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COMMENTARY The Journal of Clinical Investigation Histamine-releasing factor: a promising therapeutic target for food allergy Marsha Wills-Karp Department of Environmental Health and Engineering, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA. ety of fundamental cellular functions, such as cell­cycle progression, proliferation, and The prevalence of food allergies has been increasing at an alarming rate over malignant transformation (10). HRF is also the last few decades. Despite the dramatic increase in disease prevalence, the secreted by many cell types, and extracellu­ development of effective therapies has not kept pace. In this issue of the JCI, lar HRF has cytokine­like functions, includ­ Ando et al. provide a causal link between histamine-releasing factor (HRF) ing stimulation of histamine release and interactions with IgE and food allergy in a murine model. Successful oral type 2 cytokine production from IgE­sensi­ immunotherapy of both egg-allergic human patients and food-allergic mice tized basophils and mast cells (11). Since its was associated with sustained suppression of HRF-reactive IgE levels. These first description, HRF has been identified results support a role for HRF-IgE interactions in the amplification of intestinal in nasal lavages, bronchoalveolar lavages, and skin blister fluids of allergic individ­ inflammation and suggest HRF as a therapeutic target in food allergy. uals (12–14). Several studies have shown that decreases in HRF levels track with a reduction of allergic symptoms in those patients undergoing successful immuno­ Introduction releasing factor (HRF), which has long therapy, while HRF levels remain steady in Food allergy has become increasingly been associated with allergies, has poten­ those individuals that are nonresponsive to prevalent in recent years, with up to 8% tial as a therapeutic target for the treat­ immunotherapy (15, 16). Collectively, these to 10% of infants afflicted. Adverse food ment of food allergy. Specifically, Ando studies provide strong support for a role for reactions, such as anaphylaxis, can be life and colleagues showed that blockade of HRF in allergic pathogenesis. threatening in many individuals, especi­ HRF, which stimulates the release of his­ Although the contribution of HRF in ally children. There are no cures for food tamine and type 2 promoting cytokines the allergic diathesis has been studied for allergy, and current clinical management (IL-4, IL-13) from mast cells and basophils, more than 30 years, efforts to establish a relies heavily on avoidance of offending ameliorates food­allergy symptoms in a definitive role for this protein in allergic foods and emergency preparedness. As mouse model of food allergy and is asso­ diseases had been hampered by the lack of food avoidance, especially of nuts, in early ciated with clinical improvement in egg­ an animal model in which to explore in vivo childhood has recently been shown to actu­ allergic patients undergoing oral antigen HRF functions, as HRF­knockout mice are ally enhance sensitivity to these foods (1), immunotherapy. Although multiple stud­ embryonically lethal (10). However, mice there is a pressing need for new preventive ies have linked elevated levels of HRF with with overexpression of HRF specifically and/or treatment modalities for food allergy. allergic disorders (4–6), this study provides in club cells exhibit an enhanced allergic The classical pathway of anaphylaxis, a definitive link between HRF and food­ phenotype after antigen sensitization and which involves allergen­specific IgE cross­ induced inflammation and symptoms. challenge, concomitant with elevated IL-4 linking of FcεR1 on mast cells and baso­ and eosinophil levels when compared with phils, leading to the release of anaphylac­ HRF in allergy and disease littermate controls (17). While supportive togenic mediators such as histamine and HRF was originally classified as transla­ of a role for HRF in allergy, these studies serotonin, is thought to play a central role in tionally controlled tumor protein (TCTP) in did not allow dissection of HRF’s extracel­ food­induced adverse reactions. The clini­ mouse tumors and mouse erythroleukemia lular and intracellular actions. cal importance of IgE in human food allergy (7, 8). MacDonald and colleagues cloned Another impediment to defining has been supported by studies demonstrat­ and identified the same protein and called HRF function in allergy has been a lack of ing that the humanized anti­IgE mAb omal­ it HRF, as it promoted the release of hista­ understanding of its mode of action and izumab, which blocks IgE binding to FcεR1, mine from basophils (9). HRF is an evolu­ the nature of its receptor. While early stud­ enhances the benefits of oral immunother­ tionarily conserved protein that is widely ies hinted that HRF might work either in apy in food­allergic patients (2). expressed in mammals and has both intra­ concert with IgE or through IgE­dependent A report by Ando et al. (3) in this issue cellular and extracellular functions. Intra­ mechanisms, the fact that HRF activates demonstrates that inhibition of histamine­ cellular HRF expression is linked to a vari­ some cells that do not express Fc recep­ tors and that HRF does not interact with Related Article: p. 4541 all IgEs (18) complicated understanding the role of HRF in the context of allergic Conflict of interest: The author has declared that no conflict of interest exists. inflammation. In 2012, Kashiwakura and Reference information: J Clin Invest. 2017;127(12):4238–4241. https://doi.org/10.1172/JCI98297. colleagues (19) reported that HRF bound 4238 jci.org Volume 127 Number 12 December 2017 The Journal of Clinical Investigation COMMENTARY went on to show that pretreatment of mice in vivo with either of the HRF peptides (GST-N19, GST-H3) abrogated passive cutaneous anaphylaxis (PCA) induced by intradermal antigen challenge of mice primed with HRF­reactive IgE. As com­ bining the two reagents (N19, H3) largely ameliorated the PCA reactions, there is clearly a dose­dependent effect. Taken together, these studies definitively estab­ lished a link between HRF and allergic inflammation in vivo and suggested that HRF’s role was downstream of IgE/FcR interactions, not upstream. HRF inhibition reduces allergy symptoms In this issue, Ando et al. (3) demonstrated that treatment of food­sensitized mice with the HRF competitive inhibitor GST-N19 prior to food challenge reduced diarrhea occurrence, concomitant with reductions in mast cell numbers, mast cell protease levels, and type 2 cyto­ kines. HRF levels were 3 times higher in food­allergic mice than in controls and coincided with an increased HRF multi­ mer­to­monomer ratio. HRF inhibitors had no effect on food allergy–associated symptoms in FcεR1a­deficient mice; how­ ever, HRF inhibition suppressed allergic symptoms in FcγRIIB­deficient mice, Figure 1. HRF amplifies the allergic response. Exposure to food allergen results in release of inflam- matory cytokines, including IL-33 and TSLP, from the epithelium, which in turn promotes the develop- suggesting that the downstream effects ment of a Th2 inflammatory response that results in release of IL-4 and IL-13 and the development of of HRF-IgE interactions are mainly medi­ food allergen–specific IgE. IL-4, IL-13, and IL-33 induce secretion of HRF from the intestinal epithelium ated via FcεR1. After food allergy was and underlying fibroblasts. HRF dimerization is thought to crosslink IgE and enhance binding and established, Ando and colleagues noted activation of mast cells and basophils via FcRI. Activation of these cells results in the release of his- that therapeutic HRF inhibition did not tamine, IL-4, and IL-13, thereby resulting in allergic inflammation, such as is observed in food allergy, asthma, and allergic rhinitis. Additionally, release of IL-4 and IL-13 from mast cells and basophils reduce IgE levels, but did inhibit mast cell further exacerbates the allergic response by maintaining HRF secretion. activation and reduce clinical symptoms. These findings suggest that HRF regu­ lates the level of allergic inflammation, to a subset of IgE and IgG antibodies by N terminus (N-19) and the other in internal but is not able to reverse antigen sensiti­ interacting with the Fab region (antigen regions (H3 residues 107–135) of HRF. The zation once it has occurred. Nonetheless, binding) and not the Fc (FcR­binding discovery of these Ig­binding sites in HRF these studies support the possibility that region) regions of immunoglobulins, there­ opened up the possibility of developing therapeutic administration of HRF inhib­ by establishing immunoglobulins as recep­ competitive inhibitors of HRF-IgE inter­ itors may be effective at suppressing the tors for HRF. As previously reported (18), actions. Indeed, tagging both N19 and H3 severity of food allergy. Kashiwakura et al. showed that not all IgE peptides with glutathione S­transferase Although not a lot is known about how or Ig molecules bound HRF. Those Igs that (GST) (GST-N19 and GST-H3) inhibited the release of HRF is regulated during aller­ did bind HRF were designated as HRF­re­ the interaction between HRF and HRF­ gic inflammation, Ando and colleagues (3) active Igs, though it is not exactly clear reactive Abs. As they showed that HRF showed that the Th2 cytokines (IL-4, IL-5, what distinguishes HRF­reactive Igs from exists as a dimer, they theorized that the IL-13) induce HRF release from intestinal non–HRF­reactive Igs, and this area HRF dimer binds to two molecules of IgE epithelial cells and fibroblasts and that requires further study. and that this crosslinking of HRF to reac­ epithelial­derived IL-33 enhances HRF In the same report, Kashiwakura and tive IgE in turn promotes binding to FcεR1s secretion from fibroblasts. Based on their colleagues (19) identified two distinct on cells, such as mast cells and basophils, collective results, Ando et al. proposed a Ig­binding regions within HRF — one in the thereby leading to mediator release.

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