A Study to Evaluate Efficacy and Safety of Abatacept in Participants of Pemphigus Vulgaris (PV)

Inclusion Criteria: 1. Adults (18 through 80 years of age) with clinically-documented diagnosis of PV for \>2 months and \<10 years. 2. History of biopsy consistent with PV (Hematoxylin and Eosin staining and direct immunofluorescence). Confirmed diagnosis of PV within the previous 24 months, based on the presence of histological features of acantholysis via skin or mucosal biopsy and one of the following: tissue bound immunoglobulin G (IgG) antibodies by direct immunofluorescence on the surface of affected epithelium or serological detection of serum desmoglein-3 (DSg3) autoantibodies against epithelial cell surface either by indirect immunofluorescence microscopy or by enzyme-linked immunosorbent assay. 3. At least 1 previous episode of a failed steroid taper (ie, disease flare/relapse) at a prednisone/prednisolone dose \>10 mg/day. The following criteria must have been met as evidence of disease severity at the time of the failed steroid taper: a) A Pemphigus Severity of Clinical Disease score of moderate (2) or severe (3) (may be historical/retrospective assessment). b) Required a treatment change at the time of the failed steroid taper of at least one of the following: i) A steroid increase to \>=20 mg/day OR ii) The addition of immunosuppressive/immunomodulatory agent/treatment OR iii) A dose increase of immunosuppressive/immunomodulatory agent/treatment 4. Screening anti-Dsg antibodies consistent with a diagnosis of PV (ie, elevated antiDsg3 antibodies). 5. Has initiated and received a stable dose of prednisone/prednisolone from a minimum of 20 mg/day (example: 0.25 mg/kg/day for an 80 kg person) up to a maximum of 120 mg/day or 1.5 mg/kg/day (whichever is higher) for \>=2 weeks prior to randomization. 6. Has exhibited PV disease control, defined as no new lesions for \>=2 weeks. A female subject is eligible to enter the study if she: Is of non-child bearing potential, who 7. is either surgically sterile (bilateral tubal ligation, bilateral oophorectomy, or post-hysterectomy) or is postmenopausal without menses for \>2 years. Women who are \<2 years postmenopausal are required to have menopausal status confirmed by follicle-stimulating hormone (FSH) and estradiol levels at the screening evaluation. If FSH and estradiol levels do not provide confirmation of menopause, subject will be considered to be of childbearing potential. Exclusion Criteria: 1. Diagnosis of pemphigus foliaceus, paraneoplastic pemphigus, or other autoimmune blistering disease (other than pemphigus vulgaris). 2. Past or current history of hypersensitivity to components of the investigational product or medically significant adverse effects (including allergic reactions) from cetirizine (or antihistamine equivalent) or paracetamol/acetaminophen. 3. Prior treatment with rituximab without achieving disease control within 6 months of initiating rituximab dosing. 4. Prior treatment with immunosuppressant or immunomodulation agents within the protocol specified periods 5. Evidence or history of clinically significant infections 6. Past or current malignancy, except for cervical carcinoma Stage 1B or less, noninvasive basal cell and squamous cell skin carcinoma and cancer diagnoses with a duration of complete response (remission) \>5 years 7. Significant concurrent, uncontrolled medical condition that could affect the subject's safety, impair the subject's reliable participation in the study, impair the evaluation of endpoints, or necessitate the use of medication not allowed by the protocol. This includes subjects who require any systemic steroid treatment for a concurrent medical condition (other than pemphigus vulgaris). 8. Use of an investigational drug or other experimental therapy within 4 weeks, 5 pharmacokinetic half-lives, or the duration of biological effect (whichever is longer) prior to Screening. 9. Electrocardiogram (ECG) showing a clinically significant abnormality or showing a QTc interval ≥450 msec (≥480 msec for subjects with a bundle branch block) 10. Woman who is breastfeeding. 11. Positive test results for hepatitis B surface antigen (HBsAg), hepatitis B core antibody (HBcAb), or hepatitis C virus (HCV) serology at screening