Identification of immunologically relevant genes in mare and foal dendritic cells responding to infection by Rhodococcus equi

Rhodococcus equi is a facultative intracellular bacterial pathogen of horses; infected foals develop pyogranulomatous pneumonia, however adult horses are largely unaffected. R. equi infects and proliferates within host macrophages and dendritic cells (DCs). DCs initiate the appropriate adaptive immune response, thereby playing a critical role in determining the outcome of infection. Our aim was to identify genes that are differentially expressed in R. equi infected monocyte-derived DCs (mdDCs). Peripheral blood monocytes from mares and foals were used to derive mdDCs by culturing with recombinant equine IL-4 and recombinant human GM-CSF. RNA harvested 24 h after infection with R. equi (ATCC 33701+) was used to perform suppression subtractive hybridization (SSH) experiments. Approximately 38 unique sequences were obtained from these experiments. Differential expression of 19 immunologically relevant genes was validated by PCR. These genes are characterized by the following functions: cell adhesion, chemotaxis/migration, immune/inflammatory response, ion transport, signal transduction, T-cell regulation, and vesicular transport. In summary, we identified several novel genes that are differentially expressed in foal and adult mdDCs in response to R. equi infection. These genes provide promising targets for further research into the host response to R. equi, and the susceptibility of foals to this disease.