This study demonstrates that microbial stimulation with the TLR2 agonist peptidoglycan in vitro modulates functional immune capacities of cord blood mononuclear cells (CBMC) from children of mothers with as compared to without a doctors diagnosis of maternal atopy. In CBMC from children of mothers without a doctors diagnosis of atopy, an increase of Ppg-induced IL-10 secretion was paralleled by an increase of two markers of T regulatory cells (significantly for Foxp3 and mildly for CTLA4). In addition, Ppg stimulation was associated with a positive correlation between IL-10 and genes associated with T regulatory cells (Foxp3, GITR and CTLA4), suggesting innate modulation of T regulatory cells in CBMC. These data support the hypothesis that microbial stimulation of CBMC leads to immune modulation in association with the maternal atopic background.
Of note, the phenotype of T regulatory cells is not clearly defined to date. We acknowledge the limitation of a mixed CBMC population in this study. While this study did not address cell type, prior studies indicate that TLRs are present not just on monocytes and B cells but also on T cells, underscoring a putative link between innate and adaptive immunity [32
]. The induction of both IL-10 and IFN-γ following stimulation with Ppg in this study could indicate a role of a specific population of T cells in human CBMC. For example, it has been proposed that IL-10 and IFN-γ producing CD4+
T cells may be one of the human equivalents of the CD4+
T regulatory cells originally described in the mouse [33
]. In addition, in this study not only IL-10 but also GITR, another marker characteristic for T regulatory cells, was increased following Ppg stimulation. We present an increase of TLR2-stimulated IL-10 as well as a correlation between IL-10 and other markers of T regulatory cells. These data may indicate that microbial stimulation such as Ppg can impact T cells in the fetal immune system, potentially capable of regulating several immune processes including cytokine secretion. This is intriguing in the context that Ppg stimulation in our murine model of asthma could decrease allergic stimulation [17
IL-10 secretion may be crucial in modulating the development of the fetal immune system, and in contributing to Th2 maturation via inhibition of IL-12 production [27
]. On the other hand, regarding allergic diseases, IL-10 was demonstrated in several studies to be associated with lower risk for atopy or sensitization to egg protein in later life [28
]. The Ppg-induced increase of IL-10 in our study could indicate a role for innate stimuli in early immunomodulation. Furthermore, IL-10 was induced in chronic schistosomiasis in African children, who have a low prevalence of atopic disease [30
]. Additionally, successful allergen-desensitization therapy has been postulated to work through the induction of IL-10 secreting T regulatory cells. In support of this concept, IL-10 secreting T regulatory cells were shown to be induced by glucocorticoids and β 2-agonists, the hallmark of anti-allergic therapy [31
Furthermore, the forkhead-winged-helix family transcription factor Foxp3 may control genes encoding T regulatory cell-associated molecules (such as CD25, CTLA4 and GITR). Mutations in Foxp3 lead to the X-linked immunodeficiency syndrome IPEX in humans (immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome). Clinical features are autoimmune disease, inflammatory bowel disease, severe allergy including atopic dermatitis, food allergy, and fatal infection [34
]. Foxp3 is stably expressed in mature natural T regulatory cells; the role of Foxp3 in the development of the neonatal immune system remains to be determined. It is intriguing that both IL-10 and Foxp3 levels are decreased in cord blood of neonates of mothers with atopy in our study.
Maternal atopy is known to be an important influential factor in a child's allergic predisposition [35
]. In this study, maternal atopy is defined as doctors diagnosis of asthma, hay fever and/or eczema. Unfortunately, data on maternal sensitization were not available, which we acknowledge as a potential limitation of the study. From the literature, the prevalence of a positive skin prick test to at least one allergen is reported in up to 60% in the 20–29 year old age range in the American NHANES population not stratified as high or low risk for atopy [36
], which most closely represents the population in our study. The percentage of sensitization can therefore be much higher without having ever any atopic symptoms. Also, some studies suggest that a history of atopic symptoms may be more indicative of allergic disease than skin test positivity to allergens.
Our analysis was performed in a group of 50 mothers including 19 with maternal atopy as defined by the doctor's diagnoses asthma and/or hay fever and/or atopic eczema. Further separate analysis in the subgroups were not statistically feasible. In addition, the diagnosis of maternal atopy comprises a common immunological basis for all three diseases. Regardless, the specific immunological mechanisms by which maternal atopy may influence the development of atopy in the child remain undefined. Thus, differences in T cell regulation, possibly T regulatory cells, depending on the maternal atopic background, may be biologically important. The study of Amoudruz et al. in CBMC of 9 mothers with and 10 without allergy is consistent with this concept [14
]. In this study, cytokine secretion of IL-6 is lower after Ppg stimulation in CBMC of mothers with as compared to mothers without allergy. Importantly, Pasare et al have shown that the suppressive effects of CD25+
regulatory cells can be blocked by the presence of IL-6, produced by DC and activated through stimulation of the TLR pathways [37
]. Our study suggests that in maternal atopy, T regulatory cells may be potentially less effective as demonstrated by reduced secretion of IL-10 and by diminished expression of Foxp3.