2.1 Hematopoiesis and immune cell lineages

Hematopoiesis and Stem Cells

Hematopoiesis is the process that continuously generates all blood cells from a single source: hematopoietic stem cells (HSCs) in the bone marrow. Understanding this process is foundational to immunobiology because every immune cell you'll study traces back to these stem cells and the branching differentiation pathways they follow.

Process of Hematopoiesis

HSCs reside in specialized bone marrow niches where signals from surrounding stromal cells help maintain them. These stem cells have two defining properties: they can self-renew (making copies of themselves) and they can differentiate into any blood cell type.

When an HSC divides, it often does so asymmetrically, producing one daughter cell that remains a stem cell and one that becomes a progenitor committed to differentiation. From there, differentiation follows a hierarchy:

1. HSC divides to produce a multipotent progenitor (MPP)
2. The MPP commits to become an oligopotent progenitor, either a common myeloid progenitor (CMP) or a common lymphoid progenitor (CLP)
3. Oligopotent progenitors further narrow into lineage-committed progenitors (e.g., granulocyte-monocyte progenitor, megakaryocyte-erythroid progenitor)
4. Lineage-committed progenitors terminally differentiate into mature blood cells

This entire process is regulated by both intrinsic factors (transcription factors like GATA-1 or PU.1, plus epigenetic modifications that open or close gene regions) and extrinsic factors (cytokines, growth factors, and direct cell-cell interactions in the marrow niche).

Major Immune Cell Lineages

Three major lineages branch from the HSC. Here's how they break down:

Myeloid lineage (from the CMP):

• Granulocytes are named for their cytoplasmic granules. Neutrophils are the most abundant white blood cells and are first responders to bacterial infections. Eosinophils target parasites and modulate allergic inflammation. Basophils release histamine and other mediators in allergic responses.
• Monocytes circulate in the blood and then migrate into tissues, where they differentiate into macrophages (phagocytosis and cytokine production) or dendritic cells (antigen presentation).
• Mast cells reside in tissues near surfaces exposed to the environment (skin, gut, airways). They release preformed inflammatory mediators like histamine and play roles in allergy and parasite defense.
• Dendritic cells are the most effective antigen-presenting cells. They bridge innate and adaptive immunity by capturing antigens in peripheral tissues and presenting them to T cells in lymph nodes.

Lymphoid lineage (from the CLP):

• T cells come in several functional subsets. CD4+ helper T cells coordinate immune responses by activating other cells. CD8+ cytotoxic T cells directly kill infected or abnormal cells. Regulatory T cells (Tregs) suppress immune responses to maintain self-tolerance.
• B cells produce antibodies (immunoglobulins) and can also present antigens to T cells.
• Natural killer (NK) cells are innate lymphocytes that kill virus-infected cells and tumor cells without needing prior antigen exposure.

Megakaryocyte-erythroid lineage (also from the CMP):

• Erythrocytes (red blood cells) transport oxygen via hemoglobin.
• Platelets are fragments of megakaryocytes that facilitate clotting and wound healing.

Differentiation of Lymphocytes

Each lymphocyte type follows a distinct developmental path. Pay close attention to where maturation happens, because the site matters for the selection processes that shape the repertoire.

T cell differentiation:

1. A CLP in the bone marrow commits to the T cell lineage
2. The progenitor migrates to the thymus (this is why they're called T cells)
3. In the thymus, it passes through the double-negative (DN) stage, lacking both CD4 and CD8
4. It progresses to the double-positive (DP) stage, expressing both CD4 and CD8
5. Positive selection tests whether the T cell receptor (TCR) can recognize self-MHC molecules; cells that fail undergo apoptosis
6. Negative selection eliminates cells whose TCR binds self-antigens too strongly, preventing autoimmunity
7. Surviving cells downregulate one co-receptor, becoming single-positive (SP) CD4+ or CD8+ naive T cells that exit the thymus

B cell differentiation:

1. A CLP commits to the B cell lineage in the bone marrow
2. It progresses through pro-B and pre-B stages, rearranging immunoglobulin genes
3. It becomes an immature B cell expressing surface IgM
4. Immature B cells undergo selection against strong self-reactivity in the bone marrow
5. Surviving cells migrate to secondary lymphoid organs (spleen, lymph nodes) and mature into naive B cells co-expressing IgM and IgD

NK cell differentiation:

1. A CLP commits to the NK lineage
2. Development occurs primarily in the bone marrow, though some maturation may happen in secondary lymphoid tissues
3. Cells progress through NK precursor and immature NK stages
4. Mature NK cells acquire cytotoxic granules and activating/inhibitory receptors (such as killer immunoglobulin-like receptors) that allow them to distinguish healthy cells from stressed or infected ones

Cytokines in Immune Cell Development

Cytokines are signaling proteins that direct hematopoiesis at nearly every stage. They bind specific receptors on progenitor cells, activating intracellular signaling cascades (like JAK-STAT pathways) that regulate gene expression and cell fate decisions.

Early-acting cytokines support HSCs and multipotent progenitors:

• Stem cell factor (SCF) promotes HSC survival and maintenance
• Interleukin-3 (IL-3) stimulates proliferation of early multilineage progenitors
• GM-CSF drives differentiation of granulocyte and monocyte precursors

Lineage-specific cytokines push committed progenitors toward a single cell type:

• Erythropoietin (EPO) drives red blood cell production (produced by the kidneys in response to hypoxia)
• Thrombopoietin (TPO) stimulates megakaryocyte development and platelet production
• G-CSF promotes neutrophil production and is commonly used clinically to treat neutropenia

Lymphoid cytokines are critical for lymphocyte development:

• IL-7 is essential for both T and B cell development; without it, lymphocyte numbers drop dramatically
• IL-2 drives T cell proliferation and survival after activation
• IL-15 is required for NK cell development and homeostasis

These cytokines don't act in isolation. They form networks with synergistic effects (e.g., SCF + IL-3 together promote more proliferation than either alone) and antagonistic effects that fine-tune cell production. Feedback loops maintain homeostasis, ramping up production of specific cell types during infection and scaling back during resolution.