Inclusion Criteria:

- Participation in study 28063.

- Completion of Week 36 visit of the core study 28063.

- Willingness and ability to comply with study procedures for the duration of the
study.

- Voluntary provision of written informed consent (including, for the USA, subject
authorization under the Health Insurance Portability and Accountability Act (HIPAA)),
given before any study-related procedure that is not part of normal medical care and
with the understanding that the subject may withdraw consent at any time without
prejudice to his or her future medical care.

Exclusion Criteria:

- Premature discontinuation of core study 28063.

- Subjects who meet criteria listed below will receive IMP in study 28851.

- Subjects who are eligible for participation in extension study 28851 but do not meet
these criteria will not be treated with IMP but will undergo scheduled visits,
irrespective of their treatment.

- All subjects must satisfy the following criteria before Extension Study Day 1
(D1-EXT; defined as the first day of dosing in the extension study) to be eligible
for treatment with IMP:

- Eligibility for participation in extension study 28851.

- For women of childbearing potential, a negative urine pregnancy test at
eligibility assessment.

- Female subjects of childbearing potential must be willing to avoid pregnancy by
using an adequate method of contraception for four (4) weeks before the first
dose administered within the extension study, during the study and for twelve
(12) weeks after the last dose of trial medication. Adequate contraception is
defined as two barrier methods, or one barrier method with spermicide, or an
intrauterine device, or use of a combined oral female hormonal contraceptive (or
the definitions requested by health authorities and locally amended in the core
study 28063). For the purposes of this trial, women of childbearing potential
are defined as: "All female subjects after puberty unless they are
post-menopausal for at least two years or are surgically sterile" (For Germany
Only: Female subjects of childbearing potential must be willing to avoid
pregnancy by using highly effective methods of contraception for approximately
four (4) weeks prior to D1-EXT, during and for twelve (12) weeks after the last
dose of trial medication. This requirement does not apply to surgically sterile
subjects or to subjects who are postmenopausal for at least 2 years. Highly
effective contraception is defined as any method or combination of methods which
result in a low failure rate (i.e. less than 1% per year) when used consistently
and correctly, such as implants, injectables, combined oral contraceptives,
sexual abstinence, vasectomized partner, two barrier methods, or one barrier
method with spermicide)

- Willingness and ability to comply with study procedures for the duration of the
study.

- To be eligible for treatment with IMP in study 28851, the subjects must not meet any
of the following criteria:

- Non-eligibility for participation in extension study 28851 (premature discontinuation
of core study 28063).

- Major protocol violation or non-compliance in the core study.

- Use of prohibited immunomodulatory / immunosuppressive therapies

- Serum IgG level <3 g/L if the subject received atacicept in the core study, or serum
IgG level <6 g/L if the subject received placebo in the core study (to protect the
blinding of the core study, the IgG level will be communicated to the treating
physician only if it is too low for extension study participation and only after all
Week 36 assessments performed within the core study have been completed).

- Any condition, including laboratory findings that, in the opinion of the
Investigator, constitutes a risk or a contraindication for participation in the
extension study, or that could interfere with the study objectives, conduct or
evaluation.

- Known active clinically significant acute or chronic infection, or any major episode
of infection requiring hospitalization or treatment with parenteral anti-infectives.

- Investigator judgment that treatment of the subject with atacicept in the extension
study is not appropriate.

- Aspartate aminotransferase (AST), or alanine aminotransferase (ALT), or alkaline
phosphatase (AP) level >2.5 x upper limit of normal (ULN), or total bilirubin >1.5 x
ULN at eligibility assessment.

- Clinically significant abnormality in any hematological test (e.g. hemoglobin <100
g/L (6.21 mmol/L), white blood cells <3 x 109/L, platelets <100 x 109/L) at
eligibility assessment.

- Clinically significant abnormality on ECG performed at eligibility assessment.

- Presence of uncontrolled or New York Health Association (NYHA) class 3 or 4
congestive heart failure at Week 36 of the core study.

- Moderate to severe renal impairment (creatinine clearance <50 mL/min according to
Cockcroft-Gault equation).

- Allergy or hypersensitivity to gadolinium (Gd).

- Allergy or hypersensitivity to atacicept or to any of the components of the
formulated atacicept.

- Diagnosis or family history of Creutzfeldt-Jakob disease (CJD).