• Nature, incidence and severity of adverse events (AEs) [ Time Frame: Every 2 - 6 weeks ] [ Designated as safety issue: Yes ]
  
• Proportion of subjects who develop IgG <3 g/L [ Time Frame: Every 2 - 6 weeks ] [ Designated as safety issue: Yes ]
  
• Changes / abnormalities in vital signs/ routine safety lab parameters [ Time Frame: Every 2 - 6 weeks ] [ Designated as safety issue: Yes ]
  
• Changes over time in vaccine immunization status [ Time Frame: Every 2 - 6 weeks ] [ Designated as safety issue: Yes ]
  

• ACR and DAS28 composite scores at week 26 [ Time Frame: Every 2 - 6 weeks ] [ Designated as safety issue: Yes ]
  

Inclusion Criteria:

• Male and female subjects
• >18 years of age at the time of Informed Consent
• who have rheumatoid arthritis (satisfying American College of Rheumatology criteria)
• a disease history of at least 12 months.

• Subjects must have active disease defined by

  • >8 swollen joints (out of 66)
  • >8 tender joints (out of 68)
  • CRP >6 mg/L
  • ESR >28 mm/h.
• Subjects must have received previous treatment with rituximab and must be candidates for re-treatment with rituximab.
• Female subjects of childbearing potential must be willing to avoid pregnancy by using an adequate method of contraception for 4 weeks before SD1, during the treatment period and for 12 months after the last dose of rituximab, and must have a negative urine pregnancy test at the screening visit and SD1.

Exclusion Criteria:

• Neurological disease
• Inflammatory joint disease other than rheumatoid arthritis
• Any contraindication to rituximab as per national label
• Use of disease-modifying anti-rheumatic drugs (DMARDs; including methotrexate) for less than 3 months or change in dosing regimen within 28 days before SD1, or methotrexate dose regimen >25 mg/week
• Participation in any interventional clinical trial within 1 month before SD1 (or within 5 half-lives of the investigated compound before SD1, whichever is longer)
• Prednisone dose regimen >10 mg/day (or equivalent), or change in steroid dosing regimen within 28 days before SD1
• Active or latent tuberculosis within the year before screening or major infection requiring hospitalization or intravenous anti-infectives within 28 days before SD1
• Serum IgG below 6 g/L
• Known hypersensitivity to atacicept or to any of the components of the formulated atacicpet
• Known hypersensitivity to rituximab, to any of the components of the formulated rituximab or to murine proteins
• Breastfeeding or pregnancy