Immunological mechanism of long-term stimulation by LPS in macrophages

AIM: To investigate the molecular mechanism and the immunosuppressive phenotype of macrophages under long-term exposure to lipopolysaccharide (LPS). METHODS: We used Ficoll-Hypaque density gradient centri-fugation combined with MicroBeads Separation Kits to separate peripheral blood mononuclear cells from human blood, and then induced the monocytes into macrophages. We observed the morphology of the macrophages by treating the cells with LPS for 48 h, in comparison with a negative control and IFN-γ treatment. ELISA was used to detect the levels of cytokines, such as IL-10, IL-12, IL-6 and TNF-α, and flow cytometry was used to detect the expression of the surface molecules (HLA-DR, CD14, CCR7, HLA-ABC and CD40). To observe the effect of macrophage on T cell proliferation, co-culture experiment was carried out for 6 d. Real-time PCR was used to validate the expression levels of molecules related to MyD88-independent pathway in Toll-like receptor 4 (TLR4) signal pathway. RESULTS: The antigen-presenting ability of the macrophages was reduced and the IL-10 expression level was increased after the cells were treated with LPS for 48 h. We observed a poor proliferative capacity of CD8⁺ T cells after co-culturing of LPS-induced macrophages with CD3⁺ T cells for 6 d. The results of real-time PCR indicated that TRIF, IRF3 and CIITA were down-regulated in LPS-induced macrophages.CONCLUSION: We successfully established a macrophage model in vitro and observed that LPS-induced macrophages into an immunosuppressive phenotype with poor CD8⁺ T cell proliferative capacity, in which MyD88-independent TLR4 signaling pathway was impaired.