This study will be conducted in two stages and will test the safety/tolerability, pharmacokinetics (how the body handles study drug) and pharmacodynamics (effects on the immune system and the virus) of the study drug ABBV-181 in Human immunodeficiency virus (HIV)-1 infected participants undergoing Antiretroviral therapy (ART) interruption.

Inclusion Criteria:

• Body Mass Index (BMI) between 18.0 and 35 kg/m2.

• HIV-1 infected on antiretroviral therapy (ART) for at least 12 months prior to screening and on current ART regimen for at least 8 weeks prior to screening.

• Meets HIV-specific laboratory parameters as below:

  • Plasma HIV-1 RNA below lower limit of quantification (LLOQ) at screening and at least 6 months prior to screening.
  • CD4+ T cell count >= 500 cells/uL at screening and at least once during the 12 months prior to screening.
  • CD4+ T cell nadir of >= 200 cells/uL during chronic infection.

• Willing to undergo ART interruption.

• Agrees to use an effective barrier method of protection (male and/or female condoms) during sexual activity for protection against HIV-1 transmission throughout the entire study.

Exclusion Criteria:

• Known resistance to at least 2 classes of ART.
• History of AIDS-defining illness.
• Active or suspected malignancy or history of malignancy (other than basal cell skin cancer or cervical carcinoma in situ) in the past 5 years.
• History of or active immunodeficiency (other than HIV).
• Active autoimmune disease or history of autoimmune disease that has required systemic treatment.
• Prior receipt of immunomodulatory or immunosuppressive (including intravenous infusion or oral steroids at any dose, but excluding steroids that are inhaled, topical or by local injection) therapy within 24 weeks prior to the first dose of study drug.
• Prior therapy/exposure to ABBV-181 or any other immune checkpoint inhibitor.
• Current hepatitis B virus or hepatitis C virus infection.
• Clinically significant medical disorders that might expose the participants to undue risk of harm, confound study outcomes, or prevent the participant from completing the study (including but not limited to significant or unstable cardiac, neurologic or pulmonary disease, chronic active infectious disease except for HIV, chronic liver disease, poorly controlled diabetes mellitus and history of Stevens Johnson Syndrome toxic epidermal necrolysis (TEN), or drug reaction with eosinophilia and systemic symptoms (DRESS)).
• Known psychiatric or substance abuse disorders that would interfere with adherence to study requirements.
• Female participants must not be pregnant, breastfeeding, or considering becoming pregnant during the study.