(37,38). Colonies of transfected cells expressing MAA were identified and 
used to immunize mice syngeneic with the tumor. Following immunization, the 
animals exhibited evidence of cellular immunity toward the melanoma and 
survived significantly (P < .001) longer following challenge than non 
immunized mice or mice in various control groups (37) . Thus, transfecting 
genes that specify melanoma-associated determinants into cells that express 
allogeneic histocompatability antigens generated a cellular immunogen capable 
of generating strong anti melanoma responses and prolonging survival. 
Further evidence that growth of a potentially antigenic neoplasm results from 
a failure of immune recognition is provided by the results of experiments 
implant tumor cells with genes specifying defined cytokines. Genes for IL-2 
were transduced into weakly antigenic tumor cells by means of replication- 
defective retroviruses (39,40). The IL-2-secreting tumor cells were rejected 
by mice syngeneic with the neoplasm under conditions in which unmodified cells 
grew progressively. Significantly, animals rejecting the IL-2-secreting 
malignant cells were at least partially immune to challenge with the 
unmodified cells. Secretion of IL-2 by the tumor cells by-passed the 
requirement for CD4 + T-helper cells in the induction of the anti tumor 
response; tumor immunity developed in the absence of this ordinarily essential 
subset of T lymphocytes. Thus, two components of the cellular vaccine 
augmented immune recognition of weakly antigenic neoplasms: the expression of 
tumor-associated determinants and secretion of IL-2 by tumor cells. Cells 
modified in this manner lost malignant growth properties in immune competent 
recipients. Animals rejecting the cells exhibited both in vitro and in vivo 
evidence of anti tumor immunity. 
Does immunization with a cellular immunogen that combines IL-2 secretion and 
the expression of tumor associated determinants lead to an enhancement of the 
anti tumor immune response? Does the expression of allogeneic 
histocompatability antigens lead to an enhancement of anti tumor immunity 
response? Experiments performed in our laboratories indicate that inbred mice 
immunized with a construct that combined IL-2 secretion and co-expression of 
allogeneic antigens and melanoma associated antigens generated anti melanoma 
immune responses that were capable of prolonging the survival of mice with 
melanoma. In some instances, tumor-bearing animals rejected the neoplasm and 
survived indefinitely. The anti tumor immune response following immunization 
with a cellular immunogen that combined the immunogenic properties of IL-2- 
secretion and expression of allogeneic antigens and MAA exceeded those 
following immunization with immunogens that were deficient in one or more of 
these immunogenic properties. Constructs that did not express MAA but formed 
IL-2, or those that expressed MAA but did not secrete IL-2, were less 
immunogenic in both in vitro and in vivo tests of immunity. Immunization of 
mice syngeneic with the constructs led to weak anti tumor responses, pointing 
to the role of allogeneic antigens in the anti melanoma response. The 
rejection of the melanoma was mediated primarily by CD8+ T lymphocytes. If 
the construct secreted IL-2, CD4 + T-helper cells were not required for 
induction of the anti melanoma response. 
In analogous studies, Fearon et al. (39) transfected the IL-2 gene into a 
murine tumor cell line, CT26. Like B16 melanoma, CT26 cells ordinarily grow 
without inhibition in syngeneic (BALB/c) recipient mice. The IL-2-secreting 
tumor cells lost their malignant growth properties and were rejected by BALB/c 
mice. Significantly, mice rejecting the modified cells were partially 
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Recombinant DNA Research, Volume 18 
