Spleen as a site for hematopoiesis of a distinct antigen presenting cell type

Skeletal stem/progenitor cells provide the niche for extramedullary hematopoiesis in spleen

In bone marrow, the niche which supports hematopoiesis and nurtures hematopoietic stem cells (HSCs) contains perivascular reticular cells representing a subset of skeletal stem/progenitor cells (SSPCs). These stromal cells which provide the niche are lost or become inadequate during stress, disease or ageing, such that HSCs leave bone marrow and enter spleen and other peripheral sites to initiate extramedullary hematopoiesis and particularly myelopoiesis. Spleen also maintains niches for HSCs under steady-state conditions, evident since neonatal and adult spleen contain HSCs in low number and provide low-level hematopoiesis. In spleen, HSCs are found in the sinusoidal-rich red pulp region also in the vicinity of perivascular reticular cells. These cells resemble to some extent the known stromal elements reflecting HSC niches in bone marrow, and are investigated here for their characteristics as a subset of SSPCs. The isolation of spleen stromal subsets and the generation of cell lines which support HSCs and myelopoiesis in vitro has led to the identification of perivascular reticular cells which are unique to spleen. Analysis of gene and marker expression, as well as differentiative potential, identifies an osteoprogenitor cell type, reflective of one of several subsets of SSPCs described previously in bone, bone marrow and adipose tissue. The combined information supports a model for HSC niches in spleen involving perivascular reticular cells as SSPCs having osteogenic, stroma-forming capacity. These associate with sinusoids in red pulp to form niches for HSCs and to support the differentiation of hematopoietic progenitors during extramedullary hematopoiesis.

Hematopoietic Effects of Angelica gigas Nakai Extract on Cyclophosphamide-Induced Myelosuppression

Myelosuppression is a major adverse effect of chemotherapy. With the increasing number of cancer patients worldwide, there is a growing interest in therapeutic approaches that reduce the adverse effects of chemotherapy. Angelica gigas Nakai (AGN) roots have been widely used in oriental medicine to treat blood-related diseases, including cancer. However, the effects of AGN on myelosuppression have not been studied. Here, we investigated the effects of AGN ethanol extract (AGNEX) on cyclophosphamide-induced myelosuppression. AGNEX treatment significantly decreased white blood cell levels while increasing red blood cell and platelet levels in the peripheral blood. It inhibited thymus and spleen atrophy. It also enhanced serum levels of interleukin (IL)-6 and tumor necrosis factor (TNF)-α. qRT-PCR results showed that AGNEX decreased the expression of IL-1b and stem cell factor (SCF) in the bone marrow (BM) while increasing the mRNA expression of IL-3 and IL-6 in the spleen. Although AGNEX did not significantly decrease apoptosis and cell cycle arrest in the BM and splenocytes, AGNEX plays a positive role in cyclophosphamide-induced myelosuppression. AGNEX administration increased BM cells in the femur while decreasing apoptotic BM cells. These findings suggest that AGNEX could be used to treat myelosuppression and as a combination therapy in cancer patients.

Role of SVEP1 in Stroma-Dependent Hematopoiesis In vitro

Study of the microenvironment that supports hematopoietic stem cell (HSC) development in vivo is very difficult involving small numbers of interacting cells which are usually not well defined. While much is known about HSC niches located within the bone marrow in terms of contributing cell types and signalling molecules, very little is known about equivalent niches within spleen. Extramedullary hematopoiesis in spleen contributes myeloid cells important in the mobilisation of an immune response. As a result, it is important to develop in vitro models to identify the cells which constitute HSC niches in spleen and to identify the regulatory molecules supporting myeloid cell development. Studies described here document a model system to study the maintenance and differentiation of HSC by splenic stromal cells in vitro. The splenic stromal lines 5G3 and 3B5 differ in hematopoietic support capacity. SVEP1 and IGF2 are molecules of interest specifically expressed by 5G3 stroma. Gene knockdown technology using shRNA plasmids has been used to reduce gene expression in 5G3 and to determine specific effects on myeloid cell development following co-culture with overlaid hematopoietic progenitors in vitro. Knockdown of Svep1 gave specific inhibition of a dendritic cell (DC) population described previously in spleen (L-DC). Knockdown of Igf2 resulted in loss of production of a minor subset of conventional (c) DC. SVEP1 is now considered a marker of mesenchymal stromal cells with osteogenic differentiative capacity reflective of perivascular stromal cells. The power of this in vitro model is evidenced by the fact that it has been used to define SVEP1 as a specific adhesion molecule that regulates the hematopoietic process dependent on stromal niche interaction. The identification of stromal cells and molecules that contribute to the hematopoietic process in spleen, brings us closer to the realm of therapeutically regulating hematopoiesis in vivo, and to inhibiting niches which support cancer stem cells.

Identification of the Adult Hematopoietic Liver as the Primary Reservoir for the Recruitment of Pro-regenerative Macrophages Required for Salamander Limb Regeneration

The lack of scar-free healing and regeneration in many adult human tissues imposes severe limitations on the recovery of function after injury. In stark contrast, salamanders can functionally repair a range of clinically relevant tissues throughout adult life. The impressive ability to regenerate whole limbs after amputation, or regenerate following cardiac injury, is critically dependent on the recruitment of (myeloid) macrophage white blood cells to the site of injury. Amputation in the absence of macrophages results in regeneration failure and scar tissue induction. Identifying the exact hematopoietic source or reservoir of myeloid cells supporting regeneration is a necessary step in characterizing differences in macrophage phenotypes regulating scarring or regeneration across species. Mammalian wounds are dominated by splenic-derived monocytes that originate in the bone marrow and differentiate into macrophages within the wound. Unlike mammals, adult axolotls do not have functional bone marrow but instead utilize liver and spleen tissues as major sites for adult hematopoiesis. To interrogate leukocyte identity, tissue origins, and modes of recruitment, we established several transgenic axolotl hematopoietic tissue transplant models and flow cytometry protocols to study cell migration and identify the source of pro-regenerative macrophages. We identified that although bidirectional trafficking of leukocytes can occur between spleen and liver tissues, the liver is the major source of leukocytes recruited to regenerating limbs. Recruitment of leukocytes and limb regeneration occurs in the absence of the spleen, thus confirming the dependence of liver-derived myeloid cells in regeneration and that splenic maturation is dispensable for the education of pro-regenerative macrophages. This work provides an important foundation for understanding the hematopoietic origins and education of myeloid cells recruited to, and essential for, adult tissue regeneration.

The spleen as a sanctuary site for residual leukemic cells following ABT-199 monotherapy in ETP-ALL

B-cell lymphoma 2 (BCL-2) has recently emerged as a therapeutic target for early T-cell progenitor acute lymphoblastic leukemia (ETP-ALL), a high-risk subtype of human T-cell ALL. The major clinical challenge with targeted therapeutics, such as the BCL-2 inhibitor ABT-199, is the development of acquired resistance. We assessed the in vivo response of luciferase-positive LOUCY cells to ABT-199 monotherapy and observed specific residual disease in the splenic microenvironment. Of note, these results were confirmed by using a primary ETP-ALL patient-derived xenograft. Splenomegaly has previously been associated with poor prognosis in diverse types of leukemia. However, the exact mechanism by which the splenic microenvironment alters responses to specific targeted therapies remains largely unexplored. We show that residual LOUCY cells isolated from the spleen microenvironment displayed reduced BCL-2 dependence, which was accompanied by decreased BCL-2 expression levels. Notably, this phenotype of reduced BCL-2 dependence could be recapitulated by using human splenic fibroblast coculture experiments and was confirmed in an in vitro chronic ABT-199 resistance model of LOUCY. Finally, single-cell RNA-sequencing was used to show that ABT-199 triggers transcriptional changes in T-cell differentiation genes in leukemic cells obtained from the spleen microenvironment. Of note, increased expression of CD1a and sCD3 was also observed in ABT199-resistant LOUCY clones, further reinforcing the idea that a more differentiated leukemic population might display decreased sensitivity toward BCL-2 inhibition. Overall, our data reveal the spleen as a site of residual disease for ABT-199 treatment in ETP-ALL and provide evidence for plasticity in T-cell differentiation as a mechanism of therapy resistance.

Impact of spleen size and splenectomy on outcomes of allogeneic hematopoietic cell transplantation for myelofibrosis: A retrospective analysis by the chronic malignancies working party on behalf of European society for blood and marrow transplantation (EBMT)

The role of spleen size and splenectomy for the prediction of post-allogeneic hematopoietic stem cell transplant (allo-HCT) outcome in myelofibrosis remains under debate. In EBMT registry, we identified a cohort of 1195 myelofibrosis patients transplanted between 2000-2017 after either fludarabine-busulfan or fludarabine-melphalan regimens. Overall, splenectomy was performed in 202 (16.9%) patients and its use decreased over time (28.3% in 2000-2009 vs 14.1% in 2010-2017 period). By multivariate analysis, splenectomy was associated with less NRM (HR 0.64, 95% CI 0.44-0.93, P = .018) but increased risk of relapse (HR 1.43, 95% CI 1.01-2.02, P = .042), with no significant impact on OS (HR 0.86, 95% CI 0.67-1.12, P = .274). However, in subset analysis comparing the impact of splenectomy vs specific spleen sizes, for patients with progressive disease, an improved survival was seen in splenectomised subjects compared to those patients with a palpable spleen length ≥ 15 cm (HR 0.44, 95% CI 0.28-0.69, P < .001), caused by a significant reduction in NRM (HR 0.26, 95% CI 0.14-0.49, P < .001), without significantly increased relapse risk (HR 1.47, 95% CI 0.87-2.49, P = .147). Overall, despite the possible biases typical of retrospective cohorts, this study highlights the potential detrimental effect of massive splenomegaly in transplant outcome and supports the role of splenectomy for myelofibrosis patients with progressive disease and large splenomegaly.

Salmonid Antibacterial Immunity: An Aquaculture Perspective

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.

Transplanted spleen stromal cells with osteogenic potential support ectopic myelopoiesis

Spleen stromal lines which support in vitro hematopoiesis are investigated for their lineage origin and hematopoietic support function in vivo. Marker expression and gene profiling identify a lineage relationship with mesenchymal stem cells and perivascular reticular cells described recently in bone marrow. Stromal lines commonly express Cxcl12, Pdgfra and Pdgfr typical of bone marrow derived perivascular reticular cells but reflect a unique cell type in terms of other gene and marker expression. Their classification as osteoprogenitors is confirmed through ability to undergo osteogenic, but not adipogenic or chondrogenic differentiation. Some stromal lines were shown to form ectopic niches for HSCs following engraftment under the kidney capsule of NOD/SCID mice. The presence of myeloid cells and a higher representation of a specific dendritic-like cell type over other myeloid cells within grafts was consistent with previous in vitro evidence of hematopoietic support capacity. These studies reinforce the role of perivascular/perisinusoidal reticular cells in hematopoiesis and implicate such cells as niches for hematopoiesis in spleen.

Targeting the Spleen as an Alternative Site for Hematopoiesis

Bone marrow is the main site for hematopoiesis in adults. It acts as a niche for hematopoietic stem cells (HSCs) and contains non-hematopoietic cells that contribute to stem cell dormancy, quiescence, self-renewal, and differentiation. HSC also exist in resting spleen of several species, although their contribution to hematopoiesis under steady-state conditions is unknown. The spleen can however undergo extramedullary hematopoiesis (EMH) triggered by physiological stress or disease. With the loss of bone marrow niches in aging and disease, the spleen as an alternative tissue site for hematopoiesis is an important consideration for future therapy, particularly during HSC transplantation. In terms of harnessing the spleen as a site for hematopoiesis, here the remarkable regenerative capacity of the spleen is considered with a view to forming additional or ectopic spleen tissue through cell engraftment. Studies in mice indicate the potential for such grafts to support the influx of hematopoietic cells leading to the development of normal spleen architecture. An important goal will be the formation of functional ectopic spleen tissue as an aid to hematopoietic recovery following clinical treatments that impact bone marrow. For example, expansion or replacement of niches could be considered where myeloablation ahead of HSC transplantation compromises treatment outcomes.

Identification of genes which regulate stroma-dependent in vitro hematopoiesis

Cultured splenic stroma has been shown to support in vitro hematopoiesis in overlaid bone marrow and spleen progenitors. These co-cultures support longterm production of a novel dendritic-like cell type along with transient production of myeloid cells. They also maintain a progenitor cell population. The splenic stromal lines 5G3 and 3B5 have been identified as a supporter and a non-supporter of hematopoiesis. Based on their gene expression profile, both 5G3 and 3B5 express genes related to hematopoiesis, while 5G3 cells express several unique genes, and show upregulation of some genes over 3B5. Based on gene expression studies, specific inhibitors were tested for capacity to inhibit hematopoiesis in co-cultures. Addition of specific antibodies and small molecule inhibitors identified VCAM1, CXCL12, CSF1 and SPP1 as potential regulators of hematopoiesis, although both are expressed by 5G3 and 3B5. Through inhibition of function, SVEP1 and ALDH1 are also shown here to be deterministic of 5G3 hematopoietic support capacity, since these are uniquely expressed by 5G3 and not 3B5. The achievement of inhibition is notable given the dynamic, longterm nature of co-cultures which involve only small numbers of cells. The alternate plan, to add recombinant soluble factors produced by 5G3 back into 3B5 co-cultures in order to recover in vitro hematopoiesis, proved ineffective. Out of 6 different factors added to 3B5, only IGF2 showed any effect on cell production. The identification of differentially expressed or upregulated genes in 5G3 has provided an insight into potential pathways involved in in vitro hematopoiesis leading to production of dendritic-like cells.

Distribution of non-myelinating Schwann cells and their associations with leukocytes in mouse spleen revealed by immunofluorescence staining

The nervous system and the immune system communicate extensively with each other in order to maintain homeostasis and to regulate the immune response. The peripheral nervous system (PNS) communicates specifically with the immune system according to local interactions, including the “hardwiring” of sympathetic/parasympathetic (efferent) and sensory nerves (afferent) to lymphoid tissue and organs. To reveal this type of bidirectional neuroimmune interaction at the microscopic level, we used immunofluorescent staining of glial fibrillary acidic protein (GFAP) coupled with confocal microscopy/3D reconstruction to reveal the distribution of nonmyelinating Schwann cells (NMSCs) and their interactions with immune cells inside mouse spleen. Our results demonstrate i) the presence of an extensive network of NMSC processes in all splenic compartments including the splenic nodules, periarteriolar lymphoid sheath (PALS), marginal zone, trabecula, and red pulp; ii) the close association of NMSC processes with blood vessels (including central arteries and their branches, marginal sinuses, penicillar arterioles and splenic sinuses); iii) the close “synapse-like” interaction/association of NMSC processes with various subsets of dendritic cells (DCs; e.g., CD4⁺CD11c⁺ DCs, B220⁺CD11c⁺ DCs, and F4/80⁺ CD11c⁺ DCs), macrophages (F4/80⁺), and lymphocytes (B cells, CD4⁺ T helper cells). Our novel findings concerning the distribution of NMSCs and NMSC-leukocytes interactions inside mouse spleen should improve our understanding of the mechanisms through which the PNS affects cellular- and humoral-mediated immune responses in a variety of health conditions and infectious/non-infectious diseases.

An Atypical Splenic B Cell Progenitor Population Supports Antibody Production during Plasmodium Infection in Mice

Hematopoietic stem and progenitor cells (HSPCs) function to replenish the immune cell repertoire under steady-state conditions and in response to inflammation due to infection or stress. Whereas the bone marrow serves as the primary niche for hematopoiesis, extramedullary mobilization and differentiation of HSPCs occur in the spleen during acute Plasmodium infection, a critical step in the host immune response. In this study, we identified an atypical HSPC population in the spleen of C57BL/6 mice, with a lineage(-)Sca-1(+)c-Kit(-) (LSK(-)) phenotype that proliferates in response to infection with nonlethal Plasmodium yoelii 17X. Infection-derived LSK(-) cells upon transfer into naive congenic mice were found to differentiate predominantly into mature follicular B cells. However, when transferred into infection-matched hosts, infection-derived LSK(-) cells gave rise to B cells capable of entering into a germinal center reaction, and they developed into memory B cells and Ab-secreting cells that were capable of producing parasite-specific Abs. Differentiation of LSK(-) cells into B cells in vitro was enhanced in the presence of parasitized RBC lysate, suggesting that LSK(-) cells expand and differentiate in direct response to the parasite. However, the ability of LSK(-) cells to differentiate into B cells was not dependent on MyD88, as myd88(-/-) LSK(-) cell expansion and differentiation remained unaffected after Plasmodium infection. Collectively, these data identify a population of atypical lymphoid progenitors that differentiate into B lymphocytes in the spleen and are capable of contributing to the ongoing humoral immune response against Plasmodium infection.

Hematopoietic stem and progenitor cells can be enriched by implanting biomaterial into spatium intermusculare

Hematopoietic stem and progenitor cells (HSPCs) have been used successfully to treat patients with cancer and disorders of the blood and immune systems. In this study, we tried to enrich HSPCs by implanting biomaterials into the spatium intermusculare of mice hind limbs. Gelatine sponges were implanted into the spatium intermusculare of mice and then retrieved after 12 days. The presence of HSPCs in the migrating cells (MCs) was detected by phenotypically probing with CD34⁺Sca-1⁺ and functionally confirming the presence of using colony-forming cell assay and assessing the long-term reconstitution ability. The frequency of CD34⁺, Sca-1⁺, and CD34⁺Sca-1⁺ cells and colony formation unit in the MCs was much higher than that in the bone marrow (BM). Moreover, transplanted MCs were able to home to BM, muscle, and spleen, which induced an efficient long-term hematopoietic reconstitution in vivo. In addition, HSPCs within the MCs originated from the BM. Furthermore, the administration of G-CSF greatly reduced the time of implantation, and increased the number of MCs and frequency of HSPCs in the MCs. These data provide compelling evidence that HSPCs can be enriched by implanting biomaterial into spatium intermusculare. Implantation of biomaterial may be seen as the first step to a proof of their applicability to clinical practice in enriching HSPCs.

Characterisation of dendritic cells arising from progenitors endogenous to murine spleen

Heterogeneity amongst dendritic cell (DC) subsets leads to a spectrum of immune response capacity against pathogens. Several DC subsets in spleen have been described which differ in terms of phenotype and function. We have previously reported a distinct population of CD11c(lo)CD11b(hi)MHC-II(-)CD8(-) dendritic-like "L-DC" in murine spleen, which can also be generated in splenic stromal longterm cultures. Here, the ontogeny of L-DC development in perinatal mice has been compared with other known splenic DC subsets. Flow cytometric analysis has revealed the presence of L-DC at embryonic age (E)18.5 spleen, while plasmacytoid (p)DC and conventional (c)DC appear at 2 and 4 days following birth. Co-cultures of E18.5 spleen above splenic stroma also showed production of only L-DC, while spleen cells from D0 through D5 neonates showed production of both L-DC and cDC-like cells. Addition of an M-CSFR inhibitor to co-cultures revealed that while the development of cDC-like cells depended on M-CSF, many L-DC developed independently of M-CSF. Furthermore, purified hematopoietic stem cells (HSC) and multipotential progenitors (MPP) isolated from neonatal D1 spleen are capable of developing into L-DC in co-cultures. These studies reveal a lineage of dendritic-like cells developing in the spleen microenvironment, and which appear to arise from endogenous progenitors laid down in spleen during embryogenesis.

Investigation into the prevalence of a novel dendritic-like cell subset in vivo

A novel dendritic-like cell subset termed L-DC was recently identified in murine spleen based on marker expression of a homogeneous cell population derived from long-term culture of neonatal spleen. The function of L-DC is distinct from other splenic dendritic and myeloid cell subsets because of their high endocytic capacity and their ability to cross-present antigen to CD8⁺ T cells. This paper shows the subset to be unique to spleen and blood, with a similar, but possibly functionally distinct subset also present in bone marrow. The prevalence of the subset is low; ∼6% of all dendritic and myeloid cells in the spleen and ∼5% in blood. However, they are a distinct cell type on the basis of marker expression, and endocytic and T-cell stimulatory capacity. Attempts to identify an enriched population of these cells in mutant mouse strains with reported increases in myelopoiesis showed either a lack of L-DC or an altered phenotype reflective of the phenotype of the mouse strain.

Sphingosine kinase 2 (Sphk2) regulates platelet biogenesis by providing intracellular sphingosine 1-phosphate (S1P)

Human megakaryocytes (MKs) release trillions of platelets each day into the circulation to maintain normal homeostatic platelet levels. We have previously shown that extracellular sphingosine 1-phosphate (S1P) plays a key role in thrombopoiesis via its receptor S1pr1. In addition to its role as an extracellular mediator, S1P can also function as a second messenger in the intracellular compartment. Although signaling via intracellular S1P is involved in various cellular processes, a role in thrombopoiesis has not been examined. Sphingosine kinases are the key enzymes that produce intracellular S1P. Here we report that sphingosine kinase 2 (Sphk2) is the major messenger RNA species present in MKs. Sphk2 predominantly localizes to the nucleus and is the major source of intracellular S1P in MKs. Loss of Sphk2 significantly reduced intracellular S1P in MKs and downregulated the expression and activity of Src family kinases (SFKs). Loss of Sphk2 and inhibition of SFK activity resulted in defective intravascular proplatelet shedding, the final stage of thrombopoiesis. Correspondingly, mice lacking Sphk2 in the hematopoietic system display thrombocytopenia. Together, our data suggest that Sphk2 provides the source of intracellular S1P that controls thrombopoiesis, which is associated with SFK expression and activity in MKs.

Development of two distinct dendritic-like APCs in the context of splenic stroma

Murine splenic stroma has been found to provide an in vitro niche for hematopoiesis of dendritic-like APC. Two distinct cell types have been characterized. The novel “L-DC” subset has cross-presenting capacity, leading to activation of CD8⁺ T cells, but not activating CD4⁺ T cells, which is consistent with their CD11c^loCD11b^hiMHC-II⁻ phenotype. For L-DC, an equivalent tissue-specific APC has been found only in spleen. A second population of CD11c^hiCD11b^loMHC-II⁺ cells resembling conventional dendritic cells (cDC) can activate both CD4 and CD8 T cells. Production of L-DC but not cDC-like cells is now shown to be dependent on contact between the L-DC progenitor and stroma such that the presence of a Transwell membrane can prevent L-DC development. Since L-DC can be produced continuously in vitro in stromal co-cultures overlaid with bone marrow (BM) progenitors, it was hypothesized that L-DC progenitors are self-renewing. The L-DC progenitor is shown here to be defined by the Flt3⁻c-kit⁺Lin⁻Sca-1⁺ (F⁻KLS) subset of adult BM which contains primitive HSC. Since the less primitive F⁺KLS HSC subset also contains L-DC progenitors, Flt3 does not appear to be a defining marker for this progenitor. Precursors of the cDC-like subset are found only within the F⁺KLS subset and seed production of a transient population of APC. All data identify differentiation of L-DC from HSC, and of cDC-like cells from DC precursors, which occurs independently of inflammatory signals and is dependent on a splenic stromal microenvironment.