Influenza (H1N1)-ISCOMs enhance immune responses and protection in aged mice

Immunization with live virus vaccine protects highly susceptible DBA/2J mice from lethal influenza A H1N1 infection

BACKGROUND

The mouse represents an important model system to study the host response to influenza A infections and to evaluate new prevention or treatment strategies. We and others reported that the susceptibility to influenza A virus infections strongly varies among different inbred mouse strains. In particular, DBA/2J mice are highly susceptible to several influenza A subtypes, including human isolates and exhibit severe symptoms after infection with clinical isolates.

FINDINGS

Upon intra-muscular immunization with live H1N1 influenza A virus (mouse-adapted PR8M, and 2009 pandemic human HA04), DBA/2J mice mounted virus-specific IgG responses and were protected against a subsequent lethal challenge. The immune response and rescue from death after immunization in DBA/2J was similar to those observed for C57BL/6J mice.

CONCLUSIONS

DBA/2J mice represent a suitable mouse model to evaluate virulence and pathogenicity as well as immunization regimes against existing and newly emerging human influenza strains without the need for prior adaptation of the virus to the mouse.

Immunostimulating complexes (ISCOMs) for nasal vaccination

The immunostimulating complex (ISCOM) is documented as a strong adjuvant and delivery system for parenteral immunization. Its effectiveness for mucosal immunization has also been proven with various incorporated antigens. Lövgren et al. were the first to demonstrate the capacity of influenza virus ISCOMs to induce mucosal immune response and protection after one comparatively low nasal dose. Further studies show that similar to Cholera toxin (CT) and Escherichia coli heat-labile toxin (LT), ISCOMs break immunological tolerance and exert strong mucosal adjuvant activity, resulting in secretory IgA and systemic immune responses. Striking is the capacity of ISCOMs to induce CTL response also after nasal administration. In contrast to CT, ISCOMs initiate mucosal as well as systemic immune responses in an IL-12 dependent manner but independently of IL-4. The recombinant B subunit of cholera toxin (rCTB) was incorporated in the same ISCOM particle to explore symbiotic effects. The IgA response to rCTB in lungs was increased 100-fold when rCTB was administered nasally in ISCOMs and more than 10-fold in the remote mucosa of the genital tract. An enhanced IgA response to a passenger antigen OVA was recorded in the remote genital tract. After i.n. administration of the envelope proteins of respiratory syncytial virus in ISCOMs, high serum antibodies were induced, almost at the same levels as those following parenteral immunization and potent IgA responses were also evoked both at the local respiratory mucosa, and in the cases tested at the distant mucosae of the genital and intestinal tracts. Similar results have also been recorded with ISCOMs containing envelope proteins from Herpes simplex virus, Influenza virus and Mycoplasma mycoides. The mucosal targeting property of envelope proteins of RSV was utilized in an HIV-gp120 RSV ISCOM formulation. After nasal administration an enhanced mucosal IgA response to gp120 was observed in the female reproductive tract. In general, antigens derived from envelope viruses or cell membranes incorporated into ISCOMs retain their biological activity and conformation, encompassing the mucosal targeting and virus neutralizing properties.

Immunosenescence and infectious diseases

Infectious diseases are major causes, with malignancies, of morbidity and mortality in the elderly. Increased susceptibility to infections may result from underlying dysfunction of an aged immune system; moreover, inappropriate immunologic functions associated with aging can determine an insufficient response to vaccines. Impairments of cellular, humoral and innate immunity in the elderly, contributing to increased incidence of infectious diseases, are discussed in this review.

Severe impairment of primary but not memory responses to influenza viral antigens in aged mice: costimulation in vivo partially reverses impaired primary immune responses

Profound alterations in humoral and cellular immune responses are a hallmark of aging, and understanding the immunobiology of aging is key to the success of preventive vaccination strategies. With aging, while recall or memory responses to influenza viral antigens for the most part remained unaltered, primary immune responses are severely impaired. The impaired primary responses are partly due to a lack of costimulation, as providing costimulation at the time of induction of primary immune responses against influenza virus vaccine partially reversed aged-related immune dysfunction and conferred enhanced protection. Inclusion of immunomodulators that up-regulate the expression of costimulatory molecules must be considered to improve the efficacy of vaccination in the elderly, particularly to novel immunogens.

Enhanced antibody and cytokine responses to influenza viral antigens in perforin-deficient mice

Cytotoxic T lymphocytes (CTL) lyse virus-infected target cells by secreting the pore-forming effector molecule, perforin. Perforin-mediated cell death appears to be a major mechanism in viral clearance but its role in regulating immune responses in vivo is unclear. In this report, we show that following immunization with influenza viral antigens, perforin-deficient mice generated about 100-fold greater serum antibody responses than wild-type mice. Further, immune spleen cells from perforin knock-out mice secreted over 10-fold more IFN-gamma following in vitro restimulation than immune spleen cells from control mice. Finally, there were over 10-fold more IFN-gamma-secreting cells in cultures from perforin-deficient mice than those from control mice, suggesting that the enhanced cytokine release by T cells from perforin-deficient mice is due to an increase in the effector cell pool. Collectively, these results suggest that perforin-mediated effector function is required in the down-regulation of the immune response by way of limiting antigen-presenting cell function.