Consequences of 11β-hydroxysteroid dehydrogenase deficiency during inflammatory responses
Coutinho, Agnes Elizabeth
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Glucocorticoids profoundly influence the immune system and pharmacological doses exert potent anti-inflammatory actions. During inflammation, glucocorticoids limit oedema and influence cell trafficking, differentiation programmes and gene transcription in glucocorticoid-sensitive leukocytes. Within cells, glucocorticoid action is modulated by a pre-receptor mechanism; glucocorticoid metabolism by the enzyme 11β- hydroxysteroid dehydrogenase (11β-HSD). Two 11β-HSD isozymes exist: 11β-HSD1, which catalyses amplification of glucocorticoid levels in intact cells by oxo-reduction of intrinsically inert cortisone (11-dehydrocorticosterone in rodents) into active cortisol (corticosterone in rodents) and 11β-HSD2, which performs the opposite reaction. Thus, amplification of intracellular glucocorticoid levels by 11β-HSD1 may represent an endogenous anti-inflammatory mechanism. This hypothesis has been tested in Hsd11b1-/- mice (homozygous for a targeted disruption in the Hsd11b1 gene, encoding 11β-HSD1), using carageenan-induced pleurisy and experimental model of arthritis induced by injection of arthritogenic antibodies. In both models, Hsd11b1-/- mice showed more severe acute inflammation than control mice. During carrageenan-induced pleurisy, Hsd11b1-/- mice recruited more inflammatory cells to the pleural cavity than congenic controls, with a greater proportion of viable cells, at the onset and peak of pleurisy, suggesting a worse inflammatory response. Histological examination suggested impaired resolution of inflammation in Hsd11b1-/- mice with persistence of inflammation in the visceral pleura, activation of lymphoid aggregates, and uniquely in Hsd11b1-/- mice, formation of fibrous adhesions between lung lobes 48h after initiation of pleurisy. During experimental arthritis induced by injection of serum from arthritic K/BxN mice, clinical signs of inflammation occurred earlier in Hsd11b1-/- mice and were slower to resolve than in control mice. Histological assessment of the acute phase (2d) of arthritis showed no difference in joint pathology between genotypes, despite greater oedema and higher clinical scores in the Hsd11b1-/- mice. However, when the inflammation had resolved (21d following injection of serum), compared to control mice, Hsd11b1-/- mice showed more severe exostosis, intense periarticular inflammation, more collagen deposition and uniquely, ganglion cyst formation. At 21d, whereas basal (morning) plasma corticosterone levels were normal in control mice, they remained elevated in Hsd11b1-/- mice, suggesting ongoing inflammation and persistent activation of the hypothalamic-pituitary-adrenal axis. Mast cells are critical in the initiation of an inflammatory response and are essential in this model of arthritis. Mast cells expressed 11β-HSD1 (but not 11β-HSD2) mRNA and activity. Although mast cell number did not differ in joints or peritoneum of Hsd11b1-/- mice, 11-HSD1-deficient mast cells had a lower threshold for degranulation induced by K/BxN arthritogenic serum. As well as implicating a role for mast cell 11β-HSD1 in limiting initial inflammation in arthritis, these findings also have implications for infection, allergy and tolerance. Collectively, these data suggest that 11β-HSD1 deficiency worsens acute inflammation and results in slower resolution. Therefore, amplification of intracellular glucocorticoids levels, by 11β-HSD1, may represent an important mechanism to limit the acute inflammatory response and programme its subsequent resolution. Increasing leukocyte 11β-HSD1 or local delivery of substrate affords a novel approach for anti-inflammatory therapy.