Abatacept in ANCA Associated Vasculitis

Inclusion Criteria: * Acute AAV, presenting at first diagnosis or relapse, defined by clinical presentation * ANCA positivity (anti-MPO or anti-PR3 positive) * BVAS score of \> 8. Exclusion Criteria: * Severe life-threatening disease, i.e. lung haemorrhage at the time of presentation, renal impairment with SCr\>150 micromol/l, or severe CNS dysfunction thought to be due to vasculitis. * Current symptoms of severe, progressive, or uncontrolled renal, hepatic, hematological, gastrointestinal, pulmonary, cardiac, neurological or cerebral disease, or other medical conditions that might place the subject at unacceptable risk for participation in this study. * Any other non-vasculitic multisystem autoimmune disease * Serious acute or bacterial infection unless treated and completely resolved with antibiotics prior to enrolment * With any severe chronic or recurrent bacterial infection * With Hepatitis B or C or HIV * With Herpes zoster infection that resolved less than 2 months prior to enrolment * Subjects who have received any live vaccines within 3 months of the first dose of study medication or who will have need of a live vaccine at any time in the year following enrolment * Subjects with current clinical or laboratory evidence of active or latent tuberculosis (TB) and subjects with a history of active TB treated within the last 3 years * With any previous malignancy, with the exception of non-melanoma skin malignancies, adequately treated previously * Subjects with a mammogram that is suspicious for malignancy and in whom the possibility of malignancy cannot be reasonably excluded following additional evaluations. Mammograms (females only) must be performed within 6 months of study entry or if documentation is not on file. * With MTX treatment in prior 3 months * Subjects with prior therapy with rituximab, anti-TNF therapy, or IL-1 receptor antagonists within last year or cyclophosphamide within last six months * Subjects with a history of intolerance to methotrexate * Subjects who have at any time received treatment with abatacept * Subjects who have received treatment with any investigational drug within 28 days (or less than 5 terminal half-lives of elimination) of the Day 1 dose * Subject receiving approved or investigational biologics * Subjects with any of the following laboratory values: * Hgb \< 8.5 g/dL. * WBC \< 3,000/mm3 (3 x 109/L) * Platelets \< 100,000/mm3 (100 x 109/L). * Serum ALT or AST \> 2 times upper limit of normal. * Any other laboratory test results that, in the opinion of the investigator, might place the subject at unacceptable risk for participation in this study. * Subjects participating concurrently in another clinical trial * Pregnancy, breast feeding or inadequate contraception if female. * Allergy to a study medication