Immune response in Rhodococcus equi infected foals

Rhodococcus equi (R. equi) is an intracellular, gram-positive coccobacillus that causes pneumonia in foals aged 2 to 4 months. Neonatal foals are susceptible to R. equi infection probably due to inefficient Toll-like receptor (TLR)-2 signaling and inability to produce interferon gamma. One of the reasons for inefficient receptor signaling and recognition of R. equi by the foal’s immune system may be the inefficient sequestration of TLRs in lipid rafts, which act as signaling platforms. However, there are no protocols to isolate lipid rafts from equine cells and, therefore, no data on the association of TLRs with the lipid rafts in the lung cells of normal and infected foals. Because of the clinical importance of the disease, there is considerable interest in developing effective prophylactic methods, which in turn requires a better understanding of fundamental immunology of the foals. In this study, I have examined the effect of R. equi vaccination on the lung inflammation induced following challenge with R. equi. I also developed a protocol to isolate lipid rafts from broncho-alveolar lavage (BAL) cells and investigated the association of lipid rafts with TLRs. In the first study, 15 mixed breed draft-type foals up to 7 weeks of age were studied. The foals were divided into control (n=7) and a vaccinated (n=8). The control foals were given 10 mL phosphate buffered saline intramuscularly while the vaccinated group was vaccinated on day 0 of the study followed by a booster on day 14. All the foals were challenged with R. equi (5x106 cells/mL into the dorso-caudal region of the right lung lobe). BAL was performed on day 14, 28 and 35 and all the foals were euthanized on day 49 of the study. The study design did not leave any non-infected foal at the end of the experiment. Therefore, lung samples were obtained from two untreated control (non-vaccinated non-infected) foals from the Department of Veterinary Pathology, University of Saskatchewan were used. The data showed similar levels of lung inflammation in both the control and vaccinated foal groups based on BAL cytology, gross pathology and histopathology. Gross and histopathological studies indicated that both control and vaccinated foals developed granulomatous lesions. Immunohistology showed increased expression of TLR4, TLR2 and TNF alpha in alveolar septa and in some cases in the vascular endothelium and airway epithelium in the lungs of both groups compared to the untreated control foals. Western blots showed increased expression of TLR2 but not TLR4 in the lung extracts from both the vaccinated and the control foals. Vaccinated foals showed higher concentrations of TNF alpha(p=0.0219) in their BAL on day 28 but lower concentrations of IL-10 (p=0.0172) in their lung extracts collected on day 49 compared to the controls. There were no differences in IFN gamma and protein concentrations between the two groups. To understand the role of lipid rafts in TLR4 and TLR2 signaling, I developed an efficient and simpler protocol to isolate lipid rafts from BAL cells of foals and confirmed their identity by localizing Flotillin-1 and GM-1 (fractions 6-9), which are lipid raft markers, and transferrin receptor (fractions 1-4) which is present in non-lipid raft fractions. Lung macrophages from naïve foals lacked sequestration of Flotillin-1 and GM-1 in the higher fractions compared to the vaccinated foals. Further, the data showed that while TLR4 and TLR2 were localized in most of the fractions (1-9) in control foal BAL collected on day 14 and 28, the TLR4 and TLR2 association was restricted to fractions 6-9 in the lipid rafts isolated from BAL cells of vaccinated foals. These data suggest that BAL cells of neonatal foals may not have effective signaling machinery because of lack of association of TLR2 and TLR4 with lipid rafts. Taken together, the data show similar levels of lung inflammation in the control and vaccinated foals upon infection with R. equi. The vaccination, however, appeared to have some effect on the immunohistologic expression of TLR2, TLR4 and TNFalpha in the lung tissues, and increased association of TLR2 and TLR4 with the lipid raft fractions. Based on the higher expression of TNF alpha and lower expression of IL-10, the vaccinated foals may be more competent to mount an immune response against R. equi