Effects of opsonization of Rhodococcus equi on bacterial viability and phagocyte activation.

The research investigates the effect of the opsonization process of Rhodococcus equi bacteria with specific antibodies in plasma on bacterial survival and activation of phagocytes (immune cells) in a cell culture model of infection. The findings suggest that this process increases the immune response and reduces bacterial survival outside cells, indicating a potential role of plasma antibody in protecting against diseases caused by this bacterium.

Methods and Process

• The researchers used neutrophils and macrophages (both types of immune system cells) obtained from six healthy one-week-old foals and one adult horse for the study.
• These immune cells were exposed to either opsonized or non-opsonized Rhodococcus equi bacteria. “Opsonized” refers to bacteria that have been coated with antibodies, specifically those against R. equi. The plasma used for opsonization contained either high or low concentrations of R. equi-specific antibodies.
• The researchers then monitored the immune response triggered by these cells, including the oxidative burst activity (a defense mechanism used to kill pathogens) and the production of a protein, tumor necrosis factor-alpha (TNF-alpha), which is involved in systemic inflammation and immune system regulation.
• The measurement of oxidative burst activity was done using flow cytometry – a technique to analyze the physical and chemical characteristics of particles or cells in a fluid as they pass through a laser. The production of TNF-alpha was measured using ELISA (a common immune assay for detecting and quantifying substances such as peptides, proteins, antibodies, and hormones).

Results

• The researchers found that bacteria that had been opsonized led to an increased oxidative burst activity in adult horse phagocytes. Additionally, the level of activation in neutrophils was dependent on the concentration of the specific antibody used for opsonization.
• The study found that macrophages produced more TNF-α when they were infected with opsonized bacteria compared to non-opsonized.
• Opsonization did not have a significant impact on the survival of bacteria within macrophages. However, the viability of bacteria in the extracellular space (outside the cells) was reduced when they were in a solution that contained plasma with R. equi-specific antibodies compared to a solution without them.

Conclusions

• The researchers concluded that enriching plasma with specific antibodies to facilitate the opsonization of Rhodococcus equi enhances phagocyte activation and reduces bacterial survival in the extracellular space.
• Despite the finding that opsonized Rhodococcus equi increased TNF-α secretion and oxidative burst activity in macrophages, the researchers suggest that additional factors may be needed for an effective killing of bacteria within cells.
• This research may indicate a possible role of plasma antibodies in protecting against diseases, such as pneumonia, caused by R. equi in foals.