Adoptive T cell therapy for cancer in the clinic (original) (raw)
A premise of this Review is that clinical trials of adoptive T cell transfer based on a sufficient understanding of lymphocyte and cancer biology have only begun in recent years. Nevertheless, lessons can be learned from previous trials that failed to achieve the expected clinical efficacy. In addition, issues related to the clinical translation of adoptive T cell transfer therapy are discussed below, with an emphasis on dose and scheduling issues, potential toxicities, and the optimal antigens to target with adoptively transferred T cells.
Status. Pilot clinical trials of adoptive T cell immunotherapy were initiated in cancer soon after the discovery of IL-2, which enabled the large scale culture of T cells for the first time (81). However, until recently, the clinical trials have been carried out with populations of cells that we now know were rendered tolerant or “anergic”, senescent, or immunogenic. The major criticism of the field has been that, until recently, no randomized clinical trials had demonstrated that adoptive T cell transfer approaches were efficacious (Table 2). The adoptive transfer of EBV-specific T cell lines and CTLs for the therapy of EBV-induced lymphomas is perhaps the best demonstration of clinically efficacious adoptive T cell therapy (82, 83). However, the EBV-induced lymphomas that occur in immunosuppressed patients are a “disappearing disease,” as advances in treatment to use a CD20-specific antibody (Rituximab) have drastically reduced the incidence of this once not uncommon disorder. Therefore, in the case of EBV-associated malignancies, a randomized efficacy trial is not likely to occur. To date, there has only been one randomized clinical trial that had a positive outcome; a rigorous intent-to-treat analysis of adoptive transfer trial in cancer has been in the adjuvant setting for hepatocellular carcinoma following surgical resection of the primary tumor (84). In that study, autologous peripheral blood T cells were cultured with CD3-specific antibody and IL-2, and the risk of cancer recurrence was reduced by 41% in the group treated with surgery and a T cell infusion compared with the group treated with surgery only. However, this trial remains unconfirmed, and the mechanism of the antitumor effect remains unknown. It is critical to learn whether a specific antitumor effect or an antiviral response directed to HCV, the agent often implicated in the pathogenesis of hepatocellular carcinoma (85), was involved in the protective effect. It is also conceivable that other effects, such as a reduction in the suppressive effect of Tregs, could have occurred in this trial as well. These are important lessons to learn, as hepatocellular carcinoma is the third leading cause of cancer-related deaths worldwide.
Adoptive transfer therapy for cancer: randomized clinical trials
Dose and scheduling issues. Information on the dose and schedule dependence of adoptively transferred cells is widely scattered in the literature, and from this literature one concludes that there is no standardized dosage system. There is, however, evidence from animal models (in nonlymphopenic hosts) suggesting that multiple doses of adoptively transferred T cells are superior to a single infusion of T cells (86). Doses of adoptively transferred cells are usually reported as the total number of viable cells administered or as the total number of viable cells administered per kilogram of body weight or per square meter of body surface area. However, total endogenous lymphocyte numbers do not correlate well with body surface area but rather display a strong inverse correlation with age. Other variables add to the complexity, particularly the fact that, in the case of T cells or other adoptively transferred cells with high replicative potential, the infused dose might not relate well to the steady-state number of cells that engraft and persist. Therefore, dose considerations are more complex than in other areas of transfusion medicine, where, for example, the maximal level of transfused red cells or platelets occurs immediately following infusion. In our studies of adoptively transferred autologous CD4+ T cells, we often find that the number of cells in the host peaks two weeks after infusion of the cells (87). This is because the engraftment potential and the replicative potential of the infused cells depends on complex host variables such as the number of niches available in the host for engraftment, and the antigenic stimulus for clonal expansion or deletion. In most rodent tumor models, T cell proliferation in the host after transfer is obligatory for therapeutic efficacy (reviewed in ref. 88), and with rare exceptions (89), this is presumed to also be required in humans.
Cytokines given to the host can also have a major impact on the persistence of adoptively transferred T cells. Others have found that the persistence of adoptively transferred human CD8+ T cells is enhanced by coadministration of IL-2 (13). However, we have found that when autologous human CD4+ T and CD8+ T cells are given in combination, persistence is not increased by concomitant IL-2 therapy (49). Finally, recent studies show that IL-2 can induce the proliferation and maintenance of effector CD8+ T cells but might actually deplete memory T cells and increase the number of Tregs (90). By contrast, IL-15 and IL-7 seem to select for the persistence of memory CD8+ T cells and might decrease the ratio of Tregs to effector T cells (91).
Striking schedule-dependent increases in efficacy and the frequency of adverse effects from adoptively transferred cells have been reported when T cell infusions are given to lymphopenic hosts (7). Many studies in rodent tumor models show that the coadministration of cytotoxic therapy can enhance the effects of adoptively transferred cells (92). Cyclophosphamide and/or fludarabine are generally administered to the host several days before the adoptively transferred T cells (7, 88). The drugs have multiple effects that seem to promote the antitumor effects of the adoptively transferred T cells. There is evidence for numerous effects, including killing of host Tregs that suppress antitumor immune responses; creating “space” in the host so that the adoptively transferred T cells can engraft (93); and perhaps enhancing cross-priming of tumor antigens. Curti and colleagues (94), have studied the optimal time to harvest autologous CD4+ T cells in relation to the timing of cyclophosphamide administration in patients with advanced cancers. T cells were harvested at steady state or either when on the decline or recovery from the cyclophosphamide-induced leukopenia, and Curti et al. found the greatest in vivo CD4+ T cell expansion following infusion when cells were harvested as patients entered the cyclophosphamide-induced nadir (94). In a study of patients with stage III non–small cell lung cancer, investigators tested the sequence of adoptive therapy with autologous TIL and IL-2 followed by standard chemotherapy and radiotherapy, and perhaps not surprisingly, they found that immunotherapy followed by chemotherapy was not effective (95); the reverse schedule of therapy was not tested as a concurrent comparison in this trial, however previous randomized trials from this group had demonstrated clinical activity when chemotherapy was followed by immunotherapy (96).
Toxicity issues. Many types of adverse events have been reported following infusion of human autologous or allogeneic lymphocytes. The toxicities can be classified as those that result from extrinsic factors present in the culture process, those resulting from accompanying cytokines that can be co-infused with the cells, and those that result from the cells themselves. The spectrum of the third form of adverse effects is still being defined and for the moment seems to be related to whether the cell product is genetically engineered. For cell products that have not been genetically engineered, the adverse effects are limited and are similar to those observed with therapeutic vaccines. Cytokine release syndrome, retinitis, iritis, hepatitis, autoimmune thyroiditis with hypothyroidism, and vitiligo occur following autologous T cell infusions (7, 14, 61, 97). Respiratory obstruction has been reported following CTL infusion for EBV-related lymphomas (82). This is probably due to a T cell–induced inflammatory response that results in tumor edema and necrosis. Effector functions of infused T cells can be expected to include tissue damage similar to that encountered in T cell–mediated autoimmune diseases. In the case of allogeneic lymphocyte infusions, GVHD and bone marrow aplasia can occur (98). Theoretic toxicities associated with T cell transfer also include leukemia or lymphoma if transformation is induced consequent to the in vitro culture process. However, in human trials involving genetically modified T cells, no cases of malignant transformation of the infused T cells have been reported to date.
Finally, dose- and schedule-dependent effects have been observed with allogeneic T cell infusions vis-à-vis the induction of GVHD. Early studies showed that the infusion of donor T cells soon after a myeloablative transplant conditioning regimen resulted in the marked augmentation of acute GVHD (99). It has been well established by the work of O’Reilly and colleagues that the initial dose of infused T cells in the setting of allogeneic bone marrow transplantation has a major effect on the incidence and severity of acute GVHD (98). However, it has only been recently appreciated that donor T cells can be infused with relative freedom from acute GVHD in the setting of nonmyeloablative stem cell transplantation (100). Studies show that, in the steady-state setting of relapsed chronic myelogenous leukemia following allogeneic HSC transplantation, infusions of resting donor T cells result in a decreased incidence of acute GVHD when given by dose fractionation, starting with low doses of donor cells and escalating subsequent doses as required (101). Some of these effects might be related to recent findings in mice that effector CD8+ T cell function and presumably toxicity are related to concomitant HSC infusion (28).
Tregs. Cancer patients have increased numbers and function of CD4+CD25+ Tregs at the tumor site (8). The in vivo depletion of Tregs enhances the antitumor effects of adoptively transferred effector T cells (102). On the other hand, preclinical models show that the adoptive transfer of Tregs was able to prevent GVHD while preserving graft versus tumor activity (103). Recently, we and others have developed ex vivo culture conditions that should permit pilot trials of Treg adoptive immunotherapy for the prevention or therapy of GVHD (104, 105).
Targeting issues: public versus private antigen controversy. There is controversy in the choice of antigen to target with adoptively transferred T cells. For the past several decades, shared (also known as “public”) tumor-associated antigens have been the favored target of various immunotherapy strategies. This approach has been based largely on melanoma and has been led by a study of the CTLs obtained from a patient with melanoma (106). Most of the antigens targeted by T cells obtained from patients with regressing melanoma had expression that was shared between tumor cells and their normal cell counterparts. Implications from these shared tumor–associated antigens were that, in order to achieve tumor eradication it was necessary to expect tissue-specific toxicity, such as vitiligo in the case of melanoma and prostatitis in the case of prostate cancer. Therefore, the concept of “dispensable tissues” arose (107), meaning that in the case of some tumors, damage or destruction of normal tissue would be an accepted and expected potential toxicity. Because expression of these antigens was also shared between different individuals, the preparation of patient-independent vaccine preparations would be possible. In theory however, patient-specific (also known as “private”) tumor antigens that arise from mutations could also serve as a source of tumor-specific targets. Strategies to target patient- and tumor-specific mutations have been proposed but have not received much attention in the field (108, 109). This situation is likely to change given the striking finding that common tumors such as breast and colon cancer have, on average, about 90 mutations per tumor that generate amino acid substitutions (110), a figure much higher than was previously thought. These findings have major implications for cancer immunotherapy, as a strategy that is directed against patient- and tumor-specific antigens is likely to have fewer off target effects. In addition, it might be possible to generate T cells with much higher avidity for the tumor target, since the TCR repertoire to these putative tumor-specific antigens is not expected to have been subject to editing by thymic tolerance mechanisms. By contrast, strategies targeting shared tumor-associated antigens are hindered by T cell responses against self antigens that are generally of low avidity and susceptible to immunologic tolerance.