Converge and Contrast: Leishmania donovani’s Impacts on Uninfected Bystander Cells by Life Stage 2259952

Abstract Introduction Visceral leishmaniasis (VL) remains the second most lethal parasitic disease in the world. As a neglected tropical disease, treatments are limited. During VL, systemic infection affects primarily phagocytic cells in the liver, spleen, and bone marrow. There is massive modulation of the immune system. Some immune evasion mechanisms are already known for both the promastigote (initiation) and amastigote (chronic) forms of the parasite. Moreover, our prior observations in mouse macrophages have shown NF-κB sensing is blocked for infected and uninfected bystander cells during promastigote infection. Thus, we aimed to verify bystander immunosuppression in a human model and explore further its impact and the effect of chronic infection. Methods This project used two modes of assessing the bystander immune response. First, we used a human transgenic U937 monocyte line with Green Fluorescent Protein on a NF-κB-responsive promoter as a high-throughput detector. A Dsred2 expressing L. donovani was used for promastigote or amastigote (isolated from animals) infections in vitro. Also, we evaluated single cell RNA sequencing (scRNA-seq) data to explore gene expression from in vivo murine chronic VL in infected and bystander bone marrow monocytes. Results In vitro, our reporter cells have identified that the bystander immunosuppression effect occurs through media components < 3kDa, independent of direct contact to the promastigote. Meanwhile, in the amastigote infection, we saw in vitro NF-κB bystander activation rather than suppression. Also, scRNA-seq revealed that TGF-β is highly upregulated in bystander monocytes among other differentially expressed genes (DEGs). Conclusion We have for the first time confirmed NF-κB bystander modulation during promastigote and amastigote infections of the human model cells. We also have identified DEGs of interest in chronic infection. These various effects by life stage highlight the important role for NF-κB in VL progression. Funding Source GR110769 Topic Categories Microbial, Parasitic, and Fungal Immunology (MPF)