ORTHOTOPIC TUMOUR INDUCTION PROTOCOL SHAPES TUMOUR MICROENVIRONMENT

Robust preclinical cancer models are essential for studying tumour progression and evaluating therapeutic strategies. Orthotopic implantation of tumour cells into the mammary gland is widely used to model triple-negative breast cancer (TNBC), yet substantial variation exists in the number of cells used to initiate tumours, often without explicit consideration of how this parameter shapes tumour biology. Here, we systematically assess how tumour cell number at implantation influences the tumour microenvironment using the 4T1 orthotopic TNBC model. Mammary tumours initiated with different cell numbers exhibited comparable early growth kinetics and gross size, yet diverged markedly in cellular density, hypoxia, necrosis, proliferation, vascular integrity, and immune cell infiltration. High-density tumours displayed increased HIF-1 staining and necrosis, alongside differences in vascular-associated markers and increased macrophage recruitment. Conversely, low-density tumours remained highly proliferative, less necrotic and exhibited higher PDGFR{beta} staining. Importantly, tumour cells disseminated systemically within days of implantation across all conditions, although differences in colony recovery and ex vivo expansion were not statistically significant. Together, these findings demonstrate that tumour initiating protocols are powerful determinants of tumour state, with potential implications for dissemination frequency and tumour cell fate at distant sites. Rather than representing a source of experimental variability alone, this understanding provides an opportunity to intentionally tailor orthotopic models to investigate specific stages of disease progression and therapeutic response, thereby enhancing cross-study reproducibility and translational relevance.