Effects of inoculum size on cell-mediated and humoral immune responses of foals experimentally infected with Rhodococcus equi: a pilot study.

This research investigates how the size of an infection inoculum impacts the immune response in foals, particularly in relation to Rhodococcus equi, a bacterium causing pneumonia. The study found that varying the size of the infectious dose altered the foals’ immune response.

Research Objective

• The purpose of this pilot study was to assess the impact of varying the size of inoculum (the bacteria dose used for infection) on the immune responses in foals (young horses). Two scenarios were examined: infection with a low dose of Rhodococcus equi, which leads to a mild form of pneumonia, and a high dose, which brings about severe clinical pneumonia.

Cytokine Profiles and Immune Responses

• The researchers compared the cytokine profiles, cell-mediated responses, and antibody responses of the foals in both scenarios. Cytokines are small proteins released by cells that affect the interactions and communications between cells. Cell-mediated responses refer to the activation of immune cells to fight off pathogens, while antibody responses concern the production of proteins which neutralize specific bacteria or viruses.

Key Findings

• The study found considerable differences in the IgG(T) antibody concentration (specifically related to Rhodococcus equi) between the two groups post-infection. Foals infected with the larger inoculum demonstrated significantly higher concentration levels.
• Similarly, post-infection concentrations of another type of antibody, IgM, were significantly higher in the foals infected with the high inoculum.
• However, no significant difference was observed in the proliferative responses to R. equi antigens between the two groups. Antigens are substances that trigger the immune system to produce an immune response.
• The study noted a trend suggesting the possibility of a higher IFN-gamma/IL-4 ratio in the group infected with the low dose of Rhodococcus equi. IFN-gamma and IL-4 are two types of cytokines, with this ratio often serving as an indicator of the balance between two major types of immune responses.
• This research indicates that the size of the inoculum can influence the specific antibody response (IgG subtype) and potentially the cytokine profile in foals.