SARS vaccine based on a replication-defective recombinant vesicular stomatitis virus is more potent than one based on a replication-competent vector

Abstract

A SARS vaccine based on a live-attenuated vesicular stomatitis virus (VSV) recombinant expressing the SARS-CoV S protein provides long-term protection of immunized mice from SARS-CoV infection (Kapadia, S.U., Rose, J. K., Lamirande, E., Vogel, L., Subbarao, K., Roberts, A., 2005. Long-term protection from SARS coronavirus infection conferred by a single immunization with an attenuated VSV-based vaccine. Virology 340(2), 174-82.). Because it is difficult to obtain regulatory approval of vaccine based on live viruses, we constructed a replication-defective single-cycle VSV vector in which we replaced the VSV glycoprotein (G) gene with the SARS-CoV S gene. The virus was only able to infect cells when pseudotyped with the VSV G protein. We measured the effectiveness of immunization with the single-cycle vaccine in mice. We found that the vaccine given intramuscularly induced a neutralizing antibody response to SARS-CoV that was approximately ten-fold greater than that required for the protection from SARS-CoV infection, and significantly greater than that generated by the replication-competent vector expressing SARS-CoV S protein given by the same route. Our results, along with earlier studies showing potent induction of T-cell responses by single-cycle vectors, indicate that these vectors are excellent alternatives to live-attenuated VSV.

Fig. 1

SARS-CoV S expressed by a single-cycle VSV recombinant. (A) The VSV G gene is replaced by the SARS-CoV S gene in the VSV genome to yield the VSVΔG-S genome. The RNA sequences are shown in the (+) anti-genomic sense. (B) The VSVΔG-EGFP1 lacks the VSV G gene and has an EGFP gene inserted into the first position of the genome. The RNA sequences are shown in the (+) anti-genomic sense. (C) BHK-21 cells were infected with either VSVΔG-S or wt VSV. Cells were fixed, and SARS-CoV S was visualized by indirect immunofluorescence. The fluorescence images are shown on the left, and differential interference contrast (DIC) images are shown on the right. (D) Lysates of metabolically labeled BHK-21 cells infected with wt VSV (lanes 1 and 2), VSV-S (lanes 3 and 4) or VSVΔG-S (lanes 5 and 6) were analyzed by SDS-PAGE. The lysates were also treated with PNGase F to remove N-linked glycans from proteins (lanes 2, 4, and 6).

Fig. 2

VSV G is needed to induce a SARS-CoV neutralizing response. Mice were immunized i.m. with VSVΔG-S either G-complemented or (G) or non-complemented with any viral glycoproteins (NC). Each virus was administered live or UV-inactivated (UV). Serum SARS-CoV neutralizing antibody titers one month post-immunization were determined in each group and used as a read out for replication in vivo. Error bars indicate standard error of the mean. (N = 5).

Fig. 3

VSV neutralizing antibody response. The VSV neutralizing antibodies titers of samples collected at 5 weeks post-immunization are shown. The mean reciprocal dilution giving 100% neutralization is shown for each group. Error bars indicate standard error of the mean.

Fig. 4

Specific neutralization of VSVΔG-EGFP1/SΔtail-HA by anti-S antibody. VSVΔG-EGFP1 pseudotyped with either SΔtail-HA or VSV G proteins were incubated with antiserum from mice immunized with wt VSV, VSV-S or SARS-CoV as indicated. The pseudotypes were then transferred to Vero E6 cells. Infection was determined by EGFP expression. Both fluorescence images and differential interference contrast (DIC) images are shown for each field.

Fig. 5

SARS-CoV neutralizing antibody responses to each vector. The SARS-CoV neutralizing antibody titers of serum of individual mice at 5 weeks (A), 9 weeks (B) and 13 weeks (C) post-immunization are given. The reciprocal dilutions giving 100% neutralization for each group are shown. The mean reciprocal dilutions giving 100% neutralization for each group over time are summarized (D). Mice immunized with wt VSV, either i.m or i.n., had no detectable SARS-CoV neutralizing antibodies. Error bars indicate standard error of the mean.