The selection of tumor variants with altered expression of classical and nonclassical MHC class I molecules: implications for tumor immune escape

Tumor immune escape variants can be identified in human and experimental tumors. A variety of different strategies are used by tumor cells to avoid recognition by different immune effector mechanisms. Among these escape routes, alteration of MHC class I cell surface expression is one of the mechanisms most widely used by tumor cells. In this review we focus our attention on the T-cell immune selection of MHC class I–deficient tumor variants. Different altered MHC class I phenotypes that originate from multiple molecular mechanisms can be identified in human tumors. MHC-deficient tumor clones can escape T-cell immune responses, but are in theory more susceptible to NK-cell–mediated lysis. In this context, we also review the controversial issue of the aberrant expression of nonclassical HLA class I molecules, particularly HLA-G, in tumors. This expression may be relevant in tumor cells that have lost the capacity to interact with NK inhibitory receptors—namely, those tumor cells with no HLA-B or HLA-C expression. Most published studies have not analyzed these possibilities and do not provide information about the complete HLA-A, HLA-B, or HLA-C molecule profiles of the tumors studied. In contrast, HLA-E has been reported to be expressed in some tumor cell lines with very low HLA-A, HLA-B, and HLA-C expression, suggesting that HLA-E may indeed, in some cases, play a role by inhibiting NK lysis of cells that otherwise would be destroyed by NK cells. Finally, we provide evidence that the status of the immune system in the tumor-bearing animal is capable of defining the MHC profile of the tumor cells. In other words, MHC class I–negative metastatic colonies are produced in immunocompetent animals, and MHC class I–positive colonies in T-cell immunodeficient individuals.This article forms part of the Symposium in Writing Tumor escape from the immune response, published in Vol. 53.     

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.     

A tumor-associated glycoprotein that blocks MHC class II-dependent antigen presentation by dendritic cells

Tumors evade immune surveillance despite the frequent expression of tumor-associated Ags (TAA). Tumor cells escape recognition by CD8(+) T cells through several mechanisms, including down-regulation of MHC class I molecules and associated Ag-processing machinery. However, although it is well accepted that optimal anti-tumor immune responses require tumor-reactive CD4(+) T cells, few studies have addressed how tumor cells evade CD4(+) T cell recognition. In this study, we show that a common TAA, GA733-2, and its murine orthologue, mouse epithelial glycoprotein (mEGP), function in blocking MHC class II-restricted Ag presentation by dendritic cells. GA733-2 is a common TAA that is expressed normally at low levels by some epithelial tissues and a subset of dendritic cells, but at high levels on colon, breast, lung, and some nonepithelial tumors. We show that ectopic expression of mEGP or GA733-2, respectively, in dendritic cells derived from murine bone marrow or human monocytes results in a dose-dependent inability to stimulate proliferation of Ag-specific or alloreactive CD4(+) T cells. Dendritic cells exposed to cell debris from tumors expressing mEGP are similarly compromised. Furthermore, mice immunized with dendritic cells expressing mEGP from a recombinant adenovirus vector exhibited a muted anti-adenovirus immune response. The inhibitory effect of mEGP was not due to down-regulation of functional MHC class II molecules or active suppression of T cells, and did not extend to T cell responses to superantigen. These results demonstrate a novel mechanism by which tumors may evade CD4(+) T cell-dependent immune responses through expression of a TAA.     

Natural killer cells: detectors of stress

Natural killer (NK) cells are a key component of the innate immune system, as they are able to detect microbe-infected cells, tumors as well as allogeneic cells, without specific sensitization. NK cell effector functions (cytotoxicity, cytokine secretion) are regulated by a wide array of inhibitory and activating receptors. MHC class I molecules are the ligands of most inhibitory receptors, while activating receptors recognize either pathogen-encoded molecules, or self-proteins whose expression is up-regulated upon microbial infection or tumor development. Upon integration of these negative and positive signals, Natural Killer cells can discriminate between healthy "self" (tolerance) and autologous cells undergoing different types of cellular stress or allogeneic cells (immunosurveillance). The knowledge of the different mechanisms of target cell recognition is thus crucial to dissect NK cell involvement in homeostatic and disease conditions as well as to develop novel alternative therapeutic approaches based on NK cell manipulation.     

Cyclooxygenase inhibitors modulate NK activities that control metastatic disease

Cyclooxygenase (COX) inhibitors have demonstrated efficacy in models of human cancer but the relevant mechanisms have not all been elucidated. Both Cox-dependent as well as Cox-independent mechanisms have been implicated. Using a syngeneic model of metastatic breast cancer, we have investigated the effect of Cox inhibitors on NK functions that are critical to the control of metastatic disease. NK recognition of target cells is governed by a balance of activating and inhibiting receptors that bind ligands including MHC class I. We now show that treatment of tumor cells with the nonselective COX-1/COX-2 inhibitor indomethacin or the selective COX-2 inhibitor celecoxib leads to decreased expression of the MHC class I molecules L^d and K^d . Downregulated class I expression is associated with concomitant increased sensitivity to NK cell-mediated lysis. Both COX inhibitors limit tumor metastasis and this therapeutic effect is dependent on NK but not T cell function. Antimetastatic activity is also lost in the absence of interferon- (IFN-). Both COX inhibitors also suppress local tumor growth of subcutaneously implanted mammary tumor cells in immune competent Balb/cByJ mice. This therapeutic activity is lost in the absence of either CD4+ or CD8+ T cells, but is not compromised by the loss of NK activity. Thus, the mechanism of tumor inhibition differs in the context of local versus metastatic disease. Taken together, these findings are consistent with a mechanism not previously described, whereby COX inhibitors may relieve MHC-mediated inhibition of NK cytotoxicity leading to recognition and lysis of metastatic tumor cells.     

NK cell-based cancer immunotherapy: from basic biology to clinical application

Natural killer(NK) cells, which recognize and kill target cells independent of antigen specificity and major histocompatibility complex(MHC) matching, play pivotal roles in immune defence against tumors. However, tumor cells often acquire the ability to escape NK cell-mediated immune surveillance. Thus, understanding mechanisms underlying regulation of NK cell phenotype and function within the tumor environment is instrumental for designing new approaches to improve the current cell-based immunotherapy. In this review, we elaborate the main biological features and molecular mechanisms of NK cells that pertain to regulation of NK cell-mediated anti-tumor activity. We further overview current clinical approaches regarding NK cell-based cancer therapy, including cytokine infusion, adoptive transfer of autologous or allogeneic NK cells, applications of chimeric antigen receptor(CAR)-expressing NK cells and adoptive transfer of memory-like NK cells. With these promising clinical outcomes and fuller understanding the basic questions raised in this review, we foresee that NK cell-based approaches may hold great potential for future cancer immunotherapy.     

Evidence for an involvement of T4+ cytotoxic T cells in tumor immunity

Cell-mediated immunity against cancer cells primarily involves class I major histocompatibility complex (MHC)-restricted cytotoxic T cells and natural killer (NK) cells. To investigate whether T4+ cytotoxic T cells also have a role in tumor-specific immunity, mice were immunized with a B cell lymphoma. T cell hybridomas were constructed from the immune spleen cells and analyzed for their cytotoxic ability against the immunizing lymphoma. A T4+, Lyt-1+ hybridoma cell line was developed (103L2) which specifically killed the immunizing tumor cells but not normal B cells or a range of other tumor cells of B or non-B origin. This cytotoxic hybridoma cell line differed from Lyt-2+ cytotoxic T lymphocyte cells and NK cells, commonly identified with cytotoxicity, in a number of important ways. First, the cells were class II MHC restricted; second, interleukin-2 was released from activated effector cells; and finally but most importantly, innocent nonparticipating bystander cells were also killed. The significance of this observation was that normal cells were protected, although a broad range of tumor cell types, including tumor antigen-negative mutants, were killed. It is therefore conceivable that T4+ cytotoxic T cells might play an important role in tumor immunity through the direct recognition and lysis of tumor cells while any tumor variants, arising due to antigen loss, would remain susceptible through the bystander killing effect and normal cells would remain unaffected. These results strongly suggest that tumor-reactive T4+ cytotoxic T cells belong to a new category of effector cells with an important role in tumor-specific immunity.     

Lymphocyte activation in response to melanoma: interaction of NK-associated receptors and their ligands

In recent years, studies on the molecular and cellular mechanisms of immune responses against melanoma have contributed to a better understanding of how these tumours can be recognised by cytotoxic cells and the mechanisms they have developed to escape from innate and adaptive immunity. Lysis of melanoma cells by natural killer (NK) cells and cytolytic T cells is the result of a fine balance between signals transmitted by activating and inhibitory receptors. In addition to the T cell receptor, these were initially described as NK cell-associated receptors (NKRs) and were later also found on subsets of T lymphocytes, particularly effector-memory and terminally differentiated CD8 T cells. An increase of NKR(+)CD8(+) T cells has been found in melanoma patients, correlating with the expansion of differentiated effector CD8(+)CD28(null) CD27(null) T cells. NKRs can regulate the lysis of target cells expressing appropriate ligands. Activating receptors recognise ligands on tumours whereas inhibitory receptors are specific for MHC class I antigens and sense missing self. Altered expression of MHC class I antigens is frequently found on melanoma cells, preventing recognition by specific cytolytic T cells but favouring NK cell recognition. Changes in the expression of NKR-ligands in melanoma contribute in explaining the differences in the capacity of cytotoxic immune cells to control melanoma growth and dissemination.     

MHC-I Molecules Selectively Inhibit Cell-Mediated Cytotoxicity Triggered by ITAM-Coupled Activating Receptors and 2B4

NK cell effector functions are controlled by a combination of inhibitory receptors, which modulate NK cell activation initiated by stimulatory receptors. Most of the canonical NK cell inhibitory receptors recognize allelic forms of classical and non-classical MHC class I molecules. Furthermore, high expression of MHC-I molecules on effector immune cells is also associated with reverse signaling, giving rise to several immune-regulatory functions. Consequently, the inhibitory function of MHC class I expressed on a human NKL cell line and activated primary NK and T cells on different activating receptors are analyzed in this paper. Our results reveal that MHC-I molecules display specific patterns of “selective” inhibition over cytotoxicity and cytokine production induced by ITAM-dependent receptors and 2B4, but not on NKG2D. This contrasts with the best known “canonical” inhibitory receptors, which constitutively inhibit both functions, regardless of the activating receptor involved. Our results support the existence of a new fine-tuner inhibitory function for MHC-I molecules expressed on cytotoxic effector cells that could be involved in establishing self-tolerance in mature activated NK cells, and could also be important in tumor and infected cell recognition.     

Hybrid resistance in vitro. Possible role of both class I MHC and self peptides in determining the level of target cell sensitivity.

NK cells of F1 hybrids frequently exhibit an enhanced capacity to reject semisyngeneic parental bone marrow grafts, a phenomenon known as hybrid resistance. Attempts to define the mechanism whereby this occurs have been hampered by factors inherent in the use of an in vivo model, including the host immune response, microenvironment, immune cell trafficking patterns, and host irradiation. We show here that IL-2-activated NK cells (lymphokine-activated killer cells) can be used to establish an in vitro model that appears to mimic hybrid resistance. These effectors lyse parental lymphoblast targets in a pattern consistent with that observed in vivo, bear NK not T cell surface markers, and recognize target structures mapping to the MHC. By using this model, we then demonstrate that sensitivity to lysis of syngeneic F1 lymphoblasts can be augmented by exposing the targets to conditions that have been reported to permit the exchange of endogenous class I-bound self peptides for exogenous foreign sequences. These data suggest that both MHC class I molecules and the "self" peptides they bind contribute to determining the susceptibility of a target to hybrid resistance, and the data are discussed in the context of a model of NK recognition in which the sensitivity of a target to NK lysis is mediated by a complex of the class I MHC molecule and the self peptides it samples from the intracellular peptide pool.     

The evolution of natural killer cell receptors

Natural killer (NK) cells are immune cells that play a crucial role against viral infections and tumors. To be tolerant against healthy tissue and simultaneously attack infected cells, the activity of NK cells is tightly regulated by a sophisticated array of germline-encoded activating and inhibiting receptors. The best characterized mechanism of NK cell activation is “missing self” detection, i.e., the recognition of virally infected or transformed cells that reduce their MHC expression to evade cytotoxic T cells. To monitor the expression of MHC-I on target cells, NK cells have monomorphic inhibitory receptors which interact with conserved MHC molecules. However, there are other NK cell receptors (NKRs) encoded by gene families showing a remarkable genetic diversity. Thus, NKR haplotypes contain several genes encoding for receptors with activating and inhibiting signaling, and that vary in gene content and allelic polymorphism. But if missing-self detection can be achieved by a monomorphic NKR system why have these polygenic and polymorphic receptors evolved? Here, we review the expansion of NKR receptor families in different mammal species, and we discuss several hypotheses that possibly underlie the diversification of the NK cell receptor complex, including the evolution of viral decoys, peptide sensitivity, and selective MHC-downregulation.     

MHC class I antigens,immune surveillance,and tumor immune escape

Oncogenic transformation in human and experimental animals is not necessarily followed by the appearance of a tumor mass. The immune system of the host can recognize tumor antigens by the presentation of small antigenic peptides to the receptor of cytotoxic T-lymphocytes (CTLs) and reject the nascent tumor. However, cancer cells can sometimes escape these specific T-cell immune responses in the course of somatic (genetic and phenotypic) clonal evolution. Among the tumor immune escape mechanisms described to date, the alterations in the expression of major histocompatibility complex (MHC) molecules play a crucial step in tumor development due to the role of MHC antigens in antigen presentation to T-lymphocytes and the regulation of natural killer cell (NK) cell function. In this work, we have (1) updated information on the mechanisms that allow CTLs to recognize tumor antigens after antigen processing by transformed cells, (2) described the altered MHC class I phenotypes that are commonly found in human tumors, (3) summarized the molecular mechanisms responsible for MHC class I alteration in human tumors, (4) provided evidence that these altered human leukocyte antigens (HLA) class I phenotypes are detectable as result of a T-cell immunoselection of HLA class I-deficient variants by an immunecompetent host, and (5) presented data indicating the MHC class I phenotype and the immunogenicity of experimental metastatic tumors change drastically when tumors develop in immunodeficient mice.     

Transgenic expression of Ly49A on T cells impairs a specific antitumor response

Inhibitory MHC receptors determine the reactivity and specificity of NK cells. These receptors can also regulate T cells by modulating TCR-induced effector functions such as cytotoxicity, cytokine production, and proliferation. Here we have assessed the capacity of mouse T cells expressing the inhibitory MHC class I receptor Ly49A to respond to a well-defined tumor Ag in vivo using Ly49A transgenic mice. We find that the presence of Ly49A on the vast majority of lymphocytes prevents the development of a significant Ag-specific CD8+ T cell response and, consequently, the rejection of the tumor. Despite minor alterations in the TCR repertoire of CD8+ T cells in the transgenic lines, precursors of functional tumor-specific CD8+ T cells exist but could not be activated most likely due to a lack of appropriate CD4+ T cell help. Surprisingly, all of these effects are observed in the absence of a known ligand for the Ly49A receptor as defined by its ability to regulate NK cell function. Indeed, we found that the above effects on T cells may be based on a weak interaction of Ly49A with Kb or Db class I molecules. Thus, our data demonstrate that enforced expression of a Ly49A receptor on conventional T cells prevents a specific immune response in vivo and suggest that the functions of T and NK cells are differentially sensitive to the presence of inhibitory MHC class I receptors.     

以T细胞受体为基础的免疫疗法研究进展

T细胞是机体抗肿瘤免疫的核心,以T细胞功能调控为基础的免疫检查点疗法已经在多种肿瘤的临床治疗中取得了重大突破,以基因工程化T细胞为基础的过继性免疫细胞疗法在血液瘤治疗中取得了重要进展,免疫治疗已经对肿瘤的临床治疗产生了深刻变革,成为肿瘤临床治疗策略的重要组成部分。T细胞受体（T cell receptor,TCR）赋予了T细胞识别肿瘤抗原的特异性,能够识别由主要组织相容性复合体（major histocompatibility complex,MHC）呈递的包括胞内抗原在内的广泛肿瘤抗原,具有高度的抗原敏感性,因而具有广泛的抗肿瘤应用前景。2022年第一款TCR药物的上市开启了TCR药物开发的新纪元,多项TCR药物临床研究表现出潜在的肿瘤治疗价值。本文综述了以TCR为基础的免疫治疗策略研究进展,包括T细胞受体工程化T细胞（T cell receptor-engineered T cell,TCR-T）和TCR蛋白药物,以及基于TCR信号的其他免疫细胞疗法,以期为以TCR为基础的免疫治疗策略开发提供参考。     

Manganese is critical for antitumor immune responses via cGAS-STING and improves the efficacy of clinical immunotherapy

CD8⁺ T cell-mediated cancer clearance is often suppressed by the interaction between inhibitory molecules like PD-1 and PD-L1, an interaction acts like brakes to prevent T cell overreaction under normal conditions but is exploited by tumor cells to escape the immune surveillance. Immune checkpoint inhibitors have revolutionized cancer therapeutics by removing such brakes. Unfortunately, only a minority of cancer patients respond to immunotherapies presumably due to inadequate immunity. Antitumor immunity depends on the activation of the cGAS-STING pathway, as STING-deficient mice fail to stimulate tumor-infiltrating dendritic cells (DCs) to activate CD8⁺ T cells. STING agonists also enhance natural killer (NK) cells to mediate the clearance of CD8⁺ T cell-resistant tumors. Therefore STING agonists have been intensively sought after. We previously discovered that manganese (Mn) is indispensable for the host defense against cytosolic dsDNA by activating cGAS-STING. Here we report that Mn is also essential in innate immune sensing of tumors and enhances adaptive immune responses against tumors. Mn-insufficient mice had significantly enhanced tumor growth and metastasis, with greatly reduced tumor-infiltrating CD8⁺ T cells. Mechanically, Mn²⁺ promoted DC and macrophage maturation and tumor-specific antigen presentation, augmented CD8⁺ T cell differentiation, activation and NK cell activation, and increased memory CD8⁺ T cells. Combining Mn²⁺ with immune checkpoint inhibition synergistically boosted antitumor efficacies and reduced the anti-PD-1 antibody dosage required in mice. Importantly, a completed phase 1 clinical trial with the combined regimen of Mn²⁺ and anti-PD-1 antibody showed promising efficacy, exhibiting type I IFN induction, manageable safety and revived responses to immunotherapy in most patients with advanced metastatic solid tumors. We propose that this combination strategy warrants further clinical translation.Subject terms: Pattern recognition receptors, Immunosurveillance     

A human NK cell activation/inhibition threshold allows small changes in the target cell surface phenotype to dramatically alter susceptibility to NK cells

NK cell activation is negatively regulated by the expression of target cell MHC class I molecules. We show that this relationship is nonlinear due to an NK cell activation/inhibition threshold. Ewing's sarcoma family tumor cell monolayers, which were highly susceptible to NK cells in vitro, developed a highly resistant phenotype when cultured as three-dimensional multicellular tumor spheroid structures. This suggested that tumor architecture is likely to influence the susceptibility to NK cells in vivo. Resistance of the multicellular tumor spheroid was associated with the increased expression of MHC class I molecules and greatly reduced NK cell activation, implying that a threshold of NK cell activation/inhibition had been crossed. Reducing MHC class I expression on Ewing's sarcoma family tumor monolayers did not alter their susceptibility to NK cells, whereas increased expression of MHC class I rendered them resistant and allowed the threshold point to be identified. This threshold, as defined by MHC class I expression, was predictive of the number of NK-resistant target cells within a population. A threshold permits modest changes in the target cell surface phenotype to profoundly alter the susceptibility to NK cells. Whereas this allows for the efficient detection of target cells, it also provides a route for pathogens and tumors to evade NK cell attack.     

V gamma 9V delta 2 T cell response to colon carcinoma cells

During analysis of CD8 T cells derived from ascites of a colon cancer patient, we isolated a Vgamma9Vdelta2 T cell clone showing strong reactivity against autologous tumor cell lines. This clone killed a large fraction of allogeneic colon carcinoma and melanoma cell lines, but did not affect a normal colon cell line, colon fibroblasts, or melanocytes. Tumor cell recognition was TCR and NKG2D dependent and induced TNF-alpha and IFN-gamma secretion by the clone; accordingly, tumor targets expressed several NKG2D ligands, such as MHC class I chain-related gene A and UL16-binding protein molecules. Colon tumor recognition by Vgamma9Vdelta2 T cells was highly dependent on isopentenyl pyrophosphate production and ICAM-1 expression by target cells. Finally, similar reactivity patterns against colon carcinoma cell lines were observed using polyclonal Vgamma9Vdelta2 T cells of various origins, and Vgamma9Vdelta2 lymphocytes were present in the majority of colon tumor samples studied. Together, these results suggest that Vgamma9Vdelta2 T cells contribute to the natural immune surveillance against colon cancers. Therefore, this study provides a strong rationale for the use of Vgamma9Vdelta2 T cell agonists in immunotherapies targeting colon tumors.     

CD4+CXCR4highCD69+ T cells accumulate in lung adenocarcinoma

The chemokine receptor CXCR4 is involved in the growth and metastasis of tumor cells. However, the expression of its ligand, the chemokine CXCL12, in tumors and its role in regulating the accumulation of immune cells within the tumors is not clear. Using ELISA and immunohistochemistry we found that CXCL12 is expressed in the majority of nonsmall cell lung cancer tissue sections obtained from stage IA to IIB nonsmall cell lung cancer patients undergoing operation. Histopathologic examination of these sections indicated that high CXCL12 expression correlated with increased tumor inflammation. In addition, disease recurrence rates in a subgroup of adenocarcinoma patients showed a tendency to correlate with high CXCL12 expression in the tumor. Isolation of adenocarcinoma-infiltrating immune cells demonstrated an increase in the percentage of CD4+CD69+CXCR4+ T cells as compared with normal lung tissue. About 30% of these cells expressed the regulatory T cell markers CD25high and FoxP3. The percentage of CD8 T cells within the tumor did not change, however; the percentage of NK and NK T cells was significantly reduced. In correlation with CXCR4 expression, CD4 T cells showed increased migration in response to CXCL12 compared with CD8 T cells and NK cells. Overall, these observations suggest that CXCL12 expression may influence tumor progression by shaping the immune cell population infiltrating lung adenocarcinoma tumors.     

Role of tumor cell apoptosis in tumor antigen migration to the draining lymph nodes

Establishment of an immune response against cancer may depend on the capacity of dendritic cells to transfer tumor Ags into T cell-rich areas. To check this possibility, we used a colon cancer cell variant that yields tumors undergoing complete T cell-dependent rejection when injected into syngeneic rats. We previously demonstrated that immunogenicity of these tumors depended on the early apoptosis of a part of these tumor cells. In this paper we show that fluorescent tumor cell proteins are released from FITC-labeled tumor cells and undergo engulfment by tumor-infiltrating monocytes without a phenotype of mature dendritic cells or macrophages. Fluorescence-labeled mononuclear cells with a phenotype of MHC class II+ dendritic cells are also found in the T cell areas of the draining lymph nodes. Interestingly, no fluorescent cell can be found in lymph nodes after a s.c. injection of Bcl2-transfected apoptosis-resistant tumor cells that yielded progressive tumors. Proliferation of tumor-immune T lymphocytes was induced by dendritic cells isolated from the draining lymph nodes recovered after a s.c. injection of apoptosis-sensitive, but not apoptosis-resistant, tumor cells. These results show that tumor cell apoptosis releases proteins that are engulfed by inflammatory cells in the tumor, then transported to lymph node T cell areas where they can induce a specific immune response leading to tumor rejection.     

Induction of tumor-reactive T helper responses by a posttranslational modified epitope from tumor protein p53

Posttranslational modifications regulate the function and stability of proteins, and the immune system is able to recognize some of these modifications. Therefore, the presence of posttranslational modifications increases the diversity of potential immune responses to a determinant antigen. The stimulation of tumor-specific CD4⁺ helper T lymphocytes (HTLs) is considered important for the production of anti-tumor antibodies by B cells and for the generation and persistence of CD8⁺ cytotoxic T lymphocytes, and in some instances, HTLs can directly reduce tumor cell growth. Identification of MHC class II-restricted peptide epitopes from tumor-associated antigens including those generated from posttranslational protein modifications should enable the improvement of peptide-based cancer immunotherapy. We describe here an MHC class II binding peptide from the tumor protein p53, which possesses an acetylated lysine at position 120 (p53_{110-124/AcK120}) that is effective in eliciting CD4⁺ T cell responses specific for the acetylated peptide. Most importantly, the acetylated peptide-reactive CD4 HTLs recognized the corresponding naturally processed posttranslational modified epitope presented by either dendritic cells loaded with tumor cell lysates or directly on tumors expressing p53 and the restricting MHC class II molecules. Treatment of tumor cells with a histone deacetylase inhibitor augmented their recognition by the p53_{110-124/AcK120}-reactive CD4⁺ T cells. These findings prove that the epitope p53_{110-124/AcK120} is immunogenic for anti-tumor responses and is likely to be useful for cancer immunotherapy.