The economic impact of food allergies is staggering, estimated at $25 billion in the US annually (4). Each year 200,000 Americans attend emergency departments for food allergy-related anaphylaxis, and a 124% increase in visits was documented between 2005 and 2014 (5, 6).
In stark contrast to the impressive social, medical, and economic impact of food allergies, there is a concerning paucity of therapeutic options available for this disease. Indeed, the standard of care for food allergy is allergen avoidance, although oral immunotherapy (OIT) for peanut (PN) has been approved by the FDA in the United States (7).
The greatest impact in the food allergy field was furnished by a landmark study which provided conclusive evidence that the administration of PN to infants 4–11 months old dramatically reduces the incidence of PN allergy (8). Yet, this major advancement leaves the millions of patients diagnosed with food allergies worldwide unprotected.
Clearly, one of the greatest remaining challenges is whether established food allergy can be fundamentally mended. This challenge becomes even more pressing for allergies such as those to PN, tree nuts, fish, and shellfish which are lifelong in most patients (7, 9, 10). In this context particularly, a “disease-transformative” treatment would require the restoration of an active non-harmful immune response to foods, known as tolerance. We define a “disease-transformative” treatment as one that successfully alters the underlying disease mechanisms permanently.
The Immunobiology Underlying The Persistence of Food Allergy
Allergic sensitization begins with the disruption of homeostasis at mucosal sites or the skin resulting in the release of alarmins such as IL-33, IL-25 and TSLP (11). The mechanisms of homeostatic disruption which underly allergic sensitization have been thoroughly reviewed elsewhere (11). This breach of homeostasis leads to the differentiation of allergen-specific CD4+ Th2 cells, T follicular helper (Tfh) cells, short-lived plasma cells, and memory B cells (MBCs), all of which contribute to IgE generation and clinically active food allergy (12, 13).
Serum IgE levels are extremely low compared to other isotypes and IgE has a half-life of approximately 3 days in humans, a rapid turnover rate in comparison to other immunoglobulin isotypes (14–16). Evidence for a role of long-lived plasma cells replenishing these IgE titres, and in the persistence of food allergy has long been disputed (12, 17).
In the peripheral blood of PN-allergic patients, IgE+ plasma cells have an immature transcriptional program characterized by the upregulation of MHC and low-affinity IgE receptor (CD23) and downregulation of plasma cell survival genes (19, 20). Further, in PN-allergic mice avoiding PN, IgE+ plasma cells have a half-life of approximately 60 days (18). Once bound to mast cells, IgE-mediated mast cell degranulation upon challenge has a half-life of approximately 70 days in mice prior to any subsequent allergen exposure, indicating that undetectable serum IgE does not preclude clinical reactivity upon allergen exposure (18).
Similar evidence exists in humans allergic to galactose-α-1,3-galactose, whose serum allergen-specific IgE titres declined when avoiding the allergen (tick bites) (21). This short-lived nature of IgE+ plasma cells is also evident in humans suffering from seasonal allergic rhinitis, wherein the IgE titers decline off-season and rise during on-season (22).
Arguably then, long-lived plasma cells do not retain long-lived IgE memory. Despite the accumulation of evidence that IgE responses in both mice and humans are transient, there are some observations which do not yet have a conclusive explanation. In a model of chronic house dust mite exposure, a population of long-lived IgE-expressing cells were detected in the bone marrow, suggesting that in some contexts long-lived IgE+ PCs may be generated (23).
Thus, to definitively rule out the role of long-lived IgE+ PCs in PN allergic individuals further research is required. This is a technically challenging endeavor as the rate of accidental exposure to PN is 12.4% annually and these cells would likely be extremely rare if they do exist (24). Nonetheless, the possibility exists that some long-lasting IgE+ PCs may reside in mucosal sites or in the bone marrow of these allergic individuals, but limitations in acquiring these samples for study have precluded their detection.
IgE titres are rapidly replenished upon activation of MBCs following a secondary allergen exposure. However, IgE+ MBCs are extremely rare or non-existent in humans and are, therefore, not considered relevant to the persistence of food allergy (20, 25, 26). The recent identification of IL-13-expressing Tfh cells (Tfh13) demonstrated that Tfh13 cells promote IgE+ B cell survival in germinal centers (GC), leading IgE+ B cells to preferentially differentiate into plasma cells (27–29).
In contrast to IgE+ MBCs, non-IgE MBCs have been shown to maintain long-lived food allergy, particularly IgG1+ MBCs (17, 25, 30). Although IgE+ B cells participate transiently in GCs, IgG1+ B cells persist within GCs and differentiate into affinity matured MBCs (17, 30). Upon secondary allergen exposure, IgG1+ MBCs rapidly undergo class-switch recombination (CSR) and differentiate into IgE+ plasma cells to maintain IgE responses (30, 31).
Assessing the requirements for secondary responses to allergens is critical to understand the persistence of food allergy and, consequently, to develop novel therapies. MBC reactivation leading to IgE production is strictly dependent on CD4+ T cells and IL-4, specifically through IL-4Rα signaling, which is a fundamental requirement for IgE CSR (33). Studies in mice indicate that IgE CSR requires Tfh cell-derived IL-4 during primary responses, though the source of IL-4 during secondary responses, particularly in humans, remains unclear (34, 35).
Th2 cells are generally defined by their secretion of Th2-polarized cytokines such as IL-4, IL-5, and IL-13 and high expression of GATA3, though many diverse subpopulations have been characterized with distinct phenotypes (36). Their role in IgE production remains unclear, though they do contribute to late phase inflammation (12). Although CD4+ T cells are fundamentally required to initiate secondary B cell responses, the role of memory CD4+ T cells in this process remains unclear.
It is possible that allergen-specific MBCs may be capable of polarizing naïve CD4+ T cells toward a Th2 phenotype during recall responses. Nonetheless, it is important not to disregard the possible contribution of memory CD4+ T cells to food allergy persistence, particularly in an environment where they compete with naïve CD4+ T cells. Given that Tfh cells are critical drivers of IgE CSR, memory Tfh cells may also play a role in secondary IgE responses, though this remains to be determined.
Furthermore, a subpopulation of terminally differentiated, allergen-specific “Th2A” cells has been identified exclusively in allergic individuals which expand upon allergen exposure, suggesting their implication in allergic pathology (38). Thus, MBCs are likely not the only cells that contribute to recall responses, and hence, the persistence of food allergy.
Future Avenues For Food Allergy Treatment
Adaptive immunity evolved to be long-lasting and specific, enabling a rapid recall response to prevent any single pathogen from eliciting malady twice. For example, primary infection with varicella-zoster virus induces humoral and cellular memory which protects against viral disease despite inevitable re-exposures and the virus itself establishing latency in ganglionic neurons; only when immunological memory becomes compromised (e.g., aging, drug-induced immunosuppression, etc.) will varicella-zoster virus establish secondary illness (shingles).
However, in at least two instances – allergy and autoimmunity – immunological memory perpetuates disease, rather than protecting against it. This inherent durability of immunological memory is the greatest challenge faced in efforts to therapeutically reverse, reprogram, or cure food allergy.
Perhaps the most dramatic impact on allergy in humans is through a total factory reset of the immune system. Following hematopoietic stem cell transplants (HSCTs) for the treatment of malignant or non-malignant diseases unrelated to allergy, over 90% of allergic recipients lost allergen-specific IgE reactivity when receiving a transplant from a non-allergic donor (102). Two years post-transplant, no recipients had regained allergic reactivity.
HSCTs are preceded by total body irradiation or chemotherapy-based conditioning, which depletes cells of hematopoietic origin including memory T and B cells; however, long-lived plasma cells exhibit radiation resistance (103). The rapid loss of allergen-specific antibody titers following myeloablation and HSCT reaffirms observations in animal modeling suggesting that long-lived plasma cells do not maintain lifelong food allergies (18, 30).
Thus, the pathway which replenishes short-lived plasma cells is the critical therapeutic target. Certainly, HSCTs are only warranted in the most extreme scenarios, but the success in eliminating allergic reactivity without relapse begs the question: what strategies can be employed for the targeted removal or disruption of pathogenic lymphocytes underlying the maintenance of food allergy (Table 3)?
Innovative food allergy treatments may be inspired by the successes of targeted cancer immunotherapies. Treatment of food allergy and cancer possess a common goal of eliminating cells of a certain specificity/phenotype. The advent of antigen receptor engineering has enabled the delivery of T cells or NK cells that express antibody-like receptors called chimeric antigen receptors (CARs) which are specific to neoantigens or tumor-associated antigens (104, 105).
The inclusion of costimulatory domains in the CARs overrides the need for secondary signals provided by antigen-presenting cells (106). Upon recognition of cell surface antigen through their engineered receptors, the cells execute their intrinsic cytotoxic functions resulting in destruction of malignant cells. Theoretically, CAR-T cells could be engineered to interact with allergy-associated cell surface molecules on T and B cells.
A potential T cell target is the prostaglandin D2 receptor, CRTH2, which is expressed by allergen-reactive Th2 cells (38). An equivalent marker unique to allergen-specific B cells has not yet been defined. Although CRTH2 and other molecules can be upregulated by allergic cells, the inherent redundancy of the immune system makes it unlikely that these markers are unique to allergen-specific cells. Therefore, one consequence of using CARs built for allergy- “associated” molecules may be adverse off-target effects, such as excessive cytokine production and tissue damage (107).
An optimal approach for targeted allergy immunotherapy would exploit antigen receptors (TCRs and BCRs) specific to the allergen(s) of interest. Proof-of-concept studies have demonstrated success in engineering CAR-T cells to express B cell antigens (in place of the antibody-like binding domain) upstream to costimulatory domains.
This construct was termed B cell-targeting antibody receptors (BARs) (108). Delivery of OVA-specific BAR-Treg cells to OVA-allergic mice reduced the severity of anaphylaxis upon systemic challenge (109). While promising, the mechanism by which this protection is achieved and its longevity remain unknown.
Targeting allergen-specific T cells in this manner would be far more challenging, as it would require expression of multiple different MHC:peptide complexes to encompass each of the immunodominant peptides for a given allergen.
As opposed to cellular engineering, drug conjugation may be an alternative approach to target allergen-specific cells.
Some cancer therapies utilize a “warhead” strategy wherein chemotherapeutic drugs are conjugated to monoclonal antibodies, promoting targeted drug delivery (110).
This strategy could be adopted to allergy whereby whole allergens or allergen peptides are conjugated to cytotoxic drugs. One potential limitation of approaches involving engineered allergen expression or delivery of allergen-drug conjugates is that there are often high levels of circulating allergen-specific antibodies in allergic patients, which may severely limit bioavailability. As well, delivery of whole allergen proteins may cross-link IgE on mast cells or basophils, resulting in unintended allergic reactions, though this may be ameliorated by co-administration of omalizumab.
Furthermore, it may be possible to alter or reprogram the phenotype of allergen-specific cells in lieu of their physical elimination. The potential ability to induce cellular reprogramming originates from the concept of plasticity. Beyond the description of a terminally-differentiated Th2A cell phenotype, the plasticity of allergen-specific cells is not well understood (38).
The ability for allergy outgrowth implies some degree of functional plasticity, though it is unclear why outgrowth occurs so infrequently in peanut, tree nut, fish, and shellfish allergies in comparison to milk and egg allergies (9, 111, 112). If allergen-specific cells are functionally plastic, therapeutics could be designed to deprive cells of signals that maintain pathogenicity. For example, IL-4 is critical for the induction of allergy, including Th2 polarization and IgE production (113, 114).
In recent work with PBMCs from peanut-allergic patients, we have shown that IL-4 deprivation through therapeutic IL-4Rα blockade dampens the IL-4-responsive phenotype in allergen-reactive MBCs and upregulates IFN-γ production (19). Similarly, in a murine asthma model, use of a small molecule inhibitor of STAT6 (involved in IL-4/IL-13 signaling) reversed airway hyperreactivity (115).
Alternatively, it may be possible to design therapeutics which deliver signals to actively upregulate tolerogenic or non-Th2 phenotypes. The use of a DNA vaccine is one such example, as previously mentioned. Delivery of allergen-encoding plasmid DNA provides strong Th1 signals (IFN-γ and IgG2a) which may aid to counteract the Th2 dominant signature (116). An ideal “disease-transformative” DNA vaccine would consist of a single dose that could reprogram the immune response, though it is not clear at this early stage whether this is possible with this strategy.
Lastly, it is evident that we have reached a ceiling as to what AIT can achieve as a monotherapy (117); however, the exhaustive number of AIT studies have provided a well-defined regimen for supervised allergen exposure, which may be applicable in combination therapies. Biologics for the treatment of allergic diseases have shown efficacy when administered as a monotherapy.
For example, dupilumab monotherapy in atopic dermatitis, asthma, and chronic rhinosinusitis with nasal polyposis is highly effective in ameliorating disease score and IgE titers (118–120).
These conditions, however, are distinct from allergy in that IgE specificity is broad and production is perpetual. This is in contrast to food allergy wherein IgE titers are highly specific to the eliciting allergen(s) and is only produced upon allergen exposure. Thus, dupilumab monotherapy in food allergy will likely prevent IgE production upon accidental exposures but will forgo the benefit of allergen-specific cell reprogramming.
In those that attained clinical remission from atopic dermatitis via dupilumab, skin-resident Th2A cells persisted (121). Deliberate activation of allergen-specific cells in a tolerogenic or non-Th2 context (via biologics or other therapeutics) is more likely to facilitate reprogramming.
Currently, the development of novel, efficacious therapies is limited by an incomplete understanding of the underlying immunological mechanisms. To rationally design novel therapeutics, we propose that the following three questions must be addressed from a basic immunological standpoint:
1) What are the fundamental requirements for the perpetuation of allergic disease? It remains unclear whether allergen-specific memory T or B cells can independently maintain allergic disease. Likewise, the relative contributions of Th2A, conventional TFh, and TFh13 cells in the regeneration of IgE responses remains unknown.
Investigation of these issues will be critical to determine whether future therapies should target one or more cell types. For example, if Th2-polarized T cells are the minimal requirement, then therapeutic targeting of MBCs would be insufficient. These investigations should extend even beyond adaptive immunity, to address whether conditioning of innate cells (trained immunity) is capable of re-establishing IgE responses (122, 123).
2) What molecular cues are necessary to replenish the short-lived IgE PC pool? B cell activation occurs through intricate interactions with secreted and membrane-bound molecules. The interactions involved in MBC reactivation and PC differentiation, however, remain elusive. A more complete understanding of these processes is critical for the development and use of biologics.
As of 2012, clinical trials employing biologics for eight different targets (IgE, IL-5, IL-4, IL-13, IL-17, IL-9, GM-CSF, TNFα) were already underway for the treatment of asthma (124). Nearly 10 years later, biologics are being trialed for only four targets (IgE, IL-4R, IL-33 and Glucopyranosyl Lipid A) in food allergy.
3) Is the allergic phenotype plastic? Despite several indications that the allergic response may be reprogrammed, it has not been well established whether the induction of non-Th2 phenotypes arise from de novo responses or a reprogramming of the existing allergen-specific memory cells. This distinction will help to inform whether a non-Th2 population can effectively outcompete pathogenic allergen-specific cells or if persisting pathogenic cells will eventually undermine therapeutic reprogramming.
If it is possible to reprogram cells out of a pathogenic phenotype, it will be critical to determine if there is a risk of relapse. Without the ability to reprogram cells, lifelong treatments may be required, as appears to be the case for AIT monotherapy.
Food-specific IgA antibodies
The presence of food-specific IgA antibodies in the gut does not prevent peanut or egg allergies from developing in children, according to a Northwestern Medicine study published in Science Translational Medicine.
Scientists examined stool samples from more than 500 infants across the country and found that the presence of Immunoglobulin A, the most common antibody found in mucous membranes in the digestive tract, does not prevent peanut or egg allergies from developing later in life.
This discovery calls into question the role of Immunoglobulin A, or IgA, which was previously thought to be a protective factor against the development of food allergies.
Peanuts and eggs are the two most common allergens for infants and affect an estimated one in 13 children in the U.S., according to the Ann & Robert H. Lurie Children’s Hospital of Chicago.
While prior research had shown IgA could bind to and neutralize toxins and bacteria in the body, there was inconclusive evidence that IgA could do the same for food allergens, said Stephanie Eisenbarth, MD, Ph.D., chief of Allergy and Immunology in the Department of Medicine and senior author of the study.
“We were able to collaborate with different groups around the country to look at a number of different cohorts of children and young adults to ask: ‘Does the presence of IgA to peanut tell us that the person is tolerant to peanut?'” said Eisenbarth, who is also director of the Center for Human Immunobiology and a member of the Robert H. Lurie Comprehensive Cancer Center of Northwestern University. “We found that there really was no difference between kids who had peanut allergies and children who didn’t, and the same is true with egg allergies.”
The findings come as rates of allergies in children continue to climb: According to data from the Centers for Disease Control and Prevention, the number of children with allergies has more than doubled in the last 20 years.
Future directions for research will center on understanding the role IgA plays in people who have undergone immunotherapy and developed a tolerance to food allergens, Eisenbarth said.
“This study happened because of the hard work of lead author Dr. Elise Liu and the amazing group of collaborators that we had,” she said. “This was an impressive, multi-center effort to try and answer this question. I really want to thank the people from every corner of the U.S. that provided samples and expertise to make this happen.”
More information: Elise G. Liu et al, Food-specific immunoglobulin A does not correlate with natural tolerance to peanut or egg allergens, Science Translational Medicine (2022). DOI: 10.1126/scitranslmed.abq0599
reference link: https://www.frontiersin.org/articles/10.3389/falgy.2022.826623/full
