Fine-tuning spatial-temporal dynamics and surface receptor expression support plasma cell-intrinsic longevity

Durable serological memory following vaccination is critically dependent on the production and survival of long-lived plasma cells (LLPCs). Yet, the factors that control LLPC specification and survival remain poorly resolved. Using intra-vital two-photon imaging, we find that in contrast to most plasma cells in the bone marrow, LLPCs are uniquely sessile and organized into clusters that are dependent on April, an important survival factor. Using deep, bulk RNA sequencing, and surface protein flow-based phenotyping, we find that LLPCs express a unique transcriptome and proteome compared to bulk PCs, fine tuning expression of key cell surface molecules, CD93, CD81, CXCR4, CD326, CD44 and CD48, important for adhesion and homing, and phenotypically label LLPCs within mature PC pool. Conditional deletion of Cxcr4 in PCs following immunization leads to rapid mobilization from the BM, reduced survival of antigen-specific PCs, and ultimately accelerated decay of antibody titer. In naive mice, the endogenous LLPCs BCR repertoire exhibits reduced diversity, reduced somatic mutations, and increased public clones and IgM isotypes, particularly in young mice, suggesting LLPC specification is non-random. As mice age, the BM PC compartment becomes enriched in LLPCs, which may outcompete and limit entry of new PC into the LLPC niche and pool.

TMIGD2 is an orchestrator and therapeutic target on human acute myeloid leukemia stem cells

Acute myeloid leukemia (AML) is initiated and sustained by a hierarchy of leukemia stem cells (LSCs), and elimination of this cell population is required for curative therapies. Here we show that transmembrane and immunoglobulin domain containing 2 (TMIGD2), a recently discovered co-stimulatory immune receptor, is aberrantly expressed by human AML cells, and can be used to identify and enrich functional LSCs. We demonstrate that TMIGD2 is required for the development and maintenance of AML and self-renewal of LSCs but is not essential for normal hematopoiesis. Mechanistically, TMIGD2 promotes proliferation, blocks myeloid differentiation and increases cell-cycle of AML cells via an ERK1/2-p90RSK-CREB signaling axis. Targeting TMIGD2 signaling with anti-TMIGD2 monoclonal antibodies attenuates LSC self-renewal and reduces leukemia burden in AML patient-derived xenograft models but has negligible effect on normal hematopoietic stem/progenitor cells. Thus, our studies reveal the function of TMIGD2 in LSCs and provide a promising therapeutic strategy for AML.

TNF-α Limits Serological Memory by Disrupting the Bone Marrow Niche

Both infection and autoimmune disease can disrupt pre-existing Ab titers leading to diminished serological memory, yet the underlying mechanisms are not well understood. In this article, we report that TNF-α, an inflammatory cytokine, is a master regulator of the plasma cell (PC) niche in the bone marrow (BM). Acute rTNF-α treatment depletes previously existing Ab titers after vaccination by limiting PC occupancy or retention in the BM. Consistent with this phenomenon, mice lacking TNF-α signaling have elevated PC capacity in the BM and higher Ab titers. Using BM chimeric mice, we found that PC egress from the BM is regulated in a cell-extrinsic manner, by radiation-resistant cells via TNF-α receptor 1 signaling, leading to increased vascular permeability and CD138 downregulation on PCs. PC motility and egress in the BM are triggered within 6 h of recombinant TNF-α treatment. In addition to promoting egress, TNF-α signaling also prevented re-engraftment into the BM, leading to reduced PC survival. Although other inflammatory stimuli can promote PC egress, TNF-α signaling is necessary for limiting the PC capacity in the BM. Collectively, these data characterize how TNF-α-mediated inflammation attenuates the durability of serological memory and shapes the overall size and composition of the Ab-secreting cell pool in the BM.

CD169+ macrophages orchestrate plasmacytoid dendritic cell arrest and retention for optimal priming in the bone marrow of malaria-infected mice

Plasmacytoid dendritic cells (pDC) are the most potent producer of type I interferon (IFN), but how pDC are primed in vivo is poorly defined. Using a mouse model of severe malaria, we have previously established that upon priming by CD169+ macrophages (MP), pDC initiate type I IFN-I secretion in the bone marrow (BM) of infected mice via cell-intrinsic TLR7 sensing and cell-extrinsic STING sensing. Herein we show that CD169+ MP and TLR7-sensing are both required for pDC arrest during priming, suggesting CD169+ MP are the source of TLR7 ligands. We establish that TLR7 sensing in pDC and chemotaxis are both required for pDC arrest and functional communication with CD169+ MP in the BM. Lastly, we demonstrate that STING-sensing in CD169+ MP control pDC initiation of type I IFN production while also regulating pDC clustering and retention/egress from the BM. Collectively, these results link pDC acquisition of type I IFN secreting capacity with changes in their motility, homing and interactions with CD169+ MP during infection. Thus, targeting this cellular interaction may help modulate type I IFN to improve outcomes of microbial infections and autoimmune diseases.

Inflammation depletes humoral immunity by limiting plasma cell access to the bone marrow survival niche

The survival of plasma cells (PCs) is directly correlated to the longevity of prophylactic titers against pathogens. The bone marrow contains hematopoietic and non-hematopoietic cells that contribute to the maintenance of PCs. Studies have shown that pathogens contain a variety of mechanisms that result in the depletion of bone marrow PCs and antibody titers. Most, if not all, forms of infection induce inflammation, however, the role of inflammation in bone marrow PC depletion is unclear. Here, we show that acute Tumor Necrosis Factor Alpha (TNFa) treatment can deplete pre-existing antibody titers in a variety of models. We also found that recombinant TNFa treatment limits PC retention in the bone marrow via adoptive transfer experiments. To determine if PC retention in the bone marrow was mediated in a cell-intrinsic or − extrinsic manner, we generated bone marrow chimeras with wild-type and TNFa receptor knockout mice. Results showed that TNFa signaling through TNFa receptor 1 in non-hematopoietic cells regulates PC retention in the bone marrow, indicating that this mechanism is cell-extrinsic. Another cell-extrinsic mechanism that we identified is TNFa regulation of syndecan-1, a cell surface proteoglycan found to regulate PC survival and motility. These findings demonstrate that inflammation limits PC access to the bone marrow survival niche, ultimately leading to the reduction of antibody titers and long-term humoral protection.

Supported by grants from the NIH (R01 HLI141491) and the Eric Heyer M.D. Ph.D Translational Research Grant

Tissue-resident macrophages promote early dissemination of multiple myeloma via IL-6 and TNFα

Multiple myeloma (MM) is a plasma cell malignancy characterized by the presence of multiple foci in the skeleton. These distinct tumor foci represent cycles of tumor growth and dissemination that seed new clusters and drive disease progression. By using an intratibial Vk*MYC murine myeloma model, we found that CD169+ radiation-resistant tissue-resident macrophages (MPs) were critical for early dissemination of myeloma and disease progression. Depletion of these MPs had no effect on tumor proliferation, but it did reduce egress of myeloma from bone marrow (BM) and its spread to other bones. Depletion of MPs as a single therapy and in combination with BM transplantation improved overall survival. Dissemination of myeloma was correlated with an increased inflammatory signature in BM MPs. It was also correlated with the production of interleukin-6 (IL-6) and tumor necrosis factor α (TNFα) by tumor-associated MPs. Exogenous intravenous IL-6 and TNFα can trigger myeloma intravasation in the BM by increasing vascular permeability in the BM and by enhancing the motility of myeloma cells by reducing the adhesion of CD138. Moreover, mice that lacked IL-6 had defects in disseminating myeloma similar to those in MP-depleted recipients. Mice that were deficient in TNFα or TNFα receptor (TNFR) had defects in disseminating MM, and engraftment was also impaired. These effects on dissemination of myeloma required production of cytokines in the radiation-resistant compartment that contained these radiation-resistant BM MPs. Taken together, we propose that egress of myeloma cells from BM is regulated by localized inflammation in foci, driven in part by CD169+ MPs.

Memory CD8+ T cells mediate early pathogen-specific protection via localized delivery of chemokines and IFNγ to clusters of monocytes

While cognate antigen drives clonal expansion of memory CD8+ T (CD8+ TM) cells to achieve sterilizing immunity in immunized hosts, not much is known on how cognate antigen contributes to early protection before clonal expansion occurs. Here, using distinct models of immunization, we establish that cognate antigen recognition by CD8+ TM cells on dendritic cells initiates their rapid and coordinated production of a burst of CCL3, CCL4, and XCL1 chemokines under the transcriptional control of interferon (IFN) regulatory factor 4. Using intravital microscopy imaging, we reveal that CD8+ TM cells undergo antigen-dependent arrest in splenic red pulp clusters of CCR2+Ly6C+ monocytes to which they deliver IFNγ and chemokines. IFNγ enables chemokine-induced microbicidal activities in monocytes for protection. Thus, rapid and effective CD8+ TM cell responses require spatially and temporally coordinated events that quickly restrict microbial pathogen growth through the local delivery of activating chemokines to CCR2+Ly6C+ monocytes.

Abstract IA21: Immunomodulatory therapy of multiple myeloma with IAP antagonists

Multiple myeloma (MM) is a tumor of terminally differentiated plasma cells that underwent affinity maturation and class switch recombination processes in germinal centers before homing to the bone marrow (BM), where they secrete large amounts of monoclonal immunoglobulins, clinically detected as an M-spike by serum protein electrophoresis (SPEP). Clinical manifestations of MM also include renal impairment, anemia, and bone disease, the latter resulting from abnormal osteoclast activation, which highlights the complex crosstalk between MM cells and their BM microenvironment.

Chromosomal translocations into the immunoglobulin loci are the primary oncogenic events in ~50% of MM cases and are thought to occur as a result of errors during somatic hypermutation or class switch recombination. The other half of MM is characterized by trisomies of odd-numbered chromosomes. Both of these genomic abnormalities are present in the premalignant phase of MM called MGUS, which progresses to MM at a rate of 1% per year. Progression events include mutations of genes in the RAS/MAPK pathways; chromosomal rearrangements of the MYC locus; and mutations, amplifications, or biallelic deletions of genes in the NFkB pathway. Among the latter, the most common are deletions of TRAF3 or cIAP1 and -2 (BIRC2 and -3) and amplification or translocations of NIK (Map3k14), all resulting in NIK stabilization, phosphorylation of IKKα, processing of p100 to p52, NFKB2 nuclear translocation, and activation of the noncanonical NFKB pathway. This can be induced in MM cell lines in vitro by treatment with SMAC mimetics or IAP antagonists (IAPa), which induce cIAP1 auto-ubiquitination and degradation. We postulated that the resulting constitutive activation of the noncanonical NFKB pathway would render MM cells independent on the BM-localized survival signals provided by BAFF and APRIL, leading to extramedullary dissemination. We tested this hypothesis in vivo by exposing Vk*MYC mice with BM-localized MM to the IAP antagonist LCL161 and monitoring disease progression.

In the immunocompetent Vk*MYC transgenic mouse model MYC expression is sporadically induced in germinal center cells by AID-dependent somatic hypermutation, leading to progressive accumulation of monoclonal plasma cells restricted to the BM that can be monitored over time by SPEP and M-spike quantization, as is done clinically in MM patients. Through a very rigorous testing of drugs whose clinical activity in MM has been assessed in clinical trials, we have also validated Vk*MYC mice as a faithful model to predict drug activity in MM, with a positive predictive value of 73% and a negative predictive value of 92%. To our surprise, treatment of Vk*MYC mice with the IAPa LCL161 did not promote MM extramedullary dissemination as we predicted, but rather induced a significant reduction in M-spike levels, indicative of reduction of tumor burden. Based on these preclinical data, we conducted a phase 2 clinical trial of LCL161 in combination with cyclophosphamide in relapsed/refractory MM patients. Results from 25 patients enrolled in the study demonstrated a response in five patients (1CR, 1 VGPR, 2PR, 1MR) and a median PFS of 10 months.

Consistent with in vitro data on MM cell lines, LCL161 treatment did not promote direct apoptosis of Vk*MYC cells but phagocytosis of live cells, detectable by intravital microscopy within four hours after treatment. While LCL161 treatment of BM-derived macrophages (MΦ) in vitro promoted the acquisition of an inflammatory phenotype, with increased expression of IL6, IL12, and CD86 that resembled activation with CD40 agonists, it did not increase their phagocytic ability. In contrast, exposure of MΦ to LCL161 treated tumor cells or to the conditioned medium collected after LCL161 treatment enhanced phagocytosis. We therefore performed gene expression analysis on MM cells collected from Vk*MYC mice and MM patients before and after LCL161 treatment and identified marked upregulation of NFKB target genes, as expected, but also a type-I interferon (IFN) signature. Interestingly, the presence of blocking antibody to the IFN receptor subunit 1 (aIFNAR1) during LCL161 treatment of tumor cells inhibited their phagocytosis in vitro, and pretreatment of Vk*MYC mice with aIFNAR antibody abrogated LCL161 activity in vivo. Pretreatment of Vk*MYC mice with liposomal clodronate, known to deplete phagocytic cells, also abrogated LCL161 activity, indicating that MΦ-dependent phagocytosis, stimulated by the LCL161 induced tumor cell autonomous IFN response, is required for the anti-MM effects of LCL161.

LCL161 has been shown to promote T-cell function in vitro, but in our hand LCL161 retained full anti-MM activity in T-cell deficient mice. However, while ~15% of immunocompetent mice were cured by just two weeks of LCL161 treatment, none of the T-cell deficient mice were, indicating a role for T cells in preventing disease recurrence. Consistently, rechallenge of cured mice with specific MM tumors induced expansion of IFNγ-producing T cells, indicative of reactivation of immunologic memory. We were surprised by the hyporesponsiveness of T cells to LCL161 in vivo and attributed it to the high levels of expression of the coinhibitory molecule PD1 we noted on both CD4 and CD8 cells irrespective of treatment. We also detected high levels of PDL1 on most CD11b+ myeloid cells as well as on a fraction of MM cells, which increased following LCL161 treatment. We therefore sought to combine LCL161 with an antibody against PD1. Remarkably, the combination of LCL161 with aPD1 was curative in all the mice that completed the two-week treatment, and we are in the process of translating this discovery to the clinic by opening a new trial of LCL161 in combination with aPD1.

Citation Format: Marta Chesi, David Fooksman, Peter Leif Bergsagel. Immunomodulatory therapy of multiple myeloma with IAP antagonists [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr IA21.

STING-Licensed Macrophages Prime Type I IFN Production by Plasmacytoid Dendritic Cells in the Bone Marrow during Severe Plasmodium yoelii Malaria

Malaria remains a global health burden causing significant morbidity, yet the mechanisms underlying disease outcomes and protection are poorly understood. Herein, we analyzed the peripheral blood of a unique cohort of Malawian children with severe malaria, and performed a comprehensive overview of blood leukocytes and inflammatory mediators present in these patients. We reveal robust immune cell activation, notably of CD14⁺ inflammatory monocytes, NK cells and plasmacytoid dendritic cells (pDCs) that is associated with very high inflammation. Using the Plasmodium yoelii 17X YM surrogate mouse model of lethal malaria, we report a comparable pattern of immune cell activation and inflammation and found that type I IFN represents a key checkpoint for disease outcomes. Compared to wild type mice, mice lacking the type I interferon (IFN) receptor exhibited a significant decrease in immune cell activation and inflammatory response, ultimately surviving the infection. We demonstrate that pDCs were the major producers of systemic type I IFN in the bone marrow and the blood of infected mice, via TLR7/MyD88-mediated recognition of Plasmodium parasites. This robust type I IFN production required priming of pDCs by CD169⁺ macrophages undergoing activation upon STING-mediated sensing of parasites in the bone marrow. pDCs and macrophages displayed prolonged interactions in this compartment in infected mice as visualized by intravital microscopy. Altogether our findings describe a novel mechanism of pDC activation in vivo and precise stepwise cell/cell interactions taking place during severe malaria that contribute to immune cell activation and inflammation, and subsequent disease outcomes.

The Plasmodium parasite is the number one killer among human parasitic diseases worldwide. Protection is associated with length of exposure for people living in endemic areas, with severe disease primarily affecting young children. Inflammation is a key component in the pathophysiology in malaria, and disease severity has been linked to the degree of activation of the immune system. However, the underlying mechanisms of protection and disease outcomes remain poorly understood. We provide a comprehensive analysis of peripheral blood immune cells obtained from a cohort of children with severe malaria. Our results show heightened inflammation and immune cell activation, in particular for monocytes, natural killer cells, and plasmacytoid dendritic cells (pDCs). We have also utilized a mouse model of lethal malaria that recapitulates many features identified in this cohort of severe malaria patients to examine drivers of immune cell activation and inflammation. Our studies provide evidence that type I interferon (IFN) acts as an early switch in inducing a potent inflammatory response in the infected host. Type I IFN production is massively produced in the bone marrow and the blood of infected mice by plasmacytoid dendritic cells (pDCs), a subset of DCs. We also demonstrate that resident macrophages in the bone marrow, control type I IFN production by the pDCs. We define how both myeloid cells “sense” the parasite to initiate the host immune response and report a previously uncharacterized physical interaction between pDCs and macrophages in the bone marrow as visualized by intravital microscopy in vivo. Our results define cellular processes underlying the marked inflammation of severe malaria and could open novel therapeutic opportunities to improve outcomes in this important human infectious disease.

Aging-related changes in lymphatic collectors predispose to pathogen dissemination in tissues (INC4P.342)

Herein we analyze how the aging process affects the structure and functionality of the lymphatic collectors (LCs) with reference to their ability to maintain pathogen clearance. Ultrastructural, biochemical and proteomic analysis indicated a loss of extracellular matrix proteins, an increase in protein oxidative modifications as well as activation of nuclear factor-κB signaling as sign of “low grade” inflammation in aged LCs. This resulted in a decrease in contractile and pumping activity of LCs, as measured in vivo. Functionally, this impairment also translated into a reduced ability for in vivo bacterial transport as determined by time-lapse microscopy. Ultrastructural and proteomic analysis also indicated a decrease in the thickness of the endothelial cell glycocalyx and loss of gap-junction proteins in aged LCs. Redox proteomic analysis mapped an aging-related increase in the glycation and carboxylation of endothelial cell glycocalyx structural proteins. Functionally, these modifications translated into higher ability of the pathogen to escape from aged LCs into the surrounding tissue. Altogether, our analysis mapped the complexity of the aging-related anatomical, biochemical and functional changes in LCs. The decreased ability to transport bacteria to the draining nodes, associated with increased bacterial escape in the surrounding tissue can contribute to the decreased ability of the immune system to clear pathogens in the elderly, as observed in immunosenescence.

Myeloid cells regulate plasma cell responses in the lymph node (P1066)

Differentiation and survival of plasma cells requires the coordinated signaling from many different cell types, and these requirements vary depending on the tissue. In the lymph node, differentiating plasma cells migrate to the medullary cords where they can either arrest or exit the lymph node. Previously, we imaged and characterized plasmablast migration by intravital microscopy in the lymph node by transferring B cells expressing a YFP plasma cell reporter. Using recipient mice that express GFP in myelomonocytic cells, we observed that plasmablasts migrated along these cells in the medullary cords and arrested in contact with them. Since several cell types express GFP in this strain, we used a various ablation techniques to narrow down which subsets were engaged with plasmablasts and determine what role they perform. Ablation of one subset increased plasmablast numbers by 3 fold. This increase was largely due enhanced cell proliferation, but plasmablast apoptosis was also reduced. We have evidence to suggest that cytokine signaling and adhesion molecules play important roles. Localization of plasma cells in the lymph nodes may play an important function in controlling infection locally during early stages, while under autoimmune conditions, excessive plasma cell recruitment and survival can be detrimental. Regulating plasma cell survival and localization may provide ways to manipulating the immune response in both contexts.

Development and Migration of Pre-Plasma Cells in the Mouse Lymph Node (44.5)

Plasma cells (PCs) play important roles in the acute response to infection and the long-term protection of the host by acting as antibody factories. In this study, we imaged the differentiation and migratory behavior of nascent plasma cells (PC) in lymph nodes of mice by intravital two-photon microscopy using a Blimp-1-YFP transgene. Pre-PCs differentiate in the germinal center and migrate to the medullary cords where they become sessile plasma cells. Their migration has been shown to be chemokine-dependent based on in vitro chemotaxis assays and histological sections using chemokine receptor-deficient cells. Here we identify a novel migration pattern for pre-PCs, which is unique among previously-described lymphocytes and uses an alternative pathway for motility.

Membrane cholesterol, lateral mobility, and the phosphatidylinositol 4,5-bisphosphate-dependent organization of cell actin

Responses to cholesterol depletion are often taken as evidence of a role for lipid rafts in cell function. Here, we show that depletion of cell cholesterol has global effects on cell and plasma membrane architecture and function. The lateral mobility of membrane proteins is reduced when cell cholesterol is chronically or acutely depleted. The change in mobility is a consequence of the reorganization of the cell actin. Binding of a GFP-tagged pleckstrin homology domain specific for phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] to the plasma membrane is reduced after cholesterol depletion. This result implies that the reorganization of cytoskeleton depends on the loss or redistribution of plasma membrane PI(4,5)P2. Consistent with this observation, agents that sequester plasma membrane PI(4,5)P2 mimic the effects of cholesterol depletion on actin organization and on lateral mobility.