The monocyte binding domain(s) on human immunoglobulin G

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Monocyte binding to human immunoglobulin G (IgG) has been primarily attributed to the C gamma 3 domain.
• The inhibitory capacity of the pFc' fragment on monocyte-IgG interaction was previously considered evidence for C gamma 3 domain involvement.

Purpose of the Study:

• To re-evaluate the specific domain of human IgG responsible for monocyte binding.
• To investigate the role of Fc fragment contamination in previous findings.

Main Methods:

• Immunoaffinity purification of pFc' fragments.
• Analysis of human IgG1 paraproteins (TIM and SIZ) lacking specific domains.
• Testing inhibitory effects of modified IgG1 proteins on monocyte-IgG interaction.

Main Results:

• Purified pFc' fragments lost their inhibitory ability, indicating contamination in previous preparations.
• Human IgG1 paraproteins lacking the C gamma 2 (TIM) or C gamma 3 (SIZ) domains did not inhibit monocyte binding.
• A hinge-deleted IgG1 protein showed minimal inhibitory capacity.

Conclusions:

• The inhibitory effect of Fc fragments on monocyte binding is due to trace contamination with intact IgG or Fc, not an intrinsic property of the pFc' fragment.
• Evidence suggests the C gamma 2 domain, or the interface between C gamma 2 and C gamma 3 domains, is the primary site for monocyte interaction with human IgG.