The Impact Of Aging On Influenza Vaccine Induced Humoral And Cellular Immune Response And Its Potential Mechanism

Influenza virus is a highly contagious airborne pathogen that enters through infection of the respiratory tract. In particular, elderly humans over 65 years old are at great risk and constitute up to 90% of all mortality associated with influenza virus infection. Vaccination represents a highly effective approach to reduce the rate of influenza virus infection and the associated social and economical burdens. Immunogenicity of influenza virus-like particles (VLPs) and their protective efficacies against lethal influenza virus challenge were evaluated in young and aged mice in comparison with inactivated influenza virus (IIV) vaccines. Influenza VLPs were produced in Sf9 insect cells by co-expressing the matrix protein M1 and the surface glycoproteins hemagglutinin (HA) and neuraminidase (NA) using the recombinant baculovirus expression system. The VLPs are morphologically similar to influenza virions. Characterization of VLPs showed that both HA and NA proteins were incorporated into VLPs at about one-third to one half of the levels detected in purified influenza virions, and these proteins retained their functional activities. Further, influenza VLPs but not IIV stimulated cytokine secretion from mouse dendritic cells (DC) in vitro. Results from immunization studies showed that influenza VLPs induced strong antibody responses against HA that inhibited hemagglutination by influenza virus, similar to IIV vaccines at the same dose. Nonetheless, the levels of IgG antibody responses against HA induced by both influenza VLPs and IIV vaccines were significantly reduced in aged mice compared to young mice. On the other hand, the levels of IgM antibody responses against HA were similar between young and aged mice. After lethal challenge by homologous mouse-adapted influenza virus (A/PR/8/34), all vaccinated animals survived. These results show that influenza VLPs exhibit similar immunogenicity to IIV vaccines and are highly efficacious for protection against influenza virus infection in both young and aged mice.Immunosenescence is a hallmark of the aging process, which is manifested by reduced immune responses to vaccination and increased susceptibility to virus infection. While the underlying mechanism remains to be elucidated, cumulative evidence indicates that immunosenescence is associated with a decline in many aspects of both innate and adaptive immune responses. In this study we found the percentage of a particular subset of CD4 T cells named regulatory T cells (Treg cells) almost doubled in aged mice as compared with their young counterpart. The homeostasis of Treg cells plays key roles in maintaining peripheral tolerance, preventing autoimmune diseases and control excess or chronic inflammations related to infections. Treg cells have very broad target cells spectrum and usually down-regulate the function of its target cells. So we assume the high accumulation of Treg cells in aged mice may take some responsibility for the greatly compromised immune response after immunization. For verifying this hypothesis, Treg depletion was performed by injection of 400ug anti-mouse CD25 monoclonal antibody (PC 6.1) intraperitoneally 4 days before immunization. The antibody response showed that Treg-depleted aged mice have greatly enhanced IgG antibody (including IgG1 and IgG2a), HAI titer and antibody avidity. Correspondingly, the entire CD25 antibody treated aged mice survived and showed no sickness and weight loss after challenged with 100×LD50 mouse adapted A/PR8/34 virus as compared with those non-treated group. Now that it showed the remarkably enhanced immune response in aged mice with Treg depletion, we further tested whether one time immunization with high dose influenza VLPs combined with Treg depletion in aged mice could make them survive when facing super high dose influenza virus challenge. The results of one time immunization is also very promising: all mice received CD25 antibody prior immunization survived with slightly weight loss. In contrast the non-treated group had 2/3 survival rate and the survived mice exhibited obvious sign of sickness and underwent 10%-15% of weight loss. This result indicated that highly accumulated Treg cells do play an important role in the immunosenescence of mice and this is one of the major reasons that make the aged mice immune response compromised. Depletion of Treg cells can greatly restore the immune response in aged mice and enhance the influenza specific humoral and cellar responses. These results may shed light on the research and development of highly effective and safe vaccines for elder people.