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Cord blood immune status: predicting health or allergy?

It has become more evident that immune status at birth may be an intrinsic feature of a newborn child that predisposes to the development of allergic disease (1). By now, mainly three predictive factors for later allergy development are evident: cord blood cytokine production, regulatory T cell number/ function, and toll-like receptor (TLR) expression/function. A characteristic feature of atopy is a T helper (Th) 2 skewed immune response and its related cytokines IL-4, IL-5, and IL-13, which are involved in the induction and maintenance of the IgE synthesis. The importance of Th1 cytokines like interferon-c (IFN-c) in atopy development is to antagonize the effect of Th2 cytokines. Although published data are not completely consistent in this regard, the most common abnormality detected early in life among children, who go on to have an allergic disease, includes diminished IFN-c levels (2–4) and altered Th2 responses that are followed by skewing of immune responses toward a Th2 phenotype (5–7). Hagendorens and colleagues (8) found that a significantly lower percentage of IFN-c-producing stimulated cord blood CD4+ T cells was present in children, who developed an atopic dermatitis during the first year of life. Our own data from the LISA study corroborate these findings: children with reduced frequencies of IFN-c-producing CD4+ T cells in the cord blood had a higher risk of developing atopic dermatitis, whereas a high percentage of IL-4-producing T cells in cord blood was associated with an increased risk for atopic dermatitis during the first two years of life (7). Besides Th1/Th2 cytokines, recent publications show the involvement of high levels of cord blood Th2-associated chemokines CCL17 and CCL22 in the prediction of atopic dermatitis, allergic sensitization, and asthma (9–11). Although different studies have provided evidence of a predictive cord blood cytokine value corresponding to the development of allergic diseases, other birth cohort studies failed to show such associations (12–15). The apparent discrepancy in the reported results might be explained by differences in the subject population, the length of follow-up, and more importantly, the mode of cytokine measurement. Mainly cord blood cytokines measured at the cellular level revealed a correlation to atopy, indicating that the source of cytokines is important in the clarification of allergy development (16). Certainly, by measuring protein concentrations in the supernatant of cord blood cell cultures, cytokines from additional sources than T helper cells are also gathered that may hide/ cover possible associations (17). Recently, there has been emerging evidence that allergic disease or sensitization is also associated to regulatory T cells. These cells downregulate Th2 cells by suppressing their activity and proliferation. Consequently, attenuated neonatal regulatory T cell (Treg) function or low numbers (18–20) were shown to be associated with later allergy development. Smith et al. (18) demonstrated that children with egg allergy at the age of one had reduced neonatal regulatory T cell function. In our LINA study, reduced maternal Treg frequencies correlated with increased total IgE concentrations in cord blood (20), and, furthermore, it was evident that children developing atopic dermatitis during the first year of life had reduced Treg numbers at birth (19). Cord blood from children born to atopic mothers was characterized by lower numbers and impaired Treg cell function (21). The defective neonatal T, and in particular IFN-c response, seems to be a result of the dysfunction of antigenpresenting cells and an altered innate immune responses (22). For instance, cord blood T cells are able to produce markedly increased amounts of IFN-c when co-cultured with adult macrophages. Vice versa neonatal macrophages can inhibit the IFN-c response of adult T cells (23). Moreover, when cultured with interleukin 12 (IL-12), a cytokine secreted by macrophages and other antigen-presenting cells that stimulates T cells, neonatal T cells are capable of producing large amounts of IFN-c (24). Interleukin-12 and other macrophage or dendritic cell-derived cytokines are activated via toll-like receptors (TLRs), known as pattern-recognition receptors critically involved in innate immune responses. Thus, deviations in innate immune responses can also contribute to allergic predisposition (25,26). A reduced expression of TLR5 and TLR9 was found in the cord blood of children who later developed an atopic dermatitis (27). Furthermore, children with allergies have shown exaggerated innate cytokine production at birth following activation by TLR ligands (28). However, whether increased or rather decreased expression/ function of TLRs may be the basis of allergy development is still a matter of debate. Besides the genetic background (29–31), it is undoubtedly that environmental exposure also affects the prenatal development of immune cells, hence the allergic predisposition (32–34). Microbial products that stimulate innate immune responses via TLRs can modulate the allergic propensity toward protective effects (28,35,36). On the other hand, maternal chemical exposure during pregnancy can contribute to reduced Th1 responses, therefore increasing the risk of developing allergies (37,38). However, the underlying mechanisms of these additional environmental impacts are fare away from being understood. Collectively, these data suggest a complex interplay of Th1/Th2/Treg as well as factors of the innate immune response in relationship to immune maturation and the development of allergic diseases. An impaired Th1 and Treg func-