Polyphosphates form antigenic complexes with platelet factor 4 (PF4) and enhance PF4-binding to bacteria

Area of Science:

• Immunology
• Biochemistry
• Molecular Biology

Background:

• Short chain polyphosphates (polyP) are inorganic polymers released by platelets with pro-coagulant and pro-inflammatory roles.
• Platelet factor 4 (PF4) binds bacterial lipid A, initiating antibody-mediated defense, but can cause heparin-induced thrombocytopenia (HIT) when complexed with heparin.

Purpose of the Study:

• To investigate the role of polyphosphate (polyP) in PF4-mediated host defense mechanisms.
• To explore the structural and functional consequences of PF4 complex formation with polyP.

Main Methods:

• Circular dichroism spectroscopy to analyze structural changes in PF4 upon polyP binding.
• Isothermal titration calorimetry to quantify the binding interaction between PF4 and polyP.
• Assays to measure PF4 binding to Escherichia coli and subsequent bacterial opsonisation and phagocytosis.

Main Results:

• PF4 undergoes structural changes upon binding to polyP, similar to its interaction with heparin.
• PF4/polyP complexes expose neoepitopes that bind human anti-PF4/heparin antibodies.
• PolyP enhances PF4 binding to E. coli, promoting bacterial opsonisation and phagocytosis in the presence of specific antibodies.

Conclusions:

• Soluble short chain polyphosphates (polyP) contribute to host defense by enhancing PF4-mediated mechanisms.
• The PF4/polyP interaction mimics aspects of the PF4/heparin interaction, with implications for immune response and antibody recognition.
• PolyP plays a significant role in innate immunity by augmenting PF4's bacterial defense capabilities.