It has previously been shown that as monocytes differentiate into macrophages, they lose the ability to secrete proinflammatory cytokines in response to monosodium urate monohydrate (MSU) crystals. The purpose of this study was to investigate whether MSU crystals induce macrophages to secrete antiinflammatory factor instead.
Human monocyte or macrophage isolates were prepared from samples obtained from healthy volunteer donors either by differentiation of blood monocytes in vitro or by collecting cells from skin blisters during the early or late phase of the dermal inflammatory response to cantharidin. Monocyte or macrophage isolates were then incubated with MSU crystals for 24 hours, and culture supernatants were assayed for candidate antiinflammatory mediators (by enzyme-linked immunosorbent assay) and for the capacity to activate or suppress endothelial cell E-selectin expression and secondary neutrophil recruitment under shear flow.
Analysis of supernatants from in vitro–differentiated macrophages revealed that transforming growth factor β1 (TGFβ1) was induced following MSU crystal stimulation (mean ± SEM 1.50 ± 0.24 ng/ml/106 cells), but there was no evidence of interleukin-10 (IL-10), IL-1 receptor antagonist, or tumor necrosis factor (TNF) receptor p55 release. Macrophage TGFβ1 significantly suppressed endothelial cell E-selectin expression and secondary neutrophil capture on endothelial monolayers stimulated with supernatants from MSU-treated monocytes. Leukocytes isolated from resolving (40-hour) skin blisters similarly elaborated TGFβ1 when challenged with MSU crystals (0.66 ± 1.3 ng/ml/105 CD14+ cells). In contrast, cells isolated from acute (16-hour) skin blisters secreted TNFα (0.49 ± 0.08 ng/ml/105 CD14+ cells) but no detectable TGFβ1.
These data provide further support for the concept that differentiated macrophages play a protective role in the pathophysiology of gout, and they identify macrophage TGFβ1 as a mediator of paracrine suppression during the resolution phase of inflammation.