Accumulating evidence shows that a proportion of many common solid tumor types are spontaneously recognized and attacked by the immune system and this is associated with improved clinical outcomes. For example, in ovarian cancer, we identified intraepithelial lymphocytes (infiltrating tumor islets) in tumors expressing a molecular signature of T cell activation, including IFN-γ, IL-2 and effector lymphocyte-associated chemokines [1
]. Ovarian cancer tumor-infiltrating lymphocytes (TILs) are oligoclonal [2
], recognize autologous tumor antigens [3
], and display tumor-specific cytolytic activity ex vivo
]. The presence of intraepithelial TILs is associated with significantly longer clinical remission after chemotherapy as well as improved overall survival of the patients in ovarian cancer [1
], an observation validated by different studies in ovarian cancer [8
] and other tumors such as melanoma, breast, prostate, renal cell, esophageal and colorectal carcinoma [16
]. The association of spontaneous antitumor immune response with improved survival implies that many patients could benefit from strengthening tumor rejection through immunotherapy.
Tumors are recognized by the immune system through unique tumor associated antigens (TAAs) (reviewed in [23
]). TAAs can be divided into five major categories: (1) mutated antigens expressed uniquely by tumors; (2) overexpressed antigens, i.e. normal proteins whose expression is upregulated in tumor; (3) oncofetal antigens shared by embryonic or fetal tissues and; (4) differentiation or lineage antigens; and (5) cancer-testis antigens shared by spermatocytes/spermatogonia and tumor cells. With rapid advancements in molecular biology and the development of new genomic and proteomic interrogation technologies such as gene expression microarray, differential display, SAGE, mass spectrometry etc. as well as techniques to interrogate immune response through serum autoantibodies such as SEREX (se
rological analysis of autologous tumor antigens in serum of cancer patients by r
ecombinant cDNA ex
pression cloning), many additional TAA targets are rapidly identified and added in the design of new immunotherapeutic strategies. However, painstaking work remains to be done to fully characterize the immunogenicity of these emerging antigens in the human, identify the most immunogenic epitopes, and test their role as bona fide
tumor rejection antigens that can cause tumor regression.
The most popular and widely used TAAs for tumor vaccines are HLA-restricted immunodominant peptides. It is relatively easy to synthesize large quantities of clinical grade peptides, but there are several disadvantages associated with their use. First, only patients possessing specific HLA expression(s) are eligible. Second, the resulting immune responses are limited to the epitope(s) used for immunization that might be insufficient to rapidly eliminate tumors, and could drive the emergence of escape variants of the tumor cells. Indeed, the phenomenon of epitope spreading is only observed in very small numbers of patients after single or multiple peptide immunization [24
]. Finally, the longevity of MHC-peptide complexes in vivo
is unknown. The affinity of peptides for their various HLA molecules also varies and this could affect their immunogenicity in vivo
, should competition occur between/amongst the peptides. Some research groups have incorporated peptides encoding epitopes recognized by CD4+
T helper cells to elicit a stronger overall immune response through providing cognate help to CD8+
T cells. However, few authentic tumor antigen CD4+
epitopes have been defined to date. Peptide-based trials have met with limited success and the issues previously mentioned still need to be addressed. Vaccination with the full length protein or open reading frame RNA or cDNA of candidate TAAs is a valid alternative, but still faces similar challenges regarding the need for painstaking characterization of individual TAAs.
A promising alternative to individual TAAs is vaccination using derivatives of whole tumor cells without defining the antigens. Tumor cells express a whole array of TAAs that are both characterized and uncharacterized, and this rich source of antigens contains epitopes of both CD8+
cytotoxic T cells (CTLs) and CD4+
T helper cells. This is important, as the parallel presentation of both MHC Class I and II restricted antigens would help to generate a stronger overall anti-tumor response and long term CD8+
T cell memory via CD4+
T cell help [26
]. In addition, it could greatly diminish the chance of tumor escape compared to using single epitope vaccines. Furthermore, the use of whole tumor cells theoretically eliminates the need to define, test and select for immunodominant epitopes. The tumor cells could be autologous, i.e. obtained from the patients, or allogeneic “off-the-shelf”. The major drawback for using autologous tumor cells is that they are only useful in single patient-tailored anti-tumor immunotherapies, and they could pose problems of collection, processing, reproducibility and inter-patient variability. Nevertheless, tumor cells from each patient potentially carry gene mutations encoding for unique TAAs that are important in stimulating effective and long-lasting anti-tumor responses in the patient. On the other hand, allogeneic tumor cell lines that share one or even several of the TAAs as autologous tumor cells provide a simpler method of delivering antigens in tumor immunotherapy. Allogeneic cell lines can be propagated in large quantities in cell factories and the quality can be easily assessed and monitored in good manufacturing practice (GMP) facilities.
In an attempt to compare the efficacy of peptide-pulsed to whole tumor cell-pulsed vaccinations in cancer patients, Neller et al
examined the clinical outcomes of 173 published peer-reviewed immunotherapy trials that used either molecular defined synthetic antigens, or autologous or allogeneic tumor cells without concomitant therapies in patients with melanoma, renal cell and hepatocellular carcinomas, lung, prostate, breast, colorectal, cervical, pancreatic or ovarian cancer [28
]. They found that 138 of 1711 patients (8.1%) had objective clinical responses when whole tumor or tumor extracts were used as antigens [including DC loaded with tumor extracts, modified tumor cells or tumor mRNA], as compared to 63 of 1733 patients (3.6%) when molecularly defined tumor antigens were used such as synthetic peptides or proteins, and viral or plasmid vectors encoding peptides or proteins (P
< 0.0001, Chi-square test). As spontaneous objective clinical responses are rarely seen in most of the cancers treated, the authors concluded that most objective clinical responses were an indication of effective immunotherapy. With the same criteria, the authors further analyzed 1601 patients who enrolled in 75 published trials for advanced metastatic melanoma, and found an objective response rate of 12.6% (107/845) when whole tumor undefined antigen was used compared to 6% (41/608) when defined antigen was administered (P
< 0.001). Interestingly, they also found no significant difference in the response rate comparing autologous to allogeneic tumor sources (P
= 0.15) [see reference [28
] for the complete list of clinical trials]. These results provide encouragement for pursuing whole tumor antigen vaccination approaches. Obviously, because tumor cells express a large load of ‘self’ antigens and have evolutionally adapted to induce immune tolerance, methods to prepare whole tumor antigen become critically important to produce immunogenic vaccines.