Using congenic mice lacking the endogenous ecotropic MuLV that encodes the MHC class I and II immunodominant tumor antigens from the CT26 tumor, we showed that (1) endogenous T cells specific for a natural TAA successfully prevent tumor development and (2) changes in expression of TAAs may lead to changes in immunologic tolerance and the efficacy of corresponding immunotherapies. The absence of the genomic interval encoding gp70 resulted in an improved T cell response against the CT26 tumor which correlates with more functional, larger, higher avidity, and more diverse TAA-specific T cell responses ( and ). Antibody depletion of CD4+ or CD8+ T cells ablated the tumor protection in the gp70-deficient mice (data not shown), demonstrating this resistance to tumor growth was dependent upon induction of T cell-mediated functions. T cells that escaped negative selection in young gp70-sufficient mice () control tumor growth after vaccination. However, these T cells were not detectable in middle-aged mice () likely due to peripheral and central tolerance initiated by gp70 expression in peripheral tissues, lymph nodes, and the thymus. Loss of T cell responses to tolerance induction by delayed expression of gp70 resulted in improved survival and growth of CT26 cells in vivo ().
Because young gp70-sufficient mice clearly allow expansion of fewer gp70-specific T cells than gp70-deficient animals (), we hypothesized that gp70 protein is expressed in the thymus in concentrations below the limits of detection of our PCR assay, leading to negative selection of high avidity, antigen-specific T cells, and reduced T cell diversity. AIRE, a transcription factor in the thymus that regulates transcription of peripheral tissue antigens, may be responsible for low levels of gp70
transcription leading to negative selection of specific T cells (see [45
] for review). However, some T cells escape negative selection and can be activated by antigen-specific immunotherapies such as irradiated whole cell vaccines secreting GM-CSF [42
] or mimotope vaccines [32
]. During middle-age, control of gp70 protein expression is more widely detectable throughout lymphoid organs and peripheral tissues ( and , ), suggesting widespread loss of control over epigenetic repression mechanisms. The resulting expression of gp70 protein enhances effective control of the remaining gp70-specific T cells by normal tolerance mechanisms, possibly by peripheral deletion, resulting in decreased efficacy of antigen-specific immunotherapies.
As mentioned above, earlier research showed that this MuLV is expressed in LPS-treated splenocytes from BALB/c and C57BL/6 mice over 8 months of age ex vivo [38
]. Although expression of MuLV is low in these strains relative to the AKR strain [46
], we readily detected gp70
expression in a number of tissues from middle-aged mice ( and , ). Furthermore, using an RT-PCR assay, we found gp70
mRNA in middle-aged gp70-sufficient mice in all tissues examined except the uterus () and liver (). We did not detect gp70
expression in fetal tissues or tissues from one week-old mice (data not shown). Since gp70 does not function in normal tissue development and the signals that activate gp70
transcription are unknown, this variability in tissue expression is not surprising. Our reports differ from other reports in the literature. Other investigators could not detect gp70
mRNA in Northern blot analyses of normal colonic epithelium from C57BL/6 mice [47
]. Nor was it detected by RT-PCR analyses of BALB/c epithelial cells from gastrointestinal organs, testes, thymic stroma, and bone marrow-derived thymic elements [18
] or Northern blot analyses other tissues from adult, newborn, and fetal mice from C57BL/6 (testes, thymus, liver, kidney, and brain) [29
]. These discrepancies may result from variations in the ages and strains of the analyzed mice as well as the different sensitivities of the assays.
The utility of human endogenous retroviruses (HERVs) as targets for immunotherapies for cancer or as cancer markers is becoming more intriguing as more HERVs are being discovered. One family of HERVs, HERV-K, comprises 30-50 different proviruses many of which are recognized by the immune system [19
]. HERV-K expression, specifically HERV-K-mel, is low in normal tissues and increased in many tumors [19
]. HERV-K-specific T cells have been detected in melanoma patients [19
] and antibody responses are present in many patients with tumors [48
]. These antigens, particularly if they are cancer-specific, may provide targets for T cell-based therapies. Since endogenous retroviruses do not provide vital functions to the tumor, tumor-specific expression and subsequent recognition of these TAAs by CTL may contribute to a local tumor environment that favors immunosurveillance. However, as shown here, these endogenous viruses may also be expressed in normal peripheral tissues. Thus, responses to targeted retroviral TAAs may lead to autoimmunity [51
] or antigen-specific T cell tolerance. This range of possible responses to a protein with unknown regulation and with no necessary function to the cell may help to explain the variability in tumor-specific responses in experimental and clinical observations. Understanding when and where viral antigens are expressed is critical. New technologies are necessary to investigate these highly repetitive sequences. Thus, development of retroviral-based vaccines for the treatment of human cancers provides a promising therapeutic avenue with further basic research.