Immune response of neonates elicited by somatic transgene vaccination with naked DNA.

Neonates display lower immune responsiveness and higher susceptibility for high-dose tolerance. Quantitative as well as functional differences between the neonatal and adult lymphocytes or antigen presenting cells (APC) respectively, explain the particular immune responsiveness during the early stages of the postnatal development. Reduced numbers of lymphocytes and APCs as well as a modified responsiveness of T cells in neonates, are the main factors that account for the low threshold of tolerance in newborns. Taking into account these particularities, the design of effective vaccines for neonates poses significant difficulties. We hypothesized that a continuous exposure to low doses of antigens may avoid high-zone tolerance and may lead instead, to effective expansion of effector and memory cells. Indeed, inoculation of newborn mice with plasmids encoding nucleoprotein (NP) or hemagglutinin (HA) of influenza virus, led to the priming of specific cytotoxic (CTL), helper (Th) and B cells, rather than induction of unresponsiveness. Mice immunized as neonates with naked DNA and challenged later with lethal doses of influenza virus, displayed significant protection. Thus, DNA immunization may be a promising strategy for vaccination against serious infectious diseases of infants and children.