Activation of the guinea pig alternative complement pathway by mouse IgA immune complexes

Area of Science:

• Immunology
• Complement System Biology

Background:

• Immunoglobulin A (IgA) is a key antibody in mucosal immunity.
• The role of IgA in complement system activation, particularly the alternative pathway, remains less understood compared to other immunoglobulin classes.

Purpose of the Study:

• To investigate the capacity of IgA immune complexes to activate the alternative complement pathway.
• To elucidate the mechanisms and limitations of complement activation by IgA.

Main Methods:

• Generation of IgA immune complexes using a mouse IgA myeloma protein specific for phosphorylcholine (PC) linked to bovine serum albumin (BSA).
• Assessment of complement activation in mouse, guinea pig, and human serum by measuring complement component consumption (C3, C5).
• Analysis of C3 binding to IgA immune complexes.

Main Results:

• IgA anti-PC-BSA immune complexes activated the alternative complement pathway in mouse and guinea pig serum, but not in human serum.
• Complement activation was characterized by C3 consumption with minimal or no C5 consumption.
• C3 did not bind covalently to IgA immune complexes, unlike its binding to IgG immune complexes.

Conclusions:

• IgA immune complexes may not provide an optimal surface for the assembly of the alternative pathway C3 convertase.
• Limited C3 cleavage, occurring primarily in the fluid phase, restricts further complement cascade amplification, including C5 cleavage and the amplification loop.
• These findings highlight differences in complement activation mechanisms between IgA and IgG immune complexes.