Pak2: An Essential Regulator of T Regulatory and Natural Killer T Cell Development and Function

• Main Author: O'Hagan, Kyle Leonard
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2016
• Subjects: Immunology
• ISBN: 1339785811; 9781339785813

T cell development in the thymus selects for several distinct lineages that each possess unique and important functions in immune regulation. While the majority of research has focused on understanding the development and function of conventional CD4+ and CD8+ T cells, less is known about the signaling requirements that regulate the biology of non-conventional T cells, including regulatory T cells (Tregs) and natural killer T (NKT) cells. Recently, we have reported that p21-activated kinase 2 (Pak2) was essential for the proper maturation of conventional CD4 single positive thymocytes, particularly in the transition from a semi-mature to mature state. Herein, we extend these findings to include a role for Pak2 in regulating the development and function of non-conventional T cell subtypes, including Tregs and NKT cells. Loss of Pak2 in T cells resulted in a T cell intrinsic reduction in the numbers of CD25+Foxp3+ Tregs in both the thymus and secondary lymphoid organs, coupled with the onset of spontaneous colitis in Pak2-deficient mice. Mechanistically, loss of Tregs in Pak2-deficient mice was shown to be independent of pro-survival factors, such as Bcl2. Rather, Pak2 was essential for generating the characteristically high-affinity TCR and IL2-mediated signals that are a prerequisite for their lineage commitment in the thymus. Interestingly, Treg-specific deletion of Pak2 resulted in a more severe immunopathology, evident by significant inflammatory cell infiltration in multiple peripheral organs and a 100% mortality rate approximately 6-8 weeks following birth. This immunopathology was caused by a significant inability to control T and B cell activation in the periphery. Indeed, Treg functional assays showed that loss of Pak2 in Tregs impaired the ability to suppress conventional T cell proliferation, in vitro, and the onset of severe wasting disease, in vivo. Loss of Treg suppressive function is predicted to be a result of a loss of the characteristic Treg phenotype in the absence of Pak2, including reduced expression of Foxp3, CD25, CTLA4 and Neuropilin-1, all of which play an important role in regulating Treg function. Furthermore, Treg-specific deletion of Pak2 altered the genetic signature of Foxp3+ Tregs, with Pak2-deficient Tregs adopting a signature that more resembled Th2 cells. Similar to Tregs, loss of Pak2 in T cells resulted in a T cell intrinsic block in the development of NKT cells, particularly from stage II to stage III. Among the unique NKT cell subsets, Pak2 was required for the development and function of NKT1 and NKT2 cells, but not NKT17 cells. While Tregs required Pak2 for optimal TCR signaling leading to Nur77 induction, developing NKT cells exhibited normal TCR signaling in the absence of Pak2. Rather, Pak2 was necessary for proper NKT cell survival as well as the expression of the NKT lineage-specific transcription factor, PLZF. Mechanistically, we showed that, in the absence of Pak2, developing NKT cells reduced expression of the costimulatory receptor, SLAM6, which has been shown to be required for proper PLZF induction. Collectively, these findings highlight previously unidentified roles for Pak2 in Treg and NKT cell development and function. It is hoped that further investigation might help us better understand the signaling mechanisms transduced via Pak2 for proper Treg and NKT cell lineage commitment.