Peripheral B cell tolerance and function in transgenic mice expressing an IgD superantigen

Area of Science:

• Immunology
• Cell Biology
• Molecular Biology

Background:

• Transitional B cells are rapidly turning over and sensitive to apoptosis upon B cell receptor (BCR) ligation in vitro, but their in vivo tolerance sensitivity is less understood.
• Immunoglobulin D (IgD) is a key marker coexpressed with IgM on transitional B cells and is also present on naive follicular (B-2), marginal zone, and B-1 B cells.

Purpose of the Study:

• To investigate the in vivo role of IgD in peripheral B cell tolerance using a novel transgenic mouse model.
• To analyze the impact of blocking IgD signaling on B cell development, tolerance, and immune responses.

Main Methods:

• Generation of transgenic mice ubiquitously expressing a membrane-bound IgD-superantigen.
• Analysis of B cell populations (B-2, marginal zone, B-1, transitional B cells) in spleen.
• Assessment of B cell development, apoptosis, anergy, and receptor editing.
• Evaluation of T-independent and T-dependent immune responses, including memory responses.
• Measurement of serum immunoglobulin levels and plasma cell populations.

Main Results:

• Ubiquitous IgD-superantigen expression led to a complete block in B-2 cell development and a significant reduction in marginal zone B cells, while B-1 cells remained relatively normal.
• B cell development halted at the transitional B cell stage (T1-T2), a block resistant to Bcl-2 or BAFF overexpression.
• Arrested transitional B cells showed immature features, rapid turnover, and lacked conventional anergy or significant receptor editing.
• Type-2 T-independent responses were enhanced, whereas T-dependent responses were reduced with skewed isotype usage.
• Transgenic mice exhibited increased serum IgM and IgM+ plasma cells, but diminished IgG levels, alongside an augmented memory response to secondary challenge.

Conclusions:

• Blocking IgD signaling arrests B cell development at the transitional stage, suggesting a critical role for IgD in B cell maturation and tolerance.
• This IgD-superantigen model provides a valuable tool for studying peripheral B cell tolerance in polyclonal populations and B cells naturally lacking IgD.
• Despite developmental arrest, the model reveals complex alterations in immune responses, including enhanced T-independent immunity and altered B cell memory, highlighting the multifaceted role of IgD.