Immune control of tumors by antigen presentation improvement.

Tumor cells cannot activate T lymphocytes, since they do not usually express major histocompatibility complex (MHC) class II molecules. Thus, tumor antigens can only be presented indirectly to T cells through professional antigen-presenting cells (APC). In our laboratory, we have treated a tumor cell line (Tu1-A) - derived from an induced rat mammary sarcoma - in order to increase the expression of MHC class I and class II molecules. In our tumor model, the transference of these induced cells into normal rats generated a tumor mass that exhibited a lower tumor growth rate and an earlier regression as compared to those observed in rats inoculated with wild-type Tu1-A cells. This earlier tumor regression was associated with the development of an antigen-specific immune response. 85-87% of the rats in both groups rejected the tumor and were alive at day 60 after tumor cell inoculation. However, in rats treated with wild-type cells the rejection was delayed and took place after tumor ulceration. Rats that had rejected tumors were rechallenged with wild-type cells in order to assay the presence of a long-lived antitumor immunity. All the animals were resistant to the second tumor challenge. We conclude that the development of a specific immune response could be achieved by the superexpression of MHC molecules on tumor cells or when tumor ulceration promotes APC to take up necrotic cells and tumor antigens are presented to T lymphocytes.