Assumptions of the tumor 'escape' hypothesis

A Holistic Perspective: Exosomes Shuttle between Nerves and Immune Cells in the Tumor Microenvironment

One of the limitations of cancer research has been the restricted focus on tumor cells and the omission of other non-malignant cells that are constitutive elements of this systemic disease. Current research is focused on the bidirectional communication between tumor cells and other components of the tumor microenvironment (TME), such as immune and endothelial cells, and nerves. A major success of this bidirectional approach has been the development of immunotherapy. Recently, a more complex landscape involving a multi-lateral communication between the non-malignant components of the TME started to emerge. A prime example is the interplay between immune and endothelial cells, which led to the approval of anti-vascular endothelial growth factor-therapy combined with immune checkpoint inhibitors and classical chemotherapy in non-small cell lung cancer. Hence, a paradigm shift approach is to characterize the crosstalk between different non-malignant components of the TME and understand their role in tumorigenesis. In this perspective, we discuss the interplay between nerves and immune cells within the TME. In particular, we focus on exosomes and microRNAs as a systemic, rapid and dynamic communication channel between tumor cells, nerves and immune cells contributing to cancer progression. Finally, we discuss how combinatorial therapies blocking this tumorigenic cross-talk could lead to improved outcomes for cancer patients.

Implications of Postoperative Complications for Survival After Cytoreductive Surgery and HIPEC: A Multi-Institutional Analysis of the US HIPEC Collaborative

BACKGROUND

Postoperative complications (POCs) are associated with worse oncologic outcomes in various cancer histologies. The impact of POCs on the survival of patients with appendiceal or colorectal cancer after cytoreductive surgery (CRS) and heated intraperitoneal chemotherapy (HIPEC) is unknown.

METHODS

The US HIPEC Collaborative (2000-2017) was reviewed for patients who underwent CCR0/1 CRS/HIPEC for appendiceal/colorectal cancer. The analysis was stratified by noninvasive appendiceal neoplasm versus invasive appendiceal/colorectal adenocarcinoma. The POCs were grouped into infectious, cardiopulmonary, thromboembolic, and intestinal dysmotility. The primary outcomes were overall survival (OS) and recurrence-free survival (RFS).

RESULTS

Of the 1304 patients, 33% had noninvasive appendiceal neoplasm (n = 426), and 67% had invasive appendiceal/colorectal adenocarcinoma (n = 878). In the noninvasive appendiceal cohort, POCs were identified in 55% of the patients (n = 233). The 3-year OS and RFS did not differ between the patients who experienced a complication and those who did not (OS, 94% vs 94%, p = 0.26; RFS, 68% vs 60%, p = 0.15). In the invasive appendiceal/colorectal adenocarcinoma cohort, however, POCs (63%; n = 555) were associated with decreased 3-year OS (59% vs 74%; p < 0.001) and RFS (32% vs 42%; p < 0.001). Infectious POCs were the most common (35%; n = 196). In Multivariable analysis accounting for gender, peritoneal cancer index (PCI), and incomplete resection (CCR1), infectious POCs in particular were associated with decreased OS compared with no complication (hazard ratio [HR] 2.08; p < 0.01) or other types of complications (HR, 1.6; p < 0.01). Similarly, infectious POCs were independently associated with worse RFS (HR 1.61; p < 0.01).

CONCLUSION

Postoperative complications are associated with decreased OS and RFS after CRS/HIPEC for invasive histology, but not for an indolent disease such as noninvasive appendiceal neoplasm, and this association is largely driven by infectious complications. The exact mechanism is unknown, but may be immunologic. Efforts must target best practices and standardized prevention strategies to minimize infectious postoperative complications.

Combined treatment of murine fibrosarcoma with chemotherapy (Paclitaxel), radiotherapy, and intratumoral injection of dendritic cells

Background

New antitumor therapeutic strategies aim to combine different approaches that are able to induce tumor-specific effector and memory T cell responses that might control tumor growth. Dendritic cells (DCs) have the capacity to induce antigen-specific cytotoxic T lymphocytes. We have previously shown that the combined treatment of paclitaxel chemotherapy (Chemo) and injection of DCs led to complete tumor regression.

Objective

The goal of this study was to evaluate synergistic antitumor effect of a triple combination treatment comprising radiotherapy, paclitaxel Chemo and intratumoral injection of syngeneic bone marrow-derived DCs on murine fibrosarcoma, compared to other single or double combination treatments.

Methods

For the murine fibrosarcoma model, naïve C57BL/6 mice were inoculated intradermally with 2×10³ MCA102 cells in the right upper flank. Mice were assigned to five groups (untreatedcontrol, RT alone, RT+Chemo, RT+DC, and RT+Chemo+DC), with eight mice in each group. In vitro cytotoxicity assays were performed to assess the immune activity. The persistence of tumor-specific immunity was determined by second tumor challenge in mice with complete tumor regression.

Results

The triple combination treatment showed a significantly enhanced therapeutic efficacy by decreasing tumor size and inducing complete tumor regression, resulting in a cure of 50% of mice. The results of in vitro cytotoxicity assays and the second tumor challenge experiment strongly indicated the induction of a tumor-specific cytotoxic T lymphocyte response and acquisition of prolonged tumor immunity.

Conclusion

These findings suggest that the triple combination treatment can be a promising strategy for the treatment of murine fibrosarcoma.

Serial assessment of lymphocytes and apoptosis in the prostate during coordinated intraprostatic dendritic cell injection and radiotherapy

Local radiotherapy plus intratumoral syngeneic dendritic cell injection can mediate apoptosis/cell death and immunological tumor eradication in murine models. A novel method of coordinated intraprostatic, autologous dendritic cell injection together with radiation therapy was prospectively evaluated in five HLA-A2(+) subjects with high-risk, localized prostate cancer, using androgen suppression, 45 Gy external beam radiation therapy in 25 fractions over 5 weeks, dendritic cell injections after fractions 5, 15 and 25 and then interstitial radioactive seed placement. Serial prostate biopsies before and during treatment showed increased apoptotic cells and parenchymal distribution of CD8(+) cells. CD8(+) T-cell responses to test peptides were assessed using an enzyme-linked immunosorbent spot IFN-γ production assay, demonstrating some prostate cancer-specific protein-derived peptides associated with increased titer. In conclusion, the technique was feasible and well-tolerated and specific immune responses were observable. Future trials could further test the utility of this approach and improve on temporal coordination of intratumoral dendritic cell introduction with particular timelines of therapy-induced apoptosis.

The confluence of stereotactic ablative radiotherapy and tumor immunology

Stereotactic radiation approaches are gaining more popularity for the treatment of intracranial as well as extracranial tumors in organs such as the liver and lung. Technology, rather than biology, is driving the rapid adoption of stereotactic body radiation therapy (SBRT), also known as stereotactic ablative radiotherapy (SABR), in the clinic due to advances in precise positioning and targeting. Dramatic improvements in tumor control have been demonstrated; however, our knowledge of normal tissue biology response mechanisms to large fraction sizes is lacking. Herein, we will discuss how SABR can induce cellular expression of MHC I, adhesion molecules, costimulatory molecules, heat shock proteins, inflammatory mediators, immunomodulatory cytokines, and death receptors to enhance antitumor immune responses.

Changes in dendritic cell phenotype after a new high-dose weekly schedule of interleukin-2 therapy for kidney cancer and melanoma

High-dose intravenous interleukin-2 (IL-2) therapy (14 doses/course, 2 courses/cycle) for metastatic melanoma or kidney cancer induces infrequent, although major responses. In this trial, we evaluated a new schedule (dose of 600,000 IU/kg, 8 h between doses, 5 doses/course, 4 courses at weekly intervals/cycle) of high-dose IL-2, in which we inserted more planned breaks while maintaining high cumulative dose delivery, and investigated the relationship between dendritic cells (DC) and response to treatment. Target dose delivery was attained: median IL-2 cumulative dose per patient was 11.4 and 10.8 million units/kg (cycles 1 and 2, respectively). Major responses were observed in patients with kidney cancer (n=20; 3 complete and 2 partial responses) and melanoma (n=16; 1 partial response). Adverse events appeared comparable with those typically associated with high-dose IL-2. From this data set, we introduce the hypothesis-generating observation that patients who had more favorable outcomes had high pretreatment DC-to-myeloid-derived suppressor cell (MDSC) ratios, similar to the ratio observed in healthy individuals. However, even in patients with the most favorable outcome, after treatment, there were IL-2-induced changes in the DC-to-MDSC ratio, specifically increases in MDSCs. This modified IL-2 schedule is a feasible option, with a more uniform dose delivery over the treatment cycle, a similar toxicity profile, and observed complete, durable response in patients with renal cancer. Pretreatment assessment of DC phenotypic or maturational status may be a starting point to predicting response to high-dose IL-2 cytokine immunotherapy in patients with melanoma and kidney cancer.

Combination of external beam radiotherapy (EBRT) with intratumoral injection of dendritic cells as neo-adjuvant treatment of high-risk soft tissue sarcoma patients

PURPOSE

The goal of this study was to determine the effect of combination of intratumoral administration of dendritic cells (DC) and fractionated external beam radiation (EBRT) on tumor-specific immune responses in patients with soft-tissue sarcoma (STS).

METHODS AND MATERIAL

Seventeen patients with large (>5 cm) high-grade STS were enrolled in the study. They were treated in the neoadjuvant setting with 5,040 cGy of EBRT, split into 28 fractions and delivered 5 days per week, combined with intratumoral injection of 10(7) DCs followed by complete resection. DCs were injected on the second, third, and fourth Friday of the treatment cycle. Clinical evaluation and immunological assessments were performed.

RESULTS

The treatment was well tolerated. No patient had tumor-specific immune responses before combined EBRT/DC therapy; 9 patients (52.9%) developed tumor-specific immune responses, which lasted from 11 to 42 weeks. Twelve of 17 patients (70.6%) were progression free after 1 year. Treatment caused a dramatic accumulation of T cells in the tumor. The presence of CD4(+) T cells in the tumor positively correlated with tumor-specific immune responses that developed following combined therapy. Accumulation of myeloid-derived suppressor cells but not regulatory T cells negatively correlated with the development of tumor-specific immune responses. Experiments with (111)In labeled DCs demonstrated that these antigen presenting cells need at least 48 h to start migrating from tumor site.

CONCLUSIONS

Combination of intratumoral DC administration with EBRT was safe and resulted in induction of antitumor immune responses. This suggests that this therapy is promising and needs further testing in clinical trials design to assess clinical efficacy.

Combining carbon ion radiotherapy and local injection of α-galactosylceramide-pulsed dendritic cells inhibits lung metastases in an in vivo murine model

PURPOSE

Our previous report indicated that carbon ion beam irradiation upregulated membrane-associated immunogenic molecules, underlining the potential clinical application of radioimmunotherapy. The antimetastatic efficacy of local combination therapy of carbon ion radiotherapy and immunotherapy was examined by use of an in vivo murine model.

METHODS AND MATERIALS

Tumors of mouse squamous cell carcinoma (NR-S1) cells inoculated in the legs of C3H/HeSlc mice were locally irradiated with a single 6-Gy dose of carbon ions (290 MeV/nucleon, 6-cm spread-out Bragg peak). Thirty-six hours after irradiation, α-galactosylceramide-pulsed dendritic cells (DCs) were injected into the leg tumor. We investigated the effects on distant lung metastases by counting the numbers of lung tumor colonies, making pathologic observations, and assessing immunohistochemistry.

RESULTS

The mice with no treatment (control) presented with 168 ± 53.8 metastatic nodules in the lungs, whereas the mice that received the combination therapy of carbon ion irradiation and DCs presented with 2.6 ± 1.9 (P = 0.009) at 2 weeks after irradiation. Immunohistochemistry showed that intracellular adhesion molecule 1, which activates DCs, increased from 6 h to 36 h after irradiation in the local tumors of the carbon ion-irradiated group. The expression of S100A8 in lung tissue, a marker of the lung pre-metastatic phase, was decreased only in the group with a combination of carbon ions and DCs.

CONCLUSIONS

The combination of carbon ion radiotherapy with the injection of α-galactosylceramide-pulsed DCs into the primary tumor effectively inhibited distant lung metastases.

Photodynamic therapy of tumors can lead to development of systemic antigen-specific immune response

Background

The mechanism by which the immune system can effectively recognize and destroy tumors is dependent on recognition of tumor antigens. The molecular identity of a number of these antigens has recently been identified and several immunotherapies have explored them as targets. Photodynamic therapy (PDT) is an anti-cancer modality that uses a non-toxic photosensitizer and visible light to produce cytotoxic reactive oxygen species that destroy tumors. PDT has been shown to lead to local destruction of tumors as well as to induction of anti-tumor immune response.

Methodology/Principal Findings

We used a pair of equally lethal BALB/c colon adenocarcinomas, CT26 wild-type (CT26WT) and CT26.CL25 that expressed a tumor antigen, β-galactosidase (β-gal), and we treated them with vascular PDT. All mice bearing antigen-positive, but not antigen-negative tumors were cured and resistant to rechallenge. T lymphocytes isolated from cured mice were able to specifically lyse antigen positive cells and recognize the epitope derived from beta-galactosidase antigen. PDT was capable of destroying distant, untreated, established, antigen-expressing tumors in 70% of the mice. The remaining 30% escaped destruction due to loss of expression of tumor antigen. The PDT anti-tumor effects were completely abrogated in the absence of the adaptive immune response.

Conclusion

Understanding the role of antigen-expression in PDT immune response may allow application of PDT in metastatic as well as localized disease. To the best of our knowledge, this is the first time that PDT has been shown to lead to systemic, antigen- specific anti-tumor immunity.

Nonsurgical options for hepatocellular carcinoma: evolving role of external beam radiotherapy

BACKGROUND

Hepatocellular carcinoma (HCC) is the sixth most common cancer worldwide and thus poses a global concern. Its incidence is expected to increase in North America secondary to the increasing incidence of patients who develop hepatitis C. Patients who ultimately develop cirrhosis have an increased risk of developing hepatocellular carcinoma.

METHODS

The authors focus on nonsurgical therapies for this disease with an exploration of how external beam radiotherapy can be used alone or with other modalities. The development of partial liver strategies secondary to an explosion in radiation treatment planning and delivery advances is reviewed. Integration of advanced technology has evolved from three-dimensional conformal treatment to intensity-modulated radiation therapy and image-guided radiation therapy, along with stereotactic body radiation therapy, tomotherapy, and proton beam therapy.

RESULTS

Current data show a promising future for treatment strategies incorporating radiation with high rates of infield tumor control and low rates of radiation-induced liver disease. Radiation can be delivered in conjunction with transarterial catheter embolization for advanced-stage patients. External beam radiotherapy also has a role in the setting of patients with macrovascular tumor thrombus.

CONCLUSIONS

Future directions include how to best synergize the effects of radiation strategies with novel agents, given the hypervascularity of HCC. Downstaging of larger lesions with these therapies to resectable or transplantable disease may lead to better outcomes for patients deemed inoperable at diagnosis, and definitive radiotherapy may offer cure to patients with smaller lesions.

Systemic therapy for advanced hepatocellular carcinoma: past, present, and future

BACKGROUND

Although approximately 80% of hepatocellular carcinoma (HCC) cases occur in developing countries, the incidence of HCC in Western countries is on the rise due to the impact of hepatitis C. Challenges in developing effective therapies include the inherent chemoresistance of HCC, the pharmacologic challenges presented by a diseased liver, the presentation of most patients at advanced stages, and the difficulty in adequately measuring radiological response. While responses to traditional chemotherapeutic agents have been documented, significant survival benefit is debatable.

METHODS

The authors review the results of published clinical trials of systemic therapy and immunotherapy that have impacted the present treatment of HCC.

RESULTS

With recent progress in the elucidation of HCC molecular pathways, targeted agents show promise. The multikinase inhibitor sorafenib has provided survival benefit in patients with advanced HCC and well-preserved liver function. Sunitinib, bevacizumab, epidermal growth factor receptor inhibitors, and mammalian target of rapamycin (mTOR) inhibitors have shown activity in small patient cohorts. Immunotherapy appears to be a promising approach that can result in the regression of bulky, invasive cancer in some patients.

CONCLUSIONS

New agents with a variety of mechanisms of activity offer promising therapeutic options for patients with advanced HCC.

The role of chemokines in cancer immune surveillance by the adaptive immune system

Chemokines are key molecules involved in the migration and homeostasis of immune cells. However, also tumor cells use chemokine signals for different processes such as tumor progression and metastasis. It is thus unclear whether chemokines, through their immunostimulatory roles, contribute to the repression of tumor cells by tumor immunosurveillance or whether chemokines act primarily as growth factors and chemoattractants for primary and metastatizing tumors, respectively. Research of recent years, using gene knockout mice, recombinant chemokines, and agents able to block chemokine actions, has provided further insight into the diverse functions of chemokines. Here, we review the current knowledge on the complex actions of chemokines at the interface of the immune system and the tumor.

Immunological validation of the EpitOptimizer program for streamlined design of heteroclitic epitopes

One strategy to generate T-cell responses to tumors is to alter subdominant epitopes through substitution of amino acids that are optimal anchors for specific MHC molecules, termed heteroclitic epitopes. This approach is manually error-prone and time-consuming. In here, we describe a computer-based algorithm (EpitOptimizer) for the streamlined design of heteroclitic epitopes. Analysis of two cancer-related proteins showed that EpitOptimizer-generated peptides have enhanced MHC-I binding compared with their wild-type counterparts; and were able to induce stronger CD8+ T-cell responses against their native epitope. These data demonstrate that this approach can serve as the basis of epitope-engineering against cancer and intracellular pathogens.

Intratumoral cytokines/chemokines/growth factors and tumor infiltrating dendritic cells: friends or enemies?

The tumor microenvironment consists of a variable combination of tumor cells, stromal fibroblasts, endothelial cells and infiltrating leukocytes, such as macrophages, T lymphocytes, and dendritic cells. A variety of cytokines, chemokines and growth factors are produced in the local tumor environment by different cells accounting for a complex cell interaction and regulation of differentiation, activation, function and survival of multiple cell types. The interaction between cytokines, chemokines, growth factors and their receptors forms a comprehensive network at the tumor site, which is primary responsible for overall tumor progression and spreading or induction of antitumor immune responses and tumor rejection. Although the general thought is that dendritic cells are among the first cells migrating to the tumor site and recognizing tumor cells for the induction of specific antitumor immunity, the clinical relevance of dendritic cells at the site of the tumor remains a matter of debate regarding their role in the generation of successful antitumor immune responses in human cancers. While several lines of evidence suggest that intratumoral dendritic cells play an important role in antitumor immune responses, understanding the mechanisms of dendritic cell/tumor cell interaction and modulation of activity and function of different dendritic cell subtypes at the tumor site is incomplete. This review is limited to discussing the role of intratumoral cytokine network in the understanding immunobiology of tumor-associated dendritic cells, which seems to possess different regulatory functions at the tumor site.

Tissue- and tumor-specific targeting of murine leukemia virus-based replication-competent retroviral vectors

Replication-competent retrovirus vectors based on murine leukemia virus (MLV) have been shown to effectively transfer therapeutic genes over multiple serial infections in cell culture and through solid tumors in vivo with a high degree of genomic stability. While simple retroviruses possess a natural tumor selectivity in that they can transduce only actively dividing cells, additional tumor-targeting strategies would nevertheless be advantageous, since tumor cells are not the only actively dividing cells. In this study, we used the promiscuous murine cytomegalovirus promoter, a chimeric regulatory sequence consisting of the hepatitis B virus enhancer II and the human alpha1-antitrypsin (EII-Pa1AT) promoter, and a synthetic regulatory sequence consisting of a series of T-cell factor binding sites named the CTP4 promoter to generate replicating MLV vectors, whereby the last two are transcriptionally restricted to liver- and beta-catenin/T-cell factor-deregulated cells, respectively. When the heterologous promoters were used to replace almost the entire MLV U3 region, including the MLV TATA box, vector replication was inefficient since nascent virus particle production from infected cells was greatly decreased. Fusion of the heterologous promoters lacking the TATA box to the MLV TATA box, however, generated vectors which replicated with almost-wild-type kinetics throughout permissive cells while exhibiting low or negligible spread in nonpermissive cells. The genomic stability of the vectors was shown to be comparable to that of a similar vector containing wild-type MLV long terminal repeats, and tropism analysis over repeated infection cycles showed that the targeted vectors retained their original specificity.

Cancer cells regulate lymphocyte recruitment and leukocyte-endothelium interactions in the tumor-draining lymph node

The physiologic function of the secondary lymphoid organs to recruit large numbers of naïve lymphocytes increases the probability that antigens encounter their rare, sometimes unique, specific T lymphocytes and initiate a specific immune response. In peripheral lymph nodes (LNs), this recruitment is a multistep process, initiated predominantly within the high endothelial venules (HEVs), beginning with rolling and chemokine-dependent firm adhesion of the lymphocytes on the venular endothelium surface. We report here that, in C57BL/6 mice, the recruitment of naïve lymphocytes is impaired in LNs draining a B16 melanoma tumor. Intravital microscopy analysis of the tumor-draining LNs revealed that this effect is associated with an important defect in lymphocyte adhesion in the HEVs and a progressive decrease in the expression of the LN chemokine CCL21. In parallel with these effects, the tumor up-regulated, essentially through a P-selectin-dependent mechanism, the rolling and sticking of circulating polymorphonuclear cells within the LN low-order venules where few rolling and sticking events are usually observed. These effects of the tumor were independent of the presence of metastasis into the LN and occurred as long as the tumor developed. Together, these results indicate that the tumor proximity disturbs the LN physiology by modifying the molecular, spatial, and cellular rules that usually control leukocyte-endothelium interactions into the peripheral LNs. In addition, they emphasize a new role for the low-order venules of the peripheral LNs, which compared with the HEVs, seem to be the preferential port of entry for cells linked to inflammatory processes.

The interferon-inducible 9-27 gene modulates the susceptibility to natural killer cells and the invasiveness of gastric cancer cells

As an effort to identify immune suppressive molecules in gastric cancer cells, a signal sequence trap was employed. Among the genes identified, 9-27 gene was highly expressed in gastric tumor tissues and in cancer cell lines. It was induced by IFN-gamma treatment, but not by TNF-alpha or TGF-beta1 treatment. The overexpression of 9-27 in the gastric cancer cells rendered tumor cells more resistant to natural killer cells. In addition, the 9-27-overexpressed cells showed an increased migration and an invasive capacity when compared with the control cells. Taken together, these data indicate that 9-27 plays a role in malignant progression by suppressing natural killer cells and by increasing the invasive potential of gastric cancer cells.

Immunotherapy for malignant gliomas: emphasis on strategies of active specific immunotherapy using autologous dendritic cells

REVIEW

In this review, we discuss immunotherapy for malignant gliomas.

EMPHASIS

The emphasis is on the novel strategy of active specific immunotherapy using dendritic cells as antigen-presenting cells, especially its theoretical concepts and advantages, specific requirements, critical issues, pre-clinical and early clinical experience. Dendritic cell vaccination is situated in the diversity of other immunotherapeutical approaches.

FURTHER DISCUSSION

Future directions, challenges, and drawbacks will be discussed.

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.

Immunotherapy of cancer for the elderly patient: does allogeneic bone marrow transplantation after nonmyeloablative conditioning provide a new option?

The critical role of antigen-specific T cells in cancer immunotherapy has been amply demonstrated. Though success of clinical trials still remains far behind expectations, the continuous improvement in our understanding of the biology of the immune response will provide the basis for optimized cancer vaccines. This review focuses on active therapeutic vaccination after allogeneic bone marrow cell transplantation with nonmyeloablative conditioning. This approach could provide a major breakthrough in cancer immunotherapy, particularly of elderly patients. The senescent immune system, mainly the T-cell compartment, displays reduced responsiveness, and this has to be overcome if therapeutic vaccination is to be of benefit for the patient. Although the defects are quite well characterized, the inducing factors and ways to overcome them are still to be explored in more detail. Many questions also remain to be answered in the field of allogeneic bone marrow transplantation after nonmyeloablative conditioning to optimize this therapeutic setting in cancer immunotherapy. Current considerations to improve engraftment and to reduce graft-versus-host disease while strengthening graft-versus-tumor reactivity will be briefly reviewed. Finally, I will discuss whether tumor-reactive T cells can be "naturally" maintained during the process of T-cell maturation in the allogeneic host. Provided this hypothesis can be substantiated, a T-cell vaccine will meet a pool of virgin T cells in the allogeneically reconstituted host, which are tolerant toward the host but not anergized toward tumor antigens presented by MHC molecules of the host. Inevitably, the problem of the aged immune system would be circumvented.

Artificial T-cell receptors

Artificial T-cell receptors are generated by joining an Ag-recognizing domain (ectodomain) to the transmembrane and intracellular portion of a signaling molecule (endodomain). The ectodomain is most often derived from Ab variable chains, but may also be generated from T-cell receptor variable chains, as well as from other molecules. Various alternative ectodomain designs exist, with some comparative studies suggesting optimal forms. The endodomain most often used is the intracellular portion of CD-zeta. Although signaling by CD-zeta leads to IFN-n release and cell killing, it fails to transmit a full activation signal. Recently, unions of different signaling molecule segments have facilitated transmission of more potent signals, stimulating T-cell proliferation and overcoming this major limitation. Artificial T-cell receptors allow grafting of nearly any specificity to T cells. This allows generation of large numbers of specific T cells, without laborious selection and expansion procedures. Efficacy against tumors has been demonstrated in animal models. Phase I and II studies of T-cells transduced with artificial T-cell receptors as therapy for HIV infection have been performed. This rapidly advancing technology will make new strategies of adoptive immunotherapy possible.

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.