Interleukin-33 expression in stimulated keratinocytes and monocytes

Objectives: The IL-1 family of cytokines has at least 11 members (IL-1F1-11). IL-1 α, IL-1 β and IL-1Ra (IL-1F1-3) have well established roles in inflammatory disorders such as periodontal disease and these cytokines are potential therapeutic targets. With the exception of IL-18 (IL-1F4) there are no data on the role, if any, of the remaining IL-1 family cytokines in periodontal disease. The newly discovered IL-1 family cytokine IL-33 (IL-1F11) has a closely related structure to IL-1β (Il-1F2) and IL-18, stimulates NF-κB and MAP kinases in responsive cells and directs synthesis of T_H2 cytokines. We aimed to determine the expression of IL-33 in cells relevant to periodontal pathogenesis and to examine the effect of LPS on IL-33 expression in monocytes.

Methods: THP-1 promonocytes were differentiated into monocytes with vitamin D3 (as confirmed by upregulation of CD14 expression). Monocytes were then cultured in the presence of LPS from both P.gingivalis and E.coli (at 0.1 and 1 μg/ml) for 0.5-3 hours. Human keratinocytes (HaCaT cell line) were stimulated with IL-1β (100 μg/ml) for 24 hours. RNA was isolated from cells harvested from these experiments (and from cultured primary epidermal keratinocytes) and IL-33 expression determined by RT-PCR.

Results: IL-33 is constitutively expressed in monocytes and HaCaT cells. Expression of IL-33 is particularly strong in primary human keratinocytes. Upregulation of IL-33 expression is an early event in the stimulation of monocytes by both P.gingivalis and E.coli LPS and is maintained for at least 3 hours in culture experiments. Stimulation of keratinocytes with IL-1β also upregulates IL-33 expression.

Conclusion: IL-33 is expressed in immunocompetent cells and is upregulated when these cells are exposed to pro-inflammatory stimuli. IL-33 may therefore be important in the pathogenesis of chronic inflammatory disorders such as periodontal disease.