Multiple epitopes recognised by human monoclonal IgM anti-D antibodies

Area of Science:

• Immunology
• Hematology
• Biochemistry

Background:

• Immunoglobulin M (IgM) antibodies play a crucial role in the immune response, particularly in recognizing foreign antigens.
• Anti-D antibodies are significant in transfusion medicine and understanding RhD antigen interactions is vital.
• Previous studies have focused on the pentameric structure of IgM, but the functional characteristics of its subunits are less understood.

Purpose of the Study:

• To characterize the functional binding activity and epitope recognition of bivalent 7S subunits derived from human monoclonal anti-D IgM antibodies.
• To investigate the interaction between IgM and IgG antibodies at the epitope level on the D polypeptide.
• To determine the functional affinity constants and optimal conditions for IgM anti-D antibody binding.

Main Methods:

• Purification of 7S subunits from seven human monoclonal anti-D IgM antibodies.
• Quantification of functional binding activity and determination of the number of binding sites per red blood cell.
• Competitive binding assays using IgG and IgM antibodies to assess epitope sharing.
• Measurement of functional affinity constants at varying ionic strengths.
• Agglutination assays in microwells to determine antibody titers.

Main Results:

• Bivalent 7S subunits retained 33-63% of the total protein's functional binding activity.
• The number of recognized sites per red blood cell ranged from 9,400 to 28,500, suggesting recognition of multiple epitopes on the D polypeptide.
• Evidence of epitope sharing between IgM and IgG anti-D antibodies was observed.
• Functional affinity constants for 7S subunits ranged from 1.0 to 8.8 x 10(7) M-1, with a 3- to 16-fold increase upon reduction of ionic strength.
• Native pentamer IgM antibodies showed end-point titers at concentrations of 7-26 ng/ml.

Conclusions:

• Bivalent 7S subunits of IgM anti-D antibodies possess significant functional binding activity and recognize diverse epitopes on the D polypeptide.
• IgM and IgG antibodies compete for binding, confirming they target the same D polypeptide epitopes.
• Ionic strength significantly influences the binding affinity of IgM subunits, with lower ionic strength enhancing binding.
• The study provides valuable insights into the molecular interactions of IgM anti-D antibodies, relevant for transfusion immunology and diagnostics.