Highly aggressive rat prostate tumors rapidly precondition regional lymph nodes for subsequent metastatic growth.

The aim of this study was to examine in what ways MatLyLu (MLL) rat prostate tumors with high metastatic capacity influence regional lymph nodes prior to metastatic establishment compared to AT1 rat prostate tumors with low metastatic potential. MLL or AT1 tumor cells were injected into the ventral prostate of immunocompetent rats. Tumor and lymph node morphology, and lymph node mRNA expression of macrophage associated markers, T-cell associated markers, and cytokines were examined over time until the first microscopic signs of metastases (at day 14 for MLL- and at day 28 for AT1-tumors). Already at day 3 after tumor cell injection, when the tumors were extremely small and occupied less than 1% of the prostate volume, MLL- and AT1-tumors provoked different immune responses in both the prostate and the regional lymph nodes. MLL-tumors induced expression of immunosuppressive cytokines, suppressed T-cell accumulation, and directed T-cells towards an immunosuppressive phenotype. AT1-tumors caused a response more similar to that in vehicle-injected animals, with accumulation of T-cells in tumors and regional lymph nodes. Prostate tumors with high metastatic potential were able to precondition regional lymph nodes to subsequent metastatic growth in ways different from tumors with less metastatic potential. This may indicate the existence of a time-window when pre-metastatic changes in regional lymph nodes can aid in the prognostication of locally aggressive and potentially metastatic prostate cancer.