The retraction of >30 falsified studies by Anversa et al. has had a disheartening impact on the cardiac cell therapeutics field. The premise of heart muscle regeneration by the transdifferentiation ...of bone marrow cells or putative adult resident cardiac progenitors has been largely disproven. Over the past 18 years, a generation of physicians and scientists has lost years chasing these studies, and patients have been placed at risk with little scientific grounding. Funding agencies invested hundreds of millions of dollars in irreproducible work, and both academic institutions and the scientific community ignored troubling signals over a decade of questionable work. Our collective retrospective analysis identifies preventable problems at the level of the editorial and peer-review process, funding agencies and academic institutions. This Perspective provides a chronology of the forces that led to this scientific debacle, integrating direct knowledge of the process. We suggest a science-driven path forward that includes multiple novel approaches to the problem of heart muscle regeneration.
Acute myeloid leukemia (AML) is organized as a cellular hierarchy initiated and maintained by a subset of self-renewing leukemia stem cells (LSC). We hypothesized that increased CD47 expression on ...human AML LSC contributes to pathogenesis by inhibiting their phagocytosis through the interaction of CD47 with an inhibitory receptor on phagocytes. We found that CD47 was more highly expressed on AML LSC than their normal counterparts, and that increased
CD47 expression predicted worse overall survival in three independent cohorts of adult AML patients. Furthermore, blocking monoclonal antibodies directed against CD47 preferentially enabled phagocytosis of AML LSC and inhibited their engraftment in vivo. Finally, treatment of human AML LSC-engrafted mice with anti-CD47 antibody depleted AML and targeted AML LSC. In summary, increased CD47 expression is an independent, poor prognostic factor that can be targeted on human AML stem cells with blocking monoclonal antibodies capable of enabling phagocytosis of LSC.
Macrophages clear pathogens and damaged or aged cells from the blood stream via phagocytosis. Cell-surface CD47 interacts with its receptor on macrophages, SIRPα, to inhibit phagocytosis of normal, ...healthy cells. We find that mobilizing cytokines and inflammatory stimuli cause CD47 to be transiently upregulated on mouse hematopoietic stem cells (HSCs) and progenitors just prior to and during their migratory phase, and that the level of CD47 on these cells determines the probability that they are engulfed in vivo. CD47 is also constitutively upregulated on mouse and human myeloid leukemias, and overexpression of CD47 on a myeloid leukemia line increases its pathogenicity by allowing it to evade phagocytosis. We conclude that CD47 upregulation is an important mechanism that provides protection to normal HSCs during inflammation-mediated mobilization, and that leukemic progenitors co-opt this ability in order to evade macrophage killing.
Growth factors can cause cells to proliferate, differentiate, survive, or die. Distinguishing between these responses is difficult in multicellular, multiparameter systems. Yet this is essential to ...understand the impact on cells like hematopoietic stem cells (HSCs), which have strict and still poorly understood growth factor requirements. Single cell plating in serum-free medium allows direct assessment of growth factor responses. The range of tested factors can be expanded if the cells are protected from growth factor deprivation-induced apoptosis. BCL-2 is overexpressed in HSCs of H2K-BCL-2 transgenic mice, protecting them from many apoptotic stimuli. The response of single wild-type and transgenic HSCs to stimulations with individual factors was tested. Surprisingly, we find that high level BCL-2 expression does not prevent rapid death under serum-free conditions, even though it does in the presence of serum. We also find that transgenic, but not wild-type cells, survive and proliferate rapidly in response to steel factor (Kit ligand). These studies show that two separate signals are necessary to prevent apoptosis in HSCs, and that Kit ligand by itself provides a strong proliferative stimulus to HSCs. However, the proliferative response does not result in self-renewal, but in differentiation to all known hematopoietic oligolineage progenitors.
Atherosclerosis is the process underlying heart attack and stroke. Despite decades of research, its pathogenesis remains unclear. Dogma suggests that atherosclerotic plaques expand primarily via the ...accumulation of cholesterol and inflammatory cells. However, recent evidence suggests that a substantial portion of the plaque may arise from a subset of “dedifferentiated” vascular smooth muscle cells (SMCs) which proliferate in a clonal fashion. Herein we use multicolor lineage-tracing models to confirm that the mature SMC can give rise to a hyperproliferative cell which appears to promote inflammation via elaboration of complement-dependent anaphylatoxins. Despite being extensively opsonized with prophagocytic complement fragments, we find that this cell also escapes immune surveillance by neighboring macrophages, thereby exacerbating its relative survival advantage. Mechanistic studies indicate this phenomenon results from a generalized opsoninsensing defect acquired by macrophages during polarization. This defect coincides with the noncanonical up-regulation of so-called don’t eat me molecules on inflamed phagocytes, which reduces their capacity for programmed cell removal (PrCR). Knockdown or knockout of the key antiphagocytic molecule CD47 restores the ability of macrophages to sense and clear opsonized targets in vitro, allowing for potent and targeted suppression of clonal SMC expansion in the plaque in vivo. Because integrated clinical and genomic analyses indicate that similar pathways are active in humans with cardiovascular disease, these studies suggest that the clonally expanding SMC may represent a translational target for treating atherosclerosis.
Dendritic cells (DCs) are critical in both initiating adaptive immune responses and maintaining tolerance to self antigens. These apparently contradictory roles have been suggested to depend on ...different subsets of DCs that arise from either myeloid or lymphoid hematopoietic origins, respectively. Although DC expression of CD8α is attributed to a lymphoid origin, here we show that both CD8α+and CD8α-DCs can arise from clonogenic common myeloid progenitors in both thymus and spleen. Thus, expression of CD8α is not indicative of a lymphoid origin, and phenotypic and functional differences among DC subsets are likely to reflect maturation status rather than ontogeny.
We identified an IL-7Ralpha(+)Sca-1(low)c-Kit(low) population in E14 fetal liver, which is the phenotypical analog of common lymphoid progenitors (CLP) in adult bone marrow. After transfer into ...newborn mice, the IL-7Ralpha(+)Sca-1(low)c-Kit(low) population rapidly differentiated into CD45(+)CD4(+)CD3(-) cells, which are candidate cells for initiating lymph node and Peyer's patch formation. In addition, this population also gave rise to B, T, NK, and CD8alpha(+) and CD8alpha(-) dendritic cells. The fetal liver precursors expressed a significantly lower level of the myeloid-suppressing transcription factor Pax-5, than adult CLP, and retained differentiation activity for macrophages in vitro. We propose that the transition from fetal liver IL-7Ralpha(+)Sca-1(low)c-Kit(low) cells to adult CLP involves a regulated restriction of their developmental potential, controlled, at least in part, by Pax-5 expression.
To rigorously test the in vivo cell fate specificity of bone marrow (BM) hematopoietic stem cells (HSCs), we generated chimeric animals by transplantation of a single green fluorescent protein ...(GFP)-marked HSC into lethally irradiated nontransgenic recipients. Single HSCs robustly reconstituted peripheral blood leukocytes in these animals, but did not contribute appreciably to nonhematopoietic tissues, including brain, kidney, gut, liver, and muscle. Similarly, in$GFP^{+}:GFP^{-}$parabiotic mice, we found substantial chimerism of hematopoietic but not nonhematopoietic cells. These data indicate that "transdifferentiation" of circulating HSCs and/or their progeny is an extremely rare event, if it occurs at all.
In the adult, tissue-specific stem cells are thought to be responsible for the replacement of differentiated cells within continuously regenerating tissues, such as the liver, skin, and blood system. ...In this review, we will consider the factors that influence stem cell fate, taking as a primary example the cell fate determination of hematopoietic stem cells.
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