episomal, since plasmid DNA can be recovered by transformation 1 
month after initial injection from Hirt supernatant DNA (-100- 
1000 colonies//xg) but is not detectable in genomic DNA by 
Southern blot. 
IV. Efficacy of vector in mouse model. 
The prototype vector II containing H2-K s has been used to 
provide tumor immunity in the murine model. Our cumulative 
experience with the MCA 106 fibrosarcoma in C57BL/6 mice is 
indicated below. 
Injected ^-galactosidase H-2K S 
Plasmid: 
Responders/Total Responders/Total 
0/13 13/18 (p = .01) 
Table I. Results of H-2K S injection in MCA106 tumors. 
Responders demonstrated at least 50% reduction in tumor diameter 
1 month post-injection. 
These results, together with those reported previously and 
in the following section, indicate that the introduction of 
foreign MHC into tumors can provide protective immunity for at 
least 3 independent malignancies using 3 independent eukaryotic 
expression vectors. 
V. Protective immunity in the B16BL/6 murine melanoma: 
induction of systemic immunity. 
Lymph nodes (LN) draining immunogenic tumors contain 
sensitized but not fully functional pre-effector cells. These 
cells can acquire antitumor effector reactivity upon further 
activation with aCD3 and IL-2. However, with the poorly 
immunogenic B16BL6 melanoma (H-2 b ) , the tumor does not elicit a 
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