Study to Evaluate the Effect of XmAb®5871 on Disease Activity in Patients With IgG4-Related Disease (RD)

Inclusion Criteria:

- Active IgG4-RD

- Compatible pattern of organ involvement consistent with IgG4-RD that cannot be
attributed to other causes

- Histopathologically-proven diagnosis of IgG4-RD

- Peripheral blood plasmablast count >900 cells/mL and/or elevated IgG4-RD levels during
screening

- Able and willing to complete the entire study according to the study schedule

- Able and willing to provide written informed consent

Exclusion Criteria:

- History or evidence of a clinically unstable/uncontrolled disorder, condition or
disease other than IgG4-RD that, in the opinion of the Investigator would pose a risk
to the patient safety or interfere with the study evaluation, procedures or completion

- Malignancy within 5 years (except successfully treated in situ cervical cancer,
resected squamous cell or basal cell carcinoma of the skin, or prostate cancer with no
recurrence ≥3 years following prostatectomy)

- Presence of recurrent or chronic infections, defined as ≥3 infections requiring
antimicrobials over the past 6 months prior to screening

- Active infection requiring hospitalization or treatment with parenteral antimicrobials
within the 60 days prior to randomization or oral antimicrobials within the 21 days
prior to enrollment

- Patient is taking >40 mg of prednisone QD

- Prior use of rituximab (or other B cell depleting agents) within 6 months of
enrollment. Prior use of any B cell depleting agent greater than 6 months from
enrollment is allowed if the CD19+ B cell count is within the normal reference range
during screening

- Use of any investigational agent within 5 half-lives of the agent (or 6 months if the
half-life is unknown) prior to enrollment

- Immunosuppressive agent use within the three months prior to enrollment

- Has received live vaccines within 2 months of enrollment

- Patient is pregnant or breast feeding, or planning to become pregnant while enrolled
in the study, up to end-of-study visit

- Unable or unwilling to partake in the follow-up assessments or required protocol
procedures