Date of Award

2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Microbiology and Immunology

Abstract

Achieving immune homeostasis requires the functions and numbers of suppressive regulatory T cells (Tregs) and effector conventional T cells (conv T) to be balanced and precisely controlled. A decrease in Treg numbers is associated with autoimmune diseases such as type I diabetes and multiple sclerosis, while Treg numbers are increased in the tumor microenvironment. Conversely, an increase in conv inflammatory T cells in the tumor microenvironment is associated with better outcomes for cancer patients. Thus, maintaining the appropriate balance between Treg and conventional T cell populations is essential for controlling immune responses against non-self and self-antigens, as well as tumor antigens. Signaling mechanisms that promote the balance between regulatory and conv T cell populations remain largely unknown. One mechanism that could help control the balance between Treg and conv T cell populations is activation induced cell death (AICD). This type of apoptosis occurs when either Tregs or conv T cells undergo repeated stimulation in the presence of the T cell growth factor IL-2. If conv T cells undergo extensive AICD, then Tregs would dominate, and vice versa: if Treg cells undergo extensive AICD, then conv T cells would dominate. Our goal is to determine if conv T cells and Treg cells have different signaling mechanisms that lead to AICD.We previously discovered a novel form of AICD that occurs in vitro with plate bound stimulation of T cells with anti-CD3 and anti-CD28 coated antibodies. Under these conditions, mouse Foxp3+ Tregs expand while Foxp3- conv T cells undergo massive apoptosis, indicating that these cells respond differently to anti-CD3/anti-CD28 stimulation. Unlike classical AICD, this form of apoptosis depends on p53, and thus we termed the process PICA (p53-induced CD28-dependent apoptosis). We wanted to understand why Tregs could expand and survive, while conv T cells died. We found that the expansion and survival of Tregs requires autocrine TGF-β signaling. Conversely, addition of TGF-β could promote conv T cell survival. Our work has identified that Tregs and conventional T cells have differences in the MAPK/ERK signaling pathways, which controls survival after stimulation. Tregs maintain low RasGRP1 expression by autocrine TGF-β signaling, and RasGRP1 expression in conventional T cells is suppressed by TGF-β. Furthermore, we found that RasGRP1 signaling is required for conv T cell apoptosis during PICA. For TGF-β to promote survival of conv CD4 T cells, we found that the transcription factor FoxO3 is critical. Finally, we studied AICD in a more physiological manner in CD19-specific chimeric antigen receptor (CAR) T cells. We found that CAR T cells undergo AICD in vitro when stimulated with large numbers of tumor antigens and the surviving CAR T cells are CD25+Foxp3+. Like PICA, addition of TGF-β can promote CAR T cell survival, but markers of CAR functionality are reduced. Taken together, we have identified distinct signaling pathways that promote AICD in T cells and determined mechanisms by which TGF-β controls survival. We propose that the crosstalk between the RasGRP1 and FoxO3 pathways with TGF-β signaling are critical for T cell survival. Results from this study could inform how conventional and Treg populations survive and undergo apoptosis during AICD in memory formation and the tumor microenvironment, among other disease states.

Creative Commons License

Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.

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