This is a prospective, randomized, double-blinded, placebo-controlled, multi-center, Phase 3 study of GLSI-100 immunotherapy in HLA-A*02 positive and HER2/neu positive subjects who are at high risk for disease recurrence and have completed both neoadjuvant and postoperative adjuvant standard of care therapy. Treatment consists of 6 intradermal injections, Primary Immunization Series (PIS), over the first 6 months of treatment and 5 booster intradermal injections spaced 6 months apart. A third open-label arm will explore GLSI-100 immunotherapy in non-HLA-A*02 positive and HER2/neu positive subjects.

Inclusion Criteria:

• HLA-A*02-positive, unless being enrolled in the third non-HLA-A*02 arm
• Histologically confirmed diagnosis of HER2/neu positive primary breast cancer for all tumors biopsied (multifocal, multicentric, or synchronous contralateral disease)
• Completion of both neoadjuvant and adjuvant trastuzumab-based standard of care breast cancer therapy
• Stage I, II, or III at presentation with pathologic evidence of residual invasive carcinoma in the breast or axillary lymph nodes (residual disease) at surgery following completion of neoadjuvant therapy -OR- Stage III at presentation with pathologic complete response (pCR) at surgery following completion of neoadjuvant therapy
• The subject can begin study therapy within one year of completion of adjuvant trastuzumab-based therapy and any other standard therapies, but, study therapy can be administered concurrently with endocrine therapy.
• No clinical evidence of residual or persistent breast cancer per treating physician assessment
• ECOG 0-2
• Adequate organ function
• Negative pregnancy test or evidence of post-menopausal status
• If of childbearing potential, willing to use a form of highly effective contraception
• Subject must both reside in and have been treated for their cancer in the country in which the clinical site is located.

Exclusion Criteria:

• Stage IV cancer or metastatic breast cancer at any time
• Inflammatory breast cancer
• Receiving other investigational agents
• Receiving chemotherapy
• Requiring long-term systemic treatment with corticosteroids or other immunosuppressive therapy
• History of immunodeficiency or active autoimmune disease
• A history of serious allergic reactions, including anaphylaxis, to human granulocyte-macrophage colony-stimulating factors such as sargramostim, yeast-derived products, or any component of the investigational product
• Other malignancies except adequately treated in situ carcinoma of the cervix or basal cell or squamous cell carcinoma of the skin
• Active infection
• Known HIV infection with a detectable viral load within 6 months of the anticipated start of treatment. Note: Subjects on effective antiretroviral therapy with an undetectable viral load within 6 months of the anticipated start of treatment are eligible for this trial.

Double-blind