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MCP-1/CCL2 plays a critical role in monocyte recruitment into sites of immune responses and cancer. However, the role of other MCPs remains unclear. In this study, we generated a novel MCP-1-deficient (designated as MCP-1(Delta/Delta)) mouse model by deleting a 2.3-kb DNA fragment from the mouse genome using the Cre/loxP system. MCP-1 was not produced by LPS-activated MCP-1(Delta/Delta) macrophages; however, the production of MCP-3, coded by the immediate downstream gene, was significantly increased. In contrast, macrophages from another mouse line with a neo-gene cassette in intron 2 produced a significantly lower level of MCP-1 and MCP-3. Decreased MCP-3 production was also detected in previously generated MCP-1-deficient mice in which a neo-gene cassette was inserted in exon 2 (designated as MCP-1 knockout (KO)). Altered MCP-1 and/or MCP-3 production was also observed in vivo in each mouse model in response to i.p. injection of thioglycolate or zymosan. The up- and down-regulation of MCP-3 production in MCP-1(Delta/Delta) and MCP-1 KO mice, respectively, provided us with a unique opportunity to evaluate the role for MCP-3. Despite the increased MCP-3 production in MCP-1(Delta/Delta) mice, thioglycolate- or zymosan-induced monocyte/macrophage accumulation was still reduced by approximately 50% compared with wild-type mice, similar to the reduction detected in MCP-1 KO mice. Thus, up-regulated MCP-3 production did not compensate for the loss of MCP-1, and MCP-3 appears to be a less effective mediator of monocyte recruitment than MCP-1. Our results also indicate the presence of other mediators regulating the recruitment of monocytes in these models.
Takahashi et al. (Wed,) studied this question.