Regulators of mycobacterial granuloma formation – CCL2 and VEGF-A

Abstract

Abstract Granulomas are macrophage dominated lesions and are a central feature of mycobacterial infections. These cell aggregates are dynamically changing structures as the life span of the granuloma recruited effector cells is relatively short and the cells have to be replaced. CCL2 and VEGF-A are required for granuloma maintenance as blocking the action of these chemokines results in reduction of granuloma size and number after BCG or Mtb infections. We seek to elucidate the relevance of these two cytokines in the timeline of mycobacterial infections and their relative contribution to granuloma formation. CD11c+ cell immigration is impaired, costimulatory molecule expression is lower and granulomas are smaller while the bacterial burden is higher in the liver when CCR2KO mice are infected i.p. with M. bovis BCG. Similarly, in animals that are selectively deficient in macrophage VEGF-A production the granulomas are smaller. Granuloma cells produce VEGF-A. Caseating granulomas, like the ones induced in C3HeB/FeJ (Kramnik) mice are hypoxic and hypoxia is a strong inducer of VEGF-A production. Sarcoid lesions induced by BCG or Mtb in B6 mice are not hypoxic but we show that ATP released from dead cells induces VEGF-A in a subpopulation of macrophages in the granulomas. Additionally, VEGF production proved to be dependent on granuloma size: bigger granulomas produce larger quantities of VEGF-A while smaller lesions produce less or none. This finding suggests that unlike other chemokines such as CCL2, which is described to be involved in both granuloma initiation and maintenance, VEGF-A may support an increased lesion size in the late acute phase of infection. Regulating cellular recruitment may provide new therapies for granulomatous diseases.