were less than 15% (A). Mean increases in ear thickness are indicated by horizontal bars (B). In panel A, ▵ indicates untreated mice fed with vehicle; ▴, untreated mice fed with DNFB; ▪, mice transferred with CD4⁺ T cells and fed with DNFB; and •, mice transferred with CD4-depleted cells and fed with DNFB.