Isotype switching

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Isotype Switching (Class Switch Recombination)

Definition

Isotype switching - formally called class switch recombination (CSR) - is the process by which an antigen-stimulated B cell changes the class (isotype) of antibody it produces, shifting from the default IgM to IgG, IgA, or IgE, while retaining the same antigen-binding specificity. Only the heavy chain constant (C) region changes; the variable (V) domain, and therefore antigen recognition, remains unchanged.

Cellular and Molecular Immunology, p. 783

Where It Occurs

CSR is initiated in extrafollicular pre-germinal center Tfh cells (outside the follicle) and continues inside germinal centers, driven by follicular helper T (Tfh) cells in the light zone. B cells entering the germinal center are IgM- and IgD-expressing; their progeny switch to express downstream heavy chain genes.

Cellular and Molecular Immunology, p. 783
Roitt's Essential Immunology, p. 114

Molecular Mechanism

1. Heavy Chain Locus Architecture

The immunoglobulin heavy chain (IgH) locus is arranged in a linear order downstream of the assembled VDJ exon:

V-D-J | Sµ | Cµ | Cδ | Sγ3 | Cγ3 | Sγ1 | Cγ1 | ... | Sε | Cε | Sα | Cα

Constant region genes for IgD, IgG, IgE, and IgA lie downstream of Cµ.
Switch (S) regions - repetitive, G-rich sequences 1-10 kb long - flank the 5' end of each CH gene (except Cδ).

Class switch recombination at the IgH locus, showing excision of intervening DNA and formation of an episomal circle

Figure: CSR illustrated for an IgM to IgG2a switch. Breaks are introduced in Sµ and Sγ2a; the intervening DNA is excised as an episomal circle, and the VDJ exon is joined to Cγ2a, leading to IgG2a mRNA and protein.

2. Germline (Sterile) Transcription

Before recombination, cytokine signals induce germline transcription from an intronic promoter (I-region promoter) upstream of the target switch region. These transcripts (I exon → S region → CH exon) do not code for protein but are required to open chromatin and make the switch region accessible to AID.

Cellular and Molecular Immunology, p. 787
Roitt's Essential Immunology, p. 114

3. Role of AID (Activation-Induced Cytidine Deaminase)

AID is the key enzymatic mediator of CSR. It is induced by CD40 signaling and deaminates cytosines in the S regions, converting C→U. This leads to:

Uracil-DNA glycosylase removal of uracils
DNA double-strand breaks (DSBs) in the S regions
Non-homologous end joining (NHEJ) and other DNA repair pathways fuse the two broken S regions

The result: the VDJ exon is joined to a new downstream CH gene; the intervening DNA is excised as an episomal circle. Alternative splicing of the primary RNA then determines whether the antibody is membrane-bound or secreted.

Roitt's Essential Immunology, p. 114
Cellular and Molecular Immunology, p. 787

4. CD40 - CD40L Interaction

CD40 on B cells + CD40L (CD154) on activated Tfh cells → obligatory signal for CSR
Genetic deficiency of CD40 or CD40L → B cells remain stuck at IgM production → Hyper-IgM Syndrome (severe immunodeficiency with susceptibility to bacterial infections)
Janeway's Immunobiology 10e, p. 445

Cytokine Control of Isotype Selection

The choice of which isotype to switch to is not random - it depends on cytokines produced by Tfh cells and local tissue cells:

Cytokine	Isotype Induced (mouse)	Context
IL-4	IgG1, IgE	Th2 responses, helminths, allergy
IFN-γ	IgG3, IgG2a	Th1 responses, viruses/intracellular pathogens
TGF-β	IgG2b, IgA	Mucosal sites, regulatory environment
IL-21	IgG3, IgG1, IgA	Germinal center Tfh cells
IL-5	IgG1, IgA	Mucosal immunity

Janeway's Immunobiology 10e, p. 445 (Fig. 10.23)

Note on humans vs. mice: In mice, IFN-γ drives IgG2a switching. In humans, the specific cytokines driving differential IgG subclass switching (IgG1 vs. IgG2 vs. IgG3 vs. IgG4) are less well defined. Human Tfh cells with high IL-21:IL-4 ratio drive IgM→IgG switch. - Cellular and Molecular Immunology, p. 785

Functional Significance of Each Isotype

Isotype	Key Effector Functions
IgM	First antibody produced; efficient complement activation (classical)
IgG	Opsonization, complement activation, ADCC, placental transfer, long half-life
IgA	Mucosal immunity; exported through epithelia into secretions (secretory IgA)
IgE	Anti-helminth defense; mediates immediate hypersensitivity / allergy

B cells in mucosal tissues preferentially switch to IgA, stimulated by TGF-β (from T cells and stromal cells) and by BAFF/APRIL (from myeloid cells - enabling T-independent IgA responses).
Cellular and Molecular Immunology, p. 786

Clinical Relevance

Defect	Consequence
CD40L deficiency	Hyper-IgM syndrome - high IgM, absent IgG/IgA/IgE
AID deficiency	Same phenotype as CD40L deficiency - CSR fails
Ataxia Telangiectasia	Impaired DNA repair → defective CSR
TACI gene mutation	Selective IgA deficiency

Janeway's Immunobiology 10e, p. 445

Summary Flow

Antigen + T cell help (CD40L)
        ↓
B cell activation → CD40 signaling → AID expression
        ↓
Cytokine signal (IL-4, IFN-γ, TGF-β, IL-21...) →
Germline transcription at target switch region
        ↓
AID deaminates C→U in Sµ and target S region
        ↓
DNA double-strand breaks → NHEJ
        ↓
VDJ joined to new CH gene; intervening DNA excised as circle
        ↓
New isotype (IgG / IgA / IgE) with SAME antigen specificity

Isotype switching is irreversible at the DNA level - once a B cell switches from IgM to IgG, it cannot revert. However, it can switch further downstream (e.g., IgG → IgA, or IgG → IgE), as long as the target CH gene is still present.

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