Study Notes for Antigen Recognition by T Lymphocytes

Chapter 5: Antigen Recognition by T Lymphocytes

Overview of T-cell Functionality and Receptors

  • T-cell receptor (TCR) Diversity
    • Formation of TCR receptors through genetic mechanisms.
    • Key roles in antigen processing and presentation for T cell recognition.

T-cell Versus B-cell Comparison

Similarities
  • Structure: Both have immunoglobulins (Igs) and TCR structures.
  • Result of gene rearrangements leads to variable and diverse antigen specificity.
  • Clonal expression where each clone expresses a single species of antigen receptor.
  • Clonal distribution of receptor diversity is achieved through genetic mechanisms.
Differences

  • Igs Characteristics:

    • Variable binding sites allow interaction with a wide range of antigens.
    • Sole function is to produce secreted antibodies.
    • B-cell receptors bind to epitopes on intact molecules (proteins, carbohydrates, lipids, toxins).

  • TCR Characteristics:

    • Variable binding sites limited to one specific antigen, primarily protein-derived peptide.
    • Roles include more diverse interactions with other cells.

  • Antigen Type Requirement:

    • B-cells interact with various forms of antigens.
    • T-cells require presentation on MHC molecules (major histocompatibility complex) associated with proteins.

Major Histocompatibility Complex (MHC)

  • Definition:

    • MHC molecules are glycoproteins acting as antigen-presenting molecules.
    • MHC Class I (expressed on almost all cells) versus MHC Class II (expressed on specialized immune cells).

  • Polymorphism:

    • Genetic diversity results in numerous variants (alleles) within human populations, explaining differences in immune response among individuals.

  • Role in Transplantation:

    • MHC determinants play a significant role in tissue compatibility before the antigen presentation role was understood.

T-cell Receptors (TCR)

  • Structure of TCR:

    • Membrane-bound glycoproteins made of two polypeptide chains: alpha (α) and beta (β).
    • Functional domain akin to a single antigen-binding arm of an immunoglobulin.

  • Function:

    • TCR’s primary role is to recognize antigens, lacking a soluble form since they do not have effector functions like antibodies do.

  • Gene Rearrangement During T-cell Development:

    • Resembles the B-cell process where gene segments rearrange during T cell development; successful genes yield a uniquely specific TCR.
    • The resultant T-cells yield millions of distinct TCRs, each targeting one antigen.

  • Variability in TCR Binding:

    • TCR binding sites involve loops of hypervariable regions defined as complementary determining regions (CDR1, CDR2, and CDR3).

Mechanisms of TCR Diversity

  • Prior to Antigen Encounter:
    • Gene rearrangement occurs in the V-region.
  • After Antigen Stimulation:
    • No mutations or isotype switching in TCR, unlike in B cells.

TCR Binding Characteristics

  • TCRs bind in the context of MHC on opposing cell surfaces, facilitating multipoint attachments.

TCR Development Process

  • TCR Structure:
    • TCR is an α:β heterodimer that requires association with invariant membrane proteins for stability and signaling.
    • Known components include the CD3 complex and a zeta (ζ) chain.

TCR Complex Composition

  • Complex Structure:
    • Composed of eight polypeptides, with α and β chains forming the core and engaging with intracellular signaling molecules during antigen recognition.

Alternative T-cell Receptors

  • Gamma-Delta TCR (γ:δ TCR):
    • A distinct type of TCR expressed by a small population of T-cells, only 1-5% of total cells characterized by their unique receptor structure.
    • Typically engaged in immune function differently from α:β TCRs and not strictly limited by MHC peptide presentation.

Structure of MHC Molecules

MHC Class I and II Dysfunctions

  • MHC Class I presents intracellular antigens to CD8 T-cells.

  • MHC Class II facilitates extracellular antigen presentation to CD4 T-cells.

  • 3-D Structures:

    • Both MHC classes exhibit a similar structure with peptide-binding grooves crucial for immune recognition.

Antigen Processing and Presentation Pathways

  • Pathways:
    • Intracellular proteins are processed for MHC Class I.
    • Extracellular proteins appear via endocytosis and associate with MHC Class II.
  • Important Pathogen Categorization:
    • Example categories: Intracellular pathogens (Viruses) versus extracellular pathogens (Bacteria).

MHC Polymorphism

  • MHC polymorphism influences the ability to bind a diverse range of antigens, impacting transplant success, autoimmune responses, and resistance to diseases.

Conclusion

  • Clinical Implications:
    • MHC polymorphisms directly influence transplant compatibility and susceptibility to diseases like HIV.
    • Understanding T cell responses and MHC functionality reshapes therapeutic interventions for immune disorders and vaccines.

References to Consider

  • HLA Complex:
    • Human leukocyte antigen manifestations of varied immune functionality.
    • Inherited combinations influence individual immune responses and transplantation success rates.