Reduction in the severity of IPS by in vivo neutralization of MCP-1 after allogeneic BMT. Lethally irradiated B6D2F₁ mice received allogeneic CCR2^+/+ BM transplants and either control serum (▪) or anti–MCP-1 (▦) as described in “Patients, materials, and methods.” Syngeneic animals received control serum (□)or no treatment (data not shown). Neutralization of MCP-1 resulted in significant reductions in both lung injury (panel A) and BALF (panel B) cellularity compared with allogeneic animals treated with control serum. Data are presented as mean ± SEM and are pooled from 2 similar experiments; n = 8 to 13 per group; ^*P < .05.