Effects of weak/non-complement-binding HLA antibodies on C1q-binding

Area of Science:

• Immunology
• Complement System
• Antibody Engineering

Background:

• The complement system activation by IgG antibodies is crucial in immune responses.
• Different IgG subclasses (IgG1-4) exhibit varying abilities to bind complement.
• The inhibitory effect of non-complement-binding IgG subclasses on complement activation by binding subclasses is not well understood.

Purpose of the Study:

• To investigate the in vitro C1q-binding when IgG subclasses targeting the same HLA epitope are mixed.
• To determine the extent to which weak/non-complement-binding IgG subclasses (IgG2/IgG4) can block C1q-binding induced by complement-binding IgG subclasses (IgG1/IgG3).

Main Methods:

• In vitro analysis of C1q-binding using HLA DRB1*07:01 beads incubated with mixtures of monoclonal antibodies of different IgG subclasses targeting the same HLA epitope.
• Monitoring C1q-binding levels across varying concentrations and ratios of IgG subclass mixtures.
• Analysis of IgG subclass constellations in sera from sensitized renal allograft recipients.

Main Results:

• Complement-binding IgG1 and IgG3 induced C1q-binding, with IgG3 being more potent.
• Weak/non-complement-binding IgG2 and IgG4 showed minimal C1q-binding independently.
• An excess of IgG2/IgG4 (2- to 10-fold) significantly inhibited C1q-binding triggered by IgG1/IgG3, with a 50% inhibition at a 2- to 4-fold excess.
• In sensitized patients, IgG subclass patterns with an excess of IgG2/4 were found in 34.8%, showing slightly decreased C1q signals.
• Spiking patient sera with IgG2 targeting a different epitope synergistically increased C1q-binding.

Conclusions:

• When targeting the same epitope, excess IgG2/IgG4 actively represses IgG1/IgG3-mediated C1q-binding in vitro.
• This inhibitory phenomenon is relevant in approximately one-third of sensitized patients.
• The net effect of these IgG subclass constellations in patients is a slight inhibition of C1q-binding.