The Harry M. Zweig Memorial Fund for Equine Research

How Does the Immune System of the Foal Fight
Against Rhodococcus equi Infections?

Dr. Julia Flaminio

Foals less than 5 months of age, but not older, are susceptible to Rhodococcus equi . This bacterium causes severe pneumonia, enteritis and occasionally joint infection, with significant economic losses in the horse industry. The bacterium is prevalent in the horse environment (soil), and both virulent and avirulent forms are present. The virulent bacterium is capable of replicating inside immune cells called macrophages.

The facts that foals become resistant to R. equi infection beyond 5 months of life, and immunocompromised (e.g. AIDS) human patients are also susceptible to R. equi strongly suggest that young foals present age-dependent limitations in the protective immune response for the clearance of infection.

Our hypothesis is that adult horses have specialized immune cells (dendritic cells) that are responsible for the activation and stimulation of the effector cells involved in the protection against infection, whereas in foals, those cells are immature and cannot support the effector cells. We would like to investigate how R. equi infected dendritic cells of the foal signal to the effector cells in comparison to the adult horse.

In the last 2 years, we performed a study funded by the Harry M. Zweig Memorial Fund in which we tested the maturation and activation of foal macrophages and dendritic cells to synthetic cytosine-phosphate-guanosine oligodeoxynucleotide (CpG-ODN) motifs that mimic bacterial DNA. In vivo , CpG motifs are known to induce a strong Thl response, hence a cellular-mediated, cytotoxic response that is necessary for the killing of R. equi infected cells. Using CpG-ODN as a tool to evaluate the response to bacterial DNA in foals, and comparing that to adult horse response, we learned that foal dendritic cells have an age-dependent ability to present processed antigens to lymphocytes. These findings raised the questions about their function in disease processes, including R. equi infection.

The results of the proposed study may elucidate the limiting factor in protection against R. equi infection in young foals. The understanding of the faulty mechanisms may guide future studies for the prevention of disease, including the development of protective vaccines.

Objectives of this Proposal:

In this proposed investigation, we will attempt to measure the effect of virulent (causes disease) or avirulent (does not cause disease) R. equi on in vivo generated monocyte-derived macrophages and dendritic cells of adult horses and foals of different ages (from birth to 5 months of life).

The steps of this proposed investigation are:
1) First year- development of the in vitro model and characterization of the adult horse dendritic cell immune response to R. equi . The adult horse sample analysis will have a three-fold objective: a) establish the critical time-points for cell harvesting of R. equi infected cells for the different measurements; b) understand the dendritic cell response to R. equi ; and c) provide standard values for comparison with the foal samples.2) Second year- evaluation of the response of dendritic cell of foals of different ages (birth to 5 months of age) to virulent and avirulent R. equi . In addition to the comparison to adult horse cell values and age-dependent analysis, the effect of the virulence factor on macrophage and dendritic cells function will be tested. Using the same in vitro system, we will collect preliminary data of bacterial intracellular survival in dendritic cells.

Importance for the Horse Industry:

Although most parts of the immune system of the horse develop during fetal life, foals are very susceptible to infection within the first 5 months of age. Their susceptibility to infection in the neonatal period is partially dependent on the adequacy of transfer of maternally-derived antibodies through the colostrum. The horse placenta does not allow transfer of antibodies during gestation; hence the foal is born essentially devoid of antibodies. The maternally-derived antibodies absorbed in the first few hours of life confer short-lived, limited protection to the foal against environmental pathogens for the initial 1 to 2 months of life. After this period, the maternal antibodies reduce to very low levels, and the foal must depend on its own immune system to resist infections.

The earlier the foal can assemble its own immune system to fight infection, the more resistant it will be to disease. Conversely, the less mature the immune system, the more susceptible it will be to pathogenic and opportunistic organisms in the environment. Some organisms may take advantage of the immature elements of the immune system of the foal to cause disease, including R. equi . 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 would reduce the period of susceptibility to pathogenic organisms, and the chances of disease. Thus, 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.