Introduction:
Gout is a common form of arthritis caused by crystallization of uric acid. The uric acid crystals induce a strong inflammatory response that is hardly distinguishable from inflammation induced by bacterial infection and includes increases in serum levels of C-reactive protein (CRP). It is unclear, if the innate immune system recognizes the crystals or merely reacts to the damage they cause.
Aim:
We aimed to identify proteins of the immune system that interact with gout crystals
Methods:
We purified proteins from different body fluids including synovial fluid and identified the proteins by liquid chromatography-coupled mass spectrometry. Recombinant proteins of the identified proteins were used to test their binding to uric acid crystals and their impact on inflammatory responses induced by the crystals.
Results:
Besides strong binding of apolipoprotein B, we found specific binding of several proteins of the immune system capable of inducing inflammatory responses. Strongest binding was found for CRP. Binding was strong enough to deplete CRP from human serum by addition of uric acid crystals. Binding of CRP induced complement activation on the surface of uric acid crystals. In addition to CRP, we found immunoglobulin M (IgM) to bind to the crystals, in all donors tested. Antibodies usually arise in response to foreign antigens. However, the antibodies recognizing uric acid crystals were also found in cord blood serum, which means they are formed before birth, suggesting they are natural/innate antibodies.
Both CRP and IgM were capable of activating the complement system, which alerts the immune system by releasing inflammatory peptides, C3a and C5a. In the absence of CRP and IgM, no complement activation was induced by uric acid crystals, indication that we have found the main complement sensors for uric acid crystals. However, while IgM was only able to induce release of C3a but not C5a, CRP was able to propagate the complement cascade to the very end and induce release of the more inflammatory C5a.
Conclusion:
We have identified the main innate immune sensor proteins that link uric acid crystals to complement activation. The unique ability of CRP to drive production of the more inflammatory C5a suggests elevated CRP may enhance the ability of the immune system to detect the crystals and therefore favor the initiation of gout flares.