The programmed cell death-1 (PD-1) receptor is a key regulator of T cell activation and cytokine production. Multiple studies performed in PD-1-deficient mice demonstrate its importance in preventing autoimmunity. Evidence suggests that PD-1- mediated regulation is reduced during chronic inflammation in human diseases, such as rheumatoid arthritis (RA). In this thesis, the role of inflammation in influencing PD-1-mediated regulation of human CD4+ T cells is further characterised. First, PD-1 and PD-L1 expression, as well as the functional consequences of PD-1 ligation in rheumatoid (RA) and psoriatic arthritis (PsA) were analysed. Using flow cytometry and analysis of existing gene expression arrays, it was determined that the percentage of PD-1+ cells within the CD4+ and CD8+ T cells compartment was increased in RA and PsA synovial fluid (SF) compared to paired peripheral blood (PB). Upon in vitro T cell receptor (TCR) stimulation of HC CD4+ T cells in the presence of plate-bound PD-L1fc chimera, significantly decreased proliferation and interferon (IFN)-γ secretion was observed. In contrast, RA and PsA PB- and SF-derived CD4+ T cells appeared resistant to such PD-1-mediated inhibition. Second, it was investigated whether proinflammatory cytokines modulate PD-1-mediated regulation of healthy CD4+ T cells (HC). Addition of proinflammatory cytokines tumour necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β, which were increased in RA and PsA SF compared to HC and osteoarthritis (OA) controls, consistently abrogated PD-1- mediated suppression in HC CD4+ T cell cultures. Inhibitors of these cytokines reversed this effect. Finally, it was evaluated whether soluble PD-1 (sPD-1) negatively regulates the PD1/PD-L1 interaction. Soluble PD-1 (sPD-1) levels were increased in cell culture supernatants from TNFα and IL-6-stimulated cultures compared to untreated controls, and also in RA and PsA, but not in OA, serum and SF nor in HC serum. qPCR analysis of HC CD4+ T cells from TNFα- and IL-6-stimulated cultures also revealed increases of the PD-1Δex3 splice variant. Functionally, addition of sPD- 1fc counteracted PD-1-mediated suppression of HC CD4+ T cells, increased T cell proliferation in HC CD4+ T cell/monocyte co-cultures but had no effect on HC CD4+ Treg cell-mediated suppression. Together, the data presented in this thesis provide new evidence that the inflammatory environment of the RA and PsA joint compromises PD-1/PD-L1 mediated T cell regulation.
Inflammatory cytokines compromise programmed cell death-1 (PD-1)-mediated T cell suppression in inflammatory arthritis through up-regulation of soluble PD-1.
Bommarito, D. (Author). 2018
Student thesis: Doctoral Thesis › Doctor of Philosophy