犬特应性皮炎的研究进展

王帆

Advances in our understanding of canine atopic dermatitis

犬特应性皮炎的研究进展

作者：Rosanna Marsella

翻译：王帆

Canine atopic dermatitis (cAD) is a genetically inherited clinical syndrome that encompasses a diversity of mechanisms and can have a variety of triggers. Development of clinical disease is the result of genetic factors and environmental conditions, which shape the resulting immunological response.

Clinical disease becomes evident once a threshold of inflammatory response is achieved. Skin barrier impairment plays a role in promoting cutaneous dysbiosis and increased allergen penetration. Keratinocytes shape the response of dendritic cells and subsequent lymphocytic response. Thymic stromal lymphopoietin is one of the links between the damaged skin barrier and the modulation of a T-helper (Th)2 response. It is still unclear whether mutations in skin barrier genes exist in atopic dogs, as they do in humans, or whether the observed alterations are purely secondary to inflammation.

A dysregulated immune response with increased Th2, Th17 and CD4+ CD25+ regulatory T cells has been reported. A variety of cytokines [interleukin(IL)-31, IL-34, Macrophage migration inhibitory factor] are proposed as potential biomarkers and treatment targets because they are increased in the serum of atopic dogs when compared to controls, although a correlation between serum levels of these factors and severity of disease is not always present.

The main issue with many published studies is that atopic dogs are always only compared to normal controls. Thus, it is unclear whether the changes that we find are truly a signature of cAD or merely a manifestation of nonspecific broad inflammatory responses. Studies considering comparison with other inflammatory diseases different from cAD are urgently needed to correctly identify what is specific to this complicated syndrome.

摘要

犬特应性皮炎(cAD)是一种基因遗传性临床综合征, 包括多种机制, 可能有多种触发因素。临床疾病的发生是遗传因素和环境条件的共同结果, 这些因素塑造了产生的免疫反应。

一旦达到炎症反应的阈值, 临床疾病就变得明显。皮肤屏障损伤能加重皮肤微生态失调和增加过敏原渗透作用。角质形成细胞造成树突状细胞的反应和随后的淋巴细胞反应。胸腺基质淋巴生成素是受损的皮肤屏障与辅助性T细胞(Th)2反应调节之间的联系之一。目前尚不清楚特应性皮炎犬是否存在皮肤屏障基因突变, 就像人类一样, 或者观察到的改变是否纯粹继发于炎症。

已报告Th2、Th17和CD4 + CD25 + 调节性T细胞增加的免疫应答失调。拟定多种细胞因子[白细胞介素(IL)-31、IL-34、巨噬细胞移动抑制因子]作为潜在的生物标志物和治疗靶点, 因为与对照组相比, 其在特应性皮炎犬血清中增加, 但这些因子的血清水平与疾病严重程度之间的相关性并不总是存在。

许多已发表研究的主要问题是, 特应性皮炎犬始终仅与正常的对照组进行比较。因此, 目前尚不清楚我们发现的变化是真正的cAD的特征还是仅仅是非特异性广泛炎症反应的表现。迫切需要考虑与不同于cAD的其他炎性疾病进行比较的研究, 以正确识别这种复杂综合征的特异性。

Introduction

介绍

It is becoming increasingly clear that canine atopic dermatitis (cAD) is a clinical syndrome and not a single disease. What was once simply presented as a type I hypersensitivity to inhaled environmental allergens is now viewed as a multifactorial and complex inflammatory syndrome that may or may not be associated with a demonstrable allergic response, and in which the skin is the main avenue of allergen exposure. The realization that cAD is not simply a hypersensitivity response to environmental allergens has greatly complicated our understanding of this disease. Under this umbrella, we now have atopic dogs in which the trigger can include foods and dogs in which we cannot demonstrate a clear allergen-specific immunoglobulin (Ig)E response.

越来越清楚的是，犬特应性皮炎(cAD)是一种临床综合征，而不是单一的疾病。曾经仅仅表现为I型超敏反应吸入环境过敏原，现在视为一个多因子的和复杂的炎症综合征,可能会或可能不会证实存在过敏反应,而皮肤是过敏原暴露的主要途径。意识到cAD不仅仅是对环境过敏原的超敏反应，使我们对这种疾病的理解变得非常复杂。在这个保护伞下，我们现在认为特应性皮炎犬，其中的触发因素包括食物，并且不能证明一个明确的过敏原特异性免疫球蛋白(Ig)E反应的患犬。

Canine atopic dermatitis results from the complex interaction of genetics and environmental factors that shape the immune response and skin barrier function. In our deepening of the understanding of cAD, we now have an awareness of the role of skin barrier dysfunction and realize the complex and vital role of keratinocytes in orchestrating the immune response. Skin barrier dysfunction and aberrant immune responses are interrelated and not separate, or opposing, aspects of this disease. The keratinocyte, once considered more like an “inert physical cover”, is now understood to give key signals that can have impactful effects on the type of immune response generated by cutaneous contact with allergens. Keratinocyte-derived cytokines shape how dendritic cells process antigens and the type of lymphocytic response that is generated from the interaction. Thus, cytokines and chemokines produced by keratinocytes [thymus and activation-regulated chemokine (TARC), interleukin (IL)-33, Thymic stromal lymphopoietin (TSLP)], are being closely considered as potential therapeutic targets although a clear indication of which one is optimal is still lacking.Atopic individuals have an increased exposure and different response to environmental allergens compared to healthy controls as a consequence of an impaired skin barrier. This, in turn, shapes the microbial flora of the skin. We understand that all of these factors, from the changes in the skin barrier to the immune response and the microbiome, are interconnected, making it very difficult to pin-point the initial event which initiated the cascade. It is clear that there are vicious cycles wherein these factors aggravate each other. Thus, our treatments need to target various areas concurrently to correct or decrease the negative effects of an excessive self-perpetuating inflammatory response.

犬特应性皮炎是遗传和环境因素复杂相互作用的结果，这些因素塑造了免疫反应和皮肤屏障功能。随着我们对cAD理解的加深，我们现在已经认识到皮肤屏障功能障碍的作用，并认识到角质形成细胞在协调免疫反应中的复杂而重要的作用。这种疾病的皮肤屏障功能障碍和异常的免疫反应是相互关联的，而不是单独或对立的方面。角质形成细胞，曾经被认为更像是一种惰性的物理屏障，现在被理解为提供关键信号，可以对皮肤接触过敏原产生的免疫反应类型产生影响。角质形成细胞来源的细胞因子决定了树突状细胞如何处理抗原，以及相互作用产生的淋巴细胞反应的类型。因此，由角质形成细胞产生的细胞因子和趋化因子[胸腺和激活调节趋化因子(TARC)、白介素(IL)-33)、胸腺基质淋巴细胞生成素(TSLP)]被认为是潜在的治疗靶点，但仍然缺乏明确的指标。由于皮肤屏障受损，与健康对照犬相比，特应性皮炎患犬接触环境过敏原的程度增加，对环境过敏原的反应也不同。这反过来又影响了皮肤微生物群。我们知道，所有这些因素，从皮肤屏障的变化到免疫反应和微生物群，都是相互关联的，因此很难确定引发级联反应的初始事件。显然，这些因素之间存在着相互加剧的恶性循环。因此，我们的治疗需要同时针对不同的区域，以纠正或减少过度自我持续的炎症反应的负面影响。

We are still struggling to identify truly specific markers for cAD; as in humans, it is possible that multiple biomarkers may exist owing to the heterogeneity of this condition. In dogs, many candidates have been proposed yet the reality is that many of them may not be specific and simply be the manifestation of a generalized inflammatory response. Some do not appear to change with treatment (e.g. IL-34), which would defeat the very concept of a biomarker of disease severity which could be used in formulating a long-term treatment plan. The purpose of this paper is to discuss the advances in our understanding of cAD and propose directions for future studies.

我们仍在努力确定cAD真正的特异性标记;与人类一样，由于这种疾病的异质性，可能存在多种生物标记物。在犬上，已经提出了许多候选基因，但现实情况是，其中许多可能不是特异的，只是一种普遍的炎症反应的表现。有些似乎不会随着治疗而改变(例如IL-34)，这将推翻疾病严重程度的生物标志物的概念，而这种生物标志物可用于制定长期治疗计划。本文的目的是讨论我们在理解cAD方面的进展，并提出未来的研究方向。

The importance of the skin barrier

皮肤屏障的重要性

Several studies have focused on skin barrier function and ultrastructure in cAD. It is commonly accepted that some form of skin barrier dysfunction is present in this disease although it is difficult to determine whether this is a primary defect, secondary to inflammation or a combination of the two. Additionally skin barrier function has not been studied in dogs affected with other inflammatory skin conditions and, thus, we cannot say how specific our findings are to cAD.

近年来，有关cAD皮肤屏障功能和超微结构的研究较多。人们普遍认为，本病存在某种形式的皮肤屏障功能障碍，但很难确定这是原发性缺陷、继发于炎症，还是两者兼得。此外，皮肤屏障功能还没有在患有其他炎性皮肤病的犬上进行研究，因此，我们不能说我们的研究结果对cAD有多大的特异性。

We lack conclusive evidence of a causative effect of skin barrier dysfunction in the actual development of cAD, although in vitro studies using atopic keratinocytes showed that the behaviour of the keratinocytes between normal and atopic dogs is different in terms of growth and ability to establish connections. Many of the initial studies in veterinary medicine have been observational studies of associations. Associations are not the same as causation and it is necessary to work out how critical skin barrier impairment is per se in the actual causation of the canine disease.

虽然在cAD实际发展中，我们缺乏皮肤屏障功能障碍的因果效应的结论性证据，但使用特应性角质形成细胞进行的体外研究表明，正常犬和特应性皮炎患犬角质形成细胞的行为在生长和建立联系的能力方面是不同的。许多兽医医学的初步研究都是观察性研究。有关联并不等同于因果关系，有必要研究出皮肤屏障损伤本身在犬类疾病实际起因中的重要性。

To some degree the same applies to human medicine, even for the case of filaggrin. Mutations in this protein are considered a powerful risk factor for the development of AD, yet there is still some debate as to their “causative” role; some people with AD do not have evidence of mutations and some individuals with filaggrin mutations do not develop disease. Clearly, other mutations may be important in some patients and a “threshold of disease development” needs to be reached to manifest clinical signs. Ultimately, a combination of genetic and epigenetic factors contributes to shaping the immune response and to triggering disease development.

在某种程度上，这同样适用于人类医学，甚至是丝聚合蛋白。该蛋白的突变被认为是AD发展的一个强有力的危险因素，但对于其致病作用仍有一些争论。一些AD患者没有突变的证据，而一些丝聚合蛋白突变的人则没有发病。显然，其他突变在一些患者中可能很重要，需要达到“疾病发展阈值”才能显示临床症状。最终，遗传和外生因素共同作用形影响着免疫反应和触发疾病的发展。

Assessment of skin barrier function can be done in vivo with noninvasive strategies or by invasive methodologies requiring skin biopsy. The measurement of trans epidermal water loss (TEWL) is frequently included in clinical studies as it is noninvasive, yet the repeatability of this methodology has been questioned. In a pilot study evaluating five different methodologies (skin hydration, TEWL, pH, skin absorbance and erythema) for skin barrier function assessment done in both normal and atopic dogs it was determined that measurement of pH was the most repeatable assessment, while the measurement of TEWL was the least reliable.This finding is consistent with a study published previously which reported the variability of TEWL measurements.

皮肤屏障功能的评估可以在体内用非侵入性策略或需要皮肤活检的侵入性方法进行。经表皮水分流失(TEWL)的测量经常被纳入临床研究，因为它是无创的，但这种方法的可重复性受到质疑。在一项初步研究中，评估了五种不同的方法(皮肤水合作用、水分流失、pH值、皮肤吸收率和红斑)对正常犬和特应性皮炎犬的皮肤屏障功能评估，确定pH值测量是最可重复的评估，而水分流失测量是最不可靠的。这一发现与之前发表的一项研究一致，该研究报告了水分流失测量的多样化。

Indeed, correlation between TEWL and the severity of clinical signs in atopic dogs has been reported to be variable and inconsistent.This could possibly be a consequence of technical issues. The lack of reliability makes it difficult to use this parameter when evaluating the effect of treatment options for skin barrier improvement.Interestingly, although decreased hydration is a demonstrated finding for human atopic patients, currently we do not have evidence that this is the case in atopic dogs. Examination of biopsies is useful to observe the organization of the skin and particularly the structure of lipid lamellae. Transmission electron microscopy has shown that the organization of the lipid lamellae in atopic dogs is abnormal even in nonlesional atopic skin, and further derangement of lipid lamellae occurs after exposure to allergen and development of clinical lesions of AD. Nonlesional atopic skin is not equivalent to normal skin due to low-grade inflammatory response or, potentially, some intrinsic ultrastructural differences. Lipid alterations have been described widely in dogs with decreased free fatty acid levels and decreased ceramides. As a consequence of these differences, the packing of the lipids and the ultrastructure of the skin are affected.

事实上，据报道在特应性皮炎犬中，TEWL和临床症状的严重程度之间的相关性已被报道是多变的和不一致的。这可能是技术问题导致的。由于缺乏可靠性，在评估改善皮肤屏障的治疗方案的效果时，很难使用该参数。有趣的是，虽然水合作用减少在人特应性皮炎患者中已得到证实，但目前我们还没有证据表明在犬的特应性皮炎中也是如此。活检检查有助于观察皮肤组织，特别是脂质薄层的结构。透射电镜显示，即使在无病变的特应性皮炎患犬中，脂质薄层组织也是异常的，接触过敏原后，脂质薄层进一步发生紊乱，发展为有临床病变的AD。无病变特应性皮炎的皮肤并不是正常皮肤，这是因为炎症反应程度低，或可能存在一些内在超微结构差异。随着游离脂肪酸水平和神经酰胺水平的降低，犬的脂质改变已经被广泛描述。由于这些差异，脂质的堆积和皮肤的超微结构受到影响。

When keratinocytes harvested from canine skin biopsies are grown in cell culture, the behaviour of keratinocytes from normal and atopic individuals is different. Atopic keratinocytes in cell culture are prone to form little isolated “domes” and have more irregular shapes in comparison to normal keratinocytes that grow flatter and connect more evenly with each other.Whether this behaviour in vitro has implications for what we observe with the clinical disease is unknown at this time. The abnormal appearance of filaggrin staining in cell cultures may indicate abnormal processing of this protein. This may have an effect on the shape and differentiation, and explain why atopic keratinocytes grow differently in cell culture.

当从犬皮肤活检中获得的角质形成细胞在细胞培养中生长时，来自正常和特应性皮炎的角质形成细胞的行为是不同的。在细胞培养过程中，特应性皮炎角质形成细胞容易形成孤立的拱形结构，形状不规则，而正常的角质形成细胞则更平，彼此连接更均匀。这种体外行为是否对我们观察到的临床疾病有意义目前还不清楚。细胞培养中丝聚合蛋白染色的异常表现可能表明该蛋白的加工异常。这可能对形状和分化有影响，并解释了为什么特应性皮炎角质形成细胞在细胞培养中生长不同。

When keratinocytes are grown to confluence and the permeability of the monolayer is assessed, differences exist between the normal and the atopic keratinocytes in terms of trans epithelial electrical resistance (TEER). TEER is commonly used as an in vitro assessment of epithelial permeability. The higher the TEER, the less permeable the epithelial layer. The TEER of a monolayer made of canine keratinocytes from an atopic dog is lower than that of a normal individual, suggesting an increased permeability for the atopic monolayer. This can be linked to decreased tight junction proteins expression or linked to the morphology of the atopic keratinocytes themselves.

当角质形成细胞生长融合并评估单层的通透性时，正常和特应性皮炎角质形成细胞在跨上皮电阻值(TEER)方面存在差异。TEER通常用于体外评估上皮通透性。TEER越高，上皮通透性越差。特应性皮炎犬角质形成细胞单层的TEER低于正常犬，提示特应性皮炎单层通透性增加。这可能与紧密连接蛋白表达的减少有关，或与特应性皮炎角质形成细胞本身的形态有关。

Tight junctions have attracted a lot of attention for their role in regulating permeability of the skin and it is clear that different aberrations may be important in different subsets of patients. Several proteins within tight junctions such as claudin, occludin and Zonula Occludens 1 (ZO-1) have been investigated in dogs with AD. Decreased claudin 1 expression has been reported in samples from atopic dog skin (nonlesional skin) and in atopy patch test sites. Decreased intensity of staining for ZO-1 on immunohistochemical analysis also has been reported in atopic dogs. It is interesting to point out that the abnormalities observed so far in samples from dogs with cAD, using immunohistochemical findings and immunofluorescence patterns to visualize tight junction proteins and filaggrins, always describe a “patchy” or noncontinuous expression. Thus, the issues in atopic keratinocytes may not be something that can simply be proven by changes in quantity, and rather by their irregular and discontinuous expression patterns.

紧密连接因其在调节皮肤通透性方面的作用而引起了人们的广泛关注，很明显，不同的畸变在不同的患者群体中可能是重要的。一些紧密连接蛋白，如闭合蛋白、闭锁蛋白和闭锁小带1(ZO-1)已经在AD犬中进行了研究。在特应性皮炎犬皮肤(非病变皮肤)和特应性皮炎斑片试验部位的样本中，已经报道了闭合蛋白1表达降低。免疫组化分析中ZO-1染色强度的降低也在特应性皮炎犬中有报道。有趣的是，到目前为止，在cAD患犬的样本中观察到的异常，使用免疫组化和免疫荧光模式来观察紧密连接蛋白和丝聚合蛋白，通常描述的是斑片状或非连续的表达。因此，特应性皮炎角质形成细胞的问题可能不能简单地通过数量的变化来证明，而是通过其不规则和不连续的表达模式来证明。

By contrast with human studies, filaggrin mutations have not been clearly associated with cAD or decreased expression has not been consistently reported in dogs. Increased gene expression for filaggrin in atopic skin and increased expression of enzymes responsible for its metabolism have been reported in samples from atopic dogs, possibly as a result of the skin compensating for a defective barrier with increased degradation. In dogs and humans, two filaggrin-type proteins have been described with similar location within the epidermis and possibly overlapping functions. The exact difference in function between the two filaggrins is not known at this time. It is important to point out that some of the first publications in veterinary medicine actually referred to what is now known to be filaggrin 2 rather than filaggrin. This may cause some confusion when reading the veterinary literature.

与人类研究相比，丝聚合蛋白突变与cAD的关系并不明显，在犬上也没有一致报道过丝聚合蛋白表达降低。在特应性皮炎皮肤中丝聚合蛋白的基因表达增加和负责其代谢的酶的表达增加已经在特应性皮炎犬的样本中被报道，这可能是由于皮肤通过增加降解来弥补缺陷屏障的结果。在犬和人中，两种丝聚合蛋白在表皮内的位置相似，功能可能重叠。这两个丝聚合蛋白在功能上的确切区别目前还不清楚。需要指出的是，在兽医学中，一些最早的出版物实际上是指现在已知的丝聚合蛋白2，而不是丝聚合蛋白。在阅读兽医文献时，这可能会引起一些混淆。

The role of the microbial composition of atopic skin

特应性皮炎皮肤的微生物组成作用

When talking about skin barrier we do not simply refer to the structural organization of the keratinocytes. The skin barrier is chemical and microbiological as well as physical. For this reason, the type of microbes that are present on the skin are of critical importance in shaping the immune response of the individual and in determining the protection from outside influences. The increased pH characteristics of canine atopic skin may be the result of abnormal metabolism of filaggrin and may be linked to aberrant bacterial growth. Decreased biodiversity of the microbiome and an increased presence of Staphylococcus has been reported in atopic dogs and associated with clinical flares of the disease. Longitudinal studies in dogs with cAD showed that antipruritic treatments restored biodiversity and normalized skin barrier parameters such as TEWL and pH, demonstrating the inter-relationship between the skin barrier and microbiome.

当谈到皮肤屏障时，我们不只是指角质形成细胞的结构组织。皮肤屏障是化学的、微生物的和物理的。因此，皮肤上存在的微生物类型对于形成个体的免疫反应和决定免受外界影响的保护至关重要。犬特应性皮炎皮肤pH值的增加可能是丝聚合蛋白代谢异常的结果，也可能与异常的细菌生长有关。据报道，在特应性皮炎犬中，微生物群的生物多样性减少和葡萄球菌的存在增加，并与该疾病的临床发作相关。对cAD患犬的纵向研究表明，止痒治疗可以恢复生物多样性，并使皮肤屏障参数(如水分流失和pH值)正常化，这表明了皮肤屏障与微生物群之间的相互关系。

Allergen challenge in previously sensitized dogs leads to dysbiosis with increased amounts of Staphylococcus pseudintermedius. Topical antimicrobial therapy has been reported to increase biodiversity on the skin in atopic dogs. Although historically we have viewed staphylococcal over-colonization as a consequence of the inflammation in the skin of atopics, it is increasingly clear that Staphylococcus and its cell wall proteins and toxins can by themselves induce AD-like inflammatory responses and affect the skin barrier function. More specifically, a cell wall component of Staphylococcus can induce transcription of TSLP via TLR2 in canine keratinocytes and this could be a link to explain Th2 responses precipitated by Staphylococcus. It is now known that difference in strains of Staphylococcus are important in the type of inflammation triggered and expansion of Th2 and Th17 cells.

过敏原刺激已经被致敏的犬会导致假中间葡萄球菌数量失调与增加。已报道外部抗菌治疗能增加特应性皮炎犬皮肤的生物多样性。虽然以前我们认为葡萄球菌过度增殖是特应性皮炎皮肤炎症的结果，但越来越清楚的是，葡萄球菌及其细胞壁蛋白和毒素本身可以诱导类似AD的炎症反应，并影响皮肤屏障功能。更具体地说，葡萄球菌的一种细胞壁成分可以通过TLR2诱导犬角质形成细胞中TSLP的转录，这可能是解释葡萄球菌诱发Th2反应的一个环节。现在我们知道，葡萄球菌菌株的差异在Th2和Th17细胞的炎症触发类型和发展中起着重要作用。

The aberrant immune responses

免疫反应异常

Much progress has been made since the time in which AD was believed to be simply linked to mast cell degranulation and histamine release. Our knowledge has expanded to include an appreciation of the role of the various lymphocytic populations. Although for the longest time emphasis was placed primarily on Th2 cells, we now appreciate the intricacy and the importance of other populations, which play a role in various phases.

自AD被认为与肥大细胞脱颗粒和组胺释放简单相关以来，已经取得了很大的进展。我们的知识已经扩展到包括对各种淋巴细胞群体作用的认识。虽然很长一段时间的重点都放在Th2细胞上，但我们现在认识到其他细胞的复杂性和重要性，它们在不同阶段发挥着作用。

In humans, it is believed that involvement of Th2, Th22 and Th17 cells occurs in the acute stage, and the involvement of Th2, Th22 and Th1 cells occurs in the chronic phase. A dysregulated immune response in cAD includes increased Th2, Th17 and CD4+ CD25+ regulatory T (Treg) cells. Treg cells are significantly increased in atopic dogs compared to normal and correlate with disease severity. Th2 cytokines such as IL-4 are overexpressed in atopic skin and clinical tolerance in healthy dogs is associated with TGF-beta. In chronic lesions a combination of Th2, Th1 and Treg cells are present. Acute patch test reactions show activation of Th2/Th22 pathways and increased gene expression for IL-31, which is believed to play a role in the mediation of pruritus. IL-31 has been successfully identified as a therapeutic target to relieve pruritus in affected atopic dogs. Positive correlation was detected in active disease between severity of dermatitis and circulating levels of IL-31 in a colony of research atopic beagles and in privately owned atopic dogs. Although IL-31 has been traditionally considered for its role in pruritus it is clear that more complex functions are linked to IL-31 including epidermal differentiation and immune responses. Keratinocytes express the receptor for IL-31 and this cytokine can greatly suppress differentiation markers such as filaggrin and cornified cell envelope formation. In humans keratinocytes from atopic patients have higher expression of the receptor for IL-31 when compared to healthy controls, making them very sensitive to the effect of this cytokine.

在人类中，据认为Th2、Th22和Th17细胞的参与发生在急性期，Th2、Th22和Th1细胞的参与发生在慢性期。cAD中免疫反应失调包括Th2、Th17和CD4+ CD25+调节性T (Treg)细胞的增加。与正常犬相比，特应性皮炎犬体内Treg细胞显著增加，并与疾病严重程度相关。Th2细胞因子如IL-4在特应性皮炎皮肤中过表达，健康犬的临床耐受性与TGF- β相关。在慢性病变中存在Th2、Th1和Treg细胞的结合。急性斑贴试验反应显示Th2/Th22通路激活，IL-31基因表达增加，被认为在瘙痒的调节中发挥作用。IL-31已经被成功地确定为一种治疗靶点，以缓解特应性皮炎患犬的瘙痒。在活跃性疾病中，皮炎的严重程度与IL-31循环水平呈正相关，在一群研究特应性皮炎比格犬和家养特应性皮炎犬中。虽然传统上认为IL-31在瘙痒症中发挥作用，但很明显，IL-31与包括表皮分化和免疫反应在内的更复杂的功能有关。角质形成细胞表达IL-31受体，IL-31能显著抑制丝聚合蛋白和角质形成细胞包膜形成等分化标志物的形成。与健康对照组相比，特应性皮炎患犬的角质形成细胞IL-31受体表达更高，使他们对这种细胞因子的影响非常敏感。

Discriminating between what is specific for AD and what is an indicator of generalized inflammation has been a limitation of many of the studies published, as the vast majority of them have compared normal and atopic individuals and not always AD with another inflammatory disease. In human medicine, psoriasis often is used for disease comparison; yet we do not have an equivalent canine inflammatory disease. Nevertheless, it is important to have a sense of how much of what we report is actually relevant to the pathogenesis of AD and how much is secondary “background noise”.

判断哪些是AD的特异性，哪些是全身性炎症的指标，这是许多已发表的研究的局限，因为绝大多数研究都比较了正常个体和特应性皮炎个体，并不总是比较AD与其他炎性疾病。在人医中，银屑病常被用于疾病比较，但我们没有类似的犬炎性疾病。然而，重点是要了解我们所报道的有多少与AD的发病机制有关，有多少是次要“背景噪音”。

Interleukin-17, a pro-inflammatory cytokine that links Tcell activation to neutrophil activation, also is increased in atopic dogs and could be another target for treatment. IL-17 activates canine keratinocytes to produce several pro-inflammatory cytokines and currently it is speculated to have an important role in the development of a Th2 response.

白细胞介素-17是一种将T细胞活化与中性粒细胞活化联系起来的促炎细胞因子，它在特应性皮炎犬上也增加，这可能是另一个治疗靶点。IL-17激活犬角质形成细胞产生多种促炎细胞因子，目前推测它在Th2反应的发展中发挥重要作用。

The link between keratinocyte-derived cytokines is demonstrated by increased expression of TSLP and IL-33 in canine atopic skin.Increased expression of IL-33 can have negative consequences on the function of tight junction proteins such as claudin. Thus inflammatory cytokines can modulate skin barrier function and exacerbate existing deficiencies.

犬特应性皮炎皮肤中TSLP和IL-33的表达增加证实了源于角质形成细胞的细胞因子之间的联系。IL-33表达的增加会对紧密连接蛋白(如闭合蛋白)的功能产生负面影响。因此，炎症细胞因子可以调节皮肤屏障功能，并加重现有的缺陷。

The production of one newly described cytokine, IL-34, in the skin is largely the responsibility of keratinocytes. IL-34 is believed to play a role in the differentiation and proliferation of Langerhans cells during steady states. Studies in humans have shown that IL-34 is largely expressed in the epidermis of normal individuals and nonlesional atopic skin, and decreased in the epidermis of lesional atopic skin. Instead, it is localized in the dermis, associated with myeloid dendritic cells and macrophages. Thus, IL-34 is believed to have a function in negatively influencing expansion of inflammation and current research is considering the possibility of boosting this cytokine as a possible way to prevent development of atopic lesions in people and mice. Other studies have reported on the correlation of increased serum levels of IL-34 with inflammatory states and autoimmune diseases.

一种新发现的细胞因子，IL-34，在皮肤中的产生主要是来自角质形成细胞。IL-34被认为在稳态中对郎格罕细胞的分化和增殖发挥作用。对人类的研究表明，IL-34主要在正常人和无皮肤病变特应性皮炎皮肤的表皮中表达，在特应性皮炎皮肤病变的表皮中表达减少。相反，它位于真皮层，与髓系树突状细胞和巨噬细胞相关。因此，IL-34被认为有抑制炎症发展，目前的研究正在考虑增加这种细胞因子的可能性，作为一种可能的方法，以防止人类和小鼠的特应性皮炎病变的发展。其他研究报道了血清IL-34水平升高与炎症状态和自身免疫性疾病的相关性。

A recent study in atopic dogs reported an increased expression of IL-34 in the serum as compared with healthy controls and furthermore that this correlated with disease severity. However, concentrations did not decrease after glucocorticoid or oclacitinib therapy. The authors postulated that this may be linked to IL-34 being produced by damaged keratinocytes rather than immune cells. No evaluation of IL-34 expression in the skin of atopic dogs has been published so far. Thus, the relevance of elevated serum IL-34 and whether it correlates with cutaneous expression is unknown at this time.

最近一项对特应性皮炎犬的研究报告，与健康对照组相比，血清中IL-34的表达增加，而且这与疾病的严重程度相关。然而，糖皮质激素或奥拉替尼治疗后，浓度并没有下降。作者推测，这可能与IL-34有关，IL-34是由受损的角质形成细胞而不是免疫细胞产生的。目前还没有关于IL-34在特应性皮炎犬皮肤中表达的评价。因此，目前尚不清楚血清IL-34升高的相关性以及是否与皮肤表达相关。

Macrophage migration inhibitory factor (MIF) also has been investigated as it is a pro-inflammatory cytokine that could be considered as target for therapy. It has been shown to play a role in human AD. MIF levels were found to be increased in atopic dogs, apparently not correlated with clinical scores nor pruritus, and thus the clinical relevance of these findings is unknown.

巨噬细胞迁移抑制因子(MIF)也有被研究，因为它是一种促炎细胞因子，可以被认为是治疗的靶点。它已被证明在人AD中发挥作用。MIF水平被发现在特应性皮炎犬中增加，显然与临床评分或瘙痒无关，因此这些发现的临床相关性是未知的。

Another recent study evaluating potentially useful peripheral markers showed decrease expression of micro-RNA gene markers (PIAS1, RORA and SH2B1) and an increased expression of the phosphodiesterase 4D gene in peripheral blood mononuclear cells in addition to differential expression of cytokines in dogs with AD in comparison to healthy controls. Again, the clinical relevance of these findings needs to be further evaluated to see how relevant these changes are for AD and how much they simply reflect a state of inflammation.

另一个最近的研究评估，与健康对照组相比，在AD犬外周血单核细胞除细胞因子以外的差异表达潜在作用，即使用外周标记显示微核糖核酸基因标记表达下降(PIAS1 RORA基因和SH2B1)和磷酸二酯酶4 d基因的表达增加。同样，需要进一步评估这些发现的临床相关性，以了解这些变化与AD的相关性，以及它们在多大程度上只是反映了炎症状态。

Summary and directions for the future

总结和未来方向

Our understanding of AD has greatly advanced in the last few years. Our view has expanded from considering cAD as a histamine-driven Type I hypersensitivity triggered by inhalant allergens to a very complex multifaceted syndrome in which the skin barrier is key. In this syndrome, multiple factors play a role and the skin itself seems to play a crucial role in orchestrating the immune response to epicutaneously absorbed allergens. Keratinocytes shape the immune response and affect the type of lymphocytic response that is promoted. Skin barrier impairments have been described in atopic dogs, yet more work is needed to understand what is a primary and what is a secondary consequence. Many cytokines have been explored as potential targets for future therapies in the hope of providing customized medicine to address the specific mediators that are relevant for specific subsets of patients.

在过去的几年里，我们对AD的理解有了很大的进步。我们的观点已经从认为cAD是由吸入性过敏原引发的组胺驱动的I型超敏反应，发展到一种非常复杂的多因素综合征，其中皮肤屏障是关键。在这种综合征中，多种因素起了作用，皮肤本身似乎在协调对表皮吸收的过敏原的免疫反应中发挥了关键作用。角质形成细胞形成免疫反应，并影响被促进的淋巴细胞反应的类型。皮肤屏障损伤在特应性皮炎犬已描述，但需要更多的工作来了解什么是主要的和什么是次要的后果。许多细胞因子已被探索为未来的潜在治疗靶点，希望提供针对性的药物，以解决特定亚型患者的特定介质。

Future studies should include control groups with nonatopic inflammatory diseases in addition to healthy controls to discriminate which changes are simply the consequence of nonspecific inflammation or which ones are specific signatures of atopic disease. Owing to the complexity of this syndrome, it is clear that our approach needs to be multimodal and customized to each patient as different pathways may be relevant to various patients. As our treatments may become more targeted, it is conceivable that one approach would work very well for some patients and not for all. Importantly, we need to be mindful about the modulation of the microbiome. Restoring microbial biodiversity is crucial for proper immune system and barrier function, and to minimize repeating cycles of new sensitizations and further disruption of the skin barrier.

在未来的研究中，除健康对照组外，还应包括非特应性皮炎的炎症对照组，以区分哪些变化只是非特应性皮炎的炎症结果，哪些变化是特应性皮炎的特定特征。由于这种综合征的复杂性，很明显，我们的方法需要多模式，并针对每个患者定制，因为不同的路径可能与不同的患者相关。由于我们的治疗可能变得更有针对性，可以想象的是，一种方法将对一些患者非常有效，但不是所有的患者都有效。重点是，我们需要注意微生物群的调节。恢复微生物多样性对于正常的免疫系统和屏障功能至关重要，并可最大限度地减少新的致敏过程和进一步破坏皮肤屏障的重复循环。

Ryohei

南海

阿茵

学学人家