Human follicular dendritic cells promote the APC capability of B cells by enhancing CD86 expression levels

Akt, IL-4, and STAT Proteins Play Distinct Roles in Prostaglandin Production in Human Follicular Dendritic Cell-like Cells

This study aimed to explore the role of Akt protein in the induction and inhibition of prostaglandin (PG) in human follicular dendritic cell (FDC)-like cells. FDC-like cells and B cells were isolated from human tonsils. PG production was assessed using enzyme immunoassay, while the upstream cyclooxygenase-2 (COX-2) protein levels were measured using immunoblotting with FDC-like cells transfected with Akt siRNA to analyze the impact of Akt knockdown. The COX-2 expression and PG production induced with IL-1β were significantly increased by Akt knockdown. However, IL-1β did not significantly alter either total or phosphorylated Akt protein levels. Akt knockdown resulted in the augmentation of COX-2 expression induced by B cells, although the addition of B cells did not significantly modulate both total and phosphorylated Akt proteins. In contrast, IL-4 specifically exhibited a potent inhibitory effect on COX-2 protein induction and PG production via STAT6. The inhibitory activity of IL-4 was not hampered by Akt knockdown. Interestingly, COX-2 expression levels induced with IL-1β were markedly modulated with STAT1 and STAT3 knockdown. STAT1 silencing resulted in further augmentation of COX-2, whereas STAT3 silencing prohibited IL-1β from stimulating COX-2 expression. The current results suggest that Akt, IL-4, and STAT1 play inhibitory roles in PG production in FDC-like cells and expand our knowledge of the immune inflammatory milieu.

A paracrine effect of 15 (S)-hydroxyeicosatetraenoic acid revealed in prostaglandin production by human follicular dendritic cell-like cells

Lipid mediators play active roles in each stage of inflammation under physiological and pathologic conditions. We have investigated the cellular source and functions of several prostanoids in the immune inflammatory responses using follicular dendritic cell (FDC)-like cells. In this study, we report a novel finding on the role of 15(S)- hydroxyeicosatetraenoic acid (HETE). Our observation of 15(S)-HETE uptake by FDC-like cells prompted to hypothesize that 15(S)-HETE might have a regulatory role in the other branch of eicosanoid production. The effects of 15(S)-HETE on COX-2 expression and prostaglandin (PG) production were analyzed by immunoblotting and specific enzyme immunoassays. The addition of 15(S)-HETE resulted in elevated levels of COX-2 expression and PG production. The enhanced PG production was not due to growth stimulation of FDC-like cells since 15(S)-HETE did not modulate FDC-like cell proliferation by the culture period of PG measurement. Peroxisome proliferator-activated receptor gamma (PPARγ) seems to mediate the augmenting activity as the antagonist GW9662 dose- dependently prevented 15(S)-HETE from increasing PG production. In addition, PPARγ protein expression was readily detected in FDC-like cells. These effects of 15(S)-HETE were displayed in the combined addition with IL-1β. Based on these results, we suggest that 15(S)-HETE is an inflammatory costimulator of FDC acting in a paracrine fashion.

Prostaglandin E2 stimulates COX-2 expression via mitogen-activated protein kinase p38 but not ERK in human follicular dendritic cell-like cells

Background

Prostaglandin E2 (PGE₂) is an endogenous lipid mediator of inflammation. Its production is regulated by the rate-limiting upstream enzyme cyclooxygenase-2 (COX-2). We have recently demonstrated that the major cell type expressing COX-2 in the germinal center is follicular dendritic cell (FDC). In this study, to elucidate the molecular mechanism of PGE₂ in COX-2 production, we asked whether mitogen-activated protein kinases ERK and p38 might regulate COX-2 expression.

Results

FDC-like cells were used to analyze the phosphorylation kinetics of ERK and p38 and the impact of genetic knockdown. PGE₂ stimulation gave rise to a rapid increase of p38 but not ERK phosphorylation. In contrast, IL-1β induced phosphorylation of both MAPKs. Knockdown of p38 resulted in a marked suppression of COX-2 expression induced by either PGE₂ or IL-1β. ERK knockdown did not significantly affect the effect of PGE₂ and IL-1β on COX-2 induction. The differential results of p38 and ERK siRNA transfection were reproduced in the production of prostaglandins and in experiments performed with pharmacologic inhibitors.

Conclusions

Our data indicate that p38 is essentially required for PGE₂ to induce COX-2 expression in FDC-like cells. The current study helps to expand our understanding of the biological function of FDC at the molecular level and provides a potential rationale for the pharmacologic or genetic approaches to regulate p38 MAPK in the treatment of various inflammatory disorders.

Synaptic Interactions in Germinal Centers

The germinal center (GC) is a complex, highly dynamic microanatomical niche that allows the generation of high-affinity antibody-producing plasma cells and memory B cells. These cells constitute the basis of long-lived highly protective antibody responses. For affinity maturation to occur, B cells undergo multiple rounds of proliferation and mutation of the genes that encode the immunoglobulin V region followed by selection by specialized T cells called follicular helper T (T_FH) cells. In order to achieve this result, the GC requires spatially and temporally coordinated interactions between the different cell types, including B and T lymphocytes and follicular dendritic cells. Cognate interactions between T_FH and GC B cells resemble cellular connections and synaptic communication within the nervous system, which allow signals to be transduced rapidly and effectively across the synaptic cleft. Such immunological synapses are particularly critical in the GC where the speed of T–B cell interactions is faster and their duration shorter than at other sites. In addition, the antigen-based specificity of cognate interactions in GCs is critical for affinity-based selection in which B cells compete for T cell help so that rapid modulation of the signaling threshold determines the outcome of the interaction. In the context of GCs, which contain large numbers of cells in a highly compacted structure, focused delivery of signals across the interacting cells becomes particularly important. Promiscuous or bystander delivery of positive selection signals could potentially lead to the appearance of long-lived self-reactive B cell clones. Cytokines, cytotoxic granules, and more recently neurotransmitters have been shown to be transferred from T_FH to B cells upon cognate interactions. This review describes the current knowledge on immunological synapses occurring during GC responses including the type of granules, their content, and function in T_FH-mediated help to B cells.

Activated human B cells stimulate COX-2 expression in follicular dendritic cell-like cells via TNF-α

In spite of the potential importance of cyclooxygenase (COX)-2 expression in the germinal center, its underlying cellular and molecular mechanisms are largely unknown. COX-2 is the key enzyme generating pleiotropic prostaglandins. Based on our previous findings, we hypothesized that lymphocytes would stimulate COX-2 expression in follicular dendritic cell (FDC) by liberating cytokines. In this study, we examined the effect of tonsillar lymphocytes on COX-2 expression in FDC-like cells by immunoblotting. B but not T cells induced COX-2 protein in a time- and dose-dependent manner. Sub-fractionation analysis of B cell subsets revealed that activated but not resting B cells were responsible for the COX-2 induction. Confocal microscopy of frozen tonsils demonstrated that FDCs indeed express COX-2 in situ, in line with the in vitro results. To identify the stimulating molecule, we added neutralizing antibodies to the coculture of FDC-like cells and B cells. COX-2 induction in FDC-like cells was markedly inhibited by TNF-α neutralizing antibody. Finally, the actual production of TNF-α by activated B cells was confirmed by an enzyme immunoassay. The current study implies an unrecognized cellular interaction between FDC and B cells leading to COX-2 expression during immune inflammatory responses.

Interferon γ induced compositional changes in human bone marrow derived mesenchymal stem/stromal cells

Background

Mesenchymal stem/stromal cells (MSC) display a range of immunoregulatory properties which can be enhanced by the exposure to cytokines such interferon γ (IFN-γ). However the compositional changes associated with the ‘licensing’ of these cells have not been clearly defined. The present study was undertaken to provide a detailed comparative proteomic analysis of the compositional changes that occur in human bone marrow derived MSC following 20 h treatment with IFN-γ.

Methods

2D LC MSMS analysis of control and IFN-γ treated cells from 5 different healthy donors provided confident identification of more than 8400 proteins.

Results

In total 210 proteins were shown to be significantly altered in their expression levels (≥|2SD|) following IFN-γ treatment. The changes for several of these proteins were confirmed by flow cytometry. STRING analysis determined that approximately 30% of the altered proteins physically interacted in described interferon mediated processes. Comparison of the list of proteins that were identified as changed in the proteomic analysis with data for the same proteins in the Interferome DB indicated that ~35% of these proteins have not been reported to be IFN-γ responsive in a range of cell types.

Conclusions

This data provides an in depth analysis of the proteome of basal and IFN-γ treated human mesenchymal stem cells and it identifies a number of novel proteins that may contribute to the immunoregulatory capacity if IFN-γ licensed cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12014-017-9161-1) contains supplementary material, which is available to authorized users.

Positive feedback effect of PGE2 on cyclooxygenase-2 expression is mediated by inhibition of Akt phosphorylation in human follicular dendritic cell-like cells

Prostaglandins (PGs) are bioactive lipid mediators generated from the phospholipids of cell membrane in response to various inflammatory signals. To understand the potential role of PGs in PG production itself during immune inflammatory responses, we examined the effect of PGE₂, PGF_2α, and beraprost on COX-2 expression using follicular dendritic cell (FDC)-like HK cells isolated from human tonsils. Those three PGs specifically augmented COX-2 protein expression in a dose-dependent manner after 4 or 8h of treatment. The enhancing effect was also reflected in the actual production of PGs and the viable cell recovery of germinal center B-cells. To investigate the underlying molecular mechanism, we examined the impact of PI3K inhibitors on PG-induced COX-2 expression. Interestingly, COX-2 induction by PGE₂ and beraprost, but not PGF_2α, was enhanced by wortmannin and LY294002. In line with this result, Akt phosphorylation was inhibited by PGE₂ and beraprost but not by PGF_2α. The distinct effect of PGE₂ and beraprost from PGF_2α was reproduced in Akt-knockdowned HK cells. Our current findings imply that PGE₂ and PGI₂ stimulate COX-2 expression in FDC by inhibiting Akt phosphorylation. Additional studies are warranted to determine the potential role of Akt as a therapeutic target in patients with inflammatory disorders.

Opposing roles of TGF-β in prostaglandin production by human follicular dendritic cell-like cells

Prostaglandins (PGs) are recognized as important immune regulators. Using human follicular dendritic cell (FDC)-like HK cells, we have investigated the immunoregulatory role of PGs and their production mechanisms. The present study was aimed at determining the role of TGF-β in IL-1β-induced cyclooxygenase-2 (COX-2) expression by immunoblotting. COX-2 is the key enzyme responsible for PG production in HK cells. TGF-β, when added simultaneously with IL-1β, gave rise to an additive effect on COX-2 expression in a dose-dependent manner. However, TGF-β inhibited IL-1β-stimulated COX-2 expression when it was added at least 12h before IL-1β addition. The inhibitory effect of TGF-β was specific to IL-1β-induced COX-2 expression in HK cells. The stimulating and inhibitory effects of TGF-β were reproduced in IL-1β-stimulated PG production. Based on our previous results of the essential requirement of ERK and p38 MAPKs in TGF-β-induced COX-2 expression, we examined whether the differential activation of these MAPKs would underlie the opposing activities of TGF-β. The phosphorylation of ERK and p38 MAPKs was indeed enhanced or suppressed by the simultaneous treatment or pre-treatment, respectively. These results suggest that TGF-β exerts opposing effects on IL-1β-induced COX-2 expression in HK cells by differentially regulating activation of ERK and p38 MAPKs.

Suppressor of cytokine signaling 1 is a positive regulator of TGF-β-induced prostaglandin production in human follicular dendritic cell-like cells

PGs are emerging as important immune modulators. Since our report on the expression of PG synthases in human follicular dendritic cells, we investigated the potential immunoregulatory function of PGs and their production mechanisms. In this study, we explored the intracellular signaling molecules mediating TGF-β-induced cyclooxygenase (COX)-2 augmentation in follicular dendritic cell-like cells. TGF-β triggered phosphorylation of Smad3 and ERK, which were essential for the increase in COX-2 protein. Interestingly, depletion of suppressor of cytokine signaling 1 (SOCS1) resulted in an almost complete inhibition of Smad3 phosphorylation and COX-2 induction. Nuclear translocation of Smad3 was inhibited in SOCS1-depleted cells. SOCS1 knockdown also downregulated TGF-β-stimulated Snail expression and its binding to the Cox-2 promoter. In contrast, overexpression of SOCS1 gave rise to a significant increase in Snail and COX-2 proteins. SOCS1 was reported to be a negative regulator of cytokine signaling by various investigators. However, our current data suggest that SOCS1 promotes TGF-β-induced COX-2 expression and PG production by facilitating Smad3 phosphorylation and Snail binding to the Cox-2 promoter. The complete understanding of the biological function of SOCS1 might be obtained via extensive studies with diverse cell types.

Interferon-γ stimulates human follicular dendritic cell-like cells to produce prostaglandins via the JAK-STAT pathway

IFN-γ plays a critical role in the regulation of innate and adaptive immunity. Paying attention to the emerging role of prostaglandins (PGs) as immune regulators, we attempted to establish the effect of IFN-γ on PG production in human follicular dendritic cell-like HK cells and the underlying signaling pathway by using RNA interference technology. IFN-γ induced COX-2 protein expression in HK cells in a time- and dose-dependent manner, which was not observed in peripheral blood monocytes. Although IFN-γ induced phosphorylation of STAT1, STAT3, and STAT5, only STAT1 was essential for the COX-2 augmentation. The JAK kinases responsible for IFN-γ-triggered STAT1 phosphorylation were JAK1 and JAK2, which were also required for the COX-2 induction. The essential requirement of JAK1 and JAK2 was verified by confocal microscopic analysis, since STAT1 phosphorylation and nuclear translocation were impaired in HK cells with these two kinases knocked down. Finally, we demonstrated that JAK1, JAK2, and STAT1 were indispensable for the actual enhancement of PG production in response to IFN-γ stimulation. These results provide a novel insight into our understanding of IFN-γ under inflammatory conditions and support the emerging concept of PGs as important immune regulators.

Macrophages enhance tumor-derived autophagosomes (DRibbles)-induced B cells activation by TLR4/MyD88 and CD40/CD40L

Our previous studies have showed that tumor-derived autophagosomes (termed "DRibbles") induce B cell activation, resulting in antibody production and cytokine secretion. Unexpectedly, we found that unfractionated splenocytes produced a higher level of antibody and cytokine than that of purified B cells. In the current study, we investigated the role of accessory cells in DRibbles-induced B cell activation. We found that cognate macrophages, but not T cells, significantly enhanced the B cell activities. Such an enhancement required cell-cell contact. Furthermore, DRibbles stimulation up-regulated CD40L expression on macrophages, resulting in increased level of CD40 expressed on B cells. The accessory role of macrophages in DRibbles-activated B cells is critically dependent on the CD40/CD40L interaction. In addition, the effects of macrophages were found to be largely dependent on TLR4 and MyD88 signaling pathway. Finally, our results showed that macrophages were able to enhance the antigen presentation function of B cells for specific T cell stimulation. Thus, these results suggest that macrophages play an important accessory role for DRibbles-induced B cell immune function.

IL-4 and HDAC Inhibitors Suppress Cyclooxygenase-2 Expression in Human Follicular Dendritic Cells

Evidence for immunoregulatory roles of prostaglandins (PGs) is accumulating. Since our observation of PG production by human follicular dendritic cells (FDCs), we investigated the regulatory mechanism of PG production in FDC and attempted to understand the functions of released PGs in the responses of adjacent lymphocytes. Here, using FDC-like cells, HK cells, we analyzed protein expression alterations in cyclooxygenase-2 (COX-2) in the presence of IL-4 or histone deacetylase (HDAC) inhibitors. Both IL-4 and HDAC inhibitors suppressed COX-2 expression in dose-dependent manners. Their effect was specific to COX-2 and did not reach to COX-1 expression. Interestingly, HDAC inhibitors gave rise to an opposing effect on COX-2 expression in peripheral blood monocytes. Our results suggest that IL-4 may regulate COX-2 expression in FDCs by affecting chromatin remodeling and provide insight into the role of cellular interactions between T cells and FDC during the GC reaction. Given the growing interests in wide-spectrum HDAC inhibitors, the differential results on COX-2 expression in HK cells and monocytes raise cautions on their clinical use.

Follicular dendritic cells in health and disease

Follicular dendritic cells (FDCs) are unique immune cells that contribute to the regulation of humoral immune responses. These cells are located in the B-cell follicles of secondary lymphoid tissues where they trap and retain antigens (Ags) in the form of highly immunogenic immune complexes (ICs) consisting of Ag plus specific antibody (Ab) and/or complement proteins. FDCs multimerize Ags and present them polyvalently to B-cells in periodically arranged arrays that extensively crosslink the B-cell receptors for Ag (BCRs). FDC-FcγRIIB mediates IC periodicity, and FDC-Ag presentation combined with other soluble and membrane bound signals contributed by FDCs, like FDC-BAFF, -IL-6, and -C4bBP, are essential for the induction of the germinal center (GC) reaction, the maintenance of serological memory, and the remarkable ability of FDC-Ags to induce specific Ab responses in the absence of cognate T-cell help. On the other hand, FDCs play a negative role in several disease conditions including chronic inflammatory diseases, autoimmune diseases, HIV/AIDS, prion diseases, and follicular lymphomas. Compared to other accessory immune cells, FDCs have received little attention, and their functions have not been fully elucidated. This review gives an overview of FDC structure, and recapitulates our current knowledge on the immunoregulatory functions of FDCs in health and disease. A better understanding of FDCs should permit better regulation of Ab responses to suit the therapeutic manipulation of regulated and dysregulated immune responses.