Interactions of Streptococcus iniae with phagocytic cell line

Area of Science:

• Microbiology
• Immunology
• Aquatic Animal Health

Background:

• Streptococcus iniae is a significant aquatic pathogen causing substantial economic losses in fish.
• This bacterium poses a zoonotic risk, capable of infecting humans.

Purpose of the Study:

• To investigate the interaction between Streptococcus iniae and murine macrophage cell lines (J774-A1 and RAW 264.7).
• To understand how S. iniae modulates macrophage immune responses for intracellular survival.

Main Methods:

• Cytotoxicity assays were performed on macrophage cell lines exposed to live and killed S. iniae strains.
• Respiratory burst activity was measured post-infection.
• Immunofluorescence and ultrastructural studies confirmed intracellular bacterial localization.
• Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) arrays analyzed immune gene expression changes.

Main Results:

• Live S. iniae exhibited differential cytotoxicity compared to UV-killed strains.
• Macrophage respiratory burst activity decreased significantly after S. iniae exposure.
• Intracellular S. iniae was confirmed at 1 and 24 hours post-infection.
• Eleven of 84 immune-related genes were upregulated, including pro-inflammatory cytokines like IL-1α, IL-1β, and TNF-α, with limited anti-inflammatory response.

Conclusions:

• Streptococcus iniae can survive and replicate intracellularly within murine macrophages.
• The bacterium modulates macrophage responses, promoting pro-inflammatory cytokines and potentially evading immune clearance.
• Macrophages may act as a reservoir for S. iniae, facilitating persistent infection.