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 . 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-γ (IFN-γ) 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-γ levels [2-4] and altered Th2 responses that are followed by skewing of immune responses toward a Th2 phenotype [5-7]. Hagendorens and colleagues  found that a significantly lower percentage of IFN-γ-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-γ-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 . 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 . 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 .
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.  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 , and, furthermore, it was evident that children developing atopic dermatitis during the first year of life had reduced Treg numbers at birth . Cord blood from children born to atopic mothers was characterized by lower numbers and impaired Treg cell function .
The defective neonatal T, and in particular IFN-γ response, seems to be a result of the dysfunction of antigen-presenting cells and an altered innate immune responses . For instance, cord blood T cells are able to produce markedly increased amounts of IFN-γ when co-cultured with adult macrophages. Vice versa neonatal macrophages can inhibit the IFN-γ response of adult T cells . 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-γ . 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 . Furthermore, children with allergies have shown exaggerated innate cytokine production at birth following activation by TLR ligands . 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 function at birth may contribute to a reduced capacity to suppress Th2 responses in the early postnatal period, increasing the likelihood for the development of an allergic reactivity (Fig. 1).
Recent studies have attributed hypermethylation of CpG sites in the IFN-γ promoter in neonates as causative event responsible for the strong suppression of IFN-γ gene transcription [39-41]. However, treatment of cord blood T cells with a demethylating agent did not lead to a significant increase in IFN-γ response . In addition, atopy development by age 2 was not found to be associated with variations in methylation patterns in cord blood T cells . These data suggest that abnormalities in neonatal T cell function are regulated at the transcriptional level by so far unknown mechanisms.
A study by Martino et al.  recently published in Allergy provided a deeper insight into the regulatory cascade in neonatal T cells. To characterize the signaling cascades induced downstream of the T cell receptor, the group adopted a genome-wide approach measuring the gene expression profile of stimulated cord blood CD4+ T cells. Thereby, in a case–control study, data from children with early allergic outcomes (IgE-mediated food allergy in the first year of life) were compared with the ones from children who remained nonallergic. Furthermore, between these two groups, the allergen-specific responses to ovalbumin were compared. The production of IFN-γ, IL-13, IL-5, and IL-10 was significantly lower at birth in children developing an atopy at age one compared with the nonallergic group, reflecting the immaturity of T cell responsiveness in that group. However, at age one, Th2 as well as Th1 cytokine responses were increased in the allergic group. At birth, also at molecularly level differences between the two children groups were observed. The microarray analyses revealed that anti-CD3 antibody-induced gene responses of isolated CD4+ T cells were markedly reduced in the allergic children. Particularly, genes belonging to the NF-κB family were involved, including RELB, NFκB2, LIF, and FAS. Because of the fact that the proliferative response of T cells depends on signals through the NF-κB/Rel complex, the observed impaired lymphoproliferation in allergic children may be explained by the lower expression of these genes. In line with other studies, Martino et al.  demonstrated that the differences in gene expression detected at birth are only transient [28, 44]. By 12 month of age, differentially expressed genes in unstimulated CD4+ T cells of allergic infants included TCR pathway, the MAL T cell differentiation pathway and as expected the master regulator of Th2 differentiation GATA3. These findings clearly confirm the data of other studies reporting a Th2 bias in allergic children [45-47].
In spite of the recent progress, open questions remain. For instance, it is still unclear how the different regulated gene sets at birth and age one are linked to the identified signaling pathways, which result in impaired T cell function. However, rather imbalanced regulatory networks involved in T cell activation and differentiation than single factors seem to be responsible for priming toward an allergic reactivity. Therefore, to predict allergic outcomes from cord blood-derived factors, a pattern of different factors, including molecular and cellular markers instead of single factors, might be relevant. Furthermore, a deeper understanding of the differentially regulatory events in neonates at risk may support future allergy prevention strategies.