Autoimmunity and the immunotherapy of cancer: targeting the "self" to destroy the "other"

It is increasingly clear that immunity to "self"-antigens may result in tumor destruction in mouse and man. But which antigens should be targeted with therapeutic cancer vaccines? In the case of melanoma, recognition of melanocyte differentiation antigens (MDA) can be associated with autoimmune depigmentation (vitiligo). We propose that intersection of protein transport to melanosomes and endosomes allows for the loading of MDA-derived peptides on MHC class II molecules, resulting in the activation of MDA-specific CD4+ "helper" T cells that aid the induction of melanoma-specific CD8+ T cells. Thus, the immunogenicity of MDA may be a consequence of their unique cell biology. Studies of MDA-based vaccines can provide new insight into the development of more effective cancer vaccines.

Vaccination with a recombinant vaccinia virus encoding a "self" antigen induces autoimmune vitiligo and tumor cell destruction in mice: requirement for CD4(+) T lymphocytes

Many human and mouse tumor antigens are normal, nonmutated tissue differentiation antigens. Consequently, immunization with these "self" antigens could induce autoimmunity. When we tried to induce immune responses to five mouse melanocyte differentiation antigens, gp100, MART-1, tyrosinase, and tyrosinase-related proteins (TRP) 1 and TRP-2, we observed striking depigmentation and melanocyte destruction only in the skin of mice inoculated with a vaccinia virus encoding mouse TRP-1. These mice rejected a lethal challenge of B16 melanoma, indicating the immune response against TRP-1 could destroy both normal and malignant melanocytes. Cytotoxic T lymphocytes specific for TRP-1 could not be detected in depigmented mice, but high titers of IgG anti-TRP-1 antibodies were present. Experiments with knockout mice revealed an absolute dependence on major histocompatibility complex class II, but not major histocompatibility complex class I, for the induction of both vitiligo and tumor protection. Together, these results suggest that the deliberate induction of self-reactivity using a recombinant viral vector can lead to tumor destruction, and that in this model, CD4(+) T lymphocytes are an integral part of this process. Vaccine strategies targeting tissue differentiation antigens may be valuable in cancers arising from nonessential cells and organs such as melanocytes, prostate, testis, breast, and ovary.

Construction and characterization of a triple-recombinant vaccinia virus encoding B7-1, interleukin 12, and a model tumor antigen

BACKGROUND

Construction of recombinant viruses that can serve as vaccines for the treatment of experimental murine tumors has recently been achieved. The cooperative effects of immune system modulators, including cytokines such as interleukin 12 (IL-12) and costimulatory molecules such as B7-1, may be necessary for activation of cytotoxic T lymphocytes. Thus, we have explored the feasibility and the efficacy of inclusion of these immunomodulatory molecules in recombinant virus vaccines in an experimental antitumor model in mice that uses Escherichia coli beta-galactosidase as a target antigen.

METHODS

We developed a "cassette" system in which three loci of the vaccinia virus genome were used for homologous recombination. A variety of recombinant vaccinia viruses were constructed, including one virus, vB7/beta/IL-12, that contains the following five transgenes: murine B7-1, murine IL-12 subunit p35, murine IL-12 subunit p40, E. coli lacZ (encodes beta-galactosidase, the model antigen), and E. coli gpt (xanthine-guanine phosphoribosyltransferase, a selection gene). The effects of the recombinant viruses on lung metastases and survival were tested in animals that had been given an intravenous injection of beta-galactosidase-expressing murine colon carcinoma cells 3 days before they received the recombinant virus by intravenous inoculation.

RESULTS

Expression of functional B7-1 and IL-12 by virally infected cells was demonstrated in vitro. Lung tumor nodules (i.e., metastases) were reduced in mice by more than 95% after treatment with the virus vB7/beta/IL-12; a further reduction in lung tumor nodules was observed when exogenous IL-12 was also given. Greatest survival of tumor-bearing mice was observed in those treated with viruses encoding beta-galactosidase and B7-1 plus exogenous IL-12.

CONCLUSION

This study shows the feasibility of constructing vaccinia viruses that express tumor antigens and multiple immune cofactors to create unique immunologic microenvironments that can modulate immune responses to cancer.

Apoptotic Death of CD8+ T Lymphocytes After Immunization: Induction of a Suppressive Population of Mac-1+/Gr-1+ Cells

Following an infection or immunization, a primary CD8+ T cell response generally rises then falls rapidly before giving rise to a “memory” response. When we immunized mice with recombinant viral immunogens optimized to enhance the lytic capability of CD8+ T cells, we measured a profound depression in Ag-specific effector function after early restimulation. Indeed, a “mirror image” cytolytic capability was observed: the most powerful immunogens, as measured by cytolytic capacity 6 days after immunization, elicited the weakest secondary immune response when evaluated following an additional 6 days after restimulation. To understand the mechanism of this suppression, we examined the fate of splenocytes immunized with a vaccinia virus encoding Ag and IL-2 then restimulated ex vivo. We found that these splenocytes underwent an apoptotic cell death, upon early restimulation, that was not dependent on the engagement of the FasR (CD95). Unlike previously described mechanisms of “propriocidal cell death” and “clonal exhaustion,” the cell death we observed was not an inherent property of the CD8+ T cells but rather was due to a population of splenocytes that stained positive for both the Mac-1 and Gr-1 surface markers. Deletion of these cells in vitro or in vivo completely abrogated the observed suppression of cytolytic reactivity of Ag-specific CD8+ T cells. These observations could account for the apparent absence of Ag-specific immune responses after some current vaccination regimens employing powerful immunogens. Finally, our results may shed new light on a mechanism for the suppression of CD8+ T cell responses and its effect on vaccine efficacy and on immune memory.

Apoptotic death of CD8+ T lymphocytes after immunization: induction of a suppressive population of Mac-1+/Gr-1+ cells

Following an infection or immunization, a primary CD8+ T cell response generally rises then falls rapidly before giving rise to a "memory" response. When we immunized mice with recombinant viral immunogens optimized to enhance the lytic capability of CD8+ T cells, we measured a profound depression in Ag-specific effector function after early restimulation. Indeed, a "mirror image" cytolytic capability was observed: the most powerful immunogens, as measured by cytolytic capacity 6 days after immunization, elicited the weakest secondary immune response when evaluated following an additional 6 days after restimulation. To understand the mechanism of this suppression, we examined the fate of splenocytes immunized with a vaccinia virus encoding Ag and IL-2 then restimulated ex vivo. We found that these splenocytes underwent an apoptotic cell death, upon early restimulation, that was not dependent on the engagement of the FasR (CD95). Unlike previously described mechanisms of "propriocidal cell death" and "clonal exhaustion," the cell death we observed was not an inherent property of the CD8+ T cells but rather was due to a population of splenocytes that stained positive for both the Mac-1 and Gr-1 surface markers. Deletion of these cells in vitro or in vivo completely abrogated the observed suppression of cytolytic reactivity of Ag-specific CD8+ T cells. These observations could account for the apparent absence of Ag-specific immune responses after some current vaccination regimens employing powerful immunogens. Finally, our results may shed new light on a mechanism for the suppression of CD8+ T cell responses and its effect on vaccine efficacy and on immune memory.

gp100/pmel 17 is a murine tumor rejection antigen: induction of "self"-reactive, tumoricidal T cells using high-affinity, altered peptide ligand

Many tumor-associated antigens are nonmutated, poorly immunogenic tissue differentiation antigens. Their weak immunogenicity may be due to "self"-tolerance. To induce autoreactive T cells, we studied immune responses to gp100/pmel 17, an antigen naturally expressed by both normal melanocytes and melanoma cells. Although a recombinant vaccinia virus (rVV) encoding the mouse homologue of gp100 was nonimmunogenic, immunization of normal C57BL/6 mice with the rVV encoding the human gp100 elicited a specific CD8(+) T cell response. These lymphocytes were cross-reactive with mgp100 in vitro and treated established B16 melanoma upon adoptive transfer. To understand the mechanism of the greater immunogenicity of the human version of gp100, we characterized a 9-amino acid (AA) epitope, restricted by H-2Db, that was recognized by the T cells. The ability to induce specific T cells with human but not mouse gp100 resulted from differences within the major histocompatibility complex (MHC) class I-restricted epitope and not from differences elsewhere in the molecule, as was evidenced by experiments in which mice were immunized with rVV containing minigenes encoding these epitopes. Although the human (hgp10025-33) and mouse (mgp10025-33) epitopes were homologous, differences in the three NH2-terminal AAs resulted in a 2-log increase in the ability of the human peptide to stabilize "empty" Db on RMA-S cells and a 3-log increase in its ability to trigger interferon gamma release by T cells. Thus, the fortuitous existence of a peptide homologue with significantly greater avidity for MHC class I resulted in the generation of self-reactive T cells. High-affinity, altered peptide ligands might be useful in the rational design of recombinant and synthetic vaccines that target tissue differentiation antigens expressed by tumors.

Identification of a Kb-restricted CTL epitope of beta-galactosidase: potential use in development of immunization protocols for "self" antigens

The use of recombinant and synthetic vaccines in the treatment of cancer has recently been explored using model tumor associated antigens (TAA), many of which do not model the immunological state of affairs in which the TAA is expressed by normal tissues. One potentially useful model Ag is beta-galactosidase (beta-gal). Because the activity of this enzyme is so easily detectable, this gene has been inserted into a large number of recombinant viruses and tumors useful to the cancer vaccinologist. In addition, numerous transgenic mouse colonies that have tissue-specific expression of beta-gal have been developed, enabling the modeling of tolerance to "self" Ags. Since most of these mice have an H-2b background, we generated cytotoxic T lymphocytes (CTL) capable of recognizing beta-gal-expressing tumor cells of C57BL\6 origin and have determined that their restriction element is the K(b) molecule. Using an allele-specific epitope forecast to generate a panel of candidate peptides, we have determined that the K(b)-restricted sequence is DAPIYTNV and corresponds to amino acids 96-103 of the intact beta-gal molecule. A recombinant vaccinia virus (rVV-ES beta-gal96-103) was constructed that encoded the peptide epitope preceded by an endoplasmic reticulum insertion signal sequence. Tumor cells infected with this rVV were recognized by the original CTL that had been used to identify the epitope. Furthermore, splenocytes of mice immunized with a rVV encoding the full-length beta-gal molecule and restimulated with the DAPIYTNV peptide specifically recognized tumor cells expressing beta-gal. The identification of this immunogenic beta-gal sequence enables the modeling of immunization strategies in animal models of malignant disease in which the target antigen is a "self" protein.

Antigen expression by dendritic cells correlates with the therapeutic effectiveness of a model recombinant poxvirus tumor vaccine

Recombinant poxviruses encoding tumor-associated antigens (TAA) are attractive as candidate cancer vaccines. Their effectiveness, however, will depend upon expression of the TAA in appropriate antigen-presenting cells. We have used a murine model in which the TAA is beta-galactosidase (beta-gal) and a panel of recombinant vaccinia viruses (rVV) in which beta-gal was expressed under early or late promoters at levels that varied over 500-fold during productive infections in tissue culture cells. Remarkably, only those rVV employing early promoters were capable of prolonging the survival of mice bearing established tumors expressing the model TAA. Late promoters were ineffective regardless of their determined promoter strength. The best results were obtained when beta-gal was regulated by a strong early promoter coupled to a strong late promoter. When a variety of cell types were infected with the panel of viruses in vitro, dendritic cells were found to express beta-gal only under the control of the early promoters even though late promoters were intrinsically more active in other cell types. Furthermore, in a functional assay, dendritic cells infected in vitro with rVV encoding beta-gal regulated by an early promoter activated beta-gal-specific cytotoxic T lymphocytes, whereas similar rVV with a late promoter-regulated gene did not. These data indicate that promoter strength per se is not the most critical quality of a recombinant poxvirus-based tumor vaccine and that the use of promoters capable of driving the production of TAA in "professional" antigen presenting cells may be crucial.

Highly attenuated modified vaccinia virus Ankara (MVA) as an effective recombinant vector: a murine tumor model

Modified vaccinia virus Ankara (MVA), a highly attenuated strain of vaccinia virus (VV) that is unable to replicate in most mammalian cells, was evaluated as an expression vector for a model tumor associated antigen (TAA) and as a potential anti-cancer vaccine. We employed an experimental murine model in which an adenocarcinoma tumor line, CT26.CL25, was stably transfected with a model TAA, beta-galactosidase (beta-gal). Mice injected intramuscularly with a recombinant MVA (rMVA) expressing beta-gal (MVA-LZ), were protected from a lethal intravenous (i.v.) challenge with CT26.CL25. In addition, splenocytes from mice primed with MVA-LZ were therapeutically effective upon adoptive transfer to mice bearing pulmonary metastases of the CT26.CL25 tumor established 3 days earlier. Most importantly, i.v. inoculation with MVA-LZ resulted in significantly prolonged survival of mice bearing three day old pulmonary metastases. This prolonged survival compared favorably to mice treated with a replication competent recombinant VV expressing beta-gal. These findings indicate that rMVA is an efficacious alternative to the more commonly used replication competent VV for the development of new recombinant anti-cancer vaccines.

Cloning and characterization of the genes encoding the murine homologues of the human melanoma antigens MART1 and gp100

The recent identification of genes encoding melanoma-associated antigens has opened new possibilities for the development of cancer vaccines designed to cause the rejection of established tumors. To develop a syngeneic animal model for evaluating antigen-specific vaccines in cancer therapy, the murine homologues of the human melanoma antigens MART1 and gp100, which were specifically recognized by tumor-infiltrating lymphocytes from patients with melanoma, were cloned and sequenced from a murine B16 melanoma cDNA library. The open reading frames of murine MART1 and gp100 encode proteins of 113- and 626-amino acids with 68.8 and 77% identity to the respective human proteins. Comparison of the DNA sequences of the murine MART1 genes, derived from normal melanocytes, the immortalized nontumorgenic melanocyte line Melan-a and the B16 melanoma, showed all to be identical. Northern and Western blot analyses confirmed that both genes encoded products that were melanocyte lineage proteins. Mice immunized with murine MART1 or gp100 using recombinant vaccinia virus failed to produce any detectable T-cell responses or protective immunity against B16 melanoma. In contrast, immunization of mice with human gp100 using recombinant adenoviruses elicited T cells specific for hgp100, but these T cells also cross reacted with B16 tumor in vitro and induced significant but weak protection against B16 challenge. Immunization with human and mouse gp100 together [adenovirus type 2 (Ad2)-hgp100 plus recombinant vaccinia virus (rVV)-mgp100], or immunization with human gp100 (Ad2-hgp100) and boosting with heterologous vector (rVV-hgp100 or rVV-mgp100) or homologous vector (Ad2-hgp100), did not significantly enhance the protective response against B16 melanoma. These results may suggest that immunization with heterologous tumor antigen, rather than self, may be more effective as an immunotherapeutic reagent in designing antigen-specific cancer vaccines.