Novel animal model of lymph node metastasis by intrauterine inoculation of the actively metastatic subline PL3 separated from rat Walker 256 tumor cells.

To investigate the cellular mechanism of lymph node metastasis by tumor cells through the lymphatic vessels in the uterine corpus, we selected an active metastatic subline (PL3) from rat Walker 256 tumor cells and used it to develop a novel experimental model of lymph node metastasis induced by intrauterine inoculation of the tumor cells. Light- and electron-microscopic examinations revealed that the inoculated PL3 cells could actively infiltrate the endometrium from the uterine cavity and form a primary lesion in the uterine corpus. A few PL3 cells in the myometrium were found in the lumen of the peripheral lymphatic vessels on day 7 after inoculation. The regional lymph nodes around the uterus were then invaded by the migrated PL3 cells, and finally (after 3 weeks), most of the parenchyma of the nodes was replaced by metastasized tumor cells. By flow-cytometric analysis, the metastatic PL3 cells expressed CD44, like Walker 256 cells, but lacked integrin alphaL- and alpha4-chains. However, expression of ICAM-1 was considerably down-regulated in the PL3 cells compared to the parent cells. More aggressive invasion was shown by the PL3 cells compared to the parent cells in the in vitro invasion assay. These findings suggest that this experimental model and the separated PL3 cells are suitable for thorough investigations of the unidentified metastatic process and the related cellular behavior involved in the onset of lymphatic invasion by the primary lesion. Furthermore, our model more closely reproduces the clinical conditions related to lymph node metastasis of malignant carcinomas through the lymphatic vessels than does any previously reported animal model.