CD4⁺ cells are required for the generation of barrier activity in recipients.

(Left panel) B6-CD4^−/− mice were primed against donor BALB.B MiHA 4 weeks prior to BM-TCD transplantation. Resistance in these CD4 knock-out animals was examined by CFU-IL3 analyses. (Right panel) Spleen + LN cells containing desired number of CD4⁺ cells were transferred from naive B6-wt mice into naive B6-CD4^−/− recipient mice. Groups of B6-CD4^−/− recipients received (1) no lymphocyte transfer prior to priming, (2) a lymphocyte transfer inoculum containing 5.7 × 10⁶ wild-type CD4⁺cells, (3) a lymphocyte transfer inoculum containing 1.6 × 10⁶ wild-type CD4⁺ cells, or (4) a lymphocyte transfer inoculum depleted of CD4⁺ cells (< 4 × 10⁴ CD4⁺ cells) as described in “Materials and methods.” One day following transfer, the CD4^−/− recipient mice were immunized against donor BALB.B cells. Three weeks after priming, the allograft resistance in these adoptively transferred B6-CD4^−/− recipient groups were examined by CFU-IL3 assay following transplantation of 2 × 10⁶ BALB.B BM-TCD.