Clonal Megakaryocyte Dysplasia with Isolated Thrombocytosis Is a Distinct Myeloproliferative Neoplasm Phenotype.

INTRODUCTION: About 15% of people with a myeloproliferative neoplasm (MPN) are identified as MPN, unclassifiable using the 2016 WHO classification. METHODS: We tested whether persons with platelet concentration ≥450 × 10E+9/L, bone marrow megakaryocyte morphology typical of prefibrotic/early myelofibrosis (pre-MF), and no minor criteria of pre-MF should be classified as a distinct MPN subtype, clonal megakaryocyte dysplasia with isolated thrombocytosis (CMD-IT). RESULTS: 139 subjects meet these criteria who we compared with primary myelofibrosis (PMF) including 402 with pre-MF and 521 with overt myelofibrosis. CMD-IT subjects were more likely female and younger. They had lower frequencies of JAK2V617F compared with persons with PMF (55% vs. 70%; p < 0.001) and higher frequencies of CALR mutations (37% vs. 17%; p < 0.001). They also had lower frequency of variations associated with JAK2V617F susceptibility, JAK2 46/1 (35% vs. 47%; p = 0.021), and VEGFA rs3025039 (12% vs. 17%; p = 0.030). Subjects with CMD-IT had lower incidences of thrombotic events compared with those with pre-MF (9.7% vs. 26%; p < 0.001) and longer survival (median, not reached vs. 23 years; HR = 0.34 (0.10, 0.30); p < 0.001). CONCLUSION: Our data indicate CMD-IT is a distinct MPN subtype and should be included in the classification of myeloid neoplasms.

Clonal Megakaryocyte Dysplasia with Normal Blood Values Is a Distinct Myeloproliferative Neoplasm.

INTRODUCTION: In 1991, we reported 18 persons with a clinical-pathologic entity and termed atypical myeloproliferative disorder because they did not meet the contemporary diagnostic criteria for a myeloproliferative neoplasm. We sought to gain further knowledge on this disease entity. METHODS: This retrospective cohort study included consecutive subjects registered in the database of the Center for the Study of Myelofibrosis in Pavia, Italy, from 1998 to 2020 (June), and diagnosed with atypical myeloproliferative disorder according to our adjudicated criteria. We studied clinical, histological, cytogenetic, and molecular covariates and risks of thrombosis, disease progression, and death. Data were compared with those of concurrent subjects with prefibrotic myelofibrosis. RESULTS: Fifteen new subjects with atypical myeloproliferative disorder were identified. Seven were male. Median age was 50 years (IQR, 41-54 years). Thirteen were diagnosed with a synchronous symptomatic or incidentally detected thrombotic event. The bone marrow showed megakaryocyte hyperplasia with dysplasia. JAK2V617F was present in 10 subjects and CALR mutation in one. No other somatic mutations were identified in next generation sequencing. After a median follow-up of 101 months (IQR, 40-160 months), no subject had disease progression or blast transformation. Incidence of post-diagnosis or recurrent thrombosis was 3.9 events (95% confidence interval, 3.5-4.0) and 5.0 events (4.6-5.6) per 100 person-years. Features of subjects with atypical myeloproliferative disorder differed markedly from those of 546 subjects with prefibrotic myelofibrosis. CONCLUSION: Our data indicate that these 15 persons have a distinct myeloproliferative neoplasm. We propose naming this new disorder clonal megakaryocyte dysplasia with normal blood values.

Reduced CXCR4-expression on CD34-positive blood cells predicts outcomes of persons with primary myelofibrosis.

The expression of the CXCR4 chemokine receptor on CD34-positive blood cells is reduced in persons with primary myelofibrosis (PMF). We analyzed the relevance of cytofluorimetric assessment of the percentage of CD34-positive blood cells that had a positive CXCR4 surface expression (CD34/CXCR4-se) in a large cohort of subjects with myeloproliferative neoplasms. Mean CD34/CXCR4-se was lower in subjects with PMF compared with those with essential thrombocythemia (ET) or polycythemia vera (PV). A cutoff value of 39% was associated with a diagnosis of pre-fibrotic PMF vs. ET with a positive predictive value of 97%. In PMF male sex, older age, and MPL mutation were independent correlates of reduced CD34/CXCR4-se and associated with a briefer interval to development of severe anemia, large splenomegaly, thrombocytopenia, leukopenia, elevated CD34-positive blood cells, blast transformation and death. We constructed a prognostic model including age >65 years, hemoglobin < 100 g/L, CD34-positive blood cells > 50 × 106/L, and CD34/CXCR4-se <39% at diagnosis. The model identified three risk cohorts with greater accuracy compared with the International Prognostic Scoring System. In conclusion, CD34/CXCR4-se is a highly sensitive marker of disease activity and a new potential diagnostic and prognostic biomarker in PMF.

Primary myelofibrosis: Older age and high JAK2V617F allele burden are associated with elevated plasma high-sensitivity C-reactive protein levels and a phenotype of progressive disease.

We measured plasma levels of high-sensitivity C-reactive protein (hs-CRP) in 526 subjects with primary myelofibrosis (PMF). Thirty-eight percent had an elevated hs-CRP level (≥0.3mg/dL). Elevated hs-CRP levels were associated with a progressive disease phenotype, including anemia, high white blood cell count, low platelet count, increased splenomegaly, increased risk of blast transformation, and worse survival. Age≥52years, but no other demographic characteristics, was associated with an elevated hs-CRP level in multivariable logistic regression (odds ratio [OR], 4.29; 95% CI, 2.73-6.77; P <0.001). Subjects with JAK2V617F mutation and an allele burden≥50% had an age-independent higher incidence of elevated hs-CRP level (OR=1.97; 95% CI,1.21-3.22; P=0.006) compared with a combined cohort of subjects with JAK2V617F <50% allele burden, CALR, MPL mutations, or no detectable driver mutations. Neither ASXL1 or EZH2 sub-clonal mutations, nor JAK2 46/1 haplotype or the A3669G polymorphism of glucocorticoid receptor were significantly associated with increased hs-CRP levels. Subjects with age≥52years and JAK2V617F with≥50% allele burden had a phenotype of progressive disease. Our data indicate that older age and high JAK2V617 allele burden are major determinants of inflammation in PMF, and are associated with disease progression.

Increased plasma nicotinamide phosphoribosyltransferase is associated with a hyperproliferative phenotype and restrains disease progression in MPN-associated myelofibrosis.

Myeloproliferative neoplasm (MPN)-associated myelofibrosis is a clonal, neoplastic disorder of the hematopoietic stem cells, in which inflammation and immune dysregulation play an important role. Extracellular nicotinamide phosphoribosyltransferase (eNAMPT), also known as visfatin, is a cytokine implicated in a number of inflammatory and neoplastic diseases. Here plasma levels of eNAMPT in patients with MPN-associated myelofibrosis and their effects on disease phenotype and outcomes were examined. The concordance of eNAMPT levels with the marker of general inflammation high-sensitivity C-reactive protein (hs-CRP) was also studied. A total of 333 MPN-associated myelofibrosis patients (187 males and 146 females) and 31 age- and gender-matched normal-weight healthy subjects were enrolled in the study main body. Levels of eNAMPT and hs-CRP were simultaneously assayed in 209 MPN-associated myelofibrosis patients. Twenty-four polycythemia vera or essential thrombocythemia patients were used as controls. eNAMPT was over expressed in MPN-associated myelofibrosis, and eNAMPT expression was correlated with higher white blood cell count, higher hemoglobin, and higher platelet count, suggesting that eNAMPT is an indispensable permissive agent for myeloproliferation of MPN-associated myelofibrosis. The lack of correlation between eNAMPT and hs-CRP revealed that eNAMPT in MPN-associated myelofibrosis does not behave as a canonical inflammatory cytokine. In addition, higher levels of eNAMPT predicted longer time to blast transformation, and protected against progression toward thrombocytopenia and large splenomegaly. In conclusion, in MPN-associated myelofibrosis high levels of eNAMPT mark the myeloproliferative potential and, at variance with a high number of cancers, are protective against disease progression. Am. J. Hematol. 91:709-713, 2016. © 2016 Wiley Periodicals, Inc.

Spleen endothelial cells from patients with myelofibrosis harbor the JAK2V617F mutation.

Increased microvessel density contributes to abnormal BM and spleen microenvironment in myelofibrosis (MF). Taking advantage of the JAK2V617F mutation as a marker of malignancy, in the present study, we investigated whether splenic endothelial cells (ECs) obtained from capillaries by laser microdissection or from fresh spleen tissue by cell culture or cell sorting harbored such mutation in patients bearing the mutation in their granulocytes and undergoing splenectomy for therapeutical reasons. To extend the analysis to the ECs of large vessels, endothelial tissue from the splenic vein was also studied. We found JAK2V617F(+) ECs in 12 of 18 patients also bearing the mutation in their granulocytes. In 3 patients, the mutation was found in at least 2 different EC samples obtained by laser microdissection, cell culture, or cell sorting. The mutation was detected in the splenic vein ECs of 1 of 6 patients investigated. In conclusion, we provide evidence that some ECs from the spleen and splenic veins of patients with MF bear the JAK2V617F mutation. We suggest that splenic ECs are involved in the process of malignant transformation in MF.

Alteration of liver enzymes is a feature of the MYH9-related disease syndrome.

BACKGROUND: MYH9-related disease (MYH9-RD) is a rare autosomal dominant genetic syndrome characterized by congenital thrombocytopenia associated with the risk of developing progressive nephropathy, sensorineural deafness, and presenile cataract. During the collection of a large case-series of patients with MYH9-RD we noticed several cases with unexplained elevation of liver enzymes. Our aim was to evaluate if the alteration of liver tests is a feature of the MYH9-RD and to define its clinical significance. METHODS AND FINDINGS: Data concerning liver tests, prospectively recorded in the Italian Registry for MYH9-RD, were collected and compared with those of three control populations: patients with autoimmune thrombocytopenia, patients with inherited thrombocytopenias other than MYH9-RD, and the participants to a large epidemiologic survey in an Italian geographic isolate. Thirty-eight of 75 evaluable MYH9-RD patients (50.7%) showed an elevation of ALT and/or AST, and 17 of 63 (27.0%) an increase of GGT. The increases ranged from 1.9 ± 0.7 to 2.7 ± 1.6 fold the upper normal limit. The prevalence of liver test alterations was significantly higher in MYH9-RD patients than in each of the control populations, with odds ratios ranging from 8.2 (95% CIs 2.2-44.8) to 24.7 (14.8-40.8). Clinical follow-up and more detailed liver studies of a subset of patients, including ultrasound liver scan, liver elastography and liver biopsy in one case, did not show any significant structural damage or evolution towards liver insufficiency. CONCLUSIONS: Elevation of liver enzymes is a frequent and previously unrecognized feature of the MYH9-RD syndrome; however, this defect does not appear to have poor prognostic value.

Evidence that prefibrotic myelofibrosis is aligned along a clinical and biological continuum featuring primary myelofibrosis.

PURPOSE: In the WHO diagnostic classification, prefibrotic myelofibrosis (pre-MF) is included in the category of primary myelofibrosis (PMF). However, strong evidence for this position is lacking. PATIENTS AND METHODS: We investigated whether pre-MF may be aligned along a clinical and biological continuum in 683 consecutive patients who received a WHO diagnosis of PMF. RESULTS: As compared with PMF-fibrotic type, pre-MF (132 cases) showed female dominance, younger age, higher hemoglobin, higher platelet count, lower white blood cell count, smaller spleen index and higher incidence of splanchnic vein thrombosis. Female to male ratio and hemoglobin steadily decreased, while age increased from pre-MF to PMF- fibrotic type with early and to advanced bone marrow (BM) fibrosis. Likely, circulating CD34+ cells, LDH levels, and frequency of chromosomal abnormalities increased, while CXCR4 expression on CD34+ cells and serum cholesterol decreased along the continuum of BM fibrosis. Median survival of the entire cohort of PMF cases was 21 years. Ninety-eight, eighty-one and fifty-six percent of patients with pre-MF, PMF-fibrotic type with early and with advanced BM fibrosis, respectively, were alive at 10 years from diagnosis. CONCLUSION: Pre-MF is a presentation mode of PMF with a very indolent phenotype. The major consequences of this contention is a new clinical vision of PMF, and the need to improve prognosis prediction of the disease.

High frequency of endothelial colony forming cells marks a non-active myeloproliferative neoplasm with high risk of splanchnic vein thrombosis.

Increased mobilization of circulating endothelial progenitor cells may represent a new biological hallmark of myeloproliferative neoplasms. We measured circulating endothelial colony forming cells (ECFCs) in 106 patients with primary myelofibrosis, fibrotic stage, 49 with prefibrotic myelofibrosis, 59 with essential thrombocythemia or polycythemia vera, and 43 normal controls. Levels of ECFC frequency for patient's characteristics were estimated by using logistic regression in univariate and multivariate setting. The sensitivity, specificity, likelihood ratios, and positive predictive value of increased ECFC frequency were calculated for the significantly associated characteristics. Increased frequency of ECFCs resulted independently associated with history of splanchnic vein thrombosis (adjusted odds ratio = 6.61, 95% CI = 2.54-17.16), and a summary measure of non-active disease, i.e. hemoglobin of 13.8 g/dL or lower, white blood cells count of 7.8×10(9)/L or lower, and platelet count of 400×10(9)/L or lower (adjusted odds ratio = 4.43, 95% CI = 1.45-13.49) Thirteen patients with splanchnic vein thrombosis non associated with myeloproliferative neoplasms were recruited as controls. We excluded a causal role of splanchnic vein thrombosis in ECFCs increase, since no control had elevated ECFCs. We concluded that increased frequency of ECFCs represents the biological hallmark of a non-active myeloproliferative neoplasm with high risk of splanchnic vein thrombosis. The recognition of this disease category copes with the phenotypic mimicry of myeloproliferative neoplasms. Due to inherent performance limitations of ECFCs assay, there is an urgent need to arrive to an acceptable standardization of ECFC assessment.

Unique vascular tumor primary arising in the liver and exhibiting histopathological features consistent with so-called polymorphous hemangioendothelioma.

Reported herein is an unusual vascular tumor primary arising in the liver and exhibiting unique histopathological features. A 47-year-old woman underwent left hepatectomy because of a large hepatic mass. On histology the tumor had a composite pattern, consisting of angiomatous, retiform and solid areas, formed by oval to cuboidal to spindle cells, that expressed only endothelial markers (CD31 and factor VIII-related antigen). These findings led to the diagnosis of a low-grade vascular neoplasm with morphological features consistent with so-called polymorphous hemangioendothelioma. The tumor was completely resected. At 24 month follow up the patient was alive, without evidence of disease. Polymorphous hemangioendothelioma is a rare vascular neoplasm, with borderline malignant potential, which usually occurs in lymph nodes and, rarely, at extranodal sites. Its classification as an entity has been questioned recently. The unusual morphological features of the present case, which do not fit neatly with any other recognized hemangioendothelioma subtype, indicate that the family of vascular tumors is broader than currently accepted. In addition the present case widens the spectrum of primary vascular tumors arising in the liver.

Clinical, radiological, and biochemical features of a bilateral buttock amyloidoma emerging after 27 years of hemodialysis.

Deposition of amyloid in the buttock is a rare complication of dialysis related amyloidosis (DRA), but this localization is even rarer in other types of amyloidoses. We report here the clinical, radiological, and biochemical features of a patient who incurred into this complication after 27 years of hemodialysis. Imaging of the amyloid deposition by magnetic resonance imaging (MRI) documents the amyloid infiltration in the muscles of the buttock region and highlights a peculiar feature of amyloid fibrils deposition in the subcutaneous fat. The amyloid deposition is confirmed by biochemical and microscopic analysis of fibrils extracted from a biopsy specimen. Review of literature and the features of this case lead to speculation that the peculiar involvement of the buttock region including muscles and subcutaneous fat in DRA might derive from the propagation of amyloid initially deposited in the hip joint.

Clinical relevance of bone marrow fibrosis and CD34-positive cell clusters in primary myelodysplastic syndromes.

PURPOSE: We studied bone marrow (BM) histologic abnormalities in myelodysplastic syndromes (MDS) classified according to WHO criteria to determine their clinical correlates and prognostic value. PATIENTS AND METHODS: Three hundred one consecutive patients were retrospectively evaluated for BM fibrosis and CD34 immunoreactivity. Marrow fibrosis was assessed following the European consensus guidelines. RESULTS: Moderate to severe BM fibrosis was detected in 17% of cases and was associated with multilineage dysplasia (P = .001), high transfusion requirement (P < .001), and poor-risk cytogenetics (P = .007). CD34+ cell clusters were found in 23% of patients and were associated with WHO categories with excess of blasts (P < .001) and poor-risk cytogenetics (P = .001). In multivariable analysis, BM fibrosis and presence of CD34+ cell clusters had independent negative impact on overall survival (P < .001 and P = .019, respectively) and leukemia-free survival (P < .001 and P = .004, respectively). A hierarchical clustering analysis identified three subsets of patients with distinct clinical features. One cluster consisted mainly of patients with BM fibrosis, multilineage dysplasia, and high transfusion requirement; these individuals had lower overall survival and leukemia-free survival (P = .001 and P < .001, respectively). Within patients stratified according to International Prognostic Scoring System and WHO classification-based Prognostic Scoring System categories, BM fibrosis involved a shift to a one-step more advanced risk group. CONCLUSION: BM fibrosis identifies a distinct subgroup of MDS with multilineage dysplasia, high transfusion requirement, and poor prognosis and represents an independent prognostic factor that may be useful in clinical decision making. Furthermore, the presence of CD34+ cell clusters is an independent risk factor for progression to acute leukemia.

Bone marrow microvessel density in chronic myeloproliferative disorders: a study of 115 patients with clinicopathological and molecular correlations.

Philadelphia-negative chronic myeloproliferative disorders (CMD) include polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Angiogenesis is critical in the pathogenesis of PMF. We studied angiogenesis in 115 patients with CMD (23 PV, 24 ET, 46 PMF, 12 post-PV and 10 post-ET myelofibrosis) by assessment of microvessel density (MVD) in bone marrow (BM). Kruskall-Wallis analysis of variance showed that patients with PMF had significantly higher values of MVD than those with PV (P < 0.001), ET (P < 0.001) and controls (P < 0.001). Mann-Whitney U-test demonstrated that patients with PMF at the prefibrotic stage had significantly higher MVD values than those with ET (P = 0.02). Patients with post-PV myelofibrosis showed significantly higher MVD values than those with PV (P < 0.001), as did patients with post-ET myelofibrosis compared with ET (P < 0.001). In patients with CMD, the multivariate generalized linear regression model showed that the JAK2 (V617F) mutational burden (P = 0.01), serum lactate dehydrogenase level (P = 0.003), and anaemia (P < 0.001) independently correlated with MVD. In summary, this study indicates that assessment of BM angiogenesis, as measured by MVD, may be a useful additional tool in the histopathological definition of CMD.

B-cell posttransplant lymphoproliferative disorders in heart and/or lungs recipients: clinical and molecular-histogenetic study of 17 cases from a single institution.

BACKGROUND: Posttransplantation lymphoproliferative disorders (PTLDs) are heterogeneous lymphoid proliferations representing a major complication of solid organ transplant. This study details the clinicopathological and molecular features of 17 B-cell PTLDs observed in a single center series of 988 heart and/or lung transplant recipients. METHODS: Cases were classified according to World Health Organization lymphoma classification and tested for Epstein-Barr Virus (EBV), clonality, histogenetic phenotypic (CD10, Bcl-6, MUM1, CD138), and genotypic (immunoglobulin and BCL-6 genes somatic hypermutation) markers. RESULTS: This series of 17 PTLDs included: two B-cell monoclonal polymorphic PTLDs and 15 B-cell monomorphic PTLDs (13 diffuse large B-cell lymphomas [DLBCL] and 2 Burkitt lymphomas [BL]). EBV was detected in 9/17 cases. A monoclonal immunoglobulin variable (IGV) genes rearrangement was documented in 17/17 cases; IGV somatic hypermutation was found in 88% of cases, indicating a prevalent origin from germinal center (GC)-experienced B cells. Using immunophenotypic markers, three histogenetic profiles were identified: a) CD10/bcl-6/MUM1/CD138, mimicking GC B-cells; b) CD10-/bcl-6+/MUM1+/CD138-, reminiscent of B-cells at the latest phases of GC reaction; and c) CD10-/bcl-6-/MUM1+/CD138+/-, consistent with preterminally differentiated B-cells. CONCLUSIONS: Correlation between morphology, histogenesis, and EBV status demonstrated a high degree of homogeneity in the two GC-related groups, mostly including EBV-negative cases with BL and DLBCL-centroblastic features; the third group, consisting of post GC EBV-positive cases, was histologically less homogeneous, as it included polymorphic PTLDs and DLBCL with immunoblastic and anaplastic features. The EBV-negative cases with GC histogenetic phenotype showed a slightly better outcome; however, such less aggressive prognostic trend was not confirmed by statistical analysis.

Dyspnea secondary to pulmonary hematopoiesis as presenting symptom of myelofibrosis with myeloid metaplasia.

We report a case of a patient with myelofibrosis with myeloid metaplasia (MMM) who presented with progressive dyspnea of unexplained origin. Splenomegaly, blood smear, and bone marrow findings allowed diagnosis of MMM. High-resolution CT chest scan revealed diffuse septal thickening, while echocardiography and electrocardiogram showed no indirect evidence of pulmonary hypertension. Finally, lung biopsy revealed irregularly distributed interstitial fibrosis with islands of erythroblasts, immature granulocytic elements, and dysplastic megakaryocytes, allowing diagnosis of pulmonary extramedullary hematopoiesis (EMH). The patient received hydroxyurea as cytoreductive agent, obtaining a good hematologic response and an improvement of dyspnea. Note that, in this patient, dyspnea was the first clinical symptom of MMM; the dyspnea was not associated with pulmonary hypertension and improved following cytoreductive treatment. This case points to the importance of suspecting pulmonary EMH when unexplained progressive dyspnea occurs in a patient with MMM. Early recognition of pulmonary EMH may prevent PH and favor a better response to therapy.

EBV positive primary cutaneous CD30+ large T-cell lymphoma in a heart transplanted patient: case report.

Most post-transplant lymphoproliferative disorders (PTLDs) are of B-cell origin, whereas T-cell lymphomas rarely occur. We detail the clinicopathological features of the first case of Epstein-Barr virus (EBV)-associated primary cutaneous CD30+ anaplastic large cell lymphoma (ALCL) in the setting of heart transplant. A 71-year-old patient, 111 months after transplant, presented with multiple cutaneous lesions on the left thigh; histological and immunohistochemical examinations led to diagnosis of T-cell CD30+ ALCL. In situ hybridization demonstrated the presence of EBV-positive tumour cells. The patient received radiotherapy, but he relapsed at the same cutaneous site with loco-regional nodal spread. Chemotherapy was administered resulting in complete remission; four years later the patient is alive and well. Our findings indicate that primary cutaneous EBV+ CD30+ ALCLs should be included within the T-cell PTLDs spectrum; further studies are required to confirm whether they may be also considered, in transplantation settings, a distinct lymphoma subset with relatively favourable outcome.

Incidence of novel N-glycosylation sites in the B-cell receptor of lymphomas associated with immunodeficiency.

Novel N-glycosylation sites are introduced by somatic mutation into the V genes of the majority of follicular lymphomas. Sites are positively selected and rare in normal memory B cells, indicating a potential role in tumour survival in the germinal centre (GC). The incidence of c. 40% in diffuse large B-cell lymphomas (DLBCL) parallels the known heterogeneity of the disease. Immunodeficiency-related non-Hodgkin's lymphomas (NHL) include post-transplant lymphoproliferative disorders (PTLD) and acquired immunodeficiency syndrome-related NHL (AIDS-NHL). Most PTLD derive from B cells that carry mutated VH genes and that have completed the GC reaction. All AIDS-NHL carry mutated VH genes and variable features of GC or post-GC cells. To determine if N-glycosylation is a feature of immunodeficiency-related lymphomas, we analysed the VH genes of 19 PTLD and 36 AIDS-NHL. Novel sites were rare in PTLD (4/19), similar to memory B cells (P = 0.15). AIDS-NHL, including DLBCL and Burkitt's lymphomas (BL), showed heterogeneity with 16 of 36 (44%) having novel sites. The findings indicate no selection of N-glycosylation sites in PTLD, consistent with post-GC features. The variable incidence of N-glycosylation sites in AIDS-NHL mirrors that in DLBCL and sporadic BL of immunocompetent hosts, supporting the known heterogeneity of these disorders, and possibly pointing to distinct routes of tumour development.

Spleen neoangiogenesis in patients with myelofibrosis with myeloid metaplasia.

Neoangiogenesis is an integral component of bone marrow myeloproliferation in patients with myelofibrosis with myeloid metaplasia (MMM). As extramedullary haematopoiesis is a constitutive feature of MMM, we studied spleen neoangiogenesis by a computerized image analysis in MMM patients. Compared with five normal subjects, spleen CD34-staining capillary vascular density (CVD) was 2.1-3.03 times higher than the upper range of normal in six of the 15 (40%) MMM patients. CD8-staining sinusoidal vascular density (SVD) was constantly normal or lesser than normal and was inversely correlated with CVD (R = -0.53; P = 0.04). In MMM patients who did not receive cytoreductive or radiation therapy in the month before splenectomy (n = 9), the CVD was a significant determinant of spleen size (R = 0.88; P = 0.04). In MMM patients, the number of spleen CD34+ haematopoietic stem cells was increased from 1.2 to 98 times the upper limit of normal, and predicted the expansion of CVD (R = 0.57; P = 0.03). A population of cells expressing the CD34+/CD133+/VEGFR-2+ angiopoietic phenotype was present in the blood and spleen of five of seven patients. These results document that neoangiogenesis is an integral component of spleen re-localization of haematopoietic stem cells and suggest a cellular mechanism for spleen neoangiogenesis.