Supervisor:Julie Guignot, team Bacteria and perinatality
Streptococcus agalactiae also named Group B Streptococcus (GBS) is a commensal bacterium found in 10-30% of human gastrointestinal and urogenital tractus, and the leading cause of invasive bacterial neonatal infections. Worldwide epidemiological studies highlight a strong association between capsular serotype III GBS strains, more precisely a specific Clonal Complex-17 (CC17) hypervirulent clone, and neonatal meningitis and death cases. It is well described that macrophages are a permissive niche for some pathogens, and that GBS can survive in those after being phagocytosed. Trying to understand the hypervirulence of CC17, we deciphered in this work the specific interaction of macrophages and CC17 GBS.
Using GBS invasive neonatal strains from the French National Reference Center and human monocytes and macrophages derived from THP1 cell line or from healthy donors primary cells, we showed an increased phagocytosis of GBS CC17 strains compared to non-CC17 strains. Capsular polysaccharide type is not involved in CC17 specific hyperphagocytosis. However, this CC17-specific hyperphagocytosis is due to a higher initial adhesion step to macrophages, mediated by CC17 specific HvgA surface protein and the PI-2b pilus protein Spb1. Next, we tried to identify receptors involved in adhesion and/or phagocytosis steps. Using scavenger receptors inhibitors, we observed a defect in GBS phagocytosis level by macrophages, suggesting their implication in GBS bacterial uptake. However, siRNA screening did not allow us to identify a specific phagocytic receptor. Interestingly, after phagocytic uptake both CC17 and non-CC17 GBS strains survived at similar rates within macrophages when compared to the intracellular uptake. Fluorescence microscopy imaging revealed that bacteria localize in phagocytic LAMP-1 positive and acidic vacuoles, suggesting that bacteria do not massively escape from the phagolysosome and that this one pursues its maturation. However, we observed some bacterial egress from macrophages to extracellular medium, independently of actin or microtubules, that could be due to a stochastic macrophage lysis effect.
Our findings bring out a surprisingly high and specific propensity of CC17 GBS strains for adhesion and to undergo phagocytosis by macrophages, and provide new insights into the interplay between CC17 hypervirulent GBS and the main actors of host innate immune response. Those results reinforce the hypothesis that CC17 GBS may highjack phagocytes to promote its own dissemination and the crossing of physiological barriers, critical steps in the pathophysiology of GBS neonatal invasive infections.
Key words: Streptococcus agalactiae, Group B Streptococcus, CC17, macrophages, monocytes, phagocytosis, adhesion, survival, HvgA, Spb1, capsule, receptor