TAP off — tumors on

Mechanisms of MHC-I Downregulation and Role in Immunotherapy Response

Immunotherapy has become a key therapeutic strategy in the treatment of many cancers. As a result, research efforts have been aimed at understanding mechanisms of resistance to immunotherapy and how anti-tumor immune response can be therapeutically enhanced. It has been shown that tumor cell recognition by the immune system plays a key role in effective response to T cell targeting therapies in patients. One mechanism by which tumor cells can avoid immunosurveillance is through the downregulation of Major Histocompatibility Complex I (MHC-I). Downregulation of MHC-I has been described as a mechanism of intrinsic and acquired resistance to immunotherapy in patients with cancer. Depending on the mechanism, the downregulation of MHC-I can sometimes be therapeutically restored to aid in anti-tumor immunity. In this article, we will review current research in MHC-I downregulation and its impact on immunotherapy response in patients, as well as possible strategies for therapeutic upregulation of MHC-I.

Targeting Neoepitopes to Treat Solid Malignancies: Immunosurgery

Successful outcome of immune checkpoint blockade in patients with solid cancers is in part associated with a high tumor mutational burden (TMB) and the recognition of private neoantigens by T-cells. The quality and quantity of target recognition is determined by the repertoire of ‘neoepitope’-specific T-cell receptors (TCRs) in tumor-infiltrating lymphocytes (TIL), or peripheral T-cells. Interferon gamma (IFN-γ), produced by T-cells and other immune cells, is essential for controlling proliferation of transformed cells, induction of apoptosis and enhancing human leukocyte antigen (HLA) expression, thereby increasing immunogenicity of cancer cells. TCR αβ-dependent therapies should account for tumor heterogeneity and availability of the TCR repertoire capable of reacting to neoepitopes and functional HLA pathways. Immunogenic epitopes in the tumor-stroma may also be targeted to achieve tumor-containment by changing the immune-contexture in the tumor microenvironment (TME). Non protein-coding regions of the tumor-cell genome may also contain many aberrantly expressed, non-mutated tumor-associated antigens (TAAs) capable of eliciting productive anti-tumor immune responses. Whole-exome sequencing (WES) and/or RNA sequencing (RNA-Seq) of cancer tissue, combined with several layers of bioinformatic analysis is commonly used to predict possible neoepitopes present in clinical samples. At the ImmunoSurgery Unit of the Champalimaud Centre for the Unknown (CCU), a pipeline combining several tools is used for predicting private mutations from WES and RNA-Seq data followed by the construction of synthetic peptides tailored for immunological response assessment reflecting the patient’s tumor mutations, guided by MHC typing. Subsequent immunoassays allow the detection of differential IFN-γ production patterns associated with (intra-tumoral) spatiotemporal differences in TIL or peripheral T-cells versus TIL. These bioinformatics tools, in addition to histopathological assessment, immunological readouts from functional bioassays and deep T-cell ‘adaptome’ analyses, are expected to advance discovery and development of next-generation personalized precision medicine strategies to improve clinical outcomes in cancer in the context of i) anti-tumor vaccination strategies, ii) gauging mutation-reactive T-cell responses in biological therapies and iii) expansion of tumor-reactive T-cells for the cellular treatment of patients with cancer.

Polymorphisms in transporter associated with antigen presenting are associated with cervical intraepithelial neoplasia and cervical cancer in a Chinese Han population

The host immune system plays an important role in infectious diseases and cancers. The heterodimer formed by transporter associated with antigen presenting (TAP)1 and TAP2 is responsible for intracellular peptide loading onto MHC-I molecules. Studies have shown that single-nucleotide polymorphisms (SNPs) in TAP genes might affect the expression and function of TAP and be associated with cancer risk. We aimed to investigate the association of SNPs in the TAP1 and TAP2 genes with cervical intraepithelial neoplasia (CIN) and cervical cancer (CC) in a Chinese Han population. Six SNPs in the TAP1 gene and seven in the TAP2 gene were selected. The 13 SNPs were genotyped in 1255 healthy individuals, 575 patients with CIN and 1034 patients with CC using the SNaPshot assay. The association between these SNPs and CIN and CC risk was analysed. The allelic and genotypic distributions of rs41549617 and rs1135216 showed significant differences between the control and CC groups (P < 0.0038). The T allele of rs41549617 was associated with a decreased risk of CC (OR = 0.476, 95%CI: 0.286-0.791). Moreover, the G allele of rs1135216 appears to be associated with a decreased risk of CC (OR = 0.746; 95%CI: 0.632-0.881). The allele and genotype distribution of rs241441 showed a significant difference between the control and CC groups (P < 0.0038), and the rs241441 G allele was associated with an increased risk of CC (OR = 1.232, 95%CI: 1.092-1.398). In addition, the results of the association between TAP alleles and CC showed that TAP1*020101 and TAP1*0301 have an association with CC (P = 0.001 and P = 0.002, respectively). Our results demonstrate that the TAP1 and TAP2 genes are associated with CC in the Chinese Han population.

C19orf48 encodes a minor histocompatibility antigen recognized by CD8+ cytotoxic T cells from renal cell carcinoma patients

PURPOSE

Tumor regression has been observed in some patients with metastatic renal cell carcinoma (RCC) after nonmyeloablative allogeneic hematopoietic cell transplantation (HCT). Cellular and molecular characterization of antigens recognized by tumor-reactive T cells isolated from responding patients could potentially provide insight into the mechanisms of tumor regression.

EXPERIMENTAL DESIGN

CD8+ CTL clones that recognized a novel RCC-associated minor histocompatibility (H) antigen presented by HLA-A*0201 were isolated from two patients with metastatic RCC who experienced tumor regression or stable disease following nonmyeloablative allogeneic HCT. These clones were used to screen a cDNA library and isolate the unique cDNA encoding the antigen.

RESULTS

An alternative open reading frame in the C19orf48 gene located on chromosome 19q13 encodes the HLA-A*0201-restricted minor H antigen recognized by the RCC-reactive T cells. The differential T-cell recognition of donor- and recipient-derived target cells is attributable to a nonsynonymous single-nucleotide polymorphism within the nucleotide interval that encodes the antigenic peptide. Assays for gene expression and CTL recognition showed that the C19orf48-encoded peptide is widely expressed in renal tumors and solid tumors of other histologies. The antigenic peptide can be processed for CTL recognition via both TAP-dependent and TAP-independent pathways.

CONCLUSIONS

Donor T-cell responses against the HLA-A*0201-restricted minor H antigen encoded by C19orf48 may contribute to RCC regression after MHC-matched allogeneic HCT.

Different lineages of P1A-expressing cancer cells use divergent modes of immune evasion for T-cell adoptive therapy

Tumor evasion of T-cell immunity remains a significant obstacle to adoptive T-cell therapy. It is unknown whether the mode of immune evasion is dictated by the cancer cells or by the tumor antigens. Taking advantage of the fact that multiple lineages of tumor cells share the tumor antigen P1A, we adoptively transferred transgenic T cells specific for P1A (P1CTL) into mice with established P1A-expressing tumors, including mastocytoma P815, plasmocytoma J558, and fibrosarcoma Meth A. Although P1CTL conferred partial protection, tumors recurred in almost all mice. Analysis of the status of the tumor antigen revealed that all J558 tumors underwent antigenic drift whereas all P815 tumors experienced antigenic loss. Interestingly, although Meth A cells are capable of both antigenic loss and antigenic drift, the majority of recurrent Meth A tumors retained P1A antigen. The ability of Meth A to induce apoptosis of P1CTL in vivo alleviated the need for antigenic drift and antigenic loss. Our data showed that, in spite of their shared tumor antigen, different lineages of cancer cells use different mechanisms to evade T-cell therapy.

Expression and Functional Analysis of Human Leukocyte Antigen Class I Antigen-Processing Machinery in Medulloblastoma

Defects in the expression and/or function of the human leukocyte antigen (HLA) class I antigen-processing machinery (APM) components are found in many tumor types. These abnormalities may have a negative impact on the interactions of tumor cells with host's immune system and on the outcome of T cell-based immunotherapy. To the best of our knowledge, no information is available about APM component expression and functional characteristics in human medulloblastoma cells (Mb). Therefore, in the present study, we have initially compared the expression of APM components in Mb, an embryonal pediatric brain tumor with a poor prognosis, with that in noninfiltrating astrocytic pediatric tumors, a group of differentiated brain malignancies with favorable prognosis. LMP2, LMP7, calnexin, beta2-microglobulin-free heavy chain (HC) and beta2-microglobulin were down-regulated or undetectable in Mb lesions, but not in astrocytic tumors or normal fetal cerebellum. Two Mb cell lines (DAOI and D283) displayed similar but not superimposable defects in APM component expression as compared with primary tumors. To assess the functional implications of HLA class I APM component down-regulation in Mb cell lines, we tested their recognition by HLA class I antigen-restricted, tumor antigen (TA)-specific CTL, generated by stimulations with dendritic cells that had been transfected with Mb mRNA. The Mb cell lines were lysed by TA-specific CTL in a HLA-restricted manner. Thus, defective expression of HLA class I-related APM components in Mb cells does not impair their ability to present TA to TA-specific CTL. In conclusion, these results can contribute to optimize T cell-based immunotherapeutic strategies for Mb treatment.

Antitumor NK activation induced by the Toll-like receptor 3-TICAM-1 (TRIF) pathway in myeloid dendritic cells

Myeloid dendritic cells (mDCs) recognize and respond to polyI:C, an analog of dsRNA, by endosomal Toll-like receptor (TLR) 3 and cytoplasmic receptors. Natural killer (NK) cells are activated in vivo by the administration of polyI:C to mice and in vivo are reciprocally activated by mDCs, although the molecular mechanisms are as yet undetermined. Here, we show that the TLR adaptor TICAM-1 (TRIF) participates in mDC-derived antitumor NK activation. In a syngeneic mouse tumor implant model (C57BL/6 vs. B16 melanoma with low H-2 expresser), i.p. administration of polyI:C led to the retardation of tumor growth, an effect relied on by NK activation. This NK-dependent tumor regression did not occur in TICAM-1(-/-) or IFNAR(-/-) mice, whereas a normal NK antitumor response was induced in PKR(-/-), MyD88(-/-), IFN-beta(-/-), and wild-type mice. IFNAR was a prerequisite for the induction of IFN-alpha/beta and TLR3. The lack of TICAM-1 did not affect IFN production but resulted in unresponsiveness to IL-12 production, mDC maturation, and polyI:C-mediated NK-antitumor activity. This NK activation required NK-mDC contact but not IL-12 function in in vivo transwell analysis. Implanted tumor growth in IFNAR(-/-) mice was retarded by adoptively transferring polyI:C-treated TICACM-1-positive mDCs but not TICAM-1(-/-) mDCs. Thus, TICAM-1 in mDCs critically facilitated mDC-NK contact and activation of antitumor NK, resulting in the regression of low MHC-expressing tumors.

Identification of Mechanisms Underlying Transporter Associated with Antigen Processing Deficiency in Metastatic Murine Carcinomas

Expression of transporter associated with antigen processing (TAP) is often lost in metastatic carcinomas, resulting in defective antigen processing and presentation and escape of the cancer cells from immune surveillance. In this study, the nature of TAP deficiencies in tumors was investigated. By chromatin immunoprecipitation assay, we showed that the recruitment of RNA polymerase II to the TAP-1 gene was impaired in TAP-deficient cells derived from murine melanoma, prostate, and lung carcinomas, compared with TAP-expressing fibroblasts and lymphoma cells. This suggested that the deficiency in TAP-1 expression resulted, at least partially, from a relatively low level of transcription of the TAP-1 gene. Furthermore, levels of TAP-1 promoter activity, as assessed by stable transfections with a reporter construct containing the TAP-1 promoter, were relatively low in TAP-deficient cells. To examine genetic heritability of regulators of TAP-1 promoter activity, TAP- and MHC class I-deficient cells of H-2b origin were fused with wild-type fibroblasts of H-2k origin. Fusion with TAP-expressing cells complemented the low levels of TAP-1 promoter activity in TAP-deficient cells. However, these fused cells exhibited lower levels of TAP-1 mRNA and H-2k than unfused fibroblasts. Further analysis showed that TAP-1 mRNA stability was lower in fused carcinoma fibroblasts than in unfused fibroblasts. Based on these results, we propose that TAP deficiency in many carcinomas is caused by a decrease in activity/expression of trans-acting factors regulating TAP-1 promoter activity, as well as a decrease in TAP-1 mRNA stability. These results have significant implications for understanding immune evasion mechanisms in tumors.

A Rare Transporter Associated with Antigen Processing Polymorphism Overpresented in HLAlow Colon Cancer Reveals the Functional Significance of the Signature Domain in Antigen Processing

Transporter associated with antigen processing (TAP), a member of the ATP-binding cassette transporter superfamily, is composed of two integral membrane proteins, TAP-1 and TAP-2. Each subunit has a C-terminal nucleotide-binding domain that binds and hydrolyzes ATP to energize peptide translocation across the endoplasmic reticulum membrane. A motif comprising the sequence LSGGQ (called the signature motif) and the amino acid that is immediately C-terminal to this motif are highly conserved in the nucleotide-binding domains of ATP-binding cassette transporters. To search for natural variants of TAP-1 with alterations in or near the signature motif, we sequenced the TAP-1 exon 10 amplified from 103 human colon cancer samples. We found a rare TAP-1 allele with an R>Q alteration at a residue immediately C-terminal to the signature motif (R648) that occurred 17.5 times more frequently in colon cancers with down-regulated surface class I MHC than those with normal MHC levels (P = 0.01). Functional analysis revealed that the Q648 variant had significantly reduced peptide translocation activity compared with TAP-1 (R648). In addition, we found that mutations S644R, G645R, G646S, and G646D interfered with TAP-1 activity. TAP-1 G646D, which showed the most severe defect, resided normally in the endoplasmic reticulum and associated with the peptide loading complex, but failed to transport peptide across the endoplasmic reticulum membrane. Thus, a TAP-1 polymorphism adjacent to the signature motif may be a contributing factor for MHC class I down-regulation in colon cancer. Given the widespread defects in DNA mismatch repair in colon cancer, mutations at or near the signature domain can potentially modulate antigen processing.

Potent selection of antigen loss variants of B16 melanoma following inflammatory killing of melanocytes in vivo

We have reported that i.d. injection of plasmids encoding hsp70 and a suicide gene transcriptionally targeted to melanocytes generates specific proinflammatory killing of melanocytes. The resulting CD8+ T cell response eradicates systemically established B16 tumors. Here, we studied the consequences of that CD8+ T cell response on the phenotype of preexisting tumor. In suboptimal protocols, the T cell response selected B16 variants, which grow extremely aggressively, are amelanotic and have lost expression of the tyrosinase and tyrosinase-related protein 2 (TRP-2) antigens. However, expression of other melanoma-associated antigens, such as gp100, was not affected. Antigen loss could be reversed by long-term growth in culture away from immune-selective pressures or within 96 hours by treatment with the demethylating agent 5-azacytidine (5-Aza). When transplanted back into syngeneic animals, variants were very poorly controlled by further vaccination. However, a combination of vaccination with 5-Aza to reactivate antigen expression in tumors in situ generated highly significant improvements in therapy over treatment with vaccine or 5-Aza alone. These data show that inflammatory killing of normal cells activates a potent T cell response targeted against a specific subset of self-antigens but can also lead to the immunoselection of tumor variants. Moreover, our data indicate that emergence of antigen loss variants may often be due to reversible epigenetic mechanisms within the tumor cells. Therefore, combination therapy using vaccination and systemic treatment with 5-Aza or other demethylating agents may have significant therapeutic benefits for antitumor immunotherapy.

LMP7/TAP2 gene polymorphisms and HPV infection in esophageal carcinoma patients from a high incidence area in China

Esophageal carcinoma is characterized by a widely ranged incidence variation among the different geographic regions. Anyang is a county in Henan Province of North China with the highest prevalence of esophageal carcinoma. Human papillomavirus (HPV) infection has been linked to the etiology of esophageal cancer in this area. In this study, we investigated correlations of the polymorphisms at low molecular weight polypeptide (LMP) and transporters with antigen processing (TAP) genes, with the risk of esophageal carcinoma. DNA extracted from either tumor specimens or esophageal epithelial cells was used to test HPV infection. Peripheral blood lymphocyte DNA was used for LMP/TAP genotyping. Polymerase chain reaction was performed to analyze HPV infection and LMP/TAP gene polymorphisms. The combined effect of LMP/TAP gene polymorphisms and HPV infection on esophageal carcinoma was analyzed by using unconditional logistic regression models. The TAP2 codons 379 isoleucine carriers and LMP7 codons 145 lysine carriers were found to be more susceptible to esophageal carcinoma (OR = 2.74, 95% CI = 1.15-6.49, P = 0.023 for TAP2; OR = 2.19, 95% CI = 1.09-4.37, P = 0.027 for LMP7). Patients carrying homozygous LMP7/TAP2 haplotype C, which contained the glutamine at LMP7 codons 145 and the isoleucine at TAP2 codons 379, were more prone to develop esophageal carcinoma (OR = 2.96, 95% CI = 1.13-7.81, P = 0.027). An additive effect on the risk of esophageal carcinoma development was found among individuals carrying LMP7/TAP2 haplotype C and infected by HPV (OR = 4.33, 95% CI = 2.53-7.42, P < 0.0001). LMP7/TAP2 haplotype C may act as the risk factor in esophageal carcinoma development and it may influence the tumorigenesis in HPV infected individuals.

Sequential Immunogene Therapy with Interleukin-12– and Interleukin-15–Engineered Neuroblastoma Cells Cures Metastatic Disease in Syngeneic Mice

Purpose: To investigate the potential synergistic effects of Neuro2a neuroblastoma cells engineered with IL-12 and/or IL-15 genes in improving survival of syngeneic mice bearing neuroblastoma metastatic disease.

Experimental Design: Neuro2a cells engineered with interleukin (IL)-12 (Neuro2a/IL-12), IL-15 (Neuro2a/IL-15), or both cytokines (Neuro2a/IL-12/IL-15) were injected s.c. in syngeneic A/J mice challenged i.v. with Neuro2a parental cells (Neuro2apc) using different schedules of administration in either preventive or therapeutic settings.

Results: A single injection of Neuro2a/IL-12 or Neuro2a/IL-15 cells induced resistance to a subsequent i.v. Neuro2apc challenge in 45% and 28% of mice, respectively. Neuro2a/IL-12/IL-15 cells protected 28% of mice, showing no synergistic effect. However, sequential vaccination with Neuro2a/IL-12 (day −30) followed by Neuro2a/IL-15 (day −15) protected 71% of mice from subsequent challenge with Neuro2apc. A single dose of Neuro2a/IL-12 prolonged the mean survival time of mice bearing established metastatic neuroblastoma from 21 ± 3 to 46 ± 27 days but failed to cure mice, whereas Neuro2a/IL-15 or Neuro2a/IL-12/IL-15 were ineffective. However, sequential vaccination with Neuro2a/IL-12 (day +3) followed by Neuro2a/IL-15 (day +13) cured 43% of mice as assessed by histologic analysis of different organs from long-term surviving mice. CTL activity against Neuro2apc cells was observed in splenocytes from treated mice, and CD8+ T-cell depletion abrogated the therapeutic effect of vaccination.

Conclusions: Sequential vaccination with IL-12- and IL-15-engineered neuroblastoma cells induced optimal preventive and therapeutic effects, which may be related to the Th1 priming effect of IL-12 followed by the enhancement of CD8+ T-cell responses and their maintenance mediated by IL-15.

Vaccination of Dendritic Cells Loaded with Interleukin-12-Secreting Cancer Cells Augments In vivo Antitumor Immunity: Characteristics of Syngeneic and Allogeneic Antigen-Presenting Cell Cancer Hybrid Cells

Cancer immunotherapy by fusion of antigen-presenting cells and tumor cells has been shown to induce potent antitumor immunity. In this study, we characterized syngeneic and allogeneic, murine macrophage/dendritic cell (DC)-cancer fusion cells for the antitumor effects. The results showed the superiority of allogeneic cells as fusion partners in both types of antigen-presenting cells in an in vivo immunotherapy model. A potent induction of tumor-specific CTLs was observed in these immunized conditions. In addition, the immunization with DC-cancer fusion cells was better than that with macrophage-cancer fusion cells. Both syngeneic and allogeneic DC-cancer fusion cells induced higher levels of IFN-γ production than macrophage-cancer fusion cells. Interestingly, allogeneic DC-cancer fusion cells were superior in that they efficiently induced Th1-type cytokines but not the Th2-type cytokines interleukin (IL)-10 and IL-4, whereas syngeneic DC-cancer fusion cells were powerful inducers of both Th1 and Th2 cytokines. These results suggest that allogeneic DCs are suitable as fusion cells in cancer immunotherapy. To further enhance the antitumor immunity in the clinical setting, we prepared DCs fused with IL-12 gene-transferred cancer cells and thus generated IL-12-secreting DC-cancer fusion cells. Immunization with these gene-modified DC-cancer fusion cells was able to elicit a markedly enhanced antitumor effect in the in vivo therapeutic model. This novel IL-12-producing fusion cell vaccine might be one promising intervention for future cancer immunotherapy.

Down-regulation of MHC class I by bovine papillomavirus E5 oncoproteins

The papillomavirus E5 protein is localized in the endoplasmic reticulum (ER) and Golgi apparatus (GA) of the host cell. Transformed bovine fibroblasts expressing bovine papillomavirus (BPV) E5 are highly vacuolated and have a much enlarged, distorted and fragmented GA. Major histocompatibility complex class I (MHC I) is processed and transported to the cell surface through the GA. Given the cellular localization of E5 in the GA and the morphologically abnormal GA, we investigated the expression of MHC I in cells transformed by E5 from BPV-1 and BPV-4. Two cell lines were used: bovine cells that also express E6, E7 and activated ras, and NIH3T3 cells that express only E5. In addition, PalF cells acutely infected with a recombinant retrovirus expressing E5 were also examined. In contrast to non-transformed normal cells, or transformed cells expressing other papillomavirus proteins, cells expressing E5 do not express MHC I on their surface, but retain it intracellularly, independently of the presence of other viral or cellular oncogenes, or of whether the cells are long-term transformants or acutely infected. We conclude that expression of E5 prevents expression of MHC I to the cell surface and causes its retention within the cell. In addition, lower amounts of total MHC I heavy chain and of heavy chain RNA are detected in E5-transformed cells than in control cells. As surface expression of another glycosylated membrane protein, the transferrin receptor, is not affected, it appears that E5 targets MHC I with at least a degree of specificity. In papillomavirus lesions this effect would have important implications for antigen presentation by, and immunosurveillance of, virally infected cells.

Oncolytic herpes simplex virus vector with enhanced MHC class I presentation and tumor cell killing

Oncolytic herpes simplex virus type 1 (HSV-1) vectors are promising therapeutic agents for cancer. Their efficacy depends on the extent of both intratumoral viral replication and induction of a host antitumor immune response. To enhance these properties while employing ample safeguards, two conditionally replicating HSV-1 vectors, termed G47Delta and R47Delta, have been constructed by deleting the alpha47 gene and the promoter region of US11 from gamma34.5-deficient HSV-1 vectors, G207 and R3616, respectively. Because the alpha47 gene product is responsible for inhibiting the transporter associated with antigen presentation (TAP), its absence led to increased MHC class I expression in infected human cells. Moreover, some G47Delta-infected human melanoma cells exhibited enhanced stimulation of matched antitumor T cell activity. The deletion also places the late US11 gene under control of the immediate-early alpha47 promoter, which suppresses the reduced growth properties of gamma34.5-deficient mutants. G47Delta and R47Delta showed enhanced viral growth in a variety of cell lines, leading to higher virus yields and enhanced cytopathic effect in tumor cells. G47Delta was significantly more efficacious in vivo than its parent G207 at inhibiting tumor growth in both immune-competent and immune-deficient animal models. Yet, when inoculated into the brains of HSV-1-sensitive A/J mice at 2 x 10(6) plaque forming units, G47Delta was as safe as G207. These results suggest that G47Delta may have enhanced antitumor activity in humans.

Immunization with tumor-associated epitopes fused to an endoplasmic reticulum translocation signal sequence affords protection against tumors with down-regulated expression of MHC and peptide transporters

Treatment of human cancers with an inherent antigen-processing defect due to a loss of peptide transporters (TAP-1 and TAP-2) and/or MHC class I antigen expression remains a considerable challenge. There is now an increasing realization that tumor cells with down-regulated expression of TAP and/or MHC class I antigens display strong resistance to cytotoxic T lymphocyte (CTL)-mediated immune control, and often fail to respond to the conventional immunotherapeutic protocols based on active immunization with tumor-associated epitopes (TAE) or adoptive transfer of tumor-specific T cells. In the present study, we describe a novel approach based on immunization with either genetically modified tumor cells or naked DNA vectors encoding TAE fused to an endoplasmic reticulum (ER) signal sequence (ER-TAE) which affords protection against challenge by melanoma cells with down-regulated expression of TAP-1/2 and MHC class I antigens. In contrast, animals immunized with a vaccine based on TAE alone showed no protection against tumor challenge. Although MHC-peptide tetramer analysis showed a similar frequency of antigen-specific CTL in both ER-TAE- and TAE-immunized mice, functional analysis revealed that CTL activated following immunization with ER-TAE displayed significantly higher avidity for TAE when compared to animals immunized with the TAE alone. These observations provide a new strategy in anti-cancer vaccine design that allows activation of a highly effective and well-defined CTL response against tumors with down-regulated expression of TAP and MHC class I antigens.

Deficiency of Transporter for Antigen Presentation (TAP) in Tumor Cells Allows Evasion of Immune Surveillance and Increases Tumorigenesis

Proteins involved in class I MHC (MHC-I) Ag processing, such as the TAP, are deficient in some human tumor cells. This suggests that antitumor responses by CD8 T cells provide selection pressure to favor outgrowth of cells with defective processing of tumor Ags. Nonetheless, this evidence is only correlative, and controlled in vivo experiments have been lacking to demonstrate that TAP deficiency promotes survival of tumor cells. To explore the role of Ag processing defects in tumor progression, matched panels of TAP1-positive and TAP1-negative tumor cell lines were generated from a parental transformed murine fibroblast line. Inoculation of C57BL/6 mice with TAP1-negative cells produced large and persistent tumors. In contrast, TAP1-positive cells did not generate lasting tumors, although small tumors were detected transiently and regressed spontaneously. Both TAP1-positive and TAP1-negative cells produced tumors in athymic mice, confirming that TAP-dependent differences in tumorigenicity were due to T cell-dependent immune responses. Inoculation of C57BL/6 mice with mixtures of TAP1-positive and TAP1-negative cells produced tumors composed exclusively of TAP1-negative cells, indicating in vivo selection for cells with TAP deficiency. Thus, loss of TAP function allows some tumor cells to avoid T cell-dependent elimination, resulting in selection for tumor cells with deficient Ag processing.

Tumor Antigens are Constitutively Presented in the Draining Lymph Nodes

Tumor growth is rarely associated with a strong specific CTL response, suggesting that the immune system is ignorant of the presence of tumor because the Ags are not readily available to or are sequestered from potential effector cells. We studied the in vivo activation of naive TCR transgenic hemagglutinin (HA)-specific CD8+ T cells adoptively transferred into mice bearing HA-expressing tumor using 5,6-carboxy-succinimidyl-fluorescein-ester labeling, which allows the identification of proliferating HA-specific T cells. We demonstrate that tumor Ags are constitutively presented in the lymph nodes draining tumors and are powerfully mitogenic for responding T cells despite the absence of anti-tumor CTL responses. Importantly, this proliferative signal occurs throughout tumor growth and is still detectable 6 mo after tumor inoculation when tumor is not palpable. These results provide the first evidence that there is constitutive presentation of tumor Ags in draining lymph nodes.

Antitumor Effect of CD40 Ligand: Elicitation of Local and Systemic Antitumor Responses by IL-12 and B7

The interaction between CD40 ligand (CD40L, CD154) and its receptor CD40 has been implicated in the establishment of cell-mediated immunity as well as humoral immune responses. To examine the role of CD40L in eliciting antitumor immunity, we introduced murine CD40L gene into P815 mastocytoma (CD40L-P815). CD40L-P815 cells underwent prompt rejection when inoculated s.c. into syngenic DBA/2 mice or athymic BALB/c nu/nu mice, which was mediated by NK cells and dependent on endogenous IL-12. The primary rejection of CD40L-P815 cells in DBA/2 mice elicited CD8+ T cell-mediated protective and systemic immunity against parental tumor cells, which was induced by CD4+ T cells and endogenous B7. These results indicated a potent antitumor effect of CD40L that is mediated by potentiation of host Ag-presenting cell functions, and introduction of CD40L will be useful as a new strategy of immuno-gene therapy against tumors.

Generation of CD8+ T cells specific for transporter associated with antigen processing deficient cells

Cells with impaired transporter associated with antigen processing (TAP) function express low levels of cell surface major histocompatibility complex (MHC) class I molecules, and are generally resistant to lysis by MHC class I restricted cytotoxic T lymphocytes (CTLs). Here we report the generation of MHC class I restricted CD8(+) CTLs that surprisingly require target cell TAP deficiency for efficient recognition. C57BL/6 (B6) mice immunized with syngenic B7-1 (CD80) expressing TAP-deficient cells generated a potent CTL response against both TAP-deficient RMA-S tumor cells and TAP-deficient Con A blasts, whereas the corresponding TAP-expressing target cells were considerably less susceptible or resistant to lysis. The CTL epitopes recognized were expressed also by the human TAP-deficient cell line T2, transfected with appropriate MHC class I molecules. B6 mice immunized with B7-1-transfected TAP-deficient RMA-S cells were protected from outgrowth of a subsequent RMA-S tumor challenge. These findings are discussed in relation to the biochemical nature of MHC class I dependent CTL epitopes associated with impaired TAP function, as well as implications for immunotherapy and autoimmunity.