KATE2: This phase II trial will assess the safety and impact of using trastuzumab emtansine in combination with atezolizumab for the treatment of people with HER 2 + locallly advanced or metastatic breast cancerA Study to Evaluate the Efficacy and Safety of Trastuzumab Emtansine in Combination With Atezolizumab or Atezolizumab-Placebo in Participants With Human Epidermal Growth Factor-2 (HER2) Positive Locally Advanced or Metastatic Breast Cancer Who Have Received Prior Trastuzumab and Taxane Based Therapy

Exclusion

• Prior treatment with trastuzumab emtansine, cluster of differentiation 137 agonists, anti-programmed death-1, or anti-PD-L1 therapeutic antibody or pathway-targeting agents
• Receipt of any anti-cancer drug/biologic or investigational treatment within 21 days prior to Cycle 1 Day 1 except hormone therapy, which can be given up to 7 days prior to Cycle 1 Day 1; recovery of treatment related toxicity consistent with other eligibility criteria
• Radiation therapy within 2 weeks prior to Cycle 1, Day 1
• History of exposure to the cumulative doses of anthracyclines
• History of other malignancy within the previous 5 years, except for appropriately treated carcinoma in situ of the cervix, non-melanoma skin carcinoma, Stage I uterine cancer, or participants who have undergone potentially curative therapy with no evidence of disease and are deemed by the treating physician to be at low risk for recurrence
• Cardiopulmonary dysfunction, symptomatic pleural effusion, pericardial effusion, or ascites
• Participants with severe infection within 4 weeks prior to randomization, including but not limited to hospitalization for complications of infection, bacteremia, or severe pneumonia
• Current severe, uncontrolled systemic disease
• Major surgical procedure or significant traumatic injury within 28 days prior to randomization or anticipation of the need for major surgery during the course of study treatment
• Clinically significant history of liver disease, including cirrhosis, current alcohol abuse, autoimmune hepatic disorders, sclerosis cholangitis or active infection with human immunodeficiency virus, hepatitis B virus, or hepatitis C virus
• Need for current chronic corticosteroid therapy (>=10 mg of prednisone per day or an equivalent dose of other anti-inflammatory corticosteroids)
• Spinal cord compression not definitively treated with surgery and/or radiation, or previously diagnosed and treated spinal cord compression without evidence that disease has been clinically stable for greater than (>) 2 weeks prior to randomization
• Participants with known central nervous system disease
• Leptomeningeal disease
• History of autoimmune disease
• Prior allogeneic stem cell or solid organ transplantation
• Active tuberculosis
• Receipt of a live, attenuated vaccine within 4 weeks prior to randomization or anticipation that such a live, attenuated vaccine will be required during the study
• Treatment with systemic immunostimulatory agents within 4 weeks or five half-lives of the drug (whichever is shorter) prior to randomization
• Treatment with systemic corticosteroids or other systemic immunosuppressive medications within 2 weeks prior to randomization, or anticipated requirement for systemic immunosuppressive medications during the trial
• Participants who are breastfeeding, or intending to become pregnant during the study