Targeting antigens to phosphatidylserine triggers rapid T- and B-cell responses

Abstract Phosphatidylserine (PS) is a phospholipid that is present in each cell membrane. In intact cells, PS is kept intracellularly, in dying cells, extracellular vesicles (EVs) and activated platelets, PS flips outside where it can be accessed by PS-binding reagents, such as Milk fat globule-EGF factor 8 (MFG-E8). In previous studies, we have established in vivouse of fluorescent MFG-E8 to label and characterize EVs. Here, we measured the antigen-specific immune responses elicited by targeting antigen (Ag) to PS +structures, presumably EVs via MFG-E8 as a novel vaccination strategy. In contrast to non-targeted conventional Ag, PS-targeting by fusing Ag to MFG-E8 caused accumulation in the marginal zone of lymphoid follicles soon after injection. Hours later, Ag-MFG-E8 was deposited on the follicular dendritic cell network without the need of previous primary immunization. Mice immunized with PS-targeted Ag/Alum induced long-lasting germinal centers (GCs) and significantly more GC B cells. Furthermore, it resulted in an approx. 13,000-fold increase in Ag-specific IgG-levels and strongly accelerated antibody class switch recombination as compared to immunization with conventional Ag/Alum. Ag targeted to PS also induced more efficient CD8 T cell activation and IFN-gamma production post immunization. Therefore, targeting Ag to EVs by harnessing the PS-binding properties of MFG-E8 might turn out to be of advantage for the induction of very rapid and long-lasting immune responses. This project was supported by the Deutsche Forschungsgemeinschaft (DFG, German Research foundation) – Project-ID 210592381 - SFB 1054 B03 to Thomas Brocker and Z02 to Jan Kranich.