T Cell Activation
Summary: James Allison's research involves studies of the mechanisms regulating immunological responses mediated by T lymphocytes and the manipulation of these responses to develop novel strategies for tumor immunotherapy.
T lymphocytes play a major role in immune responses, performing both regulatory and direct effector functions. T cells recognize foreign antigens in the form of small peptides displayed on the surface of cells bound to self-proteins encoded by the major histocompatibility complex (MHC). This recognition is mediated by the T cell antigen receptor (TCR), which is generated by random rearrangements of gene segments in a clonal manner that can make trillions of different receptors. The functional repertoire of antigen receptors is shaped from this astronomical potential by a complex process of positive and negative selection of individual T cell clones in the thymus gland. Clones with reactivity toward self-antigens are eliminated, and those capable of recognizing foreign antigens in the context of self-MHC are preserved.
This process is quite efficient in eliminating T cells reactive with peptides derived from generally expressed housekeeping gene products. However, other mechanisms of inducing immunological tolerance must exist for tissue-specific antigens not expressed in the thymus. T cell clones surviving the selection process emigrate from the thymus to the periphery, where they seek out and respond to cells that express foreign peptides derived from infecting viruses or bacteria, or new antigens that are expressed by tumor cells.
Recognition of foreign antigen by the TCR is not, however, sufficient for activation of naïve T cells; a second signal is also required. This costimulatory signal is not related to antigen but is provided by engagement of the T cell surface molecule CD28 by costimulatory ligands of the B7 family in the antigen-presenting cell (APC). B7 molecules are not found on many cell types. Their expression is limited to "professional" APCs of the hematopoeitic lineage: dendritic cells, macrophages, and activated B cells. This requirement for recognition of both antigen and costimulatory ligands means that only these professional APCs will be capable of initiating T cell responses. This is an important mechanism of maintaining self-tolerance in the periphery.
Recently it has become apparent that T cell costimulation is even more complicated. In addition to CD28, its homolog CTLA-4 is also involved in the process. Like CD28, CTLA-4 also binds B7, but there are key differences between the two molecules. One main difference is cellular localization. CD28 is constitutively expressed on the T cell surface; CTLA-4 is induced only after activation, and even then is sequestered in intracellular vesicles. However, TCR engagement results in the rapid movement of both CD28 and CTLA-4 into the region of the T cell that contacts the APC. The other difference is that while CD28 costimulates T cell activation, CDLA-4 is a major down-regulator of T cell responses.
We have proposed a dynamic integration model for T cell activation where the outcome of a T cell's encounter with antigen depends on the level of B7 expressed by the APC, the strength of the antigen receptor signal, and the level of CTLA-4 expression by the T cell. Our results have shown that CTLA-4 can play two different roles in regulating T cell responses and contributing to maintenance of peripheral T cell tolerance. The first is prevention of initiation of T cell response under noninflammatory conditions where expression of the B7 ligands is limited. In these situations, CTLA-4 may prevent full activation of cells by raising the threshold of costimulation needed to obtain full activation of naïve T cells reactive with self-antigens.
Recent results also show that CTLA-4 can limit responses of activated T cells by restricting their proliferation in a manner that is directly correlated with the strength of the antigen receptor signal. This suggests a previously unrecognized biological role for CTLA-4 in contributing to a broadening of the repertoire of T cells in the early stages of an immune response by partially restricting the expansion of the best-responding cells while allowing T cells with weaker responses to be represented in the pool. This CTLA-4 role has important implications for the regulation of T cell responses to antigens derived from pathogenic viruses and bacteria as well as for self-antigens that might be involved in autoimmune diseases such as diabetes or multiple sclerosis.
The direct correlation between CTLA-4–mediated inhibition of T cell proliferation and TCR signal strength is reflected in the molecular mechanisms of its mobilization and function. We have observed a direct correlation between the TCR signal strength and the translocation of CTLA-4 into the immunological synapse that forms between the T cell and the APC with the strength of the TCR signals. Efforts are under way to dissect the molecular mechanisms involved in this translocation and signal transduction by CTLA-4.
These basic studies suggest ways in which manipulation of the inhibitory signals mediated by CTLA-4 might be used in the treatment of autoimmune disease and in tumor immunotherapy. We have shown in several settings of experimental autoimmune diseases in the mouse, including diabetes and experimental autoimmune encephalomyelitis (EAE), that blockade of CTLA-4 signals can greatly exacerbate disease. In addition, we have recently found that blockade of CTLA-4 signals allows the induction of EAE in mouse strains that are normally resistant to it. This blockade enhances expansion of autoreactive cells to a point that results in the clinical symptoms of autoimmunity, an observation that has important implications for the treatment of autoimmune disease. We are developing new strategies for the intentional delivery of the inhibitory signals of CTLA-4 to ameliorate the effects of self-reactive T cells.
We have also shown that we can use CTLA-4 blockade to greatly enhance antitumor T cell responses. We have previously shown that short-term treatment with antibodies to CTLA-4 can lead to rejection of established transplanted tumors in several mouse model systems, including colon carcinoma, fibrosarcoma, melanoma, renal carcinoma, and prostatic carcinoma. We have also found that in several cases where CTLA-4 blockade by itself was not effective, when it was combined with genetically engineered tumor cell vaccines we could achieve rejection of established tumors. Most notable was the successful treatment of a highly metastatic melanoma. Similarly, we have shown that a combination of CTLA-4 blockade and a tumor cell vaccine can significantly delay the appearance and diminish the severity of primary tumors in a transgenic model of prostate cancer. Our success in treating cancer in the animal models has led to the development of antibodies to human CTLA-4 and the initiation of clinical trials to evaluate the potential for CTLA-4 blockade in the treatment of human cancer.
An antibody to human CTLA-4, ipilimumab, has been used in clinical trials in more than 4,000 patients with a variety of types of cancer. Objective responses have been observed in metastatic melanoma, castrate-resistant prostate cancer, renal cancer, lung cancer, and ovarian cancer. Recent results of a randomized blinded phase III trial with metastatic melanoma showed a significant increase in the survival of patients, with 25 percent alive 4 years after initiation of treatment. This is the first drug to show prolongation of life in a blinded randomized trial of metastatic melanoma.
A few years ago, we identified another member of the CD28/B7 family, a molecule called B7-x. This medical inhibitory molecule is expressed in basically normal tissues and is up-regulated in many cancers. In prostate cancer, we have shown that high levels of expression of B7-H4 and the closely related molecule B7-H3 correlate with poor clinical outcome, including metastases and death. We are now working on strategies to block the negative effects of these molecules on the immune response. Recently we have begun to study the new modalities and combinations of checkpoint blockade using anti-CTLA-4. Some of the new targeted therapies, such as tyrosine kinase inhibitors, have shown high rates of initial responses, but these are almost always of short duration. By combining these with anti-CTLA-4 in animal models, we have demonstrated synergy that leads to tumor rejection that is extremely durable. It is our belief that a combination of immune checkpoint blockade with the new targeted therapies will be a powerful treatment for cancer.
This work was supported in part by grants from the National Cancer Institute and the Prostate Cancer Foundation.
As of May 30, 2012