Immunologic Targeting of ESR1 Receptor for Hormone Receptor Expressing Metastatic Breast Cancer

This is a pilot study to determine feasibility and safety of the combination of Dendritic Cell (DC1) vaccines and elacestrant in patients with hormone positive HER2 negative metastatic breast cancer.

Inclusion Criteria: * Participants must have histologically or cytologically confirmed diagnosis of hormone positive HER2 negative metastatic breast cancer per ASCO/CAP criteria, with diagnosis established through either a breast/axillary biopsy or biopsy of a metastatic lesion. 1. Estrogen Receptor (ER) or Progesterone Receptor (PR) are considered positive when expressed ≥1% on immunohistochemistry (IHC). 2. HER2 is considered negative by IHC when expression is 0 or 1+ and if equivocal 2+ then a reflex in situ hybridization should be not amplified (standard practice per ASCO/CAP criteria). * Participants must have Presence of an ESR1 mutation detected via tissue based or blood based (ctDNA) genomic profiling. * Participants must have been previously treated with at least 1 line of endocrine therapy and a CDK 4/6 inhibitor in the metastatic setting. * Participants must have measurable or nonmeasurable (evaluable) disease on imaging by RECIST v1.1. * Participants must have an Eastern Cooperative Oncology Group (ECOG) performance status 0 to 2. * Participants must be adults 18 years or older. * Participants must have the ability to understand and the willingness to sign a written informed consent document. * Participants must be able to read and speak standard English or Spanish. * Participants must have adequate organ and marrow function as defined below: 1. absolute neutrophil count ≥1,000/mcL 2. platelets ≥75,000/mcL d. AST(SGOT)/ALT(SGPT) ≤3 fold × institutional ULN e. creatinine 1.5 ≤ institutional ULN f. hemoglobin (Hb) ≥ 9 g/dL g. Total bilirubin \< 1.5 x ULN or \