Giant cell arteritis (GCA) is the most frequent vascularitis after 50 years of age The investigators recently showed that GCA was accompanied by an elevation in Th1 and Th17 response [1]. Even though a quantitative deficit in regulatory TL (Treg) was shown, there are to date no data concerning their precise phenotypic and functional characteristics and notably their ability to inhibit Th1 and Th17 polarisation. The hypothesis of the investigator is that, in GCA, there is quantitative and above all functional deficit of Treg. Recently, progress has been made in the identification of Treg with new markers (CD39), which will make it possible to better identify and to study their specific functions. In this study the phenotypic and functional characteristics of Treg in GCA will be analysed. Better understanding of the role des Treg in GCA should lead to better-targeted treatments for patients with GCA, notably via the blockage of cytokines that inhibit the differentiation and/or function of Treg.

The study is classified interventional because a lot of blood samples are taken.

Inclusion Criteria:

Patients

• Patients who have provided written consent
• Patients with national health insurance cover
• Age > 50 years
• Patients with a diagnosis of Horton's disease, before any treatment

Horton's disease is defined by the American College Rheumatology ACR criteria [2], as the association of 3 of the following 5 criteria:

• age at disease onset 50 years or older
• recent onset localized headache
• indurated temporal artery or diminished/abolition of temporal pulse
• erythrocyte sedimentation rate (ESR) greater than 50 mm during the first hour (or C Reactive protein (CRP)>20 mg/L)
• Positive temporal artery biopsy (TAB) showing vascularitis with infiltration by mononuclear cells or granulomatous inflammation with or without giant cells.

Control subjects

Control subjects will be healthy volunteers recruited among blood donors at Dijon University Hospital, voluntary hospital personnel (nurses, doctors, laboratory technicians and secretaries) and patients without infectious, inflammatory or auto-immune diseases or cancer (CRP<5mg/L) recruited in the investigating departments of Dijon Hospital. They will be matched for age and sex and must meet the following criteria:

• Age > 50 years
• Patients with national health insurance cover
• Signed written informed consent form
• Absence of an inflammatory syndrome (CRP<5 mg /L)

Exclusion Criteria:

• Adult under guardianship
• Persons without national health insurance cover
• Pregnant or breast-feeding women
• Patients treated with corticoids or immunosuppressants in the month preceding inclusion
• Patients treated with chemotherapy