AE=any new untoward medical occurrence or worsening of a pre-existing medical condition which does not necessarily have a causal relationship with this treatment. SAE=any untoward medical occurrence that at any dose: results in death, is life-threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, results in development of drug dependency or drug abuse, is an important medical event. Related SAE/AE = possibly, probably, or certainly related to study drug

All Treated Participants, all participants who received at least 1 dose of study medication

AE=any new untoward medical occurrence or worsening of a pre-existing medical condition which does not necessarily have a causal relationship with this treatment. AEs of special interest are those AEs that may be associated with the use of immunomodulatory drugs, including all infections, serious infections, and opportunistic infections; autoimmune disorders; neoplasms; acute infusional AEs (pre-specified AEs occurring within 1 hour of start of infusion) and peri-infusional AEs (pre-specified AEs occurring within 24 hours of the start of infusion).

All Treated Participants, all participants who received at least 1 dose of study medication

Upper Normal Limit (ULN), Lower Normal Limit (LLN), Baseline (BL). Marked abnormality criteria are: Hemoglobin (HGB): >3 g/dL decrease from BL; Hematocrit: <0.75 * BL; Erythrocytes: <0.75 * BL; Platelets (PLT): <0.67 * LLN/>1.5 * ULN, or if BL < LLN then use <0.5 * BL and <100,000 mm^3; Leukocytes: <0.75 * LLN/ >1.25 * ULN, or if BL<LLN then use <0.8 * BL or >ULN, or if BL>ULN then use >1.2 * BL or <LLN; neutrophils+bands: <1.0 * 10^3 c/uL; eosinophils: >0.750 * 10^3 c/uL; basophils: > 400 mm^3; monocytes: >2000 mm^3; lymphocytes: <0.750 * 10^3 c/uL/ >7.50 * 10^3 c/uL.

All Treated Population. One participant in each group was not evaluated for hematology abnormalities due to data unavailability (missing data).

ULN=upper level of normal; BL=baseline.Marked abnormality criteria: High alkaline phosphatase (ALP): >2* ULN, or if BL>ULN then use >3* BL; high aspartate aminotransferase (AST): >3* ULN (80 U/L), or if BL>ULN then use >4* BL; high alanine aminotransferase (ALT): >3* ULN (34-47 U/L), or if BL>ULN then use >4* BL; high G-Glutamyl transferase (GGT): >2* ULN, or if BL>ULN then use >3* BL; high bilirubin: >2* ULN, or if BL>ULN then use >4* BL; high blood urea nitrogen (BUN): >2* BL; high creatinine: >1.5* BL (ULN 14.6 pg/mg. AST ULN=80 U/L; ALT ULN=34-47 U/L;creatinine ULN=14.6 pg/mg.

All Treated Population. Two participants in the ABA group and 3 participants in the PLA group were not evaluated for blood chemistry abnormalities due to data unavailability (missing data).

Serum samples from all treated adult participants with active rheumatoid arthritis (RA) were screened for the presence of drug-specific antibodies using ELISA. Immunogenicity was defined as the presence of a positive anti-abatacept or anti-CTLA4 antibody.

All participants treated during DB who received at least 1 dose of abatacept and had antibody samples collected at baseline and at least 1 post-baseline visit. 561 participants were not evaluated for anti-abatacept anti-bodies during the DB.

Physical examinations were performed at the discretion of the investigator and included breast examinations for female participants. Vital sign measurements were performed for participants before and after infusion of study medication at each visit and included seated systolic blood pressure, seated diastolic blood pressure, temperature, and heart rate. Abnormalities were determined to be clinically significant by the investigator.

All Treated Population.

AE=any new untoward medical occurrence or worsening of a pre-existing medical condition which does not necessarily have a causal relationship with this treatment. SAE=any untoward medical occurrence that at any dose: results in death, is life-threatening, requires inpatient hospitalization or causes prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, results in development of drug dependency or drug abuse, is an important medical event. Related SAE/AE = possibly, probably, or certainly related to study drug

All Treated Participants

AE=any new untoward medical occurrence or worsening of a pre-existing medical condition which does not necessarily have a causal relationship with this treatment. AEs of special interest are those AEs that may be associated with the use of immunomodulatory drugs, including all infections, serious infections, and opportunistic infections; autoimmune disorders; neoplasms; acute infusional AEs (pre-specified AEs occurring within 1 hour of start of infusion) and peri-infusional AEs (pre-specified AEs occurring within 24 hours of the start of infusion).

All Treated Participants, all participants who received at least 1 dose of study medication

Upper Normal Limit (ULN), Lower Normal Limit (LLN), Baseline (BL). Marked abnormality criteria are: Hemoglobin (HGB): >3 g/dL decrease from BL; Hematocrit: <0.75 * BL; Erythrocytes: <0.75 * BL; Platelets (PLT): <0.67 * LLN/>1.5 * ULN, or if BL < LLN then use <0.5 * BL and <100,000 mm^3; Leukocytes: <0.75 * LLN/ >1.25 * ULN, or if BL<LLN then use <0.8 * BL or >ULN, or if BL>ULN then use >1.2 * BL or <LLN; neutrophils+bands: <1.0 * 10^3 c/uL; eosinophils: >0.750 * 10^3 c/uL; basophils: > 400 mm^3; monocytes: >2000 mm^3; lymphocytes: <0.750 * 10^3 c/uL/ >7.50 * 10^3 c/uL.

All Treated Population.

Marked abnormality criteria: Alkaline phosphatase (ALP): >2* ULN, or if BL>ULN then use >3* BL; aspartate aminotransferase (AST): >3* ULN, or if BL>ULN then use >4* BL; alanine aminotransferase (ALT): >3* ULN, or if BL>ULN then use >4* BL; G-Glutamyl transferase (GGT): >2* ULN, or if BL>ULN then use >3* BL; Bilirubin: >2* ULN, or if BL>ULN then use >4* BL; blood urea nitrogen (BUN): >2* BL; creatinine: >1.5* BL

All Treated Population. One participant was not evaluated for liver function abnormalities.

Marked abnormality criteria: Sodium (Na): <0.95*LLN/ >1.05*ULN, or if BL<LLN then use <0.95* BL or >ULN, or if BL>ULN then use>1.05* BL or <LLN; potassium (K): <0.9* LLN/>1.1*ULN, or if BL<LLN then use <0.9* BL or >ULN, or if BL>ULN then use>1.1* BL or <LLN; (Cl): <0.9* LLN/>1.1* ULN, or if BL<LLN then use <0.9* BL or >ULN, or if BL>ULN then use>1.1* BL or <LLN; calcium (Ca): <0.8* LLN/>1.2* ULN, or if BL<LLN then use <0.75* BL or >ULN, or if BL>ULN then use>1.25* BL or <LLN; phosphorous (P): <0.75* LLN/ >1.25* ULN, or if BL<LLN then use 0.67* BL or >ULN, or if BL>ULN then use>1.33* BL or \

All Treated Population. One participant was not evaluated for electrolyte abnormalities.

MA criteria: serum glucose (Glu): <65 mg/dL/>220 mg/dL;fasting serum Glu: <0.8* LLN/>1.5*ULN,or if BL<LLN then use 0.8*BL or >ULN,or if BL>ULN then use >2.0*BL or <LLN;total protein: <0.9*LLN/>1.1*ULN,or if BL<LLN then use <0.9*BL or >UNL,or if BL>UNL then use >1.1*BL or <LLN; albumin: <0.9*LLN,or if BL<LLN then use <0.75 BL;uric acid: >1.5*ULN,or if BL>ULN then use >2*BL. Urinalysis (Urine protein,urine Glu,urine blood,leukocyte esterase,Red Blood Cells [RBCs], White Blood Cells [WBCs]):Use ≥2 when BL value missing or when pre-dose=0 or 0.5; use ≥3 when pre-dose=1, use ≥4 when pre-dose=2 or 3

All Treated Population. N=Number of Participants Analyzed, n=number of participants with measurements at time point

Physical examinations were performed at the discretion of the investigator and included breast examinations for female participants. Vital sign measurements were performed for participants before and after infusion of study medication at each visit and included seated systolic blood pressure, seated diastolic blood pressure, temperature, and heart rate. Abnormalities were determined to be clinically significant by the investigator.

All Treated Population.

Serum samples from all treated adult participants with active rheumatoid arthritis were screened for the presence of drug-specific antibodies using ELISA. Immunogenicity was defined as the presence of a positive anti-abatacept or anti-CTLA4 antibody.

All DB participants treated on study who received at least 1 dose of abatacept and had antibody samples collected at baseline and at least 1 post-baseline visit. 68 participants were not evaluated for anti-abatacept anti-bodies on study.