In vitro cellular responses from sheep draining lymph node cells after subcutaneous inoculation with Salmonella abortusovis.

Salmonella abortusovis infection leads to ovine abortion. The basis for immunity against this infection is unknown. Immune responses were studied from prescapular lymph node (PSLN) cells of sheep infected with either a subcutaneous inoculation of virulent (15/5) or a vaccine (Rv6) strain and compared with those of uninfected sheep. PSLN cell phenotypes were characterized by immunofluorescence staining associated with flow cytometry. The in vitro responses were analysed using the PSLN cell proliferative response to several antigens, their secreted IL-2-like activities and their level of nitric oxide (NO) release. The phenotype analyses showed that the CD4(+)-T cell percentages decreased whereas B and MHC-II+ cell percentages increased in the infected sheep. This phenomenon occurred earlier for the virulent strain. The PSLN enlargement was greater and in vitro proliferation more frequent for the sheep infected with the virulent strain compared with the vaccine-infected ones. Proliferation occurred as a result of cell exposure to whole killed bacteria or to the cell wall fraction from S abortusovis but not to the homologous lipopolysaccharide. The secreted IL-2-like activities increased in parallel. The mitogenic response to concanavalin A decreased for infected sheep. For infected sheep, the level of NO release was maximal at the beginning of the infection. It was downregulated by in vitro exposure to S abortusovis antigens but remained unchanged for concanavalin A. NO release was not detected in uninfected sheep. This preliminary investigation provided some keys to the understanding of ovine response to S abortusovis infection and suggested that it shared common features with the mouse immune response.