Lymphocyte Study Notes

2 - Lymphocytes

Objectives

  1. Describe the function of lymphocytes (T cells, B cells, NK cells).
  2. Outline and describe the development of lymphocytes including distinguishing maturation and morphologic features of cells of the lymphocytic lineage.
  3. Describe the morphology of the activated or reactive lymphocyte.
  4. Summarize the distribution and state the concentration of lymphocytes in peripheral blood.
  5. List causes/conditions associated with an increase or decrease in the absolute numbers of lymphocytes found in the peripheral blood.
  6. List the age-related reference intervals for peripheral blood lymphocytes.
  7. Define antigen-presenting cell and identify cells that have this functional capability.
  8. Outline and describe the hierarchy of lymphocyte development.
  9. Describe and compare T and B lymphocyte developmental stages.
  10. Compare and contrast the immunologic features and functions of the various types of lymphocytes found in peripheral blood (B cells; CD8+ T cells; CD4+ T cells, including subsets).
  11. Describe the synthesis of immunoglobulin including heavy and light chain gene rearrangement.
  12. Summarize molecular characteristics used to differentiate lymphocyte subtypes.
  13. Summarize the kinetics of the lymphocytic lineage.
  14. Differentiate between polyclonal and monoclonal gammopathies and describe each type in relationship to a patient’s clinical condition.

Introduction

  • Lymphocytes develop from multipotential hematopoietic stem cells (HSCs) in the bone marrow.
  • They differentiate into three main types: T cells, B cells, and Natural Killer (NK) cells.
  • These cells are all involved in the immune response (IR).

Primary Function of Lymphocytes

  • Recognition and Reaction with Specific Antigens: Lymphocytes identify pathogens through specific receptors.
  • Collaboration with Macrophages: They work alongside macrophages to eliminate pathogens.
  • Provision of Long-lasting Immunity: They provide immunity against previously encountered pathogens.
  • Lifespan: Lymphocytes have a varied lifespan, with some living for several years while others may only survive for hours or days.

Effector Cells of the Adaptive Immune Response

  • Lymphocytes act as principal effector cells of the adaptive immune response:
    • Development Rate: Their development is gradual.
    • Response Generation to Eliminate Pathogens: Lymphocytes generate specific responses aimed at pathogens.
    • Involvement of Various Cells: This process involves lymphocytes, monocytes, and other antigen-presenting cells.
    • Immunologic Memory: This allows for rapid responses upon re-exposure to the same antigens.
    • Types for Immune Response: Lymphocytes are involved in two main types of immunity:
    • Humoral immunity (mediated by B cells)
    • Cell-mediated immunity (mediated by T cells).

Types of Lymphocytes

  1. T Lymphocytes (T cells):

    • Primarily involved in cell-mediated immunity (CMI).
    • Requires interaction with macrophages and antigens.
    • Three functional subsets:
      • Helper T lymphocytes (Th): Involved in supporting B cells and other T cells.
      • Cytotoxic T lymphocytes (CTLs): Responsible for directly killing infected cells.
      • Regulatory T cells (TReg): Help in regulating immune responses.

  2. B Lymphocytes (B cells):

    • Primary effector cells for adaptive humoral immune response (antibody production).
    • Need to be activated to differentiate into plasma cells.
    • Two main subsets:
      • B-1 B cell: Found in serous cavities, produces natural antibodies.
      • B-2 B cell: Typical B cells that respond to specific antigens.

  3. Natural Killer (NK) Cells:

    • Effectors of innate immunity.
    • Function independently of the adaptive immune response.
    • Responsible for the cytolysis of tumor cells and pathogen-infected cells.

Lymphopoiesis

  • Historical Perspective:

    • HSCs give rise to common lymphoid progenitors (CLP) and common myeloid progenitors (CMP).
    • CLP differentiates into T cells, B cells, NK cells, and some dendritic cells.
    • Differentiation begins during fetal liver development and primarily occurs in the bone marrow after birth.

  • New Findings:

    • Identification of Lymphoid-Primed Multipotential Progenitor (LMPP): capable of full lymphoid development with limited myeloid potential.

  • Commitment to Lymphocytic Lineage:

    • CLPs are influenced by transcription regulators and cytokines crucial in lymphocyte development, including:
      • Common gamma chain (γc) family cytokines: IL-2, -4, -7, -9, -15, -21.

  • Phases of Lymphopoiesis:

    • Antigen-Independent: Occurs in primary lymphoid organs (Bone marrow and thymus).
    • Antigen-Dependent: The interaction with specific antigens leading to differentiation into effector T and B lymphocytes.

Transcription Factor Regulation of Differentiation (Lymphoid Lineage)

  • Early lymphoid progenitor cells:
    • Transcription Factors for T and B: Ikaros, Pu.1, E2A.
    • T lymphocytes: GAT A-3, TCF1, Bcl-11b.
    • B lymphocytes: EBF, E2A, Pu.1, Pax-5.
    • Natural Killer lymphocytes: Id2, Ikaros, E4BP4, Eomes, Tbet.

Lymphocyte Types and Function

Effector Functions

  1. Effector T Cells:

    • Responsible for executing cell-mediated cytotoxic reactions (CTLs or CTCs).
    • They provide help for B cells and macrophages.
    • Regulatory subsets suppress other T-cell immune responses.

  2. B Cells:

    • Concentrate and present antigens to T cells.
    • Serve as precursors to immunoglobulin-secreting plasma cells.
    • Programmed to generate specific antibodies through gene rearrangement of immunoglobulin genes.

  3. NK Cells:

    • A form of cytotoxic lymphocyte involved in the innate immune response.
    • Play a vital role in recognizing and killing tumor cells and pathogen-infected cells.

T and B Lymphocytes Developmental Stages

  1. B Lymphocytes:

    • Precursor B cells are produced in the bone marrow and undergo antigen-independent development, exiting as naive B lymphocytes (15–30% of peripheral blood lymphocytes).
    • BCR (B Cell Receptor) allows recognition of specific antigens, comprised of Ig molecules identical to those produced by mature plasma cells.
    • Immunoglobulin consists of two heavy and two light chains linked by disulfide bonds.
    • Heavy chains exist in five types (IgA, IgD, IgE, IgG, IgM).
    • B lymphocytes undergo differentiation after antigen encounter, leading to activated B cells that can proliferate into memory cells or antibody-secreting plasma cells.

  2. T Lymphocytes:

    • Developing T cells (thymocytes) originate from the bone marrow and undergo maturation in the thymus.
    • Consist of several stages defined by TCR gene rearrangement and presence of CD markers (CD1, CD2).
    • Major histocompatibility complex (MHC) plays a significant role in T cell activation through antigen presentation.

  3. Natural Killer Cells:

    • Represent a unique population of lymphoid cells with distinct markers (CD56+, CD16+) differing from T and B cells, crucial for innate immunity.

Distribution of Lymphocytes

  • Distribution is approximately 5% in peripheral blood and 95% in extravascular lymphoid tissue (spleen/nodes).
  • Long-lived memory cells account for about 80% of lymphocytes in peripheral blood.
  • Lifespan of lymphocytes varies, from mere hours to several years according to their type and function.

Kinetics of Lymphocyte Development

  • Normal peripheral blood concentrations for adults range from 1.0 - 4.8 imes 10^3/ ext{µL}.
  • At birth, the concentration is approximately 5.5 imes 10^3/ ext{µL}, rising to 7.0 imes 10^3/ ext{µL} within six months, then decreasing until adulthood.
  • **Definitions: **
    • Lymphocytosis: Increase in absolute lymphocyte counts to > 4.8 imes 10^3/ ext{µL}.
    • Lymphocytopenia: Decrease in absolute lymphocytes to < 1.0 imes 10^3/ ext{µL}.

Morphological Characteristics of Lymphocytes

  • Lymphoblast:

    • Shape: Round to oval, Size: 10–18 µm
    • Nucleus: Centrally located, may be indented, nucleoli present.
    • Cytoplasm: Scant, agranular, with occasional azurophilic granules.

  • Prolymphocytes:

    • Shape: Round to oval, Size: 12–20 µm.
    • Nucleus: Single prominent nucleolus.
    • Cytoplasm: Moderate with occasional azurophilic granules.

  • Small Lymphocyte:

    • Shape: Round to oval, Size: 7–10 µm (5:1–2:1 N:C ratio).
    • Nucleus: Dense chromatin, nucleoli not visible.
    • Cytoplasm: Scant, agranular.

  • Large Lymphocyte:

    • Shape: Round to oval, Size: 11–16 µm.
    • Nucleus: Notched, dense chromatin.
    • Cytoplasm: Increased amount, may include azurophilic granules.

  • Reactive Lymphocyte:

    • Shape: Large and irregular, Size: 16–30 µm.
    • Nucleus: Round or notched, variable chromatin.
    • Cytoplasm: Deep blue with possible azurophilic granules.

  • Immunoblast:

    • Shape: Round to oval, Size: 12–25 µm.
    • Nucleus: Central and prominent nucleoli, fine chromatin.
    • Cytoplasm: Deep blue, extending past the cell periphery.

  • Plasmacytoid Lymphocyte:

    • Shape: Round to oval, Size: 15–20 µm.
    • Nucleus: Central with single visible nucleolus.
    • Cytoplasm: Dark and basophilic, with irregularly sized surrounding cells.

  • Plasma Cell:

    • Shape: Round and eccentrically placed, Size: 9–20 µm.
    • Nucleus: Not present, nucleoli absent.
    • Cytoplasm: Moderate to abundant, reflecting immunoglobulin production.

Conclusion

  • The functional roles, development, and morphology of lymphocytes are critical for understanding adaptive immune responses. Lymphocytes not only participate in immediate defense mechanisms but also contribute to long-term immunologic memory and protection against pathogens. Further studies on lymphocyte kinetics, concentrations, and their specific functions are essential for optimizing therapeutic strategies in immunological disorders.