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Induction of Protective Immunity to Listeria monocytogenes in Neonates1

Neonates suffer unduly from infections and also respond suboptimally to most commonly used vaccines. However, a CD8 T cell response can be elicited in neonates if the Ag is introduced into the cytoplasm of APCs. Listeria monocytogenes (Lm) targets the cytoplasm of APC and is a strong CD8 and CD4 Th1-promoting vaccine vehicle in adult mice. We hypothesized that an attenuated strain of Lm would be safe and induce long-lasting protective immunity, even in neonates. We found that neonatal mice immunized only once with the attenuated strain ΔactA-Lm developed robust primary and secondary CD8 and CD4 Th1 responses and were fully protected from lethal challenge with virulent wild-type Lm without the need for a booster immunization. Furthermore, ΔactA-Lm expressing a heterologous recombinant Ag induced a strong CD8 and Th1 memory response to that Ag. Based on these data, we propose that ΔactA-Lm or derivatives thereof might serve as a vaccine vehicle for neonatal immunization.