Background/Purpose: Thy-1 (CD90) is a cell surface marker which is found primarily on fibroblasts and whose expression has previously been shown to correlate with pathologic subsets of fibroblasts.  SSc patients have been shown to have increased circulating levels of Thy-1 and increased Thy-1 staining in skin.  Assessment of fibrosis in animal models is usually limited by the need for histologic and biochemical assays to determine the course of collagen deposition, ASMA expression, and other characteristic findings.  We used a Thy-1-YFP reporter mouse to assess whether expression of Thy-1 can serve as a surrogate for fibrosis in vivo using the bleomycin induced skin fibrosis model.

Methods: Thy-1-YFP mice were injected with intradermal bleomycin over a time-course with 0 to 10 injections and were imaged on an IVIS spectrum in vivo fluorescent imaging system three times per week for up to 35 days.  3 mice per time point were assessed histologically and biochemically at 0, 3, 8, 14, 16, 21, 28, and 35 day time points.  Skin was assessed for presence of YFP+ cells using fluorescent microscopy, for dermal thickness, for biochemical assessments of fibrosis, and for fibrogenic gene expression.

Results: SSc patients have increased Thy-1 expression in skin and this correlates with collagen and other fibrogenic gene expression and the modified Rodnan skin score (MRSS). Thy-1-YFP mice had no baseline skin Thy-1 fluorescence and began to display epi-fluorescence which was localized to the areas surrounding intradermal bleomycin injections.  This was statistically increased by IVIS at day 10 and then progressively increased until 21 days at which point it began to dissipate.  This pattern correlated closely with both dermal thickness and expression of fibrotic genes.  Fluorescence microscopy confirmed that YFP positive cells were restricted to dermal cells with spindle-shaped fibroblast morphology.

Conclusion: Thy-1 is differentially expressed in SSc skin and correlates with extent of skin disease.  In bleomycin-induced fibrosis, Thy-1-YFP mice demonstrated inducible expression with bleomycin and were useful for quantitatively tracking fibrosis progression using in vivo fluorescent imaging.  Fluorescent intensity measured in vivo correlated with histologic and biochemical outcomes of fibrosis measured terminally.   This system is able to track fibrosis in vivo and will allow for dynamic assessment of fibrosis over time in treatment trials to assess onset and resolution of fibrosis without requiring large numbers of experimental animals.  Further studies should further assess the pathophysiologic role of Thy-1 in SSc skin fibrosis.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/thy-1-cd90-as-a-novel-marker-for-tracking-in-vivo-skin-fibrosis/