M onocyte chemoattractant protein-1 (MCP-1) is a CC chemokine that stimulates monocyte recruitment when injected into tissues of healthy animals. However, the function of this chemokine in models with preexisting inflammation is not known. Therefore, MCP-1 was superfused over the mesentery of naive rats or rats with chronic adjuvant-induced vasculitis. MCP-1 elicited increased leukocyte transendothelial migration in adjuvant-immunized rats compared with naive animals. Surprisingly, histology revealed that neutrophils constituted the majority of leukocytes recruited in adjuvant-immunized animals. In vitro, MCP-1 was also able to induce chemotaxis of neutrophils isolated from adjuvant-immunized rats but not from naive rats. Flow cytometry revealed novel expression of the CC chemokine receptors CCR1 and CCR2 on neutrophils from adjuvant-immunized animals. In naive animals, an antibody against CD18 blocked leukocyte adhesion and emigration in response to MCP-1. In adjuvant-immunized animals, leukocyte adhesion was reduced by antibodies against the α₄-integrin but not by antibodies against CD18. However, the CD18 antibody did block emigration. To our knowledge, this study is the first to show increased sensitivity to a CC chemokine in a model with preexisting inflammation, and altered leukocyte recruitment profiles in response to MCP-1. It also demonstrates that CD18 is required for chemokine-induced leukocyte transendothelial migration, independent of its known role in mediating firm adhesion.J. Clin. Invest. 103:1269–1276 (1999).