Up-regulated expression of CXCL12 in human spleens with extramedullary haematopoiesis

Reduced splenic uptake on 68Ga-Pentixafor-PET/CT imaging in multiple myeloma - a potential imaging biomarker for disease prognosis

Beyond being a key factor for tumor growth and metastasis in human cancer, C-X-C motif chemokine receptor 4 (CXCR4) is also highly expressed by a number of immune cells, allowing for non-invasive read-out of inflammatory activity. With two recent studies reporting on prognostic implications of the spleen signal in diffusion-weighted magnetic resonance imaging in patients with plasma cell dyscrasias, the aim of this study was to correlate splenic ⁶⁸Ga-Pentixafor uptake in multiple myeloma (MM) with clinical parameters and to evaluate its prognostic impact.

Methods: Eighty-seven MM patients underwent molecular imaging with ⁶⁸Ga-Pentixafor-PET/CT. Splenic CXCR4 expression was semi-quantitatively assessed by peak standardized uptake values (SUV_peak) and corresponding spleen-to-bloodpool ratios (TBR) and correlated with clinical and prognostic features as well as survival parameters.

Results:⁶⁸Ga-Pentixafor-PET/CT was visually positive in all MM patients with markedly heterogeneous tracer uptake in the spleen. CXCR4 expression determined by ⁶⁸Ga-Pentixafor-PET/CT corresponded with advanced disease and was inversely associated with the number of previous treatment lines as compared to controls or untreated smouldering multiple myeloma patients (SUV_peakSpleen 4.06 ± 1.43 vs. 6.02 ± 1.16 vs. 7.33 ± 1.40; P < 0.001). Moreover, reduced splenic ⁶⁸Ga-Pentixafor uptake was linked to unfavorable clinical outcome. Patients with a low SUV_peakSpleen (<3.35) experienced a significantly shorter overall survival of 5 months as compared to 62 months in patients with a high SUV_peakSpleen >5.79 (P < 0.001). Multivariate Cox analysis confirmed SUV_peakSpleen as an independent predictor of survival (HR 0.75; P = 0.009).

Conclusion: These data suggest that splenic ⁶⁸Ga-Pentixafor uptake might provide prognostic information in pre-treated MM patients similar to what was reported for diffusion-weighted magnetic resonance imaging. Further research to elucidate the underlying biologic implications is warranted.

Unique molecular and functional features of extramedullary hematopoietic stem and progenitor cell reservoirs in humans

Rare hematopoietic stem and progenitor cell (HSPC) pools outside the bone marrow (BM) contribute to blood production in stress and disease but remain ill-defined. Although nonmobilized peripheral blood (PB) is routinely sampled for clinical management, the diagnosis and monitoring potential of PB HSPCs remain untapped, as no healthy PB HSPC baseline has been reported. Here we comprehensively delineate human extramedullary HSPC compartments comparing spleen, PB, and mobilized PB to BM using single-cell RNA-sequencing and/or functional assays. We uncovered HSPC features shared by extramedullary tissues and others unique to PB. First, in contrast to actively dividing BM HSPCs, we found no evidence of substantial ongoing hematopoiesis in extramedullary tissues at steady state but report increased splenic HSPC proliferative output during stress erythropoiesis. Second, extramedullary hematopoietic stem cells/multipotent progenitors (HSCs/MPPs) from spleen, PB, and mobilized PB share a common transcriptional signature and increased abundance of lineage-primed subsets compared with BM. Third, healthy PB HSPCs display a unique bias toward erythroid-megakaryocytic differentiation. At the HSC/MPP level, this is functionally imparted by a subset of phenotypic CD71+ HSCs/MPPs, exclusively producing erythrocytes and megakaryocytes, highly abundant in PB but rare in other adult tissues. Finally, the unique erythroid-megakaryocytic-skewing of PB is perturbed with age in essential thrombocythemia and β-thalassemia. Collectively, we identify extramedullary lineage-primed HSPC reservoirs that are nonproliferative in situ and report involvement of splenic HSPCs during demand-adapted hematopoiesis. Our data also establish aberrant composition and function of circulating HSPCs as potential clinical indicators of BM dysfunction.

The Role and Mechanism of the Vascular Endothelial Niche in Diseases: A Review

Vascular endothelial cells, forming the inner wall of the blood vessels, participate in the body’s pathological and physiological processes of immunity, tumors, and infection. In response to an external stimulus or internal pathological changes, vascular endothelial cells can reshape their microenvironment, forming a “niche”. Current research on the vascular endothelial niche is a rapidly growing field in vascular biology. Endothelial niches not only respond to stimulation by external information but are also decisive factors that act on neighboring tissues and circulating cells. Intervention through the vascular niche is meaningful for improving the treatment of several diseases. This review aimed to summarize reported diseases affected by endothelial niches and signal molecular alterations or release within endothelial niches. We look forward to contributing knowledge to increase the understanding the signaling and mechanisms of the vascular endothelial niche in multiple diseases.

Investigation and Analysis of Iron-Deficiency Anemia Complicated by Splenomegaly

Objective

This study aimed to determine the incidence of iron-deficiency anemia (IDA) complicated by splenomegaly in our hospital over the past 6 years and to analyze the possible causes of this result.

Methods

This is a retrospective study. In total, 668 patients with IDA who were hospitalized in the hematology department of our hospital from 2013 to 2019 were selected as the research subjects and included in the IDA group, and 3201 patients who underwent outpatient physical examinations in our hospital during the same period were included in the control group. The incidences of splenomegaly in the IDA and control groups were calculated, and the difference was analyzed by means of statistical methods.

Results

Among the 668 IDA patients, 46 (6.9%) had splenomegaly, and among the 3201 patients in the control group, 21 had splenomegaly (0.7%). The incidence of splenomegaly was significantly higher in the IDA group than in the control group, and the severity of anemia in the IDA group was associated with the occurrence of splenomegaly. Specifically, the incidence of splenomegaly was 12.4% among patients with severe anemia and as high as 50% among patients with extremely severe anemia.

Conclusion

IDA is correlated with the incidence of splenomegaly, and the incidence of splenomegaly significantly increases as the severity of IDA increases. This is considered to be caused by extramedullary hematopoiesis.

15-PGDH inhibition activates the splenic niche to promote hematopoietic regeneration

The splenic microenvironment regulates hematopoietic stem and progenitor cell (HSPC) function, particularly during demand-adapted hematopoiesis; however, practical strategies to enhance splenic support of transplanted HSPCs have proved elusive. We have previously demonstrated that inhibiting 15-hydroxyprostaglandin dehydrogenase (15-PGDH), using the small molecule (+)SW033291 (PGDHi), increases BM prostaglandin E₂ (PGE₂) levels, expands HSPC numbers, and accelerates hematologic reconstitution after BM transplantation (BMT) in mice. Here we demonstrate that the splenic microenvironment, specifically 15-PGDH high-expressing macrophages, megakaryocytes (MKs), and mast cells (MCs), regulates steady-state hematopoiesis and potentiates recovery after BMT. Notably, PGDHi-induced neutrophil, platelet, and HSPC recovery were highly attenuated in splenectomized mice. PGDHi induced nonpathologic splenic extramedullary hematopoiesis at steady state, and pretransplant PGDHi enhanced the homing of transplanted cells to the spleen. 15-PGDH enzymatic activity localized specifically to macrophages, MK lineage cells, and MCs, identifying these cell types as likely coordinating the impact of PGDHi on splenic HSPCs. These findings suggest that 15-PGDH expression marks HSC niche cell types that regulate hematopoietic regeneration. Therefore, PGDHi provides a well-tolerated strategy to therapeutically target multiple HSC niches, promote hematopoietic regeneration, and improve clinical outcomes of BMT.

Myeloid, mast cell, histiocytic and dendritic cell neoplasms and proliferations involving the spleen

Splenic involvement and consequent splenomegaly are usually seen as part of systemic involvement by myeloid neoplasms as well as mast cell and histiocytic neoplasms. Primary splenic involvement by these neoplasms is rare. Splenectomy is usually not performed for establishing a diagnosis of these entities. However, in rare instances, the pathologist may need to evaluate the spleen secondary to splenic rupture or palliative splenectomy to alleviate symptoms related to splenomegaly. This review article describes the clinicopathologic features of a broad group of myeloid, mastocytic, and histiocytic proliferative and neoplastic disorders.

Morphological and Transcriptional Changes in Human Bone Marrow During Natural Plasmodium vivax Malaria Infections

Background

The presence of Plasmodium vivax malaria parasites in the human bone marrow (BM) is still controversial. However, recent data from a clinical case and experimental infections in splenectomized nonhuman primates unequivocally demonstrated the presence of parasites in this tissue.

Methods

In the current study, we analyzed BM aspirates of 7 patients during the acute attack and 42 days after drug treatment. RNA extracted from CD71⁺ cell suspensions was used for sequencing and transcriptomic analysis.

Results

We demonstrated the presence of parasites in all patients during acute infections. To provide further insights, we purified CD71⁺ BM cells and demonstrated dyserythropoiesis and inefficient erythropoiesis in all patients. In addition, RNA sequencing from 3 patients showed that genes related to erythroid maturation were down-regulated during acute infections, whereas immune response genes were up-regulated.

Conclusions

This study thus shows that during P. vivax infections, parasites are always present in the BM and that such infections induced dyserythropoiesis and ineffective erythropoiesis. Moreover, infections induce transcriptional changes associated with such altered erythropoietic response, thus highlighting the importance of this hidden niche during natural infections.

Generation of Myeloid Cells in Cancer: The Spleen Matters

Myeloid cells are key components of the tumor microenvironment and critical regulators of disease progression. These innate immune cells are usually short-lived and require constant replenishment. Emerging evidence indicates that tumors alter the host hematopoietic system and induce the biased differentiation of myeloid cells to tip the balance of the systemic immune activities toward tumor-promoting functions. Altered myelopoiesis is not restricted to the bone marrow and also occurs in extramedullary organs. In this review, we outline the recent advances in the field of cancer-associated myelopoiesis, with a focus on the spleen, the major site of extramedullary hematopoiesis in the cancer setting. We discuss the functional specialization, distinct mechanisms, and clinical relevance of cancer-associated myeloid cell generation from early progenitors in the spleen and its potential as a novel therapeutic target.

Modern management of splenomegaly in patients with myelofibrosis

Myelofibrosis (MF) is a chronic myeloproliferative neoplasm which can lead to massive splenomegaly secondary to extramedullary hematopoiesis. Patients frequently exhibit debilitating symptoms including pain and early satiety, in addition to cellular sequestration causing severe cytopenias. JAK 1/2 inhibitors, such as ruxolitinib and fedratinib, are the mainstay of therapy and produce significant and durable reductions in spleen volume. However, many patients are not eligible for JAK 2 inhibitor therapy or become refractory to treatment over time. Novel therapies are in development that can reduce the degree of splenomegaly for some of these patients. However, splenectomy, splenic irradiation, and partial splenic artery embolization remain valuable therapeutic options in select patients. In this review, we will discuss currently available pharmacologic therapies and describe promising drugs currently in development. We will also delve into the efficacy and safety concerns of splenectomy, splenic irradiation, and partial splenic artery embolization. Finally, we will propose a treatment algorithm to help guide clinicians in the management of symptomatic splenomegaly in patients with MF.

Extramedullary blastic transformation of primary myelofibrosis in the form of disseminated myeloid sarcoma: a case report and review of the literature

Splenomegaly is a key clinical manifestation of myelofibrosis, and splenectomy is currently indicated in patients with drug refractory, symptomatic splenomegaly or with the aim of improving refractory cytopenias. Transformation to acute myeloid leukemia occurs in up to 20% of patients with myelofibrosis, while cases of myeloid sarcoma have been reported very unfrequently. In this manuscript, we report the case of a 60-year-old man with a history of primary myelofibrosis who underwent splenectomy because of drug-refractory massive splenomegaly, systemic symptoms and anemia. At the opening of the peritoneal cavity, the spleen resulted massively enlarged and tenaciously entrapped by a pervasive neoplastic-like tissue. The extensive involvement of the abdomen fatally complicated the surgical procedure. At postmortem examination, the spleen showed a diffuse infiltration of immature cells that were also found in the peritoneum, bowel, liver, lungs and myocardium. After immunohistochemical, cytogenetic, flow cytometric and molecular characterization of neoplastic population, a diagnosis of disseminated myeloid sarcoma of the spleen was made. This case report highlights a very unusual case of myeloid sarcoma originating from the spleen in a patient with myelofibrosis who had no evidence of bone marrow or peripheral blood involvement by leukemic cells. Molecular characterization showed that leukemic cells originated from the founding clone of the chronic phase. The sarcoma could not be suspected based on clinical findings and was diagnosed only at the time of surgical procedure and autopsy. This case suggests that leukemic transformation of myelofibrosis can originate outside the bone marrow and, presumably rarely, present as a granulocytic sarcoma.

Understanding Splenomegaly in Myelofibrosis: Association with Molecular Pathogenesis

Myelofibrosis (MF) is a clinical manifestation of chronic BCR-ABL1-negative chronic myeloproliferative neoplasms. Splenomegaly is one of the major clinical manifestations of MF and is directly linked to splenic extramedullary hematopoiesis (EMH). EMH is associated with abnormal trafficking patterns of clonal hematopoietic cells due to the dysregulated bone marrow (BM) microenvironment leading to progressive splenomegaly. Several recent data have emphasized the role of several cytokines for splenic EMH. Alteration of CXCL12/CXCR4 pathway could also lead to splenic EMH by migrated clonal hematopoietic cells from BM to the spleen. Moreover, low Gata1 expression was found to be significantly associated with the EMH. Several gene mutations were found to be associated with significant splenomegaly in MF. In recent data, JAK2V617F homozygous mutation was associated with a larger spleen size. In other data, CALR mutations in MF were signigicantly associated with longer larger splenomegaly-free survivals than others. In addition, MF patients with ≥1 mutations in AZXL1, EZH1 or IDH1/2 had significantly low spleen reduction response in ruxolitinib treatment. Developments of JAK inhibitors, such as ruxolitinib, pacritinib, momelotinib, and febratinib enabled the effective management in MF patients. Especially, significant spleen reduction responses of the drugs were demonstrated in several randomized clinical studies, although those could not eradicate allele burdens of MF.

IL-17 Promotes Differentiation of Splenic LSK- Lymphoid Progenitors into B Cells following Plasmodium yoelii Infection

Lineage^-Sca-1⁺c-Kit^- (LSK^-) cells are a lymphoid progenitor population that expands in the spleen and preferentially differentiates into mature B cells in response to Plasmodium yoelii infection in mice. Furthermore, LSK^- derived B cells can subsequently contribute to the ongoing immune response through the generation of parasite-specific Ab-secreting cells, as well as germinal center and memory B cells. However, the factors that promote their differentiation into B cells in the spleen postinfection are not defined. In this article, we show that LSK^- cells produce the cytokine IL-17 in response to Plasmodium infection. Using Il-17ra^-/- mice, IL-17R signaling in cells other than LSK^- cells was found to support their differentiation into B cells. Moreover, primary splenic stromal cells grown in the presence of IL-17 enhanced the production of CXCL12, a chemokine associated with B cell development in the bone marrow, by a population of IL-17RA-expressing podoplanin⁺CD31^- stromal cells, a profile associated with fibroblastic reticular cells. Subsequent blockade of CXCL12 in vitro reduced differentiation of LSK^- cells into B cells, supporting a direct role for this chemokine in this process. Immunofluorescence indicated that podoplanin⁺ stromal cells in the red pulp were the primary producers of CXCL12 after P. yoelii infection. Furthermore, podoplanin staining on stromal cells was more diffuse, and CXCL12 staining was dramatically reduced in Il-17ra^-/- mice postinfection. Together, these results identify a distinct pathway that supports lymphoid development in the spleen during acute Plasmodium infection.

Lymphoid differentiation of hematopoietic stem cells requires efficient Cxcr4 desensitization

The CXCL12/CXCR4 signaling exerts a dominant role in promoting hematopoietic stem and progenitor cell (HSPC) retention and quiescence in bone marrow. Gain-of-function CXCR4 mutations that affect homologous desensitization of the receptor have been reported in the WHIM Syndrome (WS), a rare immunodeficiency characterized by lymphopenia. The mechanisms underpinning this remain obscure. Using a mouse model with a naturally occurring WS-linked gain-of-function Cxcr4 mutation, we explored the possibility that the lymphopenia in WS arises from defects at the HSPC level. We reported that Cxcr4 desensitization is required for quiescence/cycling balance of murine short-term hematopoietic stem cells and their differentiation into multipotent and downstream lymphoid-biased progenitors. Alteration in Cxcr4 desensitization resulted in decrease of circulating HSPCs in five patients with WS. This was also evidenced in WS mice and mirrored by accumulation of HSPCs in the spleen, where we observed enhanced extramedullary hematopoiesis. Therefore, efficient Cxcr4 desensitization is critical for lymphoid differentiation of HSPCs, and its impairment is a key mechanism underpinning the lymphopenia observed in mice and likely in WS patients.

Truncated EphA2 likely potentiates cell adhesion via integrins as well as infiltration and/or lodgment of a monocyte/macrophage cell line in the red pulp and marginal zone of the mouse spleen, where ephrin-A1 is prominently expressed in the vasculature

We previously established a J774.1 monocyte/macrophage subline expressing a truncated EphA2 construct lacking the kinase domain. We demonstrated that following ephrin-A1 stimulation, endogenous EphA2 promotes cell adhesion through interaction with integrins and integrin ligands such as ICAM1 and that truncated EphA2 potentiates the adhesion and becomes associated with the integrin/integrin ligand complex. Based on these findings, we hypothesized that the EphA/ephrin-A system, particularly EphA2/ephrin-A1, regulates transendothelial migration/tissue infiltration of monocytes/macrophages, because ephrin-A1 is widely recognized to be upregulated in inflammatory vasculatures. To evaluate whether this hypothesis is applicable in the spleen, we screened for EphA2/ephrin-A1 expression and reexamined the cellular properties of the J774.1 subline. We found that ephrin-A1 was expressed in the vasculature of the marginal zone and the red pulp and that its expression was upregulated in response to phagocyte depletion; further, CD115, F4/80, and CXCR4 were expressed in J774.1 cells, which serve as a usable substitute for monocytes/macrophages. Moreover, following ephrin-A1 stimulation, truncated EphA2 did not detectably interfere with the phosphorylation of endogenous EphA2, and it potentiated cell adhesion possibly through modulation of integrin avidity. Accordingly, by intravenously injecting mice with equal numbers of J774.1 and the subline cells labeled with distinct fluorochromes, we determined that truncated EphA2 markedly potentiated preferential cell infiltration into the red pulp and the marginal zone. Thus, modulation of EphA2 signaling might contribute to effective transplantation of tissue-specific resident macrophages and/or monocytes.

Splenic pooling and loss of VCAM-1 causes an engraftment defect in patients with myelofibrosis after allogeneic hematopoietic stem cell transplantation

Myelofibrosis is a myeloproliferative neoplasm that results in cytopenia, bone marrow fibrosis and extramedullary hematopoiesis. Allogeneic hematopoietic stem cell transplantation is the only curative treatment but is associated with a risk of delayed engraftment and graft failure. In this study, patients with myelofibrosis (n=31) and acute myeloid leukemia (n=31) were analyzed for time to engraftment, graft failure and engraftment-related factors. Early and late neutrophil engraftment and late thrombocyte engraftment were significantly delayed in patients with myelofibrosis as compared to acute myeloid leukemia, and graft failure only occurred in myelofibrosis (6%). Only spleen size had a significant influence on engraftment efficiency in myelofibrosis patients. To analyze the cause for the engraftment defect, clearance of hematopoietic stem cells from peripheral blood was measured and immunohistological staining of bone marrow sections was performed. Numbers of circulating CD34⁺ were significantly reduced at early time points in myelofibrosis patients, whereas CD34⁺CD38^- and colony-forming cells showed no significant difference in clearance. Staining of bone marrow sections for homing proteins revealed a loss of VCAM-1 in myelofibrosis with a corresponding significant increase in the level of soluble VCAM-1 within the peripheral blood. In conclusion, our data suggest that reduced engraftment and graft failure in myelofibrosis patients is caused by an early pooling of CD34⁺ hematopoietic stem cells in the spleen and a bone marrow homing defect caused by the loss of VCAM-1. Improved engraftment in myelofibrosis might be achieved by approaches that reduce spleen size and cleavage of VCAM-1 in these patients prior to hematopoietic stem cell transplantation.

Extramedullary hematopoiesis: Elucidating the function of the hematopoietic stem cell niche (Review)

Extramedullary hematopoiesis (EMH) occurs under various circumstances, including during embryonic/developmental periods, pathological status secondary to insufficient bone marrow function or ineffective hematopoiesis, in hematological disorders, for example malignancies, as well as stromal disorders of the bone. EMH is characterized by hematopoietic cell accumulations in multiple body locations. Common EMH locations observed in clinical and pathological practice include the spleen, liver, lymph nodes and para‑vertebral regions. Among the various organs associated with EMH, the spleen offers a unique site for evaluation of hematopoietic stem cell (HSC)/niche interactions, as this organ is one of the most common sites of EMH. However, the spleen does not have a major role in embryonic/developmental hematopoiesis. A recent study by our group revealed that circulating HSCs may be trapped by chemokine (C‑X‑C motif) ligand 12 (CXCL12)‑positive cells at the margin of sinuses near CXCL12‑positive endothelial cells, resulting in the initiation of the first step of EMH, which is a similar mechanism to bone marrow hematopoiesis. The present review briefly discusses the environment of EMH in extramedullary spaces in order to investigate the mechanisms underlying HSC maintenance, and aid the elucidation of the niche‑stem cell interactions that occur in the bone marrow.

A perisinusoidal niche for extramedullary haematopoiesis in the spleen

Haematopoietic stresses mobilize haematopoietic stem cells (HSCs) from the bone marrow to the spleen and induce extramedullary haematopoiesis (EMH). However, the cellular nature of the EMH niche is unknown. Here we assessed the sources of the key niche factors, SCF (also known as KITL) and CXCL12, in the mouse spleen after EMH induction by myeloablation, blood loss, or pregnancy. In each case, Scf was expressed by endothelial cells and Tcf21(+) stromal cells, primarily around sinusoids in the red pulp, while Cxcl12 was expressed by a subset of Tcf21(+) stromal cells. EMH induction markedly expanded the Scf-expressing endothelial cells and stromal cells by inducing proliferation. Most splenic HSCs were adjacent to Tcf21(+) stromal cells in red pulp. Conditional deletion of Scf from spleen endothelial cells, or of Scf or Cxcl12 from Tcf21+ stromal cells, severely reduced spleen EMH and reduced blood cell counts without affecting bone marrow haematopoiesis. Endothelial cells and Tcf21(+) stromal cells thus create a perisinusoidal EMH niche in the spleen, which is necessary for the physiological response to diverse haematopoietic stresses.

C-X-C motif chemokine 12 influences the development of extramedullary hematopoiesis in the spleens of myelofibrosis patients

Myelofibrosis (MF) is characterized by the constitutive mobilization of hematopoietic stem cells (HSC) and hematopoietic progenitor cells (HPC) and the establishment of extramedullary hematopoiesis. The mechanisms underlying this abnormal HSC/HPC trafficking pattern remain poorly understood. We demonstrated that both splenic and peripheral blood (PB) MF CD34(+) cells equally share a defective ability to home to the marrow, but not to the spleens, of NOD/LtSz-Prkdc(scid) mice. This trafficking pattern could not be attributed to discordant expression of integrins or chemokine receptors other than the downregulation of C-X-C chemokine receptor type 4 by both PB and splenic MF CD34(+) cells. The number of both splenic MF CD34(+) cells and HPCs that migrated toward splenic MF plasma was, however, significantly greater than the number that migrated toward PB MF plasma. The concentration of the intact HSC/HPC chemoattractant C-X-C motif chemokine 12 (CXCL12) was greater in splenic MF plasma than PB MF plasma, as quantified using mass spectrometry. Functionally inactive truncated products of CXCL12, which are the product of proteolytic degradation by serine proteases, were detected at similar levels in both splenic and PB MF plasma. Treatment with an anti-CXCL12 neutralizing antibody resulted in a reduction in the degree of migration of splenic MF CD34(+) cells toward both PB and splenic MF plasma, validating the role of CXCL12 as a functional chemoattractant. Our data indicate that the MF splenic microenvironment is characterized by increased levels of intact, functional CXCL12, which contributes to the localization of MF CD34(+) cells to the spleen and the establishment of extramedullary hematopoiesis.