Outer membrane vesicles: moving within the intricate labyrinth of assays that can predict risks of reactogenicity in humans

Abstract

Outer membrane vesicles (OMV) are exosomes naturally released from the surface of Gram-negative bacteria. Since the '80s, OMVs have been proposed as powerful vaccine platforms due to their intrinsic self-adjuvanticity and ability to present multiple antigens in natural conformation. However, the presence of several pathogen-associated molecular patterns (PAMPs), especially lipid A, has raised concerns about potential systemic reactogenicity in humans. Recently, chemical and genetic approaches allowed to efficiently modulate the balance between reactogenicity and immunogenicity for the use of OMV in humans. Several assays (monocyte activation test, rabbit pyrogenicity test, limulus amebocyte lysate, human transfectant cells, and toxicology studies) were developed to test, with highly predictive potential, the risk of reactogenicity in humans before moving to clinical use. In this review, we provide a historical perspective on how different assays were and can be used to successfully evaluate systemic reactogenicity during clinical development and after licensure.

Promising Extracellular Vesicle-Based Vaccines against Viruses, Including SARS-CoV-2

“…The concerns about OMV biosafety, due to their bacterial origin and consequent possible inflammatory response, seem to be solved by chemical and genetic approaches that efficiently reduce OMV reactogenicity in humans. In particular, the use of nonionic detergents and chelating agents causes the reduction or dissolvement of OMV lipopolysaccharides (LPS) after the isolation of these bacterial vesicles, whereas the genetic engineering of genes involved in the biosynthetic pathway of LPS allows for the production of recombinant OMVs with attenuated LPS forms [69]. In case these EV-based vaccines demonstrate lower immune responses, it could be useful to consider the use of the modified mRNA coding for the prefusion conformation of the Spike protein, as employed so far by Moderna and Pfizer/BioNTech.…”

GMMA-Based Vaccines: The Known and The Unknown

“…LPS, the most abundant component of GMMA, is a key antigen in Gram-negative bacteria, but it is also the main component for systemic reactogenicity ( 29 ). Intrinsic LPS endotoxicity can be reduced by genetically modifying the lipid A structure.…”

“…Moreover, Alhydrogel has been used in preclinical and clinical studies in S. sonnei GMMA vaccine formulation as absorbent agent with the solely purpose of reducing GMMA reactogenicity, mainly due to results observed after intramuscular immunization of rabbits ( 29 ). Indeed, intramuscular immunization of rabbits with 100 µg of 1790GAHB caused only a low and transient temperature rise, comparable to what observed with 5 µg of S. sonnei GMMA delivered alone.…”

Microfluidic Tools for Enhanced Characterization of Therapeutic Stem Cells and Prediction of Their Potential Antimicrobial Secretome

“…Successful application in the biomedical field of outer membrane vesicles (OMVs), naturally secreted by Gram-negative bacteria, has led to them being proposed as the basis for a promising antigen delivery system and being found in ongoing vaccine development [134]. The advantage for a vaccine platform stems from OMV's built-in adjuvanticity [135] and sizedependent ability to induce both humoral and cell-mediated immune responses as well as from the delivery of heterologous antigens by engineered OMVs [136] in a natural conformation [137].…”