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Development of digestive and immunological function in
neonates: role of early nutrition
*
D. Kelly , A.G.P. Coutts
Department of Intestinal Cell Biology and Immunology, Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen AB21 9SB, UK
Abstract
The developing intestine undergoes morphogenesis and cytodifferentiation, events that are exquisitely regulated and highly organised, both spatially and temporally. The outermost epithelial layer of the intestine, the site of nutrient digestion and absorption, is organised into two morphological and functionally distinct compartments. Within the crypt, stem cells undergo several cycles of cell division and migrate onto the villus where they exhibit changes in phenotype and functional characteristics. Enzyme, transporter and glycosylation patterns change dramatically during the transition from birth to adulthood. These transitions permit dietary change and adaptation, and are also important determinants of disease susceptibility. In addition to its role in nutrient uptake, the intestine represents a key innate component of intestinal defence by providing a physical barrier to the external environment. Maintenance of tight junctions and the ability to secrete mucin are critical components of its protective function. Innate immunity is particularly important to the neonate, as the active or specific arm of the immune system is immature. In early life, the presence and processing of luminal antigen, particularly bacterial antigen, drives the expansion of intestinal epithelial and lymphoid tissues. Early nutrition is also recognised to play a very significant role in modulating intestinal differentiation and function and in regulating immune responses to environmental antigens. 2000 Published by Elsevier Science B.V.
Keywords: Intestine; Development; Immune system; Early nutrition; Environmental antigens
1. Introduction active enzymes and transporters, that degrade and transport nutrients from the intestinal lumen across The intestinal epithelium represents the major the epithelium, is essential to the digestive function interface between the host and the environment. It of the intestine. Many of these membrane-bound plays a crucial role in nutrient transport and in glycoproteins are developmentally regulated. A protection, by limiting the uptake and translocation change in the expression of these enzymes and of harmful agents. The expression of biologically transporters provides the cellular basis for develop-mental changes in digestive capability and capacity. Dramatic changes in glycosylation patterns also occur during development and are thought to be *Corresponding author. Tel.: 144-1224-716-648; fax: 1
44-important to intestinal health. Many of the carbohy-1224-716-687.
E-mail address: dk@rri.sari.ac.uk (D. Kelly). drate moieties displayed on gut surfaces are
portant determinants of bacterial colonisation and 2. Development of intestinal epithelium and
uptake by epithelial cells, including M cells. The digestive function
barrier function of the intestinal epithelium is an
important feature of the intestine at all stages of life, Transitions in the morphogenic, phenotypic and and is reinforced by the formation of tight junctions, functional characteristics of the intestinal epithelium which dramatically reduce the permeability of the occur in a very organised and predictable fashion intestine. The mucin gel coating the intestine pro- during pre- and postnatal development. The intrinsic vides a further protective feature that can trap genetic programming of the intestine drives these noxious agents and thus prevent their access to the developmental transitions. However, functional de-intestinal epithelium. Carbohydrate moieties, which velopment can be altered by intrinsic factors, includ-are expressed in abundance in mucin, includ-are thought to ing neural and hormonal stimuli, but also by cell– restrict the movement of harmful agents within the cell and cell–matrix crosstalk or communication. In
mucous gel. addition, diet and microbes influence both the
prolif-A major function of the intestinal epithelium is to eration and differentiation state of the epithelium and transport and present dietary and bacterial antigens to have a very significant impact on the developing the immune system. For the developing neonate, intestine.
intestinal transport of antigen is vital to drive the Extensive structural and functional development of expansion and maturation of lymphoid organs. How- the intestine occurs in utero. The degree of maturity
¨
ever, the neonate is immunologically naıve and is at birth generally reflects the length of the gestational very susceptible to infection and damage from period. For example, in the human foetus at 20 harmful antigens. Early nutrition plays an important weeks gestation, intestinal morphology and func-role in protecting the developing intestine from tional characteristics generally resemble those of the harmful agents and in modulating immune responses newborn (Lebenthal, 1989). Intestinal development following antigenic challenge. Maternal milk pro- in the pig, like the human, is also initiated early in vides a diverse range of substances that are de- foetal development. For example, at 40 days of velopmentally delayed in the neonate and are thought gestation, intestinal morphogenesis has progressed to to play a critical role in gastrointestinal defence. the point where recognisable villi are present and Bioactive molecules such as immunoglobulin A mRNA for enzymes and cytoskeletal proteins are (IgA) and lactoferrin are present in abundance and readily detectable (Perozzi et al., 1993). The mam-are potent anti-microbial agents. Maternal milk also malian intestine undergoes two main phases of contains substances, including growth factors and development during the neonatal period. The first cytokines, that promote the maturation of the intes- phase involves the preparation for extra-uterine life tine and the associated immune system. The im- when maternal colostrum and milk will provide the muno-modulatory property of maternal colostrum sole nutrient source. The second phase of intestinal and milk is now attracting considerable scientific and development is associated with a shift in the diges-commercial interest. Certain constituents of maternal tive capability of the epithelium from one which milk, with immuno-modulatory potential, are capable digests and absorbs a milk diet to one which of promoting immune function in early life, the efficiently utilises complex solid feeds. With both benefits of which may persist throughout adult life. these phases it is recognised that extensive changes Identification of these constituents would permit occur in the gross architecture, ultrastructure, growth supplementation of animal and infant milk formula- and differentiation of the intestine.
tions to improve their nutritional and health promot- During the immediate postnatal period a dramatic
ing properties. increase in the diameter, total weight and surface
content to be 4.6-fold higher at 24 h after birth The carbohydrates expressed on the surface of gut (Zhang et al., 1997). This increase in growth is cells are important determinants for bacterial coloni-driven by accelerated cell proliferation in the crypt sation as they facilitate cell recognition and bacterial stem cell compartment of the intestine. Furthermore, attachment. Many of the pathogenic bacteria associ-it has been proposed that a major component of the ated with the neonatal period have high affinity for growth response of the intestine in early life is carbohydrate moieties that are expressed in the directed towards cryptogenesis and villus remodel- intestine during this critical period. It is therefore ling (Goodlad and Wright, 1990). In addition to likely that predisposition to certain bacterial and viral extensive epithelial cell proliferation, dramatic infections in the neonatal period are, in part, attribut-changes in enzymes, receptors and transport systems able to the presence of specific carbohydrate re-occur. Many of the intestinal-specific genes, includ- ceptors. Recent studies with neonatal pigs have ing those of enzymes, show distinct temporal pat- shown that the susceptibility to K88 Escherichia terns of expression. Lactase, the principal car- coli, a major pig pathogen, is related to the presence bohydrase of the neonatal mammalian intestine and of a galactosylated receptor complex in the intestine responsible for the digestion of lactose in milk, is of disease-susceptible animals, one that is absent very high at birth and declines with age and follow- from animals that display disease resistance (Jeyasin-ing the wean(Jeyasin-ing process (Kelly et al., 1991a). A gham et al., 1999).
reciprocal pattern of expression occurs for enzymes such as sucrase and maltase. At birth, the levels of
active enzyme are low and increase as the neonate 3. Development of immune function in neonates
matures or is weaned. The expression of these
enzymes can be readily induced by presentation of At birth the structural development of the immune specific substrates (Kelly et al., 1991b; Zhang et al., system is complete. Analogous to the gut epithelium, 1997). In the intestine, oligosaccharide moieties are the degree of maturation can be correlated with found associated with enzyme and non-enzyme gestational length. The primary lymphoid organs, glycoconjugates. Many of these oligosaccharides thymus and bone marrow, are generally well formed provide a wide range of binding sites for luminal and and contain progenitor lymphomyeloid cells. circulating biologically-active ligands such as growth Lymphopoiesis is initiated early in gestation and is factors, hormones, bacteria and toxins (Kelly et al., very advanced at birth. A communicating network of 1992; Stewart et al., 1993). Age-related changes in lymphatic vessels is also present during early de-intestinal glycosylation in neonates have been exten- velopment, creating a functional circulatory system sively reported (Taatjes and Roth, 1990; Kelly and which enables rapid dissemination of lymphomyeloid King, 1991; King and Kelly, 1991). These changes precursor cells. This permits seeding of precursor play an important role in modifying the properties of cells from progenitor sites to key inductive sites such intestinal receptors for dietary constituents as well as Peyer’s patches and mesenteric lymph nodes commensal and pathogenic bacterial receptors and (MLNs).
an infectious challenge. The major immune cell Neonatal cells also have a limited ability to activate types involved in mediating innate responses are the specific adaptive arm of the immune system macrophages, neutrophils and natural killer cells. (Fairhurst et al., 1998).
These cells are capable of discriminating self from
non-self and recognise molecular arrays or patterns, 3.1. Disease susceptibility of neonates such as lipopolysaccharides or techoic acids, that
seem to be shared among groups of pathogens. These Although the cellular apparatus of the immune structures are recognised by pattern recognition system is in place and mucosal and systemic anti-receptors expressed on activated macrophages that body responses can be detected early in life, a induce killing mechanisms including phagocytosis functional deficit in immune function persists for and opsonisation. For many years the innate immune some time leaving the neonate susceptible to bacteri-system has been seen as a separate entity from the al and viral attack. Aspects of the immune function adaptive system. However, recent studies have which account for the impaired responses to antigen shown that they have a more integrative function challenge have been proposed and include enhanced where the innate system functions to initiate and intestinal permeability and immaturity in T and B regulate the adaptive response (Jullien et al., 1997). cell function. However, in the neonate, a major The adaptive immune responses are initiated follow- limitation to mounting appropriate immune responses ing antigen uptake and presentation to T and B cells. appears to relate to deficiencies in antigen presenta-The major route of antigen uptake has long been tion and, in particular, the function of APCs (Ridge considered to be via M cells which are located on et al., 1996; Jullien et al., 1997). The neonate is also organised lymphoid structures referred to as Peyer’s very deficient in anti-polysaccharide antibodies to Patches. Antigens are transported into the gut associ- encapsulated pathogens (Fairhurst et al., 1998) and ated lymphoid tissue and are presented to T and B this has been attributed to deficiencies in the CD1 cells by antigen presenting cells (APCs), a process system of antigen recognition and presentation. The that involves antigen recognition and has inherent CD1 system is also thought to be an important fine molecular specificity. Antigen-primed T and B antigen presentation pathway that links together the lymphocytes migrate through the lymph and reach innate immune system and the adaptive response the peripheral blood, where they home to mucosal (Jullien et al., 1997). This may explain why the effector sites, including the mammary gland. In this neonate fails to mount strong and appropriate im-way the maternal experience of environmental an- mune responses. The neonate is therefore immuno-tigens is passed to the sucking neonate. The role of logically suboptimal and, given an appropriate level enterocytes and dendritic cells in antigen uptake and of pathogen exposure, the mucosal surfaces are presentation to APCs (Kaiserlian and Etchart, 1999) readily colonised by harmful micro-organisms. is currently very topical and it is considered that
immunological outcome may be determined as a
result of the communication and crosstalk between 4. Early nutrition and the developing intestine
these cell types. and immune system
Resistance to infection relies on a harmonious
protection can be grouped into four main categories express the surface antigen CD45RO, which is and comprise maturational, immuno-modulatory, an- associated with immunological memory (Bertotto et ti-inflammatory and anti-microbial effects. al., 1990). They are also loaded with ligands such as CD40L (Bertotto et al., 1996). These cells are 4.1. Maternal milk and maturation of the intestine thought to be taken up by the neonate and to compensate for the immature function of neonatal Colostrum and milk feeding have been shown to cells by providing potent activation signals leading to promote the maturation of the developing intestinal strong active immune responses (Bertotto et al., epithelium. Lactase expression (specific and total) 1996; Xanthou, 1997). It has also been proposed that was found to decline significantly in colostrum-fed milk-borne cytokines are important regulators of animals when compared with colostrum-deprived immune responses. For example, human colostrum animals. The levels of sialylation and fucosylation on has been shown to stimulate the release of cytokines epithelial cells indicated that intestinal cells from from peripheral blood mononuclear cells (PBMCs), colostrum-fed animals were phenotypically mature thus altering the cytokine milieu or background (Kelly et al., 1993). Differences in glycosylation against which immunological decisions are made patterns over Peyer’s Patch epithelium were also (Bessler et al., 1996). Among all the factors that are observed. This suggests that the adherent flora in known to control T cell development and function, colostrum-fed animals may be qualitatively different cytokines are considered to be the most important over Peyer’s Patch epithelium and hence the antigens (Delespesse et al., 1998).
sampled may also differ. The possibility that such It is interesting that the defence factors in human differences influence the priming of the immune milk function without causing inflammation and in system is considerable, since the nature and dose of fact a number of maternal milk constituents have antigen have a dramatic effect on the immunological been reported to have anti-inflammatory activity.
outcome. Lactoferrin and fragments thereof has been shown to
inhibit the endotoxin-induced interleukin 6 (IL6) 4.2. Maternal milk and immune modulation release from human monocytic cells (Mattsby-Bal-tzer et al., 1996). The cytokine IL10 and TGFbhave The immuno-modulatory effects of maternal milk been reported in maternal milk and are recognised to have been investigated using antibody responses to have immuno-suppressive and anti-inflammatory ac-vaccines as indicators of immunity (Pickering et al., tivities (Letterio et al., 1994; Garofalo et al., 1995). 1998). The results from these studies are very Maternal TGFb has been shown to be important to controversial. However, a well-executed randomised the survival of TGFb-null mice by reducing the trial (Pickering et al., 1998) recently reported that diffuse and lethal inflammation caused by gene breast-fed infants do exhibit enhanced specific anti- disruption (Letterio et al., 1994).
body titre to some vaccines. Similarly,
immuno-phenotypic differences in lymphocyte populations 4.3. Maternal milk and anti-microbial function have been reported following exposure to maternal
against infection. A large number of constituents will address this question at a mechanistic level, will which interfere with pathogen binding have been unravel further virtues of maternal feeding.
reported in milk. Lactadherin is a milk glycoprotein that inhibits rotaviral infection (Newburg et al.,
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