Cross-presentation of tumour antigens: evaluation of threshold, duration, distribution and regulation

Celebrating 100 years of Immunology & Cell Biology - a special focus on the field of tumor immunology in Australia

In this Commentary article, as part of the 100-year celebrations of the journal, we reflect on the contribution of articles published in ICB in the field of tumor immunology. A highlight is a series of interviews conducted with three Australian-based ICB authors who have contributed key papers over the years: Rajiv Khanna, Delia Nelson and Ian Frazer.

How Our Continuing Studies of the Pre-clinical Inbred Mouse Models of Mesothelioma Have Influenced the Development of New Therapies

Asbestos-induced preclinical mouse models of mesothelioma produce tumors that are very similar to those that develop in humans and thus represent an ideal platform to study this rare, universally fatal tumor type. Our team and a number of other research groups have established such models as a stepping stone to new treatments, including chemotherapy, immunotherapy and other approaches that have been/are being translated into clinical trials. In some cases this work has led to changes in mesothelioma treatment practice and over the last 30 years these models and studies have led to trials which have improved the response rate in mesothelioma from less than 10% to over 50%. Mouse models have had a vital role in that improvement and will continue to play a key role in the future success of mesothelioma immunotherapy. In this review we focus only on these original inbred mouse models, the large number of preclinical studies conducted using them and their contribution to current and future clinical therapy for mesothelioma.

Tumor cells, rather than dendritic cells, deliver antigen to the lymph node for cross-presentation

It is widely accepted that generation of tumor specific CD8+ T-cell responses occur via cross-priming; however the source of tumor antigen for this event is unknown. We examined the source and form of tumor antigen required for cross-presentation in the local lymph node (LN) using a syngeneic mouse tumor model expressing a marker antigen. We found that cross-presentation of this model tumor antigen in the LN is dependent on continuous traffic of antigen from the tumor site, but without any detectable migration of tumor resident dendritic cells (DCs). Instead, small numbers of tumor cells metastasize to local LNs where they are exposed to a localized CTL attack, resulting in delivery of tumor antigen into the cross-presentation pathway.

Turning the tumor microenvironment into a self vaccine site

We aimed to determine if the tumor microenvironment could be turned into a “self”-vaccine site. We show that provoking a local inflammatory response modulates endothelia to permit the infiltration of innate and adaptive effector cells which collaborate to eradicate the inflamed tumor and other tumor deposits, and provide long-term protection.

Chemotherapy enhances cross-presentation of nuclear tumor antigens

Cross-presentation of tumor antigen is essential for efficient priming of naïve CD8⁺ T lymphocytes and induction of effective anti-tumor immunity. We hypothesized that the subcellular location of a tumor antigen could affect the efficiency of cross-presentation, and hence the outcome of anti-tumor responses to that antigen. We compared cross-presentation of a nominal antigen expressed in the nuclear, secretory, or cytoplasmic compartments of B16 melanoma tumors. All tumors expressed similar levels of the antigen. The antigen was cross-presented from all compartments but when the concentration was low, nuclear antigen was less efficiently cross-presented than antigen from other cellular locations. The efficiency of cross-presentation of the nuclear antigen was improved following chemotherapy-induced tumor cell apoptosis and this correlated with an increase in the proportion of effector CTL. These data demonstrate that chemotherapy improves nuclear tumor antigen cross-presentation and could be important for anti-cancer immunotherapies that target nuclear antigens.

Harnessing the immune response to treat cancer

It is well established that the immune system has the capacity to attack malignant cells. During malignant transformation cells acquire numerous molecular and biochemical changes that render them potentially vulnerable to immune cells. Yet it is self-evident that a growing tumour has managed to evade these host defence mechanisms. The exact ways in which the immune system interacts with tumour cells and how cancers are able to escape immunological eradication have only recently started to be fully elucidated. Understanding the relationship between the tumour and the anti-tumour immune response and how this can be altered with conventional treatments and immune-targeted therapies is crucial to developing new treatments for patients with cancer. In this review, focusing on the anti-tumour T-cell response, we summarize our understanding of how tumours, cancer treatments and the immune system interact, how tumours evade the immune response and how this process could be manipulated for the benefit of patients with cancer.

Immunotherapy for neuroblastoma: turning promise into reality

Neuroblastoma is one of the commonest and most aggressive paediatric malignancies. The majority of children present with metastatic disease for which long-term survival remains poor despite intensive multi-modal therapies. Toxicity from current treatment regimes is already significant, and there is little room to further intensify therapy. Alternative treatment strategies are therefore needed in order to improve survival. Immunotherapy is an attractive therapeutic option for these children as it potentially offers a much more specific and less toxic treatment than conventional therapies. This review discusses the different immunotherapy strategies that may be useful in neuroblastoma, their advantages and disadvantages and the challenges that need to be overcome to successfully use them clinically.

Inefficient presentation of tumor-derived antigen by tumor-infiltrating dendritic cells

BACKGROUND

Transplantable B16 melanoma is widely used as a tumor model to investigate tumor immunity. We wished to characterize the leukocyte populations infiltrating B16 melanoma tumors, and the functional properties of tumor-infiltrating dendritic cells (TIDC).

MATERIALS AND METHODS

We used the B16 melanoma cell line expressing ovalbumin protein (OVA) to investigate the phenotype and T cell stimulatory capacity of TIDC.

RESULTS

The majority of leukocytes in B16 melanoma were macrophages, which colocalized with TIDCs, B and T cells to the peripheral area of the tumor. Both myeloid and plasmacytoid DC populations were present within tumors. Most of these DCs appeared immature, but about a third expressed a mature phenotype. TIDCs did not present tumor-derived antigen, as they were unable to induce the proliferation of tumor-specific CD4+ and CD8+ T cells in vitro unless in the presence of specific peptides. Some presentation of tumor-derived antigen could be demonstrated in the tumor-draining lymph node using in vivo proliferation assays. However, while proliferation of CD8+ T cells was reproducibly demonstrated, no proliferation of CD4+ T cells was observed.

CONCLUSION

In summary, our data suggest that DCs in tumors have limited antigen-presenting function. Inefficient antigen presentation extends to the tumor-draining lymph node, and may affect the generation of antitumor immune responses.

Activation of tumor-specific CD8+ T Cells after intratumoral Ad5-TRAIL/CpG oligodeoxynucleotide combination therapy

CD8(+) T-cell activation via cross-presentation of antigens from apoptotic tumor cells is controversial. Dendritic cells capture naturally shed tumor antigens and cross-present them to CD8(+) T cells; unfortunately, the frequency of activated CD8(+) T cells is often too low to mount an effective response against the tumor. By increasing the amount of antigen for presentation, a larger T-cell response can be theoretically elicited. We used a recombinant adenovirus encoding full-length murine tumor necrosis factor-related apoptosis-inducing ligand (Ad5-mTRAIL) to induce tumor cell apoptosis, and when given intratumorally to mice bearing experimental renal cell carcinoma (Renca) tumors, Ad5-mTRAIL minimally prolonged survival and induced a low level of CTL activity. To enhance dendritic cell efficiency, an immunostimulatory CpG oligodeoxynucleotide (CpG ODN) was combined with Ad5-mTRAIL. This combination therapy significantly augmented in vivo antigen-specific T-cell proliferation and CTL activity, as well as prolonged survival of Renca tumor-bearing mice. Interestingly, depletion of CD4(+) or CD25(+) cells before therapy further enhanced survival and in vivo CTL activity. In addition, tumor-free mice depleted of CD4(+) cells were also able to reject a subsequent challenge of Renca cells, but not MHC-matched RM-11 prostate tumor cells, demonstrating the existence of immunologic memory. These results collectively show that local treatment with Ad5-mTRAIL and CpG ODN can augment tumor antigen cross-presentation resulting in T-cell proliferation, enhanced CTL activity, and increased animal survival.

Rejection of RG-2 gliomas is mediated by microglia and T lymphocytes

Immunotherapy holds great promise for the treatment of invasive brain tumors, and we are interested specifically in evaluating immune stimulation of microglial cells as one potential strategy. In order to better understand the tumor fighting capabilities of microglial cells, we have compared the responses of syngeneic (Fisher 344) and allogeneic (Wistar) rat strains after intracranial implantation of RG-2 gliomas. Animals were evaluated by clinical examination, magnetic resonance imaging (MRI) and immunohistochemistry for microglial and other immune cell antigens. While lethal RG-2 gliomas developed in all of the Fisher 344 rats, tumors grew variably in the Wistar strain, sometimes reaching considerable sizes, but eventually all of them regressed. Tumor regression was associated with greater numbers of T cells and CD8 positive cells and increases in MHC I and CD4 positive microglia. Our findings suggest that the combined mobilization of peripheral and CNS endogenous immune cells is required for eradicating large intracranial tumors.

The delivery of an antigen from the endocytic compartment into the cytosol for cross-presentation is restricted to early immature dendritic cells

Dendritic cells (DCs) are the only antigen-presenting cell population having a cross-presentation capacity. For cross-presentation, however, the intracellular antigen-processing pathway and its regulatory mechanism have not been defined. Here we report the differences in cross-presentation ability among murine bone marrow-derived immature DC, early immature day8-DC and late immature day10-DC, and fully mature day10 + lipopolysaccharide DC. Day8-DCs and day10-DCs show an immature phenotypic profile but are different in morphology. Day8-DCs can internalize an abundant volume of exogenous soluble ovalbumin (OVA) and result in cross-presentation. In contrast, day10-DCs are not able to cross-present, although they maintain efficient macropinocytosis. Exogenously internalized OVA antigens are stored in the endocytic compartments. The endocytic compartments are temporarily maintained at mildly acidic pH in day8-DCs and are rapidly acidified in day10-DCs after uptake of antigens. We show that OVA antigens accumulated in the endocytic compartments move into the cytosol in day8-DCs but do not in day10-DCs. NH(4)Cl-treatment, which neutralizes the acidic endocytic compartments and/or delays endosomal maturation, restores day10-DCs for transport the stored OVA antigens from the endocytic compartments into the cytosol. Diphenyleneiodonium chloride-treatment, which acidifies the endocytic compartments, decreases an amount of transported OVA antigen into the cytosol in day8-DCs. These data indicate that only the early immature stage of DC interferes with endosomal maturation, even after uptake of exogenous antigens, and then transports the antigens into the cytosol.

Cranking the Immunologic Engine with Chemotherapy: Using Context to Drive Tumor Antigen Cross-Presentation towards Useful Antitumor Immunity

This review shows how tumor antigen cross-presentation is affected by the major therapeutic modalities including chemotherapy, radiotherapy, and surgery. We argue that this process could affect the way that a tumor works as its own cellular vaccine, and that it is differentially modulated by the choice of treatment.

Phenotype and homing of CD4 tumor-specific T cells is modulated by tumor bulk

Technical difficulties in tracking endogenous CD4 T lymphocytes have limited the characterization of tumor-specific CD4 T cell responses. Using fluorescent MHC class II/peptide multimers, we defined the fate of endogenous Leishmania receptor for activated C kinase (LACK)-specific CD4 T cells in mice bearing LACK-expressing TS/A tumors. LACK-specific CD44(high)CD62L(low) CD4 T cells accumulated in the draining lymph nodes and had characteristics of effector cells, secreting IL-2 and IFN-gamma upon Ag restimulation. Increased frequencies of CD44(high)CD62L(low) LACK-experienced cells were also detected in the spleen, lung, liver, and tumor itself, but not in nondraining lymph nodes, where the cells maintained a naive phenotype. The absence of systemic redistribution of LACK-specific memory T cells correlated with the presence of tumor. Indeed, LACK-specific CD4 T cells with central memory features (IL-2(+)IFN-gamma(-)CD44(high)CD62L(high) cells) accumulated in all peripheral lymph nodes of mice immunized with LACK-pulsed dendritic cells and after tumor resection. Together, our data demonstrate that although tumor-specific CD4 effector T cells producing IFN-gamma are continuously generated in the presence of tumor, central memory CD4 T cells accumulate only after tumor resection. Thus, the continuous stimulation of tumor-specific CD4 T cells in tumor-bearing mice appears to hinder the systemic accumulation of central memory CD4 T lymphocytes.

Programming of CTL with heat-killed Brucella abortus and antigen allows soluble antigen alone to generate effective secondary CTL

Optimal generation of cytotoxic T cell (CTL) responses continues to be a challenge in the production of vaccines against pathogens such as HIV-1, in part because it is difficult to introduce soluble protein antigens (Ag) into the MHC class I pathway. Using heat-killed Brucella abortus (HKBA) as an adjuvant and ovalbumin (ova) protein as an Ag, we demonstrated that a high dose of Ag was required for systemic and effective CTL. In an adoptive transfer model, primary and secondary ova-specific OT-1 CD8 cell expansion by HKBA plus high dose of ova were partially CD4 T cell-dependent. Interestingly, primary stimulation with HKBA plus ova allowed effective secondary stimulation with ova alone that was equivalent to HKBA plus ova in terms of IFN-gamma production from Ag-specific CD8 cells. Thus a combination of adequate Ag dose, and selection of appropriate adjuvants can meet the threshold not only for primary effective CTL responses to soluble protein Ags but for secondary CTL responses following stimulation with protein Ag alone.

Immune surveillance and anti-tumor immune responses: an anatomical perspective

The development of adaptive immune responses against infectious agents relies on the initiation of antigen specific immune responses in secondary lymphoid organs and on the migration of effector cells at the site of infection. Similarly, the development of anti-tumor immunity depends on the recognition of tumor-derived antigens by specific lymphocytes in the context of the lymphoid tissues and on the re-localisation of the cells to the site of cell transformation. Here, we will review the preclinical studies, which have defined the spatial and temporal organisation of anti-tumor immunity, and discuss the implications of these findings in active immunotherapy.

Distinct requirements for deletion versus anergy during CD8 T cell peripheral tolerance in vivo

Activation of naive T cells by quiescent APCs results in tolerance through deletion and anergy. The underlying basis for these distinct fates is unclear. Using clone 4 TCR transgenic animals as a source of naive CD8 T cells, we examined the requirements for peripheral deletion in vivo. Our results demonstrate that independent of the amount of Ag used for stimulation, a single dose was insufficient to achieve complete clonal deletion. Instead, further antigenic exposure was required to completely eliminate all of the activated T cells. Additionally, consecutive stimulations with low doses of Ag were highly effective in promoting deletion. In contrast, although stimulation with high doses of Ag initially led to the apoptosis of many of the activated T cells, it induced hyporesponsiveness in a portion of the responding cells, thereby sparing them from further activation and deletion. These data explain why some conditions promote tolerance through clonal deletion whereas others promote anergy. Furthermore, these data provide a framework to devise protocols for effective deletion of potentially autoreactive T cells.

Tumor growth enhances cross-presentation leading to limited T cell activation without tolerance

Using a tumor model of spontaneously arising insulinomas expressing a defined tumor-associated antigen, we investigated whether tumor growth promotes cross-presentation and tolerance of tumor-specific T cells. We found that an advanced tumor burden enhanced cross-presentation of tumor-associated antigens to high avidity tumor-specific T cells, inducing T cell proliferation and limited effector function in vivo. However, contrary to other models, tumor-specific T cells were not tolerized despite a high tumor burden. In fact, in tumor-bearing mice, persistence and responsiveness of adoptively transferred tumor-specific T cells were enhanced. Accordingly, a potent T cell-mediated antitumor response could be elicited by intravenous administration of tumor-derived peptide and agonistic anti-CD40 antibody or viral immunization and reimmunization. Thus, in this model, tumor growth promotes activation of high avidity tumor-specific T cells instead of tolerance. Therefore, the host remains responsive to T cell immunotherapy.

New approaches for mesothelioma: biologics, vaccines, gene therapy, and other novel agents

Although malignant mesothelioma is not a classically immunogenic cancer, there is abundant evidence for immune recognition. The relative ease of obtaining tumor tissue makes mesothelioma ideal for studying surrogate biomarkers such as lymphocytic infiltration or expression of transduced genes. There is evidence that malignant mesothelioma patients as well as asbestos-exposed persons without mesothelioma have impaired immune responsiveness. Substantial progress has been made in animal models using several biological and immunological techniques, but clinical application has been problematic. Systems studied have included lysis by interleukin-2 (IL-2)-activated lymphokine-activated killer (LAK) cells, tumor necrosis factor-alpha (TNF-alpha), a p16-expressing adenovirus vector, suicide gene therapy using the herpes simplex virus-tyrosine kinase (HSV-tk) followed by ganciclovir, and immunomodulatory gene therapy with IL-2, IL-4, interferon-gamma (IFN-gamma), IFN-alpha, TNF-alpha, granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-6, and IL-1beta transfected into tumors. Vaccinia virus has been studied as a vector for cytokine gene transfer. Suicide gene therapy has been combined with a tumor vaccine. The University of Western Australia is initiating a pilot study of autologous vaccination in malignant mesothelioma. Novel agents under study include the angiogenesis inhibitors SU5416, bevacizumab, and thalidomide. ZD1839, an orally administered, highly selective inhibitor of the epidermal growth factor receptor (EGFR) tyrosine kinase, is being tested in a phase II trial. Since platelet-derived growth factor (PDGF) is thought to be an autocrine growth factor for mesothelioma STI-571 (Gleevec; Novartis, Basel, Switzerland), a highly selective inhibitor of the PDGF receptor tyrosine kinase, is being tested in a phase II trial. The development of more active cytotoxic combinations in this disease should facilitate further studies of chemoimmunotherapy. It seems likely that no single treatment modality will be effective by itself.

In vivo cross-presentation of a soluble protein antigen: kinetics, distribution, and generation of effector CTL recognizing dominant and subdominant epitopes

Cross-presentation of exogenous Ags via the MHC class I pathway is now recognized for its role in self-tolerance, tumor immunity, and vaccine development. However, little is known about the in vivo distribution and kinetics of cross-presented protein Ags, nor the subsequent development of CTL effector responses to dominant or subdominant epitopes. We examined the location and duration of cross-presented Ag by using 5,6-carboxy-succinimidyl-fluorescein ester-labeled T cells from class I-restricted Ag-specific TCR mice. Comparisons of results from an in vitro (51)Cr release CTL assay with an in vivo CTL assay provided physiologically relevant insights into the functional capacities of CTL specific for epitopes with differing affinities. These data demonstrate that efficient cross-presentation of a dominant class I-restricted Ag is dose related and remains largely localized, but not limited to the draining lymph nodes for up to 3 wk following a single injection of soluble protein. Within this period, dominant peptide-specific CTL are fully functional in vivo throughout the secondary lymphoid system. However, no in vivo responses are seen to a subdominant or cryptic epitope. Prolonging Ag cross-presentation via use of IFA promoted persisting in vivo dominant epitope-specific CTL activity and revealed dose-responsive precursor CTL to the subdominant, but not to a cryptic epitope. Analysis of functional in vivo CTL responses demonstrated that, in the presence of strong ongoing responses to the dominant peptide, lytic activity of CTL directed at weaker epitopes is undetectable.