Immunity to Rhodococcus equi: antigen-specific recall responses in the lungs of adult horses.

The research investigates how adult horses resist Rhodococcus equi, the bacterium causing rhodococcal pneumonia, focusing on the immune responses in their lungs. It demonstrates that these horses can rapidly clear the infection, which provides evidence to support the creation of an effective vaccine for foals.

Research Hypothesis

• The study was based on the belief that most foals develop protective immune responses to Rhodococcus equi, a bacterium they are routinely exposed to in their environments. These reactive immune responses were hypothesized to carry on throughout the adult life of these horses, preventing the development of rhodococcal pneumonia.
• Understanding the principles of immune clearance in adult horses could give a significant foundation for creating an efficient vaccine for foals, who are most vulnerable to the disease.

Study Methodology

• The researchers infected adult horses with virulent Rhodococcus equi by inoculating it directly into the right lung.
• The pulmonary immune responses were monitored for two weeks by using bronchoalveolar lavage, a method for examining the airways.
• The responses in the inoculated right lung were then compared to those in the uninfected left lung and blood circulating in the peripheral area.

Results and Observations

• the infected horses cleared the R. equi infection quickly and without displaying significant clinical indicators of illness.
• the bacteria’s clearance was linked with an increase in mononuclear cells (mainly lymphocytes) in the fluid retrieved from bronchoalveolar lavage and the normal ratio of macrophage to lymphocyte was reversed.
• no significant rise in neutrophils was noted seven days post-infection.
• analysis demonstrated that the bacteria’s clearance was linked with a significant increase in the pulmonary T-lymphocytes, both CD4+ and CD8+ types.
• the clearance also correlated with marked increases in lymphoproliferative responses to the R. equi antigen and recombinant VapA antigen, a protein associated with the virulence of R. equi and a potential candidate for an immunogen.

Conclusions and Future Work

• the observed immune response supports the previous tests conducted on mice, which deduced that both CD4+ and CD8+ T-cells are critical in clearing the R. equi infection.
• the successful recognition of VapA antigen as part of the clearance process further underscores its potential as an immunogen for future testing.
• it was noticed that cellular responses to the R. equi challenge were relatively compartmentalized, with responses being more marked and sensitivity to antigen dose being increased at the site of the challenge.
• the peripheral blood and associated mononuclear cells were found to be poor indicators of local pulmonary responses.