Peripheral Blood Immune Cell Profiling as Predictive Biomarkers for Neoadjuvant Therapy Response in Early Breast Cancer Patients

Background: In breast cancer (BC), pathological complete response (pCR) to neoadjuvant therapy (NAT) improves survival. Escalated NATs aim to increase pCR, requiring predictive biomarkers. Tumor-induced T cell exhaustion and senescence impair anti-tumor immunity, but their role in NAT response is unclear. We assessed their potential as pCR biomarkers across BC subtypes. Methods: In this prospective study at Maggiore Hospital (Novara, Italy), blood samples from BC patients were collected before and after NAT. Circulating CD3⁺CD4⁺ and CD3⁺CD8⁺ T cells were profiled by flow cytometry for exhaustion markers (PD-1, LAG-3, TIGIT) and senescence markers (CD57, KLRG-1). Myeloid-derived suppressor cells (MDSCs) were also evaluated and classified into monocytic (M-MDSCs) and polymorphonuclear (PMN-MDSCs), the latter characterized by LOX-1 expression. Systemic inflammation indices (NLR, LMR) were calculated. Results: At baseline, ER⁺ tumors showed high NLR and exhausted CD8⁺PD-1⁺TIGIT⁺ cells; HER2⁺ tumors had more senescent CD8⁺CD57⁺KLRG-1⁺ cells. TN patients had elevated LMR. Lower NLR (OR=0.57, p=0.026) and higher LMR (OR=1.36, p=0.033) correlated with pCR. In HER2⁺ cases, fewer CD8⁺CD28⁻CD57⁻KLRG-1⁺ T cells were linked to better response (OR=0.68, p=0.029). Non-responders had higher NLR and more CD4⁺PD-1⁺TIGIT⁺ cells. ER⁺ non-responders showed increased exhausted and senescent CD4⁺ subsets. Post-NAT, HER2⁺ and TN patients displayed expansions in exhausted and senescent T cells. MDSC levels did not differ by pCR status but showed subtype-specific correlations: LOX-1⁺ PMN-MDSCs correlated with exhausted T cells in ER⁺, M-MDSCs with senescent T cells in HER2⁺, and inversely with LAG-3⁺ cells in TN. Conclusion: T cell exhaustion and senescence predict poor pCR in ER+ and HER2+ BC. Low NLR and high LMR correlate with better response. MDSCs may contribute to immune suppression via subtype-specific interactions with dysfunctional T cells.