Converting HR+ Breast Cancer Into an Individualized Vaccine

Patients will be on the study for a total of 5 months, this includes 4 months on active study intervention, with breast surgery at week 16 and one month follow up period, after surgery. Patients will be randomly assigned to one of these 4 arms - 1. Anti-PD1 antibody pembrolizumab (Keytruda, Merck) will be infused day 12, at the standard dose of 200 mg IV over 30 minutes, repeated every 3 weeks until disease progression or unacceptable toxicity. 2. FLT3L (CDX-301, the recombinant human protein by Celldex) will be self-administered subcutaneously, in 5 consecutive daily injections, week 1, day 1-5. 3. For all arms radiation therapy to the breast tumor will begin on week 2 (Day 8,10,12), at dose of 8 Gy x 3 fractions, every other day. 4. Letrozole (Femara ®, Novartis) 2.5 mg tabs, once a day, daily for 4 months, until surgery, and thereafter is decided by the treating physician.