A dysregulation of the innate immune system in rosacea would unify current clinical observations. In innate immunity, the pattern recognition system, which includes the TLR (Toll-like receptor) and NLR (nucleotide-binding domain and leucine-rich repeat-containing) families, respond to environmental stimuli such as UV, microbes, and physical and chemical trauma. Triggering the innate immune system normally leads to a controlled increase in cytokines and antimicrobial molecules in the skin [56, 79]. One of these antimicrobial molecules is a peptide known as cathelicidin antimicrobial peptide (CAMP ) [30]. Some forms of CAMP were known to have a unique capacity to be both vasoactive and pro-inflammatory. Therefore, given the potential for a single molecule to affect both of the events that describe rosacea, analyses of cathelicidin in rosacea were conducted. Individuals with rosacea expressed abnormally high levels of cathelicidin [88]. Note that the cathelicidin peptides found in rosacea were not only more abundant but were different from those in normal individuals. These forms of CAMP promote and regulate leukocyte chemotaxis [24], angiogenesis [49], and expression of extracellular matrix components [34]. The presence of the vasoactive and inflammatory CAMP in rosacea was subsequently explained by abnormal production of local protease kallikrein 5 (KLK5) , which controls the production of CAMP in epidermis [88, 90]. To confirm the importance of these observations and to test the hypothesis that abnormal CAMP could induce the signs of rosacea, these abnormal CAMP or the CAMP-producing enzymes were injected into the mouse skin. This rapidly resulted in skin inflammation resembling pathological changes in rosacea [88]. Azelaic acid is topically used for rosacea treatment and decreases CAMP and KLK5 in keratinocyte in vitro and mouse skin and rosacea patient in vivo studies, which proved why azelaic acid has benefit for rosacea [20]. Combined, these findings indicated that an exacerbated innate immune response induces abnormal CAMP, and that this then leads to the clinical findings of rosacea.

The innate immune system of the skin is programmed to detect microbes, tissue damage such as UV-induced apoptosis, or damage of the extracellular matrix [19, 80]. Sun exposure, dermal matrix changes, and microbes have been recognized as triggers and exacerbating factors of rosacea. Along with the aberrant CAMP expression, TLR2 expression is altered in rosacea skin [89]. Increase in TLR2 enhances skin susceptibility to innate immune stimuli and TLR2 stimuli lead to increased cathelicidin and kallikrein production from epidermal keratinocytes [67, 89]. Hence, combination of high cathelicidin and KLK5 induced by TLR2 generate aberrant and abnormal CAMP in rosacea. Interestingly, TLR2 involvement is also suggested in other disorders with resemblance to rosacea. Perioral dermatitis and glucocorticoid-inducing rosacea-like dermatitis are adverse events by topical steroid use on the face [22, 73, 83, 86]. Although the precise molecular mechanisms of the steroid-induced dermatitis is not determined, Shibata et al. reported that glucocorticoid increases TLR2 expression in epidermal keratinocytes, and that P. acnes enhanced glucocorticoid-dependent TLR2 induction and cytokine production [70]. In acne vulgaris, TLR2-positive cells are rapidly recruited in the early lesion [48]. Thus, findings and accumulated knowledge on rosacea and dermatoses resembling rosacea in skin manifestations suggest the innate immune response in rosacea has gone awry. For these reasons the events worsening rosacea trigger innate immunity, and rosacea patients are more susceptible to environmental stimuli that do not cause reactions in normal patients.

Most rosacea patients have telangiectasia and flushing episodes, thus leading to a common hypothesis that vascular hyperreactivity and increased blood flow play a role in the susceptibility to this disease. Studies have demonstrated a measurable increase in blood flow in skin lesions of patients with rosacea [39, 71]. Some factors that trigger flushing such as emotional stress, spicy food, hot beverages, high environmental temperatures, and menopause worsen rosacea [16], thus supporting the hypothesis of vascular hyperreactivity in rosacea. Resolution of erythema and flushing by topical α-adrenergic receptor agonist application also supports the hypothesis that vascular hyperreactivity is major factor of rosacea pathology [33, 76].

Elevated expression of vascular endothelial growth factor (VEGF) , CD31 , and lymphatic endothelium maker D2-40 are observed in the skin of patients with rosacea [37]. VEGF proliferate vascular endothelial cells as well as increase permeability of vessels. CD31 is platelet/endothelial cell adhesion molecule (PECAM1 in gene symbol), and anti-CD31 antibody recognizes the endothelial cells. Anti-D2-40 monoclonal antibody identifies a 40 kDa O-linked sialoglycoprotein and has also been demonstrated to label lymphatic endothelium but not vascular endothelium. Thus, elevated expression of VEGF, CD31, and D2-40 in rosacea demonstrates rosacea skins have more stimulants to increase vascular and lymphatic endothelial cells.

Sun and UV exposure exacerbate rosacea, and UV irradiation induces VEGF in human keratinocytes and skin [15]. UVB induces cutaneous angiogenesis that is histologically similar to the telangiectasia seen in rosacea histopathology [11]. In skin, epidermal keratinocytes are a major source of angiogenic factor VEGF (vascular endothelial growth factor) and FGF2 (fibroblast growth factor 2, also known as basic FGF) [9, 27]. UVB increases VEGF and FGF2 secretion from human keratinocytes and expression in mouse epidermis [11, 15, 53].

From the aspect of innate immunity, cathelicidin is one of the triggers of hypervascularity in rosacea. Injection of CAMP LL37 in mouse skin induced vasodilatation [88]. Application of LL37 resulted in neovascularization in a rabbit model of hind-limb ischemia, and the angiogenesis by LL37 is mediated by formyl peptide receptor-like 1 (FPRL1) , a G-protein coupled receptor expressed on endothelial cells [49]. LL37 also transactivates epidermal growth factor receptor (EGFR) and downstream signaling in epithelial cells [65, 81]. EGFR signaling induces VEGF in epidermal keratinocytes [27]. Thus, cathelicidin induces endothelial cell changes through several signaling pathways; directly to endothelial cells and indirectly through keratinocyte activation.

Reactive oxygen species (ROS) has been discussed in rosacea pathology and served as the molecular explanation of why rosacea medications are effective. Tetracyclines, azelaic acid, metronidazole, and retinoids inhibit ROS generation in neutrophils [3, 4, 58, 92]. The molecular action of these medicines used for rosacea provoked the hypothesis of ROS involvement in rosacea pathology. Erythromycin and azithromycin, the other effective medicine for rosacea treatment, have been shown to have antioxidant effects [8, 40]. ROS levels were examined in skin biopsy samples from rosacea and healthy individuals, and confirmed higher ROS activity in rosacea lesional skin than healthy controls [8, 60]. The decrease of ROS in rosacea skin was also observed after azithromycin treatment [8], suggesting rosacea treatments affect ROS activity and supporting the hypothesis of ROS involvement in rosacea pathology.

Although the precise localization of ROS is not determined in rosacea skin, UV radiation generates ROS and activates cellular signaling in keratinocytes [61, 62]. ROS is a mediator of innate immune signaling and activates cellular signaling and induces chemokine production by TLR2 in monocytes [51, 91] and cytokine production by TNFα in human keratinocytes [93]. ROS simulates fibroblast and alters matrix metalloproteinases (MMP) and the tissue inhibitor of metalloproteinase (TIMP) expression. UVA radiation increased MMP-1, and ROS increased MMP-2 mRNA and suppressed TIMP-1 in human dermal fibroblast [46]. In a three-dimensional culture, normal human dermal fibroblasts increased MMP-1 and MMP-2 mRNA expression by ROS, whereas both proalpha1(I) and proalpha1(III) collagen mRNA production were suppressed by ROS. Thus, ROS production by innate immune stimuli and UV irradiation caused vascular and dermal matrix damage via upregulation of matrix metalloproteinases [59, 66, 87]. Increased ROS activity in rosacea skin would enhance inflammatory reactions by abnormal and damaged dermal matrix, which may permit accumulation and prolonged retention of inflammatory cells, cytokines, and chemokines.

UV and sun exposure are known to cause a flushing response and appear to worsen the clinical symptoms of rosacea [16]. As discussed above, UV irradiation could induce erythema in the skin by increasing expression of angiogenic factors and by degenerating extracellular matrix. Innate immunity is also involved in UV-mediated cytokine and matrix metalloproteinase expression in keratinocytes. Myeloid differentiation factor 88 (MyD88), an essential adaptor molecule for Toll-like receptors (TLR) family signaling, increases expression in UV-irradiated human culture keratinocytes as well as photoaged human skin [52]. Overexpression of dominant negative form of MyD88 prevented UV-induced expressions of IL-6 and MMP-1 in human keratinocytes, whereas overexpression of dominant positive form of MyD88 increased IL-6 and MMP-1 expression. Combined with ROS involvement in chemokine production by TLR2 stimuli [51, 91], TLRs/MyD88 signaling would be part of the link between UV irradiation to skin inflammation. The future studies of photoaging in animals lacking TLRs signaling molecules will be of great interest.

MMPs digest dermal matrices such as collagens, fibronectin, elastin, and the like, and balances between MMPs and their inhibitor TIMPs dictate dermal components and vascular remodeling [44]. MMP2 and MMP9 increase was immunohistochemically observed in granulomatous rosacea [41], and MMP-8 (collagenase 2) and MMP-9 (gelatinase B) activities are higher in the fluid of ocular rosacea than in normal subjects [2, 54, 74]. MMPs are inducible by UV irradiation and ROS stimulation in keratinocytes and fibroblasts [46, 52]. The supports of protease involvement in rosacea pathology are the evidence that tetracyclines, which are effective for rosacea treatment, inhibit several matrix metalloproteinases (MMP) and serine proteases [1, 64, 75]. Thus, the effective rosacea treatments might be partially dependent on their antiprotease properties.

In vitro study showed that doxycycline inhibits keratinocyte MMP and in turn inhibits keratinocyte proteolytic activity of tryptic kallikrein-related peptidases, which leads to cathelicidin activation [45]. Serine protease kallikrein 5 (KLK5, also known as stratum corneum tryptic enzyme SCTE ) is a representative tryptic kallikrein-related peptidase in keratinocytes. KLK5 is the processing enzyme of cathelicidin, and high KLK5 expression and hyperprotease activity are observed in rosacea skin, which is inducible by TLR2 stimuli [88, 90]. KLK5 expresses in the upper epidermis (granular to cornfield cell layer) in normal skin, and rosacea skin expresses KLK5 in the entire epidermis. KLK5 also digest corneodesmosome proteins desmocollin 1 and desmoglein 1 in epidermis and are supposed to affect desquamation of epidermal keratinocytes [18, 26]. Hence high KLK5 and serine protease activity may increase skin sensitivity by enhanced desquamation of the cornfield layer. KLK5 also efficiently digest the extracellular matrix components including collagens type I, II, III, and IV, fibronectin, and laminin [57]. Considering the high KLK5 expression in basal cells of rosacea epidermis, KLK5 would have roles in skin inflammatory reactions in rosacea by affecting dermal matrix and vascular remodeling.

Two microbes have been discussed in rosacea pathology: Demodex folliculorum and Helicobacter pylori. D. folliculorum, a mite that lives within sebaceous follicles, has been implicated as a trigger of rosacea since histological studies revealed inflammation of the pilosebaceous follicle units. Studies have shown increased density of the mites in patients with rosacea compared with control patients [14, 17, 31, 32]. Lacey et al. isolated Bacillus oleronius from D. folliculorum and identified the antigens reacting to sera from rosacea individuals but not from control individuals [50]. The extracts of the B. oleronius stimulate proliferation of mononuclear cells from patients with rosacea suggesting that rosacea individuals are exposed to the B. oleronius molecules and that B. oleronius from D. folliculorum induces inflammation in rosacea. Interestingly, they identified heat shock proteins (HSP) and a lipoprotein in the antigenic molecules of B. oleronius. HSP and lipoproteins from microbes are also known to be stimulants for TLRs [21, 36]. This report supports the hypothesis of mite and pilosebaceaous unit involvement in rosacea. Further studies are required to examine if these B. oleronius molecules evoke an innate immune reaction or if rosacea is caused by an adaptive immune reaction against B. oleronius and D. folliculorum.

Correlation of H. pylori infection and rosacea is controversial and inconsistent in clinical observation [5, 28, 43, 63, 78]. Several reports showed seropositivity to H. pylori in rosacea individuals [38]. Eradication therapy for gastric H. pylori infection showed a preferable outcome for rosacea symptoms though it is not clear if the improvement of rosacea is due to H. pylori eradication [12, 35, 82]. H. pylori produces ROS [7, 29, 55] and rosacea individuals showed higher ROS including NO (nitric oxide) in plasma than controls [10, 38]. H. pylori induces cytokine release through TLR2 and TLR4 in gastric epithelial cells [47, 72]. Thus ROS and cytokines released by TLRs stimuli in organs other than skin may be mediators that worsen rosacea by H. pylori infection. However, concrete molecular evidence is still required to support the involvement of H. pylori in rosacea pathology.