MF59 Mediates Its B Cell Adjuvanticity by Promoting T Follicular Helper Cells and Thus Germinal Center Responses in Adult and Early Life

The early life influenza disease burden calls for more effective vaccines to protect this vulnerable population. Influenza vaccines including the MF59 oil-in-water adjuvant induce higher, broader, and more persistent Ab responses in adults and particularly in young, through yet undefined mechanisms. In this study, we show that MF59 enhances adult murine IgG responses to influenza hemagglutinin (HA) by promoting a potent T follicular helper cells (T-FH) response, which directly controls the magnitude of the germinal center (GC) B cell response. Remarkably, this enhancement of T-FH and GC B cells is already fully functional in 3-wk-old infant mice, which were fully protected by HA/MF59 but not HA/PBS immunization against intranasal challenge with the homologous H1N1 (A/California/7/2009) strain. In 1-wk-old neonatal mice, MF59 recruits and activates APCs, efficiently induces CD4(+) effector T cells and primes for enhanced infant responses but induces few fully functional T-FH cells, which are mostly follicular regulatory T cells, and poor GC and anti-HA responses. The B cell adjuvanticity of MF59 appears to be mediated by the potent induction of T-FH cells which directly controls GC responses both in adult and early life, calling for studies assessing its capacity to enhance the efficacy of influenza immunization in young infants.