Update on Allergy Immunotherapy

Several mechanisms of IT have emerged over recent decades, reflecting an increased understanding of immunology. Early studies concentrated on increases in allergen-specific immunoglobulin (Ig) G, specifically that of the IgG4 isotype. These immunoglobulins were proposed as "blocking" immunoglobulins that competed with IgE for allergen binding to IgE receptor-expressing cells. However, the correlation between IgG concentrations and clinical response to treatment is weak, and even when correlated to nasal IgG (or IgA) concentrations in the allergic inflammatory milieu, no improvements in these relationships were observed.

More recently, immunodeviation has been cited, with reductions in T-cell proliferative responses to allergens and a shift from a T-helper 2 (Th2)-like response toward a Th1-like response following immunotherapy [6]. Unfortunately, this model also comes under scrutiny because (1) T cells that produce interferon (INF)-γ (Th1-like cells) are a characteristic feature of allergic inflammation, (2) IFN-γ is a potent proinflammatory compound that contributes to both the presence and severity of allergic disease [7], and (3) subsequent studies have failed to consistently confirm these findings. In contrast, the one consistent finding observed after IT is diminished responsiveness (tolerance) of the allergen-specific Th2-like cells.

A prominent role for IL-10-producing activated CD4⁺ cells was first described in studies involving bee venom IT [8]. Subsequent investigations with house dust mite IT extended the importance of IL-10 (and transforming growth factor beta [TGF-β]) production by CD4⁺Tcells to inhalant allergy and confirmed that this occurred in parallel to the suppression of Th2 proliferative responses and cytokine production [9]. In this study, IL-10 responses in healthy nonatopic individuals who had been exposed to allergen were similar to those in the IT-treated group, implying the restoration of tolerant T-cell responses in the atopic individual. Other studies have also shown IL-10 production by CD4⁺ cells without changes in grass pollen-induced proliferation or Th2 cytokine production [10]. This area of research remains confused; for example, although a role for CD25 expression has been ascribed to these IL-10-producing cells, it is unclear whether this reflects the constitutive expression of this component of the IL-2 receptor that is the signature characteristic of Treg cells or whether this reflects the induced expression of this component of the IL-2 receptor as occurs with activation of these effector T cells. However, what is consistent is that each of these studies has found cells capable of making high levels of IL-10 (with or without TGF-β) consistent with the Tr1 cell type, and current concepts therefore focus on the integral role of these IL-10-producing cells in immune tolerance to allergens after IT.

It is reasonable to speculate that since its inception, IT has been attempted with virtually all conceivable aeroallergens. However, efficacy has been established against only a relatively few extracts; for many extracts, especially mould extracts, the evidence for efficacy is sparse. In well-designed placebo-controlled studies, efficacy has been shown against ragweed, various grasses (timothy, rye grass), trees (mountain cedar, birch), dust mites, and cats. As discussed, significant clinical improvement has been shown for subjective symptom and medication diary scores in comparison to the placebo group. Efficacy largely requires definition of the nature of the specific allergen and the development of a means of generating an extract with a concentration of the relevant allergenic component sufficient to produce an immunomodulating effect (dosing requirements are discussed below). With pollens, defining the relevant allergen--the pollen grain itself--and developing the botanic techniques to obtain that allergen are relatively straightforward. Thus, the earliest studies categorically establishing the efficacy of IT were performed with ragweed, and subsequent studies have shown positive results with northern grasses and a few trees. The ability of studies to show the efficacy of IT for ragweed was rendered easier by the relative paucity of allergens responsible for autumnal seasonal AR and by the dominant role of ragweed in that disease. This is in contrast to tree allergy, for which obtaining a pollen extract in concentrations sufficient to produce efficacy should be straightforward; however, the ability to document efficacy is confounded by an inability to identify subjects that demonstrate sensitization to only one or a few trees and by the concomitant presence of innumerable tree-derived pollens, for example, in early spring in the eastern United States. The studies that have shown efficacy in cases of tree allergy involved the treatment of cohorts in locations where a single allergen (eg, birch in Scandinavia or mountain cedar in Texas) presented overwhelming and unique allergic problems. Similarly, the efficacy of IT for grass allergy is well established, reflecting the cross-reactivity of the various northern grass pollens and the relative paucity of confounding allergens during the late spring season.

In contrast to the case of pollens, the ability to show responsiveness to indoor allergens followed a more difficult course and was severely confounded by the absence of a proper characterization of the relevant allergenic proteins. Innumerable patients have received IT for "dust," extracts of which are at best described as "unstandardized" and at worst represent a conglomeration of numerous uncharacterized components, none of which are present at sufficient concentrations to provide any sort of clinical benefit. Only with the recognition of dust mites as the dominant allergen in indoor dust samples did it become possible to develop standardized extracts in concentrations sufficient to mediate clinical benefit. Similarly, the availability of extracts with well-characterized concentrations of the dominant cockroach-derived allergens should make it possible to perform effective IT with these agents. Unfortunately, little additional progress has been made in defining other allergenic proteins likely to be present in indoor dust samples and likely to be producing symptoms in large cohorts of perennial AR subjects. For example, in specific communities, allergens such as those derived from spiders, "miller" moths, and ladybugs may be extremely important sources of allergic symptoms but remain an untapped area of treatment.

Another area in which research has led to the development of efficacious extracts is the treatment of animal allergy. The treatment of subjects with cat allergy is a revealing example of the difficulties inherent in developing useful extracts and the importance of proper investigations. Cats have long been recognized as an important cause of AR, and such AR has traditionally been ascribed to cat dander, with little insight in regard to the term "dander." The assumption that the allergen was a pelt-derived protein led to the development of extracts derived from the skins of carefully cleaned animals. The subsequent recognition that the dominant cat allergen is a protein derived from skin glands led to the recognition of the paradox that the commercially available extracts were virtually devoid of Fel d I and, as such, were worthless as immunomodulating agents. Current standardized extracts, with quantifiable levels of the dominant cat allergen, have proved effective for cat allergy. The treatment of dog allergy has lagged behind that of cat allergy, but the recent characterization of the dominant dog allergens has begun to make available extracts with sufficient concentrations of these allergens to warrant investigations to document their efficacy. Rodents, either as pets or as pests, are an additional major source of animal-derived allergens. The rodent glomerulus permits the passage of small proteins, and these urinary proteins are the primary source of allergens. As in the case of cat allergy, only after the recognition that urine and not dander was the source of the dominant allergen has it become possible to develop allergy extracts that contain sufficient concentrations of relevant allergens to plausibly provide efficacy. Continuing research has defined major allergens in other mammals such as horses, cows, and rabbits, and this presumably will lead to the availability of useful extracts.

The one area of inhalant allergy that remains a daunting problem has been the treatment of mould allergies. Although a few studies have shown efficacy of IT in mould-induced allergic disease caused by Alternaria and Cladosporium, the preponderance of studies have not confirmed these results, nor have these results been extended to the innumerable other moulds. In part, this reflects the apparent absence of a single dominant allergen responsible for allergy caused by a given species of mould. Thus, different species (or even strains) of Alternaria have allergens that are sufficiently different that their allergens do not provide cross-protection when administered as IT. In fact, not only do different strains of moulds produce sufficiently different allergenic components to preclude cross-reactive efficacy, but even the same strain can express different allergenic proteins, depending on variations in growth and environmental conditions.

Several other issues pertain to the preparation of extracts for administration. It is important to avoid the inadvertent administration of subtherapeutic doses resulting from dilution caused by mixing multiple allergens. Current guidelines recommend diluting all allergic extracts into the same final volume, regardless of the number of allergens included, and adjusting the volume of diluent in accordance with this number of extracts. In addition, surprisingly little is known at present regarding the stability of these extracts. Loss of extract potency is generally viewed as being accelerated by passage of time, higher temperature, greater dilution, non-glycerine-containing preservatives, allergens with higher protease content, and (in some studies) use of excessive volume for the storage vial. The majority of published studies of concentrated extracts preserved with 50% glycerin showed the extracts to be stable for at least 12 months [28–33]. In one study, several allergens remained stable after storage for as long as 36 months at 6°C [31]. However, in contrasting studies, the potency of Der p 1 and Der p 2 fell 50% at 12 months [32], and another study found that Candida lost stability after only 10 weeks [34].

Several studies have addressed the stability of diluted extracts. Dilutions (1:100) of Russian thistle solubilized in human serum albumin, glycerin, saline, or phosphate buffer maintained potency for 12 months [35]. However, only extracts in glycerin maintained potency at a 1:10,000 dilution. In a different study, temperature had no effect, and at 3 months, diluted extracts that were stored at 4°C and those that were brought to room temperature 13 hours a week showed equivalent potency [36]. At 12 months, both groups had similarly reduced potency. There were no differences in the residual potency between single and mixed allergens except at dilutions of 1:1000, at which there was greater preservation of allergen potency with the three-allergen mix.

A more recent study addressed the influence of dilution and antigen mixing on extract stability [37]. Potency was preserved at 3 months with single-allergen extracts but was reduced by protease containing extracts; Alternaria had the broadest degradation capacity, followed by cockroach and Cladosporium. Glycerin protected the extracts from the protease-containing allergens. The best preservative was 50% glycerin, followed by 10% glycerin and 0.03% human serum albumin.

In summary, concentrated extracts appear to maintain potency for at least 12 months, and current recommendations state that purchased extracts should therefore be replaced annually. Guidelines argue against the mixing of allergens with high protease activity (such as mould and cockroach allergens) with those of low activity (grass, tree, weed, cat, and dog allergens). Glycerinated extracts have the greatest stability and may also act to protect the allergens from proteases.

Current recommendations are that inhalant allergen IT be discontinued after 3 to 5 years [5]. The most important randomized controlled trial to examine the persistence of improved symptoms after discontinuation of IT was the grass SAR study previously discussed [16]. This study demonstrated that the IT-induced relief of grass pollen rhinitis symptoms persisted for a minimum of 3 years after discontinuation of IT. No differences were observed between those patients who continued with IT and those who discontinued IT after the completion of 3 years of therapy. Persistence of improved symptoms after discontinuation of dust mite IT has also been shown [17] although not in a double-blind placebo-controlled trial. Whether IT with other aeroallergens will have the same prolonged protective effects as IT with grass or dust mite allergen has not been determined. Patients will have achieved all of the clinical benefit they can expect from a given extract after 3 to 5 years. Insufficient response to that extract clearly warrants its discontinuation and a reassessment of the diagnosis of AR, the continuing relevance of allergens present in that extract, and the adequacy of dosing of each allergen. Recrudescence of symptoms after IT is discontinued would also warrant a reevaluation of the patient's allergies and suggests that IT may need to be restarted with different agents or with doses that are more appropriate; if allergies to the same allergens remain responsible for the recurrent symptoms, this argues that the repeat IT should be maintained for a longer duration.

Poorly controlled asthma stands as the most significant risk factor for severe systemic reactions to IT (this is also true for all forms of anaphylaxis). Several studies [3] reported the presence of moderate or severe asthma in the vast majority of reported IT fatalities [38, 42]. More recently, Bernstein and colleagues found that 15 of 17 IT fatality patients had preexisting asthma [40]; 60% of these patients were reported to have suboptimal control of their asthma symptoms despite appropriate pharmacotherapy. Additionally, 50% of the asthma fatality patients in this study (for whom data were available) demonstrated a forced expiratory volume in 1 second (FEV1) of <70% prior to injection. The absence of recent spirometric or even peak flow data for many of these patients emphasizes the importance of obtaining this information on all asthmatic patients prior to administering each IT injection. Symptomatic patients, patients with a recent history of hospitalization or emergency department visits, and patients who have suffered a recent exacerbation of asthma represent the majority of fatal reactors in surveys of the safety of IT. In addition to spirometry or peak flow assessment, preinjection assessments of recent asthma symptoms, rescue inhaler use, nocturnal awakenings, recent health care utilization, and medication compliance are vital for ensuring safe IT for asthmatic patients. Careful selection of patients and the identification of patients with moderate or severe asthma and labile asthma will greatly reduce the risk of fatal systemic reactions to IT.

The potential for severe systemic reactions following IT injections necessitates the administration of IT in a proper clinical setting. The home and other medically unsupervised settings are inappropriate venues for IT; out-of-office administration has been associated with death, likely because of the lack of recognition of systemic reactions and the failure to administer epinephrine in a timely fashion. Guidelines recommend that injections be given at the prescribing allergist's office or at the office of another physician who is trained and medically equipped for the treatment of systemic IT reactions. Delayed administration of epinephrine is consistently reported as a risk factor for IT fatality. In general, epinephrine (1:1000) should be premeasured and instantly available for intramuscular administration should a systemic reaction occur. Physicians who administer IT supplied to them by an allergist must be provided with specific instructions emphasizing the importance of prompt administration of epinephrine and other resuscitative measures to ensure the safety.

Physicians must enforce the recommended 20- to 30-minute waiting period following an IT injection. In general, 20 minutes is considered sufficient for patients with AR alone whereas asthmatic patients should remain at least 30 minutes after an IT injection. However, up to 38% of systemic reactions occur after 30 minutes and thus outside the clinician's office. Current recommendations advise a prolonged postinjection waiting period for patients with a history of systemic reactions after the half-hour waiting period [5]. Although no more-specific recommendations exist, these observations argue for the appropriateness of more-prolonged waiting periods for patients with more than mild asthma or with a history of a previous systemic reaction. In addition, it has been suggested that any patient with a history of a previous systemic reaction or perhaps even all asthmatic patients receiving IT should be prescribed epinephrine and should receive appropriate training in its indications and administration.

Patient-specific IT vials and administration forms may reduce the risk of incorrect IT injections. As 32% of incorrect IT injections potentially lead to systemic reactions, specific steps must be taken at each patient encounter to ensure safety. One suggested approach involves a "one patient, one nurse, one injection" safety measure. One nurse and the patient review three identifiers, including first and last names on the patient-specific IT vial and administration form, as well as the patient's date of birth or medical record number. An additional recommendation is to use standardized color-coded IT vials according to strength (from most dilute to maintenance strength). Finally, clinic physicians should be informed immediately should there be any deviation from an appropriate injection dose.

On initial and follow-up evaluations of patients with AR or allergic asthma, close attention to comorbid medical conditions and coadministration of pharmacologic therapies are essential for safe IT. Patients may have heart disease or hypertension requiring medications such as β-blockers or angiotensin-converting enzyme inhibitors, both of which have been implicated in fatal reactions to IT [5, 38, 40]. Advanced age, nonallergic respiratory disease, or risk factors for cardiovascular disease such as hypertension, elevated cholesterol, or diabetes should be weighed in risk-benefit analyses because administration of epinephrine for systemic reactions may induce significant cardiovascular side effects. These patients may also have a reduced ability to survive a systemic reaction. Although not addressed here, Hymenoptera IT benefits may outweigh risk in patients taking β-blockers insofar as these patients are unlikely to be able to completely avoid stings in the future and are at less risk from the complications of a systemic reaction in a physician's office than they would be in the field after being stung.

Future strategies for IT are aimed at delivering increased efficacy with less risk of IgE-mediated systemic side effects. Recognition of the importance of T-cell-dependent mechanisms in immunomodulation after IT has led to studies to develop reagents that interact with T lymphocytes without engaging mast cell/basophil-bound IgE. Denatured proteins such as polymerized extracts or, more recently, short peptide fragments derived from genetically-engineered allergens have been successfully used in IT studies. These peptides retain the ability to be recognized and processed by antigen-presenting cells, to be presented to T helper lymphocytes, and to drive T cells into an anergic or tolerant state. Because these short peptides lack the complex structure of their parent compounds, they cannot be recognized by mast cell/basophil-bound IgE and are generally not associated with risk for immediate-phase anaphylactic responses. Studies of peptides in patients sensitized to cats [43, 44] and to bee venom [45] have demonstrated reductions in the proliferation of CD4⁺ T cells, diminished Th1 and Th2 cytokine production, and significant increases in IL-10. However, these compounds have been frustrated by their tendency initially to activate allergen-specific Th2-like cells and thereby produce delayed but often severe allergic reactions, including exacerbations of asthma [46]. More recent studies have used allergenic peptides coupled to synthetic bacterial deoxyribonucleic acid (DNA) sequences [47]. These are based on the concept that engagement of these immunostimulatory DNA sequences to their specific toll-like 9 (TL9) receptors will be a potent signal to drive the ensuing immune responses toward Th1 immunity. Initial studies were extremely promising in regard to efficacy, safety, and long-term immunomodulation with these agents. Interestingly, the absence of adverse effects associated with the Th1-like responses appears to be related to a concomitant induction of regulatory immune responses.

A final and potentially more immediately available possibility involves the concurrent administration of humanized anti-IgE (omalizumab) with standard IT. The concept is that the removal of IgE should dramatically reduce the risk of anaphylactic reactions and thereby may allow a more rapid induction of tolerance, the implementation of higher doses of allergen, or the administration of IT to patients who are considered to have contraindications to IT. For example, it may become easier to administer IT to patients with moderate to severe asthma or with a previous history of systemic reactions. Recent studies support such an approach and raise the exciting potential of a role for IT in patients for whom immunomodulating therapies might provide critically important clinical benefits.