Fetal Phagocytes Loaded with Salmonella typhimurium or its Flagellin Trigger Adaptive Immunity in Adult Recipients

Background: Our previous studies revealed that in utero exposure to ovalbumin could not elicit tolerance to ovalbumin but rather conferred immunity to ovalbumin. Such an event of in utero sensitization was caused by fetal phagocytes that attenuated proteolysis to sequester internalized ovalbumin and differentiated towards dendritic cells for delayed presentation.

Objective: We tried to manipulate fetal phagocytes as a strategy for cellular therapies against pathogens.

Methods: Fetal phagocytes were harvested from gestational day 14 FVB/N fetal liver and peritoneal cavity. Pooled fetal cells were pulsed with flagellin of Salmonella typhimurium (ST) or heat-killed ST overnight. Then, they were transferred intravenously into 4-6 week-old FVB/N mice. Four-six weeks later, recipients were examined for serum anti-flagellin or anti-LPS IgG₁ levels by ELISA using mouse monoclonal anti-flagellin (FLIC-1, BioLegend) or anti-LPS IgG₁ (1E6, Abcam) as the standard curve. Mice were also evaluated for the capacity of defending against lethal intraperitoneal challenge of live ST.

Results: We demonstrated that there were phagocytes within harvested fetal cells, capable of taking up bacterial bioparticles. Then, flagellin-loaded or heat-killed ST-loaded fetal cells were intravenously transferred into adult FVB/N mice. Both antigen-loaded fetal cells caused significant secretion of anti-flagellin or anti-LPS IgG_1, as compared with fetal cells only. Following lethal challenge of live ST, ST-loaded but not flagellin-loaded fetal cells endowed the recipients with significantly better survivals than the controls of fetal cells only.

Conclusion: Fetal phagocytes loaded with ST particles or its flagellin could trigger adaptive immunity in adult recipients to significantly secret antigen-specific IgG₁. However, only ST-loaded fetal phagocytes conferred effective protection against lethal challenge of ST in the generation of anti-LPS IgG₁ in recipient mice. The ineffective protection against ST in the presence of anti-flagellin IgG₁ following the transfer of flagellin-loaded fetal phagocytes might be due to its low antibody titer generated.