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The essential micronutrient zinc regulates immune responses by affecting signaling pathways. In activated monocytes and macrophages, signaling networks mediate the metabolic reprogramming that meets the demands of participation in immune responses. Here, we demonstrated that cytoplasmic, bioavailable zinc was essential for promoting IL-1β production in activated human monocytes and macrophages downstream of glycolysis induced by the kinase-containing multiprotein complex mTORC1. The concentration of cytoplasmic zinc was determined by that of extracellular zinc, which was brought into cells through the zinc-specific importer Zip8. The abundance of Zip8 was increased in monocytes from patients with rheumatoid arthritis (RA), as well as in LPS-stimulated monocytes and macrophages from healthy individuals. The mTORC1-mediated phosphorylation of S6 kinase (S6K) was enhanced by zinc-mediated inhibition of PP2A, a phosphatase that targets S6K. As a result, IL-1β production was increased due to the activation of mTORC1-induced glycolysis. In monocytes of patients with RA, the expression of Zip8 and the zinc-inducible metallothionein isoform MT2A and the phosphorylation of S6K were enhanced compared with those of healthy controls. Furthermore, Zip8 expression correlated with more severe RA clinical parameters, suggesting that Zip8-mediated zinc influx is related to inflammatory conditions. These results provide insight into the role of cytoplasmic, bioavailable zinc in the metabolic reprogramming of human monocytes and macrophages in inflammatory responses.

Monocytes are important cellular effectors of innate immune defense. Human monocytes are heterogeneous and can be classified into three distinct subsets based on CD14 and CD16 expression. The expansion of intermediate CD14 + CD16 + monocytes has been reported in chronic inflammatory diseases including rheumatoid arthritis (RA). However, the mechanism underlying induction of CD16 and its role in monocytes remains poorly understood. Here, we demonstrate that activated platelets are important for induction of CD16 on classical CD14 + CD16 - monocytes by soluble factors such as cytokines. Cytokine neutralization and signaling inhibition assays reveal that sequential involvement of platelet - derived TGF-β and monocyte-derived IL-6 contribute to CD16 induction on CD14 + CD16 - monocytes. Activated platelet-induced CD16 on monocytes participates in antibody-dependent cellular phagocytosis (ADCP) and its level is positively correlated with phagocytic activity. CD14 + CD16 - monocytes treated with activated platelets preferentially differentiate into M2 macrophages, likely the M2c subset expressing CD163 and MerTK. Lastly, the amount of sCD62P, a marker of activated platelets, is significantly elevated in plasma of RA patients and positively correlates with clinical parameters of RA. Our findings suggest an important role of activated platelets in modulating phenotypical and functional features of human monocytes. This knowledge increases understanding of the immunological role of CD14 + CD16 + cells in chronic inflammatory diseases., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2021 Lee, Yoon, Kim, Yoo, Kang and Lee.)

B7-H3, a newly identified B7 family member, has functional duality as a co-stimulator and co-inhibitor that fine-tunes T cell-mediated immune responses. Given that B7-H3 expression on human monocytes and dendritic cells is enhanced by inflammatory cytokines, its potential inmmunoregulatory role at sites of inflammation has been suggested. Further, monocytes play crucial roles in the pathophysiology of various inflammatory disorders including autoimmune diseases; however, the immunological role of B7-H3 in rheumatoid arthritis (RA) has not been defined. Thus, we aimed to investigate the possible roles of monocyte B7-H3 in the pathogenesis of RA. Synovial monocytes, but not peripheral monocytes, in RA patients predominantly express surface B7-H3. The 4Ig isoform of B7-H3 is exclusively induced on the cell surface, whereas the 2Ig B7-H3 isoform is constitutively expressed in the intracytoplasmic region of both peripheral and synovial monocytes. B7-H3 knockdown experiments reveal that surface B7-H3 has an inhibitory effect on IFN-γ production in CD4 memory cells. Moreover, surface B7-H3 expression on synovial monocytes inversely correlates with RA clinical parameters. Our findings demonstrate that activation-induced B7-H3 expression on synovial monocytes has the potential to inhibit Th1-mediated immune responses and immunomodulatory roles affecting RA pathogenesis., (© 2016 by The American Society for Biochemistry and Molecular Biology, Inc.)

Monocytes function as crucial innate effectors in the pathogenesis of chronic inflammatory diseases, including autoimmunity, as well as in the inflammatory response against infectious pathogens. Human monocytes are heterogeneous and can be classified into three distinct subsets based on CD14 and CD16 expression. Although accumulating evidence suggests distinct functions of monocyte subsets in inflammatory conditions, their pathogenic roles in autoimmune diseases remain unclear. Thus, we investigated the phenotypic and functional characteristics of monocytes derived from synovial fluid and peripheral blood in RA patients in order to explore the pathogenic roles of these cells. In RA patients, CD14+CD16+, but not CD14dimCD16+, monocytes are predominantly expanded in synovial fluid and, to a lesser degree, in peripheral blood. Expression of co-signaling molecules of the B7 family, specifically CD80 and CD276, was markedly elevated on synovial monocytes, while peripheral monocytes of RA and healthy controls did not express these molecules without stimulation. To explore how synovial monocytes might gain these unique properties in the inflammatory milieu of the synovial fluid, peripheral monocytes were exposed to various stimuli. CD16 expression on CD14+ monocytes was clearly induced by TGF-β, although co-treatment with IL-1β, TNF-α, or IL-6 did not result in any additive effects. In contrast, TLR stimulation with LPS or zymosan significantly downregulated CD16 expression such that the CD14+CD16+ monocyte subset could not be identified. Furthermore, treatment of monocytes with IFN-γ resulted in the induction of CD80 and HLA-DR expression even in the presence of TGF-β. An in vitro assay clearly showed that synovial monocytes possess the unique capability to promote Th1 as well as Th17 responses of autologous peripheral CD4 memory T cells. Our findings suggest that the cytokine milieu of the synovial fluid shapes the unique features of synovial monocytes as well as their cardinal role in shaping inflammatory T-cell responses in RA.