Neonatal CD8+ T cells are slow to develop into lytic effectors after HSV infection in vivo.

HSV is an important neonatal pathogen. We defined the kinetics of the primary CTL response to HSV-2 in vivo in neonatal mice. Using a replication-defective HSV-2 virus, we demonstrate that neonates mount a primary HSV-specific CTL effector response in the draining LN, with delayed onset and shortened peak activity, in contrast to the rapid, strong response observed in adult mice. The shortened peak neonatal CTL response is independent of HSV dose and is associated with retarded CD8(+) T cell expansion, reduced expansion of HSV-specific tetramer-positive CD8(+) T cells and a reduced CD8(+) T cell IFN-gamma response. Paradoxically, neonatal CD8(+) T cells display enhanced non-specific early activation that is not sustained. Neonatal HSV-specific TCR-transgenic CD8(+) T cells showed reduced proliferation in vivo when transferred into HSV-infected neonatal mice compared to adult T cell controls. Our data suggest that early events in CD8(+) T cell priming underlie the attenuated newborn CTL response to HSV.