S.-C. Yoon

Follicular Dendritic Cells Produce IL-15 That Enhances Germinal Center B Cell Proliferation in Membrane-Bound Form

Factors that control the survival and proliferation of Ag-stimulated B cells within the germinal center (GC) are crucial for humoral immune responses with high affinity Abs against infectious agents. The follicular dendritic cell (FDC) is known as a key cellular component of the GC microenvironment for GC-B cell survival and proliferation. In this study, we report that IL-15 is produced by human FDC in vivo and by an FDC cell line, FDC/HK cells, in vitro. IL-15 is captured by IL-15Rα on the surface of FDC/HK cells. The surface IL-15 is functionally active and augments GC-B cell proliferation. Because GC-B cells have the signal-transducing components (IL-2/15Rβγ), but not a receptor for binding of soluble IL-15 (IL-15Rα), IL-15 signaling is possibly transduced by transpresentation from FDCs to GC-B cells via cell-cell contact. Together, these results suggest that IL-15 from FDC, in membrane-bound form, plays an important role in supporting GC-B cell proliferation, proposing a new target for immune modulation as well as treatment of B cell tumors of GC origin.

Notch Ligands Expressed by Follicular Dendritic Cells Protect Germinal Center B Cells from Apoptosis

The Notch signaling pathway is one of the most conserved mechanisms to regulate cell fate in many tissues during development and postnatal life. In the immune system, Notch signaling regulates T and B cell development and modulates the differentiation of T and B cells. In this study, we investigated the functional roles of Notch signaling in human B cell differentiation within the germinal center (GC). Notch ligands, Delta-like 1 (Dll1) and Jagged 1 (Jg1), are expressed by follicular dendritic cells (FDC) but not by B cells in the GC, while GC-B cells express the Notch receptors, Notch1 and Notch2. The blockade of Notch signaling pathways using a γ-secretase inhibitor, DAPT (N-[N-(3,5-difluorophenacetyl-l-alanyl)]-S-phenylglycine t-butyl ester), reduces the survival of GC-B cells in the presence of FDC/HK cells. Jg1 has a dominant effect on GC-B cell survival mediated by Notch signaling. Furthermore, Notch cooperates with another anti-apoptotic factor, BAFF/Blys produced by FDC to support GC-B cell growth. Taken together, our data shows the important role of Notch signaling provided by FDC in the survival of GC-B cells in vitro.

Mesenchymal stem/stromal cells precondition lung monocytes/macrophages to produce tolerance against allo- and autoimmunity in the eye

Significance Mesenchymal stem/stromal cells (MSCs) are the focus of intensive efforts directed at developing cell-based therapies in immunologic disorders. However, one of the paradoxical observations made so far is that MSCs engraft transiently in the recipient after exogenous infusion, but achieve long-term therapeutic benefits. Here we demonstrate that MSCs induce the immune tolerance by activating endogenous immune regulatory system of the recipient. Specifically, i.v. administered MSCs induce a population of regulatory monocytes/macrophages in the lung, which are capable of suppressing allo- and autoimmune responses independently of regulatory T cells. The data provide a mechanistic insight into the action of MSCs in immunologic disorders, one of the most frequent indications of diseases for clinical trials using stem cells. Intravenously administered mesenchymal stem/stromal cells (MSCs) engraft only transiently in recipients, but confer long-term therapeutic benefits in patients with immune disorders. This suggests that MSCs induce immune tolerance by long-lasting effects on the recipient immune regulatory system. Here, we demonstrate that i.v. infusion of MSCs preconditioned lung monocytes/macrophages toward an immune regulatory phenotype in a TNF-α–stimulated gene/protein (TSG)-6–dependent manner. As a result, mice were protected against subsequent immune challenge in two models of allo- and autoimmune ocular inflammation: corneal allotransplantation and experimental autoimmune uveitis (EAU). The monocytes/macrophages primed by MSCs expressed high levels of MHC class II, B220, CD11b, and IL-10, and exhibited T-cell–suppressive activities independently of FoxP3+ regulatory T cells. Adoptive transfer of MSC-induced B220+CD11b+ monocytes/macrophages prevented corneal allograft rejection and EAU. Deletion of monocytes/macrophages abrogated the MSC-induced tolerance. However, MSCs with TSG-6 knockdown did not induce MHC II+B220+CD11b+ cells, and failed to attenuate EAU. Therefore, the results demonstrate a mechanism of the MSC-mediated immune modulation through induction of innate immune tolerance that involves monocytes/macrophages.

IL-21 and IL-10 have redundant roles but differential capacities at different stages of plasma cell generation from human germinal center B cells

The GC is the anatomical site where antigen‐activated B cells differentiate into PC, producing high‐affinity antibodies in physiological and pathological states. PC differentiation is regulated by multiple factors within the GC microenvironment, including cytokines. IL‐21, a recently identified type I cytokine produced by GC‐Th cells, promotes differentiation of human B cells into ISC. In this study, we investigated in detail the functional role of IL‐21 in the course of GC‐B cell differentiation into terminally differentiated PC compared with that of IL‐10, a well‐known PC differentiation factor. IL‐21 had a greater capacity to initiate PC differentiation from CD77+ centroblasts than IL‐10 by strongly inducing PC transcription factors through activation of STAT3; however, IL‐10 was more potent than IL‐21 in generating CD138+ PC from CD20–CD38++ plasmablasts in the terminal stage of GC‐B cell differentiation. This differential effect of IL‐21 and IL‐10 was reflected in receptor expression on B cell subsets emerging in the course of differentiation. Our studies have revealed that IL‐21 is a critical decision‐maker for driving initial PC differentiation at the stage of CD77+ centroblasts, yet IL‐10 is more effective in producing IgG by generating terminally differentiated CD138+ PC at the later stage of PC differentiation in the GC.

Mesenchymal Stem/Stromal Cells Protect against Autoimmunity via CCL2-Dependent Recruitment of Myeloid-Derived Suppressor Cells

Exogenously administered mesenchymal stem/stromal cells (MSCs) suppress autoimmunity despite transient engraftment. However, the mechanism is unclear. In this study, we report a novel mechanism by which MSCs modulate the immune system by recruiting myeloid-derived suppressor cells in a mouse model of experimental autoimmune uveitis (EAU). Intravenous infusion of MSCs blocked EAU development and reduced Th1 and Th17 responses. Time course analysis revealed an increase of MHC class IIloLy6G−Ly6ChiCD11b+ cells in draining lymph nodes by MSCs. These Ly6ChiCD11b+ cells suppressed CD4+ cell proliferation and Th1/Th17 differentiation and induced CD4+ cell apoptosis. Adoptive transfer of Ly6ChiCD11b+ cells ameliorated EAU, whereas depletion of Ly6ChiCD11b+ cells abrogated the effects of MSCs. 1.8% of MSCs were present in draining lymph nodes 1 d after infusion, and MSCs with CCL2 knockdown did not increase MHC class IIloLy6G−Ly6ChiCD11b+ cells and failed to attenuate EAU. Therefore, our findings demonstrate that MSCs suppress autoimmunity by recruiting myeloid-derived suppressor cells into sites of inflammation in a CCL2-dependent manner.

Rapamycin regulates macrophage activation by inhibiting NLRP3 inflammasome-p38 MAPK-NFκB pathways in autophagy- and p62-dependent manners

Excessive and prolonged activation of macrophages underlies many inflammatory and autoimmune diseases. To regulate activation and maintain homeostasis, macrophages have multiple intrinsic mechanisms, one of which is modulation through autophagy. Here we demonstrate that autophagy induction by rapamycin suppressed the production of IL-1β and IL-18 in lipopolysaccharide- and adenosine triphosphate-activated macrophages at the post-transcriptional level by eliminating mitochondrial ROS (mtROS) and pro-IL1β in a p62/SQSTM1-dependent manner. In addition, rapamycin activated Nrf2 through up-regulation of p62/SQSTM1, which further contributed to the reduction of mtROS. Reduced IL-1β subsequently diminished the activation of p38 MAPK-NFκB pathways, leading to transcriptional down-regulation of IL-6, IL-8, MCP-1, and IκBα in rapamycin-treated macrophages. Therefore, our results suggest that rapamycin negatively regulates macrophage activation by restricting a feedback loop of NLRP3 inflammasome-p38 MAPK-NFκB pathways in autophagy- and p62/SQSTM1-dependent manners.

Myeloid-Derived Suppressor Cells Mediate Inflammation Resolution in Humans and Mice with Autoimmune Uveoretinitis

Resolution of inflammation is an active process that leads to tissue homeostasis and involves multiple cellular and molecular mechanisms. Myeloid-derived suppressor cells (MDSCs) have recently emerged as important cellular components in the resolution of inflammation because of their activities to suppress T cell activation. In this article, we show that HLA-DR−CD11b+CD33+CD14+ human MDSCs and CD11b+Ly6G−Ly6C+ mouse MDSCs markedly increased in patients and mice during and before the resolution phase of autoimmune uveoretinitis. CD11b+Ly6C+ monocytes isolated from autoimmune uveoretinitis mice were able to suppress T cell proliferation in culture, and adoptive transfer of the cells accelerated the remission of autoimmune uveoretinitis in mice. Alternatively, depletion of CD11b+Ly6C+ monocytes at the resolution phase, but not CD11b+Ly6G+ granulocytes, exacerbated the disease. These findings collectively indicate that monocytic MDSCs serve as regulatory cells mediating the resolution of autoimmune uveoretinitis.

Mesenchymal stromal cells promote B-cell lymphoma in lacrimal glands by inducing immunosuppressive microenvironment

Mesenchymal stromal cells (MSCs) have therapeutic potential for various diseases because of their anti-inflammatory and immunosuppressive properties. However, the immunosuppressive microenvironment allows tumor cells to evade immune surveillance, whereas maintenance of inflammation is required for tumor development and progression. Hence, MSCs may promote or suppress tumors in a context-dependent manner. We here investigated the effects of bone marrow-derived MSCs in a murine model of lacrimal gland B-cell lymphoma. Co-injection of MSCs with B lymphoma cells enhanced tumor growth in lacrimal glands without long-term engraftment. Of note, MSCs induced greater infiltration of immune and immune-regulatory cells near tumor: CD4+ cells, CD11b+ cells, CD4+Foxp3+ regulatory T cells and CD11b+Ly6C+Ly6G− myeloid-derived suppressor cells. Concurrently, there was up-regulation of immune-related molecules including TNF-α, IL-1β, TGF-β1, and arginase in glands treated with MSCs. Apoptosis in tumor was less severe in mice treated with MSCs compared to those without MSCs; however, MSCs did not directly inhibit apoptosis of B lymphoma cells in an in vitro co-culture. Together, data demonstrate that MSCs create immunosuppressive milieu by recruiting regulatory immune cells and promote B-cell lymphoma growth in lacrimal glands.

The effect of epitope-based ligation of ICAM-1 on survival and retransplantation of pig islets in nonhuman primates

Pig islet xenotransplantation is a promising alternative to allogeneic transplantation. However, the wide immunologic barrier between pigs and primates limits the long‐term survival of the graft. MD‐3, a novel monoclonal antibody (mAb) that recognizes a particular epitope of human ICAM‐1, can render T cells tolerant to a xenograft by arresting dendritic cell maturation. We report the long‐term survival of adult wild‐type pig islets and successful retransplantation in nonhuman primates using a protocol comprising induction with MD‐3 mAb and maintenance with anti‐CD154 mAb and sirolimus.

Initial assessment of space‐based lidar CALIOP aerosol and cloud layer structures through inter‐comparison with a ground‐based back‐scattering lidar and CloudSat

This study presents results of the intercomparison of aerosol/cloud top and bottom heights obtained from a space‐borne active sensor Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard CALIPSO, and the Cloud Profiling Radar (CPR) onboard CloudSat, and the space‐borne passive sensor Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Aqua, and ground‐based 2‐wavelenght polarization lidar system (532 and 1064 nm) at Seoul National University (SNU), Seoul, South Korea. This result confirms that the CALIPSO science team algorithms for the discrimination of cloud and aerosol as well as for the detection of layer top and base altitude provide reliable information both under cloud‐free conditions and in cases of multiple aerosol layers underlying semi‐transparent cirrus clouds. Simultaneous space‐borne CALIOP, CPR and ground‐based SNU lidar (SNU‐L) measurements complement each other and can be combined to provide full information on the vertical distribution of aerosols and clouds, especially for thick opaque clouds. The aerosol extinction profiles from both lidars show good agreement for aerosols within the planetary boundary layer under cloud‐free conditions and for the night‐time CALIOP flight.