A novel liposomal influenza vaccine (INFLUSOME-VAC) containing hemagglutinin–neuraminidase and IL-2 or GM-CSF induces protective anti-neuraminidase antibodies cross-reacting with a wide spectrum of influenza A viral strains
Introduction
Resistance to influenza virus infection is primarily associated with the humoral response to the major viral surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA) [1], [2], [3], [4], [5]. Anti-HA antibodies (Abs) can effectively prevent viral infection by blocking virus attachment to epithelial cells and its subsequent penetration into the cytosol. Anti-NA Abs, on the other hand, may be less efficient in preventing the primary infection, but such Abs can significantly reduce viral spreading and morbidity by inhibiting virus release from already infected cells, thereby reducing the possibility of infecting new cells [1], [2], [5].
The commonly used influenza vaccines to date consist of inactivated whole virions, split virions (subvirion vaccines) or of HA+NA (HN) (subunit, or surface antigen vaccines), which are quite efficacious (60–90%) in healthy young adults. However, such vaccines are considerably less effective (≤50%) in the elderly, in infants, in patients with certain chronic diseases, and in immunodeficient patients [1], [4], [5], [6], [7], [8], [9], and the duration of protection is relatively short (<1 year) [1], [5]. These vaccines induce high titers of Abs against HA and low levels of Abs against NA, probably because of (a) the low content of NA in the vaccines [10]; (b) the negative influence of prior experience with HA [11], [12] and/or (c) the relatively lower immunogenicity of NA as compared with that of HA. Moreover, the current vaccines afford protection only against a narrow range of viral strains, namely, those included in the vaccine [1], [5]. In view of the vast heterogeneity of the influenza viruses (the old and the recently arising strains), and the frequent alteration in the antigenic make-up (antigenic drift) of the virus, annual vaccination against the circulating strains is highly recommended, particularly for the high-risk groups [5]. New, improved vaccines are therefore needed for (a) reducing morbidity and mortality among the high-risk groups and (b) providing long-term protection against a wider range of viral subtypes, thereby reducing the need for frequent immunization.
Since NA appears to be less antigenically variable than HA [13], vaccines capable of eliciting high levels of Abs to NA, in addition to a strong anti-HA response, may fulfill the two above-mentioned requirements. Recently, we reported on novel influenza vaccines comprising liposome-encapsulated HA+NA and IL-2 or GM-CSF (as vaccine adjuvants [14], [15]). In mice, the liposomal vaccines induced much faster, stronger and longer-lasting humoral and cellular responses, as well as protective immunity, as compared with those evoked by the standard subunit vaccines. In the present study, we demonstrate that our liposomal vaccines also elicit a high titer of anti-NA Abs, and such Abs efficiently cross-react with various influenza A viral subtypes (H3N2) that have emerged over a period of 20 years.
Section snippets
Mice
Specific pathogen-free (SPF) female BALB/c mice, aged 2 or 18 months, were used. Animals were maintained under SPF conditions.
Influenza viruses/antigens
Influenza viruses were obtained from allantoic fluid as described elsewhere [14]. Split-viron and subunit preparations were kindly provided by Dr. Palache, B.V. Solvay Duphar, Weesp, The Netherlands, and by Drs. Glück and Zurbriggen, the Swiss Serum and Vaccine Institute, Bern, Switzerland. The split virus preparations contained 50–60% HA and 25–30% NA/NP (w/w) of total
The anti-HA/NA response following vaccination with free/liposomal antigen co-administered with free/liposomal IL-2 or GM-CSF
Various preparations of non-liposomal and liposomal vaccines were compared (Table 1). Mice were immunized with subunit or split virus preparations derived from influenza A/Shangdong/9/93 (H3N2) given as free antigen (F-HN) or liposome entrapped (Lip-HN), alone and with free/liposomal cytokine. Sera were tested on days 40, 50 and 180 for specific anti-HA Abs (by HI), for anti-NA Abs (by NI), and for total anti-HA/HN Igs (by ELISA).
As shown in Table 1, experiment 1, a low anti-HA Ab titer was
Discussion
Due to the continuous variation in the antigenic make-up of the influenza viruses (antigenic drift), annual vaccination is highly recommended for high-risk groups [1], [4], [5], [6], [7], [8], [9]. In addition to the relatively low efficacy of the currently used vaccines in certain high-risk groups, such vaccines afford poor protection against viral strains that do not match those included in the vaccine. Therefore, more potent influenza vaccines are needed, especially for the high-risk groups.
Acknowledgements
Supported by research grants from the I. Horowitz Foundation and the Israel Science Foundation.
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