PEER REVIEWED

Immunotherapy for Uveal Melanoma: Obstacles and Opportunities

Immune privilege in the eye presents interesting and unique challenges.

Jerry Y. Niederkorn, PhD • Yuguang He, MD

Uveal melanoma (Figure 1) is the most common primary malignant intraocular tumor in adults.¹ One of the remarkable differences between cutaneous and uveal melanomas is their metastatic behavior. Although skin melanomas metastasize to almost any organ, uveal melanoma has a propensity to spread to the liver. Indeed, liver metastases occur in up to 95% of the patients who die from uveal melanomas.^2,3

The results of the Collaborative Ocular Melanoma Study (COMS) supported the opinions of many clinicians that the best current therapy yields five-year mortality rates of 1%, 10% and 30% for small, medium, and large tumors, respectively.⁴ Despite advances in the management of primary uveal melanomas, significant additional benefit might be realized through the introduction of new treatment modalities. These modalities include enlisting the aid of the mmune system.

Figure 1. Uveal melanoma in the posterior pole.

IMMUNE SURVEILLANCE OF NEOPLASMS

Evidence suggesting that the body might be able to rid itself of some neoplasms was demonstrated in the late 19th century by Coley, who observed that bacterial toxins injected into some sarcomas produced antitumor effects and, in some cases, cures.^5,6 The positive responses were probably due to the action of tumor necrosis factor (TNF)-α, which is present in the kinds of bacterial extracts used by Coley.

In the early 20th century—a time before the immune system was recognized as a distinct entity—the eminent microbiologist Paul Ehrlich predicted that the host's defense systems might protect against carcinomas.⁷ It would take another 50 years before the “immune surveillance” theory was formally articulated by Burnet and Thomas.^8,9

In its simplest form, the immune surveillance hypothesis posits that neoplasms express antigens that can be detected by the immune system, which eradicates the antigenic tumors in the same fashion that it eliminates pathogens. The immune surveillance hypothesis was strengthened by subsequent animal studies on chemically and virally induced cancers, which revealed the presence of tumor antigens that were capable of activating the immune system and pro tecting hosts from progressive cancers.^10,11

Today the immune surveillance hypothesis is firmly established; however, successful implementation of immunotherapy in the treatment of cancer has yet to match the enthusiasm that has surrounded this concept for the past half-century.

The immune system, like any effective defense apparatus, is composed of two distinct components: (1) a first responder component that mounts a swift but limited res ponse to control an initial threat; and (2) a subsequent multifaceted second component that, although slower to respond, employs a broad array of defensive elements that provide sustained protection against the invading force.

Elements of the innate immune apparatus represent the first responders and are characterized by their nimble but limited responses to pathogens and tumors. Compo nents of the innate immune system include macrophages, neutrophils, natural killer (NK) cells and the complement system. The innate immune system is effective in restraining bacterial and viral infections during the time in which the adaptive immune response is generated.

The adaptive immune system includes T lymphocytes, B lymphocytes and antibodies, and it has three properties that make it especially attractive as a partner in the treatment of cancer: inducibility, memory and antigen-specificity. These are the very properties that form the basis for vaccines that protect us from a wide variety of life-threatening and debilitating viral and bacterial diseases. Indeed, our planet is free of smallpox due to the efficacy of our adaptive immune apparatus, which is coaxed along with a little help from man-made vaccines.

IMMUNOTHERAPY FOR UVEAL MELANOMA

Immunotherapy is broadly defined as the manipulation of the host's immune apparatus to eliminate or control the spread of neoplasms. Both the innate and adaptive immune systems can be targeted for immunotherapy.

In the case of the innate immune system, it has become clear that the NK cell population holds promise for immunotherapy. As the name implies, NK cells are naturally cytotoxic to a wide range of tumors, including uveal melanomas.^12-14 In the past, NK cell–based immunotherapy has largely focused on the administration of cytokines, such as interferon (IFN)-γ and interleukin (IL)-2, as means of activating this cell population.

However, it has become clear that tumors elaborate a variety of soluble and cell membrane–bound factors that disarm NK cells. Accordingly, a new generation of immu notherapeutic strategies seeks to buffer tumor-derived molecules that disable the host's NK cell repertoire.

For example, some tumors produce interleukin-10 and transforming growth factor (TGF)-ß, both of which suppress NK cell activity.^12-14 Administration of anti–IL-10 and anti–TGF-β antibodies relieves the suppression of NK cells in tumor-bearing animals and results in steep reductions in metastases.^15-17 This strategy warrants consideration for use in human uveal melanoma patients.

Human tumors, including uveal melanomas, express novel tumor-specific antigens that can arouse the adaptive immune system and serve as targets for immunotherapy. Melanoma-associated antigens are a group of well-characterized cancer antigens that include more than 50 closely related proteins that are expressed on cutaneous and uveal melanomas.^18-20 One promising new immunotherapeutic approach for enlisting the adaptive immune system in the battle against uveal melanoma involves the development of “designer” tumor vaccines, using engineered MHC class 2–matched uveal melanoma cells that are selected from a bank of uveal melanoma cell lines that express a spectrum of MHC class 2 haplotypes.²¹ Uveal melanoma cells with the appropriate MHC 2 haplotype can be incorporated into a vaccine that matches the MHC class 2 haplotype of the uveal melanoma patient

Because MHC class 2 alleles are not as polymorphic as MHC class 1 alleles, a relatively small set of uveal melanoma cell lines with the most common MHC class 2 phenotypes could be assembled to serve as a repository for tai loring vaccines for individual patients. Moreover, stable uveal melanoma cell lines could be further engineered to express potent costimulatory molecules to nudge the adaptive immune response even further.

One of the major pitfalls in tumor immunotherapy is the inherently weak im mu nogenicity of most tumors. Although it may not be feasible to increase the immunogenicity of the tumor antigens themselves, recent breakthroughs in the area of costimulatory molecules suggest that it may be possible to promote a sustained immune response by blocking the immune system's “off ” switch. That is, two signals are transmitted that set the adaptive immune response into motion (Figure 2). The first signal is in the form of antigen expressed on either MHC class 1 or 2 mo lecules on the surface of antigen-presenting cells (APCs). The second signal is a costimulatory signal that is transmitted when the costimulatory molecules on APC engage their ligands on T lymphocytes.

Figure 2. Ipilimumab blocks engagement of B7 (CD80/CD86) with CTLA-4 and prevents induction of “off” signals to T cells, which leads to sustained and elevated adaptive immune responses.

The most widely studied costimulatory molecule expressed on APCs is B7 (CD80 and CD86), which engages its ligand, CD28, on T cells. The interaction between B7 and CD28 results in the ac tivation and clonal expansion of antigen-specific T cells. However, if unrestrain ed, this stimulation would lead to a relentless expansion of T cells, not unlike the un controlled proliferation of a neoplasm. However, to avoid lymphocyte “burnout” and to restore immune homeostasis, a second molecule, called CTLA-4, is subsequently upregulated on the surface of T lymphocytes. CTLA-4 has >50 times the affinity for B7 compared to CD28, and thus, it out-competes CD28 for B7 binding.

Moreover, engagement of B7 with CTLA-4 results in the generation of a potent inhibitory signal that culminates in a steep reduction in T cell proliferation and a quenching of T cell activation. However, blocking CTLA-4/B7 interactions with monoclonal antibodies has the opposite effect and produces a sustained and elevated immune response by blocking the T cells' “off ” switch.

Administering anti-CTLA-4 antibody in combination with tumor cell vaccination resulted in the regression of the weakly immunogenic melan omas in mice.²² Two anti-CTLA-4 antibodies have been tested in human cutaneous mel a noma patients, ipilimumab^23,24 and tremelimu mab.²⁵ Ipilimumab was tested in stage 4 skin melanoma patients in the first randomized phase 3 trial in melanoma ever to show a survival advantage.²⁶ Ipilimumab exhibits interesting kinetics, with slow responses in some patients that require months to years to produce regression.²⁷ Considerable enthusiasm has surrounded ipilimumab, as it represents the first new drug approved by the FDA for melanoma in over a decade.²⁷

IMMUNE PRIVILEGE IN THE EYE: AN OBSTACLE TO IMMUNOTHERAPY

Implementing immunotherapy for the treatment of intraocular tumors is confounded by the immune privilege of the eye. The eye is endowed with multiple regulatory, physiological and anatomical features that prevent the induction and expression of both adaptive and innate immune responses.^28-30

The anti-inflammatory and immunosuppressive properties of the intraocular environment shield neoplasms from immune surveillance and allow the progressive growth of highly immunogenic tumors that undergo swift immune rejection outside of the eye in experimental animals.^14,31,32 Moreover, there is mounting evidence that many uveal melanomas have already metastasized by the time that the primary tumor is diagnosed.

Thus, targeting immunotherapy to intraocular melanomas offers more obstacles than opportunities. Because metastatic disease is the leading cause of death in uveal melanoma patients, implementing immunotherapy that systemically attacks disseminated uveal melanoma cells in multiple organs is an attractive option. A two-pronged approach that incorporates both the innate and adaptive immune systems is desirable.

There is a compelling body of evidence indicating that uveal melanomas are sensitive to NK cell–mediated cytolysis and that the development of liver metastases is closely correlated with the emergence of subpopulations of uveal melanoma cells that have acquired properties that allow them to escape NK cell–mediated surveillance.^14,31,32

Among these properties is the capacity of melanoma cells to produce soluble factors, such as TGF-β and IL-10, that inhibit NK cell activity. For example, administration of anti–IL-10 antibody can reverse this suppression and produce a steep reduction in liver metastases arising from intra ocular melanomas in mice.¹⁷ It is also possible to activate the liver NK cell population through the administration of biological response modifiers, such as IFN-β, which has been shown to produce a significant reduction in liver metastases in mice with intraocular melanomas.³³ Thus, immunotherapy designed to enhance the innate immune re sponse could include strategies to neutralize inhibitory molecules and concomitantly deliver activating signals.

The second prong of immunotherapy would focus on activating the adaptive immune response. Two approaches come to mind. The first approach is to implement the aforementioned “designer” vaccines, which are engineered specifically to activate the host's T cell population using melanoma antigens displayed on the appropriate MHC class 2 molecules. The second approach is to enhance the efficacy of designer vaccines through the administration of ipilimumab, which would block the immune system's “off ” switch and thereby allow a sustained and elevated adaptive immune response.

CONCLUSIONS

Immunotherapy is still in its infancy and has yet to reach its expected potential. However, there are glimmers of hope as we learn more about tumor immunology. Evolution has driven the compartmentalization of the mammalian immune system into innate and adaptive components, each of which offers unique opportunities for immunotherapeutic development. It seems axiomatic that an optimal immunotherapeutic regimen will take full advantage of both of these components. RP

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