Mechanisms that Regulate the CD8+ T cell Response to SV40 T Antigen
Open Access
- Author:
- Tatum, Angela Megin
- Graduate Program:
- Microbiology and Immunology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 25, 2008
- Committee Members:
- Todd Schell, Dissertation Advisor/Co-Advisor
Todd Schell, Committee Chair/Co-Chair
Christopher Charles Norbury, Committee Member
David Joseph Spector, Committee Member
Kristin Ann Eckert, Committee Member - Keywords:
- SV40
T antigen
immunodominance
CD8+ T cells
tumor
immunotherapy - Abstract:
- As a component of the cellular immune response, CD8+ T cells serve an important function by defending the host against intracellular pathogens. In addition to targeting infected cells within the host, CD8+ T cells are capable of eradicating tumors. CD8+ T cells respond to antigen by recognizing distinct peptide epitopes presented on the surface of infected cells or cells specialized for antigen presentation. Often a CD8+ T cell response can be directed towards multiple epitopes. During such a response, a phenomenon termed “immunodominance” can occur in which a particular epitope is targeted by a larger frequency of CD8+ T cells compared to the other epitopes. Immunodominance is observed in a variety of systems including bacterial, viral and transplantation antigens as well as tumor antigens. Understanding the mechanisms that regulate the size and complexity of the CD8+ T cell response is important for developing CD8+ T cell-based immunotherapeutic approaches. SV40 T antigen (Tag) is a virus-derived oncoprotein and tumor antigen that lends itself to the study of both immunodominance and tumor immunotherapy. Expression of Tag as a transgene in mice promotes the development of tumors from specifically targeted tissues, providing a model of spontaneous cancer. Four well-defined epitopes recognized by CD8+ T cells can be targeted in Tag, designated as sites I, II/III, IV and V. Following immunization of C57BL/6 mice with Tag-expressing cells, a hierarchal CD8+ T cell response is induced in which epitope IV is dominant, and epitopes I and II/III are subdominant. Using the SV40 Tag model of CD8+ T cell immunity, the following two questions were addressed in this study: 1) What mechanisms regulate the size and complexity of the CD8+ T cell response to SV40 Tag? 2) How does targeting the dominant epitope of Tag lead to successful tumor immunotherapy? Previous studies that described the immunodominance hierarchy of the Tag-specific CD8+ T cell response relied on data obtained at the peak of the response (day 10), leaving unknown how quickly immunodominance to Tag is established following immunization. In the current study, we determined both the kinetics of immunodominance and mechanisms that regulate the complexity and magnitude of the CD8+ T cell response toward the two most dominant Tag epitopes, I and IV. Both the endogenous T cells as well as T cell receptor transgenic T cells specific for epitopes I and IV were used for this analysis. We found that dominance toward epitope IV is established by 3 days post-immunization and maintained throughout the response. As indicated by experiments that quantitate the number of naïve epitope-specific CD8+ T cells per mouse, this result may be explained by a higher number of naïve precursors specific for epitope IV relative to epitope I. By increasing the number of naïve precursor T cells specific for epitope I the immunodominance hierarchy was reversed, indicating that the number of precursors present at the time of immunization dramatically influences the relative magnitude of the response. The role of CD8+ T cell competition in determining the immunodominance hierarchy also was investigated, but was found to play only a minor role in limiting the ability of Tag-specific CD8+ T cells to accumulate to their maximum levels. Lastly, we found that direct presentation of the Tag epitopes by Tag-expressing cells promotes optimal accumulation of Tag-specific CD8+ T cells compared to when only cross-presented Tag is available, suggesting that direct presentation by cell-based vaccines can enhance the overall efficiency of immunization. In a second focus, we examined whether CD8+ T cells targeting a single dominant epitope could lead to effective tumor immunotherapy of established Tag-induced brain tumors. Line SV11 mice succumb to Tag-induced tumor burden at approximately 105 days of age. Previous evidence suggested the importance of epitope IV-specific CD8+ T cells in the control of tumor progression in these mice. Therefore, we examined whether epitope IV-specific CD8+ T cells were necessary and sufficient for the control of tumor progression in SV11 mice. The adoptive transfer of a donor population devoid of epitope IV-specific CD8+ T cells demonstrated the need for epitope IV-specific CD8+ T cells in the control of tumors in SV11 mice. Epitope IV-specific T cell receptor transgenic mice (TCR-IV) were used to determine whether epitope IV-specific CD8+ T cells alone could control tumor burden in SV11 mice. Purified TCR-IV T cells were able to control tumor burden in combination with conditioning whole body irradiation, but were dramatically less effective in the absence of irradiation. Although we observed no difference in the frequency of epitope IV-specific CD8+ T cells that accumulated in the brains of SV11 mice 10 days following adoptive transfer, epitope IV-specific CD8+ T cells were detected much earlier, by day 5, in the brains of SV11 mice that received irradiation than those receiving only adoptive transfer. The combination of adoptive T cell transfer and conditioning irradiation resulted in the complete regression of established brain tumors by 10 days post adoptive transfer and was accompanied by long-term persistence of epitope IV-specific CD8+ T cells in the brain of SV11 mice. Finally, using gene knockout donor lymphocytes, we found that interferon gamma (IFNγ), a cytokine with anti-tumor properties, was a necessary component of the donor cells to control tumor progression. These studies indicate that epitope IV-specific CD8+ T cells are both necessary and sufficient to control tumor burden in SV11 mice, but require preconditioning of the host with irradiation to promote complete tumor regression. The results suggest that both the magnitude of T cell infiltration at early time points and production of IFNγ are critical to achieve long-term tumor-free survival.