HemaBites showcase hematology news and short commentaries on recent high-impact articles published in international journals. This blog will keep you up to date with the latest developments and discoveries in the field of hematology.

Friday, January 11, 2019

Blood and guts: hematopoietic stem and progenitor cells resident within human intestinal allografts potentially mediate immune tolerance.

Michael D. Milsom1,2
1.Division of Experimental Hematology, German Cancer Research Center (DKFZ), Heidelberg Germany.
2.Heidelberg Institute for Stem Cell Technology and Experimental Medicine (HI-STEM), Heidelberg, Germany.

HumanInstestinal_Jan2019.pngIn mammals, the bone marrow is a major site of hematopoiesis, with the vast majority of hematopoietic stem and progenitor cells (HSPCs) residing within the medullary cavities of bones. However, alternative sites of so-called extramedullary hematopoiesis have also been defined, such as the spleen, liver and lung, which likely play distinct biological roles supporting hematopoiesis in diverse scenarios, such as during stress; at specific developmental stages; or in the local production of mature blood cell lineages. In a recently published article in Cell Stem Cell, the group of Megan Sykes has made the surprising finding that functional HSPCs reside within the adult intestine of humans. As well as experimentally characterizing HSPCs isolated from human intestine using experimental approaches such as flow cytometry, molecular analysis and xenotransplantation into immune deficient mice, the authors could demonstrate the functional relevance of these cells by characterizing the sustained production of donor-derived mature peripheral blood cells in patients who received intestinal allografts. Intriguingly, intestinal transplantation appeared to robustly result in the de novo production of circulating T cells that had apparently undergone in vivo selection, and which may play a role in modulating immune tolerance. Donor-derived HSPCs were slowly replaced with recipient HSPCs, presumably seeded from a circulating pool of stem cells. Thus, an unexpected side effect of intestinal transplantation appears to be the establishment of hematopoietic chimerism, which may impact on the outcome of the procedure. Clearly this phenomenon warrants further study in order to ascertain whether it can yield new insight into transplant rejection.

Figure legend:
Multiplex immunohistochemistry staining on an ileum specimen collected from a transplant patient 1606 day after intestinal transplantation. The image suggests that human intestinal HSPCs (CD45+ cyan; CD34+ red; and CD90+ yellow) reside in the lamina propria layer of the gut mucosa around the crypts (DAPI in blue). Given that flow cytometric analysis on the ileum specimen collected on the same day showed that all CD45+ cells in the ileum graft were recipient HLA+, the representative HSPC (white arrow) is a recipient-derived HSPC, which presumably populated the donor graft from a circulating pool. Image kindly provided by Megan Sykes, Columbia University, New York.

Fu J., et al., Human Intestinal Allografts Contain Functional Hematopoietic Stem and Progenitor Cells that are Maintained by a Circulating Pool, Cell Stem Cell, DOI: https://doi.org/10.1016/j.stem.2018.11.007

Monday, January 7, 2019

Fighting sterile inflammation in sickle cell disease

Francesca Vinchi, PhD
Lindsley F. Kimball Research Institute (LFKRI), New York Blood Center - NYBC, New York, USA.

Sickle cell disease (SCD) patients carry activated platelets. Given the crucial role of platelets in vascular inflammation and thrombosis, their exacerbated activation might contribute to acute pain crisis and acute chest syndrome in SCD. Platelets express the pattern recognition receptor nucleotide-binding domain leucine-rich repeat containing protein 3 (NLRP3) and Bruton tyrosine kinase (BTK), which regulate inflammosome activation through caspase-1 and IL-1ß cleavage (Figure 1). Inflammosome complex formation eventually results in platelet aggregation and thrombus generation. Recently, Vogel and co-workers showed that caspase-1 and NLRP3 inflammosome activation are increased in circulating platelets isolated from SCD patients as well as SCD mice, and are further aggravated during SCD pain crises. SCD platelet-free plasma stimulation was sufficient to induce inflammosome and caspase-1 activation in platelets from healthy subjects. The authors identified HMGB1-mediated TLR4 (Toll-like receptor 4) signaling induction as the underlying molecular mechanism triggering inflammosome activation in SCD patients. The application of inhibitors of HMGB1, TLR4, BTK or NLRP3 efficiently prevented caspase-1 activation in SCD plasma-treated platelets in vitro and in SCD animals (Figure 1). As a result, platelet aggregation in SCD mice was restored to levels comparable to control mice. Interestingly, the strongest inhibitory effect was achieved via combined therapy with multiple inhibitors. The observation of increased SCD platelet inflammosome activation and pathway identification unravel a novel druggable mechanism to reduce chronic vascular inflammation and abnormal coagulation in SCD. Despite the importance of inflammosome in infection response, its temporary inhibition in this patient population is likely beneficial to counteract platelet aggregation and vasculopathy. Finally, inflammosome inhibition might be effective in other cell types where this mechanism is active (e.g. macrophages), further contributing to an overall reduction of the so called ‘sterile’ inflammation that underlies SCD pathophysiology.

Figure 1. Platelet inflammosome activation drives vascular inflammation and thrombus formation in SCD. HMGB-1-mediated TLR4 pathway activation triggers inflammosome complex formation, caspase-1 activation and IL-1ß cleavage in platelets. Red symbols indicate inhibitors that can be applied to block the pathway and prevent platelet activation for therapeutic purposes in SCD. 

Sebastian Vogel, Taruna Arora, Xunde Wang, Laurel Mendelsohn, James Nichols, Darlene Allen, Arun S. Shet, Christian A.Combs, Zenaide M.N. Quezado, Swee Lay Thein. The platelet NLRP3 inflammosome is upregulated in sickle cell disease via HMGB1/TLR4 and Bruton tyrosine kinase. Blood Advances 2018. 2:2672-2680.

Wednesday, December 12, 2018

​Michel Boiron 

1925 - 2018

michel Boiron.png

It is with great sadness that we announce the passing of Professor Michel Boiron last December 3. Boiron was the founding President (1992-1994) of the European Hematology Association (EHA).

Together with Michel Symann, Bob Löwenberg and John Goldman, Michel Boiron formed the 'gang of four,' as they playfully called themselves, who founded EHA. As a convinced Europeanist, he brought together a small group of friends and colleagues who shared with him the idea of Europe as a great community not only politically but also scientifically. He initiated the first meeting leading to the process of founding the association which took place on Saturday, February 27, 1988 in the Gardeners Pavilion of the Hôpital Saint-Louis where he was then Director of the Research Institute for Leukemias and Blood Diseases. He hosted a lunch in Hôpital St Louis to which he had invited about 10-15 hematologists from different countries in Europe. He had the vision that Europe needed a dedicated society. Four years later and in June 1992, the first formal charter was established in a notary office in Brussels, Belgium.  

The reason for establishing a new association and its mission as envisioned by Michel Boiron were published in EHA's first newsletter in December 1995:

"Europe consists of many separate parts that would benefit from a unifying agent. EHA aspires to be that agent, to act as a federating force for European hematology. EHA has a firm commitment to promoting excellence in all scientific endeavors while paying no attention to other obsolete considerations, such as, for example, the geographic location of the research teams or Congress venues.  In these EHA initiatives, some of you may note an attempt to reestablish, in Europe, the norms established by the American Society of Hematology in the United States. You are correct, if you are referring to our demand for excellence without concessions. As for our other achievements and further plans, we are convinced that Europe will be able to make an original contribution to the development of hematology."

The first EHA meeting (EHA-1) took place in Brussels in 1994. EHA-2 took place in Boiron's hometown of Paris in 1996.  

With thanks to the leadership of Michel Boiron and the enthusiasm he conveyed to all those working with him, EHA eventually became "The" EHA.

In 1993, after 13 years Michel Boiron retired from his work in the Hôpital Saint-Louis. After leading EHA and serving as a Congress President in 1996 in Paris, he stepped down and took some distance from the Association that was taken over by the next generation hematologist leaders leading the organization into the 21st century.

Michel Boiron never sought personal recognition or visibility and put the association above all else, which is also part of his legacy. EHA will be indebted to Michel Boiron forever and is deeply touched by the passing of its founding father.

Pieter Sonneveld

EHA President, on behalf of the EHA Board

Wednesday, December 5, 2018

Uncovering the role of the bone marrow microenvironment in severe aplastic anemia

Francesca Vinchi, PhD
Lindsley F. Kimball Research Institute (LFKRI), New York Blood Center - NYBC, New York, USA

Severe aplastic anemia (SAA) is a rare lethal form of bone marrow (BM) failure. The only curative treatment available is BM transplantation, for which not all patients are eligible. Alternative approaches are required to fill the unmet need of SAA patients. SAA is driven by over-active T cells which promote hematopoietic stem cell (HSC) loss through the production of inflammatory cytokines, in particular IFNγ. As a consequence, thrombocytopenia, megakaryocyte loss and BM destruction occur, leading to SAA. Whether inflammation depletes HSCs directly or via a deregulated microenvironment has remained unclear until recently, when McCabe at al. demonstrated the critical function of BM niche macrophages in causing the disease. Using a mouse model of BM failure through splenocyte infusion in histocompatibility mismatched recipients, the authors elegantly showed that, despite the development of cytopenia and BM hypocellularity, macrophages were increased within the niche. When the model was applied in transgenic mice bearing macrophages unresponsive to INFg, macrophages were decreased, whereas anemia, thrombocytopenia and HSC loss were significantly improved, highlighting INFg sensing by macrophages as a key SAA driver. A similar effect was obtained upon clodronate-induced macrophage depletion. Interestingly, IFNγ increased the number of podoplanin (PDPN)-expressing macrophages. PDPN controls platelet production by regulating contractility/migration of different cell types and therefore BM stiffness, which impacts on megakaryocyte pro-platelet extensions. Targeting PDPN rescued thrombocytopenia and increased survival of SAA mice. Through the identification of a unique population of macrophages that restrict HSCs and contribute to SAA, this work provides the rationale for targeting macrophages during BM failure. These findings showed for the first time a central role of the BM microenvironment in SAA and undercovered novel potential targets and therapeutic options worthy of further development for the cure of SAA.

Figure 1. PDPN+ macrophages drive IFNg-mediated SAA. During SAA, T-cell overactivation leads to increased INFg production, which in turn promotes the expansion of PDPN-expressing macrophages. PDPN+ macrophages trigger SAA by inducing megakaryocyte (MK), mesenchymal stromal (MSC) and hematopoietic stem cell loss (HSC), leading to thrombocytopenia, bone marrow hypocellularity and anemia. 

Amanda McCabe, Julianne N.P.Smith, Angelica Costello, Jackson Maloney, Divya Katikaneni, Katherine C. MacNamara. Hematopoietic stem cell loss and hematopoietic failure in severe aplastic anemia is driven by macrophages and aberrant podoplanin expression. Haematologica 2018. 103:1451-1461.

Monday, December 3, 2018

​Discovering clonal diversity in acute myeloid leukemia

Jan Cools, VIB-KU Leuven Center for Cancer Biology, Editor-in-chief HemaSphere

Acute myeloid leukemia (AML) development is driven by the accumulation of mutations and chromosomal rearrangements in hematopoietic stem or progenitor cells. As a consequence, multiple clones emerge and evolve during AML progression and one of these clones may reappear at relapse. Bauke de Boer, Jan Jacob Schuringa and colleagues have now developed new tools to enable the tracking, isolation and characterization of such AML subclones1. They used a proteomic analysis to identify suitable cell surface markers that are specifically expressed or upregulated on AML blasts and leukemia stem cells. Based on these markers, different subclones could be isolated by cell sorting and sequence analysis revealed that the different sublcones within one AML patient harbored different mutations. In one AML case, for example, CD34+CD25+ cells were isolated that harbored DNMT3A, RUNX1, IDH2 and FLT3 mutation, while CD34+CD25- cells harbored the same mutations, except the FLT3 mutation. They next also performed gene expression and chromatin analyses in those different subclones and observed that dependent on the presence or absence of specific mutations different transcriptional programmes were active in the cells. The authors were also able to correlate response to therapy or growth characteristics with the presence or absence of certain mutations. Finally, the cell surface markers were also used to track subclones during treatment and eventually at relapse, illustrating their applicability in longitudinal studies. This approach with cell sorting based on selected cell surface markers is maybe less fancy than single-cell sequencing2, but is by no means less interesting!

1. de Boer B, Prick J, Pruis MG, Keane P, Imperato MR, Jaques J, et al. Prospective Isolation and Characterization of Genetically and Functionally Distinct AML Subclones. Cancer Cell, 2018;34(4):674-689.
2. Wilson NK, Göttgens B. Single-Cell Sequencing in Normal and Malignant Hematopoiesis. HemaSphere, 2018;2(2):e34.