The role of neutrophils in Streptococcus suis infection

Ethiopian proverb: A mighty friend can become a mighty foe.

Marêva Bleuzé, Mélanie Lehoux, Marcelo Gottschalk, Mariela Segura

Swine and Poultry Infectious Diseases Research Center, Research Group on Infectious Diseases in Production Animals; Faculté de médecine vétérinaire, Université de Montréal

Article published in La terre de chez nous on December 14, 2020: https://www.laterre.ca/chroniques/page-de-conseils/le-role-des-neutrophiles-dans-linfection-a-streptococcus-suis/#:~:text=Ils%20agissent%20comme%20un%20groupe,communiquant%20avec%20les%20autres%20cellules.

As illustrated by the COVID-19 pandemic, the microorganisms responsible for infections are a perpetual challenge for humanity. Although we have successfully controlled many infections of the human, animal, and plant kingdoms, pathogens continue to impact many areas of our lives. So what can we do? The response of researchers in the field of infectious disease often consists of seeking preventive (such as vaccines) or therapeutic (such as antibiotics or their alternatives) solutions in response to a disease: this field of research is called applied science. However, a large part of the research, such as that performed in basic science, is not aimed to the finding of solutions but to the understanding of the biological mechanisms involved in infection. Basic science is essential, upstream of applied science, to imagine new approaches in the fight against pathogens.

To illustrate, consider infections with Streptococcus suis (S. suis), a porcine pathogen responsible for economic losses in livestock farming, which causes animal welfare concerns and is increasingly becoming a niche for antibiotic resistance. Sick pigs develop endocarditis, arthritis, meningitis or die suddenly. Basic science has shown that S. suis infection causes exacerbated inflammation in the animal body. This means the immune system, which usually protects the body against infections, overreacts and damages the body.

Imagine the immune system as an army of cells, each one with a specific role: some are sentinels, patrollers, or effectors. Among these cells, neutrophils quickly spot a threat and mobilize to get rid of it: they are the enemies of pathogens. They act as a tactical intervention group and deploy an arsenal of very aggressive mechanisms in contact with pathogens. On one hand, neutrophils are able to swallow bacteria, destroy them, produce toxic compounds and capture them in nets, while communicating with other immune cells. On the other hand, the behavior of neutrophils is far from being harmless to the other cells that make up the body ("collateral damage"). Thus, neutrophils must be closely controlled to avoid exacerbating inflammation. A molecule, named G-CSF (granulocyte colony-stimulating factor), controls their circulation and their activation. G-CSF production releases a bulk of neutrophils throughout the body, ready to strike back.

However, during Streptococcus suis infection, G-CSF is strongly produced. Does this production mobilize neutrophils to effectively destroy the bacteria, or does it heighten the immune response and damage the host? Hard to say. However, basic science has not yet said its last word, and studies will soon show us whether G-CSF is the ally or the enemy of the immune defense against S. suis. In the end, applied science could then look at the use of G-CSF (or its neutralization) in the fight against Streptococcus suis.