Live attenuated H7N7 influenza vaccine primes for a vigorous antibody response to inactivated H7N7 influenza vaccine

Abstract

Background: H7 influenza viruses have emerged as potential pandemic threat. We evaluated the safety and immunogenicity of two candidate H7 pandemic live attenuated influenza vaccines (pLAIV) and their ability to prime for responses to an unadjuvanted H7 pandemic inactivated influenza vaccine (pIIV).

Methods: 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 10(7.5) 50% tissue culture infective doses (TCID50) intranasally. A subset of subjects received one 45 μg dose of H7N7 pIIV containing the A/Mallard/Netherlands/12/2000 HA intramuscularly 18-24 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.

Results: 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, naïve subjects and recipients of a single dose of the mismatched H7N3 pLAIV did not develop HAI or MN antibody after pIIV.

Conclusions: 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. Understanding the mechanisms underlying priming by pLAIV may aid in pandemic vaccine development.

Trial registration: ClinicalTrials.gov NCT01534468.

Keywords: H7N7; Influenza; Live attenuated vaccine; Pandemic; Priming.

Fig. 1.

Vaccine virus-specific (V) and H 7 HA-specific (H7) IgG ASC frequencies in peripheral blood following administration of H7N7 pLAIV. Enriched B cells were analyzed by ELISpot assay on days 6, 7, or 8 after the first dose of H7N7 pLAIV, A second dose was administered on day 28 and cells were analyzed on days 34 or 35 (days 6 or 7, respectively, after the second dose). IgG ASC frequencies are expressed as a proportion of CD19⁺ B cells.

Fig. 2.

Kinetics of the (A) hemagglutination-inhibition (HAI) against H7N7 (B) hemagglutination-inhibition (HAI) against H7N3 (C) microneutralization (MN) antibody response against H7N7 (D) microneutralization (MN) antibody response against H7N3 for each subject following inactivated H7N7 vaccine in subjects previously primed with H7N7 pLAIV. GMTs are displayed by the horizontal line. In each graph the assays were done using the H7N7 PLAIV virus or H7N3 PLAIV as the test antigen (see Section 2). Samples with a titer of <4 by HAI are assigned a value of 2, and samples with a titer of <10 by MN are assigned a value of 5

Fig. 3.

Cross reactivity of the sera from H7N7 PLAIV primed subjects who received pIIV 18 months following pLAIV. The graph shows the individual titers and the geometric mean titer (GMT) of antibody at day 14 among individuals who responded to the booster vaccine as assessed by microneutralization (A) or hemagglutination Inhibition (B) of sera collected on day 14 post pIIV against the following test antigens: A/Netherlands/219/2003 PLAIV (H7N7 ca), A/Netherlands/219/2003 wild-type virus (H7N7 wt), A/chicken/British Columbia/CN-6/2004 PLAIV (H7N3 ca), Almallard/Netherlands/12/2000 (H7N3 wt), and A/Anhui/1/2013 (H7N9 wt). Tests using the wild-type viruses were performed at the Centers for Disease Control and Prevention.