Priming and tracking the virus-specific T cell response

CD4 and CD8 T cells play a vital role in mediating the clearance of viral pathogens following infection. Mice deficient- or depleted of their CD4 and/or CD8 T cells exhibit a diminished ability to control viral replication following infection and in some cases develop a persistent viral infection. CD8 T cells upregulate cytotoxic effector molecules such as granzyme B, Fas and TNF-related apoptosis-inducing ligand (TRAIL) that them to directly kill virus-infected cells. Following a systemic virus infection the CD8 T cell response is primed within secondary lymphoid organs, such as the spleen and lymph nodes (LNs). Although, it has been shown that the LNs are important for the generation of optimal CD8 T cell responses following systemic viral infections, the relative role of the spleen versus the LN in priming the CD8 T cell response is unknown. Studies in this thesis demonstrate that LNs, but not the spleen, are critical for the optimal generation of a CD8 T cell response following a systemic intraperitoneal (i.p.) lymphocytic choriomeningitis virus (LCMV) infection. Using adoptively transferred naïve LCMV-specific CD8 T cells, we demonstrate that the mediastinal LN (MedLN) serves as the initial draining LN and is responsible for priming the majority of the virus-specific CD8 T cell response following an i.p. LCMV infection. Moreover, the draining MedLN exhibits an increased frequency of CD62L^- effector memory (T_EM) CD8 T cells for up to 8 weeks following viral clearance. I demonstrate that the increased frequency of CD62L^- T_EM CD8 T cells is not due to residual viral antigen. Furthermore, a similar increase in CD62L^- T_EM CD8 T cells is found in the ipsilateral popliteal LN following a footpad LCMV infection. I demonstrate that the increased frequency of CD62L^- T_EM CD8 T cells in the draining LN is due to increased recruitment.

CD4 T cells promote the generation of both effector and memory CD8 T cells either indirectly through their CD40-CD40L-dependent maturation of dendritic cells or through the production of cytokines such as IL-2 and IFN-γ that directly interact with CD8 T cells. CD4 T cells are also critical for the generation of germinal center B cells and promote the differentiation of activated B cells into memory B cells and plasma B cells. However, CD4 T cells often recognize epitopes derived from a broad array of pathogen-encoded proteins, making it difficult to accurately quantify the magnitude of virus-specific CD4 T cell responses. Therefore, I evaluated a large panel of activation and/or memory markers to determine a combination that could be used to reliably identify antigen-specific CD4 T cells following viral infection. I show that the integrins CD11a and CD49d are upregulated in an antigen-dependent manner on virus-specific CD4 T cells following LCMV infection. Furthermore, memory LCMV-specific CD4 T cells retain their CD11a^hiCD49d⁺ expression pattern. Using CD11a and CD49d as surrogate makers for antigen-specific CD4 T cells, I show that approximately 50% of the CD4 T cells following LCMV infection are virus-specific, indicating that the virus-specific CD4 T cell response is substantially larger than previously recognized. Furthermore, I demonstrate that CD11a and CD49d can be used to accurately track newly-activated CD4 T cells following a heterologous virus challenge.

In addition to LCMV, respiratory syncytial virus (RSV)-specific CD4 T cells are CD11a^hiCD49d⁺. The two previously identified RSV CD4 T cell epitopes only account for ~3% of the CD11a^hiCD49d⁺ CD4 T cell population during the peak of RSV infection, indicating that additional RSV-derived epitopes remain to be identified. Therefore, I used an overlapping peptide library spanning each of the RSV-derived proteins to identify novel RSV-specific CD4 and CD8 T cell epitopes. Using this approach, I identified 5 novel RSV-derived CD4 T cell epitopes and 4 novel CD8 T cell epitopes. Furthermore, I demonstrate that stimulation of CD4 T cells with 17-mer peptides results in over a 2-fold increase in the frequency of responding CD4 T cells as compared to stimulation with the commonly used 15-mer peptides. Collectively, the data shown here provides new insight into where and how the CD8 T cell response is initiated following a systemic virus infection, as well as provide a novel approach to track the endogenous CD4 T cell response following viral infections.