CD8+ T cells are the branch of the adaptive immune system responsible for recognizing and killing tumor cells or cells infected with intracellular pathogens, such as Listeria monocytogenes (LM). However, when CD8+ T cells target our own tissues, they can cause autoimmune diseases, such as type I diabetes, rheumatoid arthritis. For CD8+ T cells to fulfill these functions, the T cell receptors (TCRs) on CD8+ T cells must recognize pathogens or antigens presented on the surface of target cells. TCR ligation triggers multiple signaling pathways that lead to the activation and proliferation of CD8+ T cells. The goal of our research is to define the TCR-proximal signaling events that regulate CD8+ T cell-mediated immunity. To accomplish this goal, we are focusing on an adaptor protein Gads, which is critical for optimal TCR-mediated calcium mobilization. We reported the first analysis of the function of Gads in peripheral naïve CD8+ T cells.
To examine the function of Gads in CD8+ T cell mediated immune responses, we crossed Gads-/- mice with mice expressing an MHC class I-restricted transgenic TCR recognizing ovalbumin (OVA). The transgenic mice are called ovalbumin-specific T cell receptor-major histocompatibility complex class I restricted (OT-I) mice. We investigated the effect of Gads on the proliferation of CD8+ T cells following stimulation with peptide antigen in vivo and in vitro. We stimulated splenocytes from Gads+/+ OT-I and Gads -/- OT-I mice with the peptide agonist. The experiments revealed that Gads is required for optimal proliferation of CD8+ T cells. The regulation of Gads is most evident at the early time points of proliferation. Then we demonstrated that Gads-/- CD8+ T cells have impaired TCR-mediated exit from G0 phase of the cell cycle. In addition, Gads-/- CD8+ T cells have delayed expression of c-myc and the activation markers CD69 and CD25, upon stimulation with peptide antigen.
Next, we investigated how Gads affects CD8+ T cell-mediated immunity in the context of infection with LM. We adoptively transferred naïve CD8+ T cells from Gads+/+ OT-I mice and/or Gads -/- OT-I mice into congenic wild-type hosts. Then the recipient mice were infected with recombinant LM expressing ovalbumin (rLM-OVA). The CD8 + T cells from OT-I mice recognize and respond to the ovalbumin provided by this strain of LM. By using this system, we investigated how Gads regulates the activation of antigen-specific CD8+ T cells as well as the expansion and memory phases of CD8+ T cell-mediated immune responses following infection with rLM-OVA. We also examined the recall response of CD8+ T cells after the secondary encounter with the same pathogen. Our data demonstrated that Gads regulates the expression of activation markers CD69 and CD25 of antigen-specific CD8+ T cells but Gads is not required for the onset of accumulation of antigen-specific CD8 + T cells following infection. However, Gads is critical to sustain the expansion of CD8+ T cell-mediated immune response following infection. Although the differentiation of naïve CD8+ T cells into memory cells is independent of Gads, Gads is required for an optimal recall response.
Our data indicating that Gads regulates the initiation of proliferation of CD8+ T cells upon TCR ligation by peptide antigen seemed to contradict with our in vivo infection data showing that Gads is not required for the initiation of expansion of CD8+ T cell population. In order to explain the “discrepancy”, we hypothesized that the homotypic interactions among CD8+ T cells compensate for Gads deficiency at the initial stage of accumulation of antigen-specific CD8+ T cells upon infection. Our data indicated that the need for Gads in cell cycle progression of CD8+ T cells when total splenocytes were stimulated could be overcome by stimulating purified CD8 + T cells. These data suggested that the homotypic interactions among CD8+ T cells facilitate the TCR signaling so as to compensate for Gads deficiency in promoting cell cycle entry and proliferation.
To conclude, the role of Gads in TCR-mediated activation and proliferation of CD8+ T cells is dependent on the interactions of CD8 + T cells and their partners. Interestingly, if CD8+ T cells interact with non-CD8+ T cells, Gads regulates the kinetics of cell cycle entry; however, if CD8+ T cells interact with other CD8+ T cells, Gads is dispensable for cell cycle entry of CD8+ T cells. Overall, these studies will help us better understand how TCR-proximal signaling regulates the activation of CD8 + T cells.