FLU-v, a Broad-Spectrum Influenza Vaccine, Induces Cross-Reactive Cellular Immune Responses in Humans Measured by Dual IFN-γ and Granzyme B ELISpot Assay

Previous reports demonstrated that FLU-v, a peptide-based broad-spectrum influenza vaccine candidate, induced antibody and cellular immune responses in humans. Here, we evaluate cellular effector functions and cross-reactivity. PBMC sampled pre- (day 0) and post-vaccination (days 42 and 180) from vaccine (n = 58) and placebo (n = 27) recipients were tested in vitro for responses to FLU-v and inactivated influenza strains (A/H3N2, A/H1N1, A/H5N1, A/H7N9, B/Yamagata) using IFN-γ and granzyme B ELISpot. FLU-v induced a significant increase in the number of IFN-γ- and granzyme-B-secreting cells responding to the vaccine antigens from pre-vaccination (medians: 5 SFU/10(6) cells for both markers) to day 42 (125 and 40 SFU/10(6) cells, p < 0.0001 for both) and day 180 (75 and 20 SFU/10(6) cells, p < 0.0001 and p = 0.0047). The fold increase from pre-vaccination to day 42 for IFN-γ-, granzyme-B-, and double-positive-secreting cells responding to FLU-v was significantly elevated compared to placebo (medians: 16.3-fold vs. 1.0-fold, p < 0.0001; 3.5-fold vs. 1.0-fold, p < 0.0001; 3.0-fold vs. 1.0-fold, p = 0.0012, respectively). Stimulation of PBMC with inactivated influenza strains showed significantly higher fold increases from pre-vaccination to day 42 in the vaccine group compared to placebo for IFN-γ-secreting cells reacting to H1N1 (medians: 2.3-fold vs. 0.8-fold, p = 0.0083), H3N2 (1.7-fold vs. 0.8-fold, p = 0.0178), and H5N1 (1.7-fold vs. 1.0-fold, p = 0.0441); for granzyme B secreting cells reacting to H1N1 (3.5-fold vs. 1.0-fold, p = 0.0075); and for double positive cells reacting to H1N1 (2.9-fold vs. 1.0-fold, p = 0.0219), H3N2 (1.7-fold vs. 0.9-fold, p = 0.0136), and the B strain (2.0-fold vs. 0.8-fold, p = 0.0227). The correlation observed between number of cells secreting IFN-γ or granzyme B in response to FLU-v and to the influenza strains supported vaccine-induced cross-reactivity. In conclusion, adjuvanted FLU-v vaccination induced cross-reactive cellular responses with cytotoxic capacity, further supporting the development of FLU-v as a broad-spectrum influenza vaccine.