Background/Purpose: Programmed cell death 1 (PD-1) is a co-inhibitory receptor, which inhibits T cell proliferation and survival by inhibiting IL-2 signalling. By this PD-1 has been speculated as a key player in maintaining and restoring self-tolerance. In contrast, OX40 is a surface receptor that induces IL-2R expression and OX40 has been suggested to allow possible autoreactive effector T cells to develop into memory T cells. These two molecules could therefor represent two opposite sides of the balance in immune regulation -a balance crucial for maintaining peripheral tolerance. In systemic lupus erymathosus (SLE) this balance is disrupted, and disease progression is characterized by antibody production and autoreactive lymphocytes. In this study, we investigated cellular expression and soluble (s) levels of PD-1 and OX-40.

Methods : Plasma levels of sPD-1 and sOX40 from SLE patients (n=19) and healthy controls (HCs) (n=18) were quantified by ELISA. PBMCs from SLE patients and HC were stained with monoclonal antibodies against PD-1, OX40, CD4 and CD8. Cells where analyzed by flow cytometry. Skin biopsies from SLE patients and HCs were stained with anti-OX40 and anti-PD-1 antibodies and analyzed by confocal microscope. Statistics were assessed by Mann-Whitney’s test, and Spearman’s ranked correlation. Data are expressed as median (IQR).

Results: In SLE patients, sPD-1 was significantly increased (445 pg/ml (319 pg/ml – 897 pg/ml)) compared to HCs (244 pg/ml (173 pg/ml – 343 pg/ml), p<0.001) and correlated positively with the SLE disease activity score SLEDAI (ρ=0.69, p=0.01). Soluble OX40 was decreased (7.3 pg/ml (7.3 pg/ml – 16.4 pg/ml)) compared with HC (53,5 pg/ml (41.9 pg/ml – 72.5 pg/ml) p<0.001). Furthermore, did levels of sPD-1 and sOX40 inter-correlate positively in SLE patients (ρ=0.59, p= 0.005), which was not the case in HCs. Co-expression of PD-1 and OX40 was seen on (9.5%) CD4⁺ T cells in SLE compared with (5.2%) in HCs (P<0.01). Immunofluorescence revealed that PD-1 and OX40 co-expressing cells were present in SLE skin, further supporting that the same cell expresses both PD-1 and OX40 (Fig 1).

Conclusion: In this study, we show that T-cells expressing both PD-1 and OX40 are increased in SLE, and that the levels of the soluble isoforms intercorrelate. We also show that sPD-1 correlates strongly with SLEDAI, suggesting sPD-1 as a marker of inflammation in SLE. The observed co-expresssion of the death receptor PD-1 and the survival receptor OX40, could in part explain why autoreactive effector T cells survive, despite receiving apoptosis signals, and help elucidate the lack of self-tolerance in SLE patients.

Fig 1: co-expression of PD-1 (green) and OX-40 (red)