Tendon pathologies (enthesitis) are a characteristic component of psoriatic arthritis (PsA), and are observed in 35% to 50% of PsA patients. The Achilles tendon is one of the most commonly affected sites. This condition often causes great morbidity and loss of quality of life, and response only suboptimal to current intervention strategies. One of the main obstacles for the development of effective treatment methods is that the disease mechanisms remain poorly understood. To our knowledge, no one has yet ascertained the presence and function of immune-competent cells and inflammatory markers in tendons tissue from PsA patients suffering from Achilles enthesitis.

Aim: The main objective of the IMPAACT study is to describe the histological findings of PsA Achilles enthesitis, and to a) Characterize and b) Quantify the immune-competent cell presence in tendon specimens obtained from the mid-portion and entheseal site of the Achilles tendon from PsA patients with enthesitis using immunohistochemistry and a stereological technique for the quantification. And secondly, to compare these observations with a) Those in healthy Achilles tendons, and b) Those in mono-symptomatic, non-PsA, chronic Achilles tendinopathy. Secondly, to examine whether one or more of the immune-competent cell types in the PsA tendons are associated with 3-months remission rate of Achilles tendon pain, or with baseline a) Ultrasonic findings of the Achilles tendon; b) PsA disease activity; c) Tendon protein analyses; d) Risk factors of cardiovascular disease; e) Blood biomarkers of systemic inflammation; and f) Fecal microbiota composition.

Methods: This study (IMPAACT) is a prospective, cohort study including 30 PsA patients (fulfilling the the Classification Criteria for Psoriatic Arthritis (CASPAR criteria)) with pain at the Achilles tendon insertion. At baseline, all participants will be examined clinically regarding overall PsA disease activity (Psoriasis Area Severity Index (PASI skin score), the Research Consortium of Canada (SPARCC) Enthesitis score, swollen/tender joint count) and asked to fill out a questionnaire consisting of the validated Scandinavian (Danish) version of the Victorian Institute of Sport Assessment of Achilles tendons (VISA-A) in addition to a study-composed questionnaire comprising general questions regarding patient characteristics. An ultrasonic examination of the Achilles tendons will be performed before ultrasound guided Achilles tendon specimens will be obtained from the most painful Achilles tendon. Tendon samples will be evaluated immunohistochemically by quantifying the presence of macrophages (CD68-KP1+), T-lymphocytes (CD3+), B-lymphocytes (CD20+), natural killer cells (CD56+), neutrophils (granzyme-B+), mast cells and inflammatory markers using a stereological technique. Also, a protein analysis of the tendon tissue will be conducted. Venous blood will be analysed for levels of systemic inflammatory markers, as well as screened for cardiovascular risk factors including dyslipidaemia, and diabetes. After the baseline examination, all participants will be treated and monitored in accordance with the Danish national guideline recommendations for PsA patients. A 3-months follow-up examination will be performed to determine the remission status of the Achilles tendon pain. Data will be analysed using the STATA statistical package.

Conclusion: PsA is a painful and debilitating inflammatory disease. The current treatment remains suboptimal. We hope to provide new insight into the cellular mechanisms underlying PsA tendon and enthesis pathologies.

Inclusion Criteria:

• Fulfilling the CASPAR criteria
• Insertional Achilles tendon pain
• Ultrasonic findings of inflammatory disease at the painful Achilles tendon insertion (= enthesitis) defined as abnormally hypoechoic (loss of normal fibrillar architecture) and/or thickened tendon at its bony attachment, seen in two perpendicular planes that may exhibit a Doppler signal or bony changes, including enthesophytes, and erosions.

Exclusion Criteria:

• Other inflammatory rheumatic diseases than PsA
• Ultrasonic signs of complete rupture of the Achilles tendon
• Not wishing to participate or not suited for project evaluation

Psoriatic arthritis patients with Achilles enthesitis.

There will be no restrictions regarding previous or current local and/or systemic anti-inflammatory treatments, however, any type of such treatment will be registered and used to stratify patients into treatment groups for the statistical analysis. Patients who have never received any systemic and/or local anti-inflammatory treatment will be referred to as treatment naïve.