Influence of Streptococcus suis SLY and DltA on the crosstalk between innate immune cells in different host compartments

• Applicants: Dr. Nicole de Buhr; Desiree Schaaf, Ph.D.; Dr. Sophie Öhlmann
• Subject Area: Veterinary Medical Science
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524623524

Streptococcus suis is a frequent colonizer of the upper respiratory tract of pigs, but can also cause severe systemic diseases like meningitis and septicemia. However, pathogenesis of S. suis infection and the role of its virulence-associated factors, namely the pore-forming toxin suilysin (SLY) and the D-alanine-D-alanyl carrier ligase (DltA), is still not fully understood. So far, it is known that SLY damages different host cells by lytic pore formation and induces an inflammatory response leading to the release of cytokines, especially by monocytes/macrophages, and the recruitment of neutrophils. D-alanylation of lipoteichoic acids in the cell wall by DltA is known to increase resistance against antimicrobial peptides (AMPs) and phagocytosis of S. suis by neutrophils. Our aim is to clarify the role of SLY and DltA in S. suis colonization of the porcine respiratory tract, systemic dissemination via the bloodstream, and invasion of the central nervous system. Thereby we will focus on their influence on innate immunity cells and the crosstalk between monocytes/macrophages and neutrophils. We hypothesize that the host compartments influence the expression of sly and dltA differentially and that vice versa SLY and DltA influence the host’s innate immune response by modulating the crosstalk between neutrophils and monocytes/macrophages contributing to the resistance of. S. suis towards immune defense mechanisms. To investigate this, we will use complex in vitro cell culture systems that closely mimic the in vivo situation of the three main host compartments: a co-culture system of primary respiratory epithelial cells differentiated under air-liquid interface conditions and alveolar macrophages, porcine precision-cut lung slices, a reconstituted whole blood model, and the blood cerebrospinal fluid barrier model. Infection experiments will be performed with S. suis serotype 2 wild-type strain (wt), its isogenic mutants Δsly, ΔdltA, and ΔdltAΔsly as well as the respective complemented mutant strains. We will start with the analysis of sly and dltA expression in the three compartments by quantitative real-time PCR and Western blotting, followed by the investigation of the neutrophil and the monocyte/macrophage response towards S. suis wt and sly- and dltA-deficient mutants. We will focus on the formation of neutrophil extracellular traps, reactive oxygen species production, phagocytic activity, and the release of certain cytokines and AMPs. Finally, we will investigate how secretions from monocytes/macrophages induced by S. suis infection influence neutrophils regarding their transmigration, inflammatory response, and phagocytic capacity, and vice versa. The detailed investigation of the innate immune response towards S. suis infection in the three different compartments will help to better understand the switch of S. suis as a colonizer to an invasive pathogen, and how SLY and DltA are involved in this process.

• DFG Programme: Research Grants