Anti-inflammatory role of interleukin-15 in Crohn's disease

Interleukin (IL)-15 is overexpressed in intestinal tissue with active Crohn's disease (CD). However, its role in the pathogenesis of the disease remains uncertain. We studied the effects of IL-15 on colonic mucosal proinflammatory cytokine response in vitro using organ culture of human colonic explants.

Colonic tissue was obtained from (1) resections in pediatric CD patients (inflamed and noninflamed) and (2) rectal biopsies in patients with CD undergoing colonoscopy (n = 31) and controls (n = 9). In preliminary experiments, explants from the resections were cultured in the presence or absence of a simulated T(H)1 stimulation using ionomycin (Io) and phorbol-myristate-acetate (PMA), with or without IL-15, or in medium alone. Rectal biopsies were cultured in the same conditions as above, with or without adding a monoclonal anti-IL-15 neutralizing antibody (mAb). Levels of interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha, and IL-2R alpha were measured by enzyme-linked immunosorbent assay.

IL-15, in the absence of Io + PMA, did not induce the expression of IFN-gamma, TNF-alpha, or IL-2R alpha. Only inflamed explants from resections stimulated with Io + PMA expressed IFN-gamma, TNF-alpha, and IL-2R alpha. This T(H)1 stimulatory effect was inhibited by IL-15 in a dose-dependent fashion. In rectal biopsy explants, inflamed, noninflamed CD, and control tissue responded to stimulation with Io + PMA (P < 0.05) with increased IFN-gamma and TNF-alpha (P < 0.05). This response was again inhibited by IL-15. The inhibitory effect of IL-15 was specifically reversed by anti-IL-15 mAb (P < 0.05). The data for the CD group were also analyzed according to the severity of colonic inflammation and medication use.

Our results suggest a possible anti-inflammatory role for IL-15 in CD. We postulate that its overexpression in CD potentially represents a protective mechanism against the exaggerated T(H)1 immune response.