Immunomodulatory Effect of Minocycline on Macrophage Phenotype and Function

Minocycline is a second-generation semi-synthetic tetracycline antibiotic that displays additional immunomodulatory and anti-inflammatory properties. The benefit of these properties has proved to be clinically relevant, however, exact immunoregulatory mechanisms behind minocycline’s non-antibiotic effects remain unveiled. In the context of inflammatory bowel disease, data has previously shown that minocycline improves mucosal recovery in colitic mice, an effect related to a potentiation of the innate immune response and enhanced inflammation resolution. Minocycline initially enhanced monocyte recruitment to the intestine, whilst simultaneously increasing the presence of Ly6C- MHCII+ macrophages. In vitro, minocycline was shown to both increase and decrease proinflammatory cytokine release which was crucially seen to be time-dependant. Whether these effects derived from a direct action of minocycline on macrophages or other mechanisms is unclear. Macrophages are essential in the regulation of inflammation and resolution and can switch between M1/pro-inflammatory and M2/anti inflammatory phenotypes, with disturbances in M1/M2 homeostasis contributing to the development and maintenance of chronic inflammation. An improved understanding of minocycline’s immunoregulatory activity is key for future translational studies into human disease, thus the aim of this research was to evaluate the direct effects of minocycline on monocyte-macrophage differentiation, polarisation, and activation.

Three in vitro models were employed: THP-1, U-937 and human PBMCs. When differentiated in the presence of minocycline (10-50μM) with PMA (80nM) for 48hr, minocycline interrupted cellular adhesion and significantly decreased the expression of CD14 and CD86, while increasing the percentage of CD163⁺ cells - an M2 associated marker. When administered to M0 macrophages polarised for 24hr to M1 (20ng/mL IFN-γ+10ng/mL LPS), or M2 (20ng/mL IL-4), minocycline significantly reduced the production of pro-inflammatory cytokines IL-1β, IL-6 and TNF α, while in the M2 exclusively, reduced the expression of CD206 and IL-10, but increased IFN-γ and IL-12p70 production. The transcription factors stat2, 3 and 6 were also reduced by minocycline following M1 polarization.

Conversely, when assessing the role of minocycline upon LPS-activated macrophages (100ng/mL for 24hr), minocycline-treated M1 PBMC derived macrophages displayed maintained decreased production of IL-1β but increased TNF-α, IL-6 and IFN-y, with activated M2 macrophages also increasing and decreasing IL-12p70 and IL-10 respectively.

This data has highlighted some key regulatory effects of minocycline regarding macrophage biology and have added to the collective knowledge on minocycline’s’ immunomodulatory effects that can help provide more detailed context to the anti-inflammatory effects seen in vivo. Consequently, this advancement in knowledge will ultimately aid the development of novel therapeutic strategies for those individuals that are burdened with diseases dominated by a dysregulated immune response as seen in IBD.