Hématopoïèse et différenciation des cellules immunitaires

Hematopoiesis and Ontogenesis of Immunity Cells

Hematopoiesis is the process by which all blood cells are produced, generating to cells daily. This complex process originates from hematopoietic stem cells (HSCs), which are pluripotent cells capable of self-renewal and differentiation into various blood cell types. HSCs give rise to two main progenitor types: myeloid progenitor cells and lymphoid progenitor cells.

The immune system comprises cells from both the myeloid and lymphoid lineages, contributing to innate and adaptive immunity.

Myeloid Lineage Differentiation

Cells of the myeloid lineage are often the first responders to infection and include:

• Granulocytes: Part of the innate immune system, characterized by a multilobed nucleus and granular cytoplasm. They include:
  • Neutrophils: (40-75% of WBCs) Primarily involved in phagocytosis of bacteria and fungi. They have a short lifespan of 6 hours to a few days.
  • Eosinophils: (1-6% of WBCs) Involved in degranulation and release of enzymes, targeting parasites and allergic reactions.
  • Basophils/Mast cells: (<1% of WBCs) Release histamine and other mediators involved in inflammation and immediate hypersensitivity reactions. Basophils circulate, while mast cells reside in tissues.
• Myeloid/Antigen-Presenting Cells (APCs): Phagocytic cells that include:
  • Monocytes: Circulating cells (2-6% of WBCs) that differentiate into macrophages and dendritic cells. They are kidney-shaped and capable of phagocytosis and antigen presentation.
  • Macrophages: Resident in various tissues (e.g., Kupffer cells in the liver, microglia in the CNS). They perform phagocytosis and antigen presentation to T cells, with a lifespan of months to years.
  • Dendritic cells: Crucial for initiating immune responses by presenting antigens.
• Erythrocytes (RBCs): Derived from the common myeloid precursor, responsible for oxygen transport.
• Megakaryocytes: Large myeloid cells that produce thrombocytes (platelets), essential for blood clotting.

Myeloid Cell Characteristics Summary

Cell	% in adults	Nucleus	Functions	Lifetime	Main targets
Macrophage	Varies	Varies	Phagocytosis; Antigen presentation to T cells	Months - years	Various
Neutrophil	40-75%	Multi-lobed	Phagocytosis; Degranulation	6 hours - few days	Bacteria; Fungi
Eosinophil	1-6%	Bi-lobed	Degranulation; Release of enzymes, growth factors, cytokines	8-12 days	Parasites; Allergic tissues
Basophil	<1%	Bi- or tri-lobed	Degranulation; Release of histamine, enzymes, cytokines	Uncertain	Allergic tissues
Monocyte	2-6%	Kidney shaped	Differentiate into macrophages and dendritic cells	Hours - days	Various

Lymphoid Lineage Differentiation

Cells of the lymphoid lineage regulate the adaptive immune response and include lymphocytes, which constitute 20-40% of WBCs in blood and 99% of lymph cells.

• Natural Killer (NK) Cells: Represent up to 15% of circulating lymphocytes. They are crucial for anti-tumor and anti-infectious immunity, acting as an interface between innate and adaptive immunity. NK cells lack TCRs or BCRs but use activating and inhibitory receptors to regulate their function.
  • Killing Activity: NK cells recognize "induced self" (tumor antigens) or "missing self" (lack of MHC class I) and can induce apoptosis in target cells through granule release (perforin and granzymes) or Antibody-Dependent Cellular Cytotoxicity (ADCC).
  • Immune Modulation: They produce cytokines like IFN-, IL-15, IL-12, IL-18, regulating inflammation and orienting adaptive responses.
  • Phenotypes: CD56 DIM (CD16+) are highly cytotoxic, while CD56 BRIGHT (CD16-) are better cytokine producers.
• NKT Cells: Express both NK cell markers (CD56, CD16) and T cell markers (TCR, CD3). They recognize glycolipids presented by CD1d on APCs, leading to cytokine production.
• T Cells: Constitute 1-5% of circulating lymphocytes and are abundant in epithelia. They have a limited TCR repertoire, recognize conserved pathogen epitopes without MHC restriction, and perform cytotoxicity and cytokine production.

Adaptive Immunity Cells

These cells are responsible for specific, memory-driven immune responses.

• B Cells:
  • Maturation: Occurs in the bone marrow (BM). Pro-B cells rearrange heavy chain genes, Pre-B cells express a pre-BCR (heavy chain + pseudo-light chain), Immature B cells express IgM and undergo negative selection, and Mature B cells co-express IgM and IgD.
  • Function: Activated B cells differentiate into plasma cells, which produce antibodies (immunoglobulins, Ig) to eliminate extracellular microorganisms. They also act as APCs.
  • B Cell Receptor (BCR): A membrane complex consisting of a monomeric immunoglobulin (IgM) and a dimer of Ig-Ig for signal transduction. Each B cell expresses a unique, specific BCR that recognizes native antigens. The diversity of BCRs is generated by gene rearrangements of V, D, J, and C genes.
• T Cells:
  • Maturation: Occurs in the thymus. Double-negative (CD4-CD8-) thymocytes progress to double-positive (CD4+CD8+) stage, undergoing TCR gene rearrangement. This is followed by positive selection (recognition of self-MHC) and negative selection (elimination of self-reactive cells), leading to single-positive CD4+ or CD8+ cells.
  • Function:
    • Helper T cells (CD4+): Recognize antigens presented by MHC Class II molecules. They assist other immune cells, such as B cells, in antibody production.
    • Cytotoxic T cells (CD8+): Recognize antigens presented by MHC Class I molecules. They destroy intracellularly infected cells and tumor cells.
    • Regulatory T cells: Suppress immune responses.
  • T Cell Receptor (TCR): A heterodimer (usually ) that recognizes antigens associated with MHC molecules on APCs, forming a ternary complex (TCR + Ag + MHC). The TCR is associated with the CD3 complex, which has long intracytoplasmic regions with ITAM patterns to transmit intracellular signals, as the TCR's intracytoplasmic region is too short.

Summary of Immunity

The immune system relies on both innate immunity (epithelial barriers, phagocytes, complement, NK cells) for immediate, non-specific defense and adaptive immunity (B and T lymphocytes) for specific, memory-driven responses. HSCs are the origin of all these diverse immune cells, which undergo precise differentiation and maturation processes to fulfill their specialized roles in maintaining health and fighting pathogens.