Exosome-Mediated Antigen Delivery: Unveiling Novel Strategies in Viral Infection Control and Vaccine Design

Abstract

Exosomes are small subtypes of extracellular vesicles (EVs) naturally released by different types of cells into their environment. Their physiological roles appear to be multiple, yet many aspects of their biological activities remain to be understood. These vesicles can transport and deliver a variety of cargoes and may serve as unconventional secretory vesicles. Thus, they play a crucial role as important vectors for intercellular communication and the maintenance of homeostasis. Exosome production and content can vary under several stresses or modifications in the cell microenvironment, influencing cellular responses and stimulating immunity. During infectious processes, exosomes are described as double-edged swords, displaying both beneficial and detrimental effects. Owing to their tractability, the analysis of EVs from multiple biofluids has become a booming tool for monitoring various pathologies, from infectious to cancerous origins. In this review, we present an overview of exosome features and discuss their particular and ambiguous functions in infectious contexts. We then focus on their properties as diagnostic or therapeutic tools. In this regard, we explore the capacity of exosomes to vectorize immunogenic viral antigens and their function in mounting adaptive immune responses. As exosomes provide interesting platforms for antigen presentation, we further review the available data on exosome engineering, which enables peptides of interest to be exposed at their surface. In the light of all these data, exosomes are emerging as promising avenues for vaccine strategies.

Keywords: antigen presentation; cargoes; exosome engineering; exosomes; extracellular vesicles; flaviviruses; infection; pathological biomarkers; vaccine design; viruses.

Figure 2

Insights into exosome biogenesis and composition. (A). Exosomes biogenesis. The early endosomes produced by the endocytic pathways can mature into multivesicular bodies (MVBs). During their formation, the intraluminal vesicles (ILVs) are loaded with cytosolic material like proteins, lipids, and nucleic acids. MVBs have two possible fates. They are either directed to lysosomes, enabling the degradation of ILV constituents, in particular membrane components, or released by exocytosis, enabling the externalization of ILVs, then called exosomes. (B). Schematic representation of an exosome and its standard cargo. Exosomes are delimited by a phospholipid bilayer membrane containing proteins from the tetraspanin family, such as CD9, CD63, or CD81. The intercellular adhesion molecule 1 (ICAM-1) and major histocompatibility complex MHC I and/or II are detected in variable proportions in the membrane, depending on the emitting cells, and contribute to the biological or pathological functions of exosomes. The ALIX protein is identified within these vesicles, serving as one of the primary intravesicular markers. Additional intra-exosomal markers exist, such as the tumor susceptibility gene 101 (TSG101), which plays a critical role in exosome biogenesis and secretion. A variety of chaperones, including members of the heat shock protein 70 (HSP70) family, could use this unconventional secretion pathway. In addition to proteins, these vesicles can carry nucleic acids, including DNA and RNA such as miRNA. Adapted from Kowal et al. [57]. Created with Biorender.com.

Figure 1

Overall diagram outlining the different aims of the present review, with a particular focus on exosomes in the context of viral infection, as antigen presentation disposals, and as antigen-presenting devices in vaccine strategies.

Figure 3

Exosomes are double-edged swords during infection. On the one hand, exosomes may have detrimental effects during infection by promoting viral replication, impairing immune response, and increasing susceptibility, ultimately leading to enhanced viral spreading. On the other hand, exosomes can participate in controlling infection through their immunomodulatory effects, including antigen presentation, the delivery of cytokines and antiviral mediators, miRNA delivery, and the engagement of the stimulator of interferon genes. Created with Biorender.com.

Figure 4

Antigen presentation by exosomes. Having been primed with non-self antigens, dendritic cells may emit exosomes wherein antigens have been enclosed. Upon interaction with naïve dendritic cells, exosomes may deliver the non-self antigens, in a process called cross-priming. Although recipient dendritic cells have never been directly exposed to them, they can now present the exosome-derived antigens to CD4+ T cells and CD8+ T cells. CD4+ T cells will further activate naïve B cells and induce clonal expansion and differentiation. Cellular immunity response will thus be set up, leading to Th1 memory and Th2-like cytokines. A particularity of exosome and antigen presentation is that B-cell-derived exosomes can present allergen peptides and activate allergen-specific T cells.

Figure 5

Various strategies of exosome display: Target peptide presentation and surface binding mechanisms. (A). A fusion gene model for targeting recombinant proteins to exosomes. To display proteins at the surface of exosomes, cells undergo transfection with plasmids encoding a chimeric protein. This chimeric protein exhibits a signal peptide present in the N-terminal region, the antigen of interest, and an exosome targeting domain generally positioned in the C-terminal region if the peptide is intended for display at the exosome surface. (B). Schematic diagram showing the presentation of a target peptide via different transmembrane domains of the exosome. Exosomes exhibit the presentation of target peptides on their surface through diverse exosomal transmembrane domains, showcasing the outcome of the exosome display strategy. Moreover, target peptides can bind to the exosome surface either through the cloaking strategy or by associating with the lipid rafts spanning the exosome membrane.

Figure 6

Exosomes modified according to two strategies’ have been proposed as candidate vaccines against SARS-CoV-2. They are either designed to carry viral mRNA or engineered to expose a viral antigen, such as the Spike RBD. Both of these candidate vaccines demonstrated protection against SARS-CoV-2 infection in mice.