Probiotic bacterial strains differentially modulate macrophage cytokine production in a strain-dependent and cell subset-specific manner

Gut mucosal macrophages play a pivotal role in driving mucosal immune responses, resulting in either activation of inflammatory immune responses to pathogenic challenge or tolerance to beneficial luminal contents such as food and commensal bacteria. Macrophage responses elicited are dependent on tissue environment and the resulting cell subset, where homeostatic macrophages resemble the M2 macrophage subset and inflammatory macrophages resemble M1s. Probiotics can modulate macrophage function with outcome dependent on subset present. Using a THP-1 monocyte cell line-derived model of CD14(high/low) M1 and M2 macrophages, the aim of this study was to investigate the immunomodulatory effects of a panel of heat-killed probiotic bacteria and their secreted proteins on the subset-specific inflammatory marker profile of TNF alpha, IL-6 and NF kappa B. M1 and M2 cells were generated by differentiation of monocyte stable transfectants for high and low CD14 expression with phorbol 12-myristate 13-acetate and vitamin D-3, respectively, where the resulting CD14(lo) M2 and CD14(hi) M1s mimicked homeostatic and inflammatory mucosal macrophages. Subsets were stimulated by enteropathic lipopolysaccharides in the presence or absence of heat-killed (HK) or secreted proteins (SP) from a panel of probiotic bacteria. Regulation of cytokine expression was measured by ELISA and NF kappa B activity by reporter assay. HK probiotics suppress CD14(lo) and augment CD14(hi) M1 and M2 production of TNF alpha whereas SPs augmented CD14(hi) M1 TNF alpha and were generally suppressive in the other subtypes. M2 macrophage IL-6 production was suppressed by both HK and SPs and differentially regulated in CD14(lo) and CD14(hi) M1s. NF kappa B activation failed to parallel the regulatory profiles for TNF alpha and IL-6 which is suggestive of probiotic bacteria exerting their regulatory effects on these cytokines in an NF kappa B-independent manner. In conclusion, HK and SP probiotics differentially regulate macrophage cytokines and NF kappa B activation in a subset-dependent manner and suggest a cautionary approach to probiotic treatment of mucosal inflammation.