In less than a year from its zoonotic entry into the human population, SARS-CoV-2 has infected more than 45 million people, caused 1.2 million deaths, and induced widespread societal disruption. Leading SARS-CoV-2 vaccine candidates immunize with the viral spike protein delivered on viral vectors, encoded by injected mRNAs, or as purified protein. Johns Hopkins University researchers describe a different approach to SARS-CoV-2 vaccine development that uses exosomes to deliver mRNAs that encode antigens from multiple SARS-CoV-2 structural proteins.
Exosomes were purified and loaded with mRNAs designed to express (i) an artificial fusion protein, LSNME, that contains portions of the viral spike, nucleocapsid, membrane, and envelope proteins, and (ii) a functional form of spike. The resulting combinatorial vaccine, LSNME/SW1, was injected into thirteen weeks-old, male C57BL/6J mice, followed by interrogation of humoral and cellular immune responses to the SARS-CoV-2 nucleocapsid and spike proteins, as well as hematological and histological analysis to interrogate animals for possible adverse effects.
Exosome purification and characterization
(A) Schematic of the exosome purification process. (B) NTA analysis of purified exosomes showing a mean exosome diameter of ~115 nm. (C) Negative stain electron microscopic analysis of purified exosomes, showing two vesicles with the expected size ranges of human exosomes. Bar, 100 nm. (D) Immunoblot analysis of cell and exosome lysates probed using antibodies for CD9, CD63 and actin.
Immunized mice developed CD4+, and CD8+ T-cell reactivities that respond to both the SARS-CoV-2 nucelocapsid protein and the SARS-CoV-2 spike protein. These responses were apparent nearly two months after the conclusion of vaccination, as expected for a durable response to vaccination. In addition, the spike-reactive CD4+ T-cells response was associated with elevated expression of interferon gamma, indicative of a Th1 response, and a lesser induction of interleukin 4, a Th2-associated cytokine. Vaccinated mice showed no sign of altered growth, injection-site hypersensitivity, change in white blood cell profiles, or alterations in organ morphology. Consistent with these results, the researchers also detected moderate but sustained anti-nucleocapsid and anti-spike antibodies in the plasma of vaccinated animals.
Taken together, these results validate the use of exosomes for delivering functional mRNAs into target cells in vitro and in vivo, and more specifically, establish that the LSNME/SW1 vaccine induced broad immunity to multiple SARS-CoV-2 proteins.