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A Mechanism for the Impaired IFN-γ Production in C-C Chemokine Receptor 2 (CCR2) Knockout Mice: Role of CCR2 in Linking the Innate and Adaptive Immune Responses1

We have recently shown that mice with a targeted disruption of CCR2, the receptor for monocyte chemoattractant protein-1, have markedly impaired recruitment of macrophages to sites of inflammation. An unexpected finding in the CCR2−/− mice was a dramatic decrease in the production of IFN-γ after challenge with purified protein derivative of Mycobacterium bovis. In this study, we have investigated the mechanism of this cytokine production defect. In vitro, direct activation of splenocytes with CD3/CD28 Abs failed to reveal any differences in IFN-γ production between CCR2+/+ and CCR2−/− mice. However, after immunization, the number of Ag-specific, IFN-γ-producing cells in the draining lymph nodes was decreased by 70% in the CCR2−/− mice, suggesting an in vivo trafficking defect. Direct measurement of cell trafficking with fluorescently labeled CFA revealed a marked decrease in the number of monocytes/macrophages migrating to the site of immunization and to the draining lymph nodes in the CCR2−/− mice. The data suggest that impaired trafficking of APCs in the CCR2−/− mice contributes to the defect in IFN-γ production. These data support the idea that CCR2-positive monocytes/macrophages are critical in linking the innate and adaptive immune responses.