MHC II controls growth and differentiation of transformed B cells. (A) Frequency of MHC II⁻ leukemic cells recovered from Rag1^−/− recipients that had received A1, F6, or G7 cells, sorted on the basis of MHC II expression prior to inoculation. (B) B3, D5, F6, or G7 cells, transduced with either GFP or MHC II Ab1 and Aa chains were mixed in equal ratios and transferred into Rag1^−/− recipients. GFP and MHC II expression in mixtures of F6 cells before (pre) and after (post) transfer in Rag1^−/− recipients is shown (left). Also shown is the frequency of GFP⁺MHC II⁻ leukemic cells recovered from Rag1^−/− recipients that had received mixtures of GFP or MHC II–transduced B3, D5, F6, or G7 cells (right). *P < .001 by Student t tests. (A-B) Symbols represent individual recipient mice and group means (plus or minus SEM) are also shown. (C) B3 and F6 cells were transduced either with GFP or MHC II Ab1 and Aa chains. GFP⁺ and MHC II⁺ cells from the same cell line were mixed at equal ratios and cocultured. Mean change (plus or minus SEM) from starting frequency of MHC II⁻ cells over time in such cocultures (B3, n = 2; F6, n = 4). *P = .03; **P = .08 by 1-way ANOVA on ranks. (D) Growth rate, indicated by the specific growth rate constant, μ, of the exponential growth phase as measured with the IncuCyte live cell analysis system, of B3 or F6 cells transduced either with GFP or MHC II Ab1 and Aa chains. Symbols represent separate cultures. *P < .001 by 2-tailed Mann-Whitney rank-sum tests. (E) CD24 and BP-1 expression in parental F6 cells and those transduced either with GFP or MHC II Ab1 and Aa chains. (F) Expression of differentially expressed genes (at least twofold, q = 0.05) between MHC II⁻ and MHC II⁺ F6 cells (left), expression of selected MHC II, immunoglobulin, and B-cell differentiation genes in the same cells (middle), and expression of selected genes downregulated in MHC II⁺ F6 cells (right).