Inclusion Criteria:

          -  Patients must have a histologically documented (either primary or metastatic site)
             diagnosis of breast cancer that is HER2 non-overexpressing by immunohistochemistry,
             namely 0 or 1; if they have an equivocal immunohistochemistry, 2, the tumor must be
             non-gene amplified by fluorescence in situ hybridization (FISH) performed upon the
             primary tumor or metastatic lesion (ratio < 2 and HER2 copy number < 4); estrogen
             receptor (ER) positivity is defined as 1% or greater

          -  Patients must have measurable disease by Response Evaluation Criteria in Solid Tumors
             (RECIST) criteria

          -  Patients who are ER negative must have progressed through at least one prior
             chemotherapy regimen in the metastatic setting or within 12 months of their last
             adjuvant systemic treatment; patients who are ER positive must have progressed through
             standard hormone therapy options and have received at least one line of chemotherapy
             in the metastatic setting

          -  Completion of prior chemotherapy systemic anticancer therapy at least 2 weeks prior to
             study entry

          -  Radiation therapy must be completed at least 2 weeks prior to study entry; radiated
             lesions may not serve as measurable disease unless they have been radiated over 12
             months prior to enrollment

          -  Patients may have parenchymal brain metastases if stable (no evidence of progression)
             for at least 1 month after local therapy (radiation or surgery); leptomeningeal
             disease is excluded; must have completed any prescribed steroid taper

          -  Patients may have had a prior diagnosis of cancer if it has been > 5 years since their
             last treatment

          -  Patients must exhibit an Eastern Cooperative Oncology Group (ECOG) performance status
             of =< 2

          -  Absolute neutrophil count >= 1,000/mcL

          -  Platelets >= 50,000/mcl

          -  Total bilirubin =< 1.5 times the institutional upper limit of normal (ULN) (or =< 3
             times ULN in case of liver metastasis)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SPGT])
             =< 2.5 X institutional ULN (or =< 5 times ULN in case of liver metastasis)

          -  Creatinine =< 2 ng/ml

          -  Females of child-bearing potential (FOCBP) and males must agree to use 2 methods of
             adequate contraception prior to study entry, for the duration of study participation,
             and for number (#) days following completion of therapy; should a female patient
             become pregnant or suspect she is pregnant while participating in this study, she
             should inform her treating physician immediately

               -  NOTE: A FOCBP is any woman (regardless of sexual orientation, having undergone a
                  tubal ligation, or remaining celibate by choice) who meets the following
                  criteria:

               -  Has not undergone a hysterectomy or bilateral oophorectomy

               -  Has had menses at any time in the preceding 12 consecutive months (and therefore
                  has not been naturally postmenopausal for > 12 months)

          -  FOCBP must have a negative pregnancy test within 7 days prior to registration on study

          -  Willingness to provide a fresh biopsy prior to study enrollment and after 2 cycles of
             treatment

          -  Patients must have the ability to understand and the willingness to sign a written
             informed consent prior to registration on study

        Exclusion Criteria:

          -  Patients who have had chemotherapy or radiotherapy within 2 weeks prior to entering
             the study or those who have not recovered from adverse events due to agents
             administered more than 4 weeks earlier are not eligible.

          -  Current or prior use of immunosuppressive therapy within 2 weeks of starting
             investigational therapy

          -  Patients who are taking any herbal (alternative) medicines are NOT eligible for
             participation; patients must be off any such medications by the time of registration
             for at least 2 weeks; NOTE: Vitamin supplements are acceptable

          -  Patients may not have received any other investigational agents within 4 weeks prior
             to registration

          -  Prior treatment with immune therapy (including but not limited to cluster of
             differentiation [CD]137, OX40, programmed death [PD]-1, PD-L1 or cytotoxic
             T-lymphocyte antigen 4 [CTLA4] inhibitors)

          -  Prior severe infusion reaction to a monoclonal antibody

          -  Patients with a history of or active autoimmune disease within the past 3 years with
             the following exceptions:

               -  Vitiligo or alopecia

               -  Hypothyroidism on stable doses of thyroid replacement therapy

               -  Psoriasis not requiring systemic therapy within the past 3 years

          -  History of primary immunodeficiency disease or tuberculosis

          -  Major medical conditions that might affect study participation (uncontrolled
             pulmonary, renal, or hepatic dysfunction, uncontrolled infection) are not eligible;
             other significant comorbid condition which the investigator feels might compromise
             effective and safe participation in the study

          -  Patients who have an uncontrolled intercurrent illness including, but not limited to
             any of the following, are not eligible:

               -  Uncontrolled pulmonary, renal, or hepatic dysfunction

               -  Ongoing or active infection requiring systemic treatment

               -  Known active or chronic viral hepatitis or human immunodeficiency virus (HIV)

               -  Psychiatric illness/social situations that would limit compliance with study
                  requirements

               -  Any other illness or condition that the treating investigator feels would
                  interfere with study compliance or would compromise the patient's safety or study
                  endpoints

          -  Female patients who are pregnant or nursing are not eligible