Electron microscopic investigation of intracellular events after ingestion of Rhodococcus equi by foal alveolar macrophages.

The research focuses on the interaction between a bacteria called R. equi and alveolar macrophages (immune cells in foal’s lungs). It explores how R. equi can survive and multiply within these immune cells, contributing to lung disease in foals. The paper attempts to elucidate the mechanisms by which R. equi evades the host’s immune response.

Methodology

• Researchers conducted an ultrastructural study to understand the changes occurring within immune cells when they interact with R. equi.
• Secondary lysosomes of alveolar macrophages (immune cells from the lungs) obtained from foals were labelled with a substance called ferritin, which appears dense under an electron microscope. This aided in tracking the lysosomes.
• The immune cells were then exposed to either viable (live) R. equi or those killed using formalin, a disinfectant and preservative.
• The cells were incubated for different durations (0, 3, 8, or 24 hours), fixed, and then examined under an electron microscope.

Key Findings

• No evidence of phagosome-lysosome fusion was observed after the immune cells ingested either the viable or non-viable R. equi.
• Phagosomes are vesicles formed around a pathogen by an immune cell, while lysosomes contain enzymes to break down such pathogens. Their fusion is a critical step in destroying ingested pathogens.
• R. equi was found to survive and multiply within dilated (expanded) phagosomes, which often had elongated microvillous (small projections) structures lining them.
• After a 24-hour incubation period, 75% of the ingested bacteria were still structurally intact, implying the immune cells were unable to break down and destroy the bacteria.
• The macrophages that ingested live R. equi were irreversibly damaged and released the bacteria into the surrounding medium.

Conclusion

• The study confirmed that R. equi can exist as a facultative intracellular parasite within alveolar macrophages of foals. This means it can survive both inside and outside cells.
• The bacteria can persist and multiply within the phagosome, and may inhibit the fusion of phagosomes with lysosomes, effectively evading the host’s immune defense mechanism. The exact method by which R. equi achieves this remains unknown and requires further investigation.