The immune system of neonates is relatively under-developed and relies on maternal antibodies transmitted in utero to modify and control the severity of neonatal diseases. These antibodies are of the class IgG, which can be transported across the placenta. This is an active, selective process that involves intracellular pathways and is specifically mediated by the neonatal Fc receptor FcRn. Fc receptors are expressed by the trophoblastic cell layer which surrounds the developing foetus. Once the receptor has bound IgG it is transported to the foetal circulation.
Transfer of antibodies begins at 17 weeks of gestation and increases as gestation advances. Maximal transfer occurs from the third trimester onwards. By 33 weeks of gestation maternal and foetal IgG are at equivalent levels and by 40 weeks of gestation the foetal IgG concentration is higher than maternal IgG. Factors that may reduce transplacental antibody transfer include a) pathogen insult that affect placental integrity, such as HIV or Malaria; b) very high concentrations of maternal IgG which cause antibodies to compete for the receptor and (c) the age of the foetus.
Premature infants born before 28 weeks gestation have very low levels of maternal antibodies and prematurity is also responsible for impaired cellular immunity. Having impaired humoral and cellular responses results in a “physiological immunodeficiency” which leaves premature infants vulnerable to both viral and bacterial pathogens.
The ways in which maternal antibodies enhance neonatal immunity includes interference with growth of the pathogen and by facilitating organism removal by a process of opsonisation. The antibody-coated pathogens are also more easily taken up by phagocytes that express Fc receptors which improves antigen presentation to T cells. In addition, maternal antibody-coated antigen is trapped by follicular dendritic cells that express Fc receptors thus allowing priming of B cells. However, there are also times when maternal antibodies can inhibit the immune response in the newborn. Even though it is normal functioning for maternal antibodies to coat pathogens to reduce infectivity and mark them for destruction, this can also bring about the undesired effect of reducing vaccine efficacy when using live replicating vectors because they are rapidly destroyed or growth is impeded (eg. oral polio vaccine or measles virus vaccine). Antibody coating can also mask B cell epitopes thereby interfering with B cell priming.
Along with placental transfer of maternal antibodies, breastmilk is a source of IgA, IgM and IgG antibodies. The Fc receptor is expressed in the lactating breast and serves to transport IgG from maternal blood to breastmilk. There are also B lymphocytes present in breastmilk which can produce antibodies. The Fc receptors expressed on epithelial cells lining the infant gut facilitates transport of maternal IgG into the foetal circulation. IgM and IgA bind antigens present in the gut lumen and small amounts of IgA are also transported into the foetal circulation in the first few days of life. IgM is not transported into the foetal circulation.
Without the passive acquisition of maternal antibodies, the immature nature of neonate humoral and cellular immunity can result in the infant being highly vulnerable to bacterial and viral infections in the perinatal period. Maternal antibodies confer an immunological advantage to the child until the infant immune system has developed to the point of self-sufficiency. The following should be noted about neonatal immunity:
- The innate immune response to pathogens involving engagement of toll-like receptors expressed by neonatal antigen presenting cells, such as dendritic cells, preferentially promote the secretion of cytokines IL-6, IL-1β, IL-10 and IL-23 and very little IL-12. In turn, when T cells become activated, IL-10 and IL-23 favour the maintenance of Th2 (secretion of IL-4 and IL-10) and Th17 (IL-17) helper T cell responses rather than Th1 (secretion of IL-2, IFN-γ) responses.
- The secretion of IL-10 switches off Th1 CD4 cell development and promotes Th2 CD4 cells and counteracts the small amounts of IL-12 produced. Although the neonatal cytokine profile is polarised towards a Th2 response, an overall deficiency of cytokines or co-stimulatory molecules, such as the CD40 ligand, may explain why neonatal CD4 cells have a diminished capacity to provide help to B cells. Engagement of CD40 on B cells is essential for immunoglobulin synthesis, isotype switching and affinity maturation which is impaired in neonates (and hence the importance of maternal IgG).
- The development of anti-viral CD8 cytotoxic T-lymphocytes (CTL) is imparied in infants. For example, fewer infants under 5 months develop CTL responses to respiratory syncytial virus (RSV) compared to those between 6-24 months of age. Hence any viral infection in a child with “physiological immunodeficiency” will be highly vulnerable in the first few weeks of life. It is thought that low expression of surface molecules CD80, CD86 and CD40, which serve as co-stimulatory molecules on activated dendritic cells, fail to provide adequate activation signals for full development of T cells.
- New experimental insights using a mouse model have shown an additional mechanism contributing to a lack of Th1 responses. These findings were made in the neonatal mouse, and have yet to be shown in humans. Neonatal Th1 T cells express a unique receptor for interleukin 4 (IL-4) that induces programmed cell death (apoptosis) when stimulated by IL-4 which is abundantly secreted by activated neonatal Th2 T cells. Thus the process of apoptosis will eliminate Th1 cells.
Download Combined PDF of all Graphics