Influenza A viruses cause significant global morbidity and mortality, with endemic H1N1 and H3N2 strains causing recurrent seasonal infections. In recent years, zoonotic transmission from animal reservoirs has seen the emergence of human infections with avian influenza strains such as H5N1, H10N8 and H7N9. Such infections are generally associated with high pathogenicity and lethality, and a lack of population level immunity raises significant concerns about pandemic potential. Notably, current seasonal influenza vaccines elicit insufficient cross-reactive immunity to limit infection with newly emergent viral strains. However, low titres of serum antibodies binding H5 or H7 have been previously reported in unexposed individuals (1,2). Here, we used fluorescently-labelled, recombinant hemagglutinin (HA) proteins as flow cytometric probes to identify B cells specific for avian influenza in healthy unexposed subjects (N=18). Low frequencies of H5+ (0.02-0.12%) and H7+ (0.03-0.08%) B cells were found in all individuals studied to date, expressed surface IgA, IgM or IgG and displayed a classical resting memory phenotype (IgD- CD27+ CD21+). Rare B cells binding both H5 and H7 (0.002-0.046%) were also observed. Single B cells binding H5, H7 or both were sorted and B-cell receptor (BCR) transcripts sequenced using multiplex PCR. Representative antibodies were cloned and expressed. Influenza neutralisation activity and non-neutralising effector functions such as antibody-dependent cellular cytotoxicity and antibody-dependent phagocytosis were assessed in vitro. Our results suggest while exposure to seasonal influenza does not elicit high titres of serum antibody binding H5 and H7, it does prime a pool of highly cross-reactive memory B cells that could (a) form a substrate targetable by immunisation and (b) provide a source of cross-reactive HA-specific antibodies with therapeutic potential against future pandemics. Further studies are required into mechanisms that increase subdominant humoral responses to vaccines in order to increase pandemic preparedness against avian influenza strains.
References: 1 – Terajima et al. J Infect Dis. 2015; 212(7):1052-60, 2 – Wang et al. J Infect Dis. 2015; pii: jiv407