Has a body mass index between 18 and 32 kg/m\^2, inclusive
Systemic lupus erythematosus (SLE): Has a diagnosis of SLE for at least 6 months and met the European Alliance of Associations for Rheumatology (EULAR)/ American College of Rheumatology (ACR) 2019 classification criteria
SLE: Is taking at least one background therapy for SLE
RA: Has a diagnosis of RA for at least 6 months and meets the 2010 ACR-EULAR classification criteria for RA
You may not be if
Has a known active infection (excluding fungal infection of nail beds), or any major episode of infection requiring hospitalization or treatment with anti-infectives within 8 weeks prior to the Day 1 dosing
History of serious recurrent or chronic infection
Is known to be infected with hepatitis B virus, hepatitis C virus, or human immunodeficiency virus
Has evidence of active tuberculosis (TB), latent TB, or inadequately treated TB
Has a significant or uncontrolled medical disease in any organ system not related to RA or SLE
For RA participants, has a history of any arthritis with onset before age 17 years
Has a current inflammatory condition other than SLE or RA that could interfere with disease activity assessments
History of cancer (except fully treated nonmelanoma skin cancers or cervical carcinoma in situ after complete surgical removal) and is disease free for \<5 years before Day 1 dosing
Has had a major surgery within 3 months prior to Screening or has a major surgery planned during the study.
Has symptomatic heart failure (New York Heart Association class III or IV) or myocardial infarction or unstable angina pectoris within 6 months prior to Screening
Has a severe chronic pulmonary disease requiring oxygen therapy
Has current active lymphoproliferative disease, including lymphoma, or signs and symptoms suggestive of possible lymphoproliferative disease
