SUMMARYThe expression of H‐2 antigenic specificities on Meth‐A and meth‐A‐vaccinia were compared. Their ability to inhibit complement dependent cytotoxicity of51Cr‐labelled normal lymphoid target cells was tested by anti H‐2 sera of restricted specificity. Normal lymphocytes positive or negative for a particular specificity, were used as controls. With this assay the expression of H‐2‐like specificities of foreign haplotypes on Meth‐A cells passaged together with vaccinia virus was confirmed.In addition, using an immunofluorescence technique the expression of some foreign H‐2 antigens on non‐infected Meth‐A tumour cells was shown. Anti‐vaccinia virus sera were used to compare the reactivity of Meth‐A and Meth‐A‐vaccinia. Unexpectedly, anti‐vaccinia sera stained non‐infected Meth‐A and other tumour cells of different H‐2 origin and absorption of these sera with normal Meth‐A left little activity behind when testing against Meth‐A‐vaccinia.Finally, the r#x006f;#x030c;le that the foreign H‐2 antigenic specificities present in Meth‐A after vaccinia virus infection may have in the growth of these tumour cells in syngeneic recipients was studied. A powerful inhibitory effect was observed and regressor mice were found to be resistant to a challenge of non‐infected meth‐A. In contrast, vaccinia virus was irrelevant to the rate of growth of P815 Y in DBA/2 mice.These results are discussed in relation to the origin of the foreign H‐2 antigens on tumour cells after virus infection (derepression) and the role that
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