Histopathology of regression of tumor metastasis in the lymph nodes

A transplanted rat lymphosarcoma (SGT-4), induced by Gross virus and inoculated s.c. into the foot of a normal syngeneic rat, initially grew but ultimately regressed. The tumor cells metastasized to the regional popliteal, lumbar, and inguinal lymph nodes and formed massive metastatic foci there. These lymph node metastases also regressed spontaneously. However, in Gross-tolerant rats inoculated with Gross virus at birth, no regression was observed. Histopathologically, infiltration and proliferation of lymphoid cells, reticulum cells, and bifrocytes occurred in the regressing metastatic tumor in lymph nodes as well as in the regressing transplanted tumor in the foot. Only in lymph nodes of normal rats, in which tumor metastasis regressed, was the characteristic "starry sky" appearance observed. Our results suggest that regression of metastatic tumor in lymph nodes, as well as of transplanted tumor in syngeneic rats, was due to an immunological reaction by the host and that an immunological factor may be responsible for the "starry sky" picture.