Hygiene and Other Early Childhood Influences on the Subsequent Function of the Immune System

Abstract

Hygiene Depression a b s t r a c tThe immune system influences brain development and function. Hygiene and other early childhood influences impact the subsequent function of the immune system during adulthood, with consequences for vulnerability to neurodevelopmental and psychiatric disorders. Inflammatory events during pregnancy can act directly to cause developmental problems in the central nervous system (CNS) that have been implicated in schizophrenia and autism. The immune system also acts indirectly by "farming" the intestinal microbiota, which then influences brain development and function via the multiple pathways that constitute the gut-brain axis. The gut microbiota also regulates the immune system.Regulation of the immune system is crucial because inflammatory states in pregnancy need to be limited, and throughout life inflammation needs to be terminated completely when not required; for example, persistently raised levels of background inflammation during adulthood (in the presence or absence of a clinically apparent inflammatory stimulus) correlate with an increased risk of depression. A number of factors in the perinatal period, notably immigration from rural low-income to rich developed settings, caesarean delivery, breastfeeding and antibiotic abuse have profound effects on the microbiota and on immunoregulation during early life that persist into adulthood. Many aspects of the modern western environment deprive the infant of the immunoregulatory organisms with which humans co-evolved, while encouraging exposure to nonimmunoregulatory organisms, associated with more recently evolved "crowd" infections.

Social and physical environments early in development predict DNA methylation of inflammatory genes in young adulthood

“…The pattern of associations between postnatal microbial exposures and methylation of inflammatory genes underscores the relevance of epigenetic processes to the "hygiene" or "old friends" hypotheses, which link the development of immunoregulatory pathways to microbial environments early in development (22,53). More specifically, our findings are consistent with experimental animal models documenting negative associations between early exposure to microbes/microbial products and inflammatory processes later in life (54)(55)(56).…”

Green spaces and cognitive development in primary schoolchildren

“…Furthermore, parental psychological stress and depression have been reported to be adversely associated with cognitive development in their children (39) and exposure to green space has been associated with evidence of stress restorative effects and reduced depression in adults (3,28). A growing body of evidence also suggests that a failure of the immunoregulatory pathways due to a reduced exposure to macroorganisms and microorganisms in Westernized populations might play a role in impairment of brain development (10,40) with childhood as a particular window of vulnerability (41). Therefore, the ability of outdoor surrounding greenness to enhance immunoregulation-inducing microbial input from the environment (10) could have been another mechanism underlying our observed association between greenness exposure and cognitive development.…”

Psychoneuroimmunology of Early-Life Stress: The Hidden Wounds of Childhood Trauma?

“…Through experience in early phases of postnatal development, the brain and the immune system progressively expand their repertoire presumably to maximize adaptation to stimuli that are specific to the individual's unique environment (Bateson et al, 2004;Danese and McEwen, 2012;Greenough et al, 1987). Although these phenomena of developmental plasticity have been best studied with regard to the basic brain functions, such as vision (Hensch, 2005;Hubel and Wiesel, 2005), they are increasingly recognized as key determinants of complex brain and immune functions (Danese and McEwen, 2012;Rook et al, 2015). Here we will review evidence suggesting that early experiences are also important to set up long-term patterns of interaction between the brain and the immune system.…”