This study will determine how the intestinal microbiome differs between patients with obesity and early triple-negative breast cancer who achieve a pathologic complete response from preoperative anti-PD-1 immunotherapy (pembrolizumab) versus patients who do not.

Inclusion Criteria:

• Age ≥18 years
• Female biological sex
• Newly diagnosed, previously untreated, histologically confirmed early triple-negative breast cancer
• Plan to initiate preoperative anti-PD-1 immunotherapy using pembrolizumab
• Overweight or obesity, defined as a body mass index >=25 kg/m2
• Ability to provide written informed consent
• Allow the collection and storage of biospecimens and data for future use

Exclusion Criteria:

• Active autoimmune disease
• Concomitant conditions that require the use of antibiotics (e.g., chronic sinusitis)
• Digestive disease disorders (e.g., irritable bowel syndrome, Crohn's disease)
• Currently pregnant, breastfeeding, or planning to become pregnant within the next 24 weeks
• Recent (within 90 days) use of glucocorticoids for more than 10 consecutive days
• Any other cardiovascular, pulmonary, orthopedic, neurologic, psychiatric, or other condition that, in the investigator's opinion, would preclude participation or successful compliance with the protocol
• Any other situation that, in the opinion of the investigator, would negatively impact subject safety or successful compliance with the protocol

The study population will consist of survivors of early triple-negative breast cancer.