Variable-region-identical antibodies differing in isotype demonstrate differences in fine specificity and idiotype.

Area of Science:

• Immunology
• Molecular Biology
• Structural Biology

Background:

• The prevailing view posits that antibody (Ab) variable regions solely dictate antigen (Ag) specificity.
• Previous observations suggested otherwise, noting differing fine specificities in chimeric Abs with identical V regions and changes in Ab idiotype (Id) structure with isotype switching.

Purpose of the Study:

• To investigate whether antibody constant (C) regions influence fine specificity and idiotype (Id).
• To evaluate the reactivity of murine IgG switch variants with differing V regions against specific antigens and anti-Id mAbs.

Main Methods:

• Examined two families of murine IgG switch variants.
• Assessed antibody reactivity using Cryptococcus neoformans glucuronoxylomannan, its peptide mimetics, and anti-Id monoclonal antibodies (mAbs).
• Compared fine specificities and Id structures across different isotypes within homologous antibody families.

Main Results:

• Identified isotype-related differences in fine specificities and Id for two mAb isotype-switched families.
• Demonstrated that the C region can affect V region protein conformation, leading to altered fine specificity and Id.
• Confirmed the validity of isotype-dependent specificity modulation in homologous antibody sets.

Conclusions:

• Antibody isotype significantly impacts fine specificity and idiotype, challenging the exclusive role of variable regions.
• This finding has profound implications for understanding secondary immune responses, idiotypic network regulation, and isotype function.
• Unifies isotype class switching and somatic hypermutation as processes capable of altering antibody specificity and affinity.