In the present study, we assessed influenza A virus-specific cellular and humoral immune responses in children with CF who had been vaccinated against seasonal influenza annually and in unvaccinated control children. The antibody recognition profile was broader in vaccinated children with CF than unvaccinated control children. No differences were observed in the development of virus-specific CD4+
T cell responses. However, in unvaccinated children, an age-dependent increase in the frequency of virus-specific CD8+
T cells which was not observed in vaccinated children with CF was detected. These findings are in concordance with our results in the mouse model, in which we demonstrated that vaccination against seasonal influenza A virus prevented the development of influenza A virus-specific CD8+
T cell immunity otherwise induced by infection (4
It has been demonstrated previously that the majority of influenza A virus-specific CD8+
T cells is directed to conserved viral proteins (29
). This indicates that memory CD8+
T cells provoked against seasonal influenza A viruses will cross-react with other influenza A viruses, even with those of other subtypes (24
). Thus, vaccinated children with CF will develop lower cross-reactive virus-specific CD8+
T cell responses than unvaccinated children.
The age-dependent increase in the frequency of virus-specific CD8+
T cells in the unvaccinated children most likely reflects the increase in the number of subjects who experienced an infection with an influenza virus early in life. Of interest, a similar pattern for the development of antibodies to influenza viruses was recently observed in a large seroepidemiological study performed in children ages 0 to 7 years (3
). Indeed, two unvaccinated subjects without detectable antibodies to any of the influenza A viruses also had very low frequencies of virus-specific CD8+
T cells, which thus reflects a lack of exposure to influenza A virus. Maturation of the immune system may have contributed to the increased responsiveness observed in older children (8
). However, using SEB, we were unable to demonstrate an age-dependent increase in CD4 and CD8 T cell responses to this superantigen.
In the group of CF patients vaccinated annually, the age-dependent increase in virus-specific CD8+ T cell responses was absent. Our interpretation of these findings is that vaccination efficiently induced virus-specific antibodies which protected against infection with seasonal influenza viruses to a great extent and thereby prevented the induction of virus-specific CD8+ T cell responses.
Although it would have been more ideal to compare the immune responses of unvaccinated healthy children with those of vaccinated healthy children, it is unlikely that patients with CF responded poorly because of intrinsic immunologic defects for various reasons. First, the virus-specific CD4+
T cell response of this group was comparable to that of the unvaccinated group. This confirms that the use of inactivated vaccines induced CD4+
T cell responses but not virus-specific CD8+
T cell responses, which has been demonstrated previously (21
). Second, the antibody titers in the seropositive subjects were comparable between the two groups. The proportion of subjects with antibodies to older strains was higher in the group of vaccinated children. This confirms that patients with CF can be vaccinated effectively against seasonal influenza and the complications that these infections may cause in this vulnerable group of high-risk patients. Third, the CD8+
T cell response to SEB was not affected in the group of CF patients and was comparable to that in unvaccinated subjects. Finally, the antibody responses to various viral and bacterial vaccine antigens used in the Dutch national immunization program were similar for the two groups, indicating that there were no differences in the functionality of T and B cells between the groups. It is unlikely that subjects of the two groups have been exposed to viruses containing different B or T cell epitopes since the viruses causing influenza epidemics are highly homogeneous, especially in a small geographical region like The Netherlands.
Universal vaccination of healthy children is not practiced in The Netherlands, and therefore, this study group was not available. In addition, since universal vaccination of children 6 to 59 months of age has been recommended and practiced in some countries only since 2007, the long-term effects of vaccination of healthy children cannot be examined at present. Therefore, results from the present study warrant follow-up studies with larger cohorts of vaccinated and unvaccinated children in the future, especially since epidemiological data suggest that previous vaccination against seasonal influenza increased the risk of infection with pandemic influenza A/H1N1 virus in 2009 (10
). Such studies would also exclude potential confounding explanations for the differences observed.
Thus, annual vaccination against influenza is effective but may have potential drawbacks that have previously been underappreciated and that are also a matter of debate (7
). By no means do we suggest halting annual vaccination of children, especially those at high risk for complications, such as CF patients. A number of studies have demonstrated that annual vaccination reduces the morbidity and mortality caused by seasonal influenza in children and is (cost-)effective (23
). However, long-term annual vaccination using inactivated vaccines may hamper the induction of cross-reactive CD8+
T cell responses by natural infections and thus may affect the induction of heterosubtypic immunity. This may render young children who have not previously been infected with an influenza virus more susceptible to infection with a pandemic influenza virus of a novel subtype. Therefore, we argue for the development and use of vaccines that could induce broadly protective immune responses in children. For example, it has been demonstrated that live attenuated influenza vaccines induce virus-specific CD8+
T cell responses (21
). In addition, it has been demonstrated that live attenuated influenza vaccines are also effective against drift variants in children (1
). The development of broadly protective vaccines has been on the research agenda for some time, and progress has been made (13
). Young children, whether they are at high risk for influenza-associated complications or not, may especially benefit the most from these vaccines.