INTRADERMAL INJECTION OF MONOCYTE CHEMOATTRACTANT PROTEIN-1 INDUCES EMIGRATION AND DIFFERENTIATION OF BLOOD MONOCYTES IN RAT SKIN

Background: Monocyte chemoattractant protein-1 (MCP-1) is a potent che moattractant for blood monocytes in vitro. Recent studies in MCP-1-tra nsgenic mice revealed that the local production of MCP-1 caused monocy te infiltration. However, the kinetics of monocyte infiltration after the production of MCP-1 or the amount of MCP-1 necessary for monocyte recruitment are not known. Methods: We purified recombinant rat MCP-1 expressed in COS-7 cells, and injected it into rat skin. The infiltrat ing cells were examined by immunohistochemistry and ultrastructural pe roxidase cytochemistry. Results: Rat recombinant MCP-1 had a molecular mass of approximately 30 kD and exhibited the peak monocyte chemotact ic activity at 10(-9) M. One microgram of MCP-1 caused intra-and extra vascular accumulation of mononuclear cells 3 h after injection. The ce lls were ED1+, indicating they were blood monocytes. The infiltration of mononuclear cells peaked at 12-24 h, and most of them were TRPM-3and ED3+, characteristic to exudate macrophages. None of the cells exp ressed ED2 or Ki-M2R antigens, markers for resident macrophages, until 3 days after injection. There was no uptake of [H-3]thymidine by the infiltrating cells. Ultrastructural peroxidase cytochemistry confirmed that the infiltrating cells were monocytes and exudate macrophages. T he number of OX8+ lymphocytes also peaked at 12 h, consisting of appro ximately 9% of the total infiltrating cells. Conclusion: These results indicate that MCP-1 attracts blood monocytes as early as 3 h and the infiltrating monocytes differentiate into exudate macrophages in loco. However, this effect was transient and the infiltration of monocytes did not result in tissue damage.