Membrane Binding, Cellular Cholesterol Content and Resealing Capacity Contribute to Epithelial Cell Damage Induced by Suilysin of Streptococcus suis

Abstract

Streptococcus (S.) suis is a major cause of economic losses in the pig industry worldwide and is an emerging zoonotic pathogen. One important virulence-associated factor is suilysin (SLY), a toxin that belongs to the family of cholesterol-dependent pore-forming cytolysins (CDC). However, the precise role of SLY in host–pathogen interactions is still unclear. Here, we investigated the susceptibility of different respiratory epithelial cells to SLY, including immortalized cell lines (HEp-2 and NPTr cells), which are frequently used in in vitro studies on S. suis virulence mechanisms, as well as primary porcine respiratory cells, which represent the first line of barrier during S. suis infections. SLY-induced cell damage was determined by measuring the release of lactate dehydrogenase after infection with a virulent S. suis serotype 2 strain, its isogenic SLY-deficient mutant strain, or treatment with the recombinant protein. HEp-2 cells were most susceptible, whereas primary epithelial cells were hardly affected by the toxin. This prompted us to study possible explanations for these differences. We first investigated the binding capacity of SLY using flow cytometry analysis. Since binding and pore-formation of CDC is dependent on the membrane composition, we also determined the cellular cholesterol content of the different cell types using TLC and HPLC. Finally, we examined the ability of those cells to reseal SLY-induced pores using flow cytometry analysis. Our results indicated that the amount of membrane-bound SLY, the cholesterol content of the cells, as well as their resealing capacity all affect the susceptibility of the different cells regarding the effects of SLY. These findings underline the differences of in vitro pathogenicity models and may further help to dissect the biological role of SLY during S. suis infections.

Streptococcus suis Research: Progress and Challenges

“…SLY has been suggested to play a role in these early infection events [24][25][26]. Vötsch et al [27] investigated the susceptibility of different respiratory epithelial cells to SLY, including immortalized cell lines (HEp-2 and NPTr cells), which are frequently used in in vitro studies addressing S. suis virulence mechanisms, as well as primary porcine respiratory cells. It was observed that the human epithelial cell line HEp-2 is the most susceptible, whereas primary porcine epithelial cells were hardly affected by the toxin.…”

“…The authors suggested that the choice of cells for in vitro studies on biological functions of virulence-associated factors, such as SLY, should be performed with models which more closely represent the in vivo situation than permanent cell lines. The study of Vötsch et al [27] also emphasized the importance of choosing the proper host species, as previous studies showed that SLY is a critical virulence factor in the mouse model, but not in pigs [28,29].…”

Research Progress on the pathogenic mechanism of Streptococcus suis 2

“…Sly can lyse and damage a variety of cells and plays a vital role in SS2 invading the central system and destroying the BBB. In addition, Sly can induce the changes of host cytoskeleton (Lv et al, 2014) and the release of proinflammatory and immunomodulatory cytokines and chemokines, such as interlukin-6(IL-6), interlukin-8(IL-8), tumor necrosis factor-α(TNFα), and interlukin-8(IL-10) (Vötsch et al, 2019).…”

New Insights into Neutrophil Extracellular Trap (NETs) Formation from Porcine Neutrophils in Response to Bacterial Infections

“…S. suis produces a pore-forming cholesterol-dependent cytolysin (Suilysin) [ 41 , 42 , 43 ]. The different effects of this toxin on neutrophils are discussed [ 44 , 45 ].…”