Inclusion Criteria:  Unresectable Stage III or Stage IV Triple Negative Breast cancer Age 18 years or older Performance status ECOG 0-2 Life expectancy ≥ 12 weeks Documented progressive disease, based on radiographic, clinical or pathologic assessment, during or subsequent to last anticancer therapy. For initial safety cohort, subject is in second to third line setting of treatment for metastatic or unresectable disease, and have received 1 to 2 prior regimens for metastatic or unresectable disease. For dose expansion, subject is in first to third line setting of treatment for metastatic or unresectable disease, and have received 0 to 2 prior regimens for metastatic or unresectable disease. Among any patient enrolled in the first line treatment setting, subjects must be PD-L1 negative by either SP142 or 22C3 assay and not be eligible for FDA approved standard of care chemotherapy and anti-PD-1/PD-L1 combination therapy as alternative to this clinical trial.  Screening laboratory values must meet the following criteria  Neutrophils ≥ 1500/uL Platelets ≥ 100 x109/L Hemoglobin ≥ 8 g/dL Patients may receive erythrocyte transfusions to achieve this hemoglobin level at the discretion of the investigator. Initial treatment must not begin earlier than the day after the erythrocyte transfusion. Creatinine ≤ 2 mg/dL Creatinine clearance >30 mL/minute AST ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis ALT ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis Total Bilirubin ≤ 1.5 X ULN (except patients with Gilbert's syndrome or liver involvement, who must have a total bilirubin ≤ 2 X ULN) Alkaline phosphatase ≤ 2.5 X ULN without, and ≤ 5 x ULN with hepatic metastasis  All men as well as women of child bearing potential enrolled in this trial must agree to use effective contraception during the course of the trial and for at least 6 months after discontinuing study treatment. Patients and/or partners who are surgically sterile or postmenopausal are exempt from this requirement.  A female of child-bearing potential is any woman (regardless of sexual orientation, marital status, having undergone a tubal ligation, or remaining celibate by choice) who meets the following criteria: ( 1) has not undergone a hysterectomy or bilateral oophorectomy OR (2) has not been naturally postmenopausal for at least 12 consecutive months (i.e., has had menses at any time in the preceding 12 consecutive months). Provision of consent for pre-treatment and on-treatment biopsies. Biopsy sites must be soft tissue tumor lesions or accessible visceral diseases that can be biopsied with acceptable clinical risk (as judged by the investigator); are large enough to allow for the collection of tumor tissue for proposed correlative studies (e.g., anticipated goal of 6-8 cores preferred when feasible using a ≥ 18 gauge needle with an expected core sample length of 5 mm); and have not been irradiated prior to entry. This does not include bone lesions. This may exclude many lung lesions and small lesions. Measurable disease allowing for serial assessment of at least one target lesion(s) by RECIST 1.1 criteria [100]. Target lesions selected for tumor measurements should be those where additional (e.g., palliative) treatments are not indicated or anticipated. All residual toxicity related to prior anticancer therapies (excluding alopecia, grade 2 fatigue, vitiligo, endocrinopathies on stable replacement therapy, grade 2 neuropathy from taxanes or platinum and grade 2 hearing loss from platinum) must resolve to grade 1 severity or less (or returned to baseline) prior to receipt of study treatment. Read, understood, and provided written informed consent, and if applicable, Health Insurance Portability and Accountability Act (HIPAA) authorization, after the nature of the study has been fully explained, and must be willing to comply with all study requirements and procedures.  Exclusion Criteria:  Among any patients enrolled in the first line treatment setting, tumors should not be PD-L1+ by SP142 or 22C3 assays (at least one of these assays must be checked by treating physician per their preference) or eligible for FDA approved standard of care chemotherapy and anti-PD-1/PD-L1 combination therapy as alternative to this clinical trial. History of severe hypersensitivity reactions to mAbs. Prior treatment with any anti-CD40 antibody or rhuFlt3L product. Treatment with anthracycline in the metastatic setting. Prior progression while on anthracycline based therapy or within 6 months of completing (neo)adjuvant chemotherapy regimen. Prior history of acute myeloid leukemia (AML), or tumor with known Flt3 mutation/amplification Receipt of any antibody targeting T cell check point or co-stimulation pathways within 4 weeks, use of any other monoclonal based therapies within 4 weeks, and all other immunotherapy (tumor vaccine, cytokine, or growth factor given to control the cancer) within 2 weeks prior to the planned start of study treatment. Prior T-cell or other cell-based therapies within 12 weeks (or 2 weeks if patient experienced disease progression on the prior treatment) Systemic radiation therapy within 4 weeks, prior focal radiotherapy within 2 weeks, or radiopharmaceuticals (strontium, samarium) within 8 weeks prior to the first dose of study treatment. Chemotherapy within 21 days or at least 5 half-lives (whichever is shorter) prior to the planned start of study treatment. Any kinase inhibitors within 2 weeks prior to the first dose of study treatment. Major surgery within 4 weeks prior to the first dose of study treatment. Surgery requiring local/epidural anesthesia must be completed at least 72 hours before study drug administration and patients should be recovered. Use of other investigational drugs within 4 weeks or 5 half-lives (whichever is longer) prior to study treatment administration. Use of immunosuppressive medications within 4 weeks or systemic corticosteroids within 2 weeks prior to first dose of study treatment. Topical, inhaled or intranasal corticosteroids (with minimal systemic absorption) may be continued if the patient is on a stable dose. Non-absorbed intraarticular corticosteroid and replacement steroids (≤ 10 mg/day prednisone or equivalent) will be permitted. Other prior malignancy, except for adequately treated basal or squamous cell skin cancer or in situ cancers; or any other cancer from which the patient has been disease-free for at least 3 years. Active, untreated central nervous system metastases. Patients with known treated brain metastases should be neurologically stable for 4 weeks post-treatment and prior to study enrollment. Continued use of steroids and/or anticonvulsants (in the absence of any suspicion of progressive brain metastases) is acceptable if ≤ equivalent of prednisone 10 mg daily. Brain MRI required on screening to document lack of progression. Women who are pregnant or nursing. All female patients with reproductive potential must have a negative pregnancy test prior to starting treatment. Active autoimmune disease or a documented history of autoimmune disease, or history of potential autoimmune syndrome that required systemic steroids or immunosuppressive medications, except for patients with vitiligo, endocrinopathies, type 1 diabetes, or patients with resolved childhood asthma/atopy or other syndromes which would not be expected to recur in the absence of an external trigger (e.g., drug-related serum sickness or post-streptococcal glomerulonephritis). Patients with mild asthma who require intermittent use of bronchodilators (such as albuterol) who have not been hospitalized for asthma in the preceding 3 years will not be excluded from this study. Significant cardiovascular disease including unstable angina pectoris, uncontrolled hypertension or arrhythmia, congestive heart failure (New York Heart Association Class III or IV or EF<50%) related to primary cardiac disease, uncontrolled ischemic or severe valvular heart disease or any of the following within 6 months prior to the first dose of study treatment: myocardial infarction, severe/unstable angina, coronary artery bypass graft, congestive heart failure, cerebrovascular accident, transient ischemic attack. Prior anthracycline therapy with a cumulative doxorubicin-equivalent dose greater than 240 mg/m2. Has received a live vaccine within 30 days prior to the first dose of study drug. Examples of live vaccines include, but are not limited to the following: measles, mumps, rubella, varicella/zoster (chicken pox), yellow fever, rabies, Bacillus Calmette-Guérin (BCG), and typhoid vaccine. Seasonal influenza vaccines for injection are generally killed virus vaccines and are allowed; however, intranasal influenza vaccines (e.g., FluMist®) are live attenuated vaccines and are not allowed. The COVID-19 vaccines available in the United States are not live vaccines and are allowed if the final vaccine dose (of a regimen that requires more than 1 dose) is received at least 1 week prior to study enrollment. History of (non-infectious) pneumonitis or has current pneumonitis. This includes asymptomatic infiltrates on screening chest CT scan that are suggestive of an inflammatory process (i.e. grade 1 pneumonitis). Active infection requiring systemic therapy, known HIV infection, or positive test for hepatitis B surface antigen or hepatitis C (antibody screen and if positive confirmed by RNA analysis). If positive results are not indicative of a true active or chronic infection, the patient can be enrolled after discussion with and agreement by the Investigator. Any other acute or chronic medical or psychiatric condition or laboratory abnormality that could increase the risk associated with trial participation or trial drug administration or could interfere with the interpretation of trial results and, in the judgment of the investigator, would make the patient inappropriate for entry into the trial. Evidence of acute or chronic infection on screening chest radiography.