Efforts to develop immunotherapies for cancer have historically involved vaccines and individual cytokines, which have produced benefits to only a small percentage of patients. Recently, improved understanding of immune processes, such as the role of T-cell costimulatory molecules and regulatory molecules, has led to renewed efforts to develop immunotherapies to treat cancer. Current clinical trials are utilizing monoclonal antibodies (mAbs) to modulate costimulatory effects in the treatment of a variety of cancers. For example, two fully humanized mAbs that block cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) have advanced to phase III clinical studies in metastatic melanoma. Antitumor activity has been observed in melanoma and other malignancies after treatment with anti-CTLA-4 antibodies, as well as the potential for autoimmune-related toxicities.
Rationale for Targeting CTLA-4 in Patients with Cancer
As described in Chapter 3, B7-1 and B7-2 proteins on antigen presenting cells serve as ligands for both CD28 receptors and CTLA-4 inhibitory molecules on T-cells. In association with T-cell receptor binding of antigen presented on major histocompatibility complexes, ligation of CD28 provides "costimulation," whereas ligation of CTLA-4 receptors represents "coinhibition". CTLA-4 mediates T-cell inhibition through competition with CD28 for B7 and through specific inhibitory signaling.1,2 In addition to inhibiting effector T-cell activation and proliferation, CTLA-4 engagement also results in downregulation of interleukin-2 (IL-2) gene transcription. This immunologic "checkpoint" limits T-cell activation to areas where there is inflammation or injury.3
CTLA-4 is also present on regulatory T-cells, but it is not clear whether CTLA-4 signaling is associated with the regulatory function of these cells or contributes to their development and maintenance.4 One theory holds that CTLA-4 on regulatory T-cells may produce reverse stop signaling, which prevents these cells from exerting their inhibitory effects on antigen presenting cells. CTLA-4 may reverse signal through B7 costimulatory molecules, which leads to increased dendritic cell expression of indoleamine 2,3-dioxygenase (IDO). IDO-expressing dendritic cells can potently suppress cytotoxic T-cells, providing another pathway by which CTLA-4 engagement can inhibit T-cells.5
CTLA-4 may be one mediator of the peripheral tolerance that protects normal tissue and tumor cells against adaptive immune responses. Tumor-associated antigens are typically self-antigens that are overexpressed on tumor cells and provide a weak signal for T-cells.6 CTLA-4 may inhibit T-cell activation in the presence of tumor or attenuate an active immune response to tumor antigens. CTLA-4 may also be expressed by tumor cells, allowing them to directly suppress T-cells.5 Improved understanding of the functions of CTLA-4 led to the hypothesis that blocking its engagement could result in unopposed CD28 activation of T-cells coupled with suppression or depletion of regulatory cells. In essence, blockade of CTLA-4 leads to "taking the brakes off" the immune system.
Listen as Dr. Weber interviews Dr. Hodi about the functions of CTLA-4 and the precise mechanisms of action of these products.
Audio Interview Clip
Preclinical Animal Models
The potential antitumor effects of CTLA-4 blockade are suggested by experiments in animal tumor models and transgenic mice. CTLA-4âdeficient mice develop a lethal lymphoproliferative disorder,7,8 demonstrating the key function of CTLA-4 in balancing the immune system. The administration of CTLA-4âblocking antibodies resulted in the regression of several highly immunogenic tumor types in mice, including colon carcinoma 51BLim10 and fibrosarcoma Sa1N tumors,9 as well as transgenic adenocarcinoma mouse prostate (TRAMP)C1 cancers.10
CTLA-4 antibody blockade alone has had minimal effects in other mouse tumors, however, including the poorly immunogenic B16 melanoma11 and SM1 breast cancer.12 However, the combination of CTLA-4 blockade and vaccination with GVAX (irradiated tumor cells engineered to secrete cytokine granulocyte-macrophage colony-stimulating factor [GM-CSF]) resulted in significant tumor regression in B16 melanoma,11 TRAMP,13 and SM1 breast carcinoma.12 Experiments using one animal model suggested that CTLA-4 blockade alone may potentiate both regulatory T-cells and effector T-cells, whereas CTLA-4 blockade coupled with GM-CSF shifted the ratio to favor effector T-cells over regulatory cells.14 Another murine study indicated that response to this combination therapy was associated with increased levels of cytotoxic T-cells specific for an epitope of the melanosomal protein of tyrosinase-related protein 2, whereas no reactivity was seen to gp100, tyrosinase, Mart1/Melan A, or tyrosinase-related protein 2.15
While the combination of CTLA-4 blockade and GM-CSF tumor cell vaccines showed synergistic activity in these animal models, some of those mice also lost tolerance to normal tissues. For example, treated mice bearing the B16 melanoma model developed loss of fur pigmentation or vitiligo,11 and mice in the prostate model showed evidence of immune responses to normal prostate epithelium.13 While similar antitumor benefit with this combination was observed in the SM1 breast cancer model, there was less evidence of immune reactivity to normal tissues.12 This suggests that effective antitumor immune responses can be attained in some cases without the loss of tolerance, although autoimmune manifestations often accompany antitumor effects.
In preclinical investigations, enhanced therapeutic benefit was observed when CTLA-4 blockade was combined with:
These encouraging data supported the conduct of anti-CTLA-4 antibody trials in patients with multiple tumor types.