Classically activated macrophages (CAMphi) have been described as a major effector cell on the host's innate immunities. However, CAMphi are not generated in immunocompromised hosts whose alternatively activated macrophages (AAMphi) predominate. In this study, the mechanism by which AAMphi suppress the ability of resident macrophages (RMphi) to generate CAMphi was investigated. AAMphi were isolated from peritoneal exudates of mice 2 days after third-degree thermal injuries affecting 15% total body surface area. CAMphi were generated from RMphi (peritoneal Mphi from normal mice) through stimulation with CpG DNA, a typical CAMphi inducer. RMphi did not polarize to CAMphi when they were cultured with AAMphi in a dual-chamber Transwell even when supplemented with CpG DNA. In addition, RMphi stimulated with CpG DNA did not convert to CAMphi when they were cultured with the culture fluids of AAMphi (AAMphi Culture-Sup). AAMphi Culture-Sup contained IL-6, IL-10, CCL17, PGE(2), and TGF-beta. Among these, CCL17 and IL-10 inhibited CAMphi generation. The ability of AAMphi Culture-Sup to inhibit CAMphi generation was eliminated when the Culture-Sup was treated with a mixture of mAbs directed against CCL17 and IL-10. These results indicate that CCL17 and IL-10 released from AAMphi inhibit CAMphi generation from RMphi stimulated with CpG DNA.