Structural basis for quinine-dependent antibody binding to platelet integrin αIIbβ3

Area of Science:

• Immunology
• Hematology
• Structural Biology

Background:

• Drug-induced immune thrombocytopenia (DITP) involves drug-dependent antibodies targeting platelet glycoproteins.
• Quinine is a common trigger, but the mechanism of drug-antibody-platelet interaction remains unclear.
• Previous research faced challenges with polyclonal antibodies and demonstrating drug-integrin binding.

Purpose of the Study:

• To elucidate the molecular mechanism of quinine-induced DITP.
• To investigate how quinine binding confers antibody specificity for platelet targets.
• To define the structural basis of drug-antibody interaction in DITP.

Main Methods:

• Utilized quinine-dependent murine monoclonal antibodies mimicking human DITP antibodies.
• Demonstrated specific, high-affinity binding of quinine to antibody complementarity-determining regions (CDRs).
• Determined crystal structures of quinine-bound CDRs to visualize induced conformational changes.

Main Results:

• Quinine binds with high affinity to the CDRs of specific monoclonal antibodies.
• Crystal structures reveal drug-induced conformational changes in the antibody paratope.
• No direct binding of quinine to the target platelet integrin was detected.

Conclusions:

• A hybrid paratope, formed by quinine and reconfigured antibody CDRs, is critical for target recognition.
• The primary event in quinine-induced DITP is drug binding to antibody CDRs, not direct drug-integrin interaction.
• This mechanism explains how drugs can induce antibodies to target self-antigens, leading to immune injury.