Objective: Repeated injection of streptococcal cell wall (SCW) fragments results in chronic arthritis in mice. The objective of this study was to identify genes and pathways that determine disease progression based on gene expression profiling in this model.
Methods: Chronic arthritis was induced in mice by 4 injections of SCW fragments. RNA samples were isolated from synovial tissue obtained at various time points and were analyzed using mouse genome array and quantitative reverse transcription-polymerase chain reaction techniques. The functional role of potential key genes was evaluated in mice with specific gene deletions.
Results: Gene expression analyses revealed a shift in molecular signature. In contrast to an up-regulation of the inflammatory response pathway, the pathways involved in oxidative metabolism were significantly down-regulated during the chronic phase of arthritis. Since oxidative metabolism determines the mode of macrophage activation, we investigated phenotype switching in macrophages. Markers of alternatively activated macrophages, such as arginase 1, were at maximal levels during acute inflammation. In contrast, induction of markers of classically activated macrophages (M1), such as interleukin-1beta (IL-1beta) and inducible nitric oxide synthase (iNOS), was relatively low during the acute phase of disease, but highly increased toward the chronic phase. M1 polarization during the chronic phase was accompanied by a Th1 signature, characterized by IL-12p40, IL-12p35, and interferon-gamma. However, the absence of IL-12p40, but not IL-12p35, significantly inhibited the chronic phase of arthritis and was marked by a reduction in IL-17 and iNOS levels, as well as restored expression of oxidative metabolism genes.
Conclusion: M1 polarization accompanied by a decline in oxidative metabolism determine the chronic phase of arthritis. IL-12p40, most likely acting through the IL-23/IL-17 axis, plays a critical role in this process.