Dr. Alberto Martin Cordero received the degree of Doctor in Veterinary Medicine by the University of Guadalajara. He studied Veterinary Dermatology in the European School for Advanced Veterinary Studies, University of Luxembourg. He is a member of the American Academy of Veterinary Dermatology and a founder member of the Latin American Society of Veterinary Dermatology. Is an assistant professor in University of Guadalajara, Department of Veterinary Medicine. He has been a speaker in numerous conferences in Mexico, Latin America and Eastern Europe; and presented articles and clinical cases in Latin America, USA, and Europe. He has been a coordinator in videotoscopy wetlabs and dermatology workshops. He is the coordinator of the dermatology program in Leon Conference (CVDL), currently the second largest veterinary meeting in the world. Dr. Alberto has published articles and clinical cases national and international. He works in private practice and it is the owner of VETDERM: the first veterinary dermatology referral practice the west of Mexico. He has realized visits in the Animal Dermatology Clinics in Southern California and in the Ludwig Maximilian University in Munich and Colorado State University.

Canine atopic dermatitis (CAD) is a pruritic, chronic, inflammatory dermatosis due to an allergic response to non-pathogen environmental allergens; affects approximately 10% of the canine and human population. Histologically and clinically, several similarities have been found between human AD and CAD, which has allowed researchers to perform applicable comparisons.

One of the similarities between CAD and human AD is the presence of a dysfunctional epidermal barrier: secondary bacterial infections may take advantage of this dysfunctional barrier, colonizing and complicating cutaneous lesions in patients with AD.

Within CAD pathology, there are several factors present in patients that may exacerbate the condition.

Beyond the genetic and immunological factors in charge of mediating this pathology; a concept with the same importance has raised in the last years in veterinary dermatology: epidermal barrier.

Epidermal barrier is the front line of defense against external agents (allergens) which cause directly the immune reaction that unchains the allergic process.

There are different factors of importance regarding epidermal barrier concept:

 Keratinocytes

 Ceramides

 Antimicrobial peptides (AMPs)

 Filaggrin

 Transepidermal water loss (TEWL)

Ceramides

Ceramides are important components of the lamellar intercellular lipids of the stratum corneum. Within the lipids of the stratum corneum, ceramides are the largest group of fatty acids, which include cholesterol, free fatty acids, cholesterol sulfate, and cholesterol esters.

Ceramide deficiency is linked to an increase of TEWL, which at the same time increase dryness of the skin and predispose to allergenic penetration and secondary infections.

A study suggest that decreased levels of ceramides lead to a diminish in sphingoin levels; a natural antimicrobial within the stratum corneum, and by consequence increasing the level of bacteria in human patients with AD.¹

An electron transmission microscopy study was performed in the skin of dogs with lesions with CAD and revealed that extracellular lamellar lipids of the stratum corneum where often abnormal in this structure; and its continuity and thickness where lower compared with canine patients with no CAD.²

A recent study confirm this findings, showing that dogs with CAD with no lesions presents disorganized and decreased lamellar lipids, and further demonstrate that applying cutaneous lipids topically results in production and secretion of endogen stratum corneum lipids, improving epidermal barrier.

A simple way to imagine this concept is creating and analogy between the epidermal barrier as a brick wall, in which, bricks would be keratinocytes and the mortar between them would be lipids from the stratum corneum. In order that we find "holes" on this wall; allergen penetration become possible, in this matter we understand that patients with AD; as they have this "cellular wall" affected are more susceptible to expose to the allergens which they have previously been sensitized.

The results of another study suggests that there is ceramide efficiency, which added to an increase of cholesterol are the responsible for the lipid barrier defect, being a basic defect in dogs with CAD.³

Antimicrobial Peptides

Antimicrobial peptides (AMP) are natural antimicrobial found in nature. In mammals are expressed by epithelial cells and phagocytic leukocytes, they have a broad spectrum against bacteria, fungi and virus.

There are two main families of AMPs: cathelicidins and defensins, which show similar physical characteristics and share certain activity.

As I have mentioned before, epidermis provide a physical barrier against microbial invasion and prevent suitable conditions for pathogen colonization. If trespassing of this barrier occur, coordinated immune responses that includes neutrophils and macrophages are activated to eliminate invader organisms. However, this innate immune response is incomplete. Evidence suggest that epidermis is more than a physical barrier, and in fact, keratinocytes are equipped with pathogen recognition receptors able to detect and respond to this invaders. This response leads to the beginning of a coordinate inflammatory response, that includes leukocyte recruitment and induction of AMPs by keratinocytes.

Defensins

Defensins are small cationic peptides expressed by certain phagocytic leukocytes and epithelial cells. These peptides possess broad spectrum antimicrobial activity. Evolutionary speaking beta defensins are the oldest, and we may track its ancestors way behind to the cangrejo cacerola (horseshoe crab). Defensins are found in other places in organism such as mouth, respiratory tract, reproductive tract, and keratinocytes.

Cathelicidins

Cathelicidins are AMPs that possess broad spectrum activity and show great diversity on structure and sequence.

Bacteria has generated mechanisms of defense similar of what we see in antibiotic resistance but focus to AMPs. Proteolytic degradation of AMPs is one of the mechanisms used by bacteria to avoid this innate immune response. Several bacteria have the ability to modulate the overcharge of the cell membranes, transforming them in less anionic and by this mechanism resist cationic activity of the AMPs.

AMPs express chemotaxis to leukocytes, since leukocytes are attracted to infection sites where a gradient of expression of AMPs occur.

A recent study concluded that AMPs plays an important role in host immunity, particularly in the environmental skin interface. Not only they possess antimicrobial activity, but they participate also in host microbiota, chemotaxis, pathogen neutralization, reproductive system and hair color in dogs.⁵

Filaggrin

Filaggrins (filamentous protein aggregates) are complex polypeptides that play a crucial role in keratinization and epidermal barrier function. Profilaggrin is the precursor of filaggrin. It is contained in keratohyaline granules, an essential component of epidermis superficial layers. When differentiation of the granular cells has occurred, profilaggrin is proteolytically converted into filaggrin peptides, which is rapidly added to the keratin cytoskeleton causing collapse of the granular cells to flat, squamous, anucleated cells.

In humans with AD a diminish expression of filaggrin in lesions and no lesions skin is presented; and it is estimated that over half of children with AD present filaggrin mutations that decrease its function and expression.

Data suggest that modifications to epidermal barrier caused by expression of filaggrin variants facilitate allergen penetration in sub epidermal layers and depending of the cytosine environment, facilitate allergic sensitization.

A recent immunohistochemistry study proved that there is a diminished expression of filaggrin in dust mite sensitized atopic Beagles.⁶

Transepidermal Water Loss

In normal and allergic subjects, there is loss of water through evaporation through the skin; this process occurs through the stratum corneum and is a normal regulatory mechanism of the epidermis; this process is named transepidermal water loss (TEWL). It has been proved that in patients with AD, there is an increase of transepidermal water loss, leading to a dryer skiing allergic persons, compromising the epidermal barrier. In conclusion, this is directly linked to ceramide function, which has been previously mentioned.

Although is demonstrated that TEWL is increased in atopic individuals; a recent study tried to demonstrate the correlation between severity of CAD and TEWL and found that there is not a direct relationship.⁷

Improving the Epidermal Barrier

Now we are able to understand the basic concept and mechanism of the epidermal barrier and the components affected particularly in dogs with CAD. We can comprehend that restore or help to diminish the factors that affect epidermal barrier is fundamental in the multimodal treatment of CAD.⁸

Oral Supplements

The original studies of EFA (essentials fatty acids) deficiency performed in rats in the 1940s showed a dramatic decrease in barrier function of the skin, as characterized by increased transepidermal water loss. Essential fatty acids can be delivered as part of the diet or as an extra supplement. Incorporating the fatty acids into the diet may help increase compliance.

Use of emollients and moisturizing agents, with softening and hydration function respectively is helpful. This kind of product s are very popular in humans with AD. In dogs, oatmeal and lubricating oils are used; however there is little supporting scientific evidence regarding the efficacy.

Some keratolytic and keratoplastic products must be considered in cases that present seborrhea caused by CAD or other causes, their use in atopic patients must be evaluated in order to restore normal function. This is one of the main goals in treating CAD.

There are topical products in the veterinary market in spot on formulation; its content goes from essential oils, ceramides or phytosphingosine which is a proceramide. This kind of formulation have gain popularity in the last years due to the good efficacy and easy administration. There are some published papers supporting evidence of efficacy with these products.

In conclusion, part of the treatment of CAD must be focused on the improvement of the epidermal barrier function. Improving epidermal barrier function will diminish allergen transcutaneous penetration and by this mechanism avoid exposure to allergens that produce aberrant responses in patients with CAD.

References

1.  Arikawa J, Ishibashi M, Kawashima M, et al. Decreased levels of sphingosine, a natural antimicrobial agent, may be associated with vulnerability of the stratum corneum from patients with atopic dermatitis to colonization by Staphylococcus aureus. The Journal of Investigative Dermatology. 2002;119:433–439.

2.  Inman AO, Olivry T, Dunston SM, et al. Electron microscopic observations of stratum corneum intercellular lipids in normal and atopic dogs. Veterinary Pathology. 2001;38 720–723.

3.  Reiter LV, Torres SMF, Wertz PW. Characterization and quantification of ceramides in the nonlesional skin of canine patients with atopic dermatitis compared with controls. Veterinary Dermatology. 2009;20(4):260–266. doi:10.1111/j.1365-3164.2009.00759.x.

4.  Lai Y, Gallo RL. AMPed up immunity: how antimicrobial peptides have multiple roles in immune defense. Trends in Immunology. 2009;30:131–141.

5.  Leonard BC, Affolter VK, Bevins CL. Antimicrobial peptides: agents of border protection for companion animals. Veterinary Dermatology. 2012;23(3):177–e36. doi:10.1111/j.1365-3164.2012.01037.x.

6.  Marsella R, Samuelson D, Harrington L. Immunohistochemical evaluation of filaggrin polyclonal antibody in atopic and normal Beagles. Veterinary Dermatology. 2009;20(5–6):547–554. doi:10.1111/j.1365-3164.2009.00844.x.

7.  Marsella R. Are transepidermal water loss and clinical signs correlated in canine atopic dermatitis? A compilation of studies. Veterinary Dermatology. 2012;23(3):238–e49. doi:10.1111/j.1365-3164.2012.01055.x.

8.  Suter MM, Schulze K, Bergman W, Welle M, Roosje P, Müller EJ. The keratinocyte in epidermal renewal and defence. Veterinary Dermatology. 2009;20(5–6):515–532.