Oral paratuberculosis vaccine efficacy and mucosal immunity in cattle.

Mycobacterium avium subsp. paratuberculosis (MAP) primarily invades ruminants' small intestine via the Peyer's patches in the ileum and jejunum. Despite ongoing efforts to develop effective MAP vaccines, the effects of live-attenuated vaccines on mucosal immunity remain poorly understood. Previous studies indicate that the BacA oral vaccine confers localized protection against MAP in the ileum and ileocecal valve of calves, but not in the jejunum. This protection correlates with heightened levels of peripheral blood immune cells exhibiting pro-inflammatory and memory traits. This study aimed to evaluate immune responses induced by oral BacA vaccination in the ileum and jejunum Peyer's patches, comparing protection at both sites through mucosal immune cell profiling and RNA-seq transcriptome analyses. It represents the first exploration of mucosal immune responses in Peyer's patches following oral MAP vaccination. Oral BacA immunization increased CD4 + IFNγ+ and CD4 + TNFα+ cell frequencies, along with the T effector memory to T central memory cell ratio, in the ileum and jejunum of BacA-vaccinated animals challenged with wildtype MAP, compared to the infection control group challenged solely with wildtype MAP. Immune cells isolated from the ileum of vaccinated-challenged animals exhibited significant upregulation in IFNγ, IP-10, TNFα, IL-2, IL-15, and IL-17 expression upon restimulation compared to the uninfected control group, whereas minimal differences were observed in the jejunum under similar conditions. RNA-seq data further indicated a more robust host response in the ileum across all experimental groups. Gene ontology analyses revealed genes associated with increased phagocytic and apoptotic activities in the vaccinated-challenged group. Overall, the BacA oral vaccine's effectiveness appears to vary primarily due to differences in antigen-specific gene expression between the ileum and jejunum, with the ileum showing a more robust host response. Understanding these effects on young calves' mucosal immunity and how live vaccines modulate immune responses is crucial for advancing mucosal vaccine development against MAP.