The naive T-cell subpopulation is largely responsible for the induction of immune reconstitution disease by bulk CD4 T cells. (A) CD4 T cells from uninfected IFNγ reporter mice were FACS sorted into eYFP⁺ and eYFP⁻ cells. Purified cells (5 × 10⁵) of each subset were then separately transferred into M avium–infected TCRα^−/− mice (n = 6 control mice, n = 5 recipients of eYFP⁻, and n = 4 recipients of eYFP⁺ CD4 T cells). On day 10 after transfer, the recipient mice were analyzed for the donor cell marker and eYFP expression. (B) CD4 T cells from foxp3 reporter mice were FACS purified into CD44^lofoxp3⁻, CD44^hifoxp3⁻, and foxp3⁺ cells. Purified cells (5 × 10⁵) of each subset were then separately transferred into M avium–infected TCRα^−/− mice, and recipients were monitored for weight loss and survival (n = 5: recipients of CD44^lofoxp3⁻ or CD44^hifoxp3⁻ cells and n = 3: recipients of foxp3⁺ T cells). In panels A and B, the shaded area between the traveling error bars represents the SD. All data are representative of 2 independent experiments performed.