Marek's disease virus-induced progressive tumors and Marek's disease transplantable local regressive tumors were dispersed by treatment with collagenase and cells were examined in vitro for cytotoxic effector functions against target cells of tumor lines. Both types of tumors had adherent and non-adherent cytotoxic cells. The cytotoxicity of adherent cells was detected in an 18-hour but not in a 4-hour 51Cr-release assay. The adherent effector cells from progressive tumors were inactivated by pretreatment with carbonyl iron and carrageenan whereas the adherent effector cells from the regressive tumors were refractory to these treatments. In the progressive tumors, the 18-hour cytotoxic activity of cells of tumors and of spleens of tumor-bearing chickens was compared; the activity was higher in the tumor than in spleen. The non-adherent cell cytotoxicity detectable in a 4-hour 51Cr-release assay was associated with anti thymocyte serum-resistant natural killer cells. The incidence and levels of natural killer cell reactivity were greater in the regressive tumors than in the progressive tumors. In the regressive tumors, the natural cytotoxicity levels were higher in the tumor than in the spleen of tumor-bearing chickens. The differences in characteristics of adherent cytotoxic cells in virus-induced progressive tumors and transplantable regressive tumors and elevated levels of NK cells in regressive but not in progressive tumors may indicate a role for intratumoral immunity in tumor regression.
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