**2. Proteases**

A number of different proteases and their inhibitors have been involved in the desquamation process and to contribute to the skin's barrier function. On the basis of the catalytic domain, proteases are classified into aspartate-, cysteine-, glutamate-, metallo-, serine- and threonine proteases. Particularly serine proteases (SP) have a prominent role in epidermal permeability barrier homeostasis, as acute barrier disruption increases SP-activity

Epidermal Serine Proteases and Their Inhibitors in Atopic Dermatitis 53

One of the most important players in epidermal barrier function is profilaggrin. It is processed at the stratum granulosum/stratum corneum interphase to filaggrin monomers. These are crosslinked to form macrofibrils and eventually "natural moisturing factors, NMFs), which are important for maintaining the hydration of the SC (reviewed in (Ovaere et al. 2009). The importance of profilaggrin proteolysis to maintain epidermal structure and hydration has been underscored by human genetic studies: These have shown that loss-offunction mutations in profilaggrin cause ichthyosis vulgaris and are strongly predisposing to atopic dermatitis and asthma, possibly due to a disturbed epidermal barrier function,

Major proteases required for initiating profilaggrin processing are the type II transmembrane serine protease matriptase and prostasin, a glycosylphosphatidylinositolanchored membrane serine protease. There is now evidence that the autoactivating protease matriptase acts upstream of prostasin in a zymogen activation cascade that regulates terminal epidermal differentiation and is required for prostasin zymogen activation (Netzel-Arnett et al. 2006). A reduced matriptase expression was shown to be associated with incomplete terminal differentiation of epidermis, epidermal appendages, and oral epithelium (Bugge et al. 2007). Matriptase gene mutations lead to ichthyosis as recently reported (Alef et al. 2008). Matriptase is immediately activated by exposure to an acidic pH, as it occurs in skin, suggesting that matriptase activation may be a direct response to proton exposure (Tseng et al. 2010). Recent evidence showed that during epidermal differentiation, the matriptase-prostasin proteolytic cascade is tightly regulated by two mechanisms, either by prostasin activation temporally coupled to matriptase autoactivation or by the hepatocyte growth factor activator-inhibitor-1(HAI-1), which is rapidly inhibiting not only active matriptase but also active prostasin, resulting in an extremely brief window of opportunity for both active matriptase and active prostasin to act on their substrates (Chen et al. 2010). So far these two membrane-bound proteases are thought to be mainly involved in skin homeostasis. However, the ability of matriptase to activate Kallikrein-related proteases in the skin points to a regulatory role of matriptase in

Kallikrein-related proteases (KLKs) are the largest family of tryptic and chymotryptic serine proteases, which are encoded by 15 genes on chromosome 19q13.4. In skin, KLKs are produced by keratinocytes of the stratum granulosum (SG), where they are released into interstices of the upper SG and lower SC. To date, SC serine protease activity is attributed to human tissue KLKs (Borgono et al. 2007). At least eight KLKs have been reported to be expressed in healthy skin, of which KLK5, KLK7, KLK8, and KLK14 seem to be the most important (reviewed in (Lundwall and Brattsand 2008)). Their putative function has been extensively studied (reviewed in (Eissa and Diamandis 2008; Lundwall and Brattsand 2008). A wealth of literature revealed proteolytic function of the two serine proteases, KLK5 and KLK7, in SC. These proteases, previously termed 'stratum corneum tryptic enzyme, SCTE' (KLK5) and 'stratum corneum chymotryptic enzyme, SCCE' (KLK7), have an important role in the desquamation process, as it was shown that serine protease inhibitors were able to inhibit corneocyte shedding from human plantar skin *ex vivo* (Lundstrom and Egelrud 1988). Both enzymes are maximally expressed in the stratum granulosum, where they are released from lamellar bodies and located within stratum corneum interstices. Here they are

which allows entry of allergens and infectious agents (Sandilands et al. 2006).

**2.2.1 Matriptase and prostasin** 

inflammatory skin diseases (Sales et al. 2010).

**2.2.2 Kallikrein-related peptidases** 

in skin and inhibition by topical SP-inhibitors accelerated recovery of barrier function after acute abrogation (Hachem et al. 2006).

#### **2.1 Cysteine- and aspartate-proteases**

Cysteine peptidases represent phylogenetically ubiquitous enzymes, which can be classified into clans of independent proteins (based on the structural organization of the active site). Two of the major clans in mammal genomes are the "CA" clan, where members share an evolutionary and structural history with papain, and the "CB" clan, which includes the caspases and the legumains.

One of the most skin-relevant caspases is the cysteinyl-aspartate protease caspase-14 (Demerjian et al. 2008). In contrast to other ubiquitously expressed members of the caspase family, caspase-14 is rather specifically expressed within the epidermis, where it is of high importance in the formation of the physical skin barrier. It is expressed in keratinocytes of the uppermost stratum granulosum, where it was found to be associated with the nucleus, the keratohyalin granules and the desmosomes. Although its localization suggested a role for nuclear degradation during cornification, in caspase-14-deficient mice nuclear degradation was not affected. The observation that caspase-14 has only been found in terrestrial mammals but not in birds or reptiles and that profilaggrin is a direct substrate of caspase 14, suggests that it is important for formation of a soft stratum corneum. This could indicate a co-evolution of a soft SC and the caspase-14 gene (Denecker et al. 2007). Caspase-14 is produced as procaspase within the stratum granulosum, where it maturates in cornifying epithelia. Although it is not clear how it maturates, most likely a serine protease with elastase-like properties could be involved (Denecker et al. 2008). Thus, caspase-14 seems to be involved in the correct processing of profilaggrin, preceding its degradation into hygroscopic amino acids as well as the formation of UVB-protective compounds.

Another skin cysteine peptidase is cathepsin C, which represents a lysosomal cysteine peptidase of the papain family CIA, which is important for intracellular degradation and which has a role in the activation of serine proteases in immune cells (Rao et al. 1997). Cathepsin C knockout mice indicated that activation and processing of granzymes A and B, which are important for T-cell-mediated cell-killing, depends on cathepsin C (Pham and Ley 1999). Interestingly, Cathepsin C deficiency in humans leads to a dramatic reduction of both, levels and activities of the neutrophil serine proteases elastase, protease-3 and cathepsin G (Pham et al. 2004), which will be described below.

Cathepsin D represents the main aspartic protease of endolysosomes. It is active at the physiological acidic pH of healthy skin and is of relevance in the desquamation process (Horikoshi et al. 1999). Cathepsin D knockout mice showed reduced levels of involucrin and loricrin and lower transglutaminase 1 activity, which indicates that cathepsin D contributes indirectly to the barrier function of human skin (Egberts et al. 2004).

#### **2.2 Serine proteases**

Serine proteases represent a family of enzymes which use a catalytic triad in the substratebinding pocket (Ser, His, Asp) to cleave peptide bonds. On the basis of their substrate specificity these proteases can be subdivided into trypsin-like enzymes (cleaves Cterminally of Arg and Lys), chymotryptic enzymes (cleave behind aromatic or bulky, hydrophobic amino acids, and the elastase-like enzymes (cleave behind small or medium size non-polar amino acids. These enzymes play an important role in the terminal differentiation process and desquamation.

#### **2.2.1 Matriptase and prostasin**

52 Atopic Dermatitis – Disease Etiology and Clinical Management

in skin and inhibition by topical SP-inhibitors accelerated recovery of barrier function after

Cysteine peptidases represent phylogenetically ubiquitous enzymes, which can be classified into clans of independent proteins (based on the structural organization of the active site). Two of the major clans in mammal genomes are the "CA" clan, where members share an evolutionary and structural history with papain, and the "CB" clan, which includes the

One of the most skin-relevant caspases is the cysteinyl-aspartate protease caspase-14 (Demerjian et al. 2008). In contrast to other ubiquitously expressed members of the caspase family, caspase-14 is rather specifically expressed within the epidermis, where it is of high importance in the formation of the physical skin barrier. It is expressed in keratinocytes of the uppermost stratum granulosum, where it was found to be associated with the nucleus, the keratohyalin granules and the desmosomes. Although its localization suggested a role for nuclear degradation during cornification, in caspase-14-deficient mice nuclear degradation was not affected. The observation that caspase-14 has only been found in terrestrial mammals but not in birds or reptiles and that profilaggrin is a direct substrate of caspase 14, suggests that it is important for formation of a soft stratum corneum. This could indicate a co-evolution of a soft SC and the caspase-14 gene (Denecker et al. 2007). Caspase-14 is produced as procaspase within the stratum granulosum, where it maturates in cornifying epithelia. Although it is not clear how it maturates, most likely a serine protease with elastase-like properties could be involved (Denecker et al. 2008). Thus, caspase-14 seems to be involved in the correct processing of profilaggrin, preceding its degradation into

hygroscopic amino acids as well as the formation of UVB-protective compounds.

(Pham et al. 2004), which will be described below.

differentiation process and desquamation.

**2.2 Serine proteases** 

indirectly to the barrier function of human skin (Egberts et al. 2004).

Another skin cysteine peptidase is cathepsin C, which represents a lysosomal cysteine peptidase of the papain family CIA, which is important for intracellular degradation and which has a role in the activation of serine proteases in immune cells (Rao et al. 1997). Cathepsin C knockout mice indicated that activation and processing of granzymes A and B, which are important for T-cell-mediated cell-killing, depends on cathepsin C (Pham and Ley 1999). Interestingly, Cathepsin C deficiency in humans leads to a dramatic reduction of both, levels and activities of the neutrophil serine proteases elastase, protease-3 and cathepsin G

Cathepsin D represents the main aspartic protease of endolysosomes. It is active at the physiological acidic pH of healthy skin and is of relevance in the desquamation process (Horikoshi et al. 1999). Cathepsin D knockout mice showed reduced levels of involucrin and loricrin and lower transglutaminase 1 activity, which indicates that cathepsin D contributes

Serine proteases represent a family of enzymes which use a catalytic triad in the substratebinding pocket (Ser, His, Asp) to cleave peptide bonds. On the basis of their substrate specificity these proteases can be subdivided into trypsin-like enzymes (cleaves Cterminally of Arg and Lys), chymotryptic enzymes (cleave behind aromatic or bulky, hydrophobic amino acids, and the elastase-like enzymes (cleave behind small or medium size non-polar amino acids. These enzymes play an important role in the terminal

acute abrogation (Hachem et al. 2006).

**2.1 Cysteine- and aspartate-proteases** 

caspases and the legumains.

One of the most important players in epidermal barrier function is profilaggrin. It is processed at the stratum granulosum/stratum corneum interphase to filaggrin monomers. These are crosslinked to form macrofibrils and eventually "natural moisturing factors, NMFs), which are important for maintaining the hydration of the SC (reviewed in (Ovaere et al. 2009). The importance of profilaggrin proteolysis to maintain epidermal structure and hydration has been underscored by human genetic studies: These have shown that loss-offunction mutations in profilaggrin cause ichthyosis vulgaris and are strongly predisposing to atopic dermatitis and asthma, possibly due to a disturbed epidermal barrier function, which allows entry of allergens and infectious agents (Sandilands et al. 2006).

Major proteases required for initiating profilaggrin processing are the type II transmembrane serine protease matriptase and prostasin, a glycosylphosphatidylinositolanchored membrane serine protease. There is now evidence that the autoactivating protease matriptase acts upstream of prostasin in a zymogen activation cascade that regulates terminal epidermal differentiation and is required for prostasin zymogen activation (Netzel-Arnett et al. 2006). A reduced matriptase expression was shown to be associated with incomplete terminal differentiation of epidermis, epidermal appendages, and oral epithelium (Bugge et al. 2007). Matriptase gene mutations lead to ichthyosis as recently reported (Alef et al. 2008). Matriptase is immediately activated by exposure to an acidic pH, as it occurs in skin, suggesting that matriptase activation may be a direct response to proton exposure (Tseng et al. 2010). Recent evidence showed that during epidermal differentiation, the matriptase-prostasin proteolytic cascade is tightly regulated by two mechanisms, either by prostasin activation temporally coupled to matriptase autoactivation or by the hepatocyte growth factor activator-inhibitor-1(HAI-1), which is rapidly inhibiting not only active matriptase but also active prostasin, resulting in an extremely brief window of opportunity for both active matriptase and active prostasin to act on their substrates (Chen et al. 2010). So far these two membrane-bound proteases are thought to be mainly involved in skin homeostasis. However, the ability of matriptase to activate Kallikrein-related proteases in the skin points to a regulatory role of matriptase in inflammatory skin diseases (Sales et al. 2010).

#### **2.2.2 Kallikrein-related peptidases**

Kallikrein-related proteases (KLKs) are the largest family of tryptic and chymotryptic serine proteases, which are encoded by 15 genes on chromosome 19q13.4. In skin, KLKs are produced by keratinocytes of the stratum granulosum (SG), where they are released into interstices of the upper SG and lower SC. To date, SC serine protease activity is attributed to human tissue KLKs (Borgono et al. 2007). At least eight KLKs have been reported to be expressed in healthy skin, of which KLK5, KLK7, KLK8, and KLK14 seem to be the most important (reviewed in (Lundwall and Brattsand 2008)). Their putative function has been extensively studied (reviewed in (Eissa and Diamandis 2008; Lundwall and Brattsand 2008). A wealth of literature revealed proteolytic function of the two serine proteases, KLK5 and KLK7, in SC. These proteases, previously termed 'stratum corneum tryptic enzyme, SCTE' (KLK5) and 'stratum corneum chymotryptic enzyme, SCCE' (KLK7), have an important role in the desquamation process, as it was shown that serine protease inhibitors were able to inhibit corneocyte shedding from human plantar skin *ex vivo* (Lundstrom and Egelrud 1988). Both enzymes are maximally expressed in the stratum granulosum, where they are released from lamellar bodies and located within stratum corneum interstices. Here they are

Epidermal Serine Proteases and Their Inhibitors in Atopic Dermatitis 55

Proteolysis is also a vital element for survival of parasites, which enables them to digest resistant structural proteins. E. g. house dust mites (*Dermatophagoides pteronyssinus* and *D. farinae*) produce cysteine proteases and serine proteases (Donnelly et al. 2006), which are well known as 'group 1 house dust mite allergens' to induce allergic reactions. Several reports have shown that these proteases interact with pathways of the innate defense system

Upon skin infection or at conditions causing 'neutrophilic dermatoses', the primary cell infiltrate consists of neutrophils. A massive infiltrate in the epidermis can lead to pustule formation. Upon infection, neutrophils phagocytose microbes and then kill these microbes within the phagolysosome by oxygen-radical-generating systems, the alpha-defensins as well as proteases which are released from primary ('azurophilic') as well as secundary ('specific') granules (Faurschou and Borregaard 2003). Only primary granules contain high amounts of the serine proteases human leukocyte elastase (HLE), cathepsin G and protease 3 (PR3). These enzymes are not released upon phagocytosis. But upon 'frustrating phagocytosis' (attempts to phagocytose particles, which are bigger than leukocytes) as well as formation of "neutrophil extracellular traps" (NETs) consisting of neutrophil-derived DNA, where these cationic enzymes are bound (Brinkmann et al. 2004), a release of these enzymes can occur. Indeed, HLE activity is present at the surface of lesional skin of patients with psoriasis, a neutrophilic dermatosis (Wiedow et al. 1992). Neutrophil serine proteases have been identified as important innate immune regulators (Meyer-Hoffert 2009; Meyer-Hoffert and Wiedow 2010). Thus, neutrophil-derived enzymes may further determine the outcome of an inflammatory skin lesion – independent of possible homeostasis of

Proteolytic activity in the skin, which is often restricted to a few target proteins, its tissue localization and its enzymatic activity, needs to be properly controlled in the tissue. Although gene expression and zymogen-activation are important regulatory elements to restrict enzymatic activity, the most important one is the expression of more or less specific protease inhibitors within the skin. These inhibitors regulate more or less proteasespecifically in a timely and concentration-dependent fashion the activity of diverse proteases. This review will summarize the current knowledge on the most important

The 'lympho-epithelial Kazal-type related inhibitor' (LEKTI, today named LEKTI-1) is an effective inhibitor of multiple serine proteases (Roelandt et al. 2009). Processing of this multidomain protease inhibitor into fragments or single domains restricts the inhibitory properties to serine proteases such as trypsin, plasmin, subtilisin A, cathepsin G and human neutrophil elastase. LEKTI-1 consists of 15 complete or incomplete Kazal domains. *In vitr*o, recombinant LEKTI-1 fragments or single domains inhibit the keratinocyte-derived serine proteases KLK5, -6, -7, -13 and -14. LEKTI-1 is expressed in various stratified epithelia as three splice variants. In the epidermis LEKTI-1 is expressed in the stratum granulosum, where LEKTI-1 protein is located in lamellar bodies – separate from KLKs, but secreted into the

suggesting that these might be also directly involved in inflammatory skin reactions.

**2.4 Neutrophil serine proteases** 

**3. Protease inhibitors** 

epithelial protease-inhibitors.

**3.1 Kazal-type-realted protease inhibitors** 

keratinocyte-derived proteases and protease-inhibitors.

thought to form a proteolytic cascade in which KLK5 activates itself as well as KLK7 (Ovaere et al. 2009). Once active, both enzymes are believed to digest *in vivo* corneodesmosin, DSG1 and desmocollin-1, as these substrates have been shown to be digested *in vitro*. There is now evidence that also other KLKs participate in desquamation: It was recently shown, that KLK14 is responsible for 50% of the total trypsin-like serine protease activity in the stratum corneum. Because KLK14 can activate and be activated by KLK5, it is very likely that it also participates in the cascade pathway.

Apart from these three KLKs also KLK8 seems to be involved in a proteolytic activation cascade regulating skin desquamation: KLK8 is abundantly expressed and co-localized with other KLKs in human epidermis and sweat glands. It is also transported and exocytosed by lamellar bodies into the stratum granulosum/stratum corneum interface and thus may play a role in SC barrier functions. Very recent studies showed that recombinant KLK8 is optimally active at pH 8.5 suggesting that it plays a role in the upper stratum granulosum where the pH is rather neutral (Eissa et al. 2011). Active KLK8 has been found in SC extracts and in sweat, where until recently only KLK1 and kininase II were identified as active serine proteases. This raises its potential functional involvement in skin desquamation, although the physiological substrates need to be identified.

#### **2.3 Proteases of bacterial, fungal and parasite origin**

Apart from proteases produced by keratinocytes during differentiation and desquamation, the stratum corneum might also contain extracellular proteases originating from microbes and/or parasites residing at the skin surface. Bacterial proteases are often accessory proteins which are not fundamental for cell growth and division, but are considered to be virulence factors, which are often associated with mobile genetic elements such as plasmids, integrated phages and pathogenic islands. The clustering of bacterial protease genes in operons allows their coordinated expression, which in turn may imply a cooperation of the produced proteins (Wladyka and Pustelny 2008).

*Staphylococci* produce a number of extracellular proteases, among them epidermolytic toxins, staphylococcal serine proteases like a glutamyl endopeptidase referred to as V8, a cysteine protease in *S. aureus*. Similar and other proteases are produced by skin-relevant bacteria, including commensal *S. epidermidis*, *Streptococcus pyogenes*, *Pseudomonas aeruginosa* and others (Wladyka and Pustelny 2008). Among the proteases, serine proteases, cysteine proteases and metalloproteases represent the most abundant bacterial proteases. These affect the host's innate immune system in a bacterial species-specific manner by targeting phagocytes, cytokines and cytokine receptors, inflammatory signaling pathways, complement, contact activation as well as antimicrobial peptides (Potempa and Pike 2009).

Apart from bacteria also fungi represent an important source of proteases. Upon fungal infections, which are mostly seen at mucosal surfaces, *Candida albicans* represents the most common fungal pathogen. *Candida* species are ubiquitous commensal yeasts that reside as part of the normal mucosa microflora without causing infections. Suitable predisposing conditions will let *C. albicans* change to a 'pathogenic'stage, in which proteases represent major virulence factors. These are exclusively secreted aspartyl proteases (SAPs) (Naglik et al. 2004). It is believed that extracellular proteases of saprophytic microorganisms are primarily secreted to get nutrients from decomposition of complex materials. There is, however, strong evidence, that SAPs are also needed for invasion of the host, thereby interacting with several important host defense functions eventually causing inflammation (Naglik et al. 2004).

Proteolysis is also a vital element for survival of parasites, which enables them to digest resistant structural proteins. E. g. house dust mites (*Dermatophagoides pteronyssinus* and *D. farinae*) produce cysteine proteases and serine proteases (Donnelly et al. 2006), which are well known as 'group 1 house dust mite allergens' to induce allergic reactions. Several reports have shown that these proteases interact with pathways of the innate defense system suggesting that these might be also directly involved in inflammatory skin reactions.

### **2.4 Neutrophil serine proteases**

54 Atopic Dermatitis – Disease Etiology and Clinical Management

thought to form a proteolytic cascade in which KLK5 activates itself as well as KLK7 (Ovaere et al. 2009). Once active, both enzymes are believed to digest *in vivo* corneodesmosin, DSG1 and desmocollin-1, as these substrates have been shown to be digested *in vitro*. There is now evidence that also other KLKs participate in desquamation: It was recently shown, that KLK14 is responsible for 50% of the total trypsin-like serine protease activity in the stratum corneum. Because KLK14 can activate and be activated by

Apart from these three KLKs also KLK8 seems to be involved in a proteolytic activation cascade regulating skin desquamation: KLK8 is abundantly expressed and co-localized with other KLKs in human epidermis and sweat glands. It is also transported and exocytosed by lamellar bodies into the stratum granulosum/stratum corneum interface and thus may play a role in SC barrier functions. Very recent studies showed that recombinant KLK8 is optimally active at pH 8.5 suggesting that it plays a role in the upper stratum granulosum where the pH is rather neutral (Eissa et al. 2011). Active KLK8 has been found in SC extracts and in sweat, where until recently only KLK1 and kininase II were identified as active serine proteases. This raises its potential functional involvement in skin desquamation, although

Apart from proteases produced by keratinocytes during differentiation and desquamation, the stratum corneum might also contain extracellular proteases originating from microbes and/or parasites residing at the skin surface. Bacterial proteases are often accessory proteins which are not fundamental for cell growth and division, but are considered to be virulence factors, which are often associated with mobile genetic elements such as plasmids, integrated phages and pathogenic islands. The clustering of bacterial protease genes in operons allows their coordinated expression, which in turn may imply a cooperation of the

*Staphylococci* produce a number of extracellular proteases, among them epidermolytic toxins, staphylococcal serine proteases like a glutamyl endopeptidase referred to as V8, a cysteine protease in *S. aureus*. Similar and other proteases are produced by skin-relevant bacteria, including commensal *S. epidermidis*, *Streptococcus pyogenes*, *Pseudomonas aeruginosa* and others (Wladyka and Pustelny 2008). Among the proteases, serine proteases, cysteine proteases and metalloproteases represent the most abundant bacterial proteases. These affect the host's innate immune system in a bacterial species-specific manner by targeting phagocytes, cytokines and cytokine receptors, inflammatory signaling pathways, complement, contact activation as well as antimicrobial peptides (Potempa and Pike 2009). Apart from bacteria also fungi represent an important source of proteases. Upon fungal infections, which are mostly seen at mucosal surfaces, *Candida albicans* represents the most common fungal pathogen. *Candida* species are ubiquitous commensal yeasts that reside as part of the normal mucosa microflora without causing infections. Suitable predisposing conditions will let *C. albicans* change to a 'pathogenic'stage, in which proteases represent major virulence factors. These are exclusively secreted aspartyl proteases (SAPs) (Naglik et al. 2004). It is believed that extracellular proteases of saprophytic microorganisms are primarily secreted to get nutrients from decomposition of complex materials. There is, however, strong evidence, that SAPs are also needed for invasion of the host, thereby interacting with several important host defense functions eventually causing

KLK5, it is very likely that it also participates in the cascade pathway.

the physiological substrates need to be identified.

produced proteins (Wladyka and Pustelny 2008).

inflammation (Naglik et al. 2004).

**2.3 Proteases of bacterial, fungal and parasite origin** 

Upon skin infection or at conditions causing 'neutrophilic dermatoses', the primary cell infiltrate consists of neutrophils. A massive infiltrate in the epidermis can lead to pustule formation. Upon infection, neutrophils phagocytose microbes and then kill these microbes within the phagolysosome by oxygen-radical-generating systems, the alpha-defensins as well as proteases which are released from primary ('azurophilic') as well as secundary ('specific') granules (Faurschou and Borregaard 2003). Only primary granules contain high amounts of the serine proteases human leukocyte elastase (HLE), cathepsin G and protease 3 (PR3). These enzymes are not released upon phagocytosis. But upon 'frustrating phagocytosis' (attempts to phagocytose particles, which are bigger than leukocytes) as well as formation of "neutrophil extracellular traps" (NETs) consisting of neutrophil-derived DNA, where these cationic enzymes are bound (Brinkmann et al. 2004), a release of these enzymes can occur. Indeed, HLE activity is present at the surface of lesional skin of patients with psoriasis, a neutrophilic dermatosis (Wiedow et al. 1992). Neutrophil serine proteases have been identified as important innate immune regulators (Meyer-Hoffert 2009; Meyer-Hoffert and Wiedow 2010). Thus, neutrophil-derived enzymes may further determine the outcome of an inflammatory skin lesion – independent of possible homeostasis of keratinocyte-derived proteases and protease-inhibitors.
