Premed · Premed · Immunology
Lecture 3: Hematopoiesis and Lymphocyte Development
Immunology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the process of hematopoiesis and the hierarchy of hematopoietic stem cells and progenitors
- Identify the key cytokines and growth factors that regulate blood cell development
- Outline the stages of B cell development in the bone marrow
- Outline the stages of T cell development in the thymus
- Explain the role of the bone marrow and thymic microenvironments in supporting lymphocyte development
Lecture Content
I. Overview of Hematopoiesis
Hematopoiesis is the process by which all blood cells are generated from a common precursor. In adults, this process occurs in the bone marrow, whereas during embryonic development it takes place in the fetal liver and yolk sac. All blood cells derive from hematopoietic stem cells (HSCs), which are self-renewing, multipotent cells that reside in specialized bone marrow niches (the endosteal niche and the perivascular niche). HSCs are identified by the markers CD34+, CD38-, Lin- (lineage negative), Sca-1+ (in mice), and c-Kit+.
HSCs give rise to two major progenitor lineages. The common myeloid progenitor (CMP) generates granulocytes, monocytes, erythrocytes, and megakaryocytes. The common lymphoid progenitor (CLP) gives rise to T cells, B cells, NK cells, and innate lymphoid cells. Within the myeloid lineage, the granulocyte-monocyte progenitor (GMP) produces neutrophils, eosinophils, basophils, and monocytes, while the megakaryocyte-erythrocyte progenitor (MEP) gives rise to megakaryocytes (and hence platelets) and erythrocytes. Within the lymphoid lineage, CLPs generate B cell progenitors that develop in the bone marrow, T cell progenitors that migrate to the thymus for development, and NK cells and ILCs.
<image>A hierarchical tree diagram of hematopoiesis. At the top, a single hematopoietic stem cell (HSC) branches into a common myeloid progenitor (CMP, left branch) and a common lymphoid progenitor (CLP, right branch). The CMP further branches into: granulocyte-monocyte progenitor (giving rise to neutrophils, eosinophils, basophils, monocytes/macrophages, dendritic cells) and megakaryocyte-erythrocyte progenitor (giving rise to megakaryocytes/platelets and erythrocytes). The CLP branches into B cells, T cells, NK cells, and ILCs. Each mature cell type is illustrated with its characteristic morphology. Key cytokines are annotated along each differentiation pathway (e.g., IL-7 for lymphoid, M-CSF for monocytes, G-CSF for neutrophils, EPO for erythrocytes, TPO for megakaryocytes).</image>
II. Cytokines and Growth Factors Regulating Hematopoiesis
A network of cytokines and growth factors governs the differentiation of blood cells at every stage. Stem cell factor (SCF, or Kit ligand) binds c-Kit on HSCs and is essential for their survival, proliferation, and maintenance. Flt3 ligand supports early progenitor expansion and dendritic cell development. Interleukin-7 (IL-7) is critical for lymphoid development, promoting the survival and proliferation of both B and T cell progenitors. IL-3 is a multi-lineage growth factor that supports early hematopoietic progenitors broadly.
Within the myeloid lineage, granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulates the production of granulocytes and monocytes, while G-CSF drives neutrophil production specifically and is used clinically as filgrastim to treat neutropenia. M-CSF promotes monocyte and macrophage differentiation. Erythropoietin (EPO), produced by the kidneys in response to hypoxia, drives erythrocyte production, and thrombopoietin (TPO) drives megakaryocyte development and platelet production. IL-5 is specific for eosinophil development and activation.
III. B Cell Development in the Bone Marrow
B cells develop through an ordered series of stages characterized by progressive immunoglobulin gene rearrangement. The earliest committed B lineage cell is the pro-B cell, in which D-J rearrangement of the heavy chain locus begins, followed by V-DJ rearrangement. At this stage the cell expresses CD19 and CD10 but no surface immunoglobulin. Successful heavy chain (μ) rearrangement marks the transition to the pre-B cell stage, where the μ heavy chain pairs with a surrogate light chain (VpreB + λ5) to form the pre-BCR. Signaling through the pre-BCR drives proliferation and initiates light chain rearrangement, which proceeds through V-J joining at the κ locus first and, if unsuccessful, at the λ locus.
The immature B cell expresses complete IgM on its surface (μ heavy chain paired with a light chain) and undergoes negative selection as part of central tolerance. Immature B cells that bind self-antigen strongly face one of three fates: receptor editing (attempting a new light chain rearrangement), anergy (functional unresponsiveness), or clonal deletion (apoptosis). Cells that pass this checkpoint exit the bone marrow as transitional B cells, migrate to the spleen, and continue to mature. The final product is the mature naive B cell, which co-expresses IgM and IgD on its surface and recirculates through secondary lymphoid organs awaiting antigen encounter.
Two key checkpoints govern this process. The pre-BCR checkpoint ensures a functional heavy chain has been produced and enforces allelic exclusion so that only one heavy chain allele is expressed. The immature B cell checkpoint tests for self-reactivity and enforces central tolerance.
<image>A stepwise diagram of B cell development in the bone marrow. Five stages are shown left to right: Pro-B cell, Pre-B cell, Immature B cell, Transitional B cell, and Mature naive B cell. For each stage, the diagram shows: (1) the immunoglobulin gene rearrangement status (D-J, V-DJ for heavy chain; V-J for light chain), (2) surface molecules expressed (pre-BCR with surrogate light chain at pre-B stage, IgM at immature stage, IgM+IgD at mature stage), (3) key transcription factors (E2A, EBF, Pax5), and (4) selection events (pre-BCR checkpoint, negative selection with arrows showing receptor editing, anergy, or deletion). The bone marrow microenvironment (stromal cells providing IL-7 and SCF) is depicted as the background.</image>
IV. T Cell Development in the Thymus
T cell progenitors migrate from the bone marrow to the thymus via the blood, where development occurs through defined stages as thymocytes migrate from the cortex to the medulla. The earliest thymic cells are double-negative (DN) thymocytes (CD4-CD8-), which progress through four substages (DN1 through DN4). TCR β chain gene rearrangement occurs at the DN3 stage, and a successfully rearranged β chain pairs with pre-Tα to form the pre-TCR. Signaling through the pre-TCR triggers the β-selection checkpoint, driving proliferation and progression. TCR α chain rearrangement occurs subsequently.
At the double-positive (DP) stage (CD4+CD8+), thymocytes express a complete αβ TCR at low levels and undergo positive selection in the thymic cortex. During positive selection, the TCR must recognize self-MHC with moderate affinity: if it recognizes MHC class II, the cell becomes a CD4+ T cell; if it recognizes MHC class I, it becomes a CD8+ T cell; and if it fails to recognize any MHC at all, it dies by neglect through apoptosis. Approximately 95% of DP thymocytes die at this stage.
Surviving cells become single-positive (SP) thymocytes (either CD4+ or CD8+) and migrate to the thymic medulla, where they undergo negative selection. Thymocytes with high affinity for self-antigen bound to MHC are deleted by apoptosis. Medullary epithelial cells express AIRE (autoimmune regulator), a transcription factor that drives expression of tissue-restricted antigens in the thymus, allowing deletion of T cells that would otherwise attack peripheral organs. Dendritic cells in the medulla also participate in negative selection. Notably, some self-reactive CD4+ cells are diverted to become regulatory T cells (Tregs) rather than being deleted. Mature naive T cells that survive both selection processes exit the thymus and circulate to secondary lymphoid organs.
V. Transcription Factors in Lymphocyte Development
Specific transcription factors commit progenitors to particular lymphocyte lineages. In the B cell lineage, E2A and EBF1 initiate the B cell program, Pax5 commits progenitors to the B cell fate by repressing alternative lineages, and IRF4 and Blimp-1 drive plasma cell differentiation at later stages. In the T cell lineage, Notch1 signaling is essential for T cell commitment in the thymus, GATA-3 supports T cell development and later Th2 differentiation, TCF-1 and Bcl11b promote T cell commitment, ThPOK drives CD4 lineage commitment, Runx3 drives CD8 lineage commitment, and FoxP3 is the master regulator of Treg development and function.
VI. Bone Marrow and Thymic Microenvironments
The success of lymphocyte development depends critically on signals from the surrounding microenvironment. Bone marrow stromal cells provide contact-dependent signals through molecules such as VCAM-1, SCF, and CXCL12, and they secrete IL-7, SCF, and Flt3 ligand. These stromal cells create specialized niches for HSC maintenance and B cell development. In the thymus, thymic epithelial cells play distinct roles depending on their location. Cortical thymic epithelial cells (cTECs) mediate positive selection and express MHC I and II along with unique proteolytic machinery including the thymoproteasome (with its β5t subunit). Medullary thymic epithelial cells (mTECs) mediate negative selection and express AIRE. Additionally, nurse cells in the cortex engulf and nurture developing thymocytes, while thymic dendritic cells participate in negative selection.
<image>A schematic cross-section of the thymus showing the journey of a developing T cell. Starting at the corticomedullary junction where bone marrow progenitors enter (DN1 stage), arrows trace the path through the cortex (DN2-DN4, beta-selection, then DP stage with positive selection by cortical epithelial cells) and into the medulla (SP stage with negative selection by medullary epithelial cells expressing AIRE and dendritic cells). Failed cells are shown undergoing apoptosis at each checkpoint. Mature single-positive T cells exit via blood vessels at the corticomedullary junction. Key molecular interactions are annotated: Notch1-DLL4 (commitment), pre-TCR (beta-selection), TCR-MHC (positive and negative selection).</image>
