-  INCLUSION CRITERIA:

          -  Patients must have histologically or cytologically confirmed metastatic HER2+ breast
             cancer. HER2 positive or amplified breast cancer is defined as IHC 3+ or FISH average
             HER2 copy number greater than or equal to 6 signals per cell or HER2/CEP17 greater
             than or equal to 2.0. HER2 testing must have been performed in a laboratory accredited
             by the College of American Pathology (CAP) or another accrediting entity.

          -  Patients must have hormone receptor negative, HER2+ breast cancer. Hormone receptor
             negative is defined as estrogen receptor < 10% by IHC and progesterone receptor < 10%
             by IHC.

          -  Patients must have measurable disease, per RECIST 1.1.

          -  Patients must have at least one lesion deemed safe to biopsy and be willing to undergo
             up to three biopsies while on trial.

          -  Patients must have received front-line treatment for metastatic disease with a taxane,
             trastuzumab and pertuzumab (THP; docetaxel or paclitaxel allowed) and progressed on
             treatment or were intolerant to treatment. Patients must have received at least one
             prior therapy in the metastatic setting. Prior T-DM1 therapy is allowed.

          -  Female or male greater than or equal to 18 years.

          -  ECOG performance status 0 or 1.

          -  Patients must have adequate bone marrow function as defined below:

               -  absolute neutrophil count greater than or equal to 1,500/mcL (> 1.5X 10(6)/L)

               -  platelets greater than or equal to 100,000/mcL

               -  hemoglobin greater than or equal to 9 mg/dL (transfusion to obtain hemoglobin
                  greater than or equal to 9 mg/dL within 24 hours prior to dosing is allowed)

          -  Patients must have adequate renal function, defined as:

               -  serum creatinine less than or equal to 1.5 X upper limit of normal (ULN) OR

               -  measured or calculated creatinine clearance greater than or equal to 60 mL/min
                  for participant with creatinine levels > 1.5 X institutional ULN (GFR can also be
                  used in place of creatinine or CrCl).

          -  Patients must have adequate hepatic function, defined as AST and ALT levels less than
             or equal to 3 X ULN and total bilirubin < 1.5 X ULN, unless known diagnosis of Gilbert
             s syndrome, where bilirubin less than or equal to 5 mg/dL will be permitted. Gilbert s
             syndrome will be defined as elevated unconjugated bilirubin, with conjugated (direct)
             bilirubin within the normal range and less than 20% of the total. Total bilirubin will
             be permitted up to 5 mg/dL, if patients have historical readings consistent with the
             definition of Gilbert s syndrome prior to entering study. Adequate hepatic function
             for patients with known liver metastases is defined as AST and ALT levels less than or
             equal to 5 X ULN.

          -  Patients must have adequate cardiac function as defined by an ejection fraction
             greater than or equal to 50%.

          -  The effects of BN-Brachyury, entinostat, M7824 and T-DM1 on the developing human fetus
             are unknown. However, the two components of T-DM1 are known to have negative fetal
             effects including oligohydramnios and oligohydramnios sequence (pulmonary hypoplasia,
             skeletal malformations and neonatal death). For this reason:

               -  Women of child-bearing potential must agree to use adequate contraception study
                  entry, for the duration of study participation and for 7 months after the last
                  dose of study medication. Should a woman become pregnant or suspect she is
                  pregnant while she or her partner is participating in this study, she should
                  inform her treating physician immediately.

               -  Men should refrain from fathering a child or donating sperm during the study and
                  for 4 months after the last dose of study medications.

          -  Patients with well-controlled HIV infection are eligible for trial as long as:

               -  On an effective anti-retroviral therapy (ART) 4 weeks and with evidence of viral
                  suppression as defined as HIV viral load 400 copies/mL within the last 3 months;

               -  CD4 greater than or equal to 200 cells/microL within the last 3 months; and

               -  No reported opportunistic infections within 6 months prior to enrollment, except
                  for the following which will be allowed:

                  i. Esophageal candidiasis treated within last 6 months or currently improving
                  with antifungal treatment.

        ii. Oral and/or genital HSV treated within last 6 months or currently improving with
        antiviral treatment.

        iii. Mycobacterium avium infection in last 6 months or that has been treated for at least 1
        month.

          -  Patients with evidence of chronic hepatitis B virus (HBV) infection are eligible for
             trial as long as the HBV viral load is undetectable on suppressive therapy, if
             indicated.

          -  Patients with history of hepatitis C virus (HCV) infection must have been treated and
             cured. For patients with HCV infection who are currently on treatment, they are
             eligible if they have an undetectable or unquantifiable HCV RNA 12 weeks or longer
             after definitive treatment completion.

          -  Patients must be able to understand and willing to sign a written informed consent
             document.

        EXCLUSION CRITERIA:

          -  Patients who have received chemotherapy, including herceptin and/or pertuzumab in the
             prior 3 weeks (6 weeks for nitrosoureas or mitomycin); other investigational agents or
             a PD-1/L1 agent within 4 weeks prior to study enrollment.

          -  Patients who have received radiotherapy within 4 weeks prior to study entry.

          -  Patients with active brain metastases or leptomeningeal metastases should be excluded
             from this clinical trial because of their poor prognosis and because they often
             develop progressive neurologic dysfunction that would confound the evaluation of
             neurologic and other adverse events. However, patients with treated brain metastases
             are eligible if there is no magnetic resonance imaging (MRI) evidence of progression
             for 6 weeks after treatment is complete (no radiotherapy within 6 weeks) and within 28
             days prior to the first dose of trial drug or asymptomatic brain metastasis. Patients
             requiring immunosuppressive doses of systemic corticosteroids (> 10mg/day prednisone
             equivalent) for palliation are excluded.

          -  Patients with a history of another invasive malignancy less than or equal to 3 years
             prior to enrollment (patients with non-melanoma skin cancers, carcinoma in situ of the
             breast or cervix are eligible).

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to agents used in study. For example, reaction to prior vaccination with
             vaccinia virus or known hypersensitivity reaction to fully humanized monoclonal
             antibodies (Grade greater than or equal to 3 NCI-CTCAEv5).

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing active
             infection that requires systemic treatment with ongoing antibiotics (eligible if can
             stop antibiotics on day of enrollment), unstable angina pectoris, cardiac arrhythmia,
             or psychiatric illness/social situation that in the opinion of the primary
             investigator would prohibit the patient from complying with study requirements.

          -  Known history of a gastrointestinal illness that in the investigator s opinion would
             prevent the absorption of entinostat, which is an oral agent. (Arm 3 ONLY)

          -  Patients with bone metastases who have initiated denosumab or a bisphosphonate therapy
             within 28 days prior to or after Cycle 1 Day 1. Continuation of prior therapy is
             allowed.

          -  Patients with inherited bleeding disorders, a history of bleeding diathesis such as
             vWF deficiency or recent (within 3 months prior to enrollment) clinically significant
             bleeding events that, in the judgment of the investigator, would interfere with
             patient s ability to carry out the treatment program.

          -  Patients should have no evidence of being immunocompromised as listed below:

               -  Active, known or suspected autoimmune disease. Patients are permitted to enroll
                  if they have vitiligo, type I diabetes mellitus, residual hypothyroidism due to
                  an autoimmune condition only requiring hormone replacement, psoriasis not
                  requiring systemic treatment or conditions not expected to recur in the absence
                  of an external trigger in the opinion of the primary investigator.

               -  Altered immune function, that in the judgement of the PI that may affect a
                  patient s ability to adequately engage the immune system and respond to the
                  immunotherapy agents being administered, including but not limited to:
                  inflammatory bowel disease; active infectious enteritis; eosinophilic enteritis;
                  lupus erythematous; ankylosing spondylitis; scleroderma; multiple sclerosis.
                  These criteria do not include all disease with an immune-related component, but
                  are not autoimmune in nature or have a primary alteration in the general immune
                  function that may interfere with the vaccine mechanism of action, for example
                  celiac disease.

               -  Immunosuppressive therapy post-organ transplant.

          -  Concurrent use of chronic use of systemic steroids, except for physiologic doses of
             systemic steroids for replacement, defined as 10mg of prednisone per day or
             equivalent, or local (topical, nasal, ophthalmic or inhaled) steroid use or prior
             concomitant use with chemotherapy. Systemic steroids must have been discontinued >2
             weeks prior to trial start. Prior use of corticosteroids in short-term schemes
             (duration shorter than 3 days) for indications such as prophylaxis of reactions to
             intravenous contrast for imaging studies or chemotherapy-related AEs are not
             considered part of this exclusion. Prior use of corticosteroids for brain metastasis
             ending at least 14 days prior to enrollment is not considered part of this exclusion
             criteria.

          -  Pregnant and breastfeeding women are excluded from this study because of the potential
             for teratogenic or abortifacient effects with all of the agents involved in this
             trial.

          -  Clinically significant cardiomyopathy, coronary disease, chronic heart failure (CHF;
             New York Heart Association class III or IV or hospitalization for CHF), or
             cerebrovascular accident within 6 months prior to enrollment.

          -  Patients with a history of myocarditis are excluded due to the potential of
             myocarditis with anti-PD-L1 antibodies.

          -  Patients with pulse oximetry < 92% on room air will be excluded due to the potential
             of pneumonitis with anti-PD-L1 antibodies.

          -  Any other condition, which would, in the opinion of the Principal Investigator or
             Medical Monitor, indicated the subject is a poor candidate for the clinical trial or
             would jeopardize the subject or the integrity of the data obtained.

        ELIGIBILITY CRITERIA FOR ARMS 2 AND 3: M7824, BN-BRACHYURY, T-DM1 +/- ENTINOSTAT IN HER2+BC

        INCLUSION CRITERIA:

          -  Patients must have histologically or cytologically confirmed metastatic HER2+ breast
             cancer. HER2 positive or amplified breast cancer is defined as IHC 3+ or FISH average
             HER2 copy number greater than or equal to 6 signals per cell or HER2/CEP17 greater
             than or equal to 2.0. (117) HER2 testing must have been performed in a laboratory
             accredited by the College of American Pathology (CAP) or another accrediting entity.

          -  Patients must have hormone receptor negative, HER2+ breast cancer. Hormone receptor
             negative is defined as estrogen receptor < 10% by IHC and progesterone receptor <10%
             by IHC.

          -  Patients must have measurable disease, per RECIST 1.1.

          -  Patients in Cohort 3 must have at least one lesion deemed safe to biopsy and be
             willing to undergo up to three biopsies while on trial.

          -  Patients must have received front-line treatment for metastatic disease with a taxane,
             trastuzumab and pertuzumab (THP; docetaxel or paclitaxel allowed) and progressed on
             treatment or were intolerant to treatment. Patients must have received at least one
             prior therapy in the metastatic setting. Prior T-DM1 therapy is allowed.

          -  Female or male greater than or equal to 18 years.

          -  ECOG performance status 0 or 1

          -  Patients must have adequate bone marrow function as defined below:

               -  absolute neutrophil count greater than or equal to1,500/mcL (greater than or
                  equal to 1.5X 106/L)

               -  platelets greater than or equal to 100,000/mcL

               -  hemoglobin greater than or equal to 9 mg/dL (transfusion to obtain hemoglobin
                  greater than or equal to 9 mg/dL within 24 hours prior to dosing is allowed)

          -  Patients must have adequate renal function, defined as:

               -  serum creatinine less than or equal to 1.5 X upper limit of normal (ULN) OR

               -  measured or calculated creatinine clearance greater than or equal to 60 mL/min
                  for participant with creatinine levels > 1.5 X institutional ULN (GFR can also be
                  used in place of creatinine or CrCl).

          -  Patients must have adequate hepatic function, defined as AST and ALT levels less than
             or equal to 3 X ULN and total bilirubin < 1.5 X ULN, unless known diagnosis of Gilbert
             s syndrome, where bilirubin less than or equal to 5 mg/dL will be permitted. Gilbert s
             syndrome will be defined as elevated unconjugated bilirubin, with conjugated (direct)
             bilirubin within the normal range and less than 20% of the total. Total bilirubin will
             be permitted up to 5 mg/dL, if patients have historical readings consistent with the
             definition of Gilbert s syndrome prior to entering study. Adequate hepatic function
             for patients with known liver metastases is defined as AST and ALT levels less than or
             equal to 5 X ULN.

          -  Patients must have adequate cardiac function as defined by an ejection fraction
             greater than or equal to 50%.

          -  The effects of BN-Brachyury, entinostat, M7824 and T-DM1 on the developing human fetus
             are unknown. However, the two components of T-DM1 are known to have negative fetal
             effects including oligohydramnios and oligohydramnios sequence (pulmonary hypoplasia,
             skeletal malformations and neonatal death). For this reason:,

               -  Women of child-bearing potential must agree to use adequate contraception study
                  entry, for the duration of study participation and

        for 7 months after the last dose of study medication. Should a woman become pregnant or
        suspect she is pregnant while she or her partner is participating in this stud...