Low Dose C-Reactive Protein on Macrophage Response to Bacterial Infection

Objective: C-reactive Protein (CRP) is detected at concentrations of up to 500 µg/mL in the serum during an acute phase response. Low levels of CRP (< 3.0 µg/mL) have been reported in serum of patients suffering from chronic inflammatory diseases such as cardiovascular disease, type 2 diabetes and periodontal disease. The possible effect of low levels CRP on macrophage function is currently unknown. The objectives of this study are, 1) to determine the effects of low levels of CRP on NFκB activation and TNFα production in macrophages, 2) to determine the effects of CRP pre-exposure to subsequent immune response of macrophages to bacterial infection. Method:  RAW 264.7 stably expressing the NFκB-SEAP reporter was treated with CRP (0.5-2.5 µg/mL) for 24 hours after which SEAP assay was carried out. Culture supernatant was collected and subjected to TNFα ELISA. To determine the effects of prior CRP exposure to bacterial infection, cells were pre-exposed to CRP for 24 hours after which the cells were infected with Streptococcus sanguinis. Bacterial induced NFκB activity and TNFα production were measured by SEAP activity assay and ELISA respectively. MTS assay was carried out to determine the effects of CRP treatment or S. sanguinis infection on cell viability. Result: NFκB activity in cells treated with CRP of > 1 µg/mL was significantly up-regulated. Pre-exposure of cells to CRP led to a dose-dependent down-regulation of bacteria-induced NFκB activation in macrophages, with corresponding decrease in TNFα production. Viability of cells did not change significantly after CRP treatment or bacterial infection. Conclusion: We show for the first time that low levels of CRP desensitize macrophages, leading to a significantly poorer NFκB activation and corresponding lower TNFα production upon bacterial infection. Thus, patients who suffer from systemic low-grade chronic inflammation attributed to low concentrations of CRP may respond poorer to bacterial infections.