Modulating the immune response to treat cancer (Proceedings)

The immune system in many ways represents an ideal tool with which to fight cancer. It is capable of killing tumor cells, has the potential to provide long-lasting protection against recurrence, and because immune responses can be antigen- and thus tumor- specific, side effects in normal tissues may be better spared than with other conventional therapy. Enlisting the immune system to treat neoplasia in veterinary species is beginning to receive more and more attention by clinical and basic researchers, and this presentation will provide an overview of the tenets of using the immune system to treat tumors. The basic means by which the immune system has been recruited to treat tumors include passive and active immune response, and what will be referred to as modulation of active immune responses.

A short immunology refresher

Immune responses are typically considered as either innate or acquired; acquired immune responses are also referred to as adaptive immune responses. The key feature of innate immune responses, and the salient feature that distinguishes innate immune responses from acquired immune responses, is that they are not antigen-specific. Some of the important effector cells of the innate immune system include neutrophils, macrophages, and natural killer (NK) cells. Compared to acquired immune responses, manipulation of the innate immune response to treat tumors has received relatively little attention.

Acquired immune responses are antigen-specific, and are also characterized by memory, that is the capacity to generate long-lived antigen-specific immune cells that can respond quickly when re-exposed to antigen. The most important effector cells of the acquired immune system are B cells, which mature to plasma cells and secrete antigen-specific antibody, and T cells, which can exhibit cytotoxic (cytotoxic T cells) activity or provide help to B cells and cytotoxic T cells (so-called T helper cells). Another T cell population that has emerged in importance with regards to anti-tumor responses is T regulatory (Treg) cells, formerly referred to as T suppressor cells.  These cell types form the principle basis for historical, and more recent, strategies to use the immune system as a weapon against tumors.

Passive immunity against tumors

Passive immunity is the provision of pre-formed antibody or immune cells to a host with the goal of achieving an effective antigen-specific response. Administration of specific antiserum, or pre-stimulated cytotoxic T cells, are both examples of passive immunity. Both have been used to treat small animal tumors. Some of the earliest attempts to treat tumors in dogs involved the administration of TALL 104 cells, a human-derived cytotoxic T cell lines with broad anti-tumor activity.

A distinct disadvantage of passive immunity is that because memory cells are not administered or generated in vivo, the host derives no benefit from immunologic memory and long-lived immunity. Because of the limitations of passive anti-tumor immunity, considerable effort has been invested in finding strategies that would promote active anti-tumor immune responses.

Active immunity against tumors

Active immune responses are the result of antigen-specific stimulation of B cells and T cells, which start with the processing and presentation of antigen to B and T cells by professional antigen presenting cells such as dendritic and Langerhan's cells, macrophages and B cells. In contrast to passive immunity, active immunity is associated with the development of immunologic memory, allowing for long-lived immune responses.

The best current example of using active immunity against tumor is the canine melanoma vaccine. This vaccine contains DNA that encodes for the protein tyrosinase. Once injected into the host, host cell machinery uses the vaccine DNA to make the tyrosinase protein, which stimulates an immune response against this protein which is expressed in melanoma cells (and some normal cells as well). There are many other examples in the veterinary literature of investigator attempts to stimulate anti-tumor immune responses with vaccines (Table 1).  The many strategies represented in Table 1 reflect attempts to overcome some of the obstacles associated with obtaining an immune response against tumors, which include immune evasion, alterations in antigen presentation, and production of immunosuppressive factors by the tumor or by other cells in the tumor environment.

Table 1. Examples of some recent approaches to anti-tumor vaccines reported in the small animal literature

Canine glioma Vaccination with glioma cell lysate, local IFN-gamma gene delivery by adenovirus vector Canine TVT Xenogenic (chicken) HSP 70 DNA vaccination   Vaccination with dendritic cell fused to TVT cell Canine mammary tumor Vaccination with a dendritic cell fused to mammary carcinoma cell Anti TERT vaccine* Plasmid DNA and adenovirus vector B cell lymphoma Anti-TERT vaccine (adenovirus vector) Canine sarcoma Anti-VEGF vaccine Canine hemangiosarcoma Hemangiosarcoma cell lysates/liposomal DNA complexes

*TERT (telomere reverse transcripstase) is expressed in many tumors

Modulating immune responses to promote anti-tumor immunity

Other strategies that have been taken to stimulate ant-tumor immune responses have included the administration of compounds or drugs that help modulate existing immune responses in a manner that promotes more effective antigen-specific immune responses. With these approaches, specific antigen is not administered. Some examples of compounds that have been administered to promote anti-tumor immune responses have included acemannan, BCG, human chorionic gonadotropin, cytokines such as interferon-gamma and interleukin 2.  

Metronomic chemotherapy

A relatively new approach to the administration of chemotherapy to veterinary patients is termed metronomic chemotherapy. Metronomic chemotherapy centers around continuous (often daily) administration of cytotoxic chemotherapy agents, such as cyclophosphamide. Practical benefits of metronomic chemotherapy include the ability to give orally administered drugs so that owners can provide chemotherapy at home, and reduced toxicity as compared to administration of cytotoxic doses. There are three papers that have described metronomic chemotherapy in dogs. Metronomic administration of cyclopshophamide, in conjunction with piroxicam, to dogs with incompletely resected low-grade soft-tissue sarcomas was associated with a longer disease-free interval as compared to dogs treated with surgery alone. Bone marrow and bladder toxicity was uncommon in the population of dogs in this study. In another report, daily administration of cyclophosphamide, etoposide and piroxicam to dogs that had splenectomy for hemangiosarcoma had survival times comparable to dogs treated with cytotoxic doses of doxorubicin. Side effects were uncommon. In the last paper, which was not a report of efficacy, low daily doses of lomustine were well-tolerated in dogs. Side effects were uncommon, although some dogs that had been on this drug for approaching a year developed thrombocytopenia, and a small number of dogs developed renal failure, which has been described as a toxicity of high-dose lomustine in people.

Originally proposed to impair angiogenesis in tumors, more recent evidence indicates that metronomic chemotherapy also alters immune responses to tumors. An important cell type to have emerged in studies of tumor immunology is the Treg cell. The Treg cell generally has properties that suppress immune responses, and there is evidence in many tumor models that cancer-bearing patients have an increased proportion of Treg cells, which could foster tumor progression as a result of suppressed anti-tumor immune responses. Metronomic chemotherapy, specifically using cyclophosphamide, has been demonstrated to decrease the total number and proportion of Treg cells in the circulation of soft-tissue sarcoma-bearing dogs. There was also evidence in this paper supporting the hypothesis that metronomic chemotherapy had anti-angiogenic effects.  Evidence in other models suggests that metronomic chemotherapy can enhance lymphocyte proliferation and increase memory T cells.

The best applications of metronomic chemotherapy in veterinary patients await results of clinical studies to define which tumors would benefit, the best drug or combination of drugs, and the stage(s) of disease in which responses could be seen. It is common for the oncology service at WSU to use a metronomic approach to chemotherapy when other standard treatment options (e.g. radiation therapy, cytotoxic chemotherapy) are not available or can't be tolerated by the patient.

Summary

Within the next decade, it is likely that the burst of research activity in developing anti-tumor immune responses will bear fruit in terms of clinically useful strategies for a more broad array of tumors. Additional studies will be needed to determine the settings in which anti-tumor immune responses are most useful (e.g. in adjuvant setting), but it is expected that promoting anti-tumor immune responses will become a standard approach for many tumors and put such an approach on the menu with the other, traditional approaches to cancer treatment, surgery, radiation therapy and chemotherapy.

References and suggested reading (additional references available on request)

Burton JH, et al. Low-dose cyclophosphamide selectively decreases regulatory T cells and inhibits angiogenesis in dogs with soft tissue sarcoma. J Vet Intern Med 2011; 25:920-926.

Elmslie RE, et al. Metronomic therapy with cyclophosphamide and piroxicam effectively delays tumor recurrence in dogs with incompletely resected soft tissue sarcomas. J Vet Intern Med 2008; 22:1373-1379.

Lana S, et al. Continuous low-dose oral chemotherapy for adjuvant therapy of splenic hemangiosarcoma in dogs.  J Vet Intern Med 2007; 21:764-769.

Pasquier E, et al. Metronomic chemotherapy: new rationale for new directions. Nat Rev Clin Oncol 2010; 7:455–465.;

 Tripp CD, et al. Tolerability of metronomic administration of lomustine in dogs with cancer. J Vet Intern Med 2011; 25:278-284.