Priming against Salmonella enterica affects differentially haemocyte sub-populations in Armadillidium vulgare

Immune priming is a in which phenomenon that invertebrates exhibit a faster and stronger secondary response than that of the primary response when encountered with a given pathogen. Building on previous research, our study demonstrates a within generation immune priming response that enhances the survival of the woodlouse Armadillidium vulgare against subsequent infection with the live pathogenic bacterium Salmonella enterica. We assessed the persistence of this protective effect using inactivated S. enterica and evaluated associated changes in the host's haemocyte populations by flow cytometry. Females A. vulgare were primed with heat-killed or paraformaldehyde-treated S. enterica at doses of 10³ or 10⁶ bacteria, followed by an infection with live bacteria. We demonstrated that the survival benefit conferred by immune priming depended on both the bacterial dose and the inactivation method. Particularly, the highest survival rate, around 50%, was observed after priming with 10⁶ paraformaldehyde-treated bacteria. Based on these results, we further analysed the proportions of granular, semi-granular and hyaline haemocytes, as well as cell mortality, size and granularity of living haemocytes in females, injected with 10⁶ paraformaldehyde-treated S. enterica, measured at 2 days and 6 days after the initial injection, and 2 days after the lethal dose infection. Overall, injection negatively impacts granular cells, while the proportion of semi-granular and hyaline haemocytes increases, amplified after lethal dose infection. This study provides new insights into the cellular mechanisms and highlights the need for mechanistic investigations into the haematopoietic processes underlying immune priming.