Lecture 13: Mesenchymal Stem Cells and Other Tissue Stromal Cells PDF

🥽 Lecture 13: Mesenchymal stem cells and other tissue stromal cells Mesenchymal Stem Cells Bone Marrow Primary site of hematopoiesis and B cell development in adulthood The bone mar...

🥽 Lecture 13: Mesenchymal stem cells and other tissue stromal cells Mesenchymal Stem Cells Bone Marrow Primary site of hematopoiesis and B cell development in adulthood The bone marrow contains hematopoietic (stem cells) and stromal cells - which are surrounded by vascularised and innervated bone Bone marrow is distributed throughout the metaphysis → Minute projections of bone, known as trabeculae, are found within this region The endosteum is the interface between the bone and soft marrow Endosteum is covered by cells that line the bone surface, including: Osteoblasts: Responsible for bone formation. Osteoclasts: Responsible for bone resorption. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 1  The endosteal surface contains a rich network of blood vessels and sinusoids → this network allows cells to pass in and out of circulation Bone Marrow Composition The bone marrow consists of hematopoietic cells and stromal cells located in the hematopoietic compartment. Haematopoietic Hematopoietic Stem Cells (HSCs): Responsible for the formation of blood cells, including RBC, WBC, platelets Supportive stroma The bone marrow contains stromal cells with supportive functions, including: Fibroblasts: Form reticular connective tissue. Macrophages: Phagocytic cells. Adipocytes: Fat cells. Osteoblasts: Cells that form bone. Osteoclasts: Cells that resorb bone. Endothelial Cells: Form sinusoids, the blood vessels in the bone marrow. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 2  Haematopoietic stem cell niche Definition: The HSC niche refers to the local tissue microenvironment that maintains and regulates hematopoietic stem cells. Function: Control HSC quiescence (rest), proliferation, self-renewal and differentiation Mechanisms of Crosstalk in the HSC Niche Cell-to-Cell Contact: Communication via adhesion molecules and gap junctions. Cytokines and Chemokines: Signaling molecules such as CXCL12 mediate crosstalk. Growth Factors: Facilitate communication and regulation. Extracellular Matrix (ECM) Components: Include proteins such as fibronectin, which support the niche environment. The HSC niche is made up of several cell types, including mesenchymal stem cells (MSCs). Lecture 13: Mesenchymal stem cells and other tissue stromal cells 3  Role of Mesenchymal Stem Cells (MSCs) in the Hematopoietic Stem Cell Niche MSCs express chemokines, growth factors, and cytokines crucial for maintaining HSC function. MSCs can be found in multiple regions, including: Perivascular Locations: Near blood vessels. Central Marrow: In the central region of the bone marrow. Endosteal Region: Near the bone surface. MSCs are tightly associated with sympathetic nerve fibers, which play a role in regulating HSC mobilisation. Depletion of MSCs decreases the content of HSCs in the bone marrow Several populations of MCS are found in different anatomical regions of the bone marrow, and these MCS expressed different marker Lecture 13: Mesenchymal stem cells and other tissue stromal cells 4  Not All MSCs Are Stem Cells Physiological role of MSCs: Maintain HSCs and support haematopoiesis Replenish osteoblasts and adipocytes Generate cartilage under specific conditions Defining MSCs True MSCs must demonstrate the ability to generate fully differentiated tissues in vivo as a test of their multipotency. MSCs must reconstitute cells in vivo that are identical in phenotype and potency, as a test of self-renewal. When transplanted in vivo, MSCs must be able to form heterotopic bone, also known as an ossicle. The ossicle is composed of bone, adipocytes, and fibroblasts. This bone is formed from a single clonogenic progenitor. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 5  Impact of Ageing With aging, there is an increase in adipocytes in the bone marrow, a process known as fatty degeneration. Implication → the increase in adipocytes affects HSC engraftment after stem cell transplantation and bone repair following trauma. Effects of Adipocyte Accumulation on Hematopoiesis Bone marrow adipocyte deposits suppress hematopoiesis (negative regulation) Key findings of a study: Fatless Mice: Mice with fewer adipocyte deposits following lethal irradiation showed enhanced hematopoietic recovery after stem cell transplantation. PPAR Gamma Inhibition: Pharmacological inhibition of adipocyte formation with a PPAR gamma inhibitor enhanced hematopoietic recovery (shown by black line) compared to control mice (shown by yellow line). Lecture 13: Mesenchymal stem cells and other tissue stromal cells 6  MSCs in the Aging Process → reduced osteogenic potential → less bone (more fat) Changes in MSC Population: Increased MSC Markers in Aged Mice: An increase in PDGFRα and CD51-positive MSCs was observed in aged mouse bone marrow (left panel). Lower Clonogenic Potential: Aged MSCs have reduced capacity to form fibroblastic colony-forming units (middle panel). Reduced Expression of Essential Genes: Aged MSCs express lower levels of key genes required for HSC maintenance, including: CXCL12, Stem Cell Factor, Angiopoietin Lecture 13: Mesenchymal stem cells and other tissue stromal cells 7  Impact of Aging on HSC function Young HSCs: HSCs maintain balanced lineage output. Increased homing to bone marrow Aged HSCs: Increase in the number of HSCs. Aged HSCs mobilize in higher numbers into the circulation but are less effective. Aged HSCs are skewed toward the myeloid lineage, resulting in decreased lymphoid output. Reduced ability to home to the bone marrow. Reside further away from the endosteum. Decreased competitive repopulating ability. “Home” here refers to the process by which hematopoietic stem cells (HSCs) migrate to, recognise, and establish themselves within their specific niche in the bone marrow. Homing is crucial after stem cell transplantation or during normal circulation, as it allows HSCs to return to the bone marrow, where they can settle and function properly to support blood cell production. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 8  SUMMARY - MESENCHYMAL STEM CELLS BM is the site of adult haematopoiesis and B cell development. HSC niche maintains and regulates HSCs via cell-cell contact, cytokines and chemokines, growth factors and extracellular matrix components. MSCs are a component of HSC niche. Not all MSCs are stem cells In vivo test for multipotency and self-renewal – formation of bone, adipocytes and fibroblasts (ossicle) from a single clonogenic progenitor. Aged MSCs make less bone, more fat, less clonogenic, less able to support HSCs Increased adipocytes negatively affect HSC engraftment and haematopoiesis. Aged HSCs skew to myeloid lineage, less repopulating capacity, less able to home to BM Stromal cells Adipose-derived stromal cells (ADSCs) Form an essential part of the stromal microenvironment in adipose tissue by: Providing structural support. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 9  Regulating immune homeostasis of adipose tissue. Isolation of ADSCs from Fat ADSCs are commonly isolated by spinning down lipoaspirate, which separates: Oil, adipose tissue, and fluid. The stromal vascular fraction (SVF), which contains various cell types: hematopoietic cells, preadipocytes, endothelial cells, pericytes, ADSCs ADSCs lack in vivo demonstration of multipotency and self-renewal, distinguishing them from true stem cells ⇒ they are NOT stem cells Immunoregulatory role of ADSCs ADSCs are the primary source of Interleukin 33 (IL-33), an alarmin essential for: The function and expansion of various immune cells. Supporting immune cell types, including: Regulatory T cells (Tregs) Innate lymphoid cells (ILCs) Lecture 13: Mesenchymal stem cells and other tissue stromal cells 10  Macrophages Impact of ageing and sex AGING Aging leads to an accumulation of ADSCs. This accumulation results in an increase in IL-33 levels, which causes: Expansion of adipose tissue regulatory T cells (Tregs). Metabolic abnormalities. SEX In males, there is a higher number of IL-33-producing ADSCs and an increase in adipose Tregs. This predisposes males to: Obesity-induced inflammation. Insulin resistance. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 11  Stromal cells in the lymph node Role of Lymph Nodes in Immunity: Lymph nodes are distributed throughout the body, serving as sentinels of the immune system. They play a key role in immune surveillance by allowing lymphatic vessels to carry lymph into the node. Lymphatic System Function: Lymph drains from peripheral tissues into lymph nodes, bringing: Antigens, Soluble mediators, Antigen-presenting cells These components are brought into close proximity with lymphocytes, which is essential for lymphocyte activation and the initiation of an immune response. Lymph Node Organisation: Lymph nodes are highly organised structures: B cells are found in follicles. T cells are located in the paracortex. Macrophages and plasma cells reside in the medulla. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 12  Fibroblastic reticular cells (FRCs) Mesenchymal origin. FRCs create and maintain the structure of the lymph node. Form conduits for dissemination of fluid, chemokines, cytokines and antigens. Produce chemokines and cytokines for immune cell survival and trafficking. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 13  Thymus Primary site of T cell development It is essential for immune function, as the absence of the thymus leads to a lack of T cells and we become immunocompromised. Eg) Patients with DiGeorge syndrome lack a functional thymus and thus lack T cells, resulting in immunodeficiency. Structure of the Thymus: The thymus is a bilobed organ located behind the sternum, in front of the heart, and between the lungs. The lobules of the thymus are separated by trabeculae. Organisation of the Thymus: The thymus is a highly organised structure with two main regions: Outer Cortex, Inner Medulla Lecture 13: Mesenchymal stem cells and other tissue stromal cells 14  Stromal cells in the thymus The thymic stromal compartment includes various cell types: Epithelial Cells: Cortical epithelial cells in the cortex. Medullary epithelial cells in the medulla. Mesenchymal Cells or Fibroblasts Endothelial Cells Neuroendocrine Cells Lecture 13: Mesenchymal stem cells and other tissue stromal cells 15  Thymic cross-talk Definition: Reciprocal interactions between thymic stromal cells and developing thymocytes, known as thymic cross-talk, are crucial for the development and function of both compartments. T Cell Development and Spatial Cues: T cell development requires spatial cues from thymic stromal cells. As thymocytes migrate through different regions of the thymus during differentiation and maturation, they rely on specific growth factors and cytokines provided by the stromal cells in each region. Reciprocal Influence on Thymic Epithelium: The development and maintenance of the thymic epithelium also require interactions with thymocytes. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 16  Thymic T cell Development Intro: T cell development in the thymus is essential for producing self- tolerant, immune-competent T cells that can respond to pathogens but do not react to self-antigens Process: (1) Bone marrow-derived precursors enter the thymus through the cortical-medullary junction (2) The precursors migrate through the cortex to the subcapsular region as they undergo T cell receptor (TCR) gene rearrangement, committing to the T cell lineage. At this stage - thymocytes do not express CD4 or CD8, hence are termed double negative (DN). Lecture 13: Mesenchymal stem cells and other tissue stromal cells 17  (3) Thymocytes upregulate both CD4 and CD8, becoming double positive (DP) cells. These cells undergo positive selection in the cortex, where cells are selected for their ability to interact with self major histocompatibility complex (MHC). (4) Following positive selection, cells differentiate into either: CD4- positive cells or CD8-positive cells These single positive (SP) cells then move to the medulla for negative selection. In the medulla, negative selection removes autoreactive cells that bind too strongly to self MHC, ensuring self-tolerance. Summary: 1. Commitment to the T Cell Lineage: Requires Delta-like 4 ligand (DLL4) expressed by cortical epithelial cells in the cortex. 2. Positive Selection: Occurs in the cortex. T cell receptors (TCRs) on thymocytes must recognize self- MHC molecules presented by cortical epithelial cells. 3. Negative Selection: Takes place in the medulla. Mediated by: Lecture 13: Mesenchymal stem cells and other tissue stromal cells 18  Medullary epithelial cells, which express peripheral tissue antigens in an ectopic manner. Thymic dendritic cells, which present self-antigens to eliminate autoreactive thymocytes Dysregulation of thymic stroma Disruptions in the thymic stroma can affect thymocyte selection events, leading to the escape of autoreactive thymocytes. Example: In the non-obese diabetic (NOD) mouse model: Severe abnormalities in thymic architecture occur before the onset of spontaneous autoimmunity. The thymic microenvironment is highly sensitive to damage from: Chemotherapy, Immunosuppressants, Infections, Pregnancy, Graft- versus-host disease (GVHD) The thymus, particularly in younger individuals, has a capacity for regeneration after damage. This recovery depends on the production of growth factors by thymic stromal cells. Impact of ageing on thymus - thymic atrophy Thymic atrophy - a gradual decline in thymic function with age Characteristics of Thymic Atrophy: Loss of Thymocytes. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 19  Collapse of the Thymic Epithelium. Loss of a Distinct Cortical-Medullary Junction. Impact on Cellularity: In mice, the aged thymus retains only 5% of the cellularity of an adult thymus. In humans: By age 70, the epithelial space occupies less than 10% of the total thymic tissue. There is a significant increase in non-epithelial stromal cells. Major invasion of adipocytes is observed. Overall impact: Reduction in T Cell Progenitors - Decrease in bone marrow and intrathymic T cell progenitors. Decline in Thymopoiesis - Decreased proliferation and increased apoptosis of thymocytes. Decrease in T Cell Export - Results in an overall reduction in the diversity of the T cell receptor (TCR) repertoire in the peripheral T cell pool. Reduced diversity T-cell receptor repertoire Why Do We Care About Thymic Atrophy? 1. T Cell Diversity and Immunity: Lecture 13: Mesenchymal stem cells and other tissue stromal cells 20  T cells display enormous diversity due to T cell receptor (TCR) gene rearrangement. Each T cell has a unique receptor that recognizes one specific peptide antigen. Broad immunity requires large numbers of T cells, each recognizing different antigens. 2. Impact of Thymic Atrophy on T Cell Diversity: As the thymus degenerates with age, the production of new T cells with diverse specificities decreases. 3. Compensatory Mechanism: Peripheral T cells undergo homeostatic expansion, involving selective clonal expansion without the replacement of new, diverse T cells. This leads to the peripheral T cell pool being biased toward past exposures and developing immunologic memory. 4. Consequences of Reduced T Cell Diversity: Decreased ability to respond to evolving pathogens and vaccinations. Delayed recovery of T cell diversity in cases such as: Radiation or chemotherapy. Bone marrow transplantation SUMMARY - STROMAL CELLS IN OTHER TISSUES Lecture 13: Mesenchymal stem cells and other tissue stromal cells 21  MSCs have been isolated from other tissues Not strictly stem cells in the absence of in vivo multipotency and self- renewal. Stromal cells provide structural support and chemical cues for immune cell regulation ADSCs express IL-33 necessary for Treg expansion in adipose tissue. Ageing and androgens have important consequences (more ADSCs, increased IL-33 levels, adipose Treg expansion). Metabolic abnormalities, obesity-induced inflammation, insulin resistance. Lymph nodes filter lymph and bring together antigens, antigen-presenting cells and antigen responsive cells to initiate immune responses. FRCs create conduits, produce chemokines to direct movement of immune cells, and cytokines for immune cell survival and function. The thymus produces competent and self-tolerant T cells. Thymic stromal cells crucial for T cell commitment, positive and negative selection, and thymic recovery from damage. Ageing has important consequences (thymic atrophy, loss of T cell diversity). Hormonal regulation provides opportunities for regeneration. Lecture 13: Mesenchymal stem cells and other tissue stromal cells 22