Background: T helper 1 (Th1)/T helper 2 (Th2)-biased cytokine regulation may be another reason that neonates are much more susceptible to infectious disease than are adults.

Objectives: We attempted to determine the ability of neonatal mice to direct the Th1 phenotype against Listeria monocytogenes (LM), because LM, an intracellular Gram-positive bacterium, induces profound cellular immunity by Th1 cells in vivo.

Methods: In order to determine whether neonatal mice evidence strong Th1 activity during LM infection, neonatal mice were compared with adult mice with regard to susceptibility to LM, cytotoxic T lymphocyte activity, and cytokine profiles. Neonatal gene profiles relevant to Th1 and Th2 differentiation during LM infection were also compared between neonatal and adult mice, via real-time PCR and RT-PCR.

Results: Neonatal mice were found to be far more susceptible to LM infection than adult mice, due to a lack in the induction of cytotoxic T cell activity, coupled with poor IFN-gamma secretion. Further, LM-infected neonatal mice evidenced much lower levels of expression of Th1-type immune components, including IL-12, IFN-gamma, Delta-4 and T-bet, as compared to those features in adult mice. These results may be due to the comparably lower expressions of mannose-bind lectins and some of toll-like receptors (TLRs) such as TLR-5, -6 and -9, necessary mediators to develop Th1 immune responses.

Conclusions: Neonatal mice may not mount an adequate Th1 type immune response due to a significantly lower expression of Th1-type immune components as compared to adult mice, even when forced into a Th1-prone environment.