Objective. In this in vivo study, we investigated changes in the proteins that coat monosodium urate (MSU) crystals in human synovial fluid samples and rat air pouch fluid samples obtained sequentially during periods of active and resolving inflammation, in order to evaluate whether in vivo findings are consistent with hypotheses on roles of protein coating based on in vitro findings.

Methods. Crystals from patients with gout were isolated from joint fluids with acute inflammation, and subsequently from the same joints at the time inflammation was resolving. Crystals were also obtained using the rat subcutaneous air pouch model. Immunogold was used to label proteins coating MSU crystals, for light microscopy (LM) and transmission electron microscopy (TEM) studies.

Results. Dense immunogold–silver labeling for IgG was observed under LM on crystals from fluid with acute inflammation, whereas other proteins (apolipoproteins [Apo], fibronectin, fibrinogen, albumin) were not labeled significantly. Apo B became strongly positive on crystals as the inflammation subsided, whereas other proteins were only weakly positive and IgG became absent or weakly positive. Quantitative TEM evaluation confirmed the LM observations.

Conclusion. This study provides the first in vivo evidence supporting the notion derived from previous in vitro studies that proteins coating MSU crystals change as inflammation evolves. Protein coatings may play an important role in the self-limited nature of gouty inflammation. IgG coating MSU crystals may enhance the inflammation. As the inflammation subsides, Apo B could displace the IgG by competitively coating sites on crystals and could contribute in part to the resolution of the acute gouty arthritis.