Monocytes in Neonatal Immunity

Abstract

Monocytes have been recognized as fundamental components of the immune system. These cells belong to the mononuclear phagocytic system, which includes monocytes, tissue macrophages, and myeloid dendritic cells. Adhesion, chemotaxis, phagocytosis, intracellular killing, and secretion of cytokines and other biologically active molecules constitute the major functions of monocytes. Studies of the regulation of monocyte development and differentiation have yielded insights into the heterogeneity of these cells. This article examines functional and biochemical characteristics of monocytes (and to lesser extent macrophages), with particular emphasis on the neonatal period, and discusses functional discrepancies in relation to adult cells. Experimental data suggest impaired phagocytosis and killing of group B streptococci by neonatal monocytes. In addition, chemotactic activity in the neonate is hindered. On the other hand, the major immune defect of macrophages results from a defect of T-helper cells and natural killer cells to produce adequate amounts of proinflammatory cytokines coupled with a marked hyporesponsiveness of macrophages to activation by interferon-gamma. Recent research also suggests that impaired toll-like receptor-4-mediated signaling in lipopolysaccharide-exposed newborn monocytes result in immature responses to pathogen-associated molecular patterns expressed by the microorganisms.