Myeloid Stroma Research Articles

IMMU-38. LEVERAGING INNATE IMMUNE SENSORS TO GENERATE DURABLE ANTI-GLIOMA ADAPTIVE IMMUNE RESPONSES

Abstract Glioblastoma harbors a tumor microenvironment (TME) that is mostly devoid of T cell effector infiltration but is dominated by immune suppressive microglia, macrophages, and myeloid-derived suppresser cells. The innate immune cells in the TME have impaired phagocytosis and antigen presentation and are inadequate for triggering the subsequent immune activating signals needed for anti-tumor effector responses. Phagocytic activity can also be blocked by the upregulation of CD47 on the tumor cells. In the immune checkpoint resistant Qki-/- Pten-/- P53-/- (QPP) glioma model, intratumoral treatment with the synthetic cyclic di-nucleotide STING agonist IACS-8803 (8803) induces global immunological reprogramming and long-term survival resistant to tumor rechallenge. More specifically, in the infiltrating myeloid stroma 8803 increases costimulatory protein CD86 while reducing the expression of suppressive markers CD163 and CD206. Conventional type 1 dendritic cells are increased in both the TME (p<0.05) and the draining cervical lymph node (p<0.01). Intratumoral infiltration of both CD8 T (p<0.05) and NK (p<0.001) cells increases. However, only T cells, but not NK cells, are necessary for 8803 therapeutic activity when assessed with in vivo NK cell depletion studies and treatment in the RAG-/- background. STING activation increases microglial phagocytic capacity, but also induces a compensatory increase in SIRPα expression (p<0.001) – the ligand for CD47. 8803 synergizes with anti-CD47 blockade to enhance microglia phagocytosis of QPP glioblastoma cells in vitro. Finally, the combination of 8803 and anti-CD47 synergizes to extend both median survival and the fraction of long-term survivors in orthotopic QPP-bearing mice (QPP8v; vehicle control MS=39d, anti-CD47 MS=40d, 8803 MS=67d 20% long-term survival, 8803 + anti-CD47 MS=107.5d 40% long-term survival). These data demonstrate that 8803 induces proinflammatory conversion of the glioblastoma TME, generates T cell-dependent survival benefits in QPP-bearing mice, and synergizes with anti-CD47 blockade to enhance microglia phagocytosis of glioblastoma cells and extend survival in QPP-bearing mice.

291 High Potency STING Agonists Induce Adaptive Immunity-Dependent Curative Responses in an Immune Checkpoint Blockade-Refractory Glioblastoma Model

OBJECTIVES/GOALS: Glioblastoma (GBM) is the most common and aggressive adult primary brain malignancy. Clinically, GBM is refractory to T cell immune checkpoint blockade (ICB), in part due to its dense immune suppressive myeloid stroma. Here we show that myeloid-targeting STING agonists can repolarize the GBM microenvironment to cure ICB-refractory GBM models. METHODS/STUDY POPULATION: Using the synthetic cyclic di-nucleotide STING agonist IACS-8803 (8803) we treated orthotopic ICB-refractory QPP8 orthotopic murine GBM tumors intratumorally. We then analyzed survival and performed high parameter flow cytometry profiling of the tumor immune microenvironment following STING agonist treatment. To assess the contribution of adaptive immunity to STING agonist therapeutic efficacy, we treated orthotopic QPP8 tumors implanted in RAG1 KO mice and monitored survival. RESULTS/ANTICIPATED RESULTS: We found that STING agonist therapy cured murine orthotopic QPP8 tumors, in contrast to ICB that showed no survival benefit. In RAG1-/- mice bearing QPP8 tumors STING agonist therapy extended survival, however, the curative effect observed in wild-type mice was lost in the absence of adaptive immunity. STING agonist-treated QPP8 tumors displayed increased counts of CD8 T cells and NK cells, and decreased CD8 T cell PD1 expression. Infiltration of STING-treated gliomas by Ly6C+ F4/80+ Mono-MDSC substantially increased; however, these cells expressed reduced CD206 and CD163, suggestive of reduced immuno-suppression. Finally, in the cervical LN of QPP8-treated mice the frequency and CD80/CD86 expression of cDC1 cells increased.​ DISCUSSION/SIGNIFICANCE: ICB has failed in GBM, and the suppressive myeloid stroma remains a major barrier to generating anti-GBM T cell responses. Our work shows that STING activation, which primarily targets innate immunity myeloid cells 'upstream' of T cells in the antitumor immunity cycle, can cure ICB-refractory GBM tumors in an adaptive immunity-dependent manner.

Novel murine glioblastoma models that reflect the immunotherapy resistance profile of a human disease.

The lack of murine glioblastoma models that mimic the immunobiology of human disease has impeded basic and translational immunology research. We, therefore, developed murine glioblastoma stem cell lines derived from Nestin-CreERT2QkL/L; Trp53L/L; PtenL/L (QPP) mice driven by clinically relevant genetic mutations common in human glioblastoma. This study aims to determine the immune sensitivities of these QPP lines in immunocompetent hosts and their underlying mechanisms. The differential responsiveness of QPP lines was assessed in the brain and flank in untreated, anti-PD-1, or anti-CTLA-4 treated mice. The impact of genomic landscape on the responsiveness of each tumor was measured through whole exome sequencing. The immune microenvironments of sensitive (QPP7) versus resistant (QPP8) lines were compared in the brain using flow cytometry. Drivers of flank sensitivity versus brain resistance were also measured for QPP8. QPP lines are syngeneic to C57BL/6J mice and demonstrate varied sensitivities to T cell immune checkpoint blockade ranging from curative responses to complete resistance. Infiltrating tumor immune analysis of QPP8 reveals improved T cell fitness and augmented effector-to-suppressor ratios when implanted subcutaneously (sensitive), which are absent on implantation in the brain (resistant). Upregulation of PD-L1 across the myeloid stroma acts to establish this state of immune privilege in the brain. In contrast, QPP7 responds to checkpoint immunotherapy even in the brain likely resulting from its elevated neoantigen burden. These syngeneic QPP models of glioblastoma demonstrate clinically relevant profiles of immunotherapeutic sensitivity and potential utility for both mechanistic discovery and evaluation of immune therapies.

ATR-mediated CD47 and PD-L1 up-regulation restricts radiotherapy-induced immune priming and abscopal responses in colorectal cancer.

Radiotherapy (RT) of colorectal cancer (CRC) can prime adaptive immunity against tumor-associated antigen (TAA)-expressing CRC cells systemically. However, abscopal tumor remissions are extremely rare, and the postirradiation immune escape mechanisms in CRC remain elusive. Here, we found that irradiated CRC cells used ATR-mediated DNA repair signaling pathway to up-regulate both CD47 and PD-L1, which through engagement of SIRPα and PD-1, respectively, prevented phagocytosis by antigen-presenting cells and thereby limited TAA cross-presentation and innate immune activation. This postirradiation CD47 and PD-L1 up-regulation was observed across various human solid tumor cells. Concordantly, rectal cancer patients with poor responses to neoadjuvant RT exhibited significantly elevated postirradiation CD47 levels. The combination of RT, anti-SIRPα, and anti-PD-1 reversed adaptive immune resistance and drove efficient TAA cross-presentation, resulting in robust TAA-specific CD8 T cell priming, functional activation of T effectors, and increased T cell clonality and clonal diversity. We observed significantly higher complete response rates to RT/anti-SIRPα/anti-PD-1 in both irradiated and abscopal tumors and prolonged survival in three distinct murine CRC models, including a cecal orthotopic model. The efficacy of triple combination therapy was STING dependent as knockout animals lost most benefit of adding anti-SIRPα and anti-PD-1 to RT. Despite activation across the myeloid stroma, the enhanced dendritic cell function accounts for most improvements in CD8 T cell priming. These data suggest ATR-mediated CD47 and PD-L1 up-regulation as a key mechanism restraining radiation-induced immune priming. RT combined with SIRPα and PD-1 blockade promotes robust antitumor immune priming, leading to systemic tumor regressions.

Abstract 4231: Hypoxia reduction in tandem with anti-angiogenic therapy remodels the pancreatic tumor microenvironment

Abstract Pancreatic cancer patients remain largely unresponsive to single agent immune checkpoint blockade (ICB) therapy. This refractory state originates from PDAC’s unique immunosuppressive microenvironment that is densely populated by suppressive myeloid cells, exhibits poor vascularity, and is highly hypoxic. We previously showed that the hypoxia-activated prodrug TH-302 (Evofosfamide) reduces intratumoral hypoxia through a tissue remodeling process, initiates tumor vasculature reorganization, and sensitizes aggressive, spontaneous murine models of prostate cancer to ICB. In a clinical trial testing the combination of Evofosfamide with cytotoxic T-lymphocyte-associated protein (CTLA-4) blockade (NCT03098160) a subset of metastatic, immune checkpoint blockade refractory patients showed prolonged progression free survival. While these studies highlight hypoxia reduction as therapeutically tractable, we lack complete understanding of the contribution of the tumor vasculature to hypoxia reduction therapy, as well as the downstream consequences of hypoxia reduction on the cellular composition of the tumor microenvironment and the responsiveness to immunotherapy. We used a transplantable, orthotopic pancreatic tumor model derived from Kras+/LSL-G12D; Trp53+/LSL-R172H; Pdx1-Cre mice and a syngeneic prostate tumor model to study tumor responses to hypoxia-reduction, anti-angiogenic therapy, and combination immunotherapy. We find that Evofosfamide with anti-VEGFR-2 (DC101) significantly extends mouse survival. Combination therapy reduces intratumoral hypoxia, improves vessel integrity, and increases intratumoral DNA damage. In response to the improved metabolic microenvironment, CD8 T cells gain enhanced effector function and lose expression of exhaustion-associated features. Like other anti-angiogenic regimens, combination Evofosfamide and DC101 leads to an increased frequency of PD-L1 expressing cells within the tumor, however blockade of PD-1 failed to prolong survival. Bulk-tumor RNA sequencing and tumor infiltrating lymphocyte analysis implicates immature myeloid cells as mediators of resistance. Re-education of the infiltrating myeloid stroma with CD40 agonist could improve responses to hypoxia-reducing, anti-angiogenic therapy. Put together, these data indicate that targeted hypoxic reduction with anti-angiogenic therapy remodels the pancreatic tumor microenvironment. In this setting, CD40 agonist over PD-1 blockade provides an additive benefit in prolonging mouse survival. Citation Format: Arthur J. Liu, Michael A. Curran. Hypoxia reduction in tandem with anti-angiogenic therapy remodels the pancreatic tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4231.

High-potency synthetic STING agonists rewire myeloid stroma in the tumour microenvironment to amplify immune checkpoint blockade efficacy in refractory pancreatic cancer.

Abstract Pancreatic ductal adenocarcinoma (PDAC) is clinically unresponsive to immune checkpoint blockade (ICB) immunotherapy. Dense immunosuppressive myeloid stroma (MS) and consequent T cell exclusion from the tumour microenvironment renders PDAC resistant to immune-based therapies. Innate immune activation of the MS via cyclic dinucleotide (CDN) agonists of the STING (Stimulator of Interferon Genes) pathway can trigger T cell infiltration into cold tumours leading to robust anti-tumour immunity. Despite proven therapeutic efficacy in preclinical models, the cellular mechanisms of how CDNs reprogram the suppressive MS to sensitise tumours to ICB is poorly understood. Using multi-omic profiling of MDSCs and M2 Macrophages of human and murine origin, we show that the synthetic STING agonist, IACS-8803, rewires these populations from immunosuppressive to immune-permissive phenotypes in part through inhibition of c-Myc signaling, energy metabolism modulation, and antagonism of cell cycle. Furthermore, dimensionality reduction analyses of multiparameter flow cytometry data and survival studies in a KPC-derived model of PDAC show a proinflammatory remodeling of the MS and an increased T/NK cell infiltration and function by synthetic CDN STING agonists into the tumour that underlies an amplified therapeutic response to ICB. We are the first to report that synthetic CDN STING agonists affect MDSC and M2 macrophage repolarization through altering energy metabolism, cell cycle dynamics and c-Myc signalling. This study uncovers molecular and cellular mechanisms by which high-potency STING agonists drive a proinflammatory conversion of the tumour MS to overcome resistance to ICB in an aggressive orthotopic tumour model of PDAC. Supported by grants from PanCan (206100-90-100300-54-00006617-11111)

291 Dual-specific antibodies blocking both PD-L1 and PD-L2 engagement of PD-1 restore anti-tumor immunity

BackgroundInhibition of T cell activation and effector function via engagement of the co-inhibitory receptor PD-1 is a critical mechanism enabling tumors to evade host immunity. The two ligands for PD-1, PD-L1 and PD-L2, can be expressed by a variety of immunosuppressive stromal cells, particularly of the myeloid lineage, endothelial cells, and by tumors themselves. In addition to PD-1, PD-L1 engages B7-1 in an additional co-inhibitory interaction. Blocking only PD-1 or only PD-L1 thus does not relieve all inhibitory components of this pathway. We hypothesized that bispecific antibodies blocking both PD-L1 and PD-L2 could more fully restore tumor-specific T cell activation and potentiate anti-cancer immunotherapy. Furthermore, we speculated that enhancing the cytotoxic effector function of these antibodies might further enhance their efficacy through the depletion of tumor cells and supportive stroma.MethodsWe investigated the capacity of monoclonal antibodies capable of bivalent binding to both PD-L1 and PD-L2 to restore the function of PD-1-suppressed T cells in vitro. To assess the in vivo therapeutic efficiency of bispecific PD-Ligand antibodies with ADCC capacities, mouse IgG2a and modified human IgG1 versions were generated. We assessed their ADCC activity in vitro using a bioluminescent reporter assay, and their therapeutic efficiency in vivo in syngeneic or human-cell derived tumors.ResultsThe bispecific antibodies we generated restore the function of PD-1-suppressed T cells in vitro with equivalent efficiency to the FDA approved PD-1 antibody Pembrolizumab. Moreover, our modified human bispecific antibodies lead to significantly higher FcγRIIa activation than FDA-approved clinical human IgG1 PD-L1 antibodies in vitro. In vivo, ADCC-capable PD-Ligand bispecific antibodies suppress the growth of U2940 lymphoma in immunodeficient mice more efficiently than Rituximab, and in a syngeneic model of PD-L1/PD-L2 double positive colon carcinoma, these antibodies demonstrate superiority to PD-1 blocking antibodies to limit tumor growth and increase survival. Furthermore, treatment with our bispecific antibodies increases T cell proliferation and cytotoxicity and reduces density of immunosuppressive myeloid stroma in vivo.ConclusionsADCC-capable PD-Ligand bispecific antibodies display higher therapeutic potential than existing anti-PD-1 antibodies and represent a new class of PD-1 pathway therapeutics with significant potential for the treatment of a variety of human cancers.AcknowledgementsI would like to thank Anupallavi Srinivasamani, PhD student in Michael Curran’s lab, who performed a considerable amount of the work on this project before I joined.

High potency STING agonists engage unique myeloid pathways to reverse pancreatic cancer immune privilege

BackgroundIntratumoral injection of cyclic dinucleotide (CDN) agonists of the stimulator of interferon genes (STING) pathway engages innate immune activation and priming of adaptive immune effectors to foster local and distal...

Pharmacologic Normalization of Pancreatic Cancer-Associated Fibroblast Secretome Impairs Prometastatic Cross-Talk With Macrophages.

Background & AimsCancer-associated fibroblasts (CAFs) from pancreatic adenocarcinoma (PDA) present high protein synthesis rates. CAFs express the G-protein–coupled somatostatin receptor sst1. The sst1 agonist SOM230 blocks CAF protumoral features in vitro and in immunocompromised mice. We have explored here the therapeutic potential of SOM230, and underlying mechanisms, in immunocompetent models of murine PDA mimicking the heavy fibrotic and immunosuppressive stroma observed in patient tumors.MethodsLarge-scale mass spectrometry analyses were performed on media conditioned from 9 patient PDA-derived CAF primary cultures. Spontaneous transgenic and experimental (orthotopic co-graft of tumor cells plus CAFs) PDA-bearing mice were longitudinally ultrasound-monitored for tumor and metastatic progression. Histopathology and flow cytometry analyses were performed on primary tumors and metastases. Stromal signatures were functionally validated through bioinformatics using several published, and 1 original, PDA database.ResultsProteomics on the CAF secretome showed that SOM230 controls stromal activities including inflammatory responses. Among the identified secreted proteins, we validated that colony-stimulating factor 1 (CSF-1) (a macrophage growth factor) was reduced by SOM230 in the tumor and plasma of PDA-harboring mice, alongside intratumor stromal normalization (reduced CAF and macrophage activities), and dramatic metastasis reduction. In transgenic mice, these SOM230 benefits alleviate the chemotherapy-induced (gemcitabine) immunosuppressive stroma reshaping. Mechanistically, SOM230 acts in vivo on CAFs through sst1 to disrupt prometastatic CAF production of CSF-1 and cross-talk with macrophages. We found that in patients, stromal CSF-1 was associated with aggressive PDA forms.ConclusionsWe propose SOM230 as an antimetastatic therapy in PDA for its capacity to remodel the fibrotic and immunosuppressive myeloid stroma. This pharmacotherapy should benefit PDA patients treated with chemotherapies.

TLR9 activation cooperates with T cell checkpoint blockade to regress poorly immunogenic melanoma

Tumors that lack pre-existing immune infiltration respond poorly to T cell checkpoint blockade immunotherapy. These cancers often surround themselves with high densities of suppressive myeloid stroma while excluding immunostimulatory dendritic cells. Tumor-resident myeloid cells and selected lymphocyte populations retain expression of Toll-like Receptors (TLR) that sense common features of pathogens and activate innate immunity in response. We explored whether agonists of TLR9 could augment innate immunity to promote tumor regression alone or in combination with T cell checkpoint blockade. In the setting of the immunogenic B16-Ova (Ovalbumin) expressing melanoma model, local injection of the CpG oligonucleotide TLR9 agonist ODN1826 combined with systemic CTLA-4 blockade cured 45% of mice of both their treated and an untreated tumor on the opposite flank demonstrating the synergistic potential of this combination. Next, in the non-immunogenic B16-F10 melanoma model, we showed that only intra-tumoral, but not systemic TLR9 activation augments the therapeutic potential of checkpoint blockade. In this setting, intra-tumoral TLR9 activation cooperated equally with either CTLA-4 or PD-1 blockade co-administered locally or given systemically; however, the uninjected tumor rarely regressed. Anti-CTLA-4 combinations were associated with improved intra-tumoral CD8 to regulatory T cell ratios, while anti-PD-1 combinations elicited improved ratios of CD8 T cells relative to suppressive myeloid stroma. Using both a TLR9 agonist (MGN1703) and a CTLA-4 antibody (9D9-IgG2a) of increased potency cured 50% of bi-lateral B16-F10 melanoma. These findings suggest that intra-tumoral TLR9 agonists can improve sensitivity of poorly immunogenic tumors to T cell checkpoint blockade, and that newer, higher potency TLR agonists and checkpoint antibodies can raise the therapeutic ceiling for this combination therapy.

Abstract A050: Intratumoral delivery of a novel STING agonist synergizes with checkpoint blockade to regress multifocal pancreatic cancer

Abstract Immunosuppressive myeloid populations including tumor-associated macrophages (TAM) and myeloid-derived suppressor cells (MDSC) are abundant within pancreatic adenocarcinoma (PDAC) tumors and play critical roles in constraining cytotoxic T-cell function in the tumor microenvironment. We hypothesized that intratumoral engagement of innate pathogen recognition receptors such as Stimulator of Interferon Genes (STING) could induce proinflammatory polarization of the myeloid stroma and liberate the antitumor T-cell response to regress refractory PDAC tumors in the presence of checkpoint blockade.We developed and characterized a novel cyclic dinucleotide (CDN) STING agonist IACS-8803, and found that 8803 activates downstream STING signaling in both human (THP-1) and murine (J774) myeloid reporter cells with over 10-fold greater potency than ML-RR-S2-CDA, the first-in-class CDN currently undergoing clinical evaluation (NCT02675439, NCT03172936). Intratumoral delivery of 8803 into subcutaneous B16 melanoma and PDAC tumors additionally revealed a greater capacity to induce tumor regression relative to ML-RR-S2-CDA. In order to evaluate the specific effects of 8803 on the phenotype and function of suppressive myeloid populations, we generated in vitro polarized human M2 macrophages and murine bone marrow-derived MDSC. Upon exposure to 8803, we observe downregulation of M2 markers CD163, LAP/TGF-β, and Arginase on human M2 macrophages, concomitant with upregulation of M1 markers CD86, CD80, and IL-6. Additionally, 8803-stimulated murine MDSC exhibit reduced T-cell suppressive capacity compared to unstimulated MDSC. In these studies, we consistently observe that the magnitude of phenotypic and functional repolarization by 8803 is superior to that of ML-RR-S2-CDA as well as other known CDN, 2’3’-cGAMP and c-di-GMP. Therefore, we describe IACS-8803 as a novel, highly potent STING agonist with the capacity to induce inflammatory repolarization in suppressive myeloid cells of both human and murine origin. We next investigated the capacity for intratumoral delivery of IACS-8803 to sensitize murine pancreatic cancer to checkpoint blockade and to mobilize systemic immunity against disseminated lesions. We utilized mT4-2D, a novel pancreatic cancer cell line from Kras+/LSL-G12D Tp53+/LSL-R172H Pdx1-Cre tumor organoids. We isolateda single cell clone of mT4-2D with reduced in vivo growth kinetics (termed mT4-LS), as well as a clone that maintains the aggressive nature of the parental line (termed mT4-LA). Mice bearing 10-day established orthotopic and subcutaneous mT4-LS tumors received standard regimens of αCTLA-4, αPD-1, or combined αCTLA-4/αPD-1 in the presence or absence of 8803 CDN injected into the orthotopic pancreatic tumor. We find single-agent treatment with 8803, αCTLA-4, αPD-1, or αCTLA-4/αPD-1 can cure 40-60% of mice of both orthotopic and subcutaneous tumors in this system; however, combining 8803 with checkpoint blockade completely eradicates both injected and distal mT4-LS tumors in all mice. We replicated these studies using the highly aggressive and refractory mT4-LA model, and found that combination therapy with intra-pancreatic 8803 and systemic αCTLA-4/αPD-1 significantly extends survival compared to 8803 or αCTLA-4/αPD-1 alone (p=0.001, p=0.0086, respectively). Analysis of treated mT4-LA tumors by flow cytometry reveals that combination therapy enhances the cytotoxic potential of CD8 T-cells at both injected and uninjected lesions, and promotes dendritic cell proliferation within the pancreatic milieu. These studies provide a preclinical rationale for pursuing the use of STING-activating CDN as a localized approach to sensitize refractory PDAC tumors to checkpoint blockade immunotherapy. Citation Format: Casey R. Ager, Maria E. Di Francesco, Philip Jones, Michael A. Curran. Intratumoral delivery of a novel STING agonist synergizes with checkpoint blockade to regress multifocal pancreatic cancer [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A050.

Transforming growth factor-beta and matrix metalloproteinases: functional interactions in tumor stroma-infiltrating myeloid cells.

Transforming growth factor-beta (TGF-β) is a pleiotropic factor with several different roles in health and disease. In tumorigenesis, it may act as a protumorigenic factor and have a profound impact on the regulation of the immune system response. Matrix metalloproteinases (MMPs) are a family that comprises more than 25 members, which have recently been proposed as important regulators acting in tumor stroma by regulating the response of noncellular and cellular microenvironment. Tumor stroma consists of several types of resident cells and infiltrating cells derived from bone marrow, which together play crucial roles in the promotion of tumor growth and metastasis. In cancer cells, TGF-β regulates MMPs expression, while MMPs, produced by either cancer cells or residents' stroma cells, activate latent TGF-β in the extracellular matrix, together facilitating the enhancement of tumor progression. In this review we will focus on the compartment of myeloid stroma cells, such as tumor-associated macrophages, neutrophils, and dendritic and mast cells, which are potently regulated by TGF-β and produce large amounts of MMPs. Their interplay and mutual implications in the generation of pro-tumorigenic cancer microenvironment will be analyzed.

Polyglobuly versus polycythemia vera

Differentiation of an elevated hemoglobin/hematocrit level warrants not only the determination of relevant laboratory values, including erythropoietin, but also an elaborate evaluation of bone marrow histopathology. Particularly in the initial stages of polycythemia vera (PV), when not all stringent clinical criteria are completely fulfilled, a more refined analysis of morphological features helps to distinguish autonomous (PV) from spurious or secondary erythrocytosis - polyglobuly (PG). PV is characterized by a panmyelosis, i.e., a trilinear proliferation of all cell lineages, whereas in PG erythropoiesis predominates. Megakaryopoiesis exerts a significant impact on differentiation, because in PV there is a conspicuous grouping associated with a strikingly expressed pleomorphous appearance. This peculiar feature implicates assemblies of small- to medium-sized megakaryocytes lying adjacent to giant cells with extensively lobulated, staghorn-like nuclei. A further discriminating parameter is presented by the interstitial lesions frequently occurring in PG. These, according to the usually underlying inflammatory cause (chronic bronchitis-recurrent bronchopneumonias), include iron-laden macrophages, a prominent perivascular plasmacytosis, phagocytosis of cell debris by histiocytic reticular cells and often an in- crease in the number of eosinophils. Comparable findings are not, or only to a minor ex-tent, detectable in PV. In conclusion, by regarding hematopoiesis and the myeloid stroma initial stages of PV may be definitively distinguished from PG.

In aplastic anemia progenitor cells have a reduced sensitivity to the effects of growth factors.

We have recently shown that in patients with aplastic anemia (AA) recovering following immunosuppressive therapy, the persistent reduction in the bone marrow clonogenic potential is unrelated to suppressive effects of the myeloid stroma and intrinsic to the hematopoietic progenitors. We examined the mechanisms of this defect by determining the response of the aplastic CD34+ clonogenic precursors to proliferative signals induced by hematopoietic growth factors and comparing their results with those of a control population. Light density bone marrow mononuclear cells were lymphocyte and monocyte depleted and enhanced for the CD34+ progenitors by immunomagnetic selection. Selected progenitors were then cultured in the mixed colony assay with incremental concentrations of combinations containing erythropoietin (Epo), granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3) and c-kit ligand. Bone marrow from aplastic patients had significantly fewer light density cells displaying the CD34 antigen (mean 0.65%, SD 0.35 vs. 1.62%, SD 1.4; p=0.002). Dose response studies on aplastic CD34+ cells demonstrated that at low concentrations of Epo, IL-3 and GM-CSF, clonogenic growth was significantly impaired but achieved normal values at concentrations giving plateau growth in control cultures. However, for all colony types, responses to effective concentrations of c-kit ligand corresponded with those of controls. These data suggest abnormalities at the receptor or signal transduction levels.

Clinicopathological Impact of the Interaction Between Megakaryocytes and Myeloid Stroma in Chronic Myeloproliferative Disorders: A Concise Update

In this report an attempt has been made to discuss some of the issues pertinent to myelofibrosis complicating chronic myeloproliferative disorders (CMPDs) that are significantly associated with megakaryocyte function. In this context, biochemical, clinical and particularly morphological features were reviewed. Morphological findings based on elaborate techniques were in keeping with the assumption that in chronic myeloid leukemia (1) the number of CD61-positive megakaryocytes, and in particular their precursors were the parameters most closely associated with myelofibrosis (2) an increased content of reticulin fibers in follow-up biopsies significantly correlated with laboratory data indicative of a high tumor burden (anemia, peripheral blasts, hepatosplenomegaly) and thus a more advanced stage of the disease process (3) even a slight increase in reticulin, i.e. doubling of the normal fiber density was associated with a worse prognosis independent of therapeutic regimens given (4) Dynamics of myelofibrosis was significantly influenced by treatment. In this context, calculation of the myelofibrosis progression index (MPI) revealed a higher score following interferon therapy compared with busulfan. In addition, in idiopathic myelofibrosis (5) the evolution of myelofibrosis was unpredictable and according to the MPI, progression occurred at a relatively low rate (6) proliferation and dilatation of sinusoids accompanying intravascular hematopoiesis and collagen type IV deposits were predominant features in later (fibro-osteosclerotic) stages in the course of disease (7) transmural migration of megakaryocytes demonstrated by three dimensional reconstruction revealed a mole-like tunneling through the thickened sinusoidal wall. A very careful assessment of the numerous correlations between bone marrow features and laboratory data will allow clinicians and pathologists to gain a better insight into the mutual relationships between hematological and morphological findings in CMPDs.

Vascular architecture and collagen type IV in primary myelofibrosis and polycythaemia vera: an immunomorphometric study on trephine biopsies of the bone marrow.

Using a monoclonal antibody against collagen type IV, an immunohistochemical and morphometric study was performed on trephine biopsies of the bone marrow to determine the vascular architecture of the myeloid stroma as well as the occurrence of subendothelial collagen type IV deposits. In comparison with a control group (20 patients), we investigated specimens from 20 patients with polycythaemia vera (P. vera) and 40 patients showing primary (idiopathic) osteomyelofibrosis/-sclerosis (OMF). Morphometric analysis disclosed a significant increase in the number of sinusoids and also in the extent of collagen type IV deposits in the latter disorder. Sinusoids of P. vera showed only a dilatation of their lumina corresponding with enlarged areas of sectioned vascular profiles. Contrary to this finding, in OMF sinusoidal distension was accompanied by an excessive deviation from a circular profile and prominent intravascular haematopoiesis. Furthermore, there was a significant formation of basement membrane collagen (collagen type IV). All these features were far less pronounced in early hypercellular stages of OMF than in the late fibro-osteosclerotic subtypes. Our morphometric findings confirm haemodynamic studies on advanced stages of OMF which revealed an increased vascularity. Moreover, these results are in keeping with the assumption that angiogenesis in the evolution of the fibroosteosclerotic marrow process is mediated by certain growth factors. The increase in collagen type IV formation is thought to be generated by the pronounced endothelial cell proliferation in the course of neovascularization.

Myeloid dysplasia (MD): a hematological disorder preceding acute and chronic myeloid leukemia. A morphological study on sequential core biopsies of the bone marrow in 27 patients.

Light- and electron microscopic appearances of core biopsies of the bone marrow in 27 selected patients out of about 195 cases with a clinically suspected preleukemic syndrome. The correct diagnosis of “ preleukemia” was established retrospectively by sequential biopsies of the iliac crest or autopsy in those patients who developed overt leukemia in periods ranging from 2 to 36 months. As a final diagnosis chronic myelogenous leukemia (CML) was established in 10, with accompanying blast crisis in 6 and acute non-lymphocytic leukemia (ANLL) in 11 cases. Histomorphology of the resin embedded cores of bone marrow showed hypercellularity in 21 specimens, a hypocellular marrow in 5 and a normal bone marrow in 1 case. There was also a conspicuous macrocytic or megaloblastoid maturation of erythropoiesis with frequent sideroblasts. Ultrastructural abnormalities included atypical nuclear clefts, dense iron deposits in the mitochondrial matrix and an increase of ferritin uptake. Neutrophilic granulopoiesis showed a shift to the left and often a remarkable aberration of nuclear segmentation consistent with a pseudo-Pelger-Huet anomaly. Electron microscopy displayed atypias of granulogenesis in comparison with maturation and segmentation of the nuclei, abnormal nuclear loops and blebs and very conspicuous nuclear fibrillar appendages (so called Nebenkerne). There was also an increase in eosinophilic granulocytes, monocytic elements, edema and a remarkable perivascular plasmacytosis of the myeloid stroma. Our results suggest that characteristic morphological features of the bone marrow exist before onset of overt, acute and chronic leukemia. These alterations are identical in CML and ANLL and are the morphological substrate of a maturation defect of hematopoiesis which precedes the establishment of the leukemic clone. The clinical term preleukemia should be replaced by myeloid dysplasia (MD), thus indicating transformation into overt leukemia in only a certain proportion of patients (only 27 of 195 clinically suspected patients who displayed an identical histopathology of MD in the bone marrow in 93 cases to date) and has to include ANLL as well as CML.