During inflammation, macrophages respond to stimuli from the environment and take on specialized states. Their function then ranges from furthering inflammation to the promotion of wound-healing. This functional heterogeneity has been extensively studied in mice and human, but the research also showed that results cannot easily be translated between species. The aim of this study was to identify phenotypic markers and functional distinctions in porcine macrophage activation states.
We cultured porcine peripheral blood monocytes with orthologous serum and with or without growth factors (macrophage colony-stimulating factor, M-CSF, or granulocyte-macrophage colony-stimulating factor, GM-CSF) to investigate effects on macrophage differentiation. Then we polarized the macrophages with interferon γ (IFNγ) or interleukin 4 (IL-4) to induce a pro-inflammatory “M(IFNγ)” or wound-healing “M(IL-4)” functional status, respectively.
Using flow cytometry, we identified CD203a as a marker for IL-4 polarization and MHC-I, MHC-II, CD11a and CD40 upregulation on M(IFNγ). Interestingly, CD1 was downregulated by M(IFNγ). RT-PCR demonstrated upregulation of Arginase (Arg1) in response to IL-4. As had been previously described, we could not detect any induction of inducible nitric oxide synthase (iNOS) in M(IFNγ). Addition of M-CSF was only necessary in serum-free cultures. GM-CSF delayed macrophage polarization and changed the phenotype towards pro-inflammatory activation, with little plasticity to stimulation with either IFNγ or IL-4. Cytokine profiling after stimulation with LPS induced no clear distinctions between unpolarized and IL-4-polarized macrophages, while IFNγ-priming was necessary for a strong IL12p40 and IL6 response.
These findings will help to further characterize macrophage plasticity in vivo in the pig, and also demonstrate some differences to other species.