In vitro analysis of CD4 SP thymocyte survival. (A) CD4 SP thymocytes from GIMAP1^fl/flhCD2-iCre⁻ (■) and GIMAP1^fl/flhCD2-iCre⁺ (□) mice were FACS-purified and cultured with (ii,iv) or without (i,iii) IL-7. Immature (i,ii) and mature (iii,iv) CD4 SP cells were defined on the basis of Qa2 and CD24 expression. Live cells were determined by annexin V staining. Plots show mean percentage of live cells (± SD) for 3 mice of each genotype. *P < .05; **P < .01; ***P < .001 (unpaired 2-tailed Student t test). (B) IL-7 receptor expression and function in thymocytes. (Left) Expression of IL-7R (CD127; white histogram) compared with control stain (gray histogram) on CD4 SP thymocytes (as determined by flow cytometry) from (i) GIMAP1^fl/flhCD2-iCre⁻ and (ii) GIMAP1^fl/flhCD2-iCre⁺ mice. (Right) Responses of total thymocytes to IL-7 as determined by pSTAT5 induction. Total thymocytes were incubated at 37°C in medium with or without IL-7 for 30 minutes before fixing, permeabilizing, staining with anti–phospho-STAT5 antibody and analyzing by flow cytometry.