BackgroundWith the exception of some live vaccines, e.g. BCG, subunit vaccines formulated with “classical” adjuvants do not induce similar responses in neonates as in adults. The usual neonatal profile is characterized by lower levels of TH1-associated biomarkers. This has hampered the development of new neonatal vaccines for diseases that require early protection. Tuberculosis is one of the major targets for neonatal immunization. In this study, we assessed the immunogenicity of a novel candidate vaccine comprising a mycobacterial fusion protein, Ag85B-ESAT-6, in a neonatal murine immunization model.Methods/FindingsThe Ag85B-ESAT-6 fusion protein was formulated either with a classical alum based adjuvant or with the novel IC31® adjuvant. Following neonatal or adult immunization, 3 parameters were studied in vivo: (1) CD4+ T cell responses, (2) vaccine targeting/activation of dendritic cells (DC) and (3) protection in a surrogate mycobacterial challenge model. Conversely to Alum, IC31® induced in both age groups strong Th1 and Th17 responses, characterized by multifunctional T cells expressing IL-2 and TNF-α with or without IFN-γ. In the draining lymph nodes, a similarly small number of DC contained the adjuvant and/or the antigen following neonatal or adult immunization. Expression of CD40, CD80, CD86 and IL-12p40 production was focused on the minute adjuvant-bearing DC population. Again, DC targeting/activation was similar in adults and neonates. These DC/T cell responses resulted in an equivalent reduction of bacterial growth following infection with M. bovis BCG, whereas no protection was observed when Alum was used as adjuvant.ConclusionNeonatal immunization with the IC31®- adjuvanted Ag85B-ESAT-6 subunit vaccine elicited adult-like multifunctional protective anti-mycobacterial T cell responses through the induction of an adult pattern of in vivo DC activation.

Vaccine efficacy largely depends upon DC targeting and activation. The most potent TLR soluble ligands induce diffuse DC activation, which may be associated with marked pro‐inflammatory responses and possibly adverse effects. This raises the concern that effective vaccine adjuvants may similarly rely on widespread DC activation. Using a promising candidate vaccine against tuberculosis (fusion protein of Ag85B and 6‐kDa early secretory antigenic target (ESAT‐6)) formulated in the potent IC31® adjuvant, DC targeting and activation was studied in vivo, following the fate of antigen and adjuvant in the draining lymph nodes, to define the magnitude of DC targeting/activation required in vivo to induce protective vaccine responses. Unexpectedly, protective IFN‐γ‐mediated Ag85B‐ESAT‐6/IC31® responses were associated to the activation of a minute population (less than 0.3%) of CD11c+ lymph node DC, without detectable systemic pro‐inflammatory responses. This activated peripheral tissue‐derived DC population, characterized by enhanced CD80, CD86, CD40 and IL‐12p40 expression, was only identified when focusing on adjuvant‐ or antigen‐labeled CD11c+ DC, which were found to support T cell proliferation. Immunization with aluminum hydroxide adjuvant (Alum) resulted in a similar proportion of antigen‐associated DC but without detectable enhancement of CD80, CD86, CD40 or IL‐12p40 expression. Thus, potent protective IFN‐γ‐producing responses may be elicited by the exquisite activation of a minute number of in vivo targeted DC.