The activation of cytotoxic T lymphocyte (CTL) normally involves an initial expansion of antigen-specific CTL clones and their acquisition of cytotoxic activity. Subsequently, the activated CTL population undergoes activation induced cell death (AICD), resulting in final stabilization of a small antigen-experienced CTL population. This coupling of CTL activation and AICD ensures the delicate CTL homeostasis. However, the rapid clearance of activated CTL curbs the usage of CTL in anti-virus and anti-cancer therapies.
We explored factors that contribute to the long-term survival of CTL and their efficacies in cancer therapies using two model systems. First, we studied the potential regulators of human CTL survival in the context of a rare leukemia, T-cell large granular lymphocyte (T-LGL) leukemia. T-LGL leukemia features a clonal expansion of antigen-primed, competent CTL. In order to understand the pathogenesis of T-LGL leukemia and key mediators that determine CTL survival, we constructed a signaling network with special interests in two known deregulated pathways in T-LGL leukemia: Ras pathway and Fas/FasL pathway. Surprisingly, we found that all known deregulations in T-LGL leukemia can be connected in a single signaling network. Subsequent network simplification revealed that most of the deregulated components were connected in the CTL activation-AICD signaling process.
To understand the survival mechanisms of leukemic T-LGL in the context of CTL activation-AICD, we constructed a T-LGL survival signaling network by integrating the signaling pathways involved in normal CTL activation and the known deregulations of survival signaling in leukemic T-LGL. This network was subsequently translated into a predictive discrete dynamic model. Our model suggests that the persistence of interleukin (IL)-15 and platelet derived growth factor (PDGF) is sufficient to reproduce all known deregulations in leukemic T-LGL. This finding leads to the following predictions: (1) Inhibiting PDGF signaling induces apoptosis in leukemic T-LGL. (2) Sphingosine kinase 1 (SPHK1) and nuclear factor kappa-B (NFκB) are essential for the long-term survival of CTL in T-LGL leukemia. (3) NFκB functions downstream of phosphoinositide-3-kinase (PI3K) and prevents apoptosis through maintaining the expression of myeloid cell leukemia sequence 1 (Mcl-1). (4) T-box expressed in T-cells (T-bet) should be constitutively activated concurrently with NFκB activation to reproduce the leukemic T-LGL phenotype. We validated these predictions experimentally. Our study provides the first model describing the signaling networks involved in maintaining the long term survival of competent CTL in human.
In the second model system, we studied the potential relationships between recognition of tumor-associated antigen (TAA), survival of tumor-specific CTL, and efficacy of tumor immunotherapy through modeling the immune responses against simian virus 40 (SV40) large T antigen (Tag) in mice. Tag is a potent virus-encoded oncoprotein that can transform a variety of cell types. Transgenic mice expressing Tag under different promoters autonomously develop various tumors. Furthermore, Tag is capable to mount MHC I-restricted CTL activity similar to TAA. Tag can be processed into four different epitopes, which ignite CTL with different survival behaviors and tumor control activities. We integrated CTL response data obtained from both wild type mice and Tag-transgenic mice tumor models (including pancreatic cancer, osteosarcoma and choroid plexus tumor) into a unifying mathematical model. This model reproduced most of experimental observations. Our model predicted that prolonged survival rather than increased proliferation contributed to the different immune responses of CTL specific for different epitopes. We experimentally validated this prediction. This study offers a novel strategy though which different CTL parameters and their effects on successful cancer immunotherapy can be quantitatively analyzed in silico.
Taken together, these studies initiated our understanding of CTL long-term survival and its application in cancer immunotherapy at a systematic level. This knowledge is important not only for identifying therapeutic targets of rare hematological disorders involving mature CTL, but also for generating long-lived CTL necessary for virus and tumor vaccine development. Methodologically, this systems biology approach greatly facilitated information integration, hypothesis formation and experiment design, which would otherwise be inaccessible to intuitive logic deduction.