Functional profile of tissue-infiltrating and circulating CD68⁺ cells in giant cell arteritis: Evidence for two components of the disease

Macrophages represent a critical component in the inflammatory lesions of giant cell arteritis. By combining immunohistochemistry and in situ hybridization, we have analyzed the functional heterogeneity of tissue-infiltrating macrophages in patients with untreated vasculitis. 20% of macrophages in temporal artery tissue synthesized IL-6-specific mRNA and produced IL-6 and IL-1β proteins. IL-6 and IL-1β production was not limited to CD68⁺ cells in the lymphoid aggregates but was a feature of CD68⁺ cells dispersed throughout the tissue. 50% of tissue-infiltrating CD68⁺ cells synthesized 72-kD type IV collagenase. Only a small subset of CD68⁺ cells produced cytokines as well as collagenase, indicating functional specialization or distinct differentiation stages of CD68⁺ cells in the inflamed tissue. Activation of CD68⁺ cells was not restricted to tissue-infiltrating cells. Expression of IL-6 and IL-1β was found in 60-80% of circulating monocytes of patients with untreated giant cell arteritis, whereas collagenase production was restricted to tissue macrophages. IL-6 and IL-1β production by the majority of circulating monocytes was a shared feature of patients with giant cell arteritis and polymyalgia rheumatica but was not found in rheumatoid arthritis. These data suggest that giant cell arteritis has two components of disease, an inflammatory reaction in vessel walls and a systemic activation of monocytes. Systemic monocyte activation can manifest independently without vasculitis as exemplified in patients with polymyalgia rheumatica.