Requisite Role for Complement C5 and the C5a Receptor in Granulomatous Response to Mycobacterial Glycolipid Trehalose 6,6′‐Dimycolate

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Abstract
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‐1α), interleukin‐1β (IL‐1β) and CXC chemokine KC (CXCL1) correlated with development of granulomas, along with modest change in tumor necrosis factor‐alpha (TNF‐α). 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‐1α, KC, IL‐1β and TNF‐α 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.