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The Effect of Age on Equine Dendritic Cell Interactions With Rhodococcus equi
Department of Veterinary Sciences
Rhodococcus equi is considered the most common cause of severe pneumonia in young foals and remains one of the most important challenges to the equine industry. Current control methods for R. equi are expensive, inconvenient, and not always effective.
There is no effective vaccine against R. equi pneumonia of foals. The relative immaturity of the foal's immune system and the fact that initial exposure to the bacterium likely occurs in the first weeks of life diminish the prospects for developing an effective vaccine. Alternative strategies, including non-specifically enhancing the resistance of foals to this infection, are needed.
We have determined that part of the susceptibility of foals to this infection likely involves their inability to produce interferon-gamma, a key cytokine needed for resistance to this infection. Here we propose to identify the underlying mechanism responsible for this defect. Our approach is to examine the functionality of those cells which play a key role in regulating interfeorn-gamma production in the lung. Once the source of the defect in their ability to induce interferon-gamma is identified, a therapeutic approach to overcome this deficiency and increase interferon-gamma expression in foals might be found.
2010 Project Description
The reason for the unique susceptibility of young foals to infection with Rhodococcus equi remains unknown. An impairment of dendritic cell (DC) function resulting in a failure to induce Th1-type immune responses is characteristic of neonates. While it was initially reported that neonatal T cells appear to be heavily biased toward Th2 responses both in vitro and in vivo (1-2), this more likely reflects an inability to generate a Th1 (interferon-gamma) response due to insufficient accessory signaling by antigen presenting cells (3-4).
Exposure to microbial antigens appears to facilitate dendritic cell maturation in neonates leading to improved Th1 signaling (4-5). In situ stimulation of the cells with environmental microbial antigens could lead to maturation of these cells. To test this hypothesis, we collected and compared peripheral blood mononuclear cells and bronchoalveolar lavage samples from foals exposed to two different environments (Barn and Pasture) and compared their phenotype and in vitro responses to stimulation. Cytokine and chemokine receptor expression was determined using RT-PCR. All FACS (percent positive, MFI) and RT-PCR (fold increase) data were analyzed to identify foal age and treatment effects. We also developed a technique to analyze dendritic cells (DC) in the lungs using both BAL and samples isolated from lung sections. Cells were stained with monoclonal antibodies to identify DC populations.
While exposure to microorganisms in the air of the barn did not induce recruitment of more lymphocytes into the lungs of the foals, exposure did increase the frequency of interferon-gamma (IFNg)+ lymphocytes amongst BAL cells. Overall, the frequency of IFNg cells BAL was significantly (p=.0486) higher for the foals exposed to barn air compared to those left on the pasture. There was also a significant (p=0.045) difference between the two groups of foals at the 12 weeks.
The effect of barn air on IFNg production likely reflects the multiple and various microbial components it contains. The effect of environment on IFNg production was also observed in peripheral blood. It was found that the overall percentage of IFNg+ lymphocytes in the PBMC was significantly (p=0.0029) higher in those foals exposed to barn air. This effect lasted after the foals were returned to the pasture at the end of the exposure period with p<0.001 at 12 weeks, though eventually the pasture group of foals caught up to the level of IFNg production as those foals kept in the barn at 16 weeks. Similarly, IFNg production by whole blood cells increased more rapidly in infants that were exposed to higher environmental concentrations of endotoxin (1-2)
The mechanism whereby exposure to barn air promotes an increase in IFNg expression is unknown. One possibility is that environmental effects on gene expression occur via epigenetic regulation (3). Since the Ifng locus is highly methylated in neonatal T cells (4), increased demethylation of Ifng may be the mechanism behind the increased IFNg production following exposure to environmental microbial components within the barn. Other factors in epigenetic regulation may also contribute to the mechanism, such as regulation of chromatin structure and expression of transcription factors (3). Future studies will need to focus on these possible mechanisms.
Sun, L, A. Betancourt, A. Page, E. Oberst, A. Adams, N. Combs, D. W. Horohov. 2010. Environment affects interferon-gamma production in neonatal foals through possible effects on dendritic cells. Conference of Research Workers in Animal Diseases, December 6, 2010, Chicago, IL.