Pilot Study to Evaluate the Safety and Efficacy of Abatacept in Adults and Children 6 Years and Older With Excessive Loss of Protein in the Urine Due to Either Focal Segmental Glomerulosclerosis (FSGS) or Minimal Change Disease (MCD)

Mean Change From Baseline in Urine Protein/Creatinine Ratio (UPCR) at Day 113

Time frame: From baseline (measured at day 1 of study) to 113 days following first dose administered in the indicated treatment period (113 days for Double-Blind Period, 226 days for Open Label Period)

Mean Change From Baseline in Serum Albumine at Day 113

Time frame: From baseline (measured at day 1 of the study) to 113 days following first dose administered in the indicated treatment period (113 days for Double-Blind Period, 226 days for Open Label Period)

Percentage of Participants Achieving Complete Remission at Day 113

Complete Remission is defined as the presence of all the following criteria: PROTEINURIA: Urine protein/creatinine ratio (UPCR) ≤ 0.3. RENAL FUNCTION: No worsening of baseline estimated glomerular filtration rate (eGFR) defined as within normal range if normal at baseline or ≥ 75% baseline value if below normal at baseline.

Time frame: From first dose to 113 days following first dose of the indicated period (113 days for Double-Blind Period, 226 days for Open Label Period)

Mean Change From Baseline in Patient Reported Outcomes Measurement Information System (PROMIS) at Day 113 - Adult Participants

PROMIS was used to capture patient reported outcomes relevant to participants with nephrotic syndrome. Mean change from baseline is reported for the following measurements: Fatigue: Short From (SF) Fatigue v1.0 Adult 8a, composed of 8 questions, each one scored from 1 (Not at all) to 5 (Very much). Output presented as Total score, ranging from 8 (most desirable outcome) to 40 (least desirable outcome). Pain Interference: SF Pain Interference v1.0 Adult 8a, (measuring how much pain is interfering with daily activities). Composed of 8 questions, each one scored from 1 (Not at all) to 5 (Very much). Output presented as Total score, ranging from 8 (most desirable outcome) to 40 (least desirable outcome). Physical Function: SF Physical Function v1.2 Adult 8b, composed of 8 questions, each one scored from 1 (Without any difficulty) to 5 (Unable to do). Output presented as Total score, ranging from 8 (most desirable outcome) to 40 (least desirable outcome).

Time frame: From baseline (measured at day 1 of study) to 113 days following first dose administered in the Double-Blind Period

Mean Change From Baseline in Patient Reported Outcomes Measurement Information System (PROMIS) at Day 113 - Pediatric Participants

PROMIS was used to capture patient reported outcomes relevant to participants with nephrotic syndrome. Mean change from baseline is reported for the following measurements: Fatigue: Short From (SF) Fatigue v1.0 Peds 10a, composed of 10 questions, each one scored from 0 (Never) to 4 (Almost Always). Output presented as Total score, ranging from 0 (most desirable outcome) to 40 (least desirable outcome). Pain Interference: SF Pain Interference v1.0 Peds 8a, ( measuring how much pain is interfering with daily activities). Composed of 8 questions, each one scored from 0 (Never) to 4 (Almost always). Output presented as Total score, ranging from 0 (most desirable outcome) to 32 (least desirable outcome). Mobility: SF Physical Function-Mobility v1.0 Peds 8a, composed of 8 questions, each one scored from 0 (Not able to do) to 4 (With no trouble). Output presented as Total score, ranging from 0 (least desirable outcome) to 32 (most desirable outcome).

Time frame: From baseline (measured at day 1 of study) to 113 days following first dose administered in the Double-Blind Period

Number of Participants Experiencing Adverse Events

This outcome describes the number of participants experiencing various types of any grade Adverse Events (AEs). The Cumulative Abatacept Safety Period starts at the time of the first dose of Abatacept (either in the Double-Blind Period or in the Open Label Period) and lasts 56 days after the last dose of Abatacept

Time frame: From first dose in the indicated period to 56 days following last dose in the indicated period (169 days for the Double-Blind Period, up to 337 days for the Cumulative Abatacept Safety Period)

Number of Participants Experiencing Adverse Events of Special Interest

Number of participants experiencing various types of Adverse Events of interest, including infections, malignancies, autoimmune disorders, and infusional related reactions.

Time frame: From first dose on day 1 to 56 days following last dose (approximately 330 days)

Percentage of Participants With Positive Antibody Response Relative to Baseline

A positive response relative to baseline is defined as a positive response at a post-baseline visit that has a titer value greater than the positive baseline titer value. If the baseline titer is missing or negative, any post-baseline positive titer value is considered a positive response relative to baseline. Immunogenicity response is presented as the total of immunogenicity response for "CTLA4 and possibly Ig" and "Ig and/or Junction Region".

Time frame: From baseline (day 1) to 168 days following last dose (up to 15 months). Results at day 113 from first dose, day 56, day 84 and day 168 after last dose are presented

Minimum Blood Plasma Concentration (Cmin) of Abatacept - Adult Participants

Time frame: From first dose to 113 days after first dose in the Double Blind Period. Data collected on day 15, day 29, day 57, day 85 and day 113

Minimum Blood Plasma Concentration (Cmin) of Abatacept - Pediatric Participants

Time frame: From first dose to 113 days after first dose in the Double Blind Period. Data collected on day 15, day 29, day 57, day 85 and day 113

Maximum Observed Serum Concentration (Cmax) of Abatacept

Time frame: Day 85 after first dose in the Double Blind Period

Area Under the Serum Concentration Time Curve Over a Dosing Interval (AUC(TAU)) of Abatacept

Time frame: From Day 85 to Day 113 in the Double Blind Period

Time to Reach Peak Serum Concentration (Tmax(h)) of Abatacept

Time frame: Day 85 after first dose in the Double Blind Period