Dendritic Cell Precursors Research Articles

Characterization and ontogeny of a novel lymphoid follicle inducer cell during development of the bursa of Fabricius

The avian bursa of Fabricius (BF) is a primary lymphoid organ, where B-cell development occurs within bursal follicles of epithelial origin. During embryogenesis the epithelial anlage of the BF emerges as a diverticulum of the cloaca surrounded by undifferentiated tail bud mesenchyme. While it is believed that the epithelial-mesenchymal BF primordium provides a selective microenvironment for developing B cells, the initial events inducing lymphoid follicle formation are not fully elucidated. Using wild type and CSF1R-eGFP transgenic chick embryos, we find that separate B cell, macrophage and dendritic cell precursors enter the BF mesenchyme, migrate to the surface epithelium, and colonize the lymphoid follicle buds. Detailed immunocytochemical characterization revealed a novel EIV-E12+ blood-borne cell type, colonizing the surface epithelium of the BF rudiment before the entry of myeloid and lymphoid lineages and the appearance of this cell type coincides with the onset of follicle bud formation. Chick-duck chimeras and chick-quail tissue recombination experiments suggest that EIV-E12+ cells represent a transient lymphoid inducer cell population. They are not dendritic or B cells precursors, and they are capable of follicle bud induction in both dendritic cell- and B cell-depleted bursae.

Abstract We055: Role of Classical Dendritic Cells in the Sterile Inflammatory Response Following Myocardial Infarction

Myocardial infarction and the subsequent development of chronic ischemic cardiomyopathy is the most common cause of congestive heart failure. Infarction is associated with an intense sterile inflammatory response which can lead to adverse remodeling over the long term. Dendritic cells (DCs) are antigen presenting cells that function as a bridge between innate and adaptive immunity. Precursor DCs give rise to several distinct classical DC (cDC) subsets that mediate a variety of functions. The cDC1 subset is essential for cross-presentation of antigens to CD8 + T cells, while the cDC2 lineage is a potent inducer of CD4 + T cell proliferation. The aim of the study was to characterize the profile of the cDC populations present during the various stages of the development of chronic ischemic cardiomyopathy. We hypothesized that cDCs are essential mediators of chronic LV remodeling in the period following ischemic myocardial injury. We sought to determine which specific cDC subtype (cDC1 vs cDC2) forms the predominant population in the heart and secondary lymphoid tissues following ischemic injury. Adult male C57BL/6 (WT) mice underwent non-perfused myocardial infarction by ligating the left anterior descending artery (LAD) to induce sterile injury, with sham operated mice used as controls. Left ventricular (LV) remodeling was assessed by echocardiography at 1 w and 8 w following surgery. Alterations in cDC1 (CD45+CD11c+CD103-XCR1-) and cDC2 (CD45+CD11c+CD103+XCR1+) profile were examined by flow cytometry. When compared with the sham operated mice, mice with ischemic cardiomyopathy following LAD ligation exhibited the following features. There is an initial influx of cDC2 into the myocardium 1 w after MI (mean of 3.0% vs 0.26% of CD45 + leukocytes, p=0.02). This is followed by a subsequent influx of cDC1 into the myocardium 4 weeks after MI (mean of 1.98% vs 0.16% of CD45 + leukocytes, p=0.001). This influx of cDC into the heart is accompanied by an egress of cDC from the spleen (mean of 0.37% vs 0.76% of CD45 + leukocytes, P=0.001) at 4 weeks following MI. In conclusion, the findings of our study suggest that there is an initial influx of cDC2 followed by a more sustained influx of cDC1 into the myocardium in the weeks following an infarct. Antigen presentation by cDC likely plays a critical role in driving the inflammatory response and inducing chronic LV remodeling in the period following MI.

Blastic Plasmacytoid Dendritic Cell Neoplasm, from a Dermatological Point of View.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive hematological malignancy derived from the precursors of plasmacytoid dendritic cells. Although disease awareness has increased over time, BPDCN represents a rare disease with an aggressive clinical course and a dismal prognosis. Due to the overlap in clinical and histological features with a large spectrum of inflammatory and neoplastic diseases, BPDCN is difficult to diagnose. Furthermore, given the rarity of the disease, treatment options for BPDCN are limited, sometimes changing by practitioner and hospitals. Treatment options range from conventional chemotherapy to the recently approved biologic agent tagraxofusp and stem cell transplantation. Therefore, a multidisciplinary approach with coordination among dermatologists, pathologists, and hematologists is ultimately imperative to reach the correct diagnosis and management of BPDCN.

Novel genetically glycoengineered human dendritic cell model reveals regulatory roles of α2,6-linked sialic acids in DC activation of CD4+ T cells and response to TNFα.

Dendritic cells (DCs) are central for the initiation and regulation of appropriate immune responses. While several studies suggest important regulatory roles of sialoglycans in DC biology, our understanding is still inadequate primarily due to a lack of appropriate models. Previous approaches based on enzymatic- or metabolic-glycoengineering and primary cell isolation from genetically modified mice have limitations related to specificity, stability, and species differences. This study addresses these challenges by introducing a workflow to genetically glycoengineer the human DC precursor cell line MUTZ-3, described to differentiate and maturate into fully functional dendritic cells, using CRISPR-Cas9, thereby providing and validating the first isogenic cell model for investigating glycan alteration on human DC differentiation, maturation, and activity. By knocking out (KO) the ST6GAL1 gene, we generated isogenic cells devoid of ST6GAL1-mediated α(2,6)-linked sialylation, allowing for a comprehensive investigation into its impact on DC function. Glycan profiling using lectin binding assay and functional studies revealed that ST6GAL1 KO increased the expression of important antigen presenting and co-stimulatory surface receptors and a specifically increased activation of allogenic human CD4 + T cells. Additionally, ST6GAL1 KO induces significant changes in surface marker expression and cytokine response to TNFα-induced maturation, and it affects migration and the endocytic capacity. These results indicate that genetic glycoengineering of the isogenic MUTZ-3 cellular model offers a valuable tool to study how specific glycan structures influence human DC biology, contributing to our understanding of glycoimmunology.

The cholera toxin B subunit induces trained immunity in dendritic cells and promotes CD8 T cell antitumor immunity.

Innate immune training is a metabolic, functional, and epigenetic long-term reprogramming of innate cells triggered by different stimuli. This imprinting also reaches hematopoietic precursors in the bone marrow to sustain a memory-like phenotype. Dendritic cells (DCs) can exhibit memory-like responses, enhanced upon subsequent exposure to a pathogen; however, whether this imprinting is lineage and stimulus-restricted is still being determined. Nevertheless, the functional consequences of DCs training on the adaptive and protective immune response against non-infectious diseases remain unresolved. We evaluated the effect of the nontoxic cholera B subunit (CTB), LPS and LTA in the induction of trained immunity in murine DCs revealed by TNFa and LDH expression, through confocal microscopy. Additionally, we obtained bone marrow DCs (BMDCs) from mice treated with CTB, LPS, and LTA and evaluated training features in DCs and their antigen-presenting cell capability using multiparametric cytometry. Finally, we design an experimental melanoma mouse model to demonstrate protection induced by CTB-trained DCs in vivo. CTB-trained DCs exhibit increased expression of TNFa, and metabolic reprogramming indicated by LDH expression. Moreover, CTB training has an imprint on DC precursors, increasing the number and antigen-presenting function in BMDCs. We found that training by CTB stimulates the recruitment of DC precursors and DCs infiltration at the skin and lymph nodes. Interestingly, training-induced by CTB promotes a highly co-stimulatory phenotype in tumor-infiltrating DCs (CD86+) and a heightened functionality of exhausted CD8 T cells (Ki67+, GZMB+), which were associated with a protective response against melanoma challenge in vivo. Our work indicates that CTB can induce innate immune training on DCs, which turns into an efficient adaptive immune response in the melanoma model and might be a potential immunotherapeutic approach for tumor growth control.

Identification of macaque dendritic cell precursors in blood and tissue reveals their dysregulation in early SIV infection

Distinct dendritic cell (DC) subsets play important roles in shaping immune responses. Circulating DC precursors (pre-DCs) are more susceptible to HIV infection invitro, which may explain the inefficiency of immune responses against HIV. However, the interplay between HIV and pre-DC is not defined invivo. We identify human pre-DC equivalents in the cynomolgus macaque and then analyze their dynamics during simian immunodeficiency virus (SIV) infection to illustrate a sharp decrease of blood pre-DCs in early SIV infection and accumulation in lymph nodes (LNs), where they neglect to upregulate CD83/CD86 or MHC-II. Additionally, SIV infection attenuates the capacity of stimulated LN pre-DCs to produce IL-12p40. Analysis of HIV cohorts provides correlation between costimulatory molecule expression on pre-DCs and Tcell activation in spontaneous HIV controllers. These findings pinpoint certain dynamics and functional changes of pre-DCs during SIV infection, providing a deeper understanding of immune dysregulation mechanisms elicited in people living with HIV.

Retinoic acid-loaded liposomes induce human mucosal CD103+ dendritic cells that inhibit Th17 cells and drive regulatory T-cell development in vitro.

The active vitamin A metabolite, all-trans-retinoic acid (RA), primes precursor dendritic cells (DCs) into a mucosal phenotype with tolerogenic properties characterized by the expression of integrin CD103. CD103+ DCs can counteract pathogenic Th1 and Th17 in inflammatory bowel disease (IBD) or celiac disease (CD). Tolerogenic manipulation of DCs using nanoparticles carrying tolerogenic adjuvants and disease-specific antigens is a valuable treatment strategy to induce antigen-specific mucosal tolerance in vivo. Here, we investigated the effects of RA-loaded liposomes on human DC phenotype and function, including DC-driven T-cell development, both during the generation of monocyte-derived DCs (moDCs) as well as by priming immature moDCs. RA liposomes drove CD103+ DC differentiation as well as ALDH1A2 expression in DCs. Neutrophil-dependent Th17 cell development was reduced by RA-liposome-differentiated and RA-liposome-primed DCs. Moreover, RA liposome treatment shifted T-cell development toward a Th2 cell profile. Importantly, RA liposomes induced the development of IL-10-producing and FoxP3+ regulatory T cells (Tregs) of various Treg subsets, including ICOS+ Tregs, that were potent inhibitors of bystander memory T-cell proliferation. Taken together, RA-loaded liposomes could be a novel treatment avenue for IBD or CD patients.

Chromatin-binding deubiquitinase MYSM1 acts in haematopoietic progenitors to control dendritic cell development and to program dendritic cell responses to microbial stimulation.

Dendritic cells (DCs) are the most significant antigen presenting cells of the immune system, critical for the activation of naïve T cells. The pathways controlling DC development, maturation, and effector function therefore require precise regulation to allow for an effective induction of adaptive immune response. MYSM1 is a chromatin binding deubiquitinase (DUB) and an activator of gene expression via its catalytic activity for monoubiquitinated histone H2A (H2A-K119ub), which is a highly abundant repressive epigenetic mark. MYSM1 is an important regulator of haematopoiesis in mouse and human, and a systemic constitutive loss of Mysm1 in mice results in a depletion of many haematopoietic progenitors, including DC precursors, with the downstream loss of most DC lineage cells. However, the roles of MYSM1 at the later checkpoints in DC development, maturation, activation, and effector function at present remain unknown. In the current work, using a range of novel mouse models (Mysm1flCreERT2, Mysm1flCD11c-cre, Mysm1DN), we further the understanding of MYSM1 functions in the DC lineage: assessing the requirement for MYSM1 in DC development independently of other complex developmental phenotypes, exploring its role at the later checkpoints in DC maintenance and activation in response to microbial stimulation, and testing the requirement for the DUB catalytic activity of MYSM1 in these processes. Surprisingly, we demonstrate that MYSM1 expression and catalytic activity in DCs are dispensable for the maintenance of DC numbers in vivo or for DC activation in response to microbial stimulation. In contrast, MYSM1 acts via its DUB catalytic activity specifically in haematopoietic progenitors to allow normal DC lineage development, and its loss results not only in a severe DC depletion but also in the production of functionally altered DCs, with a dysregulation of many housekeeping transcriptional programs and significantly altered responses to microbial stimulation.

Breakthrough in Blastic Plasmacytoid Dendritic Cell Neoplasm Cancer Therapy Owing to Precision Targeting of CD123.

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive hematologic cancer originating from the malignant transformation of plasmacytoid dendritic cell precursors. This malignancy progresses rapidly, with frequent relapses and a poor overall survival rate, underscoring the urgent need for effective treatments. However, diagnosing and treating BPDCN have historically been challenging due to its rarity and the lack of standardized approaches. The recognition of BPDCN as a distinct disease entity is recent, and standardized treatment protocols are yet to be established. Traditionally, conventional chemotherapy and stem cell transplantation have been the primary methods for treating BPDCN patients. Advances in immunophenotyping and molecular profiling have identified potential therapeutic targets, leading to a shift toward CD123-targeted immunotherapies in both clinical and research settings. Ongoing developments with SL-401, IMGN632, CD123 chimeric antigen receptor (CAR) T-cells, and bispecific antibodies (BsAb) show promising advancements. However, the therapeutic effectiveness of CD123-targeting treatments needs improvement through innovative approaches and combinations of treatments with other anti-leukemic drugs. The exploration of combinations such as CD123-targeted immunotherapies with azacitidine and venetoclax is suggested to enhance antineoplastic responses and improve survival rates in BPDCN patients. In conclusion, this multifaceted approach offers hope for more effective and tailored therapeutic interventions against this challenging hematologic malignancy.

BCL2 inhibition stimulates dendritic cell function for improved anticancer immunotherapy.

Recently we developed a dendritic cell (DC) genotype-phenotype screening platform that is based on CRISPR/Cas9-mediated gene editing of immortalized DC precursors. Whole genome screening for gain-of-function phenotypes led to the identification of BCL2 as a DC-specific immune checkpoint. Genetic or pharmacological inhibition of BCL2 similarly enhanced the antigen presentation capacity of conventional type-1 dendritic cells (cDC1) and mediated T cell-dependent anticancer immunity. The therapeutic anticancer efficacy of the BCL2 inhibitor venetoclax in mice was further increased when combined with a PD-1-targeted immune checkpoint inhibitor. In sum, we delineated a novel strategy of dual checkpoint blockade for cancer immunotherapy in which improvement of DC antigen presentation and avoidance of T cell exhaustion can be advantageously combined.

Variegate expression of Cre recombinase in hematopoietic cells in CD11c-cre transgenic mice

In this study, we performed an in-depth analysis of Cre expression in the widely used CD11c-Cre transgenic mice generated by the group of Boris Reizis. In contrast to previous observation, using the highly sensitive Rosa-26-floxed-tdTomato reporter mouse line, we show variegated expression of Cre in multiple hematopoietic linage cells starting in hematopoietic stem cells. Indeed, we found that in the CD11c-Cre driver mice, Cre is expressed in cDC linage cells and pDC starting from the myeloid dendritic cell precursor, as expected, but also in a substantial fraction of hematopoietic stem cells and common lymphoid progenitors and, consequently, in >50% of all leukocytes. Hence, this study indicates that the reporter mice used to characterize Cre expression in Cre-driver mice should be selected with caution and considering the sensitivity of the reporter system. This study also suggests that the interpretation of some reports using this CD11c-Cre transgenic mice may need to be re-considered based on a careful evaluation of the cell type-specificity of Cre-mediated in their model.

Phenotypic and functional differences of dendritic cells in tumor.

Dendritic cells (DCs) are a unique class of immune cells vital to the immune system, functioning as antigen-presenting cells that play a key role in launching both cellular and humoral immune responses. They are crucial in preventing infectious diseases and regulating tumor growth. DCs can be categorized based on various criteria such as phenotype, function, and tissue location, resulting in several subgroups. Generally, DCs are divided into two primary groups: plasmacytoid DCs (pDCs) and conventional DCs (cDCs), which are further classified into Type I classical DCs (cDC1) and Type II classical DCs (cDC2). cDC1 cells are distinguishable by specific gene programs and associated markers, while cDC2 cells display more diversity. Moreover, there is an ongoing debate surrounding a recently identified subgroup called DC3, and whether it can be considered a distinct cell type in the maturation process of DCs remains uncertain. Most of these DC subgroups rely on the growth factor Fms-like tyrosine kinase 3 ligand (FLT3L) for differentiation from a common DC precursor (CDP), guided by various cytokines. Although the general classification of DC subgroups is similar in both humans and mice, numerous phenotypic and functional variations exist within each subgroup. Therefore, comprehending these differences between DC subgroups in humans and mice holds the potential to significantly advance relevant research.

Identification of dendritic cell precursor from the CD11c+ cells expressing high levels of MHC class II molecules in the culture of bone marrow with FLT3 ligand.

Dendritic cells (DCs) are readily generated from the culture of mouse bone marrow (BM) treated with either granulocyte macrophage-colony stimulating factor (GM-CSF) or FMS-like tyrosine kinase 3 ligand (FLT3L). CD11c+MHCII+ or CD11c+MHCIIhi cells are routinely isolated from those BM cultures and generally used as in vitro-generated DCs for a variety of experiments and therapies. Here, we examined CD11c+ cells in the BM culture with GM-CSF or FLT3L by staining with a monoclonal antibody 2A1 that is known to recognize mature or activated DCs. Most of the cells within the CD11c+MHCIIhi DC gate were 2A1+ in the BM culture with GM-CSF (GM-BM culture). In the BM culture with FLT3L (FL-BM culture), almost of all the CD11c+MHCIIhi cells were within the classical DC2 (cDC2) gate. The analysis of FL-BM culture revealed that a majority of cDC2-gated CD11c+MHCIIhi cells exhibited a 2A1-CD83-CD115+CX3CR1+ phenotype, and the others consisted of 2A1+CD83+CD115-CX3CR1- and 2A1-CD83-CD115-CX3CR1- cells. According to the antigen uptake and presentation, morphologies, and gene expression profiles, 2A1-CD83-CD115-CX3CR1- cells were immature cDC2s and 2A1+CD83+CD115-CX3CR1- cells were mature cDC2s. Unexpectedly, however, 2A1-CD83-CD115+CX3CR1+ cells, the most abundant cDC2-gated MHCIIhi cell subset in FL-BM culture, were non-DCs. Adoptive cell transfer experiments in the FL-BM culture confirmed that the cDC2-gated MHCIIhi non-DCs were precursors to cDC2s, i.e., MHCIIhi pre-cDC2s. MHCIIhi pre-cDC2s also expressed the higher level of DC-specific transcription factor Zbtb46 as similarly as immature cDC2s. Besides, MHCIIhi pre-cDC2s were generated only from pre-cDCs and common DC progenitor (CDP) cells but not from monocytes and common monocyte progenitor (cMoP) cells, verifying that MHCIIhi pre-cDC2s are close lineage to cDCs. All in all, our study identified and characterized a new cDC precursor, exhibiting a CD11c+MHCIIhiCD115+CX3CR1+ phenotype, in FL-BM culture.

Epidemiology and Survival Outcomes in Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN): A US Population-Based Study

Background: Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare, clinically aggressive hematological malignancy arising from plasmacytoid dendritic cell precursors. BPDCN remains poorly understood due to its rarity, constantly changing nomenclature, and defining criteria. This study using the Surveillance, Epidemiology, and End Results Program (SEER) database is the largest and most recent population-based study in BPDCN. We aimed to determine the incidence, clinical characteristics, overall survival, and prognostic factors in BPDCN. Methods: Patients diagnosed with BPDCN from 2000-2019 were identified from the SEER 17 database using the International Classification of Disease for Oncology, 3 rd edition (ICD-O-3) code 9727. Patient characteristics, treatment, and survival information were obtained. Age-adjusted incidence rate per 100,000 population and incidence rate ratio (IRR) were calculated using SEER stat software with the year 2000 US population as the reference standard. Overall survival and prognostic factors were evaluated based on univariate survival analysis using Kaplan-Meier plot with log-rank test, and Cox proportional hazard regression model for multivariate survival analysis. All statistical analyses were conducted with a two-sided significant p value of <0.05 using SEER stat software and STATA software version 15.1. Results: A total of 844 patients with BPDCN were included in the study. The median age at diagnosis was 40 (IQR: 15-68) with majority < 20 years (32.5%) or > 60 years (34.3%) . Majority were males (68.1%) and Caucasians (83.5%). The lymph node was the most common primary site of involvement (56.6%) followed by the bone marrow (19.1%) and skin (13%). . Eighteen percent of the cohort received radiation therapy and 83.6% received chemotherapy. The overall incidence of BPDCN was found to be 0.05 cases per 100,000 population. Incidence was significantly lower in females than males (IRR 0.62, p < 0.01) and in African Americans compared to Caucasians (IRR 0.70m, p<0.01). There was no significant change in disease incidence from 2005 to 2019. Survival rates at 1 year, 2 years and 5 years were found to be 70%, 56% and 41% respectively. On univariate analysis, age > 60 years (p <0.01) and male gender (p< 0.01) was associated with significantly worse overall survival while survival did not differ by race (p=0.10). On multivariate analysis after adjusting for age, sex, race, marital status, radiation therapy, chemotherapy and disease primary site, age > 60 years (HR 2.70; 95% CI 2.01-3.54, p<0.01) and African American race (HR 1.44; 95% CI 1.01-2.04, p<0.04) was associated with worse overall survival. Conclusion: To our knowledge, this is the largest and most recent population-based study in BPDCN. Our study revealed a population level incidence of 0.05 cases per 100,000 population. Majority are Caucasian males with bimodal age distribution (<20 years and above 60 years). The lymph nodes are the most common site of involvement. Older age and African American ethnicity are negative prognostic factors for overall survival. Racial disparities in OS warrant further prognosis. More prospective studies are needed to determine disease characteristics and optimal treatment approaches for this rare life-threatening hematological malignancy.

A Rare Case of CD1a +/CD207 + and S100 - Langerhans Cell Histiocytosis with Multi-System Risk Organ Involvement in Adult

Introduction: Langerhans Cell Histiocytosis (LCH) is a rare neoplastic disorder of myeloid dendritic cell precursors, leading to a spectrum of organ involvement and dysfunction. The nonspecific presentations result in missed or delayed diagnoses in adults, and data to guide the clinical care of adults with LCH is limited to case studies and series. Case Description: A 40-year-old Hispanic woman presented to the Emergency Department with ongoing symptoms of shortness of breath, muscle cramps, and intermittent chest pain for two weeks. She was found to have microcytic-hypochromic anemia requiring packed Red Blood Cells (pRBC) transfusion. A CT pulmonary angiogram showed incidental findings of splenomegaly and right axillary lymphadenopathy. A month before this presentation, she was evaluated for symptoms of polyuria, polydipsia, nocturia, and amenorrhea, ongoing for eight months. Arginine Vasopressin Deficiency (AVP-D) was diagnosed, and desmopressin was initiated. MRI Brain had revealed enhancement of the infundibular stalk. On follow-up in the clinic, an ultrasound of the right axilla showed a 2.4 cm lymph node (LN) with thickened cortex. Over the following month, she continued to require multiple pRBC transfusions. Further workup with Fluorodeoxyglucose (FDG)-Positron Emission Tomography (PET) revealed hypermetabolic splenomegaly with multifocal uptake, including the right axilla suspicious for metastatic disease. ( Figure 1-A) A biopsy of the right axillary LN ( Figure 2 - A, B) showed clusters of Langerhans cells and immunohistochemistry positive for CD1a, CD3, CD20, CD68, keratin, and CD 207 (Langerin) staining and variable positivity for S100. Next-Generation Sequencing showed BRAF V600E missense mutation and NOTCH3, DICER1, CREBBP, JAK1, and CTNNA1 as Variants of Unknown Significance (VUS). Bone marrow (BM) aspirate and flow cytometry were unremarkable. A diagnosis of Multi-System LCH with Risk Organ Involvement (MS-RO +) was made. Cytarabine therapy was started at 150 mg/m 2(days 1-5, 28-day cycle). She was hospitalized twice due to sepsis, leukocytosis, severe anemia: Hemoglobin (Hb) 3.1, and grade 4 thrombocytopenia. Cycle 2 was delayed, and the dose was reduced to 50%. Post-infusions, Hb decline necessitated multiple pRBC transfusions. Repeat FDG-PET after cycles 2 and 5 ( Figure 1-B, C) showed continued metabolic and anatomic response to therapy. A repeat brain MRI showed no change in pituitary findings. The patient continues to improve clinically with plans to complete a total of one year of chemotherapy. Discussion: LCH is defined by the presence of CD1a +/CD207 + histiocytes. Although usually positive for S-100, rare S-100 negative variants have been reported in skin-limited LCH and LC-Sarcoma. An aberrant maturation process was proposed as a possible explanation, but the exact pathology remains unknown. In children <15 years, the incidence is 2.6-8.9 cases/million/year; in adults, the incidence is less defined at 0.07 cases/million/year and is higher in Hispanics. Adult LCH typically presents after the fourth decade; two-thirds have multi-system involvement. Hematopoietic dysfunction, cytopenias, and liver involvement carry a poor prognosis. AVP-D is the most frequently associated endocrine abnormality. Whole-exome sequencing has shown very low somatic mutation frequency in LCH compared to other neoplastic disorders. BRAF, a central kinase in the RAS-RAF-MEK pathway, has a missense V600E mutation in roughly 50% of LCH, rendering it constitutionally active. Other mutations include BRAF, MAP2K1, ARAF, ERBB3, NRAS, and KRAS, but the VUS mentioned above have been reported infrequently. The LCH-III trial confirmed Vinblastine/Prednisone as the standard regimen for MS-LCH patients <18 years, with or without RO involvement. While it is effective in adults, its limited tolerance favors the use of cytarabine or cladribine. NCT02670707 is an ongoing trial comparing cytarabine monotherapy versus Vinblastine/Prednisone for frontline treatment. BRAF inhibitors are also being increasingly investigated. Prospective trials to optimize the duration of therapy (LCH-IV) and potential combination therapies are currently underway.

Abstract 16602: The PlGF-Nrp1 Interaction Promotes the Splenic Immune Response in Hypertension After Neuroimmune Activation

Introduction: We previously discovered that Placental Growth Factor (PlGF) is released in the spleen by a sympathetic discharge to elicit hypertension. Neuropilin1 (Nrp1) is one of PlGF receptors and, interestingly, is a transmembrane protein expressed on precursors of Dendritic cells (DCs). Hypothesis: Here we investigated the involvement of Nrp1 as the PlGF receptor mediating the neuroimmune activation of immune response during hypertension. Methods: We used flow cytometry and tail cuff to evaluate the involvement of PlGF-Nrp1 pathway in neuroimmune activation of splenic immune response in hypertension. Results: To define Nrp1 role in transducing PlGF effect during hypertension, we evaluate its expression by flow cytometry finding that monocyte-derived DCs (MoDCs) increased Nrp1 expression in response to AngII (12.7±1.3 vs 28.8±3 % Nrp1 Veh vs AngII; p<0.01). This effect was blocked by splenic sympathetic denervation obtained by celiac ganglionectomy (CGX) (28.8±3 vs 14±1.5 % Nrp1 Sham AngII vs CGX AngII; p<0.01). To test the relevance of PlGF-Nrp1 pathway in hypertension, we generated a transgenic mouse model with a selective deletion of Nrp1 in myeloid lineage utilizing the LysMcre (Nrp1 myeKO mice). By using an in vitro standard differentiation protocol of maturation of monocytes in DCs, we observed that CD86 and MHCII expression was up-regulated in MoDCs from the spleen of WT mice cultured in the standard medium plus rmPlGF (0.014±0.001 vs 0.035±0.005 CD86+ DCs/total cells count respectively, p<0.05; 0.010±0.003 vs 0.030±0.003 MHCII+ DCs/total cells count respectively, p<0.01). On the contrary, CD86 and MHCII were similarly expressed on MoDCs from Nrp1 myeKO mice cultured with and without rmPlGF indicating that PlGF is involved in the maturation of MoDCs through its interaction with Nrp1. To confirm the involvement of Nrp1 in blood pressure (BP) increase, we infused AngII to Nrp1 myeKO mice, finding that they were protected from hypertension as compared to WT control mice (Systolic BP 106±1 vs 126±2 mmHg respectively, p<0.001). Conclusions: These data demonstrate that PlGF-Nrp1 pathway is a key mechanism that promote the immune response involved in hypertension and blocking the splenic innervation, hampered this response and BP increase.

Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN):Multi-Center Collaboration and Global Registry Program

Background BPDCN is a rare, aggressive hematologic malignancy derived from the precursors of plasmacytoid dendritic cells (pDC), typically presenting with primary cutaneous dissemination and involvement of bone marrow (BM) as well as extramedullary sites including central nervous system (CNS). In 2008, it was recognized by WHO as a distinct entity and is now recognized under Myeloid/Histiocytic/Dendritic neoplasms in the 2022 5th edition update of WHO Classification. Since the recognition of this unique entity, recent efforts have been put forward for greater understanding and international collaboration, however usually focused on North America and Europe(Pemmaraju at al and Pagano et al), with this current effort representing the largest known international efforts including more parts of the world, including Asia, Middle East and Africa. Objective Due to its historical rarity and heterogeneous presentation of disease, as well as differences in availability of therapies across geo-locations, at present there is no worldwide consensus on the management of BPDCN. The outcomes remain poor, and the optimal therapy of disease remains to be determined. Launched on July 1st, 2022, the aim of our global registry is through a multi-center international collaboration to build a large database of patients with BPDCN in order to generate data-driven diagnostic and treatment recommendations. Materials and Methods The registry collects retrospective and prospective data on clinical presentation, diagnostics, treatment regimens and outcomes of patients diagnosed with BPDCN after January 1st, 2010. The data is collected globally through eCRF and Excel form. The Registry is included in ClinicalTrials.gov database (NCT05430971). Results Through July 15 th, 2023, 15 centers from 11 countries (Armenia, Canada, Cyprus, Egypt, Georgia, India, Italy, Taiwan, Turkey, UK and USA) have joined the registry and another 13 centers from USA, UK, Egypt, Iraq, Canada, Netherlands, Tajikistan, Czech Republic, Guatemala, Brazil, Serbia, Germany are onboarding. 27 retrospective and 2 prospective pts are currently included in the registry. 76 % of pts are male, in keeping with historical expectation BPDCN. Median age at diagnosis was 62 years (4-97). In 90 % of patients the tumor cells were triple positive with CD4+, CD56+, CD123+ immunophenotype. 53.6% of patients presented with cutaneous manifestation. 21% of pts received the initial treatment with AML-based regimens, 66% with ALL-based and 7% with lymphoma-based regimens. 21% of pts underwent alloSCT. 35% of pts were diagnosed after 2018, only 1 patient has received tagraxofusp, a CD123-directed therapy that received FDA approval for previously untreated or relapsed/refractory BPDCN in 2018(Pemmaraju N et al NEJM 2019). 69 % of patients achieved complete response (CR), from which 90 % were treated with ALL-based regimens. 52% of patients experienced relapse. Conclusion Current analysis of a 29-patient worldwide cohort of BPDCN demonstrates a high degree of heterogeneity in treatment regimens based on a multitude of considerations including geographic, socio-economic, drug availability, and varied clinical preferences. The outcome of ALL-based treatment was superior as compared to AML-based, but relapse rate remained high. Further global collaboration is needed to collect additional data and to identify the best diagnostic and therapeutic approaches for this challenging disease.

Lymph node medulla regulates the spatiotemporal unfolding of resident dendritic cell networks

Unlike macrophage networks composed of long-lived tissue-resident cells within specific niches, conventional dendritic cells (cDCs) that generate a 3D network in lymph nodes (LNs) are short lived and continuously replaced by DC precursors (preDCs) from the bone marrow (BM). Here, we examined whether specific anatomical niches exist within which preDCs differentiate toward immature cDCs. In situ photoconversion and Prtn3-based fate-tracking revealed that the LN medullary cords are preferential entry sites for preDCs, serving as specific differentiation niches. Repopulation and fate-tracking approaches demonstrated that the cDC1 network unfolded from the medulla along the vascular tree toward the paracortex. During inflammation, collective maturation and migration of resident cDC1s to the paracortex created discontinuity in the medullary cDC1 network and temporarily impaired responsiveness. The decrease in local cDC1 density resulted in higher Flt3L availability in the medullary niche, which accelerated cDC1 development to restore the network. Thus, the spatiotemporal development of the cDC1 network is locally regulated in dedicated LN niches via sensing of cDC1 densities.

Transitional dendritic cells are distinct from conventional DC2 precursors and mediate proinflammatory antiviral responses.

High-dimensional approaches have revealed heterogeneity amongst dendritic cells (DCs), including a population of transitional DCs (tDCs) in mice and humans. However, the origin and relationship of tDCs to other DC subsets has been unclear. Here we show that tDCs are distinct from other well-characterized DCs and conventional DC precursors (pre-cDCs). We demonstrate that tDCs originate from bone marrow progenitors shared with plasmacytoid DCs (pDCs). In the periphery, tDCs contribute to the pool of ESAM+ type 2 DCs (DC2s), and these DC2s have pDC-related developmental features. Different from pre-cDCs, tDCs have less turnover, capture antigen, respond to stimuli and activate antigen-specific naïve T cells, all characteristics of differentiated DCs. Different from pDCs, viral sensing by tDCs results in IL-1β secretion and fatal immune pathology in a murine coronavirus model. Our findings suggest that tDCs are a distinct pDC-related subset with a DC2 differentiation potential and unique proinflammatory function during viral infections.

Comparison and Development of Immunohistochemical Diagnostic Criteria for Blastic Plasmacytoid Dendritic Cell Neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare hematological malignancy derived from the precursors of plasmacytoid dendritic cells. Diagnostic criteria for BPDCN have not been fully established. BPDCN is often diagnosed without other BPDCN markers than the 3 conventional markers (CD4, CD56, and CD123) in practice and case reports, although acute myeloid leukemia/myeloid sarcoma (AML/MS), which is always considered in the differential diagnosis of BPDCN, can express them. We reviewed published case reports on BPDCN and found that the diagnosis was made without any other BPDCN markers than the conventional markers in two-thirds of the cases. Next, 4 representative existing diagnostic criteria were applied to 284 cases of our cohort of BPDCN and mimics. The results differed in 20% (56/284) of the cases. The criterion based on the 3 conventional markers alone had a low concordance rate (80%-82%) with the other 3 criteria, which were almost concordant with each other. However, newly found minor limitations in these criteria prompted us to devise new diagnostic criterion for BPDCN composed of TCF4, CD123, TCL1, and lysozyme. We also revealed that CD123-positive AML/MS patients had a significantly poorer outcome than those with BPDCN and that 12% (24/205) of the cases were non-BPDCN even if all 3 conventional markers were positive, thus clarifying the risk of diagnosing BPDCN without more specific markers. In addition, histopathological features, such as the reticular pattern, which is not seen in BPDCN and suggests AML/MS, were also identified.