Primary adhered neutrophils increase JNK1-MARCKSL1-mediated filopodia to promote secondary neutrophil transmigration

During inflammation, leukocytes extravasate the vasculature to areas of inflammation in a process termed transendothelial migration. Previous research has shown that transendothelial migration hotspots exist, areas in the vasculature that are preferred by leukocytes to cross. Several factors that contribute to hotspot-mediated transmigration have been proposed already, but whether one leukocyte transmigration hotspot can be used subsequently by a second wave of leukocytes and thereby can increase the efficiency of leukocyte transmigration is not well understood. Here, we show that primary neutrophil adhesion to the endothelium triggers endothelial transmigration hotspots, allowing secondary neutrophils to cross the endothelium more efficiently. Mechanistically, we show that primary neutrophil adhesion increases the number of endothelial apical filopodia, resulting in an increase in the number of adherent secondary neutrophils. Using fluorescence resonance energy transfer (FRET)-based biosensors, we found that neutrophil adhesion did not trigger the activity of the small GTPase Cdc42. We used kinase translocation reporters to study the activity of mitogen-activated protein (MAP) kinases and Akt in endothelial cells on a single-cell level with a high temporal resolution during the process of leukocyte transmigration and found that c-Jun N-terminal kinase (JNK) is rapidly activated upon neutrophil adhesion, whereas extracellular regulated kinase (ERK), p38, and Akt are not. Additionally, we show that short-term chemical inhibition of endothelial JNK successfully prevents the adhesion of neutrophils to the endothelium. Furthermore, we show that neutrophil-induced endothelial JNK1 but not JNK2 increases the formation of filopodia and thereby the adhesion of secondary neutrophils. JNK1 needs its downstream substrate MARCKSL1 to trigger additional apical filopodia and consequently neutrophil adhesion. Overall, our data show that primary neutrophils can trigger the endothelial transmigration hotspot by activating JNK1 and MARCKSL1 to induce filopodia that trigger more neutrophils to transmigrate at the endothelial hotspot area.

Endothelial ICAM-1 Adhesome Recruits CD44 for Optimal Transcellular Migration of Human CTLs

The endothelial lining of blood vessels is covered with a thin polysaccharide coat called the glycocalyx. This layer of polysaccharides contains hyaluronan that forms a protective coat on the endothelial surface. Upon inflammation, leukocytes leave the circulation and enter inflamed tissue by crossing inflamed endothelial cells, mediated by adhesion molecules such as ICAM-1/CD54. To what extent the glycocalyx participates in the regulation of leukocyte transmigration is not clear. During extravasation, leukocyte integrins cluster ICAM-1, resulting in the recruitment of a number of intracellular proteins and subsequent downstream effects in the endothelial cells. For our studies, we used primary human endothelial and immune cells. With an unbiased proteomics approach, we identified the full ICAM-1 adhesome and identified 93 (to our knowledge) new subunits of the ICAM-1 adhesome. Interestingly, we found the glycoprotein CD44 as part of the glycocalyx to be recruited to clustered ICAM-1 specifically. Our data demonstrate that CD44 binds hyaluronan to the endothelial surface, where it locally concentrates and presents chemokines that are essential for leukocytes to cross the endothelial lining. Taken together, we discover a link between ICAM-1 clustering and hyaluronan-mediated chemokine presentation by recruiting hyaluronan to sites of leukocyte adhesion via CD44.

Endothelial transmigration hotspots limit vascular leakage through heterogeneous expression of ICAM-1

Upon inflammation, leukocytes leave the circulation by crossing the endothelial monolayer at specific transmigration "hotspot" regions. Although these regions support leukocyte transmigration, their functionality is not clear. We found that endothelial hotspots function to limit vascular leakage during transmigration events. Using the photoconvertible probe mEos4b, we traced back and identified original endothelial transmigration hotspots. Using this method, we show that the heterogeneous distribution of ICAM-1 determines the location of the transmigration hotspot. Interestingly, the loss of ICAM-1 heterogeneity either by CRISPR/Cas9-induced knockout of ICAM-1 or equalizing the distribution of ICAM-1 in all endothelial cells results in the loss of TEM hotspots but not necessarily in reduced TEM events. Functionally, the loss of endothelial hotspots results in increased vascular leakage during TEM. Mechanistically, we demonstrate that the 3 extracellular Ig-like domains of ICAM-1 are crucial for hotspot recognition. However, the intracellular tail of ICAM-1 and the 4th Ig-like dimerization domain are not involved, indicating that intracellular signaling or ICAM-1 dimerization is not required for hotspot recognition. Together, we discovered that hotspots function to limit vascular leakage during inflammation-induced extravasation.

Bone Marrow Harbors a Unique Population of Dendritic Cells with the Potential to Boost Neutrophil Formation upon Exposure to Fungal Antigen

Apart from controlling hematopoiesis, the bone marrow (BM) also serves as a secondary lymphoid organ, as it can induce naïve T cell priming by resident dendritic cells (DC). When analyzing DCs in murine BM, we uncovered that they are localized around sinusoids, can (cross)-present antigens, become activated upon intravenous LPS-injection, and for the most part belong to the cDC2 subtype which is associated with Th2/Th17 immunity. Gene-expression profiling revealed that BM-resident DCs are enriched for several c-type lectins, including Dectin-1, which can bind beta-glucans expressed on fungi and yeast. Indeed, DCs in BM were much more efficient in phagocytosis of both yeast-derived zymosan-particles and Aspergillus conidiae than their splenic counterparts, which was highly dependent on Dectin-1. DCs in human BM could also phagocytose zymosan, which was dependent on β1-integrins. Moreover, zymosan-stimulated BM-resident DCs enhanced the differentiation of hematopoietic stem and progenitor cells towards neutrophils, while also boosting the maintenance of these progenitors. Our findings signify an important role for BM DCs as translators between infection and hematopoiesis, particularly in anti-fungal immunity. The ability of BM-resident DCs to boost neutrophil formation is relevant from a clinical perspective and contributes to our understanding of the increased susceptibility for fungal infections following BM damage.

Age-Related Variation in Sympathetic Nerve Distribution in the Human Spleen

Introduction: The cholinergic anti-inflammatory pathway (CAIP) has been proposed as an efferent neural pathway dampening the systemic inflammatory response via the spleen. The CAIP activates the splenic neural plexus and a subsequent series of intrasplenic events, which at least require a close association between sympathetic nerves and T cells. Knowledge on this pathway has mostly been derived from rodent studies and only scarce information is available on the innervation of the human spleen. This study aimed to investigate the sympathetic innervation of different structures of the human spleen, the topographical association of nerves with T cells and age-related variations in nerve distribution.

Materials and Methods: Spleen samples were retrieved from a diagnostic archive and were allocated to three age groups; neonates, 10–25 and 25–70 years of age. Sympathetic nerves and T cells were identified by immunohistochemistry for tyrosine hydroxylase (TH) and the membrane marker CD3, respectively. The overall presence of sympathetic nerves and T cells was semi-automatically quantified and expressed as total area percentage. A predefined scoring system was used to analyze the distribution of nerves within different splenic structures.

Results: Sympathetic nerves were observed in all spleens and their number appeared to slightly increase from birth to adulthood and to decrease afterward. Irrespective to age, more than halve of the periarteriolar lymphatic sheaths (PALSs) contained sympathetic nerves in close association with T cells. Furthermore, discrete sympathetic nerves were observed in the capsule, trabeculae and red pulp and comparable to the total amount of sympathetic nerves, showed a tendency to decrease with age. No correlation was found between the number of T cells and sympathetic nerves.

Conclusion: The presence of discrete sympathetic nerves in the splenic parenchyma, capsule and trabecular of human spleens could suggest a role in functions other than vasoregulation. In the PALS, sympathetic nerves were observed to be in proximity to T cells and is suggestive for the existence of the CAIP in humans. Since sympathetic nerve distribution shows interspecies and age-related variation, and our general understanding of the relative and spatial contribution of splenic innervation in immune regulation is incomplete, it remains difficult to estimate the anti-inflammatory potential of targeting splenic nerves in patients.

Overexpression of Transmembrane TNF Drives Development of Ectopic Lymphoid Structures in the Bone Marrow and B Cell Lineage Alterations in Experimental Spondyloarthritis

TNF is important in immune-mediated inflammatory diseases, including spondyloarthritis (SpA). Transgenic (tg) mice overexpressing transmembrane TNF (tmTNF) develop features resembling human SpA. Furthermore, both tmTNF tg mice and SpA patients develop ectopic lymphoid aggregates, but it is unclear whether these contribute to pathology. Therefore, we characterized the lymphoid aggregates in detail and studied potential alterations in the B and T cell lineage in tmTNF tg mice. Lymphoid aggregates developed in bone marrow (BM) of vertebrae and near the ankle joints prior to the first SpA features and displayed characteristics of ectopic lymphoid structures (ELS) including presence of B cells, T cells, germinal centers, and high endothelial venules. Detailed flow cytometric analyses demonstrated more germinal center B cells with increased CD80 and CD86 expression, along with significantly more T follicular helper, T follicular regulatory, and T regulatory cells in tmTNF tg BM compared with non-tg controls. Furthermore, tmTNF tg mice exhibited increased IgA serum levels and significantly more IgA⁺ plasma cells in the BM, whereas IgA⁺ plasma cells in the gut were not significantly increased. In tmTNF tg × TNF-RI^-/- mice, ELS were absent, consistent with reduced disease symptoms, whereas in tmTNF tg × TNF-RII^-/- mice, ELS and clinical symptoms were still present. Collectively, these data show that tmTNF overexpression in mice results in osteitis and ELS formation in BM, which may account for the increased serum IgA levels that are also observed in human SpA. These effects are mainly dependent on TNF-RI signaling and may underlie important aspects of SpA pathology.

Transendothelial migration induces differential migration dynamics of leukocytes in tissue matrix

Leukocyte extravasation into inflamed tissue is a complex process that is difficult to capture as a whole in vitro. We employed a blood-vessel-on-a-chip model in which human endothelial cells were cultured in a tube-like lumen in a collagen-1 matrix. The vessels are leak tight, creating a barrier for molecules and leukocytes. Addition of inflammatory cytokine TNF-α (also known as TNF) caused vasoconstriction, actin remodelling and upregulation of ICAM-1. Introducing leukocytes into the vessels allowed real-time visualization of all different steps of the leukocyte transmigration cascade, including migration into the extracellular matrix. Individual cell tracking over time distinguished striking differences in migratory behaviour between T-cells and neutrophils. Neutrophils cross the endothelial layer more efficiently than T-cells, but, upon entering the matrix, neutrophils display high speed but low persistence, whereas T-cells migrate with low speed and rather linear migration. In conclusion, 3D imaging in real time of leukocyte extravasation in a vessel-on-a-chip enables detailed qualitative and quantitative analysis of different stages of the full leukocyte extravasation process in a single assay. This article has an associated First Person interview with the first authors of the paper.

Endothelial junctional membrane protrusions serve as hotspots for neutrophil transmigration

Upon inflammation, leukocytes rapidly transmigrate across the endothelium to enter the inflamed tissue. Evidence accumulates that leukocytes use preferred exit sites, alhough it is not yet clear how these hotspots in the endothelium are defined and how they are recognized by the leukocyte. Using lattice light sheet microscopy, we discovered that leukocytes prefer endothelial membrane protrusions at cell junctions for transmigration. Phenotypically, these junctional membrane protrusions are present in an asymmetric manner, meaning that one endothelial cell shows the protrusion and the adjacent one does not. Consequently, leukocytes cross the junction by migrating underneath the protruding endothelial cell. These protrusions depend on Rac1 activity and by using a photo-activatable Rac1 probe, we could artificially generate local exit-sites for leukocytes. Overall, we have discovered a new mechanism that uses local induced junctional membrane protrusions to facilitate/steer the leukocyte escape/exit from inflamed vessel walls.

Extracellular Vesicles Derived From Adult and Fetal Bone Marrow Mesenchymal Stromal Cells Differentially Promote ex vivo Expansion of Hematopoietic Stem and Progenitor Cells

Recently, we and others have illustrated that extracellular vesicles (EVs) have the potential to support hematopoietic stem and progenitor cell (HSPC) expansion; however, the mechanism and processes responsible for the intercellular communication by EVs are still unknown. In the current study, we investigate whether primary human bone marrow derived mesenchymal stromal cells (BMSC) EVs isolated from two different origins, fetal (fEV) and adult (aEV) tissue, can increase the relative low number of HSPCs found in umbilical cord blood (UCB) and which EV-derived components are responsible for ex vivo HSPC expansion. Interestingly, aEVs and to a lesser extent fEVs, showed supportive ex vivo expansion capacity of UCB-HSPCs. Taking advantage of the two BMSC sources with different supportive effects, we analyzed the EV cargo and investigated how gene expression is modulated in HSPCs after incubation with aEVs and fEVs. Proteomics analyses of the protein cargo composition of the supportive aEV vs. the less-supportive fEV identified 90% of the Top100 exosome proteins present in the ExoCarta database. Gene Ontology (GO) analyses illustrated that the proteins overrepresented in aEVs were annotated to oxidation-reduction process, mitochondrial ATP synthesis coupled proton transport, or protein folding. In contrast, the proteins overrepresented in fEVs were annotated to extracellular matrix organization positive regulation of cell migration or transforming growth factor beta receptor (TGFBR) signaling pathway. Small RNA sequencing identified different molecular signatures between aEVs and fEVs. Interestingly, the microRNA cluster miR-99b/let-7e/miR-125a, previously identified to increase the number of HSPCs by targeting multiple pro-apoptotic genes, was highly and significantly enriched in aEVs. Although we identified significant differences in the supportive effects of aEVs and fEVs, RNAseq analyses of the 24 h treated HSPCs indicated that a limited set of genes was differentially regulated when compared to cells that were treated with cytokines only. Together, our study provides novel insights into the complex biological role of EVs and illustrates that aEVs and fEVs differentially support ex vivo expansion capacity of UCB-HSPCs. Together opening new means for the application of EVs in the discovery of therapeutics for more efficient ex vivo HSPC expansion.

Hematopoietic stem and progenitor cells use podosomes to transcellularly cross the bone marrow endothelium

Bone marrow endothelium plays an important role in the homing of hematopoietic stem and progenitor cells upon transplantation, but surprisingly little is known on how the bone marrow endothelial cells regulate local permeability and hematopoietic stem and progenitor cells transmigration. We show that temporal loss of vascular endothelial-cadherin function promotes vascular permeability in BM, even upon low-dose irradiation. Loss of vascular endothelial-cadherin function also enhances homing of transplanted hematopoietic stem and progenitor cells to the bone marrow of irradiated mice although engraftment is not increased. Intriguingly, stabilizing junctional vascular endothelial-cadherin in vivo reduced bone marrow permeability, but did not prevent hematopoietic stem and progenitor cells migration into the bone marrow, suggesting that hematopoietic stem and progenitor cells use the transcellular migration route to enter the bone marrow. Indeed, using an in vitro migration assay, we show that human hematopoietic stem and progenitor cells predominantly cross bone marrow endothelium in a transcellular manner in homeostasis by inducing podosome-like structures. Taken together, vascular endothelial-cadherin is crucial for BM vascular homeostasis but dispensable for the homing of hematopoietic stem and progenitor cells. These findings are important in the development of potential therapeutic targets to improve hematopoietic stem and progenitor cell homing strategies.

GITR shapes humoral immunity by controlling the balance between follicular T helper cells and regulatory T follicular cells

Follicular helper CD4⁺ T-cells (Tfh) control humoral immunity by driving affinity maturation and isotype-switching of activated B-cells. Tfh localize within B-cell follicles and, upon encounter with cognate antigen, drive B-cell selection in germinal centers (GCs) as GC-Tfh. Tfh functionality is controlled by Foxp3-expressing Tfh, which are known as regulatory T follicular cells (Tfr). Thus far, it remains unclear which factors determine the balance between these functionally opposing follicular T-cell subsets. Here, we demonstrate in human and mouse that Tfh and GC-Tfh, as well as their regulatory counterparts, express glucocorticoid-induced TNF receptor related protein (GITR) on their surface. This costimulatory molecule not only helps to identify follicular T-cell subsets, but also increases the ratio of Tfh vs. Tfr, both within and outside the GC. Correspondingly, GITR triggering increases the number of IL-21 producing CD4⁺ T-cells, which also produce more IFN-γ and IL-10. The latter are known switch factors for IgG2c and IgG1, respectively, which corresponds to a concomitant increase in IgG2c and IgG1 production upon GITR-mediated costimulation. These results demonstrate that GITR can skew the functional balance between Tfh and Tfr, which offers new therapeutic possibilities in steering humoral immunity.

Nuclear shape, protrusive behaviour and in vivo retention of human bone marrow mesenchymal stromal cells is controlled by Lamin-A/C expression

Culture expanded mesenchymal stromal cells (MSCs) are being extensively studied for therapeutic applications, including treatment of graft-versus-host disease, osteogenesis imperfecta and for enhancing engraftment of hematopoietic stem cells after transplantation. Thus far, clinical trials have shown that the therapeutic efficiency of MSCs is variable, which may in part be due to inefficient cell migration. Here we demonstrate that human MSCs display remarkable low migratory behaviour compared to other mesodermal-derived primary human cell types. We reveal that specifically in MSCs the nucleus is irregularly shaped and nuclear lamina are prone to wrinkling. In addition, we show that expression of Lamin A/C is relatively high in MSCs. We further demonstrate that in vitro MSC migration through confined pores is limited by their nuclei, a property that might correlate to the therapeutic inefficiency of administered MSC in vivo. Silencing expression of Lamin A/C in MSCs improves nuclear envelope morphology, promotes the protrusive activity of MSCs through confined pores and enhances their retention in the lung after intravenous administration in vivo. Our findings suggest that the intrinsic nuclear lamina properties of MSCs underlie their limited capacity to migrate, and that strategies that target the nuclear lamina might alter MSC-based cellular therapies.

Maintenance of memory CD8 T cells: Divided over division

Once generated during an infection, memory CD8⁺ T cells can provide long-lasting protection against reinfection with an intracellular pathogen, but the longevity of this defense depends on the ability of these pathogen-specific memory cells to be maintained. It is generally believed that the bone marrow plays an important role in this respect, where memory CD8 T cells receive reinvigorating signals from cytokines that induce homeostatic proliferation. However, in the current issue of the European Journal of Immunology, Siracusa et al. (Eur. J. Immunol. 2017. 47: 1900-1905) argue against this dogma, as they provide evidence that CD8 memory T cells in murine bone marrow are not proliferating, but largely quiescent, which protects them from elimination by the cytostatic drug Cyclophosphamide. Interestingly, this is in sharp contrast to the proliferating cell counterparts in the spleen, which are eliminated by this treatment. Here, we will discuss the impact of these results, how they relate to opposing findings by others in the field, and what the relevance of these findings is for humans and clinical applications.

Peripheral and systemic antigens elicit an expandable pool of resident memory CD8+ T cells in the bone marrow

BM has been put forward as a major reservoir for memory CD8⁺  T cells. In order to fulfill that function, BM should "store" memory CD8⁺ T cells, which in biological terms would require these "stored" memory cells to be in disequilibrium with the circulatory pool. This issue is a matter of ongoing debate. Here, we unequivocally demonstrate that murine and human BM harbors a population of tissue-resident memory CD8⁺ T (T_RM ) cells. These cells develop against various pathogens, independently of BM infection or local antigen recognition. BM CD8⁺ T_RM cells share a transcriptional program with resident lymphoid cells in other tissues; they are polyfunctional cytokine producers and dependent on IL-15, Blimp-1, and Hobit. CD8⁺ T_RM cells reside in the BM parenchyma, but are in close contact with the circulation. Moreover, this pool of resident T cells is not size-restricted and expands upon peripheral antigenic re-challenge. This works extends the role of the BM in the maintenance of CD8⁺ T cell memory to include the preservation of an expandable reservoir of functional, non-recirculating memory CD8⁺ T cells, which develop in response to a large variety of peripheral antigens.

CXCR4, but not CXCR3, drives CD8+ T-cell entry into and migration through the murine bone marrow

The BM serves as a blood-forming organ, but also supports the maintenance and immune surveillance function of many T cells. Yet, in contrast to other organs, little is known about the molecular mechanisms that drive T-cell migration to and localization inside the BM. As BM accumulates many CXCR3-expressing memory CD8⁺ T cells, we tested the involvement of this chemokine receptor, but found that CXCR3 is not required for BM entry. In contrast, we could demonstrate that CXCR4, which is highly expressed on both naive and memory CD8⁺ T cells in BM, is critically important for homing of all CD8⁺ T-cell subsets to the BM in mice. Upon entry into the BM parenchyma, both naïve and memory CD8⁺ T cells locate close to sinusoidal vessels. Intravital imaging experiments revealed that CD8 T cells are surprisingly immobile and we found that they interact with ICAM-1+VCAM-1+BP-1+ perivascular stromal cells. These cells are the major source of CXCL12, but also express key survival factors and maintenance cytokines IL-7 and IL-15. We therefore conclude that CXCR4 is not only crucial for entry of CD8⁺ T cells into the BM, but also controls their subsequent localization toward BM niches that support their survival.

Functional Heterogeneity of CD4+ Tumor-Infiltrating Lymphocytes With a Resident Memory Phenotype in NSCLC

Resident memory T cells (T_RM) inhabit peripheral tissues and are critical for protection against localized infections. Recently, it has become evident that CD103⁺ T_RM are not only important in combating secondary infections, but also for the elimination of tumor cells. In several solid cancers, intratumoral CD103⁺CD8⁺ tumor infiltrating lymphocytes (TILs), with T_RM properties, are a positive prognostic marker. To better understand the role of T_RM in tumors, we performed a detailed characterization of CD8⁺ and CD4⁺ TIL phenotype and functional properties in non-small cell lung cancer (NSCLC). Frequencies of CD8⁺ and CD4⁺ T cell infiltrates in tumors were comparable, but we observed a sharp contrast in T_RM ratios compared to surrounding lung tissue. The majority of both CD4⁺ and CD8⁺ TILs expressed CD69 and a subset also expressed CD103, both hallmarks of T_RM. While CD103⁺CD8⁺ T cells were enriched in tumors, CD103⁺CD4⁺ T cell frequencies were decreased compared to surrounding lung tissue. Furthermore, CD103⁺CD4⁺ and CD103⁺CD8⁺ TILs showed multiple characteristics of T_RM, such as elevated expression of CXCR6 and CD49a, and decreased expression of T-bet and Eomes. In line with the immunomodulatory role of the tumor microenvironment, CD8⁺ and CD4⁺ TILs expressed high levels of inhibitory receptors 2B4, CTLA-4, and PD-1, with the highest levels found on CD103⁺ TILs. Strikingly, CD103⁺CD4⁺ TILs were the most potent producers of TNF-α and IFN-γ, while other TIL subsets lacked such cytokine production. Whereas, CD103⁺CD4⁺PD-1^low TILs produced the most effector cytokines, CD103⁺CD4⁺PD-1⁺⁺ and CD69⁺CD4⁺PD-1⁺⁺ TILs produced CXCL13. Furthermore, a large proportion of TILs expressed co-stimulatory receptors CD27 and CD28, unlike lung T_RM, suggesting a less differentiated phenotype. Agonistic triggering of these receptors improved cytokine production of CD103⁺CD4⁺ and CD69⁺CD8⁺ TILs. Our findings thus provide a rationale to target CD103⁺CD4⁺ TILs and add co-stimulation to current therapies to improve the efficacy of immunotherapies and cancer vaccines.

Trigger-happy resident memory CD4+ T cells inhabit the human lungs

Resident memory T cells (T_RM) reside in the lung epithelium and mediate protective immunity against respiratory pathogens. Although lung CD8⁺ T_RM have been extensively characterized, the properties of CD4⁺ T_RM remain unclear. Here we determined the transcriptional signature of CD4⁺ T_RM, identified by the expression of CD103, retrieved from human lung resection material. Various tissue homing molecules were specifically upregulated on CD4⁺ T_RM, whereas expression of tissue egress and lymph node homing molecules were low. CD103⁺ T_RM expressed low levels of T-bet, only a small portion expressed Eomesodermin (Eomes), and although the mRNA levels for Hobit were increased, protein expression was absent. On the other hand, the CD103⁺ T_RM showed a Notch signature. CD4⁺CD103⁺ T_RM constitutively expressed high transcript levels of numerous cytotoxic mediators that was functionally reflected by a fast recall response, magnitude of cytokine production, and a high degree of polyfunctionality. Interestingly, the superior cytokine production appears to be because of an accessible interferon-γ (IFNγ) locus and was partially because of rapid translation of preformed mRNA. Our studies provide a molecular understanding of the maintenance and potential function of CD4⁺ T_RM in the human lung. Understanding the specific properties of CD4⁺ T_RM is required to rationally improve vaccine design.

Interferon-Gamma Impairs Maintenance and Alters Hematopoietic Support of Bone Marrow Mesenchymal Stromal Cells

Bone marrow (BM) mesenchymal stromal cells (MSCs) provide microenvironmental support to hematopoietic stem and progenitor cells (HSPCs). Culture-expanded MSCs are interesting candidates for cellular therapies due to their immunosuppressive and regenerative potential which can be further enhanced by pretreatment with interferon-gamma (IFN-γ). However, it remains unknown whether IFN-γ can also influence hematopoietic support by BM-MSCs. In this study, we elucidate the impact of IFN-γ on the hematopoietic support of BM-MSCs. We found that IFN-γ increases expression of interleukin (IL)-6 and stem cell factor by human BM-MSCs. IFN-γ-treated BM-MSCs drive HSPCs toward myeloid commitment in vitro, but impair subsequent differentiation of HSPC. Moreover, IFN-γ-ARE-Del mice with increased IFN-γ production specifically lose their BM-MSCs, which correlates with a loss of hematopoietic stem cells' quiescence. Although IFN-γ treatment enhances the immunomodulatory function of MSCs in a clinical setting, we conclude that IFN-γ negatively affects maintenance of BM-MSCs and their hematopoietic support in vitro and in vivo.

The TNF Receptor Superfamily-NF-κB Axis Is Critical to Maintain Effector Regulatory T Cells in Lymphoid and Non-lymphoid Tissues

After exiting the thymus, Foxp3⁺ regulatory T (Treg) cells undergo further differentiation in the periphery, resulting in the generation of mature, fully suppressive effector (e)Treg cells in a process dependent on TCR signaling and the transcription factor IRF4. Here, we show that tumor necrosis factor receptor superfamily (TNFRSF) signaling plays a crucial role in the development and maintenance of eTreg cells. TNFRSF signaling activated the NF-κB transcription factor RelA, which was required to maintain eTreg cells in lymphoid and non-lymphoid tissues, including RORγt⁺ Treg cells in the small intestine. In response to TNFRSF signaling, RelA regulated basic cellular processes, including cell survival and proliferation, but was dispensable for IRF4 expression or DNA binding, indicating that both pathways operated independently. Importantly, mutations in the RelA binding partner NF-κB1 compromised eTreg cells in humans, suggesting that the TNFRSF-NF-κB axis was required in a non-redundant manner to maintain eTreg cells in mice and humans.

Impact of T cells on hematopoietic stem and progenitor cell function: Good guys or bad guys?

When hematopoietic stem and progenitor cells (HSPC) are harvested for transplantation, either from the bone marrow or from mobilized blood, the graft contains a significant number of T cells. It is these T cells that are the major drivers of graft-vs-host disease (GvHD). The risk for GvHD can simply be reduced by the removal of these T cells from the graft. However, this is not always desirable, as this procedure also decreases the engraftment of the transplanted HSPCs and, if applicable, a graft-vs-tumor effect. This poses an important conundrum in the field: T cells act as a double-edged sword upon allogeneic HSPC transplantation, as they support engraftment of HSPCs and provide anti-tumor activity, but can also cause GvHD. It has recently been suggested that T cells also enhance the engraftment of autologous HSPCs, thus supporting the notion that T cells and HSPCs have an important functional interaction that is highly beneficial, in particular during transplantation. The underlying reason on why and how T cells contribute to HSPC engraftment is still poorly understood. Therefore, we evaluate in this review the studies that have examined the role of T cells during HSPC transplantation and the possible mechanisms involved in their supporting function. Understanding the underlying cellular and molecular mechanisms can provide new insight into improving HSPC engraftment and thus lower the number of HSPCs required during transplantation. Moreover, it could provide new avenues to limit the development of severe GvHD, thus making HSPC transplantations more efficient and ultimately safer.

Enhanced uptake of blood coagulation factor VIII containing immune complexes by antigen presenting cells

Essentials Anti-factor (F) VIII antibody formation is a major complication in the treatment of hemophilia A. We investigated uptake of FVIII and FVIII immune complex by bone marrow derived dendritic cells. Immune complex formation increased uptake of FVIII 3-4 fold in a Fcγ receptor dependent manner. FVIII immune complex binding to Fcγ receptors may modulate immune tolerance induction.

SUMMARY

Background A major complication in the treatment of hemophilia A is the development of inhibitory antibodies targeting coagulation factor VIII (FVIII). Eradication of these inhibitors can be established by immune tolerance induction (ITI), which consists of daily administration of high dosages of FVIII. FVIII immune complexes (FVIII-IC) could be formed following FVIII infusion in patients with pre-existing anti-FVIII antibodies. Objectives Here we studied endocytosis of FVIII-IC by bone marrow-derived dendritic cells (BMDCs). Methods BMDCs were pulsed with FVIII/FVIII-IC and uptake was assessed by flow cytometry and confocal imaging. Results BMDCs were able to efficiently internalize FVIII-IC in a dose-dependent manner, 3-4-fold more efficiently when compared with equimolar concentrations of non-complexed FVIII. Uptake of FVIII-IC, but not FVIII alone, could be inhibited with anti-Fcγ receptor (FcγR) antibody 2.4G2, indicating functional involvement of FcγR. No internalization of FVIII-IC was observed in BMDCs lacking FcγRI, FcγRIIb, FcγRIII and FcγRIV. Genetic ablation of FcγRIIb, FcγRIII or FcγRIV individually did not affect the ability of anti-FVIII IgG to promote the uptake of FVIII. BMDCs lacking FcγRI showed lower FVIII-IC uptake levels when compared with other single FcγR null BMDCs. Expression of the inhibitory FcγRIIb alone was sufficient to internalize FVIII-IC more efficiently than FVIII. Conclusions FcγR are critical in the internalization of FVIII-IC by BMDCs and multiple FcγR can contribute independently to this process. Our findings provide a basis for future studies to address whether the outcome of ITI is dependent on the interplay between FVIII-IC and inhibitory and activating FcγR.

Impact of Viral Infections on Hematopoiesis: From Beneficial to Detrimental Effects on Bone Marrow Output

The ability of the bone marrow (BM) to generate copious amounts of blood cells required on a daily basis depends on a highly orchestrated process of proliferation and differentiation of hematopoietic stem and progenitor cells (HSPCs). This process can be rapidly adapted under stress conditions, such as infections, to meet the specific cellular needs of the immune response and the ensuing physiological changes. This requires a tight regulation in order to prevent either hematopoietic failure or transformation. Although adaptation to bacterial infections or systemic inflammation has been studied and reviewed in depth, specific alterations of hematopoiesis to viral infections have received less attention so far. Viruses constantly pose a significant health risk and demand an adequate, balanced response from our immune system, which also affects the BM. In fact, both the virus itself and the ensuing immune response can have a tremendous impact on the hematopoietic process. On one hand, this can be beneficial: it helps to boost the cellular response of the body to resolve the viral infection. But on the other hand, when the virus and the resulting antiviral response persist, the inflammatory feedback to the hematopoietic system will become chronic, which can be detrimental for a balanced BM output. Chronic viral infections frequently have clinical manifestations at the level of blood cell formation, and we summarize which viruses can lead to BM pathologies, like aplastic anemia, pancytopenia, hemophagocytic lymphohistiocytosis, lymphoproliferative disorders, and malignancies. Regarding the underlying mechanisms, we address specific effects of acute and chronic viral infections on blood cell production. As such, we distinguish four different levels in which this can occur: (1) direct viral infection of HSPCs, (2) viral recognition by HSPCs, (3) indirect effects on HSPCs by inflammatory mediators, and (4) the role of the BM microenvironment on hematopoiesis upon virus infection. In conclusion, this review provides a comprehensive overview on how viral infections can affect the formation of new blood cells, aiming to advance our understanding of the underlying cellular and molecular mechanisms to improve the treatment of BM failure in patients.

Constitutive GITR Activation Reduces Atherosclerosis by Promoting Regulatory CD4 + T-Cell Responses—Brief Report

Objective—

Glucocorticoid-induced tumor necrosis factor receptor family-related protein (GITR) is expressed on CD4 + effector memory T cells and regulatory T cells; however, its role on these functionally opposing cell types in atherosclerosis is not fully understood.

Approach and Results—

Low-density lipoprotein receptor–deficient mice ( Ldlr −/− ) were lethally irradiated and reconstituted with either bone marrow from B-cell–restricted Gitrl transgenic mice or from wild-type controls and fed a high-cholesterol diet for 11 weeks. Chimeric Ldlr −/− Gitrl tg mice showed a profound increase in both CD4 + effector memory T cells and regulatory T cells in secondary lymphoid organs. Additionally, the number of regulatory T cells was significantly enhanced in the thymus and aorta of these mice along with increased Gitrl and Il-2 transcript levels. Atherosclerotic lesions of Ldlr −/− Gitrl tg chimeras contained more total CD3 + T cells as well as Foxp3 + regulatory T cells overall, leading to significantly less severe atherosclerosis.

Conclusions—

These data indicate that continuous GITR stimulation through B cell Gitrl acts protective in a mouse model of atherosclerosis by regulating the balance between regulatory and effector memory CD4 + T cells.