In vivo uptake of an experimental microencapsulated diphtheria vaccine following sub-cutaneous immunisation.

Previous studies demonstrated in vitro phagocytosis of poly(lactide-co-glycolide) (PLGA) microspheres (MS) by macrophages and dendritic cells and the biodistribution of fluorescent PLGA particles following oral or intranasal administration. In this study, we report the uptake and biodistribution of sub-cutaneously administered fluorescent labelled PLGA MS loaded with diphtheria toxoid (DT). The cell type and percentage of fluorescent positive cells were determined by flow cytometry and confirmed by fluorescent microscopy. Fluorescent particles were detected inside cells of the peritoneal flush as early as 10 min post-inoculation, predominantly in cells of macrophage morphology. In vivo trafficking of PLGA particles following a sub-cutaneous immunisation of mice appeared to be governed by macrophages. However, in the first week after inoculation, dendritic cells played a significant role in the uptake and digestion of the microspheres, thereby triggering the immune response against the antigen. Fluorescent PLGA MS were also observed in cells of lymphoid tissues such as mesenteric lymph nodes (MLN) and spleen (S). However, microsphere fluorescence in lymphoid tissues decreased rapidly, as they were degraded inside the cells, thereby enabling the presentation of the antigen to specific cells of the immune system. To our knowledge, this is the first time the fate of immunogenic PLGA microspheres was studied in vivo following a sub-cutaneous injection route.