The Harry M. Zweig Memorial Fund for Equine Research

The Phagocyte Response Against R. Equi in Foals

Dr. Julia Flaminio and Dr. Ute Schwab

FlaminioDr. Julia Flaminio

Susceptibility to R. equi pneumonia in foals is exclusive to the first 2-3 months of life. This bacterium causes severe pneumonia, enteritis and occasionally joint infection, with significant economic losses to the horse industry, and critical concerns about equine health and well-being. R. equi is prevalent in the horse environment (soil), and foals are exposed to it immediately after birth. Importantly, R. equi survives and replicates inside immune cells (macrophages); therefore, this bacterium has developed a mechanism to escape the immune system and cause disease. 

Our hypothesis is that phagocytes of the susceptible foal have impaired killing activity against R. equi because of inadequate signaling from the airway epithelial cells.

Our previous studies funded by the Harry M. Zweig Memorial Fund for Equine Research revealed that specialized phagocytes present an age-dependent limitation in their activation, which may be a consequence of inadequate stimulus. This age-dependent development of the immune system of the foal may be involved, at least in part, in the exclusive susceptibility to disease in early life. We know from studies in mice that efficient intracellular killing of R. equi is dependent on the activation of phagocytes by immune mediators (cytokines), for the generation of pathogen killing products. Cytokines can be produced by both immune cells and respiratory epithelial cells. 

We would like to include in our studies the cells of the respiratory tract that encounter the bacterium in the early stages of infection. We will test the direct effect of cytokines on the immune cells for killing of R. equi. In addition, we want to learn how the epithelial cells that line the respiratory tract signal phagocytes for the presence of R. equi. The failure of these cells in the foal may lead to the progression of the infection and development of the disease. Achieving these aims will readily enhance our ability to understand this disease, such that it can be prevented and managed more effectively. 

Objectives of this Proposal:

In this proposed investigation, we will: 1) test the capacity of foal phagocytes to kill R. equi in the presence or absence of cytokines in culture. We will measure bacterial survival, and the generation of pathogen killing products by the foal phagocytes, and compare those to adult horse cells. 2) examine how the function of phagocytes is affected by contact with the airway epithelium and, conversely, the modulation of epithelial cells by the presence of phagocytes and R. equi. For this purpose, we have already established a three dimensional laboratorial model of equine bronchial epithelial tissue, which expresses a fully differentiated cell phenotype with ciliary beating and mucus production.  This is a very unique model that utilizes excess tissues from horses in necropsy, therefore with no need to euthanize animals for this purpose. This system will allow us to make observations of the phagocytic and bactericidal capacity of macrophages in living airway tissues from foals and adult horses. Bacterial survival and the generation of pathogen killing products by phagocytes will be measured as above.  In addition, we will determine the cytokines produced by macrophages and epithelial cells.

Importance for the Horse Industry:

The results of this study may elucidate the limiting factors in protection against R. equi disease in young foals. The understanding of the faulty mechanisms may guide future studies for the prevention of disease, including modulation of the immune system of the foal. Conventional vaccines for Rhodococcus equi have been designed and tested, but result in only marginal protection early in life. Therefore, the identification of an effective means to augment the immune system of foals soon after birth (for instance, an immunostimulant) would reduce the period of susceptibility to pathogenic organisms, and the chances of disease. This project will attempt to identify the areas of the immune system of the foal that are not competent to fight R. equi infection. The information from this study will support future investigations on prophylaxis for R. equi infection in the young foal.