Innate immune responses to the intracellular pathogens Brucella abortus and Rhodococcus equi: Directing research through gene discovery

Brucella abortus and Rhodococcus equi are intracellular pathogens of cattle and horses, respectively, that evade the innate immune system and proliferate within macrophages and dendritic cells (DC). DCs are responsible for initiating the adaptive immune response, therefore playing a critical role in determining the outcome of infection.;Previous studies have shown human and murine DCs to be highly permissive for Brucella proliferation, however this has not previously been studied in cattle. Brucella abortus infection in bovine mdDC was characterized. MdDC infected with B. abortus (strain 2308) cleared infection by 24 hours PI. At 6 hours PI, significantly less IL-10, IL-12p40, and IFNgamma were expressed in infected mdDC compared to LPS stimulated mdDC. Flow cytometric analysis of mdDC cultures showed approximately 70% DEC205+, MHC II+ cells. Surface expression of CD80 and CD86 in response to infection was impaired compared to LPS stimulation. SSH identified 24 immunorelevant genes in infected and control mdDC cultures. We conclude that cultured bovine mdDC are not permissive for intracellular proliferation of B. abortus, and infected mdDC exhibit signs of impaired activation and maturation. Additionally, suppression subtractive hybridization identified novel genes to direct future investigations into the pathogenesis of R. equi and B. abortus.;Suppression subtractive hybridization (SSH) was used to isolate differentially expressed genes in R. equi (ATCC 33701+) infected equine monocyte derived DCs (mdDCs). Differential expression was validated using real-time PCR. One of the genes identified, indoleamine 2,3 dioxygenase (IDO), a tryptophan catabolism enzyme, was chosen for further study. Extracellular growth of R. equi does not require tryptophan and inhibition of IDO had no affect on intracellular proliferation of R. equi. To investigate an immune-regulatory role for IDO in R. equi infection, IDO-/- and strain matched control mice were infected for 3 and 6 days. There was no difference in bacterial counts in liver or spleen between the two groups. Prolonged liver inflammation in IDO-/- mice corresponded to decreased expression of TGFbeta, FOXP3, and fewer FOXP3+ regulatory T cells. We conclude that IDO expression by activated macrophages and DC plays a role in dampening the inflammatory response to R. equi infection in mice.