Growth and spontaneous regression of swine melanoma: relationship of in vitro leukocyte reactivity.

A visual microcytotoxicity technique was used to evaluate the leukocyte reactivity of melanoma swine against allogeneic swine melanoma target cells. Peripheral blood leukocytes, which had been collected and cryopreserved in liquid nitrogen at various times during in vivo tumor growth and regression, were thawed and tested on the same day. Comparison of longitudinal leukocyte reactivity with in vivo tumor volume indicated that swine with regressing melanomas exhibited increased leukocyte reactivity during tumor regression. Swine with maximum tumor volumes less than 30,000 mm3 exhibited patterns of leukocyte reactivity that paralleled the patterns of in vivo tumor growth and regression. However, swine with maximum tumor growth and regression. However, swine with maximum tumor volumes greater than 30,000 mm3 demonstrated increased in vitro leukocyte reactivity at the time of maximum in vivo tumor volume. Histopathologic analyses revealed that increases in tumor volume were frequently a result of host inflammatory cells, particularly pigment-laden macrophages, infiltrating the tumors. Thus at the time of maximum tumor volume, malignant melanocytes were proportionately decreasing in number while pigment-laden macrophages were proportionately increasing. These studies provide additional evidence that the spontaneous tumor cell regression of swine melanoma is associated with immunologic events and that assays of leukocyte reactivity are useful in vitro correlates of host antitumor immunity.