Group A Streptococcus Prevents Acidification of the Phagolysosome: Role of Lysosomal Leakage

Abstract

Group A Streptococcus (GAS) is a Gram‐positive bacterium that is among the top 10 causes of infection‐related mortality in humans, with infections ranging from minor illnesses such as pharyngitis, to life threatening diseases such as necrotizing fasciitis and streptococcal toxic shock syndrome. As the first response to infection, the innate immune system recruits leukocytes such as macrophages to the site of GAS infection, which are able to phagocytose GAS. Once GAS is inside the phagosome, the lysosome fuses with the phagosome to form the phagolysosome. To degrade pathogens such as GAS, the phagolysosome reduces its internal pH to activate proteolytic enzymes. To monitor this process, we infected THP‐1 macrophages with M1 serotype GAS transformed with a plasmid (pWASABI) that expresses a pH‐sensitive green fluorescent protein that is degraded at low pH. Our results showed that GAS expressing pWASABI has a persistent fluorescent signal when bacteria were present in phagolysosomes. In comparison, the non‐pathogenic Gram‐positive species Lactococcus lactis expressing pWASABI had an appropriate decreased fluorescence signal. These results suggest that GAS inhibits the acidification of the phagolysosome. To further confirm the pH of the phagolysosome, we stained infected macrophages with Lysotracker, an acidotropic fluorescent dye. The preliminary results of this experiment confirmed that phagolysosomes containing GAS are not acidified. Previous research has suggested that pore‐forming toxins such as streptolysin O (SLO) and streptolysin S (SLS) induce phagolysosomal leakage of protons. To examine this, we transformed a mutant strain lacking SLO (ΔSLO) with pWASABI. Our preliminary results showed no difference in fluorescence signal when the ΔSLO strain was present in phagolysosomes, suggesting that SLO may not be inducing lysosomal leakage. We are currently exploring whether pore formation by SLS plays a role in lysosomal leakage. The results of these experiments will help provide a clearer image of the molecular mechanism of innate immune resistance of GAS, and may aid in the development of therapeutics aimed to enhance the innate immune system's ability to degrade GAS.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.