Protection of thymic stroma and secondary lymphoid organs by regulatory T cells improves T-cell immunity. (A) H&E staining of thymus extracted from Balb/c recipients which received FVB TCD-BM alone (BM), with Tcons (Tcon), or with Tcons and Tregs (Tcon + Treg) (200 × magnification). Results from day 60 show small and involuted thymus with disorganized medullary, M, and cortical, C, regions in Tcon recipients compared with animals that received Tcons and Tregs. (B,C) Survival of adult thymectomized Balb/c recipients co-transplanted with BM alone (▾), with Tcons (●) or with Tcons and Tregs (O), uninfected (B). ●, n = 10 vs O, n = 9, P = .001), or challenged with MCMV on day 30 after transplantation (C): ●, n = 14 vs O, n = 13, P < .001). P is NS for BM versus Tcon + Treg. Data combined from 2 separate experiments. (D) H&E staining of peripheral lymph node sections from Balb/c recipients, which had received donor FVB TCD-BM with Tcons alone (Tcon) or with Tcons and Tregs (Tcon + Treg) on day 60 after transplantation (200 × magnification). In Tcon recipients, peripheral lymph nodes are collapsed with hyalinalization and hypoplasia. (E,F) Number of CD4⁺ and CD8⁺ T cells in peripheral lymph node of animals with Tcons alone (Tc, ▭) or with Tcons and Tregs (Tc + Tr, —) (CD4⁺, P = .002; CD8⁺, P = .002). (G-J). Spleen to body weight ratio (P = .037) and number of CD4⁺ (P = .011), CD8⁺ (P = .032), and CD19⁺ cells (.006) in spleens of animals that received BM and Tcons alone (Tc, ▭) or with BM with Tcons and Tregs (Tc + Tr, —). Data shown for day 60, representing at least 4 animals, with mean values (± SE).