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)

Renal Response is defined as the presence of all the following criteria:

PROTEINURIA: Reduction of baseline urine protein/creatinine ratio (UPCR) of >= 50% and to less than 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.

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.

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).

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).

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

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

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".