• Percentage of Participants with Adverse Events [ Time Frame: Up to approximately 37 months after first participant enrolled ]
• Complete Remission (CR) [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  The CR rate will be assessed as the percentage of participants who achieve a CR, complete remission with incomplete platelet recovery (CRp), complete remission with incomplete hematologic recovery (CRi), or complete remission with partial hematologic recovery (CRh) (as defined by the IWG 2003 and ELN 2010 criteria) after up to six cycles of combination therapy.
• Duration of Response (DOR) [ Time Frame: Up to approximately 37 months after first particpant enrolled ]
  DOR is defined as the time from the initial response (CR, CRp, CRi, CRh, or partial remission [PR]) to the time of disease progression or death, whichever occurs first

• Percentage of Participants with Adverse Events [ Time Frame: Up to approximately 32 months after first participant enrolled ]
• Complete Remission (CR) [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  The CR rate will be assessed as the percentage of participants who achieve a CR, complete remission with incomplete platelet recovery (CRp), complete remission with incomplete hematologic recovery (CRi), or complete remission with partial hematologic recovery (CRh) (as defined by the IWG 2003 and ELN 2010 criteria) after up to six cycles of combination therapy.
• Duration of Response (DOR) [ Time Frame: Up to approximately 32 months after first particpant enrolled ]
  DOR is defined as the time from the initial response (CR, CRp, CRi, CRh, or partial remission [PR]) to the time of disease progression or death, whichever occurs first

• Serum Concentrations of Atezolizumab [ Time Frame: C1 D22 PTFI of Hu5F9-G4 and atezolizumab, and 30 minutes after atezolizumab infusion; C2 D8 PTFI; C2 D22 PTFI; C3 D22 PTFI; C4 D22 PTFI; C8 D22 PTFI; C12 D22 PTFI; C16 D22 PTFI; TDV (up to C16 D21);120 days after final dose of atezolizumab (UTA 37M) ]
  C=cycle (cycle=28 days) ; D=day; PTFI=prior to first infusion; TDV=treatment discontinuation visit; UTA=up to approximately; M=month
• Serum Concentrations of Hu5F9-G4 [ Time Frame: C1D1 PTFI&1H AEOI; C1D8 PTFI,&1H AEOI; C1D11 PTFI,&1H AEOI; C1D22 1H AEOI; C2D1 PTFI,&1H AEOI; C2D8 PTFI; C3D1 PTFI,&1H AEOI; C5D1 PTFI; C7D1 PTFI, C9D1 PTFI; C11D1 PTFI; C13D1 PTFI; C15D1 PTFI; C17D1&D1 E 2C T PTFI(UTA 37M);TDV(up to C16D21)(UTA 37M) ]
  C=cycle (cycle=28 days); D=Day; PTFI=prior to first infusion; H=hour; AEOI=after end of infusion; TDV=treatment discontinuation visit; UTA=up to approximately; M=months; E=every; T=thereafter
• Objective Response Rate [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as the percentage of patients with a partial remission (PR) or better (i.e., CR + CRp + CRi + CRh+ PR)
• Event-Free Survival [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as the time from study entry to the date of induction treatment failure or relapse from CR, CRp, CRh, CRi, or death from any cause
• Leukemia-Free Survival [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined (only for patients achieving a CR, CRp, CRh, or CRi) as the time from the date of achievement of remission (CR, CRp, or CRi) until the date of relapse from CR, CRp, CRh, CRi, or death from any cause
• Overall Survival [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as time from study entry to the date of death from any cause.
• Progression-Free Survival [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as the time from the first day of study treatment to disease progression or death, whichever occurs first.
• Rate of Transfusion Independence [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as the percentage of patients who achieve transfusion independence (i.e., achieving any continuous 56-day window without requiring platelet or RBC transfusions) at any time during study treatment.
• Duration of Transfusion Independence [ Time Frame: Up to approximately 37 months after first participant enrolled ]
  Defined as the number of consecutive days of transfusion independence, measured from 1 day after the last transfusion to disease progression or subsequent transfusion.
• Incidence of Anti-Drug Antibodies (ADAs) Against Atezolizumab During the Study Relative to the Prevalence of ADAs at Baseline [ Time Frame: Baseline up to approximately 37 months ]
• Incidence of ADAs Against Hu5F9-G4 During the Study Relative to the Prevalence of ADAs at Baseline [ Time Frame: Baseline up to approximately 37 months ]

• Serum Concentrations of Atezolizumab [ Time Frame: C1 D22 PTFI of Hu5F9-G4 and atezolizumab, and 30 minutes after atezolizumab infusion; C2 D8 PTFI; C2 D22 PTFI; C3 D22 PTFI; C4 D22 PTFI; C8 D22 PTFI; C12 D22 PTFI; C16 D22 PTFI; TDV (up to C16 D21);120 days after final dose of atezolizumab (UTA 32M) ]
  C=cycle (cycle=28 days) ; D=day; PTFI=prior to first infusion; TDV=treatment discontinuation visit; UTA=up to approximately; M=month
• Serum Concentrations of Hu5F9-G4 [ Time Frame: C1D1 PTFI&1H AEOI; C1D8 PTFI,&1H AEOI; C1D11 PTFI,&1H AEOI; C1D22 1H AEOI; C2D1 PTFI,&1H AEOI; C2D8 PTFI; C3D1 PTFI,&1H AEOI; C5D1 PTFI; C7D1 PTFI, C9D1 PTFI; C11D1 PTFI; C13D1 PTFI; C15D1 PTFI; C17D1&D1 E 2C T PTFI(UTA 32M);TDV(up to C16D21)(UTA 32M) ]
  C=cycle (cycle=28 days); D=Day; PTFI=prior to first infusion; H=hour; AEOI=after end of infusion; TDV=treatment discontinuation visit; UTA=up to approximately; M=months; E=every; T=thereafter
• Objective Response Rate [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as the percentage of patients with a partial remission (PR) or better (i.e., CR + CRp + CRi + CRh+ PR)
• Event-Free Survival [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as the time from study entry to the date of induction treatment failure or relapse from CR, CRp, CRh, CRi, or death from any cause
• Leukemia-Free Survival [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined (only for patients achieving a CR, CRp, CRh, or CRi) as the time from the date of achievement of remission (CR, CRp, or CRi) until the date of relapse from CR, CRp, CRh, CRi, or death from any cause
• Overall Survival [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as time from study entry to the date of death from any cause.
• Progression-Free Survival [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as the time from the first day of study treatment to disease progression or death, whichever occurs first.
• Rate of Transfusion Independence [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as the percentage of patients who achieve transfusion independence (i.e., achieving any continuous 56-day window without requiring platelet or RBC transfusions) at any time during study treatment.
• Duration of Transfusion Independence [ Time Frame: Up to approximately 32 months after first participant enrolled ]
  Defined as the number of consecutive days of transfusion independence, measured from 1 day after the last transfusion to disease progression or subsequent transfusion.
• Incidence of Anti-Drug Antibodies (ADAs) Against Atezolizumab During the Study Relative to the Prevalence of ADAs at Baseline [ Time Frame: Baseline up to approximately 32 months ]
• Incidence of ADAs Against Hu5F9-G4 During the Study Relative to the Prevalence of ADAs at Baseline [ Time Frame: Baseline up to approximately 32 months ]

• Drug: Atezolizumab
  Atezolizumab will be administered to participants by IV infusion at a fixed dose starting on Day 22 of Cycle 1. In subsequent cycles (Cycles 2 and beyond), IV atezolizumab will be given every 2 weeks (Q2W) on Days 8 and 22 of each 28-day cycle.
  Other Name: Tecentriq
• Drug: Hu5F9-G4
  Two priming doses of 1 mg/kg of Hu5F9-G4 will be administered to participants by continuous IV infusion on Days 1 and 4 of Cycle 1, followed by loading doses of 15 mg/kg IV on Day 8 and 30 mg/kg IV on Day 11. Starting on Day 15 of Cycle 1, Hu5F9-G4 maintenance will be given by IV infusion at a dose of 30 mg/kg once a week (QW) of each 28-day cycle. Dosing for de-escalation, if needed: Hu5F9-G4 will be given as two priming doses of 1 mg/kg IV on Days 1 and 4, followed by loading doses of 10 mg/kg IV on Day 8 and 15 mg/kg on Day 11. Starting on Day 15, maintenance treatment with Hu5F9-G4 will be given by IV infusion at a dose of 15 mg/kg once a week (QW).

Inclusion Criteria:

• Life expectancy of at least 12 weeks
• Eastern Cooperative Oncology Group Performance Status 0-2
• Documented and confirmed R/R AML per WHO classification, except acute promyelocytic leukemia, and lack of response to all therapies of known benefit
• Adequate end-organ function
• Negative HIV test at screening
• Negative hepatitis B surface antigen (HBsAg) test at screening
• Negative total hepatitis B core antibody (HBcAb) test at screening, or positive total HBcAb test followed by quantitative hepatitis B virus (HBV) DNA <500 IU/mL at screening
• Negative hepatitis C virus (HCV) antibody test at screening, or positive HCV antibody test followed by a negative HCV RNA test at screening
• Willingness and ability to provide pretreatment bone marrow aspirate and biopsy and agreement to provide subsequent bone marrow aspirates and biopsies during study treatment
• For women of childbearing potential: agreement to remain abstinent or use contraceptive methods, and agreement to refrain from donating eggs
• For men: agreement to remain abstinent or use contraceptive measures and agreement to refrain from donating sperm
• For women who are not postmenopausal or surgically sterile: requirement for a negative serum pregnancy test result within 14 days prior to initiation of study treatment

Exclusion Criteria:

• Previous allogeneic hematopoietic stem cell transplant within 6 months prior to enrollment, active graft versus host disease, or requiring transplant-related immunosuppression
• Prior solid organ transplant
• Evidence of active central nervous system (CNS) involvement by leukemia
• Pregnancy or lactation or intention to become pregnant during the study or within 5 months after the final dose of atezolizumab and/or Hu5F9-G4, whichever is longer
• History of idiopathic pulmonary fibrosis, organizing pneumonitis, drug-induced pneumonitis, or idiopathic pneumonitis
• History of autoimmune disease. Patients with a history of autoimmune-related hypothyroidism who are on a stable dose of thyroid replacement may be eligible for this study. Patients with controlled Type 1 diabetes mellitus who are on a stable insulin regimen may be eligible for this study. Patients with eczema, psoriasis, lichen simplex chronicus, or vitiligo with dermatologic manifestations only are eligible for the study provided all of the following conditions are met: (1) Rash must cover <10% of body surface area, (2) Disease is well controlled at baseline and requires only low-potency topical corticosteroids, (3) No occurrence of acute exacerbations of the underlying condition that require psoralen plus ultraviolet A radiation, methotrexate, retinoids, biologic agents, oral calcineurin inhibitors, or high-potency or oral corticosteroids within the previous 12 months.
• Treatment with investigational therapy within 14 days prior to initiation of study drug
• Any approved AML-related therapy within 14 days prior to enrollment. Granulocyte colony-stimulating factor to treat neutropenic fever and/or infection is permitted. Hydroxyurea may be used throughout the trial to control peripheral blood blast counts in response to the first dose of study treatment and during the first 4 weeks of study treatment.