Interaction of Rhodococcus equi with phagocytic cells from R. equi-exposed and non-exposed foals.

The study investigates how Rhodococcus equi, a bacterial parasite, interacts with specific immune cells in both adult horses and foals that have been previously exposed and those not exposed to the organism. It reveals that R. equi can survive and multiply within certain immune cells by impeding a specific immune response, and this aiding process can be disrupted by a protein from the body, leading to a better killing response of the immune cells.

Understanding Rhodococcus equi Interaction

• The research investigates how Rhodococcus equi (R. equi), a bacterium that causes pneumonia in foals, interacts with alveolar macrophages, a type of immune cell found in the lungs. The researchers examined this interaction in adult horses, foals that were experimentally exposed to R. equi (sensitized foals), and foals that were not exposed.
• R. equi was shown to be a facultative intracellular parasite, meaning that it can survive and multiply inside the host’s cells, specifically the alveolar macrophages.
• This survival strategy involves interfering with the fusion of the phagosome, a vesicle formed around the bacteria by the immune cell, with the lysosome, the part of the cell that destroys bacteria. This interference allows R. equi to avoid being killed and digested by the host cell.

Role of Opsonization and Impact on Non-Exposed Foals

• Opsonization, the process by which pathogens are marked for destruction by the immune system, appears to be crucial in combating R. equi. When the researchers opsonized R. equi with an antibody against elements of its capsule, the fusion of the phagosome and lysosome was significantly enhanced, leading to better killing of the bacteria.
• However, macrophages from foals that were not exposed to R. equi struggled to effectively kill the bacteria, even though they were capable of ingesting it. These cells could only kill up to 65% of R. equi after 6 hours of exposure.

Response of Sensitized Foals and Role of Lymphocyte Factors

• Conversely, sensitized foals displayed a more effective response, with their macrophages able to kill over 95% of the R. equi bacteria within the same time period. This is comparable to the response of adult horses.
• The study also highlighted the significant role of lymphocyte factors, substances produced by a type of white blood cell upon exposure to the antigens (surface proteins) of R. equi. The presence of these factors increased the ability of macrophages to kill the bacteria, with non-exposed foals showing a 50% increase, and sensitized ones showing more than 100% increase in killing capacity.