Somatic Recombination and Immune Cell Maturation

Somatic Recombination of T and B Cells

  • Definition: Somatic recombination refers to the process where T and B cells undergo genetic rearrangement to produce unique receptors for recognizing antigens.

T Cells vs. B Cells

  • T Cells:

    • Composed of an alpha chain and a beta chain.

    • The alpha chain contains V (Variable) and J (Joining) segments.

    • The beta chain consists of V, D (Diversity), and J segments.

    • Somatic recombination occurs in variable regions that result in unique T cell receptors.

  • B Cells:

    • Composed of a light chain and a heavy chain.

    • The light chain has V and J segments.

    • The heavy chain includes V, D, and J segments.

Recombination Segments

  • Examples of segments:

    • Light chain: V1, V2, V3, J1, J2, J3.

    • The sequence segments are flanked by Recombination Signal Sequences (RSS), which are highly conserved DNA sequences.

  • Rag Proteins:

    • Rag proteins bind to RSS, facilitating the process by recognizing these sequences.

    • They help in recruiting the enzyme V(D)J recombinase, which is crucial for the recombination process.

Function of V(D)J Recombination

  • Endonuclease Function: The V(D)J recombinase degrades nucleic acid between the selected V, D, and J segments, leading to the formation of a loop and enabling unique binding sites for T and B cells.

  • Outcome: Unique binding sites are crucial, as T cells bind specifically to peptides and B cells can bind to any biomolecule, necessitating diverse B cells due to variability in binding domains.

Maturation Process of T and B Cells

  • The maturation process involves three key steps:

    1. Repertoire assembly (somatic recombination)

    2. Negative selection (removal of auto-reactive cells)

    3. Positive selection (selection of functional cells).

T Cell Maturation

  • Repertoire Assembly:

    • Involves D to J recombination and then V to DJ recombination for the beta chain.

    • The process is initiated by the pTα (surrogate alpha chain) that tests the beta chain's compatibility, forming a super dimer potentially including CD3 proteins.

  • Positive Selection:

    • Checks if the T cell receptor can associate with MHC molecules. Each T cell has to recognize the class I or II MHC molecules inherited from parents.

    • Fails if it does not recognize the MHC correctly, leading to cell death.

    • The outcome is differentiated between CD4+ T helper cells (recognizing MHC II) and CD8+ cytotoxic T lymphocytes (recognizing MHC I).

  • Negative Selection:

    • Utilizes the AIRE (autoimmune regulator) to express self-peptides on MHC. T cells that recognize self-peptides (auto-reactive) are eliminated.

  • Successful navigation through these stages results in a mature naive T cell ready to function in the immune response.

B Cell Maturation

  • Repertoire Assembly:

    • Starts with heavy chain recombination, where recombination follows a D to J and then V to DJ sequence.

    • The surrogate light chain (VpreB + λ5) tests the heavy chain's functionality before proceeding to light chain recombination (V to J).

  • Negative Selection:

    • IgM is tested against self-antigens present in the bone marrow. Auto-reactive B cells are eliminated.

  • Positive Selection:

    • Mature naive B cells leave the bone marrow and enter secondary lymphoid tissues (e.g., spleen, lymph nodes).

    • They interact with dendritic cells in primary follicles for survival signals required for maturation.

Immunoglobulin Structure and Function

  • The surface of a naive B cell can express IgM and IgD. These two immunoglobulins are the only ones co-expressed prior to cell activation.

  • Immunoglobulins consist of:

    • Fab region: Fragment antigen binding; includes variable regions for specific antigen binding.

    • Fc region: Determines isotype functionality (IgM, IgD, IgG, IgA, IgE).

  • IgM:

    • Pentameric structure with ten binding sites; involved in classical complement activation and neutralization.

    • Tests for auto-reactivity in the bone marrow before leaving.

  • IgD:

    • Monomeric structure involved in B cell activation, may have opsonizing capabilities.

  • IgA:

    • Usually exists as a dimer; primarily functions in mucosal immunity.

  • IgG:

    • Monomeric structure; versatile in functions (neutralization, opsonization, complement activation) used for deep-tissue infections.

  • IgE:

    • Mediates allergic responses and antiparasitic immunity; binds to mast cells and basophils in the absence of antigens, leading to rapid degranulation upon allergen exposure.

Cytokine and Co-stimulatory Signals in Activation

  • T cells require multiple signals for full activation:

    • Signal 1: T cell receptor recognizing the peptide-MHC complex on antigen-presenting cells (APCs).

    • Signal 2: Co-stimulatory interaction between CD28 on T cells and B7 on APCs leads to enhanced activation.

  • Clonal Expansion: Following successful activation, T cells proliferate rapidly due to interleukin-2 and associated signaling pathways.

  • B cells also require T cell signals for optimal activation leading to their differentiation into plasma cells, producing antibodies, and undergoing affinity maturation.

Summary of Key Concepts

  • The generation of diverse and specific T and B cell receptors through somatic recombination and effective maturation processes, alongside proper signaling and selection mechanisms, is critical for a functional adaptive immune response.