Innate cellular immune responses in newborns.

Innate immunity assures the first line of defense against pathogenic microorganisms. Innate immune responses induced by bacteria, fungi, or viral replication are triggered by granulocytes, monocytes, macrophages, dentritic cells, and natural killer cells. Neonatal deficiency of innate cellular immunity includes a decreased production of interferons, IL-12/IL-23, and IL-18, and other proinflammatory cytokines, an impaired type-1 response of macrophages to IFN-gamma, the most potent macrophage-activating agent in vivo, and to lipopolysaccharide, the primary constituent of the outer membrane of Gram-negative bacteria. An increasing body of evidence suggests impaired responses of neonatal monocytes and macrophages to multiple TLR ligands. This review will discuss recent advances in understanding innate cellular immunity in human neonates, with respect to selected aspects of immune functions that may be related to increased susceptibility to infections. Components of TLR signaling and the immune consequence that may result from neonatal deficiencies will be highlighted. A better understanding of innate immunity can make the development of techniques possible by which physicians more accurately tailor prevention and treatment of neonatal infections.