Host B cells inhibit donor T-cell proliferation. Purified CFSE-labeled Balb/c T cells were transplanted into lethally irradiated μMT (n = 4) or wild-type (n = 4) recipient mice. (A) Four hours after BMT, donor (H-2^d+) T cells were recovered from the spleens of wild-type (solid line) or μMT recipients (dotted line) and the expression of the activation marker CD69 determined relative to isotype control (solid histogram). (B) At 4 and 120 hours after BMT, proliferation of donor T cells (H-2^d+) was determined by reduction of CFSE intensity. (C) ModFit analysis of proliferation index in μMT (□, n = 5) and wild-type (▪, n = 5) allogeneic recipients. Results represent mean ± SE of individual animals *P < .05 versus wild-type. (D) Donor cells (H-2^d+) were stained for annexin V and 7-AAD and proportions of annexin V⁺/7-AAD^– apoptotic cells determined as described in “FACS and cytokine analysis.” (E) Absolute numbers of apoptotic cells per spleen in μMT (□, n = 5) and wild-type (▪, n = 5) allogeneic recipients. Results represent mean ± SE of individual animals.