Purpose: The aim of this study is to establish a murine model of oral LM infection to evaluate dose-response and immune response during listeriosis between the aged population and the younger populations.
Methods: Young-adult (3 months age), 14-months old, and 22-months old C57BL/ 6 mice were orally gavaged for two consecutive days with doses ranging from 2 x 106 to 2 x 109 CFU/mouse with a murinized LM strain (Lmo-InlAm) and monitored for body-weight changes and survivability. Tissues were collected and assayed for bacterial burden, histology. Also, in vitro stimulation (LM Ag/lysate with/wo anti-CD3/CD28) assays were performed using splenocytes isolated from uninfected or LM-infected young and old mice to compare immune biomarkers and proliferative responses. Supernatants were assayed for cytokine secretion (IL-2, IL-1β, TNF-α, IL-17, IL-10 and IFN-γ) using BioPlex assays.
Results: Compared to adult mice, the mortality rate for the older (14-months) C57BL/6 mice was relatively higher (7% vs 46%). Older mice lost significantly (P=0.005, ANOVA) more body-weight in a dose-dependent manner and had a higher bacterial burden in liver (P<0.05) and spleen (P=0.05) when compared to adult infected mice. LM-lysate treated splenocytes from older mice produced significantly (P<0.05) higher levels of the anti-inflammatory cytokines (e.g., IL-10) and higher levels of IL17, a pro-inflammatory cytokine, but produced less IFN-g. Splenocytes from the older mice proliferated significantly less and produced less IL-2 as compared to young mice.
Significance: These data suggest that older C57BL/6 mice are more susceptible to LM infection due to an imbalance of Th cell responses with disproportionate increase of anti-inflammatory responses and decreased proliferative capacity of immune cells.