Loss of functional beta 2-microglobulin in metastatic melanomas from five patients receiving immunotherapy

Immunophenotype of lymphocytic infiltration in medullary carcinoma of the breast

Medullary carcinoma (MC) of the breast is characterized by large anaplastic cells and infiltration by benign lymphocytes. Patients with this pattern of breast carcinoma are considered to have a better prognosis than those with other histological subtypes. We reviewed cases of primary breast carcinoma that were surgically resected between 1990 and 2004. Of these, 13 cases of medullary carcinoma of the breast with lymphocyte infiltration were reported. Tests for CD3, CD4, CD8, CD20, CD56, TIA-1, and granzyme B were performed on paraffin sections. We found that the MC contained very few NK cells, as assessed by their reactivity with the CD56 antibodies. However, MC had a significantly greater percentage of CD3, CD8, TIA-1, and granzyme B lymphocytes infiltrating the stroma of the tumor. Furthermore, more CD8-positive than CD4-positive T-cell lymphocytes were present within the tumor cell nests in MC, as opposed to the proportion in usual ductal carcinoma. The infiltrating cytotoxic/suppressor T cells in MC represent host resistance against cancer, and the high grading of the T-cell infiltration could explain, in part, a key mechanism controlling the good prognosis for this type of tumor and solve the pathological paradox of MC.

Identification of multiple antigens recognized by tumor-infiltrating lymphocytes from a single patient: tumor escape by antigen loss and loss of MHC expression

The authors describe a patient who experienced recurrence of metastatic melanoma after an initial dramatic response to immunotherapy using peptides derived from gp100, MART-1, and tyrosinase emulsified in incomplete Freund's adjuvant, and present data to support the hypothesis that the progression of disease in this patient was due to in vivo immunoselection for immunoresistant tumor variants. The authors previously demonstrated the existence of T-cell clones in this patient's peripheral blood and tumor-infiltrating lymphocytes (TILs) reactive against multiple antigens, including gp100, the tyrosinase-related protein (TRP)-2, a novel TRP-2 isoform-TRP-2-6b, SOX10, and the melanoma antigen NY-ESO-1. In addition to the multiple HLA-A2 restricted T-cell clones, the authors have now identified additional HLA-B/C-restricted as well as class II (HLA-DP)-restricted anti-melanoma antigen T-cell clones from this patient's TIL. One recurrent tumor showed loss of expression of multiple tumor antigens but retention of HLA class I expression. The other recurrent lesion showed total loss of HLA class I expression even though the tumor cells still expressed many melanoma antigens. This paper thus provides evidence for both the effectiveness of the immune destruction of cancer as well as problems associated with antigen-loss tumor escape mechanisms.

Escape from immunotherapy: possible mechanisms that influence tumor regression/progression

Tumor escape is one major obstacle that has to be addressed prior to designing and delivering successful immunotherapy. There is compelling evidence to support the notion that immunogenic tumors, in murine models and cancer patients, can be rejected by the immune system under optimum conditions for activating adaptive and nonadaptive antitumor immune responses. Despite this capability, a large number of tumors continue to grow and evade recognition and/or destruction by the immune system. The limited success in current immunotherapeutic strategies may be due to a variety of reasons: failure of effector cells to compete with the growing tumor burden, production of humoral factors by tumors that locally block cytotoxicity, antigen/MHC loss, T-cell dysfunction, production of suppressor T cells-to name but a few causes for therapeutic ineffectiveness for the particular malignancy being treated. To optimize immunotherapy strategies, correction of immune-activating signals, eradication of inhibitory factors, and the evasion from newly developed immunoresistant tumor phenotypes need to be simultaneously considered.

HLA class I antigen expression in malignant cells: why does it not always correlate with CTL-mediated lysis?

HLA class I antigen defects are frequently found in malignant cells. They appear to play a role in the clinical course of the disease, probably because they provide tumor cells with a mechanism to escape cytotoxic T lymphocyte (CTL) recognition and destruction. Expression of HLA class I antigens, however, is not always associated with the susceptibility of tumor cells to CTL lysis. Many mechanisms may underlie this finding, including the lack of tumor antigen (TA)-derived peptide presentation by a given HLA class I allospecificity, and/or the expression of immunosuppressive molecules such as HLA-G. These findings emphasize the need to develop probes to measure HLA class I allospecificity-TA peptide complex expression in malignant cells. Furthermore, the evaluation of the role of HLA class I antigens in the interaction of malignant cells with host immune cells should take into account the potential interference of tumor-derived immunomodulators.

Effective immunotherapy against cancer: a question of overcoming immune suppression and immune escape?

During the last decade, the breakthroughs in understanding of the molecular mechanisms responsible for immune activation and the advent of recombinant DNA technologies have changed the view on immunotherapy from "a dream scenario" to becoming a clinical reality. It is now clear that both cellular immunity comprising T and NK cells, as well as strategies based on antibodies, can provide strong antitumoral effects, and evidence is emerging that these strategies may also cure patients with previously incurable cancers. However, there are still a number of issues that remain unresolved. Progress in immunotherapy against cancer requires a combination of new, improved clinical protocols and strategies for overcoming mechanisms of immune escape and tumor-induced immune suppression. This review discusses some of the salient issues that still need to be resolved, focusing on the role of oxidative stress and the use of antioxidants to alleviate the immune hyporesponsiveness induced by reactive oxygen species (ROS).

Mechanisms of tumor evasion from the immune response

The results from in vitro immunological experiments, murine tumor models and patients with cancer clearly demonstrate that tumors have multiple mechanisms to evade the immune response. During the early stages of tumor development malignant cells can be poor stimulators, present poor targets or become resistant to the innate immune response, while at later stages, progressively growing tumors impair the adaptive immune response by blocking the maturation and function of APCs and causing alterations in T-cell signal transduction and function. Preliminary results also suggest a correlation between some of these changes and an increased metastatic potential of the tumor cells, a diminished response to immunotherapy, and poor prognosis. Carefully coordinated basic research studies and clinical immunotherapy trials will be required to fully determine the impact of these mechanisms of tumor evasion on the outcome of the disease and the response to treatment. However, understanding the mechanisms used by tumor cells to evade the immune system could result in new therapeutic approaches for preventing and/or reversing these immune alterations and could have the potential of improving the current results of immunotherapy trials.

MHC Class I-Independent Recognition of NK-Activating Receptor KIR2DS4

Natural killer cells are capable of killing tumor and virus-infected cells. This killing is mediated primarily via the natural cytotoxicity receptors, including NKp46, NKp44, NKp30, and by the NKG2D receptor. Killer cell Ig-like receptors (KIRs) are mainly involved in inhibiting NK killing (inhibitory KIRs) via interaction with MHC class I molecules. Some KIRs, however, have been found to enhance NK killing when interacting with MHC class I molecules (activating KIRs). We have previously demonstrated that KIR2DS4, an activating KIR, recognizes the HLA-Cw4 protein. The interaction observed was weak and highly restricted to HLA-Cw4 only. These findings prompted us to check whether KIR2DS4 might have additional ligand(s). In this study, we show that KIR2DS4 is able to also interact with a non-class I MHC protein expressed on melanoma cell lines and on a primary melanoma. This interaction is shown to be both specific and functional. Importantly, site-directed mutagenesis analysis reveals that the amino acid residues involved in the recognition of this novel ligand are different from those interacting with HLA-Cw4. These results may shed new light on the function of activating KIRs and their relevance in NK biology.

Stable expression of chimeric anti-CD3 receptors on mammalian cells for stimulation of antitumor immunity

Expression of CD80 or CD86 costimulatory molecules on tumor cells can produce rejection of immunogenic but not poorly immunogenic tumors. We have previously shown that anti-CD3 single-chain antibodies expressed on the surface of cells can directly activate T cells. We therefore investigated whether anti-CD3 "receptors" could enhance CD86-mediated rejection of poorly immunogenic tumors. Expression of anti-CD3 receptors on cells was increased by introduction of membrane-proximal "spacer" domains containing glycosylation sites between the single-chain antibody and the transmembrane domain of the chimeric receptors. Removal of glycosylation sites in the spacer reduced surface expression due to increased shedding of chimeric receptors from the cell surface. Induction of T-cell proliferation by anti-CD3 receptors did not correlate with the expression level of chimeric protein, but rather depended on the physical properties of the spacer. Anti-CD3 receptors effectively induced T-cell cytotoxicity, whereas coexpression with CD80 or CD86 was required for generating T-cell proliferation and IL-2 secretion. Although expression of CD86 did not significantly delay the growth of poorly immunogenic B16-F1 tumors, expression of anti-CD3 receptors with CD86 produced complete tumor rejections in 50% of mice and induced significant protection against wild-type B16-F1 tumor cells. Our results show that spacer domains can dramatically influence the surface expression and the biological activity of chimeric antibody receptors. The strong antitumor activity produced by anti-CD3 receptors and CD86 on tumor cells indicates that this strategy may be beneficial for the gene-mediated therapy of poorly immunogenic tumors.

Immunotherapy for melanoma

The immunogenicity of melanoma and the identification of melanoma-associated antigens is the basis for immunotherapy. This review will discuss the current status of melanoma immunotherapy with a focus on non-specific cytokines and highly specific vaccines, including peptides, viruses, dendritic cells, and whole cell vaccines. The passive transfer of melanoma-specific monoclonal antibodies and T-cells will also be reviewed. The problem of tumor escape and the association of immunotherapy to autoimmunity will be discussed. The use of immunotherapy in combination with other therapeutic agents and genetic profiling to predict responses suggests that immunotherapy will continue to play a role in the treatment of melanoma.

Differential downregulation of endoplasmic reticulum-residing chaperones calnexin and calreticulin in human metastatic melanoma

Characterization of the molecular basis of tumor recognition by T cells has shown that major histocompatibility complex (MHC) class I molecules play a crucial role in presenting antigenic peptide epitopes to cytotoxic T lymphocytes. MHC class Ia downregulation has been repeatedly described on melanoma cells and is thought to be involved in the failure of the immune system to control tumor progression. Proper assembly of MHC class I molecules is dependent on several cofactors, e.g. the chaperones calnexin and calreticulin residing in the endoplasmic reticulum. Alterations in the expression of these chaperones may have important implications for MHC class I assembly, peptide loading, and presentation on the tumor cell surface and thus may contribute to the immune escape phenotype of tumor cells. In the present study, we compared melanoma lesions representing different stages of tumor progression with regard to the expression of calnexin and calreticulin in tumor cells by means of immunohistochemistry. Metastatic melanoma lesions exhibited significant downregulation of calnexin as compared to primary melanoma lesions. In contrast, chaperone calreticulin was expressed in melanoma cells of primary as well as of metastatic lesions. Our data suggest that chaperone-downregulation, particularly calnexin-downregulation, may contribute to the metastatic phenotype of melanoma cells in vivo. Consistently, conserved chaperone expression in metastatic melanoma lesions may be a useful criterion for selection of patients for treatment with T cell-based immunotherapies.

High frequency of HLA-B44 allelic losses in human solid tumors

Human leukocyte antigen (HLA) class I downregulation, a frequent phenomenon observed in a variety of human tumors, favors tumor immune escape from T-lymphocyte recognition. However, it is not known whether a particular HLA class I allele is lost more frequently than others. To address this question we analyzed HLA class I expression in tumor tissues derived from 300 patients diagnosed as having breast, colorectal, or laryngeal carcinomas. Cryostatic tumor sections and a broad panel of anti-HLA class I monoclonal antibodies were used. We found that the HLA-B44 allele was lost more frequently than other HLA class I alleles, and that the difference was not related with changes in HLA-B44 allele frequencies between patients and controls. In addition, we observed that 35% of the HLA-B44 negative tumors presented HLA haplotype loss associated with loss of heterozygosity. These tests were performed on DNA samples obtained from microdissected tumor tissues. The results seem to indicate that HLA class I allelic losses are not randomly distributed during tumor development but that some HLA class I alleles, and HLA-B44 in particular, are more frequently downregulated and may play an important role in immune escape mechanisms.

Does the immune system see tumors as foreign or self?

Given the vast number of genetic and epigenetic changes associated with carcinogenesis, it is clear that tumors express many neoantigens. A central question in cancer immunology is whether recognition of tumor antigens by the immune system leads to activation (i.e., surveillance) or tolerance. Paradoxically, while strong evidence exists that specific immune surveillance systems operate at early stages of tumorigenesis, established tumors primarily induce immune tolerance. A unifying hypothesis posits that the fundamental processes of cancer progression, namely tissue invasion and metastasis, are inherently proinflammatory and thus activating for innate and adaptive antitumor immunity. To elude immune surveillance, tumors must develop mechanisms that block the elaboration and sensing of proinflammatory danger signals, thereby shifting the balance from activation to tolerance induction. Elucidation of these mechanisms provides new strategies for cancer immunotherapy.

Successful elimination of large established tumors and avoidance of antigen-loss variants by aggressive adoptive T cell immunotherapy

Utilization of ex vivo-expanded epitope-specific cytotoxic T lymphocytes has become a clinical standard in the adoptive immunotherapy of tumors. One of the obstacles faced by T cell-based immunotherapy is the development of tumor immune-escape variants. Using our previously reported CMS5 tumor/DUC18 CD8(+) TCR transgenic system, we sought to investigate whether large established tumors can be successfully eliminated before the development of escape variants. Using BALB/c mice that were s.c. transplanted with two tumors that had been growing for 8 days (double 8-day tumors), we assessed the in vivo anti-tumor activity of in vitro peptide-stimulated DUC18 T cells. A single infusion of activated DUC18 T cells showed a modest effect against the double 8-day tumors, whereas two and three administrations led to regression of both tumors within 10 days. However, in some mice, the tumors re-grew approximately 10 days after the regression. We found these tumors to be antigen-loss variants. These relapsed tumor cells progressively grew in DUC18 transgenic mice and did not express tERK-specific message. When four doses of activated DUC18 T cells were infused, the double 8-day tumors were successfully eliminated and the tumors did not grow out in any mice. Our results demonstrate that mono-specific CD8(+) T cells can effectively eliminate large established tumors before the development of antigen-loss variants when a high number of T cells is rapidly administered.

Tumor-associated antigen profiling in breast and ovarian cancer: mRNA, protein or T cell recognition?

PURPOSE

The absence of tumor-associated antigens (TAA) which might elicit an immune response is one reason for the disappointing results of therapeutical vaccines in cancer patients. Moreover, impaired expression of MHC class-I and components involved in antigen processing, such as TAP-1, -2, LMP-2, -7, and MECL-1, may lead to tumor escape from immune recognition. Expression profiling of TAA is one approach towards the design of well-defined and individualized anti-tumor vaccines.

METHODS

Quantitative polymerase chain reaction (qRT-PCR) is the method of choice to characterize immunologically relevant properties of individual tumors. However, the application of qRT-PCR as a surrogate parameter for the expression of TAAs depends upon the assumption that the level of an mRNA species correlates with the cellular level of the protein it encodes. Therefore, we additionally analyzed TAA expression by immunofluorescence and T cell recognition.

RESULTS

In the present study we were unable to confirm that impaired TAP-1 or -2 (transporter associated with antigen processing) expression characterized at the mRNA level is an appropriate surrogate parameter for down-regulated MHC class-I expression in breast cancer. In addition, we analyzed the expression pattern of TAAs in breast and ovarian cancer cell lines. Besides the well-known over-expression of HER-2/neu, CEA, and MUC-1, multiple antigens of the MAGE-family were frequently co-expressed. We investigated whether detection of TAAs by qRT-PCR correlates with monoclonal antibody staining, and which method could predict T cell recognition. We demonstrated a correlation between tumor cell lysis by HLA-A*0201-restricted, MUC-1-specific CTL and threshold levels of MUC-1-specific mRNA.

CONCLUSION

MUC-1 is an example that TAA profiling by RT-PCR and flow cytometry can fail to correlate with each other and are of limited value in the prediction of T cell recognition.

Evidence for whole chromosome 6 loss and duplication of the remaining chromosome in acute lymphoblastic leukemia

HLA class I molecules serve the essential immunological function of presenting antigen to CD8+ T lymphocytes. Tumor cells may present tumor-specific antigen to T cells via these molecules, but many tumors show a loss or down-regulation of HLA class I expression and this may serve as an immune escape mechanism. Using a microsatellite marker-based method, we have searched for loss of heterozygosity (LOH) mutations at 3 genomic regions implicated in HLA class I expression in a cohort of 56 acute lymphoblastic leukemia (ALL) samples. The regions analyzed consisted of the HLA class I heavy chain genes located within the MHC genomic region on chromosome arm 6p, the HLA class I light chain (beta-2-microglobulin, B2M) gene on chromosome arm 15q, and the putative HLA modifier of methylation gene (MEMO1) located on chromosome arm 1q. Results revealed low frequencies of B2M (2/55) and MEMO1 (5/42) LOH but a high frequency of MHC LOH (19/56) that was usually associated with whole chromosome 6 loss (13/19). Cytogenetic data were available for 30 samples, including nine of those that exhibited apparent whole chromosome 6 loss. No cases of chromosome 6 monosomy were observed. We propose that whole chromosome 6 loss with reduplication of the remaining chromosome is common in ALL and that it is driven by the presence of tumor-inhibiting factors on chromosome arm 6p (the HLA loci) along with previously localized tumor-suppressor genes on chromosome arm 6q.

Melacine: an allogeneic melanoma tumor cell lysate vaccine

Cancer vaccines have represented the holy grail of tumor immunology to many. In 2003, there are innumerable approaches to active specific immunotherapy for cancer. The identification of tumor antigens recognized by and able to activate human T-lymphocytes has heightened the enthusiasm for this approach. Melanoma is one of the diseases that has been primarily targeted by vaccine approaches. The identification of peptides and proteins associated with cancer has provided strategies to target specific components of the cancer cell. However, older vaccines utilizing products from whole cells may have a number of advantages. Melacine (Corixa Corp.) is an allogeneic melanoma tumor cell lysate combined with the adjuvant DETOX. Pioneered by Malcolm Mitchell, it has rare but confirmed antitumor activity in metastatic melanoma (5-10%). However, previous analysis of responses in advanced disease and a recent analysis in early-stage adjuvant trials suggest that Melacine may have its greatest benefit in a large subset of melanoma patients who express either the human leukocyte antigen (HLA) class I antigens A2 and/or HLA-C3. This finding must now be confirmed in a large perspective observation-controlled clinical trial to solidify the role of Melacine as an effective cancer vaccine in melanoma.

Hybrids of dendritic cells and tumor cells generated by electrofusion simultaneously present immunodominant epitopes from multiple human tumor-associated antigens in the context of MHC class I and class II molecules

Hybrid cells generated by fusing dendritic cells with tumor cells (DC-TC) are currently being evaluated as cancer vaccines in preclinical models and human immunization trials. In this study, we evaluated the production of human DC-TC hybrids using an electrofusion protocol previously defined for murine cells. Human DCs were electrically fused with allogeneic melanoma cells (888mel) and were subsequently analyzed for coexpression of unique DC and TC markers using FACS and fluorescence microscopy. Dually fluorescent cells were clearly observed using both techniques after staining with Abs against distinct surface molecules suggesting that true cell fusion had occurred. We also evaluated the ability of human DC-TC hybrids to present tumor-associated epitopes in the context of both MHC class I and class II molecules. Allogeneic DCs expressing HLA-A*0201, HLA-DR beta 1*0401, and HLA-DR beta 1*0701 were fused with 888mel cells that do not express any of these MHC molecules, but do express multiple melanoma-associated Ags. DC-888mel hybrids efficiently presented HLA-A*0201-restricted epitopes from the melanoma Ags MART-1, gp100, tyrosinase, and tyrosinase-related protein 2 as evaluated by specific cytokine secretion from six distinct CTL lines. In contrast, DCs could not cross-present MHC class I-restricted epitopes after exogenously loading with gp100 protein. DC-888mel hybrids also presented HLA-DR beta 1*0401- and HLA-DR beta 1*0701-restricted peptides from gp100 to CD4(+) T cell populations. Therefore, fusions of DCs and tumor cells express both MHC class I- and class II-restricted tumor-associated epitopes and may be useful for the induction of tumor-reactive CD8(+) and CD4(+) T cells in vitro and in human vaccination trials.

Cell transfer therapy for cancer: lessons from sequential treatments of a patient with metastatic melanoma

The development of effective autologous cell transfer therapies for the treatment of patients with cancer has been difficult, in part because the cells used to treat each patient are different, as are the patient's tumor and immune status. Much can thus be learned by sequential treatments of the same patient with the same cells, making single modifications in the treatments to determine which factors are critical. The authors have treated a single patient with five sequential administrations of the same cells with minor modifications in the mode of administration and the immune status of the patient. The treatment of this patient strongly suggested that 1) the highly avid recognition of tumor antigens in vitro by a transferred lymphocyte population does not necessarily predict in vivo antitumor activity; 2) the administration of highly avid antitumor autologous lymphocyte populations can be far more active in mediating tumor regression in vivo when administered after nonmyeloablative chemotherapy than when administered without this prior chemotherapy; 3) intra-arterial administration of highly avid antitumor autologous lymphocytes into the blood supply of the tumor can be more effective in mediating tumor regression than the intravenous administration of these same tumor infiltrating lymphocytes; 4) one mechanism of tumor escape from immunotherapy is loss of class I MHC antigen expression by the tumor due to mutation of the beta-2 microglobulin gene.

HLA class I antigen loss, tumor immune escape and immune selection

Poor clinical response rates have been observed in the majority of the T cell-based immunotherapy clinical trials conducted to date. One reason might be the presence of abnormalities in HLA class I antigen presentation in malignant lesions. An increased frequency of HLA class I abnormalities has been observed in malignant lesions from patients treated with T cell-based immunotherapy and in lesions which have recurred in patients who had experienced clinical responses following T cell-based immunotherapy. These observations are compatible with the possibility that the outgrowth of a patient's tumor reflects immune selection of tumor cells which have acquired escape mechanisms from immune recognition.

Lack of effector cell function and altered tetramer binding of tumor-infiltrating lymphocytes

Tumor-specific CD8 T cell responses to MCA102 fibrosarcoma cells expressing the cytotoxic T cell epitope gp33 from lymphocytic choriomeningitis virus were studied. MCA102(gp33) tumors grew progressively in C57BL/6 mice, despite induction of peripheral gp33-tetramer(+) T cells that were capable of mediating antiviral protection, specific cell rejection, and concomitant tumor immunity. MCA102(gp33) tumors were infiltrated with a high number ( approximately 20%) of CD11b(+)CD11c(-) macrophage-phenotype cells that were able to cross-present the gp33 epitope to T cells. Tumor-infiltrating CD8 T cells exhibited a highly activated phenotype but lacked effector cell function. Strikingly, a significant portion of tumor-infiltrating lymphocytes expressed TCRs specific for gp33 but bound MHC tetramers only after cell purification and a 24-h resting period in vitro. The phenomenon of "tetramer-negative T cells" was not restricted to tumor-infiltrating lymphocytes from MCA102(gp33) tumors, but was also observed when Ag-specific T cells derived from an environment with high Ag load were analyzed ex vivo. Thus, using a novel tumor model, allowing us to trace tumor-specific T cells at the single cell level in vivo, we demonstrate that the tumor microenvironment is able to alter the functional activity of T cells infiltrating the tumor mass.

Natural selection of tumor variants in the generation of "tumor escape" phenotypes

The idea that tumors must "escape" from immune recognition contains the implicit assumption that tumors can be destroyed by immune responses either spontaneously or as the result of immunotherapeutic intervention. Simply put, there is no need for tumor escape without immunological pressure. Here, we review evidence supporting the immune escape hypothesis and critically explore the mechanisms that may allow such escape to occur. We discuss the idea that the central engine for generating immunoresistant tumor cell variants is the genomic instability and dysregulation that is characteristic of the transformed genome. "Natural selection" of heterogeneous tumor cells results in the survival and proliferation of variants that happen to possess genetic and epigenetic traits that facilitate their growth and immune evasion. Tumor escape variants are likely to emerge after treatment with increasingly effective immunotherapies.

Immunity to cancer: attack and escape in T lymphocyte-tumor cell interaction

Tumor cells may express antigens which are recognized in a form of HLA/peptide complexes by T cells. The frequency at which different antigens are seen by T cells of melanoma patients and healthy donors was evaluated by human leukocyte antigen (HLA)/peptide tetramer technology which stains T cells bearing the specific receptor for a given epitope. By this technique, it was found that the majority of metastatic melanoma patients can recognize differentiation antigens (particularly Melan-A/MART-1), whereas such a recognition is scanty in the early phase of the disease and in healthy subjects. Despite the presence of melanoma-specific T cells infiltrating tumor lesions, tumor rejection rarely occurs. Among the different mechanisms of such inefficient antitumor response, this review discusses the possible anti-T-cell counterattack mediated by FasL-positive tumor cells, and shows that FasL is located in the cytoplasm of melanoma cells and is transported in the tumor microenvironment through the release of melanosomes. Additionally, mechanisms of suboptimal T cell activation through tumor cell expression of peptide analogs with antagonist activity are described, together with the possibility of overcoming such anergy induction by the usage of optimized tumor epitopes. Down-modulation of HLA expression by target tumor cells and its multiple mechanisms is also considered. Finally, we discuss the role of inducible nitric oxide synthases in determining the inhibition of apoptosis in melanoma cells, which can make such tumor cells resistant to the T-cell attack.

Recruitment of CTL activity by tumor-specific antibody-mediated targeting of single-chain class I MHC-peptide complexes

The MHC class I-restricted CD8 CTL effector arm of the adaptive immune response is uniquely equipped to recognize tumor cells as foreign and consequently initiates the cascade of events resulting in their destruction. However, tumors have developed sophisticated strategies to escape immune effector mechanisms; their most well-known strategy is down-regulation of MHC class I molecules. To overcome this and develop new approaches for immunotherapy, we have constructed a recombinant molecule in which a single-chain MHC is specifically targeted to tumor cells through its fusion to cancer-specific recombinant Ab fragments. As a model we used a single-chain HLA-A2 molecule genetically fused to the variable domains of an anti-IL-2Ralpha subunit-specific humanized Ab, anti-Tac. The construct, termed B2M-aTac(dsFv), was expressed in Escherichia coli, and functional molecules were produced by in vitro refolding in the presence of HLA-A2-restricted antigenic peptides. Flow cytometry studies revealed the ability to decorate Ag-positive, HLA-A2-negative human tumor cells with HLA-A2-peptide complexes in a manner that was entirely dependent upon the specificity of the targeting Ab fragment. Most importantly, the B2M-aTac(dsFv)-mediated coating of the target tumor cells made them susceptible for efficient and specific HLA-A2-restricted, melanoma gp100 peptide-specific CTL-mediated lysis. These results demonstrate the concept that Ab-guided, Ag-specific targeting of MHC-peptide complexes on tumor cells can render them susceptible and more receptive and thus potentiate CTL killing. This type of approach may open the way for the development of new immunotherapeutic strategies based on Ab targeting of natural cognate MHC ligands and CTL-based cytotoxic mechanisms.

Frequency and genetic basis of MHC, beta-2-microglobulin and MEMO-1 loss of heterozygosity in sporadic breast cancer

The Human Leukocyte Antigen (HLA) class I molecules are critical factors in T cell recognition of abnormal, including neoplastic, cells. Loss of HLA class I expression phenotypes, as defined by immunohistochemistry-based tests, have been previously described in many types of cancer. Here we describe a microsatellite marker DNA-based loss of heterozygosity (LOH) analysis of three distinct chromosomal regions which have been implicated in HLA class I expression on a cohort of 99 unselected sporadic breast cancer samples. These regions comprise the 4Mb major histocompatibility complex (MHC) region on chromosome 6p, which contains the HLA class I heavy chain loci and other genes responsible for antigen processing, the HLA class I light chain (beta-2-microglobulin, beta2m) gene on chromosome 15q, and the putative HLA class I modifier of methylation gene (MEMO-1) on chromosome 1p. Additional chromosome 6 markers were also employed to determine the likely genetic mechanism for MHC loss. We show that 25/99 (25%) of samples show allelic loss within the MHC, 28/95 informative samples (29%) show allelic loss of beta2m and 21/76 informative samples (28%) show allelic loss of MEMO-1. Approximately half of the samples are predicted to have compromised HLA class I gene expression due to LOH at one and/or other of these three loci. Sequencing of the remaining beta2m allele in samples displaying beta2m LOH failed to detect any additional intragenic mutations. Analysis of the frequency of samples showing LOH at either 0, 1, 2 or 3 of the genomic regions analyzed suggested clustering of tumors into either 'no loci loss' or '3 loci loss' categories. These results reveal major underlying genetic causes for the high level of HLA class I expression loss seen in breast cancer.

Prostate-specific antigen vaccines for prostate cancer

Prostate cancer is the most common malignant tumour in men and there are few treatment options available once the tumour becomes refractory to hormonal manipulation. Prostate-specific antigen (PSA) is a secretory glycoprotein that is commonly expressed by prostatic epithelial cells and is found in elevated levels in the serum of men with prostate cancer. The identification of T cell specific epitopes within the coding sequence of PSA has led to the development of various vaccine strategies that target PSA in an attempt to treat established prostate cancer. These strategies have included human leukocyte antigen-restricted PSA peptides, dendritic cells pulsed with PSA, recombinant viruses expressing PSA and combinations of different vectors. In addition to PSA, several other antigens have been described that may be useful for targeting prostate tumours by vaccines. Animal studies have established the feasibility and safety for many of these agents and clinical trials are now in progress to evaluate the immunological and clinical responses of PSA vaccines. Further research in manipulating anti-PSA immunity with cytokines, costimulatory molecules and other immune modulating agents will likely improve the therapeutic effectiveness of PSA vaccines. Clinical trials designed to evaluate the effects of vaccination in different stages of disease and through different routes of administration need to be performed to define the optimal schedule for PSA vaccines in patients with prostate cancer, or for those at high risk of developing the disease.

Challenges in the development of effective peptide vaccines for cancer

The ability of the immune system to recognize malignant cells has opened the door to development of tumor vaccines to treat or prevent various types of cancer. In the era of molecular biology, the tumor antigens recognized by the immune system have been identified, allowing the generation of subunit vaccines that may improve safety and efficacy compared with more crude vaccines such as irradiated tumor cells and tumor cell lysates. Synthetic peptides corresponding to defined antigenic epitopes for tumor-reactive lymphocytes represent one of the new types of vaccines currently being developed to treat or prevent various types of malignant disorders. The design of peptide-based vaccines to stimulate antitumor T-cell responses has many attractive features such as ease of manufacturing and characterization (ie, quality control), as well as an excellent safety profile in past clinical studies. However, ambiguous results from initial clinical trials indicate that these vaccines are far from optimal and that considerable efforts for their optimization lie ahead. We attempt to address the 8 most important challenges we currently face for developing peptide-based vaccines that would effectively induce immune responses leading to antitumor effects.

CD66a interactions between human melanoma and NK cells: a novel class I MHC-independent inhibitory mechanism of cytotoxicity

NK cells are able to kill virus-infected and tumor cells via a panel of lysis receptors. Cells expressing class I MHC proteins are protected from lysis primarily due to the interactions of several families of NK receptors with both classical and nonclassical class I MHC proteins. In this study we show that a class I MHC-deficient melanoma cell line (1106mel) is stained with several Ig-fused lysis receptors, suggesting the expression of the appropriate lysis ligands. Surprisingly, however, this melanoma line was not killed by CD16-negative NK clones. The lack of killing is shown to be the result of homotypic CD66a interactions between the melanoma line and the NK cells. Furthermore, 721.221 cells expressing the CD66a protein were protected from lysis by YTS cells and by NK cells expressing the CD66a protein. Redirected lysis experiments demonstrated that the strength of the inhibitory effect is correlated with the levels of CD66a expression. Finally, the expression of CD66a protein was observed on NK cells derived from patients with malignant melanoma. These findings suggest the existence of a novel class I MHC-independent inhibitory mechanism of human NK cell cytotoxicity. This may be a mechanism that is used by some of the class I MHC-negative melanoma cells to evade attack by CD66a-positive NK cells.

Cis elements for transporter associated with antigen-processing-2 transcription: two new promoters and an essential role of the IFN response factor binding element in IFN-gamma-mediated activation of the transcription initiator

Expression of cell surface MHC class I:peptide complex requires coordinated expression of multiple genes such as MHC class I heavy chain, beta(2)-microglobulin (beta(2)m), transporters associated with antigen-processing (TAP)-1 and TAP-2, and proteosomal components low-molecular weight polypeptide (LMP)-2 and LMP-7. All of these genes are expressed at defined and distinct levels in normal tissues, and are inducible by IFN-gamma. While the cis elements involved in transcription of the MHC class I heavy chain, beta(2)m, TAP-1 and LMP-2 have been analyzed extensively, those for TAP-2 and LMP-7 have not been well studied. Here we systematically analyzed the cis elements for TAP-2 transcription. We found at least two independent elements that are sufficient to activate transcription of a reporter gene. One (hereby called TAP-2 P1) is located 5' to the TAP-2 exon 1, while the other (hereby called TAP-2 P2) is a transcription initiator residing in intron 1. Analysis of the 5' sequence of TAP-2 mRNA indicates that both promoters are active. Moreover, while the TAP-2 promoter region contains cis elements that can mediate TAP-2 induction by IFN-gamma, such as gamma-activation site and IFN response factor binding element (IRFE), only the IRFE is required for IFN-gamma induction of TAP-2 promoter in vitro. The IRFE appears to work as an enhancer for the initiator (P2). Together with another promoter recently identified by others, TAP-2 therefore has three independent promoters that can be differentially regulated.

Assumptions of the tumor 'escape' hypothesis

The reasons why cancer cells are not destroyed by the immune system are likely to be similar, in most cases, to the reasons why normal cells are not destroyed by the immune system. Unfortunately for tumor immunologists, these reasons have not yet been fully elucidated. What is known, however, is that the lack of autoimmune destruction of normal tissue after immune activation is a finely regulated, highly orchestrated sequence of events. Viewed in this light, it is interesting to conceptualize the derangement of the tumor genome not merely as an engine that enables cancer cells to dodge immune recognition. The dysregulation characteristic of the transformed genome is also what makes tumor immunity, a specialized form of autoimmunity, possible.

Promoter methylation controls the expression of MAGE2, 3 and 4 genes in human cutaneous melanoma

Cancer-testis antigens expressed by different-histotype transformed cells are suitable targets for tumor immunotherapy. However, their heterogeneous expression in neoplastic lesions limits the eligibility of patients for cancer-testis antigen-directed vaccination, and low levels of cancer-testis antigens' expression may impair immune recognition of malignant cells. Because of the primary clinical relevance of cancer-testis antigens' expression in neoplastic tissues, 68 unrelated or sequential metastatic lesions from 56 patients were used to characterize the molecular mechanisms regulating the presence and levels of expression of different cancer-testis antigens of the MAGE family (i.e., MAGE2, 3 and 4) in cutaneous melanoma. Polymerase chain reaction-based methylation analyses showed that methylation status of specific cytosine-guanine dinucleotides in the promoters of investigated cancer-testis antigens correlated with their heterogeneous expression within unrelated metastatic melanoma lesions, and with their homogeneous expression among sequential metastases from three patients with melanoma. Unlike methylated promoters, unmethylated promoters of MAGE2, 3 and 4 genes drove the expression of reporter gene-enhanced green fluorescent protein after transient transfection of cancer-testis antigen-positive Mel 142 melanoma cells. Furthermore, de novo expression of MAGE3 gene induced by the treatment of Mel 195 melanoma cells with the DNA hypomethylating agent 5-aza-2'-deoxycytidine was associated with a 6%-12% demethylation of selected cytosine-guanine dinucleotides in its promoter. Finally, 5-aza-2'-deoxycytidine induced a 16-fold increase of MAGE3 expression in Mel 313 melanoma cells expressing constitutively low levels of the antigen, but did not affect that of Mel 275 melanoma cells expressing high baseline levels of MAGE3. Overall, these findings identify promoter methylation as a shared mechanism directly regulating the expression of therapeutic cancer-testis antigens in metastatic melanomas, and foresee the clinical use of 5-aza-2'-deoxycytidine to design new chemoimmunotherapeutic strategies in patients with melanoma.

Decreased intraindividual HLA class I expression is due to reduced transcription in advanced melanoma and does not correlate with HLA-G expression

The presentation of endogenously synthesized peptides in association with HLA class I molecules allows the activation of CD8(+) lymphocytes. Tumor cells often fail to present antigenic peptides resulting in the immune escape of metastasizing cells. The aim of this study was to elucidate possible molecular mechanisms leading to reduced antigen presentation in melanoma. Melanoma cell short-time cultures were genotypically and phenotypically HLA-typed by sequence-specific primer polymerase chain reaction and complement-mediated microlymphocytotoxicity assays, respectively. Flow cytometric analysis of HLA-A2 and HLA-A3 allospecificities were performed to confirm typing results. Transcriptional levels of classical HLA-A, HLA-B genes and nonclassical HLA-G genes were detected using quantitative real-time reverse transcriptase polymerase chain reaction (LightCycler). We found loss or downregulation of HLA proteins in 18% (for HLA-A) and 53% (for HLA-B) of all tested metastases. Genomic analysis, however, revealed the presence of the corresponding HLA class I gene in six out of seven cases. On the level of gene transcription we observed a differential regulation of HLA-A, HLA-B, and HLA-G mRNA expression. There was no correlation between classical and nonclassical HLA gene transcription, but the transcriptional levels of classical HLA corresponded to the protein expression levels. Furthermore, an overall reduced amount of HLA class I gene transcription was observed in melanoma metastases during disease progression in three individuals. We postulate that there is a transcriptional regulation of HLA class I gene expression in melanoma cells. These data suggest that treatment approaches aimed at activating specific cytotoxic T lymphocytes are most successful in early disease.

The analysis of the natural killer-like activity of human cytolytic T lymphocytes revealed HLA-E as a novel target for TCR alpha/beta-mediated recognition

Cytolytic T lymphocytes (CTL) are known to recognize antigen peptides in association with major histocompatibility complex (MHC) class I molecules expressed on target cells. However, a fraction of human CD8(+) CTL has been shown to lyse certain natural killer (NK)-susceptible target cells via still undefined mechanism(s). These CD8(+) T cells, hereafter referred to as NK-CTL, are frequently composed of cells expressing one single TCR Vbeta expansion (different in different individuals), display a memory phenotype and express HLA class I-specific inhibitory NK receptors. Here we show that cell populations or clones of NK-CTL isolated from three healthy donors homogeneously expressed Vbeta16, Vbeta9 and Vbeta3 TCR, respectively. Various clones isolated under limiting dilution conditions from Vbeta16(+) cells of donor 1 displayed identical TCR Vbeta and Valpha rearrangements, thus suggesting a substantial monoclonality of the NK-CTL subset analyzed. NK-CTL lysed a number of NK-susceptible tumor target cells with the exception of those characterized by beta2-microglobulin (beta2m) deficiency. However, the latter targets became susceptible to lysis upon beta2m transfection. Using monoclonal antibodies specific for the relevant TCR Vbeta or beta2m we provide evidence suggesting that target cell lysis by NK-CTL is mediated by the TCR itself upon recognition of beta2m-associated proteins. The cellular distribution of the potential beta2m-associated proteins in susceptible target cells suggested, as a likely candidate for TCR-mediated recognition, the non-classical HLA-E molecule. The use, as target cells, of the murine TAP2-deficient RMA-S cells, either untransfected or transfected with HLA-E, and loaded with an appropriate HLA-E-binding peptide, provided the direct demonstration that HLA-E represents a ligand recognized by the TCR expressed by NK-CTL. This is the first evidence that human TCR alpha/beta can recognize HLA-E molecules, thus revealing a novel type of TCR-mediated recognition, which may offer new insight in immune responses in both normal and disease conditions.

beta(2)-microglobulin induces apoptosis in HL-60 human leukemia cell line and its multidrug resistant variants overexpressing MRP1 but lacking Bax or overexpressing P-glycoprotein

In this study, we examined whether exogenous beta(2)-microglobulin (beta(2)m) can induce apoptosis in the drug sensitive HL-60 leukemia cell line and its drug resistant variants and investigated the molecular mechanism of beta(2)m-induced apoptosis. Our data revealed that beta(2)m is very significantly down-regulated in two multidrug resistant variants of the HL-60 cells: (a) the MRP1-bearing, Bax-deficient HL-60/ADR cell line, and (b) the P-glycoprotein (P-gp) overexpressing HL-60/VCR cell line. However, exogenous beta(2)m induced similar levels of apoptosis in HL-60 cells and these drug resistant variants. beta(2)m-induced apoptosis in HL-60 and HL-60/VCR cells was associated with decreased mitochondrial membrane potential (Deltapsim) but did not affect Deltapsim in HL-60/ADR cells. Surprisingly, cyclosporin A (CsA), a known inhibitor of the mitochondrial permeability transition (MPT) pore, inhibited beta(2)m-induced apoptosis in HL-60/ADR cells but not in HL-60 and HL-60/VCR cells, suggesting that the pro-apoptotic effect of beta(2)m in these cells is not through MPT pore formation. Furthermore, beta(2)m induced the release of cytochrome c and the apoptosis-inducing factor (AIF) from mitochondria in HL-60 and HL-60/VCR cells, but not in HL-60/ADR cells. Additionally, Z-VAD-fmk, a general inhibitor of caspases which inhibited cytochrome c release in HL-60 and HL-60/VCR cells, had no effect on AIF release in any of these cell lines, but inhibited beta(2)m-induced apoptosis in all three cell lines. However, Western blot analysis revealed that caspases-1, -3, -6, -8, and -9 are not activated during beta(2)m-induced apoptosis in these cells. Therefore, beta(2)m-induces apoptosis through an unknown caspase-dependent mitochondrial pathway in HL-60 and HL-60/VCR cells and by a Bax-independent, non-mitochondrial, caspase-dependent pathway in HL-60/ADR cells.

Metabolic fate of (-)-[4-(3)H]epigallocatechin gallate in rats after oral administration

After oral administration of [4-(3)H]EGCg to rats, the radioactivity in blood, major tissues, urine, and feces was measured over time. The radioactivity in blood and most tissues remained low for 4 h postdose, began to increase after 8 h, peaked at 24 h, and then decreased. Major urinary excretion of radioactivity occurred in the 8-24 h period, and the cumulative radioactivity excreted by 72 h was 32.1% of the dose. The radioactivity in the feces was 35.2% of the dose within 72 h postdose. In the case of rats pretreated with antibiotics (antibiotic-pretreated rats), the radioactivity levels of the blood and urine were definitely lower than those in rats not pretreated with antibiotics (normal rats). The radioactivity recovered in the antibiotic-pretreated rat urine was estimated to be only (1)/(100) of that in the normal rat urine. These results clearly demonstrated that the radioactivity detected in the blood and urine of normal rats mostly originated from degradation products of EGCg produced by intestinal bacteria. Furthermore, a main metabolite in the normal rats was purified and identified as 5-(5'-hydroxyphenyl)-gamma-valerolactone 3'-O-beta-glucuronide (M-2). In feces of the normal rats, EGC (40.8% of the fecal radioactivity) and 5-(3',5'-dihydroxyphenyl)-gamma-valerolactone (M-1, 16.8%) were detected. These results suggested that M-1 was absorbed in the body after degradation of EGCg by intestinal bacteria, yielding M-1 with EGC as an intermediate. Furthermore, M-2 was thought to be formed from M-1 in the intestinal mucosa and/or liver, then to enter the systemic circulation, and finally to be excreted in the urine. Taking into account all of the above findings, a possible metabolic route of EGCg orally administered to rats is proposed.

A novel autocrine pathway of tumor escape from immune recognition: melanoma cell lines produce a soluble protein that diminishes expression of the gene encoding the melanocyte lineage melan-A/MART-1 antigen through down-modulation of its promoter

We have observed that malignant melanoma cells produce a soluble protein factor(s), which down-regulates melanocyte lineage Melan-A/MART-1 Ag expression by melanoma cells with concomitant loss of recognition by Melan-A/MART-1-specific T cells. This down-modulation of Melan-A/MART-1 expression, which we refer to as "Ag silencing," is mediated via its minimal promoter, whereas the promoter for the restricting Ag-presenting HLA-A2 molecule is not affected. Significantly, this Ag silencing is reversible, as removal of factor-containing supernatants from Melan-A/MART-1-expressing cells results in up-regulation of the promoter for the gene encoding this Ag, and renewed expression of the protein. We have evaluated over 20 known factors, none of which accounts for the Ag-silencing activity of the melanoma cell culture supernatants. The existence of this autocrine pathway provides an additional novel explanation for melanoma tumor progression in vivo in the presence of CTL specific for this melanocyte lineage Ag. These observations may have important implications for Melan-A/MART-1-specific CTL-mediated immunotherapy of melanoma tumors.

Beta2-microglobulin gene mutation is not a common mechanism of HLA class I total loss in human tumors

One hundred and sixty-two tumor samples were analyzed for HLA class I expression using immunohistological techniques. HLA class I total loss (phenotype no. I) was detected in 31 cases (19%), comprising 20 colorectal, 3 laryngeal, and 2 bladder carcinomas and 6 melanomas. Twenty-one cases were selected for molecular analysis due to a higher proportion of tumor cells versus stroma cells (75%). We investigated whether beta2-microglobulin mutation was responsible for HLA downregulation. Single-strand conformation polymorphism and sequencing analysis of DNA samples was performed. Alterations were detected only in melanomas M78 (a point mutation in the initiation ATG sequence), M79 (a mutation in codon 31 producing a stop codon), and M34 (a TTCT deletion introducing a termination codon signal). We found no beta2-microglobulin gene mutation in the other 18 samples. Loss of heterozygosity in 15q close to the beta2-microglobulin gene was found in 5 cases. We conclude that HLA class I total loss can frequently occur without beta2-microglobulin gene mutations.

Clinical significance of MART-1 and HLA-A2 expression and CD8+ T cell infiltration in melanocytic lesions in HLA-A2 phenotype patients

MART-1 is a good candidate antigen for immunotherapy against HLA-A2 patients with melanoma, since it is a highly immunogenic antigen recognized by HLA-A2 and HLA-B45 restricted CD8+ cytotoxic T cells and expressed in the majority of melanoma lesions. In the present study the expression of MART-1 and HLA-A2 on melanocytic cells and CD8+ T cell infiltration was immunohistochemically analyzed. MART-1 was expressed in most melanocytic lesions, while HLA-A2 was down-regulated with melanoma disease progression. Furthermore, concomitant down-regulation of MART-1 and HLA-A2 in melanoma cells was correlated with poor prognosis. These findings suggest both MART-1 and HLA-A2 expression in melanoma lesions should be analyzed for selection of patients eligible for MART-1 based immunotherapy and monitoring for emergence of melanoma cells resistant to T cell therapy.

T cell-mediated tumor rejection displays diverse dependence upon perforin and IFN-gamma mechanisms that cannot be predicted from in vitro T cell characteristics

Experimental pulmonary metastases have been successfully treated by adoptive transfer of tumor-sensitized T cells from perforin knockout (KO) or Fas/APO-1 ligand(KO) mice, suggesting a prominent role for secretion of cytokines such as IFN-gamma. In the present study we confirmed that rejection of established methylcholanthrene-205 (MCA-205) pulmonary metastases displayed a requirement for T cell IFN-gamma expression. However, this requirement could be obviated by transferring larger numbers of tumor-sensitized IFN-gamma (KO) T cells or by immunosensitizing sublethal irradiation (500 rad) of the host before adoptive therapy. Extrapulmonary tumors (MCA-205 s.c. and intracranial) that required adjunct sublethal irradiation for treatment efficacy also displayed no requirement for host or T cell expression of IFN-gamma. Nonetheless, rejection of MCA-205 s.c. tumors and i.p. EL-4 tumors, but not MCA-205 pulmonary or intracranial tumors, displayed a significant requirement for T cell perforin expression (i.e., CTL participation). The capacity of T cells to lyse tumor targets and secrete IFN-gamma in vitro before adoptive transfer was nonpredictive of the roles of these activities in subsequent tumor rejection. Adoptive therapy studies employing KO mice are therefore indispensable for revealing a diversity of tumor rejection mechanisms that may lack in vitro correlation due to delays in their induction. Seemingly contradictory KO data from different studies are reconciled by the capacity of anti-tumor T cells to rely on alternative mechanisms when treated in larger numbers, the variable participation of CTL at different anatomic locations of tumor, and the apparent capacity of sublethal irradiation to provide a therapeutic alternative to host or T cell IFN-gamma production.

Parameters involved in the recognition of fresh human leukemic blasts by tumor-specific cytolytic T cell clones: a model study

Although clinical experience and in vitro data provide evidence of an anti-leukemic activity of T cells, there are few examples of recognition of leukemic cells by tumor-specific T cells in vitro. Tumor antigens encoded by the MAGE genes are useful tools to study this recognition. We tested the sensitivity to recognition and lysis by anti-MAGE CTL clones of MAGE-A1 positive cell lines HL60 and K562, after transfection with an HLA-A1 construct, and of fresh leukemic blasts from 10 HLA-A2 patients, after incubation with a peptide encoded by gene MAGE-A3. The presentation of MAGE antigens by leukemic cell lines and fresh leukemic blasts induced TNF secretion and cytotoxicity by MAGE-specific CD8(+) CTL clones. The amount of peptide presented by the leukemic blasts, more than the level of expression of HLA class I, adhesion or costimulatory molecules, was the major limiting factor for recognition. These data indicate that leukemic cells may be targeted by T cells showing specificity for a leukemia antigen.

Cross-presentation of tumor antigens to effector T cells is sufficient to mediate effective immunotherapy of established intracranial tumors

The systemic adoptive transfer of tumor-sensitized T cells, activated ex vivo, can eliminate established intracranial tumors. Regression of MHC class II negative MCA 205 fibrosarcomas occurs optimally following adoptive transfer of both CD4 and CD8 tumor-sensitized T cells, indicating an important function for tumor-infiltrating APC. Here, we demonstrate that during an effector response, indirect presentation of tumor Ags to transferred T cells is sufficient to mediate intracranial tumor regression. BALB/c --> CB6F1 (H-2bxd) bone marrow chimeras were challenged with the MCA 205 fibrosarcoma (H-2b). The tumor grew progressively in the H-2b-tolerant chimeras and stimulated an immune response in tumor-draining lymph nodes. Tumor-sensitized lymph node T cells were activated ex vivo with anti-CD3 and IL-2, then adoptively transferred to sublethally irradiated BALB/c or C57BL/6 recipients bearing established intracranial MCA 205 tumors. The transferred T cells eradicated MCA 205 tumors in BALB/c recipients and demonstrated tumor specificity, but had no therapeutic efficacy in the C57BL/6 recipients. These data establish that tumor-associated host cell constituents provide sufficient Ag presentation to drive effector T cell function in the complete absence of direct tumor recognition. This effector mechanism has an evident capacity to remain operative in circumstances of immune escape, where the tumor does not express the relevant MHC molecules, and may have importance even at times when direct CTL recognition also remains operative.

Heterogeneity in expression of human leukocyte antigens and melanoma-associated antigens in advanced melanoma

The study of tumor immunology has led to many innovative therapeutic strategies for the treatment of melanoma. The strategies are primarily dependent on melanoma-associated antigen peptide vaccination or T-cell-based therapy. These immunotherapies are totally reliant on proper copresentation of human leukocyte antigen class I molecules in sufficient quantity and the presence and availability of melanoma-associated antigenic peptides. Altered expression of either HLA class I molecules or melanoma antigens is known to occur. These defects lead to altered manufacture and copresentation of HLA class I molecules with melanoma-associated antigens to T-cells. Defects in any one combination can lead to loss of recognition of melanoma cells and their subsequent destruction by cytotoxic T-lymphocytes. Thus, these immunotherapy strategies can be thwarted by defects or heterogeneity of expression of human leukocyte antigen class I or of melanoma-associated antigens.

T-cell recognition of melanoma-associated antigens

In this review, we summarize the significant progress that has been made in the identification of melanoma-associated antigens (MAA) recognized by cytotoxic T-lymphocytes (CTL). These antigens belong to three main groups: tumor-associated testis-specific antigens (e.g. , MAGE, BAGE, and GAGE); melanocyte differentiation antigens (e.g., tyrosinase, Melan-A/MART-1); and mutated or aberrantly expressed molecules (e.g, CDK4, MUM-1, beta-catenin). Although strong CTL activity may be induced ex vivo against most of these antigens, often in the presence of excess cytokines and antigen, a clear understanding of the functional status of CTL in vivo and their impact on tumor growth, is still lacking. Several mechanisms are described that potentially contribute to tumor cell evasion of the immune response, suggesting that any antitumor efficacy achieved by immune effectors may be offset by factors that result ultimately in tumor progression. Nevertheless, most of these MAA are currently being investigated as immunizing agents in clinical studies, the conflicting results of which are reviewed. Indeed, the therapeutic potential of MAA has still to be fully exploited and new strategies have to be found in order to achieve an effective and long-lasting in vivo immune control of melanoma growth and progression.

Activation of lymphocytes by anti-CD3 single-chain antibody dimers expressed on the plasma membrane of tumor cells

Activation of cytotoxic T cells without MHC restriction was attempted by expressing single-chain antibodies (scFv) against CD3 on the surface of tumor cells. A chimeric protein consisting of a scFv of mAb 145.2C11, the hinge-CH2-CH3 region of human IgG1, and the transmembrane and cytosolic domains of murine CD80 formed disulfide-linked dimers on the plasma membrane of cells and specifically bound lymphocytes. Anti-CD3 scFv dimers expressed on the cell surface induced CD25 (IL-2 receptor alpha-chain) expression and proliferation of splenocytes. CT26 tumor cells engineered to express surface scFv dimers (CT26/2C11) also induced potent lymphocyte cytotoxicity with or without addition of exogenous IL-2. Splenocytes activated by CT26/2C11 cells also killed wild-type CT26 cells, indicating that activated splenocytes could kill bystander tumor cells. Immunization of BALB/c mice with irradiated CT26/2C11 cells did not protect against a lethal challenge of CT26 cells, suggesting that systemic immunity was not induced. However, the growth of CT26 tumors containing 50% CT26/2C11 cells was significantly retarded compared with CT26 tumors, whereas CT26/2C11 tumors did not grow in syngeneic mice. These results suggest that expression of anti-CD3 scFv dimers on tumors may form the basis for a novel therapeutic strategy for tumors that exhibit defects in antigen processing or presentation. Gene Therapy (2000) 7, 339-347.

Reduced recognition of metastatic melanoma cells by autologous MART-1 specific CTL: relationship to TAP expression

Class I expression in context with T-cell receptor expression is crucial for peptide presentation and induction of CD8+ cytotoxic T lymphocytes (CTL). Presentation of class I bound peptides is dependent on transporter-associated proteins (TAP) expression and function. Tumor infiltrating lymphocytes from a patient with melanoma were isolated, expanded in vitro in the presence of interleukin-2, and tested for cytotoxicity against HLA-A2 positive, MART-1 positive autologous tumor cells, an HLA-A2-positive, MART-1 positive melanoma cell line (Mel-501), and HLA-A2-negative melanoma cells. Significant killing occurred against both A2-positive cell lines (63% and 65%, respectively), but not against the A2-negative line (18%) or A2-positive autologous tumor (1.5%). These CTL preferentially recognized the MART-1 peptide F119, 27-35, and gp100 peptide F125, 280-288, resulting in a 30% to 60% enhancement of lysis when autologous tumor or major histocompatibility complex class I "empty" T2 cells were pulsed with either peptide. To address whether the deficiency in autologous tumor recognition might be related to a deficiency in Ag presentation, we screened for the presence of TAP1 and TAP2 transcripts by polymerase chain reaction, Southern blotting, and scanning densitometry using sequence-specific primers and probes. Both TAP1 and TAP2 expression levels in the autologous tumor were minimal, yet were upregulated 7- to 18-fold, respectively, by interferon-gamma. Despite this increase, a similar increase in cytotoxicity did not occur. In short, deficiencies in TAP presentation may have functional significance for tumor escape from immunosurveillance and with respect to impending vaccine trials.