In the last two decades, we have learned much about the pathogenesis of allergies, particularly canine atopic dermatitis. There are several pathogenetic mechanisms involved. While there are general features we accept as characteristic for CAD, each patient is an individual and some pathogenetic pathways may be more important in one than another; these have been manifested as distinct breed variations in the manifestation of the disease.¹

Atopic dermatitis is a lifelong inflammatory disease that is influenced by genetics and environmental factors, including the cutaneous microbiome, the microbial organisms that live on the skin.^2-5 Learning that canine atopic dermatitis has many of the features of human atopic dermatitis has allowed veterinary scientists to take advantage of the large body of research available for the human disease, and step into it to verify key findings in the canine disease. This ability to utilize information across species is enhancing our ability to improve the quality of life for atopic dogs and their owners.

An Imbalance in the Immune System: Contribution of Genetics and Environment

We have known for years that atopic patients have an abnormal immune response, making IgE against a variety of environmental allergens (pollens, molds, dusts, danders, mites, foods, and microbes) that unaffected individuals ignore (reviewed^6,7).These allergen-specific IgE antibodies are the basis by which we select allergens for immunotherapy, whether we use intradermal testing or serum allergy testing. In the past, we focused heavily on the role of IgE binding to mast cells in the dermis. The cell-bound IgE when binding to its allergen triggers mast cell release of inflammatory compounds, including histamine. In the clinic, however, we found it frustrating when antihistamines seemed to work so poorly to control itch in these dogs. Now we understand that the immune response is so much more complex in these dogs. The immune aberrations in atopic dermatitis involve a number of genes which regulate the amount of IgE expressed, the cytokines that regulate the adaptive immune response, and even cytokines of the innate immune response. These gene products interact with each other and the environmental to influence disease expression and severity (for dogs; reviewed⁸).

Atopic disease reflects a skewing of the adaptive immune response toward the T-helper 2 phenotype.^7,9 T-helper cells are instrumental in the coordination of the immune response, and there are several of them; we will focus on the 3 best understood. T-helper 1 cells produce cytokines (including IL-2, IFNg, and TNFb) support cellular responses involved in protection against viruses, intracellular pathogens, and tumor surveillance. T-helper 2 cells produce a different subset of cytokines (IL-4, IL-5, IL-6, IL-10, IL-13, and IL-31) which mediate humoral responses, supporting the development and maturation of B lymphocytes and the production of antibodies. This pathway dominates the immune system in atopic pathways, and IgE production is promoted; this pathway is believed to protect against multicellular parasites. T-helper 17 cells produce IL-17 and IL-22, and this pathway is believed to help regulate extracellular pathogens especially on surfaces; it is pro-inflammatory and has been implicated in several autoimmune diseases. When T-helper 2 responses predominate, the other pathways tend to be repressed, providing one explanation for why atopic patients are predisposed to infections (viral in human, and viral, fungal, and bacterial in humans and dogs). But the disease is complex and as it progresses, T-helper 1 cytokines can contribute to inflammation. The immunologic findings in atopic dogs have been reviewed recently.^6,9

Why do these immune imbalances result? As we have mentioned, polymorphisms in immune response genes are involved, but the environment plays an important role. Multiple environmental factors have been implicated in atopic disease, including microbes, pollutants in the air, mites, and helminthes (reviewed⁸). Microbes can play a dual role, either promoting the disease phenotype or inhibiting it.¹⁰ The hygiene hypothesis suggests that exposure to microbial endotoxins and other stimulators of the innate immune response mature the immune system, so that production of IgE and the allergic phenotype is repressed. Several studies have documented the higher incidence of human atopic diseases in urban areas and developed countries compared to rural and less developed countries. But in an atopic individual, exposure to Staphylococcus and Malassezia often exacerbate the disease, likely due to toxins, enzymes, and in the case of Staphylococcus, superantigens. In fact, over time, human and canine patients can develop IgE antibodies against Staphylococcus and Malassezia, which contribute to disease severity (reviewed³).

Support for the hygiene hypothesis in the canine disease is just emerging. Higher endotoxin levels in the coats of dogs appeared to be associated with lower disease severity, and dogs that live indoors or in cities may be more likely to develop atopic diseases than dogs that live in rural areas (reviewed⁸). Pollution is another factor implicated in atopic diseases, including atopic dermatitis and asthma. Tobacco smoke, toluene, volatile organic compounds, formaldehyde, nitrogen dioxide, and particulates are risk factors for the development and exacerbation of atopic dermatitis in people.¹¹ The mechanism of action is not known, but it is suspected that these compounds may induce oxidative damage in the skin; another hypothesis is that pollutants may break down pollens more quickly, releasing higher levels of protein to contact the skin. Pollution tends to be higher in industrialized countries, correlating with increase prevalence of atopic disease. We don't know a lot about the role of pollutants in the canine disease, but a recent paper examined the effects of passive smoke on canine atopic dermatitis. While the authors could not prove statistical significance in this one study, the topic is of interest in veterinary dermatology.¹²

The allergens of house dust mites and storage mites are proteases, enzymes which have multiple effects on the skin. Proteases can reduce adhesion molecules allowing antigen presenting cells to migrate out of the epidermis on their way to the lymph node more readily, and the proteases are pro-inflammatory. Helminths are of interest because it has been suggested that treatment with whipworm (Trichuris vulpis) eggs can attenuate atopic dermatitis in humans; however, results are conflicting. One studied showed similar results in dogs but others were not able to reproduce the results (reviewed⁸). Likely the effects of the helminths on immune function are complex and would depend on the timing of the infection, the number of parasites, and how long the infection existed.

A Disrupted Skin Barrier: "Inside-Outside-Inside"

The immunocentric view of atopic dermatitis is called the "inside-out' view and states that the immune abnormalities drive the disease and all changes in the skin are secondary to immunologic action. We have learned that the problem is much more complex. Atopic patients, both human and canine, have been shown to have a disrupted skin barrier. This defect permits absorption of allergens more deeply into the skin where they have access to immune cells, particularly the antigen presenting cell of the skin, the Langerhans cell. The skin barrier defect as a major contributor to CAD is termed "the outside-in" hypothesis. However, the disease is complex and many investigators favor the combination "outside-inside-outside" hypothesis, as both skin barrier and immune defects are required for the atopic phenotype, and they are closely intertwined. Gene abnormalities have been identified in both human and canine atopic patients, suggesting that there is a primary barrier defect, but it is also clear that the T-helper 2 cytokines can further disrupt the barrier and exacerbate the disease (reviewed³). The skin barrier defect results in a dry scaly skin that is prone to secondary infection and irritation. Infections as well as allergens with proteolytic activity (mites, fungi) can further exacerbate the defect. So what is this skin barrier?

The stratum corneum provides the physical, chemical, and antimicrobial skin barrier. It consists of the cornified epidermal cells (corneocytes) and the organized lipid layers that lie between them. It is biologically active, containing enzymes that help skin desquamate normally and invisibly, and it inhibits overgrowth of pathogens while allowing colonization by non-pathogens. There are several genes identified in the human disease which affect the structure and function of the barrier. The most commonly affected is filaggrin, a structural protein with a multitude of functions. Its metabolites decrease skin pH, which provides antimicrobial activity and the optimum environment for the enzymes involved in the production of skin lipids and proper desquamation. Filaggrin loss of function mutations contribute to the abnormal barrier structurally and chemically, and its expression is further downregulated by the inflammatory cytokines associated with the T-helper 2 phenotype. We have learned that atopic dogs have filaggrin abnormalities; in some cases, decreased expression of mRNA for the protein and in other cases, abnormal and reduced distribution of filaggrin in the skin. But it is important to note that there are breed differences in these changes, reinforcing the idea that pathogenesis and thus treatment may vary among individuals. Filaggrin changes in canine atopic dermatitis have been reviewed recently.³

Decreased ceramide levels are another critical part of the skin barrier defect. As in humans, atopic dog skin has decreased expression of ceramides and this is manifested by disruption and disorganization of the lipid layers between the corneocytes when examined by electron microscopy. Decreased sphingosine-1-phosphate levels have also been demonstrated; this lipid is important in signal transduction.¹³ It is a metabolite of ceramide, and interestingly may regulate the function of Langerhans cells, the antigen-presenting cells of the skin, as well as affect differentiation of keratinocytes. Lipid changes seen in canine atopic skin have been reviewed and compared with those in the human disease.³ Replenishing the skin with lipids has been shown to improve the skin barrier^14,15 and seems to help reduce the rate of recurrent infections in atopic dogs (personal observation).

Connections Among Nerves, Immune Cells, and Keratinocytes: Cytokines As The Conduit for Itch Stimulation

Why do atopic dogs itch? There is a complex interaction between nerves, immune cells, and keratinocytes that is tied together by cytokines. Cytokines are protein molecules secreted by cells that are used for communication. They are usually active over short distances, unlike hormones, and they bind to surface receptors on recipient cells to activate the internal signal transduction mechanisms that result in stimulation or inhibition of cell function. The immune cells in atopic skin and the keratinocytes produce a number of cytokines which influence each other's function but also bind directly to nerves to stimulate itch. IL-31 is the major immune cytokine that stimulates itch; it is produced by T-helper cells primarily but recently mast cells were shown to secrete this cytokine as well. Interleukin-31 is a major stimulator of itch in dogs.^16-18 Injection of purified IL-31 into dogs induced itch, and a monoclonal antibody against IL-31 ameliorates itch in atopic dogs (see www.zoetisus.com/products/dogs/il-31/index.aspx). IL-31 binds to its receptors on the surface of nerves to rapidly induce itch in an allergic reaction. Thus, T-helper 2 cells communicates directly and quickly with the nerves, bypassing the need for mast cell degranulation.

Monoclonal antibody therapy is a biologic therapy used widely in human medicine for the treatment of several inflammatory diseases and cancer; these antibodies are genetically engineered to replace the mouse protein with human sequences, thus reducing the likelihood of adverse reactions to a foreign protein; this is an up-and-coming approach for veterinary medicine.¹⁹ The monoclonal antibody against canine IL-31 has received conditional licensure for the treatment of itch in dogs with atopic dermatitis. Monoclonal antibodies are being studied for use in human atopic dermatitis. Dupilumab is a human monoclonal antibody that binds to the alpha subunit of the IL-4 receptor, and it blocks the ability of IL-4 and IL-13 to activate cells which carry the receptor. It reduces clinical severity in humans without significant safety concerns.^20,21 Also, a monoclonal antibody directed against the IL-31 receptor ameliorates itch in atopic humans.²²

IL-31, as well as other cytokines produced by T-helper 2 cells utilize Janus kinases as part of their signal transduction mechanism, particularly JAK1. Oclacitinib, a JAK inhibitor that is selective for JAK1, ameliorates itch in dogs injected with IL-31 and in dogs with allergic dermatitis, including CAD, further cementing the notion that IL-31 is a major inducer of itch in allergic dogs.^23-26

JAK inhibitors affect other cytokines as well, and one of them, produced by the keratinocyte, has also been shown to mediate itch. Thymic stromal lymphopoietin (TSLP) binds to its receptors on nerves; one chain of its receptor is closely related to the common gamma chain present in the receptors the allergic cytokines IL-4, IL-6, IL-10, IL-13, and IL-31 and thus could be affected by oclacitinib as well, as these receptors utilize the JAK-STAT pathway for signal transduction.²⁷ TSLP levels are increased in atopic humans and in mouse models of atopic dermatitis; we need to learn more about this in dogs.

Summary

While CAD it is a tremendously complex disease, our new knowledge is opening pathways to effective treatment for a disease which is lifelong. At last the future is bright for dogs afflicted with atopic dermatitis.

References

References are available on request.