Phagocyte chemotaxis in the perinatal period

Neonatal monocytes demonstrate impaired homeostatic extravasation into a microphysiological human vascular model

Infections are most frequent at the extremes of life, especially among newborns, reflecting age-specific differences in immunity. Monocytes maintain tissue-homeostasis and defence-readiness by escaping circulation in the absence of inflammation to become tissue-resident antigen presenting cells in vivo. Despite equivalent circulating levels, neonates demonstrate lower presence of monocytes inside peripheral tissues as compared to adults. To study the ability of monocytes to undergo autonomous transendothelial extravasation under biologically accurate circumstances we engineered a three-dimensional human vascular-interstitial model including collagen, fibronectin, primary endothelial cells and autologous untreated plasma. This microphysiological tissue construct enabled age-specific autonomous extravasation of monocytes through a confluent human endothelium in the absence of exogenous chemokines and activation. Both CD16- and CD16+ newborn monocytes demonstrated lower adherence and extravasation as compared to adults. In contrast, pre-activated tissue constructs were colonized by newborn monocytes at the same frequency than adult monocytes, suggesting that neonatal monocytes are capable of colonizing inflamed tissues. The presence of autologous plasma neither improved newborn homeostatic extravasation nor shaped age-specific differences in endothelial cytokines that could account for this impairment. Newborn monocytes demonstrated significantly lower surface expression of CD31 and CD11b, and mechanistic experiments using blocking antibodies confirmed a functional role for CD31 and CD54 in neonatal homeostatic extravasation. Our data suggests that newborn monocytes are intrinsically impaired in extravasation through quiescent endothelia, a phenomenon that could contribute to the divergent immune responsiveness to vaccines and susceptibility to infection observed during early life.

Postnatal Innate Immune Development: From Birth to Adulthood

It is well established that adaptive immune responses are deficient in early life, contributing to increased mortality and morbidity. The developmental trajectories of different components of innate immunity are only recently being explored. Individual molecules, cells, or pathways of innate recognition and signaling, within different compartments/anatomical sites, demonstrate variable maturation patterns. Despite some discrepancies among published data, valuable information is emerging, showing that the developmental pattern of cytokine responses during early life is age and toll-like receptor specific, and may be modified by genetic and environmental factors. Interestingly, specific environmental exposures have been linked both to innate function modifications and the occurrence of chronic inflammatory disorders, such as respiratory allergies. As these conditions are on the rise, our knowledge on innate immune development and its modulating factors needs to be expanded. Improved understanding of the sequence of events associated with disease onset and persistence will lead toward meaningful interventions. This review describes the state-of-the-art on normal postnatal innate immune ontogeny and highlights research areas that are currently explored or should be further addressed.

Immune vulnerability of infants to tuberculosis

One of the challenges faced by the infant immune system is learning to distinguish the myriad of foreign but nonthreatening antigens encountered from those expressed by true pathogens. This balance is reflected in the diminished production of proinflammatory cytokines by both innate and adaptive immune cells in the infant. A downside of this bias is that several factors critical for controlling Mycobacterium tuberculosis infection are significantly restricted in infants, including TNF, IL-1, and IL-12. Furthermore, infant T cells are inherently less capable of differentiating into IFN- γ -producing T cells. As a result, infected infants are 5-10 times more likely than adults to develop active tuberculosis (TB) and have higher rates of severe disseminated disease, including miliary TB and meningitis. Infant TB is a fundamentally different disease than TB in immune competent adults. Immunotherapeutics, therefore, should be specifically evaluated in infants before they are routinely employed to treat TB in this age group. Modalities aimed at reducing inflammation, which may be beneficial for adjunctive therapy of some forms of TB in older children and adults, may be of no benefit or even harmful in infants who manifest much less inflammatory disease.

First line of defense in early human life

Innate antimicrobial peptides are considered to play an important role in host defense against microbial invasion. They are expressed in a wide variety of organisms. In the case of human beings, defensins and the cathelicidin LL-37 appear to be the major microbicidal peptides. With respect to human neonates, only few investigations have been performed in this context, revealing the presence of alpha-defensins and LL-37 in neutrophils and vernix caseosa. In addition, beta-defensins are present in tracheal aspirates and breast milk, whereas LL-37 has been detected in the skin of the newborn baby. During recent years, immunomodulatory activities such as chemotaxis have emerged as important functions of antimicrobial peptides. Thus, these innate effectors may work synergistically to provide a first line of defense against infection, as well as to promote interactions between the innate and adaptive immunity in newborn infants.

Neonatal skin in mice and humans expresses increased levels of antimicrobial peptides: innate immunity during development of the adaptive response

The expression of antimicrobial peptides and proteins is an important innate immune defense mechanism that has recently been shown to be essential for cutaneous defense against invasive bacterial disease. Newborns have an immature cellular immune defense system that leads to increased susceptibility to infections. Here we show that skin from embryonic and newborn mice, as well as human newborn foreskin, express antimicrobial peptides of the cathelicidin and beta-defensin gene families. Immunohistochemistry and in situ hybridization demonstrated abundant cathelicidin protein and mRNA is present in normal skin during the perinatal period. Quantitative real-time PCR showed mouse cathelicidin expression (CRAMP) is 10- to 100-fold greater in the perinatal period than adult. Murine beta-defensins-1 and -4 and human beta-defensin-2 were also present in newborn skin. Combined, human cathelicidin (LL-37/hCAP/18) and beta-defensin-2 demonstrated synergistic antimicrobial activity and efficiently killed group B Streptococcus, an important neonatal pathogen. Antimicrobial peptides may therefore provide a compensatory innate defense mechanism during development of cellular immune response mechanisms in the newborn period.

Phagocytic activities in neonatal monocytes

Monocytes play an essential role in cellular host defense as circulating phagocytes, as well as precursors of macrophages. We investigated the principal phagocytic activities in monocytes from cord blood of term infants by analysing adherence, random migration, chemotaxis, bactericidal activity, phagocytosis-associated chemiluminescence, production of superoxide anion (O-2) and generation of hydrogen peroxide (H2O2). All phagocytic functions of monocytes from neonates were shown quantitatively to be comparable to those of cells from healthy adult volunteers. The increased susceptibility of the human neonate to serious systemic infections cannot be related to an abnormality in monocyte function.

Oxidative metabolism in cord blood monocytes and monocyte-derived macrophages

Little is known about phagocytosis-associated oxidative metabolism in mononuclear phagocytes from the human neonate. We investigated this phenomenon in monocytes from the cord blood of term newborn infants by measuring generation of superoxide anion (O2-) and hydroxyl radical (X OH) after stimulation with opsonized zymosan or phorbol myristate acetate. Production of these microbicidal oxygen metabolites by monocytes from neonates and healthy adult volunteers was equivalent. When cultured in the presence of the macrophage activator lipopolysaccharide or muramyl dipeptide, monocytes from neonates and adults differentiated into cells with the appearance of macrophages and with an enhanced capacity to release O2- compared with cells cultured in the absence of an activator. Monocyte-derived macrophages from neonates produced only slightly less O2- than did adult cells. Thus, unlike the cord blood neutrophil, which exhibits abnormalities in oxidative metabolism, the cord blood mononuclear phagocyte has a respiratory burst that is quantitatively comparable to that of the adult cell.

The status of pulmonary host defense in the neonatal sheep: cellular and humoral aspects

In consideration of the sheep neonate as a compromised host, we have examined the status of cellular and humoral pulmonary host defense components at selected developmental time points. The dynamic character of the early neonatal LFC population, reflected in changes in subpopulations and proliferative capacity, most probably contributed to the observed changes in in vitro cell function. While certain cell responses, e.g., blood and LFC PMN chemotaxis, appeared intact by day 1, others developed subsequently. The ability of AMs to elaborate a chemotactic factor(s) was first noted at day 21. Bacteria binding and killing presented a biphasic maturation pattern, with full competence not present until day 180. Although the in vitro binding and killing activity of day 8 LFCs was comparable to that of the adult, it may be a poor indicator of in vivo host defense capacity, given the relative paucity of endogenous opsonins at that age. In fact, the interdependence of mediators suggests that the sheep neonate may remain a compromised host during the first 3 months of life. Thereafter, cellular and humoral parameters begin to approximate those of adult sheep and by 180 days of life pulmonary defense, as assessed in this study, is fully developed.