Minimal role for the alternative pathway in complement activation by HIT immune complexes

Area of Science:

• Immunology
• Complement System Biology

Background:

• Heparin-induced thrombocytopenia (HIT) involves anti-platelet factor 4 (PF4)/heparin immune complexes activating the classical complement pathway.
• The alternative complement pathway is known to amplify classical pathway activation via the C3b feedback cycle.

Purpose of the Study:

• To investigate the specific contribution of the alternative complement pathway to complement activation mediated by HIT antibodies.
• To determine if inhibiting the alternative pathway impacts complement activation triggered by HIT immune complexes.

Main Methods:

• Utilized IgG monoclonal (KKO) and patient-derived HIT antibodies.
• Compared the effects of classical pathway inhibitors (BBK32, C1-esterase inhibitor [C1-INH]), alternative pathway inhibitors (anti-factor B [fB], factor D [fD]), and a combined inhibitor (soluble complement receptor 1 [sCR1]).
• Assessed complement activation in whole blood and plasma by measuring C3/C5 activation products, immune complex binding to B cells, and neutrophil activation.

Main Results:

• Classical pathway inhibitors (BBK32, C1-INH) and the combined inhibitor (sCR1) effectively blocked complement activation by HIT immune complexes.
• Alternative pathway inhibition using anti-fB or anti-fD did not affect complement activation by HIT immune complexes.
• Alternative pathway inhibition also showed no impact on complement activation by unrelated anti-dinitrophenyl (DNP) immune complexes.

Conclusions:

• The alternative complement pathway appears to contribute minimally to complement activation by HIT immune complexes.
• Further research is needed to ascertain if this is specific to HIT immune complexes or if it applies to other IgG-containing immune complexes, particularly those involving multivalent antigens.