Although the study subjects were seronegative to the hemagglutinins in the vaccine viruses and were predicted to be susceptible to infection with the pLAIVs because the HA and NA genes were derived from novel influenza virus subtypes from avian/animal species, the vaccine viruses were highly restricted in replication. The immune responses to the pLAIVs were variable, ranging from a high seroconversion rates to H9N2 and H7N3 to low seroconversion rates for H5N1, H6N1 and H2N2 vaccines. In a phase I clinical trial, an H5N1 pandemic live attenuated influenza virus (pLAIV) VN2004 vaccine bearing avian influenza H5N1 HA and NA genes on the A/Ann Arbor cold-adapted vaccine backbone displayed very restricted replication. We evaluated T cell responses to H5N1 pLAIV vaccination and assessed pre-existing T cell responses to determine whether they were associated with restricted replication of the H5N1 pLAIV. ELISPOT assays were performed using pools of overlapping peptides spanning the entire H5N1 proteome and the HA proteins of relevant seasonal H1N1 and H3N2 viruses. We tested stored peripheral blood mononuclear cells (PBMCs) from 21 study subjects who received two doses of the H5N1 pLAIV. The PBMCs were collected 1 day before and 7 days after the first and second pLAIV vaccine doses, respectively. T cell responses to conserved internal proteins M and NP were significantly boosted by vaccination (p=0.036). In addition, H5N1 pLAIV appeared to preferentially stimulate and boost pre-existing seasonal influenza virus HA-specific T cell responses that showed low cross-reactivity with the H5 HA. We confirmed this observation by T cell cloning and identified a novel HA-specific epitope. However, we did not find any evidence that pre-existing T cells prevented pLAIV replication and take. In summary, we found that cross-reactive T cell responses could be boosted by pLAIV regardless of the induction of antibody. The impact of the original antigenic sin phenomenon in a subset of volunteers, with preferential expansion of seasonal influenza-specific but not H5N1-specific T cell responses merits further investigation. We had previously evaluated pandemic live attenuated influenza vaccines (pLAIV) against H7 subtype avian influenza viruses with pandemic potential for safety and immunogenicity in healthy adults; unfortunately, few vaccinees shed infectious virus and/or seroconverted by hemagglutination inhibition or neutralization. We sought to determine whether the vaccines primed or established long lasting immunity that could be detected by administration of an unadjuvanted H7 pandemic inactivated influenza vaccine (pIIV). Healthy seronegative adults received two doses of A/Netherlands/219/03 (H7N7) or one dose of A/chicken/British Columbia/CN-6/04 (H7N3) pLAIV all given as 107.5 50% tissue culture infective doses intranasally. A subset of subjects received one 45 micrograms dose of H7N7 pIIV containing the A/Mallard/Netherlands/12/2000 HA intramuscularly 1824 months after pLAIV. Viral shedding was assessed by culture and real-time polymerase chain reaction (rRT-PCR), B cell responses following pLAIV were evaluated by ELISPOT and flow cytometry. Serum antibody was assessed by hemagglutination-inhibition (HAI), microneutralization (MN) and ELISA assays after each vaccine. Serum HAI or MN responses were not detected in any subject following one or two doses of either H7 pLAIV, although some subjects had detectable H7 specific B cells after vaccination. However, 10/13 subjects primed with two doses of H7N7 pLAIV responded to a subsequent dose of the homologous H7N7 pIIV with high titer HAI and MN antibody that cross-reacted with both North American and Eurasian lineage H7 viruses, including H7N9. In contrast, nave subjects and recipients of a single dose of the mismatched H7N3 pLAIV did not develop HAI or MN antibody after pIIV. Thus, while pLAIVs did not elicit detectable serum MN or HAI antibody, strain-specific pLAIV priming established long term immune memory that was cross-reactive with other H7 influenza strains. To better understand this priming effect, we compared H7 HA head and stalk domain-specific B cell responses in H7N7 pLAIV-primed subjects and non-H7-primed controls after a single dose of H7N7 pIIV. We found that the quantity, epitope diversity, and affinity of H7 head-specific antibodies increased rapidly in only H7N7 pLAIV-primed subjects after the pIIV. However, all cohorts generated a vigorous, high affinity stalk-specific antibody response. Consistent increases in circulating memory B cell frequencies after the pIIV reflected the specificity of high affinity antibody production. Our findings emphasize the value of pLAIVs as a vehicle for prepandemic vaccination.
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