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Lymphoid cell infiltration during breast cancer growth: A syngeneic rat model

The systematic study of potential alterations in lymphoid infiltrates during tumour growth is extremely limited in humans. Therefore, development of a model utilizing a spontaneously arising mammary adenocarcinoma in Dark Agouti rats was adopted for the study of the dynamics of lymphoid cell infiltration during tumour development. Syngeneic rats were inoculated with tumour cell suspensions and the tumours were resected from 5 to 15 days. Serial sections were immunohistochemically stained using a panel of monoclonal antibodies. Irrespective of tumour age, ED2 (macrophages) and W3/25 (CD4)‐positive cells were the most prominent cell infiltrates in tumours. There were no significant differences in cell counts for any marker between 8‐day and 15‐day tumours. However, in 5‐day tumours there were significantly fewer macrophages, OX19+ T cells, W3/25+ cells, OX8+ (CD8) cells and OX62+ dendritic cells. Interleukin‐2 receptor α chain expression was low at all examined stages of tumour growth, indicating a lack of tumour infiltrating lymphocyte (TIL) activation and/or possible TIL anergy. B cell staining was absent in all tumours, negating the possibility of these cells mediating coregulatory signals for TIL activation in the micro‐environment of established tumours. The results parallel previous immunohistochemical findings in humans, suggesting that a dysfunctional local immune response in breast cancer may be determined very early during tumour development.