11beta-hydroxysteroid dehydrogenases are regulated during the pulmonary granulomatous response to the mycobacterial glycolipid trehalose-6,6'-dimycolate

OBJECTIVE

Tuberculosis has a staggering influence on world health, resulting in nearly 2 million deaths per year. The influence of glucocorticoids during Mycobacterium tuberculosis infection has been under investigation for decades; however, the identity of mycobacterial factors and the mechanism by which glucocorticoids are tissue specifically regulated to influence immune function during acute granuloma formation are unknown.

METHODS

One factor implicated in initiating immunopathology during M. tuberculosis infection is trehalose-6,6'-dimycolate (TDM), a glycolipid component of the mycobacterial cell wall. Intravenous administration of TDM causes inflammatory responses in lungs of mice similar to M. tuberculosis infection and has been used as a successful model to examine proinflammatory regulation and early events involved in the manifestation of pathology.

RESULTS AND CONCLUSION

IL-6, IL-1alpha and TNF-alpha mRNA and protein peaked during the initiation of granuloma formation. Pulmonary corticosterone levels were elevated when the proinflammatory response was greatest, dropping to half of that upon the establishment of granuloma pathology on day 7. It is hypothesized that once corticosterone reaches the site of inflammation, the enzymes 11beta-hydroxysteroid dehydrogenases (11betaHSDs) can influence bioavailability by interconverting corticosterone and the inert metabolite 11-dehydrocorticosterone. RT-PCR demonstrated that pulmonary 11betaHSD type 1 mRNA decreased 4-fold and 11betaHSD type 2 (11betaHSD2) mRNA expression increased 2.5-fold on day 3 after injection, suggesting that corticosterone regulation in the lung, specifically the reduction of active corticosterone by 11betaHSD2, may influence the progression of granuloma formation in response to the mycobacterial glycolipid.

Requisite role for complement C5 and the C5a receptor in granulomatous response to mycobacterial glycolipid trehalose 6,6'-dimycolate

The development of pulmonary granulomatous lesions during mycobacterial infection is a complex phenomenon, in part caused by responses elicited towards the surface glycolipid trehalose 6,6'-dimycolate (TDM; cord factor). The molecular mechanisms underlying granuloma formation following challenge with TDM are not yet completely understood. The present study defines pathologic differences in acute response to Mycobacterium tuberculosis TDM in C57BL/6 mice and mice lacking the C5a receptor (C5aR-/-). Mice were intravenously injected with TDM prepared in water-in-oil-in-water emulsion and examined for histologic response and changes in proinflammatory cytokines and chemokines in lung tissue. Control C5a receptor-sufficient mice demonstrated a granulomatous response that peaked between days 4 and 7. Increased production of macrophage inflammatory protein-1 alpha (MIP-1alpha), interleukin-1beta (IL-1beta) and CXC chemokine KC (CXCL1) correlated with development of granulomas, along with modest change in tumor necrosis factor-alpha (TNF-alpha). In contrast, the C5aR-/- mice revealed markedly exacerbated inflammatory response. The receptor-deficient mice also demonstrated a lack of coherent granulomatous response, with severe oedema present and instances of lymphocytic cuffing around pulmonary vessels. Lung weight index was increased in the C5aR-/- mice, correlating with increased MIP-1alpha, KC, IL-1beta and TNF-alpha over that identified in the congenic C5aR-sufficient controls. Correlate experiments performed in C5-deficient (B10.D2-H2d H2-T18c Hco/oSnJ) mice revealed similar results, leading to the conclusion that C5 plays a significant role in mediation of chemotactic and activation events that are the basis for maturation of granulomatous responses to TDM.