Neonatal innate immunity

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Macpherson, Stephanie
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The neonatal period represents a critical time period in the development of the immune system. Adaptive human immune responses are generally viewed as immature at birth. However little is known about innate immune capacity at birth. The TLR system plays an integral role as pattern recognition receptors in the innate immune response. It is also critical in initiating and regulating the adaptive immune response. Preterm birth is associated with increased risk of developing infections in early life and has been associated with increased risk of development of other chronic disorders in later life; however the underlying mechanisms are not at all well understood. Recently, late preterm neonates (34-36 weeks gestation vs full term, 37+ weeks) have been identified as having significantly greater risks of morbidity and mortality in the perinatal period than their full term counterparts. Hence, we focus on examination of TLR responses in late preterm and full term neonates to better understand immune potential and function in these populations. We examined cord blood cytokine and chemokine responses following stimulation with a broad range of TLR agonists. Our results show for the first time that late preterm neonates have reduced capacity to produce both pro- and anti-inflammatory cytokines following stimulation with a panel of TLR agonists. This reduced responsiveness was not due to a reduction in the number of responding cells, but instead appears to be mediated by a reduction in the intrinsic levels of expression of TLRs and associated adaptor proteins. Because little is known about how the innate immune system develops throughout life, we next compared TLR responses in full term neonates to 7 children, adolescents and adults. We found that neonates had selective impairments in TLR responses, most notably in anti-inflammatory cytokine production and anti-viral immune responses compared to the other age groups. Epigenetic modifications, such as the addition or removal of acetyl groups to histone proteins by histone acetyl transferase (HAT) and histone deactylase (HDAC) respectively, are able to modify the expression of genes. Hence, environmental stimuli have been shown to influence gene expression in part by modifying the level or activity of these epigenetic regulators. Currently there are no studies which have examined how epigenetic modifications may influence neonatal innate immune responses. Hence, we sought to determine how modulation of endogenous HDAC activity would affect neonatal innate immune responses. We found that inhibition of HDAC had both inhibitory and enhancing effects on cytokine expression depending on the TLR pathway activated, indicating that the endogenous HDAC expression does not have a global inhibitory impact on all TLR-dependent responses. In summary, this body of work demonstrates that neonatal innate immune responses vary depending on gestational age, indicating that the final few weeks of gestation are crucial for maturation of responses to both bacteria and viruses. Neonates respond differently to TLR stimuli than do older individuals, further highlighting a maturation process of the innate immune system which continues throughout life. Finally, we have shown that environmental exposures may have powerful effects on immune responses in early life.
Innate Immunity, Pediatrics, Immunology, Cytokine