Are cytotoxic T cells a common homeostatic mechanism in responses to viruses, homografts and tumours?

Cytotoxic T cells (Tc) have been shown to be important in the rejection of histoincompatible tissue grafts. They are also generated in mice during infection with viruses that are known to express viral coded antigens at the infected cell surface, although their presence is much easier to demonstrate with some viral infections than with others. Cell-mediated lysis only occurs if the Tc and virus-infected target cells share gene products coded for in the K or D region of the H-2 complex. In the case of both ectromelia and influenza virus infection of mice, transfer of specific Tc to histocompatible, infected mice has been shown to significantly lower the virus titre in target organs (spleen and lungs respectively). In experimental animals with some tumours, there is increasingly good evidence for the expression of tumour specific transplantation antigens (TSTA) in the membrane of the malignant cells, yet there is little evidence for the presence of specific Tc which might control the growth of the tumour. Possible reasons for the lack of generation or expression of Tc in these situations are discussed.