Repeated fine-needle aspiration of solid tumours in mice allows the identification of multiple infiltrating immune cell types

• Analysing immune cells in tumours by fine-needle aspiration (FNA) + flow cytometry.
• We show good concordance between results from FNAs and the whole tumour.
• Repeated FNAs do not affect survival or frequency of metastasis.
• The number of mice used per experiment is greatly reduced.
• Can correlate key immune cells in tumour to increased survival of individual mice.

This paper describes a novel method for following the changes in mouse tumour-infiltrating immune cell populations by repeated sampling of tumours by fine needle aspiration (FNA), followed by flow cytometry. Using this technique we were able to collect samples from P815 mouse mastocytomas, and identify and enumerate six tumour-infiltrating immune cell types at multiple time points for each mouse. We demonstrate good agreement between cell percentages obtained from FNA samples and matched whole tumour digests (WTDs). We also demonstrate that neither survival nor the incidence of liver metastasis is adversely affected by the procedure. Our method has a clear advantage over the common practice of sacrificing mice and collecting tissue at pre-determined time points, as the technique allows 1) repeated sampling of each mouse over time, thus many fewer mice are required, and 2) the correlation of survival data with tumour-infiltrating immune cell types at different time points. This potentially allows immune cell types associated with increased or decreased survival to be identified. Therefore, our technique should greatly facilitate the characterisation of anti-tumour immunity induced in response to cancer therapy in small animal models.