Abstract
Conventional vaccines to prevent the pneumonia caused by Rhodococcus equi have not been successful. We have recently demonstrated that immunization with Salmonella enterica Typhimurium expressing the VapA antigen protects mice against R. equi infection. We now report that oral vaccination of mice with this recombinant strain results in high and persistent fecal levels of antigen-specific IgA, and specific proliferation of the spleen cells of immunized mice in response to the in vitro stimulation with R. equi antigen. After in vitro stimulation, spleen cells of immunized mice produce high levels of Th1 cytokines and show a prominent mRNA expression of the Th1 transcription factor T-bet, in detriment of the Th2 transcription factor GATA-3. Following R. equi challenge, a high H2O2, NO, IL-12, and IFN-γ content is detected in the organs of immunized mice. On the other hand, TNF-α and IL-4 levels are markedly lower in the organs of vaccinated mice, compared with the non-vaccinated ones. The IL-10 content and the mRNA transcription level of TGF-β are also higher in the organs of immunized mice. A greater incidence of CD4+ and CD8+ T cells and B lymphocytes is verified in vaccinated mice. However, there is no difference between vaccinated and non-vaccinated mice in terms of the frequency of CD4+CD25+Foxp3+ T cells. Finally, we show that the vaccination confers a long-term protection against R. equi infection. Altogether, these data indicate that the oral vaccination of mice with S. enterica Typhimurium expressing VapA induces specific and long-lasting humoral and cellular responses against the pathogen, which are appropriately regulated and allow tissue integrity after challenge.
Highlights
Rhococcus equi, a gram-positive bacterium, is a facultative intracellular coccobacillus that causes bronchopneumonia in foals aged 1 to 6 months
We have previously reported that oral vaccination of mice with an attenuated S. enterica Typhimurium vaccine strain expressing the VapA protein confers protection against virulent Rhodococcus equi [27]
For the preparation of bacterial suspensions for administration in mice, overnight cultures of the S. enterica Typhimurium x3987 strains were precipitated by centrifugation (30006g; 15 min), and the pellet was re-suspended in phosphate-buffered saline (PBS) to a final cell density of 1-561010 CFU/mL
Summary
A gram-positive bacterium, is a facultative intracellular coccobacillus that causes bronchopneumonia in foals aged 1 to 6 months. The pathogenic mechanisms of R. equi remain largely unknown, there is evidence that virulent strains contain a large 85- to 90-kb plasmid bearing a 27.5-kb pathogenicity island that encodes, among others, nine genes of the virulence-associated protein (vap) family [6,7]. One member of this family is VapA, a highly immunogenic 15–17 kDa protein that is abundantly expressed on the bacterial surface [6,8] and plays a crucial role in pathogen growth inside macrophages as well as disease development [9,10]. VapA is thought to be important in generating immunity against R. equi [11,12]
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