Enhanced pulmonary RANTES expression correlates with the influx of donor T cells into the lung after allogeneic SCT. Lethally irradiated B6D2F1 mice received SCT from either syngeneic B6D2F1 (□) or allogeneic B6 (▪) donors. RNA was isolated from the lungs of animals on days 2, 7, 14, 28, and 42 after transplantation, and RANTES expression was determined by RNase protection assay. Protein levels of RANTES present in BALF were determined by ELISA. On days 2, 7, and 14, T-cell chimerism was determined in the BALF of mice that received transplants using flow cytometry and antibodies to CD45.1 (donor marker) and CD45.2 (host marker). (A) Two days after SCT, RANTES mRNA expression in the lung is low in each SCT group and correlates with low BALF cellularity and with the lack of donor T cells in the bronchoalveolar space. (B) RANTES mRNA expression is increased by day 7 after allo-SCT, remains elevated compared to syngeneic controls throughout the observation period, and is associated with increases in BALF protein expression on days 14 and 42. (C) The early rise in RANTES mRNA expression correlates with the influx of donor-derived T cells by day 7 (see first paragraph under “Results”) and day 14 after SCT. Data are presented as mean ± SEM; n = 3 to 10 animals per group; ^*P < .05, (▪, allogeneic) versus (□, syngeneic).