Follicular dendritic cells control engulfment of apoptotic bodies by secreting Mfge8

Transplants foster B cell alloimmunity by relaying extracellular vesicles to follicular dendritic cells

B cells play fundamental roles in transplant rejection. However, how allogeneic (allo)-antigens (Ags) are transported from allografts to follicular dendritic cells (FDCs) in lymphoid tissues for development of B cell responses remains unknown. We demonstrated that graft allo-Ags are relayed to FDCs via small extracellular vesicles (sEVs), which activate complement via immunoglobulin M (IgM) bound to vesicle phospholipids. Complement-opsonized allo-sEVs bind splenic marginal-zone B cells that shuttle the vesicles to FDCs, which retain and recycle the allo-sEVs so they are recognized by B cells. Accordingly, graft release of allo-sEVs promoted allo-major histocompatibility complex (MHC) accumulation in FDCs, germinal center formation, Ig switch and affinity maturation, and donor-specific antibodies, which decreased in allografts with impaired sEV secretion or when allo-Ags were delivered via disrupted sEVs. Importantly, human spleen FDCs bound allo-sEVs opsonized with human serum bearing active complement. Our findings provide insight into the mechanisms that lead to antibody-mediated rejection, for which there are no FDA-approved therapies.

Delayed progression of prion disease in mice by polyarginine-facilitated prevention of PrPSc propagation in the spleen

Prions are infective agents composed of abnormally folded prion proteins (PrPSc), which are pathogenic isoforms of normal cellular prion proteins (PrPC) that cause incurable, transmissible, neurodegenerative conditions in mammals called prion diseases. The spread of PrPSc within a host is facilitated by the lymphoreticular system, which uptakes and propagates PrPSc in the periphery and transmits them to the central nervous system. Our previous study showed that poly-l-arginine (PLR), a cationic amino acid polymer, inhibits PrPSc accumulation in neuroblastoma cells with persistent prion infection (ScN2a). Here, we report the beneficial effect of PLR against prions. In the in vitro prion infection experiment, PLR efficiently reduced the titer of prions inoculated to infect cultured N2a cells. In animal experiments, PLR inhibited the accumulation of PrPSc in the spleens of mice intraperitoneally inoculated with prions during asymptomatic periods. Prophylactic administration of PLR significantly prolonged incubation periods in mice intraperitoneally infected with prions, mitigating vacuolation and astrogliosis, although PrPSc level was not dramatically reduced in the brain. However, PrPSc level was reduced and the marginal zone distortion associated with prion infection was prevented in spleens of mice that was intraperitoneally infected with prions and received PLR, even at the terminal stage. Expression of follicular dendritic cell (FDC)-M1 antigens, a marker of FDC activation, and the level of PrPSc colonized within the white pulp of the spleens, as well as co-localization of FDC-M1 antigens and PrPSc, were reduced in these mice during the course of disease, suggesting that PLR counteracts the ability of FDCs that support PrPSc propagation in the spleen. Overall, prophylactically administered PLR suppresses prions in vivo, presumably through cellular control of pathological processes that occur in the spleen and eventually delay prion spread to the brain. This study presents implications for modulating the progress of prion diseases acquired peripherally.

Rapid increase of MFGE8 secretion from endometrial epithelial cells is an indicator of extracellular vesicle mediated embryo maternal dialogue

Successful embryo implantation relies on synchronized dialog between the embryo and endometrium, and the role of extracellular vesicles (EVs) in facilitating this cross-talk has been recently established. In our previous study, milk fat globule-EGF factor 8 protein (MFGE8) was identified as increasing in receptive endometrial epithelial cells (EECs) in response to trophoblastic EVs. However, the dynamics of MFGE8 protein in this context are not completely understood. Therefore, we examined its expression and secretion in EECs exposed to estrogen, progesterone, and trophoblastic EVs to gain deeper insights into its potential as an indicator of EV-mediated embryo-maternal dialogue. Our findings revealed that MFGE8 secretion is sensitive to estrogen and progesterone, and that trophoblastic EVs stimulate their release in both receptive and non-receptive EECs. Furthermore, trophoblast EV function was dose and time-dependent. Notably, the secretion of MFGE8 increased within a short timeframe of 30 min after addition of EVs, suggesting the possibility of rapid processes such as binding, fusion or internalization of trophoblastic EVs within EECs. Interestingly, MFGE8 released from EECs was associated with EVs, suggesting increased EV secretion from EECs in response to embryonic signals. In conclusion, increased MFGE8 secretion in this embryo implantation model can serve as an indicator of EV-mediated embryo-maternal dialogue.

Exploring the potential of MFG-E8 in neurodegenerative diseases

Milk fat globule-epidermal growth factor 8 (MFG-E8) is a multifunctional glycoprotein regulating intercellular interactions in various biological and pathological processes. This review summarizes the effects and mechanisms of MFG-E8 in neurodegenerative diseases (NDDs), emphasizing its roles in inflammation, apoptosis, and oxidative stress. In this review, will also explore the potential of MFG-E8 as a diagnostic biomarker and its therapeutic applications in neurodegenerative disorders. Recent studies have revealed intriguing characteristics of using MFG-E8 as a potential drug for treating various brain disorders. While the discovery, origin, expression, and physiological functions of MFG-E8 in various organs and tissues are well defined, its role in the brain remains less understood. This is particularly true for NDDs, indicating unmet medical needs. Elucidating its role in the brain could position MFG-E8 as a potential treatment for NDDs.

Prion diseases disrupt glutamate/glutamine metabolism in skeletal muscle

In prion diseases (PrDs), aggregates of misfolded prion protein (PrPSc) accumulate not only in the brain but also in extraneural organs. This raises the question whether prion-specific pathologies arise also extraneurally. Here we sequenced mRNA transcripts in skeletal muscle, spleen and blood of prion-inoculated mice at eight timepoints during disease progression. We detected gene-expression changes in all three organs, with skeletal muscle showing the most consistent alterations. The glutamate-ammonia ligase (GLUL) gene exhibited uniform upregulation in skeletal muscles of mice infected with three distinct scrapie prion strains (RML, ME7, and 22L) and in victims of human sporadic Creutzfeldt-Jakob disease. GLUL dysregulation was accompanied by changes in glutamate/glutamine metabolism, leading to reduced glutamate levels in skeletal muscle. None of these changes were observed in skeletal muscle of humans with amyotrophic lateral sclerosis, Alzheimer's disease, or dementia with Lewy bodies, suggesting that they are specific to prion diseases. These findings reveal an unexpected metabolic dimension of prion infections and point to a potential role for GLUL dysregulation in the glutamate/glutamine metabolism in prion-affected skeletal muscle.

Extracellular vesicles containing MFGE8 from colorectal cancer facilitate macrophage efferocytosis

BACKGROUND

Colorectal cancer (CRC) commonly exhibits tolerance to cisplatin treatment, but the underlying mechanisms remain unclear. Within the tumor microenvironment, macrophages play a role in resisting the cytotoxic effects of chemotherapy by engaging in efferocytosis to clear apoptotic cells induced by chemotherapeutic agents. The involvement of extracellular vesicles (EVs), an intercellular communicator within the tumor microenvironment, in regulating the efferocytosis for the promotion of drug resistance has not been thoroughly investigated.

METHODS

We constructed GFP fluorescent-expressing CRC cell lines (including GFP-CT26 and GFP-MC38) to detect macrophage efferocytosis through flow cytometric analysis. We isolated and purified CRC-secreted EVs using a multi-step ultracentrifugation method and identified them through electron microscopy and nanoflow cytometry. Proteomic analysis was conducted to identify the protein molecules carried by CRC-EVs. MFGE8 knockout CRC cell lines were constructed using CRISPR-Cas9, and their effects were validated through in vitro and in vivo experiments using Western blotting, immunofluorescence, and flow cytometric analysis, confirming that these EVs activate the macrophage αvβ3-Src-FAK-STAT3 signaling pathway, thereby promoting efferocytosis.

RESULTS

In this study, we found that CRC-derived EVs (CRC-EVs) enhanced macrophage efferocytosis of cisplatin-induced apoptotic CRC cells. Analysis of The Cancer Genome Atlas (TCGA) database revealed a high expression of the efferocytosis-associated gene MFGE8 in CRC patients, suggesting a poorer prognosis. Additionally, mass spectrometry-based proteomic analysis identified a high abundance of MFGE8 protein in CRC-EVs. Utilizing CRISPR-Cas9 gene edition system, we generated MFGE8-knockout CRC cells, demonstrating that their EVs fail to upregulate macrophage efferocytosis in vitro and in vivo. Furthermore, we demonstrated that MFGE8 in CRC-EVs stimulated macrophage efferocytosis by increasing the expression of αvβ3 on the cell surface, thereby activating the intracellular Src-FAK-STAT3 signaling pathway.

CONCLUSIONS

Therefore, this study highlighted a mechanism in CRC-EVs carrying MFGE8 activated the macrophage efferocytosis. This activation promoted the clearance of cisplatin-induced apoptotic CRC cells, contributing to CRC resistance against cisplatin. These findings provide novel insights into the potential synergistic application of chemotherapy drugs, EVs inhibitors, and efferocytosis antagonists for CRC treatment.

Neutrophils Mediate Protection Against Colitis and Carcinogenesis by Controlling Bacterial Invasion and IL22 Production by γδ T Cells

Neutrophils are the most abundant leukocytes in human blood and play a primary role in resistance against invading microorganisms and in the acute inflammatory response. However, their role in colitis and colitis-associated colorectal cancer is still under debate. This study aims to dissect the role of neutrophils in these pathologic contexts by using a rigorous genetic approach. Neutrophil-deficient mice (Csf3r-/- mice) were used in classic models of colitis and colitis-associated colorectal cancer and the role of neutrophils was assessed by histologic, cellular, and molecular analyses coupled with adoptive cell transfer. We also performed correlative analyses using human datasets. Csf3r-/- mice showed increased susceptibility to colitis and colitis-associated colorectal cancer compared with control Csf3r+/+ mice and adoptive transfer of neutrophils in Csf3r-/- mice reverted the phenotype. In colitis, Csf3r-/- mice showed increased bacterial invasion and a reduced number of healing ulcers in the colon, indicating a compromised regenerative capacity of epithelial cells. Neutrophils were essential for γδ T-cell polarization and IL22 production. In patients with ulcerative colitis, expression of CSF3R was positively correlated with IL22 and IL23 expression. Moreover, gene signatures associated with epithelial-cell development, proliferation, and antimicrobial response were enriched in CSF3Rhigh patients. Our data support a model where neutrophils mediate protection against intestinal inflammation and colitis-associated colorectal cancer by controlling the intestinal microbiota and driving the activation of an IL22-dependent tissue repair pathway.

The follicular dendritic cell: At the germinal center of autoimmunity?

Autoimmune diseases strain healthcare systems worldwide as their incidence rises, and current treatments put patients at risk for infections. An increased understanding of autoimmune diseases is required to develop targeted therapies that do not impair normal immune function. Many autoimmune diseases present with autoantibodies, which drive local or systemic inflammation. This indicates the presence of autoreactive B cells that have escaped tolerance. An important step in the development of autoreactive B cells is the germinal center (GC) reaction, where they undergo affinity maturation toward cognate self-antigen. Follicular dendritic cells (FDCs) perform the essential task of antigen presentation to B cells during the affinity maturation process. However, in recent years, it has become clear that FDCs play a much more active role in regulation of GC processes. Here, we evaluate the biology of FDCs in the context of autoimmune disease, with the goal of informing future therapeutic strategies.

The follicular lymphoma tumor microenvironment at single-cell and spatial resolution

ABSTRACT

Follicular lymphoma (FL) is a generally incurable malignancy that originates from developmentally blocked germinal center B cells residing, primarily, within lymph nodes (LNs). During the long natural history of FL, malignant B cells often disseminate to multiple LNs and can affect virtually any organ. Nonmalignant LNs are highly organized structures distributed throughout the body, in which they perform functions critical for host defense. In FL, the malignant B cells "re-educate" the lymphoid environment by altering the phenotype, distribution, and abundance of other cells such as T cells, macrophages, and subsets of stromal cells. Consequently, dramatic anatomical changes occur and include alterations in the number, shape, and size of neoplastic follicles with an accompanying attenuation of the T-cell zone. Ongoing and dynamic interactions between FL B cells and the tumor microenvironment (TME) result in significant clinical heterogeneity observed both within and across patients. Over time, FL evolves into pathological variants associated with distinct outcomes, ranging from an indolent disease to more aggressive clinical courses with early death. Given the importance of both cell-intrinsic and -extrinsic factors in shaping disease progression and patient survival, comprehensive examination of FL tumors is critical. Here, we describe the cellular composition and architecture of normal and malignant human LNs and provide a broad overview of emerging technologies for deconstructing the FL TME at single-cell and spatial resolution. We additionally discuss the importance of capturing samples at landmark time points as well as longitudinally for clinical decision-making.

PTP1B mediates the inhibitory effect of MFGE8 on insulin signaling through the β5 integrin

Integrins are cell adhesion receptors that dimerize to mediate cell-cell interactions and regulate processes, including proliferation, inflammation, and tissue repair. The role of integrins in regulating insulin signaling is incompletely understood. We have previously shown that binding of the integrin ligand milk fat globule epidermal growth factor like 8 (MFGE8) to the αvβ5 integrin promotes termination of insulin receptor signaling in mice. Upon ligation of MFGE8, integrin β5 complexes with the insulin receptor beta (IRβ) in skeletal muscle, resulting in dephosphorylation of IRβ and reduction of insulin-stimulated glucose uptake. Here, we investigate the mechanism by which the interaction between β5 and IRβ impacts IRβ phosphorylation status. We show in in vitro and in vivo in skeletal muscle in mice that antibody-mediated blockade of the β5 integrin inhibits and recombinant MFGE8 promotes PTP1B binding to and dephosphorylation of IRβ resulting in increased or reduced insulin-stimulated glucose uptake, respectively. The β5-PTP1B complex is recruited by MFGE8 to IRβ leading to termination of canonical insulin signaling. β5 blockade enhances insulin-stimulated glucose uptake in wildtype but not Ptp1b KO mice indicating that PTP1B functions downstream of MFGE8 in modulating insulin receptor signaling. Furthermore, in a human cohort, we report serum MFGE8 levels correlate with indices of insulin resistance. These data provide mechanistic insights into the role of MFGE8 and β5 in regulating insulin signaling.

Long-term retention of antigens in germinal centers is controlled by the spatial organization of the follicular dendritic cell network

Germinal centers (GCs) require sustained availability of antigens to promote antibody affinity maturation against pathogens and vaccines. A key source of antigens for GC B cells are immune complexes (ICs) displayed on follicular dendritic cells (FDCs). Here we show that FDC spatial organization regulates antigen dynamics in the GC. We identify heterogeneity within the FDC network. While the entire light zone (LZ) FDC network captures ICs initially, only the central cells of the network function as the antigen reservoir, where different antigens arriving from subsequent immunizations colocalize. Mechanistically, central LZ FDCs constitutively express subtly higher CR2 membrane densities than peripheral LZ FDCs, which strongly increases the IC retention half-life. Even though repeated immunizations gradually saturate central FDCs, B cell responses remain efficient because new antigens partially displace old ones. These results reveal the principles shaping antigen display on FDCs during the GC reaction.

Salmonella infection induces the reorganization of follicular dendritic cell networks concomitant with the failure to generate germinal centers

Germinal centers (GCs) are sites where plasma and memory B cells form to generate high-affinity, Ig class-switched antibodies. Specialized stromal cells called follicular dendritic cells (FDCs) are essential for GC formation. During systemic Salmonella Typhimurium (STm) infection GCs are absent, whereas extensive extrafollicular and switched antibody responses are maintained. The mechanisms that underpin the absence of GC formation are incompletely understood. Here, we demonstrate that STm induces a reversible disruption of niches within the splenic microenvironment, including the T and B cell compartments and the marginal zone. Alongside these effects after infection, mature FDC networks are strikingly absent, whereas immature FDC precursors, including marginal sinus pre-FDCs (MadCAM-1+) and perivascular pre-FDCs (PDGFRβ+) are enriched. As normal FDC networks re-establish, extensive GCs become detectable throughout the spleen. Therefore, the reorganization of FDC networks and the loss of GC responses are key, parallel features of systemic STm infections.

Tingible body macrophages arise from lymph node-resident precursors and uptake B cells by dendrites

Antibody affinity maturation depends on the formation of germinal centers (GCs) in lymph nodes. This process generates a massive number of apoptotic B cells, which are removed by a specialized subset of phagocytes, known as tingible body macrophages (TBMs). Although defects in these cells are associated with pathological conditions, the identity of their precursors and the dynamics of dying GC B cell disposal remained unknown. Here, we demonstrate that TBMs originate from pre-existing lymph node-resident precursors that enter the lymph node follicles in a GC-dependent manner. Intravital imaging shows that TBMs are stationary cells that selectively phagocytose GC B cells via highly dynamic protrusions and accommodate the final stages of B cell apoptosis. Cell-specific depletion and chimeric mouse models revealed that GC B cells drive TBM formation from bone marrow-derived precursors stationed within lymphoid organs prior to the immune challenge. Understanding TBM dynamics and function may explain the emergence of various antibody-mediated autoimmune conditions.

Apoptotic cell fragments locally activate tingible body macrophages in the germinal center

Germinal centers (GCs) that form within lymphoid follicles during antibody responses are sites of massive cell death. Tingible body macrophages (TBMs) are tasked with apoptotic cell clearance to prevent secondary necrosis and autoimmune activation by intracellular self antigens. We show by multiple redundant and complementary methods that TBMs derive from a lymph node-resident, CD169-lineage, CSF1R-blockade-resistant precursor that is prepositioned in the follicle. Non-migratory TBMs use cytoplasmic processes to chase and capture migrating dead cell fragments using a "lazy" search strategy. Follicular macrophages activated by the presence of nearby apoptotic cells can mature into TBMs in the absence of GCs. Single-cell transcriptomics identified a TBM cell cluster in immunized lymph nodes which upregulated genes involved in apoptotic cell clearance. Thus, apoptotic B cells in early GCs trigger activation and maturation of follicular macrophages into classical TBMs to clear apoptotic debris and prevent antibody-mediated autoimmune diseases.

Leveraging Antibody, B Cell and Fc Receptor Interactions to Understand Heterogeneous Immune Responses in Tuberculosis

Despite over a century of research, Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), continues to kill 1.5 million people annually. Though less than 10% of infected individuals develop active disease, the specific host immune responses that lead to Mtb transmission and death, as well as those that are protective, are not yet fully defined. Recent immune correlative studies demonstrate that the spectrum of infection and disease is more heterogenous than has been classically defined. Moreover, emerging translational and animal model data attribute a diverse immune repertoire to TB outcomes. Thus, protective and detrimental immune responses to Mtb likely encompass a framework that is broader than T helper type 1 (Th1) immunity. Antibodies, Fc receptor interactions and B cells are underexplored host responses to Mtb. Poised at the interface of initial bacterial host interactions and in granulomatous lesions, antibodies and Fc receptors expressed on macrophages, neutrophils, dendritic cells, natural killer cells, T and B cells have the potential to influence local and systemic adaptive immune responses. Broadening the paradigm of protective immunity will offer new paths to improve diagnostics and vaccines to reduce the morbidity and mortality of TB.

MFGE8 decreased neuronal apoptosis and neuroinflammation to ameliorate early brain injury induced by subarachnoid hemorrhage through the inhibition of HMGB1

AIM

Both MFGE8 and HMGB1 were vital players for aneurysmal subarachnoid hemorrhage. However, whether HMGB1 was served as the downstream target of MFGE8 was unknown. To test this new mechanism, we performed the SAH model in rats.

METHOD

All treatments were injected intraventricularly into the right lateral ventricles. SAH grade, brain water content, and neurological function scores were evaluated. HMGB1 expression was studied by double immunofluorescence staining. HE and Nissl's staining were performed to observe the pathological change. Inflammatory factors were measured by ELISA method.

RESULTS

High expression of MFGE8 could improve neurological function and reduce the brain edema and pro-inflammatory factors. Injection of rhMFGE8 inhibited HMGB1. To further verify the regulation of MFGE8 in HMGB1, we used rhHMGB1 and glycyrrhizin, and the results indicated MFGE8 produced excellent effect on SAH rats via inhibiting HMGB1.

CONCLUSION

In a word, MFGE8 improved EBI caused by SAH, depending on HMGB1 that was the potential mechanism.

Follicular dendritic cell differentiation is associated with distinct synovial pathotype signatures in rheumatoid arthritis

Follicular dendritic cells (FDCs) fundamentally contribute to the formation of synovial ectopic lymphoid-like structures in rheumatoid arthritis (RA) which is associated with poor clinical prognosis. Despite this critical role, regulation of FDC development in the RA synovium and its correlation with synovial pathotype differentiation remained largely unknown. Here, we demonstrate that CNA.42⁺ FDCs distinctively express the pericyte/fibroblast-associated markers PDGFR-β, NG2, and Thy-1 in the synovial perivascular space but not in established follicles. In addition, synovial RNA-Seq analysis revealed that expression of the perivascular FDC markers was strongly correlated with PDGF-BB and fibroid synovitis, whereas TNF-α/LT-β was significantly associated with lymphoid synovitis and expression of CR1, CR2, and FcγRIIB characteristic of mature FDCs in lymphoid follicles. Moreover, PDGF-BB induced CNA.42⁺ FDC differentiation and CXCL13 secretion from NG2⁺ synovial pericytes, and together with TNF-α/LT-β conversely regulated early and late FDC differentiation genes in unsorted RA synovial fibroblasts (RASF) and this was confirmed in flow sorted stromal cell subsets. Furthermore, RASF TNF-αR expression was upregulated by TNF-α/LT-β and PDGF-BB; and TNF-α/LT-β-activated RASF retained ICs and induced B cell activation in in vitro germinal center reactions typical of FDCs. Additionally, FDCs trapped peptidyl citrulline, and strongly correlated with IL-6 expression, and plasma cell, B cell, and T cell infiltration of the RA synovium. Moreover, synovial FDCs were significantly associated with RA disease activity and radiographic features of tissue damage. To the best of our knowledge, this is the first report describing the reciprocal interaction between PDGF-BB and TNF-α/LT-β in synovial FDC development and evolution of RA histological pathotypes. Selective targeting of this interplay could inhibit FDC differentiation and potentially ameliorate RA in clinically severe and drug-resistant patients.

Follicular dendritic cells

Follicular dendritic cells (FDCs) are unique accessory immune cells that contribute to the regulation of humoral immunity. They are multitasker cells essential for the organization and maintenance of the lymphoid architecture, induction of germinal center reaction, production of B memory cells, and protection from autoimmune disorders. They perform their activities through both antigen-driven and chemical signaling to B cells. FDCs play a crucial role in the physiological regulation of the immune response. Dis-regulation of this immune response results when FDCs retain antigens for years. This provides a constant antigenic stimulation for B cells resulting in the development of immune disorders. Antigen trapped on FDCs is resistant to therapeutic intervention causing chronicity and recurrences. Beyond their physiological immunoregulatory functions, FDCs are involved in the pathogenesis of several immune-related disorders including HIV/AIDS, prion diseases, chronic inflammatory, and autoimmune disorders. FDCs have also been recently implicated in rare neoplasms of lymphoid and hematopoietic tissues. Understanding FDC biology is essential for better control of humoral immunity and opens the gate for therapeutic management of FDC-mediated immune disorders. Thus, the biology of FDCs has become a hot research area in the last couple of decades. In this review, we aim to provide a comprehensive overview of FDCs and their role in physiological and pathological conditions.

Immune Niche Within the Peritoneal Cavity

There are numbers of leukocytes present in peritoneal cavity, not only protecting body cavity from infection but also contributing to peripheral immunity including natural antibody production in circulation. The peritoneal leukocytes compose unique immune compartment, the functions of which cannot be replaced by other lymphoid organs. Atypical lymphoid clusters, called "milky spots", that are located in visceral adipose tissue omentum have the privilege of immune niche in terms of differentiation, recruitment, and activation of peritoneal immunity, yet mechanisms underlying the regulation are underexplored. In this review, I discuss the emerging views of peritoneal immune system in the contexts of its development, organization, and functions.

Follicular dendritic cell sarcoma

Follicular dendritic cells (FDC) are mesenchymal-derived dendritic cells located in B-follicles where they play a pivotal role in triggering and maintaining B-cell adaptive immune response. In 1986 Dr. Juan Rosai first reported a series of neoplasms showing features of FDC and defined it as Follicular Dendritic Cell Tumor, subsequently renamed as "sarcoma" (FDCS). In its seminal and subsequent articles Rosai and colleagues highlighted the heterogeneous microscopic appearance of FDCS and its immunohistochemical and ultrastructural features.

FDCS mostly occurs in extranodal sites (79.4% of cases) and lymph nodes (15.1%); in about 7%-10% of cases it is associated with hyaline-vascular Castleman disease. Given its significant growth pattern and cytological variability, FDCS can be confused with various neoplasms and even inflammatory processes. The diagnosis requires the use of a broad spectrum of FDC markers (e.g. CD21, CD23, CD35, clusterin, CXCL13, podoplanin), particularly considering that tumor antigen-loss is frequent. The inflammatory-pseudotumor-like (IPT-like) variant of FDCS, in addition to its peculiar histopathological and clinical features, is characterized by positivity of tumor cells for Epstein-Barr virus, representing a diagnostic requisite.

No distinctive genetic and molecular anomalies have been identified in FDCS. It often carries an aberrant clonal karyotype and chromosomal structural alterations, frequently involving onco-suppressor genes. Direct or next generation sequencing showed alterations on genes belonging to the NF-κB regulatory pathway and cell-cycle regulators. In contrast to hematopoietic-derived histiocytic and dendritic cells tumors, FDCS typically lacks mutations in genes related to the MAPK pathway.

FDCS recurs locally in 28% and metastasizes in 27% of cases. Extent of the disease, surgical resectability and histopathological features are significantly associated with the outcome. IPT-like FDCS behaves as an indolent tumor, even if it often recurs locally over years.

Complete surgical excision is the gold standard of treatment. Data on targeted therapies (e.g.: tyrosine kinase inhibitors) or immune checkpoint inhibitors are very limited and responses are variable. A better understanding of the molecular drivers of this tumor may lead to potential new therapeutic strategies.

Unboxing dendritic cells: Tales of multi-faceted biology and function

Often referred to as the bridge between innate and adaptive immunity, dendritic cells (DCs) are professional antigen-presenting cells (APCs) that constitute a unique, yet complex cell system. Among other APCs, DCs display the unique property of inducing protective immune responses against invading microbes, or cancer cells, while safeguarding the proper homeostatic equilibrium of the immune system and maintaining self-tolerance. Unsurprisingly, DCs play a role in many diseases such as autoimmunity, allergy, infectious disease and cancer. This makes them attractive but challenging targets for therapeutics. Since their initial discovery, research and understanding of DC biology have flourished. We now recognize the presence of multiple subsets of DCs distributed across tissues. Recent studies of phenotype and gene expression at the single cell level have identified heterogeneity even within the same DC type, supporting the idea that DCs have evolved to greatly expand the flexibility of the immune system to react appropriately to a wide range of threats. This review is meant to serve as a quick and robust guide to understand the basic divisions of DC subsets and their role in the immune system. Between mice and humans, there are some differences in how these subsets are identified and function, and we will point out specific distinctions as necessary. Throughout the text, we are using both fundamental and therapeutic lens to describe overlaps and distinctions and what this could mean for future research and therapies.

Redefining the Role of Lymphotoxin Beta Receptor in the Maintenance of Lymphoid Organs and Immune Cell Homeostasis in Adulthood

Lymphotoxin beta receptor (LTβR) is a promising therapeutic target in autoimmune and infectious diseases as well as cancer. Mice with genetic inactivation of LTβR display multiple defects in development and organization of lymphoid organs, mucosal immune responses, IgA production and an autoimmune phenotype. As these defects are imprinted in embryogenesis and neonate stages, the impact of LTβR signaling in adulthood remains unclear. Here, to overcome developmental defects, we generated mice with inducible ubiquitous genetic inactivation of LTβR in adult mice (iLTβR^Δ/Δ mice) and redefined the role of LTβR signaling in organization of lymphoid organs, immune response to mucosal bacterial pathogen, IgA production and autoimmunity. In spleen, postnatal LTβR signaling is required for development of B cell follicles, follicular dendritic cells (FDCs), recruitment of neutrophils and maintenance of the marginal zone. Lymph nodes of iLTβR^Δ/Δ mice were reduced in size, lacked FDCs, and had disorganized subcapsular sinus macrophages. Peyer`s patches were smaller in size and numbers, and displayed reduced FDCs. The number of isolated lymphoid follicles in small intestine and colon were also reduced. In contrast to LTβR^-/- mice, iLTβR^Δ/Δ mice displayed normal thymus structure and did not develop signs of systemic inflammation and autoimmunity. Further, our results suggest that LTβR signaling in adulthood is required for homeostasis of neutrophils, NK, and iNKT cells, but is dispensable for the maintenance of polyclonal IgA production. However, iLTβR^Δ/Δ mice exhibited an increased sensitivity to C. rodentium infection and failed to develop pathogen-specific IgA responses. Collectively, our study uncovers new insights of LTβR signaling in adulthood for the maintenance of lymphoid organs, neutrophils, NK and iNKT cells, and IgA production in response to mucosal bacterial pathogen.

S. Typhi derived vaccines and a proposal for outer membrane vesicles (OMVs) as potential vaccine for typhoid fever

Typhoid fever is a serious systemic infection caused by Salmonella Typhi (S. Typhi), spread by the feco-oral route and closely associated with poor food hygiene and inadequate sanitation. Nearly 93% of S. Typhi strains have acquired antibiotic resistance against most antibiotics. Vaccination is the only promising way to prevent typhoid fever. This review covers the nature and composition of S. Typhi, pathogenecity and mode of infection, epidemiology, and nature of drug resistance. Several components (Vi-polysaccharides, O-antigens, flagellar antigens, full length OMPs, and short peptides from OMPs) of S. Typhi have been utilized for vaccine design for protection against typhoid fever. Vaccine delivery systems also contribute to efficacy of the vaccines. In this study, we propose to develop S. Typhi derived OMVs as vaccine for protection against typhoid fevers.

Lymph node fibroblastic reticular cells steer immune responses

Lymph nodes (LNs), where immune responses are initiated, are organized into distinctive compartments by fibroblastic reticular cells (FRCs). FRCs imprint immune responses by supporting LN architecture, recruiting immune cells, coordinating immune cell crosstalk, and presenting antigens. Recent high-resolution transcriptional and histological analyses have enriched our knowledge of LN FRC genetic and spatial heterogeneities. Here, we summarize updated anatomic, phenotypic, and functional identities of FRC subsets, delve into topological and transcriptional remodeling of FRCs in inflammation, and illustrate the crosstalk between FRCs and immune cells. Discussing FRC functions in immunity and tolerance, we highlight state-of-the-art FRC-based therapeutic approaches for maintaining physiological homeostasis, steering protective immunity, inducing transplantation tolerance, and treating diverse immune-related diseases.

The role of macrophage scavenger receptor 1 (Msr1) in prion pathogenesis

The progression of prion diseases is accompanied by the accumulation of prions in the brain. Ablation of microglia enhances prion accumulation and accelerates disease progression, suggesting that microglia play a neuroprotective role by clearing prions. However, the mechanisms underlying the phagocytosis and clearance of prion are largely unknown. The macrophage scavenger receptor 1 (Msr1) is an important phagocytic receptor expressed by microglia in the brain and is involved in the uptake and clearance of soluble amyloid-β. We therefore asked whether Msr1 might play a role in prion clearance and assessed the scavenger function of Msr1 in prion pathogenesis. We found that Msr1 expression was upregulated in prion-infected mouse brains. However, Msr1 deficiency did not change prion disease progression or lesion patterns. Prion deposition in Msr1 deficient mice was similar to their wild-type littermates. In addition, prion-induced neuroinflammation was not affected by Msr1 ablation. We conclude that Msr1 does not play a major role in prion pathogenesis. KEY MESSAGES: Msr1 expression is upregulated in prion-infected mouse brains at the terminal stage Msr1 deficiency does not affect prion disease progression Msr1 does not play a major role in prion clearance or prion pathogenesis Microglia-mediated phagocytosis and clearance of Aβ and prion may adopt distinct molecular pathways.

B cell/stromal cell crosstalk in health, disease, and treatment: Follicular lymphoma as a paradigm

Stromal cells organize specific anatomic compartments within bone marrow (BM) and secondary lymphoid organs where they finely regulate the behavior of mature normal B cells. In particular, lymphoid stromal cells (LSCs) form a phenotypically heterogeneous compartment including various cell subsets variably supporting B-cell survival, activation, proliferation, and differentiation. In turn, activated B cells trigger in-depth remodeling of LSC networks within lymph nodes (LN) and BM. Follicular lymphoma (FL) is one of the best paradigms of a B-cell neoplasia depending on a specific tumor microenvironment (TME), including cancer-associated fibroblasts (CAFs) emerging from the reprogramming of LN LSCs or poorly characterized local BM precursors. FL-CAFs support directly malignant B-cell growth and orchestrate FL permissive cell niche by contributing, through a bidirectional crosstalk, to the recruitment and polarization of immune TME subsets. Recent studies have highlighted a previously unexpected level of heterogeneity of both FL B cells and FL TME, underlined by FL-CAF plasticity. A better understanding of the signaling pathways, molecular mechanisms, and kinetic of stromal cell remodeling in FL would be useful to delineate new predictive markers and new therapeutic approaches in this still fatal malignancy.

Dermal fibroblast phagocytosis of apoptotic cells: A novel pathway for wound resolution

The effective clearance of apoptotic cells is an essential step in the resolution of healing wounds. In particular, blood vessel regression during wound resolution produces a significant number of apoptotic endothelial cells (ApoEC) that must be cleared. In considering the fate of ApoEC and the presence of fibroblasts during wound resolution, we hypothesized that fibroblasts might serve as phagocytes involved in endothelial cell removal. The current study investigated whether dermal fibroblasts engulf ApoEC, whether this uptake alters the phenotype of dermal fibroblasts, and the biological molecules involved. In both in vitro and in vivo studies, following ApoEC engulfment, fibroblasts acquired a pro-healing phenotype (increased cell migration, contractility, α-smooth muscle actin expression, and collagen deposition). In addition, fibroblast uptake of ApoEC was shown to be mediated in part by the milk fat globule-EGF factor 8 protein/integrin αv β5 pathway. Our study demonstrates a novel function of fibroblasts in the clearance of ApoEC and suggests that this capability has significant implications for tissue repair and fibrosis.

Compartments and Connections Within the Germinal Center

Protective high affinity antibody responses emerge through an orchestrated developmental process that occurs in germinal centers (GCs). While GCs have been appreciated since 1930, a wealth of recent progress provides new insights into the molecular and cellular dynamics governing humoral immunity. In this review, we highlight advances that demonstrate that fundamental GC B cell function, selection, proliferation and SHM occur within distinct cell states. The resulting new model provides new opportunities to understand the evolution of immunity in infectious, autoimmune and neoplastic diseases.

Inflammatory pseudotumor-like follicular dendritic cell sarcoma: Literature review of 67 cases

Inflammatory pseudotumor (IPT)-like follicular dendritic cell (FDC) sarcoma is rare. The 2017 World Health Organization classification of tumors of hematopoietic and lymphoid tissues noted that data on its clinical outcome are limited, but that the tumor appears to be indolent. The aim of this study was to summarize the clinical characteristics, treatment outcomes, and prognostic factors for IPT-like FDC sarcoma. A literature review was conducted on retrospective analyses of clinical data and prognostic information on IPT-like FDC sarcoma reported between 2001 and 2020. A total of 67 cases of IPT-like FDC sarcoma were retrieved from the literature, documenting that it occurs predominantly in middle-aged adults, with a marked female predilection. Six patients had a separate malignancy and five had an autoimmune disease. Typically involving the spleen and/or liver, it may also selectively involve the abdomen, gastrointes-tinal tract, pancreas, retroperitoneum, and mesentery. Necrosis, hemorrhage, noncaseating epithelioid granulomas, and fibrinoid deposits in blood vessel walls are often present. The neoplastic cells are predominantly positive for follicular dendritic cell markers such as cluster of differentiation 21 (CD21), CD23, CD35 and CNA.42 and are consistently Epstein-Barr virus (EBV)-positive. Mitoses were very rare in most cases. Most patients were treated by surgery alone. Disease status at the time of last follow-up was known for 57 patients with follow-up time ranging from 2 to 144 mo. Local and/or distant recurrence after initial treatment was seen in 15.8% of the patients. The 1- and 5-year progression-free survival for the entire group was 91.5% and 56.1%, respectively. Kaplan-Meier and multivariate analyses showed that age, sex, tumor size, and pathological features were not risk factors for disease progression. IPT-like FDC sarcoma appears to be mildly aggressive and requires annual surveillance. Surgery is the most effective treatment modality, and the role of adjuvant chemotherapy for postoperative management is unclear. EBV is likely to play an important role in the etiology of IPT-like FDC sarcoma.

Stromal cell control of conventional and ectopic germinal centre reactions

The germinal centre (GC) is a specialized cellular structure that forms in response to antigenic stimulation. It generates long-term humoral immunity through the production of memory B cells and long-lived antibody-secreting plasma cells. Conventional GCs form within secondary lymphoid organs, where networks of specialised stromal cells that form during embryogenesis act as the stage upon which the various GC immune cell players are brought together, nurtured and co-ordinated to generate a productive response. In non-lymphoid organs, ectopic GCs can form in response to persistent antigenic and inflammatory stimuli. Unlike secondary lymphoid tissues, non-lymphoid organs do not have a developmentally programmed stromal cell network capable of supporting the germinal centre reaction; therefore, the local tissue stroma must be remodelled by inflammatory stimuli in order to host a GC reaction. These ectopic GCs produce memory B cells and plasma cells that form a critical component of the humoral immune response.

Germinal center B cell initiation, GC maturation, and the coevolution of its stromal cell niches

Throughout the developing GC response, B cell survival and fate choices made at the single cell level are dependent on signals received largely through interactions with other cells, often with cognate T cells. The type of signals that a given B cell can encounter is dictated by its location within tissue microarchitecture. The focus of this review is on the initiation and evolution of the GC response at the earliest time points. Here, we review the key factors influencing the progression of GC B cell differentiation that are both stage and context dependent. Finally, we describe the coevolution of niches within and surrounding the GC that influence the outcome of the GC response.

An integrin/MFG-E8 shuttle loads HIV-1 viral-like particles onto follicular dendritic cells in mouse lymph node

During human immunodeficiency virus-1 (HIV-1) infection lymphoid organ follicular dendritic cells (FDCs) serve as a reservoir for infectious virus and an obstacle to curative therapies. Here, we identify a subset of lymphoid organ sinus lining macrophage (SMs) that provide a cell-cell contact portal, which facilitates the uptake of HIV-1 viral-like particles (VLPs) by FDCs and B cells in mouse lymph node. Central for portal function is the bridging glycoprotein MFG-E8. Using a phosphatidylserine binding domain and an RGD motif, MFG-E8 helps target HIV-1 VLPs to αv integrin bearing SMs. Lack of MFG-E8 or integrin blockade severely limits HIV-1 VLP spread onto FDC networks. Direct SM-FDC virion transfer also depends upon short-lived FDC network abutment, likely triggered by SCSM antigen uptake. This provides a mechanism for rapid FDC loading broadening the opportunity for rare, antigen reactive follicular B cells to acquire antigen, and a means for HIV virions to accumulate on the FDC network.

Dendritic Cell-Targeted pH-Responsive Extracellular Vesicles for Anticancer Vaccination

Immunotherapy can potentially treat cancers on a patient-dependent manner. Most of the efforts expended on anticancer vaccination parallel the efforts expended on prototypical immunization in infectious diseases. In this study, we designed and synthesized pH-responsive extracellular vesicles (EVs) coupled with hyaluronic acid (HA), 3-(diethylamino)propylamine (DEAP), monophosphoryl lipid A (MPLA), and mucin 1 peptide (MUC1), referred to as HDEA@EVAT. HDEA@EVAT potentiated the differentiation and maturation of monocytes into dendritic cells (DCs) and the priming of CD8⁺ T-cells for cancer therapy. MPLA and HA enabled HDEA@EVAT to interact with the toll-like receptor 4 and the CD44 receptor on DCs, followed by endosomal escape, owing to the protonation of pH-sensitive DEAP on the EV in conjunction with MUC1 release. The MUC1 was then processed and presented to DCs to activate CD8⁺ T-cells for additional anticancer-related immune reactions. Our findings support the anticancer vaccine activity by which HDEA@EVAT expedites the interaction between DCs and CD8⁺ T-cells by inducing DC-targeted maturation and by presenting the cancer-associated peptide MUC1.

Unaltered prion disease in mice lacking developmental endothelial locus-1

Progression of prion diseases is driven by the accumulation of prions in the brain. Ablation of microglia or deletion of the eat-me-signal, milk-fat globule epidermal growth factor VIII (Mfge8), accelerates prion pathogenesis, suggesting that microglia defend the brain by phagocytosing prions. Similar to Mfge8, developmental endothelial locus-1 (Del-1) is a secreted protein that acts as an opsonin bridging phagocytes and apoptotic cells to facilitate phagocytosis. We therefore asked whether Del-1 might play a role in controlling prion pathogenesis. We assessed the anti-inflammatory and phagocytosis-promoting functions of Del-1 in prion disease and determined whether Del-1 complements Mfge8 in prion clearance in mice with a C57BL/6J genetic background. We found that Del-1 deficiency did not change prion disease progression or lesion patterns. In addition, prion clearance and scrapie prion protein deposition were unaltered in Del-1-deficient mice. In addition, prion-induced neuroinflammation was not affected by Del-1 deficiency. We conclude that Del-1 is not a major determinant of prion pathogenesis in this context.

Regulation of the Germinal Center Response

The germinal center (GC) is a specialized microstructure that forms in secondary lymphoid tissues, producing long-lived antibody secreting plasma cells and memory B cells, which can provide protection against reinfection. Within the GC, B cells undergo somatic mutation of the genes encoding their B cell receptors which, following successful selection, can lead to the emergence of B cell clones that bind antigen with high affinity. However, this mutation process can also be dangerous, as it can create autoreactive clones that can cause autoimmunity. Because of this, regulation of GC reactions is critical to ensure high affinity antibody production and to enforce self-tolerance by avoiding emergence of autoreactive B cell clones. A productive GC response requires the collaboration of multiple cell types. The stromal cell network orchestrates GC cell dynamics by controlling antigen delivery and cell trafficking. T follicular helper (Tfh) cells provide specialized help to GC B cells through cognate T-B cell interactions while Foxp3⁺ T follicular regulatory (Tfr) cells are key mediators of GC regulation. However, regulation of GC responses is not a simple outcome of Tfh/Tfr balance, but also involves the contribution of other cell types to modulate the GC microenvironment and to avoid autoimmunity. Thus, the regulation of the GC is complex, and occurs at multiple levels. In this review we outline recent developments in the biology of cell subsets involved in the regulation of GC reactions, in both secondary lymphoid tissues, and Peyer's patches (PPs). We discuss the mechanisms which enable the generation of potent protective humoral immunity whilst GC-derived autoimmunity is avoided.

Experimental Stroke Differentially Affects Discrete Subpopulations of Splenic Macrophages

Changes to the immune system after stroke are complex and can result in both pro-inflammatory and immunosuppressive consequences. Following ischemic stroke, brain resident microglia are activated and circulating monocytes are recruited to the injury site. In contrast, there is a systemic deactivation of monocytes/macrophages that may contribute to immunosuppression and the high incidence of bacterial infection experienced by stroke patients. The manipulation of macrophage subsets may be a useful therapeutic strategy to reduce infection and improve outcome in patients after stroke. Recent research has enhanced our understanding of the heterogeneity of macrophages even within the same tissue. The spleen is the largest natural reservoir of immune cells, many of which are mobilized to the site of injury after ischemic stroke and is notable for the diversity of its functionally distinct macrophage subpopulations associated with specific micro-anatomical locations. Here, we describe the effects of experimental stroke in mice on these distinct splenic macrophage subpopulations. Red pulp (RP) and marginal zone macrophages (MZM) specifically showed increases in density and alterations in micro-anatomical location. These changes were not due to increased recruitment from the bone marrow but may be associated with increases in local proliferation. Genes associated with phagocytosis and proteolytic processing were upregulated in the spleen after stroke with increased expression of the lysosome-associated protein lysosomal-associated membrane proteins specifically increased in RP and MZM subsets. In contrast, MHC class II expression was reduced specifically in these populations. Furthermore, genes associated with macrophage ability to communicate with other immune cells, such as co-stimulatory molecules and inflammatory cytokine production, were also downregulated in the spleen after stroke. These findings suggest that selective splenic macrophage functions could be impaired after stroke and the contribution of macrophages to stroke-associated pathology and infectious complications should be considered at a subset-specific level. Therefore, optimal therapeutic manipulation of macrophages to improve stroke outcome is likely to require selective targeting of functionally and spatially distinct subpopulations.

Some news from the unknown soldier, the Peyer's patch macrophage

In mammals, macrophages (MF) are present in virtually all tissues where they serve many different functions linked primarily to the maintenance of homeostasis, innate defense against pathogens, tissue repair and metabolism. Although some of these functions appear common to all tissues, others are specific to the homing tissue. Thus, MF become adapted to perform particular functions in a given tissue. Accordingly, MF express common markers but also sets of tissue-specific markers linked to dedicated functions. One of the largest pool of MF in the body lines up the wall of the gut. Located in the small intestine, Peyer's patches (PP) are primary antigen sampling and mucosal immune response inductive sites. Surprisingly, although markers of intestinal MF, such as F4/80, have been identified more than 30 years ago, MF of PP escaped any kind of phenotypic description and remained "unknown" for decades. In absence of MF identification, the characterization of the PP mononuclear phagocyte system (MPS) functions has been impaired. However, taking into account that PP are privileged sites of entry for pathogens, it is important to understand how the latter are handled by and/or escape the PP MPS, especially MF, which role in killing invaders is well known. This review focuses on recent advances on the PP MPS, which have allowed, through new criteria of PP phagocyte subset identification, the characterization of PP MF origin, diversity, specificity, location and functions.

Self-Reactive B Cells in the Germinal Center Reaction

Maintenance of immunological self-tolerance requires lymphocytes carrying self-reactive antigen receptors to be selectively prevented from mounting destructive or inflammatory effector responses. Classically, self-tolerance is viewed in terms of the removal, editing, or silencing of B and T cells that have formed self-reactive antigen receptors during their early development. However, B cells activated by foreign antigen can enter germinal centers (GCs), where they further modify their antigen receptor by somatic hypermutation (SHM) of their immunoglobulin genes. The inevitable emergence of activated, self-reactive GC B cells presents a unique challenge to the maintenance of self-tolerance that must be rapidly countered to avoid autoantibody production. Here we discuss current knowledge of the mechanisms that enforce B cell self-tolerance, with particular focus on the control of self-reactive GC B cells. We also consider how self-reactive GC B cells can escape self-tolerance to initiate autoantibody production or instead be redeemed via SHM and used in productive antibody responses.

Impaired lymphoid extracellular matrix impedes antibacterial immunity in epidermolysis bullosa

Genetic loss of collagen VII causes recessive dystrophic epidermolysis bullosa (RDEB), a skin fragility disorder that, unexpectedly, manifests also with elevated colonization of commensal bacteria and frequent wound infections. Here, we describe an unprecedented systemic function of collagen VII as a member of a unique innate immune-supporting multiprotein complex in spleen and lymph nodes. In this complex, collagen VII specifically binds and sequesters the innate immune activator cochlin in the lumen of lymphoid conduits. In genetic mouse models, loss of collagen VII increased bacterial colonization by diminishing levels of circulating cochlin LCCL domain. Intraperitoneal injection of collagen VII, which restored cochlin in the spleen, but not in the skin, reactivated peripheral innate immune cells via cochlin and reduced bacterial skin colonization. Systemic administration of the cochlin LCCL domain was alone sufficient to diminish bacterial supercolonization of RDEB mouse skin. Human validation demonstrated that RDEB patients displayed lower levels of systemic cochlin LCCL domain with subsequently impaired macrophage response in infected wounds. This study identifies an intrinsic innate immune dysfunction in RDEB and uncovers a unique role of the lymphoid extracellular matrix in systemic defense against bacteria.

Spontaneous germinal centers and autoimmunity

Germinal centers (GCs) are dynamic microenvironments that form in the secondary lymphoid organs and generate somatically mutated high-affinity antibodies necessary to establish an effective humoral immune response. Tight regulation of GC responses is critical for maintaining self-tolerance. GCs can arise in the absence of purposeful immunization or overt infection (called spontaneous GCs, Spt-GCs). In autoimmune-prone mice and patients with autoimmune disease, aberrant regulation of Spt-GCs is thought to promote the development of somatically mutated pathogenic autoantibodies and the subsequent development of autoimmunity. The mechanisms that control the formation of Spt-GCs and promote systemic autoimmune diseases remain an open question and the focus of ongoing studies. Here, we discuss the most current studies on the role of Spt-GCs in autoimmunity.

How do PrPSc Prions Spread between Host Species, and within Hosts?

Prion diseases are sub-acute neurodegenerative diseases that affect humans and some domestic and free-ranging animals. Infectious prion agents are considered to comprise solely of abnormally folded isoforms of the cellular prion protein known as PrP^Sc. Pathology during prion disease is restricted to the central nervous system where it causes extensive neurodegeneration and ultimately leads to the death of the host. The first half of this review provides a thorough account of our understanding of the various ways in which PrP^Sc prions may spread between individuals within a population, both horizontally and vertically. Many natural prion diseases are acquired peripherally, such as by oral exposure, lesions to skin or mucous membranes, and possibly also via the nasal cavity. Following peripheral exposure, some prions accumulate to high levels within the secondary lymphoid organs as they make their journey from the site of infection to the brain, a process termed neuroinvasion. The replication of PrP^Sc prions within secondary lymphoid organs is important for their efficient spread to the brain. The second half of this review describes the key tissues, cells and molecules which are involved in the propagation of PrP^Sc prions from peripheral sites of exposure (such as the lumen of the intestine) to the brain. This section also considers how additional factors such as inflammation and aging might influence prion disease susceptibility.

The Peyer's Patch Mononuclear Phagocyte System at Steady State and during Infection

The gut represents a potential entry site for a wide range of pathogens including protozoa, bacteria, viruses, or fungi. Consequently, it is protected by one of the largest and most diversified population of immune cells of the body. Its surveillance requires the constant sampling of its encounters by dedicated sentinels composed of follicles and their associated epithelium located in specialized area. In the small intestine, Peyer’s patches (PPs) are the most important of these mucosal immune response inductive sites. Through several mechanisms including transcytosis by specialized epithelial cells called M-cells, access to the gut lumen is facilitated in PPs. Although antigen sampling is critical to the initiation of the mucosal immune response, pathogens have evolved strategies to take advantage of this permissive gateway to enter the host and disseminate. It is, therefore, critical to decipher the mechanisms that underlie both host defense and pathogen subversive strategies in order to develop new mucosal-based therapeutic approaches. Whereas penetration of pathogens through M cells has been well described, their fate once they have reached the subepithelial dome (SED) remains less well understood. Nevertheless, it is clear that the mononuclear phagocyte system (MPS) plays a critical role in handling these pathogens. MPS members, including both dendritic cells and macrophages, are indeed strongly enriched in the SED, interact with M cells, and are necessary for antigen presentation to immune effector cells. This review focuses on recent advances, which have allowed distinguishing the different PP mononuclear phagocyte subsets. It gives an overview of their diversity, specificity, location, and functions. Interaction of PP phagocytes with the microbiota and the follicle-associated epithelium as well as PP infection studies are described in the light of these new criteria of PP phagocyte identification. Finally, known alterations affecting the different phagocyte subsets during PP stimulation or infection are discussed.

Identification of novel follicular dendritic cell sarcoma markers, FDCSP and SRGN, by whole transcriptome sequencing

Follicular dendritic cell (FDC)-sarcoma is a rare neoplasm with morphologic and phenotypic features of FDCs. It shows an extremely heterogeneous morphology, therefore, its diagnosis relys on the phenotype of tumor cells. Aim of the present study was the identification of new specific markers for FDC-sarcoma by whole transcriptome sequencing (WTS). Candidate markers were selected based on gene expression level and biological function. Immunohistochemistry was performed on reactive tonsils, on 22 cases of FDC-sarcomas and 214 control cases including 114 carcinomas, 87 soft tissue tumors, 5 melanomas, 5 thymomas and 3 interdigitating dendritic cell sarcomas. FDC secreted protein (FDCSP) and Serglycin (SRGN) proved to be specific markers of FDC and related tumor. They showed better specificity and sensitivity values than some well known markers used in FDC sarcoma diagnosis (specificity: 98.6%, and 100%, respectively; sensitivity: 72.73% and 68.18%, respectively). In our cohorts CXCL13, CD21, CD35, FDCSP and SRGN were the best markers for FDC-sarcoma diagnosis and could discriminate 21/22 FDC sarcomas from other mesenchymal tumors by linear discriminant analysis. In summary, by WTS we identified two novel FDC markers and by the analysis of a wide cohort of cases and controls we propose an efficient marker panel for the diagnosis of this rare and enigmatic tumor.

Human papillomavirus 16 E6 and E7 oncoprotein expression alters microRNA expression in extracellular vesicles

Extracellular vesicles released by cancer cells are mediators of intercellular communication that have been reported to contribute to carcinogenesis. Since they are readily detected in bodily fluids, they may also be used as cancer biomarkers. The E6/E7 oncoproteins drive human papillomavirus (HPV)-associated cancers, which account for approximately 5% of all human cancers worldwide. Here, we investigate how HPV16 E6/E7 oncogene expression in primary human epithelial cells alters miR expression in extracellular vesicles and compare these to changes in intracellular miR expression. Examining a panel of 68 cancer related miRs revealed that many miRs had similar expression patterns in cells and in extracellular vesicles, whereas some other miRs had different expression patterns and may be selectively packaged into extracellular vesicles. Interestingly, the set of miRs that may be selectively packaged in HPV16 E6/E7 extracellular vesicles is predicted to inhibit necrosis and apoptosis.

Stromal networking: cellular connections in the germinal centre

Secondary lymphoid organs are organized into distinct zones, governed by different types of mesenchymal stromal cells. These stromal cell subsets are critical for the generation of protective humoral immunity because they direct the migration of, and interaction between, multiple immune cell types to form the germinal centre. The germinal centre response generates long-lived antibody-secreting plasma cells and memory B cells which can provide long-term protection against re-infection. Stromal cell subsets mediate this response through control of immune cell trafficking, activation, localization and antigen access within the secondary lymphoid organ. Further, distinct populations of stromal cells underpin the delicate spatial organization of immune cells within the germinal centre. Because of this, the interactions between immune cells and stromal cells in secondary lymphoid organs are fundamental to the germinal centre response. Herein we review how this unique relationship leads to effective germinal centre responses.