Intracytoplasmic delivery of antigens
Several murine studies suggest that a key requirement for induction of effective neonatal adaptive response is entrance of Ag into the cytoplasm of APC. Chen and co-workers studied adult and neonatal (1 week old) BALB/c mice immunized i.p. with inactivated split-product influenza vaccine followed by a booster dose after 3 weeks or with intramuscular injection and in vivo electroporation of plasmid DNA [71]. Vaccination of neonates with hemagglutinin or neuraminidase DNA protected mice against influenza infection in the presence of MatAb. The authors concluded that in order to overcome potential inhibition of adaptive immune responses by MatAb, mothers and their offspring should be immunized with different influenza vaccines targeting distinct Ag (e.g. inactivated vaccine versus DNA vaccine; or use of DNA vaccines targeting different influenza products). If the same Ag is to be used, a study by Pertmer suggests that maternal antibodies do not blunt DNA vaccine-based responses to intracellularly expressed Ag [72].
Study of neonatal C57BL/6 and BALB/c mice immunized (i.p. and s.c.) within 24 hours of birth with disabled infectious single cycle HSV-1 variant reveals that a single round of viral replication dramatically enhances protective responses [73]. CD4+ and CD8+ T cells from neonatally vaccinated mice transferred to naïve recipients conferred protection against lethal viral challenge. UV-inactivated viral particles at up to 104-fold higher doses were not able to achieve this response, suggesting that cytoplasmic delivery of Ag can enhance neonatal immune responses.
Kollman and co-workers have demonstrated a novel approach to neonatal vaccination, employing an attenuated strain of the intracellular pathogenic bacterium Listeria monocytogenes to deliver Ag to the cytoplasm of APC [74]. Importantly, this approach appeared to be safe in neonatal mice in that they survived high-dose infection with the ΔactA strain of L. monocytogenes without any sign of disease nor any recoverable bacteria in spleen or liver 7 days post-vaccination. Neonatal mice vaccinated a single time with attenuated L. monocytogenes strain ΔactA mounted strong CD8+ and CD4+ T cell responses and were protected against subsequent challenge with wild type L. monocytogenes. Moreover, ΔactA served as an effective vehicle for delivery of heterologous Ag resulting in a strong CD8 and CD4 Th1-type memory response, suggesting that this strain may serve as an effective vaccine vehicle for neonatal immunization. Of note, recombinant attenuated strains of L. monocytogenes induce specific immunity even in the presence of preexisting immunity, potentially overcoming the hurdle of preexisting maternal immunity that might interfere with neonatal vaccine responses [75].