The mechanisms leading to
granuloma caseation, a hallmark of
tuberculosis (TB) in humans, are poorly understood. Lung histopathology of C57BL/6 (WT) mice 16 weeks after
aerosol infection with Mycobacterium avium strain TMC724 is uniquely characterized by centrally necrotizing
granulomas, strongly resembling human TB lesions. However, IFN-gamma-deficient (GKO) and IFN-gamma-receptor-deficient (GRKO) mice did not develop
granuloma necrosis following M. avium
infection. Comparison of differentially expressed genes in infected WT and GKO lungs by
DNA microarray and
RNase protection assays revealed that the angiostatic
chemokines CXCL9-11 were significantly reduced in GKO mice. In contrast, angiogenic mediators such as
angiopoietin and
vascular endothelial growth factor, and angiogenic
chemokines such as CXCL2, CCL3, and CCL4, remained unchanged or were expressed at higher levels than in infected WT mice, suggesting impaired neovascularization of the
granuloma as a possible mechanism for caseation in WT mice.
Granuloma vascularization was significantly decreased in central, but not peripheral, areas of
granulomas of infected WT compared to GKO mice. In contrast to GRKO mice, WT mice showed signs of severe
hypoxia in cells immediately surrounding the necrotic core of
granulomas as measured immunohistochemically with a
reagent detecting
pimonidazole adducts. To test the hypothesis that CXCR3, the common receptor for the angiostatic
chemokines CXCL9-11, is involved in
granuloma caseation, histomorphology was assessed in M. avium-infected mice deficient for CXCR3 (CXCR3-KO). 16 weeks after
infection, these mice developed caseating
granulomas similar to WT mice. We conclude that IFN-gamma causes a dysbalance between angiostatic and angiogenic mediators and a concomitant reduction in
granuloma vascularization, but that CXCR3-targeted
chemokines are not sufficient to induce
granuloma necrosis in a mouse model of mycobacteria-induced immunopathology.