Establishment of memory CD8+ T cells with live attenuated influenza virus across different vaccination doses.

FluMist has been used in children and adults for more than 10 years. As pre-existing CD8+ T cell memory pools can provide heterologous immunity against distinct influenza viruses, it is important to understand influenza-specific CD8+ T cell responses elicited by different live attenuated influenza virus (LAIV) regimens. In this study, we immunized mice intranasally with two different doses of live-attenuated PR8 virus (PR8 ts, H1N1), low and high, and then assessed protective efficacy by challenging animals with heterosubtypic X31-H3N2 virus at 6 weeks post-vaccination. Different LAIV doses elicited influenza-specific CD8+ T cell responses in lungs and spleen, but unexpectedly not in bronchoalveolar lavage. Interestingly, the immunodominance hierarchy at the acute phase after immunization varied depending on the LAIV dose; however, these differences disappeared at 6 weeks post-vaccination, resulting in generation of comparable CD8+ T cell memory pools. After vaccination with either dose, sufficient numbers of specific CD8+ T cells were generated for recall and protection of mice against heterosubtypic H1N1→H3N2 challenge. As a result, immunized mice displayed reduced weight loss, diminished inflammatory responses and lower viral titres in lungs, when compared to unvaccinated animals. Interestingly, the higher dose led to enhanced viral clearance on day 5 post-challenge, though this was not associated with increased CD8+ T cell responses, but with higher levels of non-neutralizing antibodies against the priming virus. Our study suggests that, while different LAIV doses result in distinct immune profiles, even a low dose produces sufficient protective CD8+ T cell memory against challenge infection, though the high dose results in more rapid viral clearance and reduced inflammation.

Generation of Adaptive Immune Responses Following Influenza Virus Challenge is Not Compromised by Pre-Treatment with the TLR-2 Agonist Pam2Cys.

Immunostimulatory agents provide a new category of anti-microbial agents that activate the host's innate immune system allowing control of viral and/or bacterial infections. The TLR-2 agonist PEG-Pam2Cys has been shown to mediate potent anti-viral activity against influenza viruses when administered prophylactically (1). Here, we demonstrate that the treatment of mice with PEG-Pam2Cys does not compromise their ability to generate adaptive immune responses following subsequent challenge with influenza virus. The antibody induced in mice pre-treated with Pam2Cys possessed hemagglutination-inhibiting activities and the CD8(+) T-cell responses that were elicited provided protection against heterologous viral challenge. In the absence of an effective influenza vaccine, an agent that provides immediate protection against the virus and does not compromise the induction of influenza-specific immunity on exposure to infectious virus provides an opportunity for population immunity to be achieved through natural exposure to virus.