Complement factor C7 contributes to lung immunopathology caused by Mycobacterium tuberculosis

Abstract

Mycobacterium tuberculosis (MTB) remains a significant global health burden despite the availability of antimicrobial chemotherapy. Increasing evidence indicates a critical role of the complement system in the development of host protection against the bacillus, but few studies have specifically explored the function of the terminal complement factors. Mice deficient in complement C7 and wild-type C57BL/6 mice were aerosol challenged with MTB Erdman and assessed for bacterial burden, histopathology, and lung cytokine responses at days 30 and 60 post-infection. Macrophages isolated from C7 -/- and wild-type mice were evaluated for MTB proliferation and cytokine production. C7 -/- mice had significantly less liver colony forming units (CFUs) at day 30; no differences were noted in lung CFUs. The C7 deficient mice had markedly reduced lung occlusion with significantly increased total lymphocytes, decreased macrophages, and increased numbers of CD4+ cells 60 days post-infection. Expression of lung IFN-γ and TNF-α was increased at day 60 compared to wild-type mice. There were no differences in MTB-proliferation in macrophages isolated from wild-type and knock-out mice. These results indicate a role for complement C7 in the development of MTB induced immunopathology which warrants further investigation.

Figure 1

Decreased early bacterial dissemination to the liver in C7 −/− mice. C7 −/− mice had significantly reduced bacterial CFUs in the liver at day 30 after infection with MTB. There were no differences in bacterial burden in the lung or spleen. *P < 0.05 with comparison to wild-type C57BL/6 mice.

Figure 2

Lung histopathology of wild-type C57BL/6 and C7 −/− mice after the-challenge with MTB Erdman. (a) C57BL/6 mice demonstrate extensive granulomatous inflammation at 60 days post infection, 100x. (b) High power images of wild-type mice show that infiltrates are largely macrophages, 400x. (c) C7 −/− mice have reduced lung immunopathology at day 60 after the infection, 100x. (d) Abundant areas with clusters of lymphocytes are observed in C7 −/− mice, 400x. (e) Rare multinucleated giant cell in C7 −/− mice, 400x.

Figure 3

Lung expression of proinflammatory mediators TNF-α and IFN-γ in wild-type and C7 −/− mice. Lung cytokine expression of TNF-α, and IFN-γ as measured by quantitative PCR in wild type (solid bars) and C7 −/− (open bars) mice at 60 days post challenge with virulent MTB. Data are expressed as fold change relative to naïve mice after normalization to β-actin. Data are presented as the mean with SD, n = 6 mice per group, per time point. *P < 0.05, comparisons are made to control mice. There were no significant differences in lung expression of IL-6, IL-12p40, TGF-β, or IL-17 (data not shown).

Figure 4

Cytokine production from infected bone-marrow-derived macrophages. Macrophages were isolated from wild-type and C7 −/− mice and infected with MTB at a MOI of 1 : 1. Cytokines production was measured 24 and 72 hours after infection by ELISA. *P < 0.05, comparisons are made to control mice.