Introduction & Goals

Mycobacterial tissue infections lead to the formation of granulomas, which are complex immune cell structures. Key player of the granuloma is the macrophage (MΦ). Nevertheless, a mature granuloma with a necrotic core not only induces the accumulation of immune cells but also leads to a remodelling of the tissue with an impact on fibroblasts and stromal cells (SC). In this project, we aim to characterize the role of SC for granuloma formation and control of mycobacteria, as well as the interaction of MΦ and non-immune cells in mycobacterial infections.

Methods

To address these questions, we used murine M. bovis BCG infection models and in vitro co-cultures of SC and MΦ. We further characterized SC in mycobacterial infections by flow cytometry analysis, confocal and live cell imaging as well as transcriptomic analysis.

Results

Mycobacterial infections lead to the accumulation of bone-marrow derived monocytes at the site of infection, which then further contribute to granuloma formation. We found high expression levels of CCL2 and CCL7 in SC for up to 16 weeks post infection, indicating an involvement in the recruitment of monocytes. Although in infection mostly macrophages take up mycobacteria, flow cytometry and microscopy revealed that BCG could infect SC in vitro. With the help of adoptive cell transfers, we were able to prove that infections of SC with mycobacteria also occur in vivo. Additionally, killing assays suggested that the bacteria could survive within SC over several days. Of interest, SC accumulated lipids and showed changes in lipid metabolism during the infection, which might be relevant for the intracellular survival of mycobacteria as those are highly lipid-dependent.

Currently, we are performing transcriptome analyses to elucidate mechanisms of bacterial uptake in SC as well as MΦ-SC interactions.

Summary

Our data indicate that SC play a crucial role in mycobacterial infections and that they influence the MΦ immune response. It is highly relevant to identify potential host cells of mycobacteria as this is of importance for the duration of antibiotic treatment and the pathogenesis of latency in mycobacterial infections.