Science recently highlighted cancer immunotherapy as the “2013 Breakthrough of the Year.” More to the point, they stated “this year marks a turning point in cancer, as long-sought efforts to unleash the immune system against tumors are paying off–even if the future remains a question mark”.1 Most of the celebration involves the compelling results of targeted agents designed to reactivate the immune system by manipulating the PD-1/PD-L1 and CTLA-4 pathways. By blocking these recently identified suppressor molecules, these targeted therapies are designed to unleash the immune system either as monotherapies or in combination with traditional cytotoxic chemotherapy. The ultimate result of either strategy should improve the treatment of established, late stage disease, a patient population that has yet to be adequately addressed with modern modalities. While these investigations have provided a novel direction for enhancing cancer treatment, additional technologies still need to be developed to specifically identify tumor-associated antigens (TAAs) to harness the full power of the immune system. Active specific immunotherapy (ASI) has the potential to be that transformative technology by embracing the recently demonstrated genomic heterogeneity of tumor cells, through the use of live, metabolically active, autologous tumor cells which represent the entire antigenic diversity of each patients’ primary tumor.
ASI involves generating a robust, cell-mediated, cytotoxic immune reaction against tumor cells. This concept is rooted in the reality that patient-derived vaccines can induce a potent and lasting immune response against TAAs capable of halting or even eliminating tumors to prevent recurrence. If immunomodulatory agents are capable of rearming the immune system against cancer, then ASI serves as the guidance system. However, do we have physical evidence to suggest killer T-cells are capable of interacting with malignant cells in a therapeutic manner? There is a considerable amount of literature discussing the quantitative effect of in vitro T-cell mediated cytotoxicity, but there is a paucity of direct evidence. In a previous study, Bucana et al.,2,3 harvested immune lymphocytes and other white blood cells from the peritoneal cavity of guinea pigs that had previously been immunized and cured of a lethal, transplantable, syngeneic hepatocarcinoma. These primed immune cells, with proven in vivo cytotoxic capabilities, were then combined in vitro with the same tumor cell line and followed sequentially with time-lapse cinematography, scanning- and transmission-electron microscopy. In this manner, the authors were able to directly observe lymphocyte and macrophages interact with “familiar” cancer cells in vitro.
During co-culture, considerable migration of lymphocytes and other monocytes (later identified as macrophages), occurred on and around the tumor cells (Fig. 1). Two important observations characterized the interaction of lymphocytes and activated macrophages with the tumor cell surface. First, active extension and contraction of monocyte cytoplasmic processes was observed around and into the tumor cell surface. This “probing” was sustained for long periods of time without significant lateral movement of the activated monocytes. Thus, considerable clasmatosis of the T-cells and macrophages was observed at the tumor cell surface. Second, exocytosis of osmiophilic organelles from the monocytes to the tumor cells was observed Figure 2. These organelles had the morphologic and histochemical characteristics of primary and/or secondary lysosomes. Additionally, it was clear that some of the lysosomal organelles were still within segments of monocyte cytoplasm that had detached at the tumor cell surface. Clasmatosis of macrophage or monocyte cytoplasmic extensions with these organelles would allow for tumor cell internalization of toxic proteinases initiating the dramatic cytotoxic events that were observed during the study. Other experimental systems have also reached similar conclusions concerning the cytotoxic events occurring between the innate immune response and cancer cells.4,5 Following cell lysis, the existing macrophages further phagocytized the ruptured tumor cell cytoplasm enabling the identification of secondary or previously “hidden” TAAs. Based on the studies above, the innate immune system already possesses the tools to recognize and destroy malignant cells if appropriately directed.
Figure 1.
(A) Time-lapse cinematography of transplantable, L 10 hepatocarcinoma of syngeneic Strain 2 guinea pigs with 3 peritoneal monocytes from L 10 immune animals. Note the extensions from the monocytes (clasmatosis) (B) Implosion of the tumor cell. (C) Cytoplasm ...
Figure 2.
(A) A lysosome trapped between a monocyte and tumor cell. (B) Lysosome like organelle vacuoles in monocyte and organelle in tumor cell. (C) An extension from the monocyte to and possible into the tumor cell. Intracytoplasmic organelles in the probing ...
The early claims of immunotherapy for cancer came from reports of infectious agents reducing or eliminating localized tumors both in animal models and man (for review see Hanna et al.,6). In fact, the first vaccine approved by the US Food and Drug Administration for the treatment of cancer was Bacillus Calmette-Guerin (BCG). In 1976, Morales et al.,7 first reported the use of BCG for treatment of non-muscle invasive superficial bladder tumors. They reported a 12-fold reduction in recurrence rate of superficial bladder tumors following combined intravesical and intradradermal administration of BCG. Subsequently, numerous prospective randomoized clinical trials demonstrated the efficacy of intravesical BCG therapy for therapy of Carcinoma-in situ (CIS) and later for preventing the progression and recurrence of superficial papillary bladder cancer. It seems that the immune system is triggered by the admixing of the BCG attaching to the tumor at the wall of the bladder and this is often considered to be more inflammation by the innate immune response, thus categorized as active nonspecific immunotherapy.
These results supported the enthusiasm for the specificity of ASI as a rational modality for cancer treatment and developing cancer vaccines as a means of achieving tumor-specific immune responses for disseminated disease. However, the majority of cancer vaccines have failed in practice (Fig. 3). Over the last decade, the failure rate of these treatments in phase II/III clinical trials is over 70%. If we intend to make meaningful progress with vaccine-based cancer treatments, we need to resolve this glaring discrepancy between theory and practice.
Figure 3.
In the 2 rectangles on the right, cancer vaccine candidates on the left and declared failed candidates, blue letters, on the right. Both of these used to treat patients with advanced disease. OncoVAX an autologous tumor cell vaccine used to treat occult ...
First, almost all of these trials were conducted in patients with advanced, late stage disease as a primary or salvage treatment to improve overall survival. These patients are often heavily pretreated with extensive disseminated disease. However, we must understand these immune-based treatments are expected to be effective within a well-established, tumor microenvironment that is often immunosuppressive. As mentioned above, we now have considerable evidence that tumor-infiltrating lymphocytes (TILs) demonstrate an “exhausted” phenotype initiated by molecular interactions within the tumor cells. Specifically molecules, such as members of the PD-1/PD-L1 axis, negatively regulate the efficacy of these immune responses.8,9 This critical interaction prevents cytotoxic T-cell responses against cancer cells, essentially cloaking them from the immune system. Thus, even with a systemic, robust immune response, the functional immunocompetant cells are suppressed within the primary tumor.
The second issue complicating cancer vaccine effectiveness is the staggering degree of heterogeneity observed within established tumors and between patients of a given cancer type. In 2007, a major review of active cancer vaccines outlined the various disappointing results in the field.10 One of the first general considerations of this review highlighted the importance of antigen discovery: “select the most informative targets.” The authors point out that ideal targets should be tumor-specific and “it is important to use the intended study population to assess the proportion of tumors that express the target of choice and the proportion of cells within each tumor that express it.” Thus, it should be a common goal within the field to actively search for a convenient number of shared antigens that most effectively define a patient population of interest. However, this stipulation would require a disease with significant inter- and intra-patient homogeneity.
