ABSTRACT: Vaccines are a promising but still experimental treatment for melanoma. They are intended to stimulate immune responses against melanoma and by so doing, increase resistance against and slow the progression of this cancer. Key requirements for vaccines to be effective are that they contain antigens that can stimulate tumor-protective immune responses and that some of these antigens are present on the tumor to be treated. Unfortunately, these antigens are still not known. To circumvent this problem, polyvalent vaccines can be constructed containing a broad array of melanoma-associated antigens. Several strategies are available to construct such polyvalent vaccines; each has advantages and disadvantages. Clinical trials have shown that vaccines are safe to use and have much less toxicity than current therapy for melanoma. Vaccines can stimulate both antibody and T-cell responses against melanoma, with the type of response induced, its frequency, and its magnitude depending on the vaccine and the adjuvant agent used. A growing body of evidence suggests that vaccines can be clinically effective. This evidence includes correlations between vaccineinduced antibody or T-cell responses and improved clinical outcome, clearance of melanoma markers from the circulation, improved survival compared to historical controls, and most convincingly, two randomized trials in which the recurrence-free survival of vaccine-treated patients was significantly longer than that of control groups.
There is an urgent need for better treatments for melanoma. No therapy prolongs survival in patients with widely disseminated disease. Only one treatment is approved by the US Food and Drug Administration (FDA) for patients with resected disease at high risk of progression-interferon alfa-2b (Intron A)-and it has limited effectiveness as well as frequent and potentially severe side effects.
Vaccines are receiving increasing attention as a still-experimental treatment for this cancer. Theoretically, vaccines should permit the selective and safe destruction of melanoma cells. The rationale for believing that melanoma vaccines can be effective, the relative advantages and disadvantages of the different strategies used to construct melanoma vaccines, and the results obtained to date are summarized below.
Rationale for a Melanoma Vaccine
The progression of melanoma is influenced by immune factors, and stimulation of these factors with vaccines can increase resistance to this cancer.[ 1] The two most compelling observations that support this belief are:
(1) In vivo mechanisms can kill melanoma cells selectively, without harming normal melanocytes. This is evidenced by the partial regression of melanoma in 15% to 20% of primary lesions,[2,3] and by the rare but dramatic, spontaneous, and complete regression of advanced tumors. Partial regression in primary melanomas is actually visible as areas of white depigmentation within the tumor (Figure 1). The white areas are due to the destruction of melanoma cells. As regression occurs spontaneously without any treatment, it clearly indicates that humans possess protective mechanisms that have the ability to destroy melanoma cells. These defense mechanisms are very selective, as they destroy melanoma cells without harming adjuvant normal melanocytes. This is evident from the skin adjacent to areas of regression retaining its normal pigmentation. The selectivity of this process, which destroys malignant melanocytes without harming normal melanocytes, indicates it is mediated by immune mechanisms, as only the immune system has the exquisite ability to recognize the difference between malignant and normal cells. Stimulating these immune defenses is the purpose of vaccines.
(2) Vaccines can markedly increase resistance to melanoma-at least in animals. Murine B16 melanoma is invariably fatal when injected into mice, killing all within 6 to 8 weeks. By contrast, almost all mice preimmunized to vaccines against melanoma can survive, as illustrated in Figure 2. The protection is specific for melanoma. Melanoma vaccine- immunized mice are not protected against an unrelated tumor. This specificity indicates that the protective mechanisms stimulated by vaccine treatment are immunologic in nature.
Tumor Regression vs Absence of Progression
An incidental observation in mice has an important implication for the clinical impact of vaccine treatment in humans and the end point of such trials. That is, vaccine-treated mice can live in an apparent state of good health for months while bearing large tumors that would invariably and rapidly kill nonimmunized mice. This is apparent from examining vaccinetreated mice that survive challenge with lethal doses of melanoma cells. Such mice appear clinically healthy, gain weight, and have no evidence of tumor. Yet autopsy performed months after tumor challenge reveals that many of the animals have very large melanomas in their internal organs, of a size that would kill nonimmunized mice within days.
Thus, the degree of resistance induced by vaccine treatment in these animals was insufficient to cause tumor regression, but was sufficient to prevent tumors from continuing to progress and kill the animals. The implication of this observation extended to humans is that vaccine treatment may improve survival by slowing tu- mor progression rather than by causing tumor regression. It implies that the key end point in evaluating vaccine clinical trials should be absence of tumor progression rather than the conventional end point of tumor regression.
Additional observations supporting the idea that immune mechanisms play an important role in slowing the progression of melanoma include the presence on melanoma cells of antigens that are either unique or present in larger amounts than on normal melanocytes, the ability of these antigens to stimulate antibody and/or T-cell responses in patients with melanoma, and the infiltration of lymphocytes into melanoma nodules of vaccinetreated patients. Investigators have noted correlations between the presence of these immune responses and an improved clinical outcome, indicating that these responses can play an active role in controlling the progression of melanoma.[7-11]
Thus, there are theoretical reasons for believing that vaccines can increase resistance to melanoma and practical observations that they actually do so in animals. The challenge is to develop vaccines that will be as effective in humans.
What Is Required for Melanoma Vaccines to Be Effective
Melanoma vaccines are intended to stimulate patients' immune systems to react more effectively against their own melanoma and, by so doing, destroy the tumor or slow its progression. To do so, the vaccines must satisfy several requirements. The two most important are:
(1) The vaccine must contain antigen(s) that can stimulate tumorprotective immune responses.
(2) Some of these antigens must be present on a patient's own tumor; otherwise, the vaccine-induced immune responses will be unable to recognize and attack the tumor.
A number of other requirements must be satisfied for a vaccine to be effective and practical to use, as discussed subsequently. Vaccines must also be safe to use, their composition well characterized, and their manufacture reproducible. To retain their potential to provide cost-effective therapy, the vaccines should be relatively simple to prepare and administer.
Challenges in the Design of Melanoma Vaccines
Unfortunately, there are major problems satisfying the two major requirements described above.
• Selection of Antigens Used to Prepare the Vaccine—This is the most critical issue in preparing a cancer vaccine, as the vaccine must contain antigen that can stimulate tumor-protective immunity or it will not work. Although many antigens associated with melanoma have been identified, we do not know which ones are appropriate for this purpose. Establishing this ability is arduous, requiring a large-scale phase III randomized clinical trial of each antigen, to demonstrate objectively whether it slows the progression of melanoma. Taking into account the number of candidate antigens, the size and expense of phase III trials, and the limited number of melanoma patients available, it is not possible to conduct such trials for every currently known melanoma-associated antigen and for new ones that will be discovered. Thus, it will be difficult to find out which melanoma antigens actually stimulate protective immunity against this cancer and should be used to construct vaccines.
• Antigenic Heterogeneity—Another complication in selecting antigens to construct melanoma vaccines is that some of the unknown antigen(s) that stimulate protective immunity must also be expressed by the tumor to be treated. Unfortunately, the expression of tumor antigens by melanoma cells is heterogeneous. It varies between melanomas in different individuals, between different tumor nodules in the same individual, and between different melanoma cells within the same tumor nodule.[12-14] Furthermore, the actual antigens expressed by residual melanoma cells in a patient at the time treatment is instituted cannot be known. A solution suggested to circumvent this problem is to prepare autologous vaccines from a patient's own tumor. However, this would not resolve the problem, because there is no assurance that the antigens present in the tumor tissue used to prepare the vaccine will be the same as those in the residual tumor(s) that need treatment.
• Antigen Modulation—The pattern of antigens expressed by tumor cells can change during tumor progression.[ 15] This reflects the changes that occur in tumors as they metastasize, in part due to immunologic pressures on the tumor. Vaccine-induced immune responses destroy tumor cells bearing the targeted antigen(s), resulting in the selection and expansion of surviving cells that lack these targets and that now resist the action of the vaccine.
Thus, it is unknown which antigens should be used to construct melanoma vaccines, whether these are expressed by the tumor to be treated, or whether they will still be there once treatment begins.
• HLA Restriction—Some immune responses that are critical in tumorprotective immunity, such as antigenspecific CD8+ and CD4+ T-cell responses, are human leukocyte antigen (HLA) restricted. As a consequence, each vaccine peptide antigen will induce these responses only in patients who have the particular type of HLA molecule that can bind that peptide. As there are a large number of HLA molecules, and as the expression of each varies from individual to individual, only a minority of patients express the HLA molecule required to bind a particular peptide. Even the most common HLA molecule (HLA-A0201) is expressed by only 40% of white individuals, the population most at risk for melanoma. Many HLA molecules are expressed by only a small percentage of patients to be treated.
Thus, even if one could identify a peptide capable of stimulating a tumor- protective cellular response, a vaccine made from that peptide would at best be effective in less than half of the population at risk for melanoma. Because of the variable expression of antigens on melanoma cells, the proportion of patients who can theoretically benefit from vaccines prepared from any one antigen is even smaller.
• HLA-Unrelated Heterogeneity in Immune Responses—Independent of HLA restriction, investigators have also found heterogeneity in the ability of different patients to develop cellular immune responses to antigens.[ 16] That is, patients who express the same HLA phenotype and who are immunized identically to the same antigen can vary in their ability to develop a CD8+ T-cell response to that antigen. This is not due to lack of immune competence by the patients, as patients who do not respond to one peptide will respond well to another peptide presented by the same HLA molecule. Similar heterogeneity is seen in the ability of individuals to develop antibody responses. This heterogeneity further limits the proportion of patients who can develop effective antitumor immune responses to any single antigen.
• Number of Antigens Required to Induce Effective Protective Immunity—It is unknown whether clinically effective tumor-protective immunity in humans can be induced by immunization to a single antigen or whether responses against multiple antigens are required for cancer cells to be killed. Even if immune responses to a single antigen can kill melanoma cells, stimulating responses to multiple antigens should do so more effectively. Thus, the larger the number of antigens in a vaccine, the better it should work. However, there is a price to be paid for increasing the number of antigens in a vaccine, as described below.