Cytokine gene expression in response to SnSAG1 in horses with equine protozoal myeloencephalitis.

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on a Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. In a previous study, cell-mediated immune responses to SnSAG1 were shown to be statistically significantly reduced in horses with EPM in comparison to EPM-negative control horses. It therefore appears as though the parasite is able to induce immunosuppression towards parasite-derived antigens as parasite-specific responses are decreased. Isolated peripheral blood lymphocytes from 21 EPM (cerebrospinal fluid [CSF] Western blot)-negative horses with no clinical signs and 21 horses with clinical signs of EPM (CSF Western blot positive) were cocultured with SnSAG1 for 48 and 72 h, and the effect on cytokine production was investigated by means of reverse transcriptase PCR. Cytokines assayed include gamma interferon (IFN-gamma), tumor necrosis factor alpha, interleukin (IL)-2, IL-4, and IL-6. beta-Actin was used as the housekeeping gene. A Wilcoxon signed-rank test of the findings indicated that there was a statistically significant decrease in IFN-gamma production after 48 h in culture for samples from horses with clinical disease. There was also a statistically significant increase in IL-4 production after 72 h in culture for samples from horses with EPM. These results further support the notion that this parasite is able to subvert the immune system in horses with clinical disease.

Cell-mediated immune responses in horses with equine protozoal myeloencephalitis.

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Cell-mediated immune responses to mitogens have been shown to be reduced in horses with EPM, although it is not known whether the parasite causes this immunosuppression or if the immunosuppression is required for disease manifestation. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. Isolated peripheral blood lymphocytes from 43 EPM-negative horses and 28 horses with clinical EPM were cocultured with a mitogen or SnSAG1, and lymphocyte blastogenic responses to these antigens was measured by tritiated thymidine uptake. The ability of SnSAG1 to induce gamma-interferon (gammaIFN) production was also investigated with reverse transcriptase-polymerase chain reaction. There was no significant differences between EPM-positive and -negative horses in lymphocyte responses to ConcanavalinA. However, lymphocytes from EPM-negative horses responded significantly higher to SnSAG1 than lymphocytes from EPM-positive horses. GammaIFN production was detectable by 24 hr in culture in response to SnSAG1 in all EPM-negative horses. There was still no detectable gammaIFN production in EPM-positive horses after 72 hr in culture. It appears that the parasite is also able to induce an immunosuppression toward parasite-derived antigens as parasite-specific responses are decreased.