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Immune-checkpoint blockade (ICB) combined with neoadjuvant chemotherapy improves pathological complete response in breast cancer (BC). To understand why only a subset of tumors respond to ICB, patients with hormone receptor-positive or triple-negative BC were treated with anti-PD1 prior to surgery. Paired pre-versus on-treatment biopsies from treatment-naïve patients receiving anti-PD-1 (n=29) or patients receiving neoadjuvant chemotherapy prior to anti-PD1 (n=11) were subjected to single-cell transcriptome, T-cell receptor and proteome profiling. One-third of tumors contained PD1-expressing T-cells, which clonally expanded upon anti-PD1 treatment irrespective of tumor subtype. Expansion mainly involved CD8 + T-cells with pronounced expression of cytotoxic-activity (PRF1, GZMB), immune-cell homing (CXCL13) and exhaustion markers (HAVCR2, LAG3), and CD4 + T-cells characterized by expression of T-helper-1 (IFNG) and follicular-helper (BCL6, CXCR5) markers. In pre-treatment biopsies, the relative frequency of immunoregulatory dendritic cells (PD-L1), specific macrophage phenotypes (CCR2 or MMP9) and cancer cells exhibiting MHC class I/II expression correlated positively with T-cell expansion. Conversely, undifferentiated preeffector/memory T-cells (TCF7, GZMK) or inhibitory macrophages (CXCR3, C3) were inversely correlated. Collectively, our data identify various immunophenotypes and associated gene sets that are positively or negatively correlated with T-cell expansion following anti-PD1. We shed light on the heterogeneity in treatment response to anti-PD1 in breast cancer.