To improve psoriasis care, diagnostic methods are needed that can facilitate personalized medicine. Such diagnostic method should be objective, accurate, cost-effective and easy-to-use for both patients and health-care professionals. Proteins, such as interleukins, chemokines, cell surface receptors and anti-microbial peptides drive the biological processes underlying both the physical and visual hallmarks of psoriatic skin. As such, this psoriasis ‘molecular footprint’ may be very suitable for the development of diagnostic methods that can monitor disease progression, as well as measure response to treatment. Particularly suitable may be proteins that can be assessed non-invasively from the skin surface (i.e. without disrupting the skin). A prerequisite is that skin surface molecules follow the state of disease like. Therefore, the primary aim of this study was to assess whether expression patterns of proteins known to be involved in psoriasis, and that can be measured non-invasively from the skin surface, correlate with physical and visual hallmarks of psoriatic skin.
The panel of IL-1α, IL-1RA, CXCL-1/2 and hBD-1 was arbitrarily chosen based on their role psoriasis, as reported in the literature, as well as because these proteins can be measured from the skin surface. IL-1α and IL-1RA are examples of a pro-inflammatory and anti-inflammatory interleukins, respectively, that are known to play important roles in skin homeostasis and skin inflammation, including psoriasis 36−40; 43. The combination of chemokines CXCL-1 and − 2 was chosen because of their roles in attracting neutrophils to psoriatic skin lesions 38, 41. The anti-microbial peptide hBD-1 was chosen as a representative of beta-defensins in psoriasis 44, 45. The choice for measuring skin surface proteins using FibroTx TAP was based on the fact that FibroTx TAP is a non-invasive sampling technology that does not affect skin, i.e. protein measurements are not biased by skin responding to the measurement method, and do not interfere with biological processes in and on the skin 40. To underline this, no adverse events were reported during FibroTx TAP measurements, neither on normal appearing skin nor on lesional skin, neither in patients nor in healthy individuals were reported, neither by visual assessment (e.g. signs of redness) or upon inquiry (e.g. irritation, itching, pain).
Expression patterns of IL-1α, IL-1RA, CXCL-1/2 and hBD-1 on the skin surface, as measured by FibroTx TAP, reflect reported protein expression patterns as assessed by more invasive technologies, such as mRNA analyses and immuno-histochemistry (IHC) using skin biopsies and protein-analyses after tape-stripping of the stratum corneum 38, 40, 42− 45. It appears thus that protein expression in the skin is reflected both qualitatively and quantitatively on the skin surface.
Importantly, the fact that we find some proteins, like IL-1RA, CXCL-1/2 and hBD-1, are present in higher amounts on lesional skin, whereas others, like IL-α, are found in reduced amounts on lesional skin in comparison with non-lesional and healthy skin, indicates that differences in proteins measured cannot simply be attributed to e.g. differences in skin texture, skin barrier function or amounts of dead cells on lesional skin. Instead, these differences rather indicate that amounts of proteins found on skin reflect regulation in the skin. This is supported by reports in the literature, describing an increase in IL-1RA, CXCL-1/2 and hBD-1, and a decrease in IL-1α, in psoriasis lesional skin in comparison with non-lesional skin, or skin or healthy individuals have been reported in the literature. Thus, it appears that non-invasive measurements of soluble proteins found on the skin, e.g. as measured by FibroTx TAP, both qualitatively and quantitatively correlate with proteins found in the skin, as measured by invasive methods such as immunohistochemistry and qPCR from skin biopsies 38; 40; 42−45.
The decrease in pro-inflammatory IL-1α and increase in anti-inflammatory IL-1RA levels detected on psoriasis plaques in comparison with non-lesional skin, may appear counter-intuitive at first. Also, using a skin-lavage technique, Portugal-Cohen et al found a clear increase in IL-1α on lesional skin in comparison with non-lesional skin, or skin of healthy individuals 23. In the literature, however, there is ample evidence that IL-1α is found in decreased levels, and IL-1RA in increased levels in psoriatic lesional skin in comparison with non-lesional skin 39, 42, 43. FibroTx TAP measurements of IL-1α and IL-1RA thus fit the bulk of evidence in the literature. The reason for the discrepancy between FibroTx TAP measurements and the observations of Portugal-Cohen is unclear, also because using a similar skin-lavage approach as Portugal-Cohen, we found the same pattern for IL-1α and IL-1RA as we found using FibroTx TAP 34.
Despite the very clear association between disease and expression patterns of IL-1α, IL-1RA, CXCL-1/2 and hBD-1, as evidenced by the statistically significant differences in expression of these proteins on non-lesional and lesional skin, no firm correlations could be established between IL-1α, IL-1RA, hBD-1or CXCL-1/2 and PASI scores of the patients in the present study. A simple conclusion is that measurements of analysed biomarkers on a single lesion, or the ratio between IL-1RA and IL-1α, may not be representative for ‘whole body’ diagnostic purposes. This, however, is contradicted by our observation that clinical scoring of a single lesion, either for redness or thickness, significantly correlated with PASI in our study. Rather, the lack of correlation between measurements of IL-1α, IL-1RA, CXCL-1/2 and hBD-1 and PASI may be explained by the lack of significant correlations with clinical assessment of redness, thickness and scaling of the same lesions, which are elements that comprise the PASI in addition to scoring other lesions and body-surface area affected by disease. Nevertheless, the patient cohort of current study was limited, and a study with larger cohort of patients is needed for firm conclusions.
Ultrasound measurements clearly showed a statistically significant thickening of epidermis, SLEB and dermis in lesional skin in comparison with non-lesional skin. Despite a similar trend for FibroTx TAP measurements of IL-1α, IL-1RA and hBD-1, no clear quantitative correlations could be found between protein measurements and ultrasound measurements of epidermis, SLEB or dermis, neither with respect to thickness nor to quality of individual skin layers. At least not with the small number of patients used. Mild positive correlation between CXCL-1/2 and SLEB thickness of lesional skin was observed. Interestingly, neither ultrasound measurements and visual assessments of lesional skin correlated in a highly statistically significant sense; only a mild correlation between skin thickness and SLEB thickness was observed, and thus it appears that visual -, ultrasound – and protein-measurements quantify disease intensity in their own sense.
To address if measurements of skin-surface IL-1α, IL-1RA, CXCL-1/2 and hBD-1 merely reflect disease in a qualitative state, i.e. inflamed or not-inflamed, or that these measurements reflect disease-intensity quantitatively, we followed patients during the course of short-wave UVB treatment. There were clear patterns of normalisation observed for IL-1RA and CXCL-1/2. This pattern was gradually, thus confirming that skin-surface measurements of these proteins can be used to assess psoriasis-intensity in a qualitative way. Skin-surface measurements of IL-1RA and CXCL-1/2 displayed a different pattern than achieved by visual scoring of local inflammation. Visual scores for redness, thickness and scaling decreased after 2 weeks of treatment, whereas IL-1RA and CXCL-1/2 normalized more gradually. This confirms that measuring the ‘molecular root’ of inflammation appears to have value as an objective, non-invasive biomarker measurement for scoring disease intensity on its own right. The difference in kinetics between IL-1α, unchanged during treatment, IL-1RA and CXCL-1/2, both changed albeit with different kinetics, suggest that changes in skin-surface proteins are not a uniform reflection of skin-healing, but rather reflect individual changes of expression in the skin.