Systemic lupus erythematosus (SLE) is a chronic, systemic autoimmune disease characterized by B cell hyperactivity and Sjorgren's disease (SjD) is a chronic, multisystem autoimmune disease characterized by lacrimal and salivary gland inflammation, with resultant dryness of the eyes and mouth and occasional glandular enlargement. ABBV-319 exhibits potential B cell depletion in SLE and SjD which are characterized by B cell hyperactivity. The purpose of this study is to assess the pharmacokinetics, safety, and efficacy of ABBV-319 in adult participants with SLE or SjD.

ABBV-319 is an investigational drug being developed for the treatment of SLE and SjD. Participants are placed in 1 of 6 groups called treatment arms. Each group receives a different dose of ABBV-319 depending on whether they have SLE or SjD. Around 36 adult participants with SLE or SjD will be enrolled at approximately 10 sites worldwide.

Participants will receive 2 doses of IV ABBV-319 21 days apart and will be followed for up to 343 days.

There may be higher treatment burden for participants in this trial compared to their standard of care (due to study procedures). Participants will attend regular visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, blood tests, checking for side effects and completing questionnaires.

Inclusion Criteria:

• Systemic Lupus Erythematosus (SLE) Population - Clinical diagnosis of SLE at least 6 months prior to Screening as defined by the 2019 European Alliance of Associations for Rheumatology (EULAR)/American College Of Rheumatology (ACR) classification criteria for SLE and a positive antinuclear antibody (ANA) >= 1:80 drawn at Screening.
• SLE Population - Presence of anti-double stranded DNA (dsDNA), anti-Smith (Sm), anti-ribonucleoprotein (RNP), or anti-Sjogren's syndrome Ag A (SSA) Abs above the upper limit of normal (ULN).
• Sjogren's Disease (SjD) Population - Primary diagnosis of SjD at least 6 months prior to Screening as defined by the ACR/EULAR 2016 Criteria.
• SjD Population - EULAR Sjogren's Syndrome Disease Activity Index (ESSDAI) >= 5 at Screening.
• SjD Population - EULAR Sjogren's Syndrome Patient Reported Index (ESSPRI) >= 5 at Screening.

Exclusion Criteria:

• History of infection as defined in the protocol.
• Any of the medical diseases or disorders listed in the protocol.
• History of clinically significant (per investigator's judgment) drug or alcohol abuse within the 6 months prior to Screening.
• Any planned elective surgery that would impact study procedures or assessments through the completion of the Day 365 assessments.
• Any clinically significant ECG abnormalities at Screening.