Background/Purpose:  Gouty arthritis is a common inflammatory joint disease that arises in response to the deposition of monosodium urate (MSU) crystals in soft joints, periarticular and articular joints of individuals with hyperuricemia¹. Studies have shown that the inflammatory response occurs when macrophages within the joint space phagocytose MSU crystals². Macrophages (THP-1 monocytes or Mφ) are a functionally heterogeneous cell population that have two different phenotypes related to different stimuli: M1 (classically activated) and M2 (alternatively activated)³. Based on their activation status, macrophages can influence a response cascade in gout. The objective of this study was to investigate MSU-induced responses on macrophages, and M1 and M2 polarized macrophages by using cytokine array, immunohistochemistry, and Western blot analysis.

Methods:  The human monocytic cell line THP-1 differentiated into macrophages using 10 ng/ml PMA. Once differentiated, macrophages were incubated with LPS and IFN-γ for classical macrophage activation (M1) or IL-4 and IL-13 for alternative activation (M2). MSU-crystals were induced 48 hours later to both M1 and M2 polarized macrophages and re-incubated. After the incubation, the impact of MSU-crystals on macrophage polarization was studied and assessed by performing cytokine array, immunohistochemistry, and Western blotting.

Results:  Immunohistochemistry shows that there is a differential effect produced by MSU crystals as seen by either having a pro-stimulatory effect or an anti-inflammatory effect based on the cytokine or chemokine. Macrophages show a 2-fold increase in IL-1β, IL-8, TNF-α, IL-6, GRO, and GRO-α compared to M1 and M2. IL-10, an anti-inflammatory cytokine, increased on M2 compared to macrophages and M1. Additionally, TGF-β2 did not differ between THP-1 monocytes, M1 and M2. Osteopontin, an anionic phosphoprotein, mediates biomineralization. There was a 2 fold increase in osteopontin in M1 and M2 compared to macrophages.

Conclusion: Our results from immunohistochemistry and Western blot analysis confirm that there are significant differences between crystal activation in macrophages, M1, and M2 due to their inherent functions. Upon MSU-crystal induction, a more robust effect is seen on the macrophages compared to M1 and M2 polarized macrophages. This area of research warrants further investigation. References 1. Roddy E, Choi HK. Epidemiology of Gout. Rheum Dis Clin North Am. 2014;40:155-75. 2. Liu-Bryan R, Scott P, Sydlaske A, Rose DM, Terkeltaub R (2005) Innate immunity conferred by Toll-like receptors 2 and 4 and myeloid differentiation factor 88 expression is pivotal to monosodium urate monohydrate crystal-induced inflammation. Arthritis Rheum. 52(9):2936–46

3. Laria, Antonella et al. “The Macrophages in Rheumatic Diseases.” Journal of Inflammation Research 9 (2016): 1–11. PMC. Web. 19 June 2016.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/differential-effect-of-msu-crystal-induced-inflammation-on-macrophage-polarization/