Plasmacytoid dendritic cells (pDC) express not only TLR9 molecules through which ligation with CpG DNA favors Th1 responses but also possess IgE receptors (FcεRI) implicated in allergen presentation and induction of Th2 responses. This dichotomy prompted an investigation to determine whether TLR9- and IgE receptor-mediated responses oppose one another in pDC by affecting receptor expression and associated functional responses. Results showed that IgE cross-linking reduced TLR9 in pDC and inhibited the capacity of these cells to secrete IFN-α when stimulated with the CpG oligodeoxynucleotide (ODN)-2216. In contrast, an ∼15-fold reduction in FcεRIα mRNA and a loss in surface protein were seen in pDC first exposed to TLR9 ligation with ODN-2216. Results indicated that type I IFNs partly mediated this effect, as rIFN-α also caused a significant ∼4-fold reduction in FcεRIα mRNA. Finally, this reduction in FcεRIα mediated by ODN-2216 correlated with a selective suppression of allergen-induced CD4+ T cell proliferation, but not of responses resulting from tetanus toxoid. Overall, these results imply mechanisms by which specific innate and IgE-dependent immune responses counterregulate one another at the dendritic cell level and may have significant impact on whether an ensuing response is either of Th1 or Th2 in nature.

Although IL-3 is commonly recognized for its growth factor-like activity, in vitro studies have long demonstrated a unique capacity for this cytokine to also augment the proinflammatory properties and phenotype of human basophils. In particular, basophils secrete mediators that are hallmarks in allergic disease, including vasoactive amines (e.g., histamine), lipid metabolites (e.g., leukotriene C4), and cytokines (e.g., IL-4/IL-13), which are all markedly enhanced with IL-3 pretreatment. This priming phenomenon is observed in response to both IgE-dependent and IgE-independent stimulation. Additionally, IL-3 directly activates basophils for IL-13 secretion and enhanced CD69 expression, two markers that are elevated in allergic subjects. Lymphocytes are commonly thought to be the source of the IL-3 that primes for these basophil responses. However, we demonstrate herein for the first time that basophils themselves rapidly produce IL-3 (within 4 h) in response to IgE-dependent activation. More importantly, our findings definitively show that basophils rapidly bind and utilize the IL-3 they produce, as evidenced by functional and phenotypic activity that is inhibited in the presence of neutralizing anti-IL-3 receptor (CD123) Abs. We predict that autocrine IL-3 activity resulting from low-level IgE/FcεRI cross-linking by specific allergen represents an important mechanism behind the hyperreactive nature of basophils that has long been observed in allergic disease.

Impaired TLR9-dependent immune responses in human pDCs are associated with allergic status and inversely correlated with Fc epsilon RI alpha expression. This impaired innate immune response among dendritic cells of allergic subjects may lead to more targeted therapeutic approaches and could provide a better understanding of the mechanisms underlying conventional and CpG-based immunotherapy.

Background Studies suggest that oral (OIT) and sublingual (SLIT) immunotherapy for food allergy hold promise; however, the immunologic mechanisms underlying these therapies are not well understood. Objective To generate insights into the mechanisms and duration of immunologic suppression to peanut during immunotherapy (IT). Methods Blood was obtained from subjects at baseline and at multiple timepoints during a placebo-controlled trial of peanut OIT and SLIT. Immunologic outcomes included spontaneous and stimulated basophil activity by automated fluorometry (histamine) and flow cytometry (activation markers, IL-4), allergen-induced cytokine expression in dendritic cell (DC)-T cell co-cultures by multiplexing technology, and expression of MHC II and costimulatory molecules on DCs by flow cytometry. Results Spontaneous and allergen-induced basophil reactivity (histamine release, CD63 expression, and IL-4 production) were suppressed during dose escalation and after 6 months of maintenance dosing. Peanut- and dust mite-induced expression of TH2 cytokines was reduced in DC-T cell co-cultures during IT. This was associated with decreased levels of CD40, HLA-DR, and CD86 expression on DCs, and increased expression of CD80. These effects were most striking in myeloid DC-T cell co-cultures from subjects receiving OIT. Many markers of immunologic suppression reversed following withdrawal from IT, and in some cases during ongoing maintenance therapy. Conclusion OIT and SLIT for peanut allergy induce rapid suppression of basophil effector functions, dendritic cell activation, and Th2 cytokine responses during the initial phases of IT in an antigen non-specific manner. While there was some inter-individual variation, in many patients, suppression appeared to be temporary.