We have characterized the cell types in which efficient growth of Leishmania major takes place in the infected skin. Using intravital 2-photon imaging of the tissue infected with Leishmania major that expresses a biosensor for growth, we could determine that the growth rate of the pathogen is strongly linked to specific cell types. Using a series of experiments to synchronize the arrival of cells, mainly monocytic phagocytes (monocytes) at the site of infection, and by in-depth single cell analysis, we could show that the monocytes infected by Leishmania major at the site of infection are heterogenic, ranging from pro- through anti-inflammatory phenotypes which coexist at the same tissue site. These findings are not only important for the elucidation of the cell tropism of the pathogen Leishmania major, but also expand our knowledge on how recruited monocytes are activated and exert their function upon entry of the site of infection.
The pro-inflammatory, newly recruited monocytes were revealed to be the cell type which harbours mainly fast-growing pathogens. This suggested that high pathogen proliferation is linked with the transfer of Leishmania major to new host cell, which we could show is the case and, since it was outside of the focus of the funded research, lead to the application of a follow-up project dealing exclusively with the question of pathogen exit from the infected host cell. As efficient Leishmania proliferation required newly skin-recruited monocytes, and since the antimicrobial effector nitric oxide (NO) can inhibit entry of these cells into infected tissues, we investigated, whether NO dampens Leishmania proliferation indirectly, limiting the pathogen’s cellular niche. Indeed, we could show that restriction of proliferation-permissive host cell recruitment by NO represents a mechanism that controls Leishmania major infection.
The anti-inflammatory cells on the other hand, infected with mainly slow-proliferating Leishmania major were found to be the cell type most different from any other monocyte subset investigated. We found a number of genes, including IL7R and Cathepsin B specifically up-regulated in this cell population. Ablation of these genes, both globally and limited to myeloid cells (of which monocytes constitute the largest part at the site of infection), resulted in an enhanced control of Leishmania major. For IL7R it was shown that a main effect in the cell population infected by slow proliferating Leishmania major (termed LowP) might be the reorganization of the extracellular matrix at the site of infection, while Cathepsin B seems to inhibit efficient T cell activation. Therefore, the LowP population might serve as a “control centre” that enables pathogen persistence at the site of infection, possibly via IL7R and Cathepsin B mediated mechanisms.
The results obtained in the project were included in nine publications, seven of which included first and/or corresponding authorships by members of the workforce (Heyde et al., 2018; Brandt et al., 2020; Seiß et al., 2019; Baars et al., 2021; Regli et al., 2020; Handschuh et al., 2020; Kleinholz et al., 2021; Formaglio*, Alabdullah* et al., 2021; Krone*, Fu* et al., 2022).
The results were also presented in grant applications for competitive funding by the German research foundation DFG, resulting in two successful DFG research grants to the PI (DFG MU 3744/5-1. DFG MU 3744/6-1).
