Background/Purpose:

Calcium pyrophosphate (CPP) crystal-induced inflammation is mainly driven by interleukin (IL)-1β. IL-1β production involves a two-step process including the formation of pro-IL-1β through NF-κB activation and its’ maturation through NLRP3 inflammasome activation. Two CPP crystal phases, namely monoclinic and triclinic CPP dehydrated (m- and tCPPD) crystals are identified in synovial fluids. They display different inflammatory potentials.

We aimed to assess the inflammatory properties of different CPP crystal phases and the intracellular pathways involved in CPP crystal-induced IL-1β production

Methods:

Four synthesized and pyrogen-free CPP crystals (aCPP (amorphous), mCPPD or tetrahydrated (mCPPT) and tCPPD) (Gras P et al. 2014) and monosodium urate crystals (MSU) were used, in vitro, to stimulate THP-1 cell line or mouse bone marrow-derived macrophages (BMDM) in the presence or not of pharmacological inhibitors: Bay-117085 (Bay, NF-κB inhibitor); SB203580 (SB, MAPK p38 inhibitor); SP600125 (SP, JNK inhibitor) and PD98059 (PD, p42/44 MAPK inhibitor). The expression of pro- and anti-inflammatory cytokine genes was assessed by qRT-PCR, IL-1β and IL-8 production by ELISA and MAPK activation by immunoblot. NF-κB activation was assessed in THP-1 cell line containing a gene reporter plasmid. In vivo, we used air pouch model to assess the effects of NF-κB inhibition in CPP crystal-mediated inflammation.

Results:

In vitro mCPPD crystals were the most inflammatory CPP crystals and induced a higher production of mature IL-1b and IL-8 and a higher expression of inflammatory cytokine genes (IL-1β, IL-6, IL-8, IL-33, TNF, COX-2) than tCPPD, mCPPT and MSU crystals. aCPP crystals did not have inflammatory property. Similarly, mCPPD crystals induced a higher activation of NF-κB, a higher production of NLRP3 inflammaome and a stronger phosphorylation of p38, p42/44 and JNK MAPKs. Inhibition of NF-κB with Bay completely abrogated mature IL-1b and IL-8 secretion and pro-IL-1β synthesis induced by all CPP crystals. Inhibition of JNK and p42/44 MAPK with SP and PD, respectively, decreased both IL-1β secretion and NF-κB activation induced by CPP crystals. In vivo IL-1β and IL-8 production and neutrophil infiltration induced by mCPPD crystals were dramatically decreased by NF-κB inhibitor.

Conclusion:

Our results suggested that the inflammatory potential of different CPP crystals relied on their capacity to activate MAPK pathways and NF-κB. Studies are ongoing to investigate the underlying mechanisms.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-inflammation-induced-by-four-types-of-calcium-pyrophosphate-crystals-depends-on-their-capacity-to-stimulate-nf-%ce%bab-and-mapk-pathways/