Development of intranasal Sendai virus vector-based vaccine against COVID-19

Abstract

Sendai virus (SeV) vectors are able to infect a diverse range of cells. They have a high affinity for epithelial cells in the respiratory tract, which provides advantageous properties for intranasal inoculation. Vaccination of the respiratory tract, the main route of infection for Coronavirus (COVID-19), can strongly induce mucosal immunity, which is difficult to induce through injected vaccines, in addition to systemic immunity in a manner similar to innate immunity. A SeV vector vaccine carrying part of the SARS-CoV-2 spike-protein gene showed high immunogenicity in pharmacological studies using intranasally inoculated rodents and is a promising next-generation vaccine.

Highlights

Intranasal inoculation of a vaccine against COVID-19 is considered helpful in preventing infection and serves as a measure against diverse SARS-CoV-2 variants. The Sendai virus (SeV) vector exhibits the best advantage for developing vaccines for intranasal inoculation. Rodents were intranasally inoculated with a vaccine created using the characteristics of the SeV, and strong immunogenicity was confirmed.

Introduction

The research and development of vaccines against Coronavirus (COVID-19) is underway worldwide. Approximately 300 vaccines of different types of modalities are listed on the website of the World Health Organization (WHO). This includes products currently in both preclinical and clinical developmental phases. When focusing on the inoculation routes of the 114 vaccines for which clinical trials have been initiated, injections (including intramuscular injections) account for the majority (77%), followed by intranasal (7%), oral (3%), and unknown administration methods (13%) [1]. The main purpose of immunization by injection is to prevent the onset and aggravation of an infectious disease mainly by inducing systemic immunity through an increase in immunoglobulin G (IgG) in the blood. However, with injected vaccinations, it is difficult to effectively induce mucosal immunity with immunoglobulin A (IgA) antibody as the main product in the upper respiratory tract. This is an infection route of the coronavirus, and the effect of an injected vaccine cannot be expected to prevent initial infection or suppress virus shedding from infected people. Intranasal inoculation can effectively induce both systemic and mucosal immunity through a mechanism similar to innate immunity and may be considered an ideal route of inoculation for respiratory infections such as COVID-19. However, the nasal mucosa is equipped with a mechanism to eliminate foreign substances, and it is challenging to induce a useful level of immune response as an infection control measure. Ingenuity and improvement of the formulation or an increase in the dose are required to induce and enhance the efficacy of intranasal inoculation. It is also perceived that the development of this administration route is not cost-effective. The Sendai virus (SeV) vector exhibits the best advantage for developing vaccines for intranasal inoculation because of its high affinity for airway epithelial cells.

Properties of SeV Vectors Favorable for Intranasal Inoculation Vaccine Development

Sendai virus (SeV) is a respiratory virus [2] isolated from mice in the first half of the 1950s in Japan and is classified as mouse parainfluenza I type of the Paramyxoviridae family [3]. SeV is a single, minus-strand, non-segmented RNA which consists of 15,384 bases and encodes six genes, namely NP, P, M, F, HN, and L, in order from the 3′-end [4]. Each of the six genes encodes a nucleocapsid protein (NP), a phosphoprotein (P), which is a subunit of RNA polymerase, a matrix protein (M) that backs up the viral particle structure, a membrane fusion protein (F) involved in invasion of the host cell, a hemagglutinin neuraminidase (HN) involved in binding to the host cell, and a large subunit (L) of the RNA polymerase, respectively.

Although the SeV vector used in the development of this vaccine was improved for medical use [5], the advantages are derived from the properties of the SeV itself, which make it safer and more efficient than existing viral vectors. A major benefit in terms of safety is that genome replication and transcription are carried out only in the cytoplasm by RNA polymerase carried by the SeV-vector itself, and there is no integration into the host chromosome [6]. SeV vectors are referred to as “cytoplasmic vectors” as they have no genotoxic concerns. This is a crucial difference from “DNA-type vectors,” which utilize existing lentiviruses, retroviruses, and adenoviruses. In addition, because the properties of F and HN allow infection of a variety of mammalian cells in a short time, including human-derived cells [7, 8], they are highly directional to airway epithelial cells [9, 10] and the SeV-vector itself acts as an adjuvant [11, 12]. These are the efficient and highly advantageous properties of the SeV vector when applied for intranasal inoculation. Thus, when a vaccine using a SeV vector is inoculated intranasally, it is unnecessary to add adjuvants to enhance efficacy and to devise formulations to enhance colonization and absorption at the inoculation site.

Results of Immunogenicity Studies in Rats with IRO-202

To date, vaccines against several variants of coronavirus (SARS-CoV-19) have been produced using the medical SeV vector [5], and pharmacological studies on rodents have been conducted. Here, we will introduce the results of an immunogenicity study in which rats were intranasally inoculated with IRO-202, a candidate vaccine.

Seven-week-old female Sprague Dawley (SD) rats were randomly assigned to three groups: two vaccinated groups and one non-vaccinated group. The prescribed doses of IRO-202 were inoculated into the nasal cavity of the rats in the vaccinated groups on day 1 and day 29. On day 43 they were euthanized and the key immunogenicity items were evaluated (Table 1A, 1B). The measurement results of SARS-CoV-2 specific neutralizing antibodies in serum samples are shown in Fig. 1. Neutralizing activity was measured using the microneutralization (MN) assay with Vero E6 cells, USA_WA1/2020 virus (10³TCID/ml), and rat serum mixtures. The reciprocal of the highest serum dilution that was positive for neutralizing activity was used to calculate the titer for each serum sample (MN assay was performed by the Southern Research Institute, USA). The neutralizing antibody titers in the two vaccinated groups (L and H) were higher than those of the non-vaccinated group (C) on day 29 after the first vaccination (5 in the C group, 40 in the L group, and 56 in the H group [median]). The neutralizing antibody titers on day 43, 14 days after the second vaccination, increased 8-fold (median 320) in the L group and 22-fold (median 1,280) in the H group compared to day 29 in both vaccinated groups, suggesting a strong boosting efficacy. In addition, neutralizing antibodies that were present from day 15 were observed in the H group, and the neutralizing antibody titers were higher than those in group L throughout the observation period, suggesting that IRO-202 exhibits a dose-dependent efficiency. Furthermore, this study confirmed that there was a strong splenic T-cell response based on the assessment of the number of interferon-gamma (IFN-γ)-producing cells using ELISpot assays, and that there was a strong T helper type 1 (Th1) cytokine response based on the analysis of serum IgG subclasses (data not presented).

Table 1. Immunogenicity of IRO-202 in rats

A. Experimental schedule
Group	N	Test article	Vaccine dose (/body)	Vaccination day	Termination day (Euthanasia and Necropsy)
C	5	Non-Vaccinated	-	-	43
L	5	Low dose	2 × 107 ciu	Day 1, 29	43
H	5	High dose	5 × 107 ciu	Day 1, 29	43
B. Key immunogenicity evaluation items
1. Neutralizing titer in serum
2. Interferon-γ production inducing ability in spleen
3. Th1/Th2 balance in serum

Seven-week-old female Sprague Dawley (SD) rats were randomly assigned to three groups: two vaccinated groups and one non-vaccinated group. The prescribed doses of IRO-202 were inoculated into the nasal cavity of the rats in the vaccinated groups on day 1 and day 29. They were euthanized and key immunogenicity items were evaluated on day 43.

Fig. 1.

Change in neutralizing activity in rats serum samples via microneutralization (MN) assay after intranasal IRO-202 vaccination.

Future Perspectives of Intranasal Inoculation of SeV Vector Vaccine

The favorable properties of the SeV vector for intranasal inoculation and the extremely high immune response IRO-202 have shown in studies using rodents indicate that the SeV vector has great potential as a vaccine against COVID-19 as well as a platform technology for intranasal inoculation vaccines. In the future, we aim to advance the improvement of a large-scale manufacturing system and for the practical application of the vaccine against COVID-19 as early as possible.

Conflict of Interest

The authors declare that there are no conflicts of interest.

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