In this clinical and mechanistic study, peanut OIT induced clinical desensitization in the 29 peanut-allergic subjects who completed the study. Ninety-three percent successfully completed an OFC to 3.9 g peanut protein, and all subjects had a significant increase in the amount of peanut they tolerated during food challenge. Peanut OIT was also safe; mild symptoms were relieved with diphenhydramine or albuterol. Our results are consistent with prior studies in which OIT led to clinical desensitization to foods such as egg and cow’s milk.14–16,22
Furthermore, the humoral and cellular responses associated with peanut OIT suggest that OIT also induces the transition from short-term desensitization to long-term tolerance. For this analysis, we did not perform OFCs after cessation of therapy, when sensitivity could return. Per the protocol, these definitive challenges are planned for subjects who complete 3 years of maintenance therapy and have a significant drop in serum IgE. However, compared with prior OIT studies, our study had a longer duration of maintenance therapy, which we hypothesize has a significant impact on the immunologic parameters indicative of long-term tolerance.
In our study, titrated prick skin tests showed a significant decrease at 6 months and remained decreased throughout the study. Similarly, basophil activation, a measure of IgE-dependent response, decreased significantly within 4 months, and the decline continued beyond 6 months. IgE-mediated hypersensitivity responses are known to be downregulated during drug desensitization,23
and chronic FcεRI signaling induces a downregulation of Syk-dependent signal transduction in vitro.24
With peanut OIT, peanut-specific IgE, IgG, and IgG4 increased by approximately 3 months, and then the IgE declined by 18 months. IgG began to decrease by the end of the study, while IgG4 remained elevated. Increased levels of specific IgG4 with or without decreased IgE have been associated with successful venom immunotherapy,25
lower levels of atopy in the presence of parasite infection,26
transient rather than persistent milk allergy,27
and the apparent protective effect of high levels of cat allergen exposure.28,29
Previous reports have demonstrated that fractionated IgG4 antibodies from serum of patients who received grass pollen immunotherapy inhibit IgE–FAB binding to B cells,30–33
suggesting a functional role of IgG4 in inhibiting IgE–FAB.
Traditional allergen-injection immunotherapy appears to act through downregulation of allergen-specific Th2
responses or increased Th1
responses or through the induction of T regulatory cells. Populations of both thymus-derived CD25+ “natural” T cells and antigen-specific T cells become CD25+, express FoxP3, secrete IL-10, and have suppressive function. IL-10+ T cells are induced during venom, dust mite, birch, and grass pollen immunotherapy.34–36
In our study, FoxP3 regulatory T cells increased after the induction of OIT and then eventually decreased. IL-10 was significantly increased over 6 to 12 months, as were a number of inflammatory cytokines/chemokines, such as IL-1β, IL-5, TNF-α, MIP-1β, and the growth factors G-CSF and GM-CSF. These changes did not reflect the typical transition toward a Th1
profile that we expected. However, the early induction of regulatory T cells expressing FoxP3 and the associated increase in IL-10 indicate an immunologic change induced by OIT, with transition away from a Th2
-type profile that was seen with both nonspecific (ConA) and antigen-specific (peanut) stimulation over time.
Our microarray data demonstrating downregulation of genes in several apoptosis pathways in patient T cells following 6 months of OIT are intriguing and may reflect involvement of programmed cell death in peanut OIT. However, it is unclear from these results if the observed changes in total peripheral blood T cell transcription patterns include altered apoptosis of antigen-specific T cells. To help clarify this point, studies are underway to compare transcript patterns before and after OIT in peanut-specific T cells isolated using MHC class II/Ara h 2 peptide tetramers. The lack of treatment-related changes in expression of Treg-, Th1-, or Th2-specific genes by microarray versus by protein assays likely reflects the small number of FoxP3-producing cells and low cytokine transcription levels in unstimulated CD3+ T cells analyzed in microarrays.
To our knowledge, no other oligonucleotide microarray analyses of patient T cell transcription patterns pre- and post-treatment of food allergy have been reported. In one microarray study of PBMC transcripts in 8 subjects with allergic rhinitis, several apoptosis-related genes were underexpressed compared with control PBMC prior to allergen immunotherapy.37
A small number of studies have noted increased in vitro apoptosis of stimulated peripheral blood Th2
cells following standard allergen immunotherapy in subjects with either grass pollen allergy38,39
or dust mite–sensitive asthma.40
We plan similar flow cytometric analyses of apoptosis in patient T cells following in vitro stimulation with peanut antigen.
Taken together, our results suggest that OIT induces a progression toward tolerance starting with desensitization at approximately 3 months. During this time, the threshold of antigen needed to induce an allergic response changes drastically, as reflected by diminished reaction to skin prick tests and activation of basophils. Subsequent immunologic changes over 6-to-12 months reflect a pro-inflammatory, rather than Th2, profile.
In our study, results of titrated prick skin tests, levels of allergen-specific IgE, IgG, and IgG4 over time, and FAB data are similar to those reported from studies of traditional subcutaneous immunotherapy.25,30–36
Our cytokine data, with a significant increase in IL-10 and a number of inflammatory cytokines/chemokines, are not reflective of the typical transition toward a Th1
profile. The increase in IL-10 could support an initial increase in Treg cells, leading to tolerance, but the overall increase in the inflammatory cytokines/chemokines is not really suggestive of this expected change. The inflammatory response may result from the oral versus subcutaneous route of exposure, although exactly how is unclear. No similar studies comparing OIT and traditional subcutaneous allergy immunotherapy have been done that might provide a context for our basophil and microarray data.
Clinical desensitization, which we defined as raising the threshold of food antigen needed to cause allergic symptoms, can provide an improved margin of safety in case of accidental food ingestion. This is an important therapeutic benefit to patients and their families. Blinded, placebo-controlled studies with peanut OIT are underway now, as are studies to determine the ability of OIT to induce long-term clinical tolerance after discontinuing OIT.