Supporting Information Table S1 Primers for qPCR Found at: doi:10.1371/journal.pone.0004725.s001 (0.03 MB DOC) Figure S1 Diffusion model Found at: doi:10.1371/journal.pone.0004725.s002 (0.35 MB DOC) Text S1 Calculation of hBD-2 mass transport in reconstructed skin model Found at: doi:10.1371/journal.pone.0004725.s003 (0.03 MB DOC) ; Background Previous studies have extensively documented antimicrobial and chemotactic activities of beta-defensins. Human beta-defensin-2 (hBD-2) is strongly expressed in lesional psoriatic epidermis, and recently we have shown that high beta-defensin genomic copy number is associated with psoriasis susceptibility. It is not known, however, if biologically and pathophysiologically relevant concentrations of hBD-2 protein are present in vivo, which could support an antimicrobial and proinflammatory role of beta-defensins in lesional psoriatic epidermis. Methodology/Principal Findings We found that systemic levels of hBD-2 showed a weak but significant correlation with beta defensin copy number in healthy controls but not in psoriasis patients with active disease. In psoriasis patients but not in atopic dermatitis patients, we found high systemic hBD-2 levels that strongly correlated with disease activity as assessed by the PASI score. Our findings suggest that systemic levels in psoriasis are largely determined by secretion from involved skin and not by genomic copy number. Modelling of the in vivo epidermal hBD-2 concentration based on the secretion rate in a reconstructed skin model for psoriatic epidermis provides evidence that epidermal hBD-2 levels in vivo are probably well above the concentrations required for in vitro antimicrobial and chemokine-like effects. Conclusions/Significance Serum hBD-2 appears to be a useful surrogate marker for disease activity in psoriasis. The discrepancy between hBD-2 levels in psoriasis and atopic dermatitis could explain the well known differences in infection rate between these two diseases. ; This work was supported by grants from the Radboud University Nijmegen Medical Centre, the Netherlands Organization for Scientific Research, VENI grant 902.11.092 to P.Z., and the ZONMW alternatives to animal experiments programme, grant 114000084. ; 85442