Inhibition of antigen-induced cutaneous responses of ponies with insect hypersensitivity by the histamine-1 receptor antagonist chlorpheniramine

Equine allergic skin diseases: Clinical consensus guidelines of the World Association for Veterinary Dermatology

BACKGROUND

Allergic skin diseases are common in horses worldwide. The most common causes are insect bites and environmental allergens.

OBJECTIVES

To review the current literature and provide consensus on pathogenesis, diagnosis, treatment and prevention.

MATERIALS AND METHODS

The authors reviewed the literature up to November 2022. Results were presented at North America Veterinary Dermatology Forum (2021) and European Veterinary Dermatology Congress (2021). The report was available to member organisations of the World Association for Veterinary Dermatology for feedback.

CONCLUSIONS AND CLINICAL RELEVANCE

Insect bite hypersensitivity (IBH) is the best characterised allergic skin disease. An immunoglobulin (Ig)E response against Culicoides salivary antigens is widely documented. Genetics and environmental factors play important roles. Tests with high sensitivity and specificity are lacking, and diagnosis of IBH is based on clinical signs, seasonality and response to insect control. Eosinophils, interleukin (IL)-5 and IL-31 are explored as therapeutic targets. Presently, the most effective treatment is insect avoidance. Existing evidence does not support allergen-specific immunotherapy (ASIT) using commercially available extracts of Culicoides. Hypersensitivity to environmental allergens (atopic dermatitis) is the next most common allergy. A role for IgE is supported by serological investigation, skin test studies and positive response to ASIT. Prospective, controlled, randomised studies are limited, and treatment relies largely on glucocorticoids, antihistamines and ASIT based on retrospective studies. Foods are known triggers for urticaria, yet their role in pruritic dermatitis is unknown. Recurrent urticaria is common in horses, yet our understanding is limited and focussed on IgE and T-helper 2 cell response. Prospective, controlled studies on treatments for urticaria are lacking. Glucocorticoids and antihistamines are primary reported treatments.

Molecular mechanisms and treatment modalities in equine Culicoides hypersensitivity

Equine Culicoides hypersensitivity (CH) is the most common allergic condition in horses affecting the skin. This review focuses on immunopathology and molecular mechanisms of equine CH. The role of eosinophils is emphasized, as well as disease severity and the influence of long-term chronic allergen exposure on T helper (Th) 2 cells. Using current knowledge from human allergic disorders, similar effects are hypothesized in equine patients. Key aspects of CH diagnosis and treatment are discussed, focusing on allergen specific immunotherapy and allergen-independent approaches, such as targeting hypereosinophilia through interleukin-5 and allergic non-histaminic pruritus though interleukin-31.

Active vaccination against interleukin-5 as long-term treatment for insect-bite hypersensitivity in horses

Background

Insect‐bite hypersensitivity (IBH) in horses is a chronic allergic dermatitis caused by insect bites. Horses suffer from pruritic skin lesions, caused by type‐I/type‐IV allergic reactions accompanied by prominent eosinophil infiltration into the skin. Interleukin‐5 (IL‐5) is the key cytokine for eosinophils and we have previously shown that targeting IL‐5 by vaccination reduces disease symptoms in horses.

Objective

Here, we analyzed the potential for long‐term therapy by assessing a second follow‐up year of the previously published study.

Methods

The vaccine consisted of equine IL‐5 (eIL‐5) covalently linked to a cucumber mosaic virus‐like particle (VLP) containing a universal T cell epitope (CuMV_TT) using a semi‐crossover design to follow vaccinated horses during a second treatment season. Thirty Icelandic horses were immunized with 300 μg of eIL‐5‐CuMV_TT without adjuvant.

Results

The vaccine was well tolerated and did not reveal any safety concerns throughout the study. Upon vaccination, all horses developed reversible anti‐eIL‐5 auto‐antibody titers. The mean course of eosinophil levels was reduced compared to placebo treatment leading to significant reduction of clinical lesion scores. Horses in their second vaccination year showed a more pronounced improvement of disease symptoms when compared to first treatment year, most likely due to more stable antibody titers induced by a single booster injection. Hence, responses could be maintained over two seasons and the horses remained protected against disease symptoms.

Conclusion

Yearly vaccination against IL‐5 may be a long‐term solution for the treatment of IBH and other eosinophil‐mediated diseases in horses and other species including humans.

Treating insect-bite hypersensitivity in horses with active vaccination against IL-5

BACKGROUND

Insect-bite hypersensitivity is the most common allergic dermatitis in horses. Excoriated skin lesions are typical symptoms of this seasonal and refractory chronic disease. On a cellular level, the skin lesions are characterized by massive eosinophil infiltration caused by an underlying allergic response.

OBJECTIVE

To target these cells and treat disease, we developed a therapeutic vaccine against equine IL-5 (eIL-5), the master regulator of eosinophils.

METHODS

The vaccine consisted of eIL-5 covalently linked to a virus-like particle derived from cucumber mosaic virus containing the tetanus toxoid universal T-cell epitope tt830-843 (CMV_TT). Thirty-four Icelandic horses were recruited and immunized with 400 μg of eIL-5-CMV_TT formulated in PBS without adjuvant (19 horses) or PBS alone (15 horses).

RESULTS

The vaccine was well tolerated and did not reveal any safety concerns but was able to induce anti-eIL-5 autoantibody titers in 17 of 19 horses. This resulted in a statistically significant reduction in clinical lesion scores when compared with previous season levels, as well as levels in placebo-treated horses. Protection required a minimal threshold of anti-eIL-5 antibodies. Clinical improvement by disease scoring showed that 47% and 21% of vaccinated horses reached 50% and 75% improvement, respectively. In the placebo group no horse reached 75% improvement, and only 13% reached 50% improvement.

CONCLUSION

Our therapeutic vaccine inducing autoantibodies against self IL-5 brings biologics to horses, is the first successful immunotherapeutic approach targeting a chronic disease in horses, and might facilitate development of a similar vaccine against IL-5 in human subjects.

Characterization of IgE-mediated cutaneous immediate and late-phase reactions in nonallergic horses

OBJECTIVE

To characterize the response of skin of nonallergic horses following ID injection of polyclonal rabbit anti-canine IgE (anti-IgE) and rabbit IgG.

ANIMALS

6 healthy horses.

PROCEDURES

Skin in the cervical area was injected ID with anti-IgE and IgG. Wheal measurements and skin biopsy specimens were obtained before and 20 minutes and 6, 24, and 48 hours after injection. Tissue sections were evaluated for inflammatory cells at 4 dermal depths. Immunohistochemical analysis for CD3, CD4, and CD8 was performed, and cell counts were evaluated.

RESULTS

Anti-IgE wheals were significantly larger than IgG wheals at 20 minutes and 6 and 24 hours after injection. There were significantly more degranulated mast cells after anti-IgE injection than after IgG injection. There were significantly more eosinophils at 6, 24, and 48 hours and neutrophils at 6 hours after anti-IgE injection, compared with cell numbers at those same times after IgG injection. There were significantly more eosinophils in the deeper dermis of anti-IgE samples, compared with results for IgG samples. No significant differences between treatments were detected for CD3(+), CD4(+), or CD8(+) cells.

CONCLUSIONS AND CLINICAL RELEVANCE

Injection of anti-IgE antibodies was associated with the development of gross and microscopic inflammation characterized by mast cell degranulation and accumulation of inflammatory cells, particularly eosinophils and neutrophils. This pattern appeared to be similar to that of horses with naturally developing allergic skin disease, although lymphocytes were not increased; thus, ID injection of anti-IgE in horses may be of use for evaluating allergic skin diseases of horses.

Equine insect bite hypersensitivity: what do we know?

Insect bite hypersensitivity (IBH) is an allergic dermatitis of the horse caused by bites of insects of the genus Culicoides and is currently the best characterized allergic disease of horses. This article reviews knowledge of the immunopathogenesis of IBH, with a particular focus on the causative allergens. Whereas so far hardly any research has been done on the role of antigen presenting cells in the pathogenesis of IBH, recent studies suggest that IBH is characterized by an imbalance between a T helper 2 (Th2) and regulatory T cell (T(reg)) immune response, as shown both locally in the skin and with stimulated peripheral blood mononuclear cells. Various studies have shown IBH to be associated with IgE-mediated reactions against salivary antigens from Culicoides spp. However, until recently, the causative allergens had not been characterized at the molecular level. A major advance has now been made, as 11 Culicoides salivary gland proteins have been identified as relevant allergens for IBH. Currently, there is no satisfactory treatment of IBH. Characterization of the main allergens for IBH and understanding what mechanisms induce a healthy or allergic immune response towards these allergens may help to develop new treatment strategies, such as immunotherapy.

Cetirizine in horses: Pharmacokinetics and pharmacodynamics following repeated oral administration

The pharmacokinetics of the histamine H(1)-antagonist cetirizine and its effect on histamine-induced cutaneous wheal formation were studied in six healthy horses following repeated oral administration. After three consecutive administrations of cetirizine (0.2 mg/kg body weight, bw) every 12h, the trough plasma concentration of cetirizine was 16+/-4 ng/mL (mean+/-SD) and the wheal formation was inhibited by 45+/-23%. After four additional administrations of cetirizine (0.4 mg/kg bw) every 12 h, the trough plasma concentration was 48+/-15 ng/mL and the wheal formation was inhibited by 68+/-11%. The terminal half-life was about 5.8 h. A pharmacokinetic/pharmacodynamic link model showed that the maximal inhibition of wheal formation was about 95% and the EC(50) about 18 ng/mL. It is concluded that cetirizine in doses of 0.2-0.4 mg/kg bw administered at 12 h intervals exhibits favourable pharmacokinetic and pharmacodynamic properties without causing visible side effects, and the drug may therefore be a useful antihistamine in equine medicine.

Equine recurrent airway obstruction and insect bite hypersensitivity: understanding the diseases and uncovering possible new therapeutic approaches

Recurrent airway obstruction (RAO) and insect bite hypersensitivity (IBH) are allergic conditions that are commonly encountered in the horse. Whilst complete allergen avoidance is an effective management strategy for both diseases, this may not be achievable in all cases and treatment options are therefore required. The inflammatory response is the main therapeutic target for glucocorticoids given to horses with RAO and severe cases of IBH, whilst the bronchodilators used in RAO primarily target airway smooth muscle. Such drugs are effective in most but not all individuals and there may be unwanted adverse effects. This article will review how knowledge of drug action and the pathogenesis of RAO and IBH can be utilised to identify potential targets for novel therapeutic agents that, in the longer term, may be safer and/or more effective in managing the allergic horse.

Cetirizine in horses: pharmacokinetics and effect of ivermectin pretreatment

The pharmacokinetics of the histamine H(1)-antagonist cetirizine and the effects of pretreatment with the antiparasitic macrocyclic lactone ivermectin on the pharmacokinetics of cetirizine were studied in horses. After oral administration of cetirizine at 0.2 mg/kg bw, the mean terminal half-life was 3.4 h (range 2.9-3.7 h) and the maximal plasma concentration 132 ng/mL (101-196 ng/mL). The time to reach maximal plasma concentration was 0.7 h (0.5-0.8 h). Ivermectin (0.2 mg/kg bw) given orally 1.5 h before cetirizine did not affect its pharmacokinetics. However, ivermectin pretreatment 12 h before cetirizine increased the area under the plasma concentration-time curve by 60%. The maximal plasma concentration, terminal half-life and mean residence time also increased significantly following the 12 h pretreatment. Ivermectin is an inhibitor of P-glycoprotein, which is a major drug efflux transporter in cellular membranes at various sites. The elevated plasma levels of cetirizine following the pretreatment with ivermectin may mainly be due to decreased renal secretion, related to inhibition of the P-glycoprotein in the proximal tubular cells of the kidney. The pharmacokinetic properties of cetirizine have characteristics which are suitable for an antihistamine, and this substance may be a useful drug in horses.

Fexofenadine in horses: pharmacokinetics, pharmacodynamics and effect of ivermectin pretreatment

The pharmacokinetics and the effects on inhibition of histamine-induced cutaneous wheal formation of the histamine H1-antagonist fexofenadine were studied in horse. The effect of ivermectin pretreatment on the pharmacokinetics of fexofenadine was also examined. After intravenous infusion of fexofenadine at 0.7 mg/kg bw the mean terminal half-life was 2.4 h (range: 2.0-2.7 h), the apparent volume of distribution 0.8 L/kg (0.5-0.9 L/kg), and the total body clearance 0.8 L/h/kg (0.6-1.2 L/h/kg). After oral administration of fexofenadine at 10 mg/kg bw bioavailability was 2.6% (1.9-2.9%). Ivermectin pretreatment (0.2 mg/kg, p.o.) 12 h before oral fexofenadine decreased the bioavailability to 1.5% (1.4-2.1%). In addition, the area under the plasma concentration-time curve decreased 27%. Ivermectin did not affect the pharmacokinetics of i.v. administered fexofenadine. Ivermectin may influence fexofenadine absorption by interfering in intestinal efflux and influx pumps, such as P-glycoprotein and the organic anion transport polypeptide family. Oral and i.v. fexofenadine significantly decreased histamine-induced wheal formation, with a maximal duration of 6 h. A pharmacokinetic/pharmacodynamic link model indicated that fexofenadine in horse has antihistaminic effects at low plasma concentrations (EC50 = 16 ng/mL). However, oral treatments of horses with fexofenadine may not be suitable due to the low bioavailability.

Protein kinase C (PKC) isotype profile in eosinophils from ponies with sweet itch and role in histamine-induced eosinophil activation

Eosinophils have been implicated in the pathogenesis of the seasonal equine allergic skin disease, sweet itch. Protein kinase C (PKC) is involved in regulating eosinophil function and antigen challenge has been reported to alter PKC isotype expression in blood eosinophils from allergic human subjects. Here we have compared the pattern of PKC isotype expression in eosinophils from sweet itch ponies with that in cells from normal ponies both during the active and inactive phases of the disease. A role for PKC in histamine-induced eosinophil activation was also investigated. Conventional PKCs alpha and beta, novel PKCs delta and epsilon and atypical PKCs iota and zeta were identified in eosinophils pooled from four allergic ponies during the inactive phase, when no clinical signs were evident. The PKC isotypes, like those in eosinophils from normal ponies, were located primarily in the particulate fraction of the cell. Isotype expression in cells from normal and allergic animals did not appear to be different. In contrast, during the active phase of the disease, when the sweet itch ponies had clinical signs, the expression of PKCs beta, epsilon and iota in eosinophils from these animals appeared to be increased relative to that in cells from normal ponies. When PKC expression in eosinophils from five individual normal and sweet itch ponies was compared, small, but statistically significant, increases in PKC epsilon and PKCdelta expression were evident in eosinophils from the sweet itch ponies during the active and inactive phases, respectively. The non-selective PKC inhibitors, staurosporine and Ro31-8220, significantly reduced histamine-induced superoxide production. Use of Gö6976, an inhibitor of conventional PKCs, suggested that PKCalpha and/or beta were involved and that there was significantly greater inhibition of the response in eosinophils obtained from sweet itch ponies during the active phase. There was no significant difference in histamine-induced superoxide production by eosinophils from allergic and normal ponies and the functional significance of the increased PKC isotype expression in eosinophils from sweet itch ponies relative to that in cells from healthy animals remains to be established.

Cloning, expression and biological activity of equine interleukin (IL)-5

The cytokine, interleukin (IL)-5 stimulates eosinophil differentiation, activation and survival and can prime these cells, increasing the response to other mediators. In view of its many effects on eosinophils, IL-5 has been implicated in the pathogenesis of allergic disease in man. Here we report the cloning of equine IL-5 and expression of the recombinant protein by transfection of Chinese hamster ovary (CHO) cells. The cloned cDNA sequence consisted of 405 nucleotides and encoded a protein of 135 amino acids. There is >85% identity with feline, bovine, ovine, canine, and human IL-5 sequences at the nucleotide and protein level. Supernatants containing equine IL-5 were also examined for biological activity. CHO supernatant containing equine recombinant (eqr) IL-5, like the human ortholog (hrIL-5), induced concentration dependent equine eosinophil adherence to autologous serum-coated plastic (9.7+/-1.5% with a 1:100 dilution of eqrIL-5 and 9.1+/-1.6% adherence with 1 nM hrIL-5; n = 4). The eqr protein also caused concentration dependent superoxide production (11.9+/-2.4 nmol (reduced cytochrome (cyt) C)/10(6) cells at a 1:50 dilution, n = 4). In contrast, hrIL-5 only caused significant superoxide production when diluted in conditioned CHO medium, an effect that was inhibited by the anti-human mAb, TRFK5 (4.4+/-0.3 versus 0.3+/-0.4 nmol/10(6) cells for 0.5 nM hrIL-5 in the presence of the isotype matched IgG1 control (10 microM) and TRFK5 (10 microM), respectively). TRFK5 also significantly inhibited hrIL-5 induced adherence at concentrations of 0.3 microg/ml and above but had no significant inhibitory effect on either superoxide or adherence caused by eqrIL-5. These results demonstrate that equine IL-5 expressed by CHO cells stimulates equine eosinophils, suggesting that this cytokine could play a role in eosinophil recruitment and activation in equine allergic disease. The anti-human and murine moAb TRFK5 does not appear to recognise the equine protein.

Pharmacokinetics and pharmacodynamics of clemastine in healthy horses

Clemastine is an H1 antagonist used in certain allergic disorders in humans and tentatively also in horses, although the pharmacology of the drug in this species has not yet been investigated. In the present study we determined basic pharmacokinetic parameters and compared the effect of the drug measured as inhibition of histamine-induced cutaneous wheal formation in six horses. The most prominent feature of drug disposition after intravenous dose of 50 microg/kg bw was a very rapid initial decline in plasma concentration, followed by a terminal phase with a half-life of 5.4 h. The volume of distribution was large, Vss = 3.8 L/kg, and the total body clearance 0.79 L/h kg. Notably, oral bioavailability was only 3.4%. There was a strong relationship between plasma concentrations and effect. The effect maximum (measured as reduction in histamine-induced cutaneous wheal formation) was 65% (compared with controls where saline was injected) and the effect duration after i.v. dose was approximately 5 h. The effect after oral dose of 200 microg/kg was minor. The results indicate that clemastine is not appropriate for oral administration to horses because of low bioavailability. When using repeated i.v. administration, the drug has to be administered at least three to four times daily to maintain therapeutic plasma concentrations because of the short half-life. However, if sufficient plasma concentrations are maintained the drug is efficacious in reducing histamine-induced wheal formations.

Intradermal skin tests in equine dermatology: a study of 83 horses

Allergic diseases are often diagnosed clinically in the horse without performing diagnostic tests. The purpose of this work was to contribute to the validation of intradermal skin tests in the horse. Eighty-three horses, 14 showing skin or respiratory signs of supposed allergic origin, were subjected to an intradermal skin test using 6 different allergens, positive and negative controls. The tests were read for all animals after 20 min, and for 29 horses after 1 and 4 h. Additionally, 19 horses were tested a few months apart. The comparison after 20 min of the cutaneous reactions to allergens and to positive and negative controls allowed us to propose a threshold of positivity; skin reactions with a diameter above or equal to 15 mm, or with at least a 13 mm diameter and a skin thickness similar to the positive control. There was a marked difference between the healthy group and the allergic group for Culex pipiens and Dermatophagoides farinae, although positive reactions were not rare in the healthy group. Tabanus sp. gave positive reactions in both healthy and allergic animals. There was no significant variation in the reactions observed after 20 min and after 60 min. After 4 h, the progression of the reactions was highly variable and negative controls showed a certain number of positive reactions, which negated their reliability at the point in time, and made it difficult to interpret the other allergens. Finally, the repeatability of intradermal skin tests reactions after 20 min was poor, and probably related to the influence of seasonality for some of the allergens. Further studies are required, notably with others allergens, to interpret intradermal skin tests' responses in clinical practice.

Culicoides antigen extract stimulates equine blood mononuclear (BMN) cell proliferation and the release of eosinophil adherence-inducing factor(s)

Intradermal injection of a Culicoides antigen extract (CAgX) induces T lymphocyte and eosinophil accumulation in the skin of horses with sweet itch. Blood mononuclear (BMN) cells from normal ponies proliferate when stimulated by mitogen (phytohaemagglutinin, PHA) or antigen (tetanus toxoid, TT) and, as shown here, release soluble factor(s) that induce eosinophil adherence. CAgX also caused concentration dependent proliferation of BMN cells from sweet itch and normal ponies [stimulation index: 29 (13) and 17 (7) for BMN cells from sweet itch and normal ponies, respectively during the active phase of disease; 4 microg protein ml(-1)CAgX; 168 h]. A heat labile factor(s) which caused eosinophil adherence was also released [sweet itch ponies: 6.0 (1.6) per cent adherence versus 1.3 (0.4) per cent; normal ponies: 6.6 (0.5) per cent adherence versus 0.9 (0.1) per cent for supernatants from CAgX (4 microg protein ml(-1); 48 hours) stimulated versus unstimulated BMN cells, respectively]. These results suggest that soluble proteins released from T lymphocytes could affect eosinophil function in the lesional skin of sweet itch horses.

Hypersensitivity disorders in horses

Hypersensitivity is an exaggerated immunologic response to a foreign agent that results in inflammation and organ dysfunction. Hypersensitivity disorders are broadly divided into antibody-mediated and T-cell-mediated reactions. The inflammatory pathways that result in disease are initiated in an antigen-specific manner through Fab portions of antibodies or the T-cell receptor, causing the up-regulation of effector mechanisms designed to clear the offending agent. Effector mechanisms include the generation of inflammatory chemicals such as cytokines and chemokines and the attraction of leukocytes and potentiation of their function. This article reviews current concepts in the immunopathogenesis of hypersensitivity disorders and demonstrates these mechanisms as they apply to equine disease.

Characterisation of lymphocyte subpopulations in the skin and circulation of horses with sweet itch (Culicoides hypersensitivity)

Circulating lymphocyte numbers are elevated in horses with the allergic skin disease sweet itch and skin lesions are typified by an infiltrate of eosinophils and mononuclear cells, the latter of which have not been fully characterised. The aim of the present study was to characterise the lymphocyte subpopulations in the circulation and skin of ponies with sweet itch by flow cytometry and a newly developed modified alkaline phosphatase immunohistochemical technique. Sweet itch ponies were found to have significantly greater numbers of circulating CD5+ and CD4+ T-lymphocytes than normal animals. Increased numbers of CD3+ T-lymphocytes, most of which were CD4+, and eosinophils were present in the skin of these animals following intradermal injection of a Culicoides antigen extract (97 +/- 21 vs. 449 +/- 49 CD3+ T-lymphocytes/mm2 in deep dermis of vehicle vs. antigen injected sites; 83 +/- 8% CD4+ T-lymphocytes at antigen injected site). T-lymphocytes, which are thought to be important in the pathogenesis of human allergic skin disease, may therefore contribute to the development of sweet itch lesions via the release of cytokines which can cause eosinophil accumulation and activation. An understanding of the pathology of this disease may lead to a more rational approach to therapy.