Research in the Weissman Lab

Stem cell biology in health and disease and the development of macrophage-based immunotherapy:

Tissue stem cells are rare and only make the maturing and mature cells of the tissue they serve [HSC for blood, CNS stem cells for brain, skeletal stem cells for bone and cartilage and bone marrow inductive microenvironments, etc.]. Within a tissue they are the only cells that can self-renew throughout life. While the usual outcome is tissue and organ homeostasis, stem cells can accumulate and propagate mutations over many years, and those mutant stem cells can contribute to disease. Mutations that arise in any non-stem cell in a tissue are lost via the limited lifespan of non self-renewing cells. In leukemic cells, the lab discovered up-regulation of CD47 that acts as a dont eat me signal for macrophages and allows cancer cells to evade an immune response. They developed anti-CD47 as a cancer immunotherapy and continue to investigate how macrophages recognize and eliminate unhealthy cells, with a prospect of advancing medicine.

The biology of HSC and their niche:Weissman was first to identify, prospectively isolate, and transplant hematopoietic [blood-forming] stem cells, called HSC from mice and humans. HSCs generate and regenerate the entire blood and immune systems throughout life. The lab has then isolated and studied the progenitor steps between the HSC and each of the blood cell type produced. He has also traced the formation of HSC during mouse embryogenesis and fetal development, and for all of these candidate stem and progenitor cells has optimized single cell RNAseq and utilized ATACseq and CHIPseq to elucidate the steady state expression of suites of genes that characterize each stem and progenitor cell in both species. These technologies have allowed the lab to propose candidate genes whose expression either characterize the molecular fingerprints of those cells, but point toward the events required for stem cell self-renewal and for differentiation to the next cell in the developmental pathways.To study HSC and their niche, the lab generated unique reporter mouse models in which HSC expressing a fluorescent marker, HoxB5.mCherry exist as single cells attached to a subtype of bone marrow blood vessel cell, surrounded also by stromal cells from the skeletal stem cell. In parabiotic pairs, mice with a joined vasculature to each other, these marked HSC migrate from one mouse to the partner bone marrow to occupy one of these vascular niches. Working out how these interactions and migrations occur and the molecules that are responsible is a current interest of the lab.

Human Brain Stem Cells [CNS SC]: Weissman and colleagues identified and prospectively isolated human fetal brain stem cells, and upon their transplantation into the brains of immune deficient mice, found that these CNS SC home to mouse brain stem cell niches near the lateral ventricles and in the dentate gyrus of the hippocampus. The progeny of these human CNS SC self-renew in these niches, migrate their progeny in a site appropriate manner long distances through the brain, and differentiate to neurons, astrocytes, and oligodendrocytes in site appropriate manners as well. This allows one to begin to understand adult human CNS SC behaviors. These human CNS SC can engraft in patients and have regenerative and neuroprotective properties. Each CNS SC can be propagated in vitro into clonal spheres of CNS SC. Recently students in the Weissman lab have found how to identify, isolate grow, and transplant human CNS SC from surgical samples.

Stem Cells, Clonal Precancer Cells, and Progeny Cancer Stem Cells:Over the past 20 years theWeissman lab developed a method to identify mutations in single cells, and using that showed that preclinical progression occurs due to stepwise accumulation ofdrivermutationsin a clone of HSCsleading to clonal expansions of preleukemic HSCs, competing with normal HSC for the single HSC cell niches, with the last step forming leukemia stem cells [LSC].While random passenger mutations also occur, sometime creating new antigens; passenger mutations do not contribute to clonal expansions.The preleukemic HSC clones can become disease cells in CML, MDS, and acute leukemias.This model of preclinical cancer progression through accumulation of mutations in stem cells should apply to any somatic tissue. Work in the lab is currently focused on studying neural stem cells, their generation of oligolineage progenitors to form the brain neuropoietic tree similar to the hematopoietictree, to use them for study single cell RNAseq to discover genes enforcing or preventing each step of differentiation, and to identify genes that allow their progeny to self-renew, migrate, and differentiate in a site-appropriate manner. A current project in the lab is to isolate CNS-SC from surgical samples of brain tumors resected from patients with incurable brain cancers, to look for the order in which driver and passenger mutations occur, and to use their clonal expansion to neurospheres to work out the in vivo biological consequences of each driver mutation.

Macrophage regulation and its therapeutic application:By comparing leukemic to healthy hematopietic stem cells, the Weissman lab has identified CD47 overexpression on LSC, and then on all cancers tested. They showed that CD47 is a cell surface molecule used by cancer cells to evade macrophage phagocytosis by binding to its receptor, SIRPaon macrophages. This led to the development of a new type of immunotherapy based on macrophage checkpoint inhibition through blockade of CD47 which is perhapsthe first targetexpressed on all human cancers tested. In pre-clinical research using patient-derived xenografts, we showed dramatic effects in the treatment of diverse types of human cancer with anti-CD47 antibodies or blocking agents that neutralize the inhibitory effect of CD47-SIRPainteraction and unleashes the ability of macrophages to engulf and eliminate cancer cells.Importantly, antibody blockade of CD47 did not affect normal cells expressing CD47. This suggested that cancer cells but not normal cells display an eat me signal, which they discovered to be calreticulin, a signal recognized by macrophage prophagocytic receptor CD91. Blockade of CD47 allows macrophage removal only of cells that express calreticulin on their surface. Weissman et al discovered that calreticulin is mainly produced and secreted by activated macrophages, and that it binds to nearby cancer cells through recognition of surface asialoglycans, presumably via sialic acid removing enzymes that create the calreticulin binding sites. The molecular and cellular mechanisms for this system are currently under study. The lab has also found 3 additional dont eat me molecules and their macrophage receptors.

Our findings in collaboration with the Leeper lab re Atherosclerosis:This collaboration has shown that atherosclerotic plaque formation involves the clonal expansion of arterial smooth muscle cells from local stem or progenitor cells. These cells display both CD47 and calreticulin on their surface. Treatment with anti-CD47 antibodies in a mouse model (high fat diet on a genetically susceptible background), resulted in the elimination of atherosclerotic lesions, and preliminary studies implicate this process in human atherosclerosis, a process that leads to heart attacks, stroke, aortic aneurysms, and loss of tissues in diabetic atherosclerosis.Current research in the lab continues to explore the role of macrophages in disease prevention and treatment, through understanding how macrophages recognize their target cells, and the signals that impact the ability of these cells to maintain tissue integrity and sustain a state of health.

Stem cell research in a marine model organism:At the Hopkins Marine Station, the Weissman lab has space where its been conducting stem cell research in the marine model organismBotryllus schlosseri. This model organism has very interesting stem cell biology and immunology as related above.. It is a colonial organism, in which each individual within the colony undergoes a complete regeneration cycle weekly through a process of budding. When two adjacent colonies develop vascular anastomoses, stem cells from one colony can compete with stem cells of the other; for the germline stem cells, there is always a winner and a loser strain. Practically what this means is that stem cells from one colony invade the other and can take over the germline so that the invaded colony will now produce gametes, reproductive cells, of the genotype of its neighbor.

The discovery of this process is what led Weissman to realize that stem cells can compete and to hypothesize and then prove that both spermatogenic stem cells compete in mice, and led to the concept and proof of competition in preleukemic clonal expansions, in the leukemias and in aging.

A current list of publications

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Irving Weissman Laboratory | Institute for Stem Cell ...

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