In vitro model of Rich foci: mycobacterium-induced granuloma formation in the central nervous system

Abstract

Abstract Central nervous system (CNS) involvement is the most devastating manifestation of tuberculosis (TB) infection. 5–10 % of the extrapulmonary cases affect immunocompromised individuals and children. Although CNS TB is a global phenomenon with extremely high mortality, understanding how bacteria disseminate into the brain parenchyma is not well understood. In the 1930s Rich and McCordock proposed that bacteria travel via the blood and form a tuberculoma (Rich foci). These foci may rupture resulting in diffuse meningitis. Previously our laboratory showed that bone marrow derived dendritic cells (DCs) migrate across the blood-brain barrier (BBB) into the CNS during inflammation in vitro and in vivo. We also described Mycobacterium bovis (BCG) and M. tuberculosis (Mtb) dissemination from pulmonary granulomas by DCs. We hypothesize that DCs play a main role in the dissemination of mycobacteria and formation of Rich foci. Our goal was to understand the mechanisms and cellular interactions between brain endothelial cells and leukocytes in the formation of the foci in the CNS. Here we show that using an in vitro, primary mouse cell based BBB co-culture system, cellular aggregates resembling granulomas form on the endothelial cell layer, which after four days consisted of infected cells including DCs that were surrounded by leukocytes. More importantly aggregates migrated together to the other side of the monolayer as a migrating island of cells showing that cellular migration of mycobacterium infected cells across the BBB involves relocalization of cellular aggregates. The potential interference through the formation of inflammatory foci on the surface of the brain endothelial layer may provide a way to decrease Mtb dissemination.