PD-1 Expression on CD8+CD28- T cells within inflammatory synovium is associated with Relapse: A cohort of Rheumatoid Arthritis

Highlights

• Different CD8 + T cell immunophenotypes were observed in early and established RA states.

• The higher frequency of CD8+CD28- T cells was observed in PB vs. SF of relapsed patients.

• Relapsed patients also showed higher CXCR3 and especially PD-1 expression on their CD8+CD28- T cells.

• Higher level of PD-1+CD8+CD28- T cell at inflammatory synovium may be associated with RA relapse.

• IFN-γ elevation in the RA synovium probably poses a key role in disease initiation rather than progression.

Abstract

Defect in T lymphocyte homeostasis could implicate initiation and development of rheumatoid arthritis (RA). Since PD-1 plays a key role in the regulation of T lymphocytes, its expression pattern in various CD8+ T cell subsets could be so effective in RA pathogenesis. Here, we investigated the expression of PD-1 and CXCR3 on CD8+CD28- T cells in association with the IFN-γ levels in patients with RA. A total of 42 RA patients, including 10 newly-diagnosed (ND) and 32 relapsed (RL) cases and also 20 healthy donors were enrolled. Phenotypic characterization of CD8+ T cells derived from peripheral blood (PB) and synovial fluid (SF) was performed by flow cytometry. The plasma and SF IFN-γ levels were also assessed by ELISA. The frequency of CD8+CD28- T cells showed no significant differences between patients and controls while its higher levels were observed in PB, versus SF of RL patients. Relapsed patients also showed higher CXCR3 and especially PD-1 expression on their CD8+CD28- T cells. The IFN-γ concentration was elevated in SF of ND patients while its plasma level was significantly lower in RL subgroup than controls. Although PD-1 could induce immune suppression in effector T cells, it is upregulated during inflammation and its overexpression on CD8+CD28- T cells within inflammatory synovium is associated with severity of disease in our cohort of RA patients.

Introduction

Rheumatoid arthritis (RA) is a systemic and chronic autoimmune disease (1), characterized by inflammation of synovium, resulting in cartilage and bone damage, and joint destruction (2). Despite the complex pathogenesis of RA, dysregulation of lymphocyte activation is involved in the onset and progression of the disease (3). Numerous studies have observed the accumulation of T cells within the inflamed joints and their participation in RA pathogenesis (4). The antigen-specific activation of T cells could possibly result in the breakdown of tolerance, synovial inflammation, and autoantibody production (5). The involvement of IFN-γ as a major pro-inflammatory cytokine which mainly produce by activated T lymphocytes (6), has been highlighted in the pathogenesis of RA (7,8). However, its association to the synovitis onset along with the higher number of IFN-γ producing T cells in RA patients could accentuate its crucial role in the disease initiation (6).

Migration, recruitment and activation of effector T lymphocytes to the sites of inflammation including synovium is mediated by the expression of certain chemokine receptors. The ligands/CXCR3 is considered as a major axis in the migration and recruitment of effector T cells within the milieu of an inflamed synovium (9,10). The increased expression of CXCR3 and its ligands has been shown in serum, synovial fluid (SF) and synovial tissue of RA patients and also animal models (11,12). CXCR3 has been also expressed by the most effector CD8+ T cells and linked to their recruitment to inflamed joints (13).

Programmed death receptor-1 (PD-1; CD279) is a cell-surface co-inhibitory receptor which is expressed on activated and exhausted T cells, B cells, monocytes and natural killer T cells (4,5). T lymphocyte activation and IFN-γ production modulate PD-1_PD-L ligation or its related signaling pathway by different ways (14,15). Any defect in these molecules could contribute to T cell hyperactivity and loss of self-tolerance (16). PD-1 is considered to possess a crucial role in the pathogenesis of autoimmune diseases such as RA, systemic lupus erythematosus (SLE), type 1 diabetes mellitus (T1D) and multiple sclerosis (MS) (4,5). Moreover, increased expressions of PD-1 on synovial T cells and macrophages have been reported in RA patients (4,17). Despite the significance in the regulation of T cells and induction of peripheral tolerance (5), its regulatory activity in CD8+CD28- T cells is not well studied. Chronic inflammation, immune exhaustion and senescence could also be associated to the production of certain types of CD8+CD28- T cells ([18], [19], [20]). On the other hand, there are some evidences that CD8+CD28- phenotype could be one of the subsets of CD8+ regulatory T cells (Tregs) which have been already observed in the peripheral blood and synovium of RA patients ([21], [22], [23]). Acording to the some recent investigations focused on the function of CD8+CD28- T cell subsets, they could be a hybrid population and little is known about their role in RA pathogenesis.

At present, we investigated the expression of PD-1 and CXCR3 molecules on CD8+CD28- T cells in the peripheral blood (PB) and synovial fluid (SF) of newly-diagnosed (ND) and relapsed (RL) RA patients, and also assessed IFN-γ levels to evaluate their association with the disease development.