Capsular polysaccharide mediates Streptococcus agalactiae to resist Nile tilapia macrophage phagocytosis

Streptococcus agalactiae (S. agalactiae) is a highly pathogenic gram-positive bacterium for farmed tilapia. The surface capsular polysaccharide (cps) containing terminal sialic acid (sia) residues is a crucial virulence factor for S. agalactiae. However, there is limited knowledge regarding the effects of cps and sia on the phagocytosis of S. agalactiae by teleost macrophages (MCYRILLIC CAPITAL LETTER EF). In this study, we investigated the role of cps and sia in tilapia MCYRILLIC CAPITAL LETTER EF phagocytosis of S. agalactiae using cps-deficient mutant Delta cps and sia-deficient mutant Delta neuA. Nonopsonic phagocytosis of wild type (WT), Delta cps and Delta neuA was initially analyzed by flow cytometry. The results indicated that MCYRILLIC CAPITAL LETTER EF phagocytosis of Delta cps and Delta neuA was significantly higher than that of WT. The proportion of MCYRILLIC CAPITAL LETTER EF with higher intracellular reactive oxygen species (ROS) or nitric oxide (NO) levels, and overall mean fluorescence intensity (MFI) of the ROS or NO in MCYRILLIC CAPITAL LETTER EF increased higher during nonopsonic phagocytosis in the Delta cps and Delta neuA. The expression levels of pro-inflammatory factors (IL-1 beta, IL-6, TNF-alpha) during S. agalactiae infection revealed that S. agalactiae could dampen the inflammatory response of MCYRILLIC CAPITAL LETTER EF through the cps and sia. Increased susceptibility of the Delta cps and Delta neuA to phagocytic killing indicated that S. agalactiae could impair the phago-cytic clearance of MCYRILLIC CAPITAL LETTER EF through the cps or sia. Furthermore, opsonic phagocytosis of the Delta cps and Delta neuA by MCYRILLIC CAPITAL LETTER EF was dramatically enhanced compared to the WT under complement or antibody mediation. Moreover, relative to Delta cps and Delta neuA, nonopsonic and opsonic phagocytic receptors were significantly down-regulated during MCYRILLIC CAPITAL LETTER EF phagocytosis of WT, indicating that S. agalactiae might resist phagocytosis of MCYRILLIC CAPITAL LETTER EF by directly or indirectly inhibiting the expression of phagocytic receptors through the cps or sia. Our data suggest that cps plays a crucial role in the interference of S. agalactiae on the phagocytosis of tilapia MCYRILLIC CAPITAL LETTER EF, and sia may be a significant immune escape site. These results provide valuable insights into the mechanism of S. agalactiae attempting to escape the defenses of teleost innate immune system and offer clues for S. agalactiae vaccine development.