Time to disease flare was defined as the number of days from Day 1 (dosing) to the date of the first treated and Adjudication Committee (AC)-determined IgG4-RD flare within the 52-week RCP. The date of disease flare was determined by the initiation of any flare treatment, including new or increased glucocorticoid (GC) treatment, other immunotherapy, or an interventional procedure, as deemed necessary by the Investigator to address the flare.

Kaplan-Meier (KM) method was used to estimate the median time to flare, and 95% confidence interval (CI).

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

The annualized rate of treated and AC-determined flares during the 52-week RCP was calculated by dividing the total number of treated and AC-determined flares by the total time at risk (in years) for all participants. This measure reflected the frequency of disease flares per person per year during the RCP.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

Flare-free, treatment-free complete remission at Week 52 was defined as the absence of evident disease activity at Week 52, no AC-determined flare during the RCP, and no treatment for flare or disease control, except for the required 8-week glucocorticoid taper. Lack of evident disease activity was determined by either a score of 0 on the IgG4-RD Responder Index (score range: 0-4, with lower scores indicating better disease control) or an investigator's assessment that no disease activity was present based on physical, laboratory, pathology, or other evidence.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

Flare-free, corticosteroid-free complete remission at Week 52 was defined as the absence of evident disease activity at Week 52, no AC-determined flare during the RCP, and no corticosteroid treatment for flare or disease control, except for the required 8-week glucocorticoid taper. Lack of evident disease activity was determined by either a score of 0 on the IgG4-RD Responder Index (score range: 0-4, with lower scores indicating better disease control) or an investigator's assessment that no disease activity was present based on physical, laboratory, pathology, or other evidence.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

An adverse event (AE) referred to any untoward medical occurrence in a clinical study participant, regardless of a causal relationship with the study treatment. TEAEs included any event occurring after the participant received the study treatment. Clinically significant changes in vital signs, electrocardiograms, and laboratory tests recorded after treatment administration were documented as TEAEs. Serious AEs (SAEs) were untoward medical occurrences after the first dose, irrespective of a causal link to the study treatment, that led to death, were life-threatening, required hospitalization or its prolongation, caused significant disability, resulted in congenital anomalies, or were considered other medically important events. AEs were graded (Grade 3: severe; Grade 4: life-threatening; Grade 5: death) using the Common Terminology Criteria for Adverse Events [CTCAE]).

Safety Analysis Set RCP: All participants who received any dose of investigational product during the RCP. Participants were analyzed according to the treatment they actually received. Specifically, participants randomized to the inebilizumab group who received all placebo dose were included in the placebo group.

An AE referred to any untoward medical occurrence in a clinical study participant, regardless of a causal relationship with the study treatment. TEAEs included any event occurring after the participant received the study treatment. Clinically significant changes in vital signs, electrocardiograms, and laboratory tests recorded after treatment administration were documented as TEAEs. SAEs were untoward medical occurrences after the first dose, irrespective of a causal link to the study treatment, that led to death, were life-threatening, required hospitalization or its prolongation, caused significant disability, resulted in congenital anomalies, or were considered other medically important events. AEs were graded (Grade 3: severe; Grade 4: life-threatening; Grade 5: death) using the CTCAE).

Incidence was the proportion of the participants with ADA positive post-baseline only or boosted their preexisting ADA (at least 4-fold over the baseline titer) during the study period. Baseline was defined as the last valid value on or before the first dose of RCP.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

Time to initiation of the first treatment (medication or procedure) for new or worsening disease activity, as determined by the investigator, within the RCP, was measured from day 1 (dosing) to the date the first treatment was administered. It Included any treatment initiated by the investigator for disease activity, regardless of the AC determination of flare.

KM method was used to estimate the median time to the initiation of first treatment or worsening of disease activity, and 95% CI.

Full Analysis Set: all participants who received any dose of investigational product during the RCP, and experienced IgG4-RD flares, regardless of the AC determination of flare.

The annualized flare rate for AC-determined IgG4-RD flares, whether or not treated, during the RCP was calculated by dividing the total number of AC-determined flares by the total time at risk (in years) for all participants. This measure reflected the frequency of IgG4-RD flares, irrespective of treatment, per participant per year during the study period.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.

GC use was calculated as the cumulative glucocorticoid dose (in milligrams) taken for the purpose of IgG4-RD disease control during the RCP. This measure accounted for all GC treatments administered to participants to manage disease activity throughout the study period.

Full Analysis Set: all participants who received any dose of investigational product during the RCP.