Investigation on antigen-specific T cell responses induced by Outer Membrane Vesicles from Escherichia coli Δ60 strain

Michele Tomasi¹Lorenzo Croia²Ilaria Zanella³Assunta Gagliardi¹Giulia Boscato Sopetto³Mattia Benedet¹Gabriele di Lascio¹Gaia Gambini³Alvise Berti³Riccardo Corbellari³Guido Grandi^3,4*Alberto Grandi^1,4

• ¹Toscana Life Sciences Foundation, Siena, Italy
• ²Shenzhen Institute of Advanced Technology, Shenzhen, China
• ³Department of Cellular, Computational and Integrated Biology, University of Trento, Povo, Italy
• ⁴BiOMViS Srl, Siena, Italy

There is a growing interest in the exploitation of bacterial Outer Membrane Vesicles (OMVs) for the design of vaccines and novel anti-tumor immunotherapeutic products. Such interest is motivated by their potent immunostimulatory properties, which promote elevated immune responses against heterologous antigens combined to OMVs by genetic engineering, chemical coupling or absorption. However, for a full exploitation of OMVs, a few questions remain to be fully addressed: Which is the OMVs/heterologous antigen ratio needed to obtain the optimal antigen-specific immune response? To what extent OMVs endogenous proteins interfere or favor antigen-specific immunity? Using OMVs derived from our E. coli Δ60 strain, we recently addressed these questions, focusing on the humoral immune responses, and we determined the concentrations of the OMVs-associated proteins necessary and sufficient to elicit saturating levels of specific antibodies. In this work we focused on the cell-mediated immunity. We show that, because of the numerous OMVassociated MHC II epitopes, OMV immunization elicited detectable levels of IFN-γ + epitopespecific CD4 + T cells provided that epitope concentrations were >10% of total OMV proteins (w/w). Such elevated concentrations could be achieved by mixing synthetic peptides to OMVs but not by genetic manipulation of OMVs. By contrast, most likely thanks to the crosshelp of the polyclonal CD4 + T cell population, elevated frequencies of epitope-specific CD8 + T cells were found even when the MHC I epitopes were present at concentrations lower than 1% of total OMV proteins. Our data provide a mechanistic inside of the OMV-mediated immune responses and have important implication in vaccine design.