The purpose of this study is to determine the safety and tolerability of XmAb657 in healthy participants and participants with autoimmune diseases. Participants will be given XmAb657 subcutaneously (SC) by injection under the skin.

This is a Phase 1 study to determine the safety, tolerability, pharmacokinetic (PK), and pharmacodynamics (PD), and immunogenicity of XmAb657 in healthy adult participants and participants with autoimmune diseases.

Inclusion Criteria:

Healthy participants - Adult participants in good health

Idiopathic inflammatory myopathy (IIM) participants - Adult participants that meet the 2017 European Alliance of Association Rheumatology (EULAR)/American College of Rheumatology (ACR) Classification Criterion for IIM

Systemic sclerosis (SSc) participants - Adult participants that meet the 2013 ACR/EULAR Classification Criteria for SSc

Sjogren's Disease participants that meet the 2016 ACR/EULAR Classification Criteria for Primary Sjogren's Syndrome

All participants - Use of highly effective methods of contraception

Exclusion Criteria:

• Major surgery within 12 weeks prior to dosing or planned within the study
• Recurrent infections or active clinically significant infection
• Active or untreated latent tuberculosis
• Cancer or history of cancer or lymphoproliferative disease within the previous 5 years
• Uncontrolled cardiovascular, pulmonary, renal, hepatic, endocrine, or gastrointestinal disease

Note: Additional, more specific inclusion/exclusion criteria are defined in the protocol.