Background/Purpose:

Disturbances in the regulatory mechanisms of the immune system play a major role in rheumatoid arthritis (RA). Programmed Death 1 (PD-1) on lymphocytes negatively regulate T cell receptor (TCR)-induced activation through interaction with two ligands (PD-L1 and PD-L2) on activated antigen-presenting cells, including fibroblasts. PD-1 exists in a soluble form (sPD-1), and is increased in chronic RA. Here, we investigate whether sPD-1 plays a role in the generation of immunological memory in early RA.

Methods: In a longitudinal set of steroid- and DMARD naïve RA patients (eRA) (n=76, <3 months of disease) we measured plasma levels of sPD-1 by ELISA (R&D systems) at baseline and after 3, 6 and 12 months of treatment with methotrexate (MTX) + placebo (PLA) or MTX + adalimumab (ADA). After 12 months of treatment, ADA/PLA was discontinued and patients were followed every third month for flare. sPD-1 was also measured in chronic RA (cRA) in plasma and synovial fluid (SF) (n=24, >8 years of disease). Data were expressed as median (IQR) and correlations were assessed by Spearman’s rho. T cells were stimulated with anti-CD3/CD28 and co-cultured with RA fibroblast-like synoviocytes (FLS) for 3 days with addition of sPD-1 (20ng/ml), sPD-L1 (20ng/ml) or TNFα (10ng/ml). T cells were examined by flow cytometry for expression of PD-1, CD25, CD69 and proliferation, and FLS were examined for PD-L1 expression.

Results:

The plasma levels of sPD-1 was increased in eRA (0.52 ng/ml (0.3-1.1 ng/ml)) compared with cRA (0.14 ng/ml (0.07 ng/ml -0.33 ng/ml), p<0.001) and decreased following treatment (0.26 ng/ml (0.2-0.6 ng/ml), p<0.001)). Treatment with ADA did not influence this decrease. Baseline sPD-1 correlated tightly with anti-CCP (r=0.42) and IgM-RF (r=0.60, both p<0.001). The correlation persisted at 3, 6 and 12 months. Patients with high baseline sPD-1 showed increased risk of disease flares (p<0.05). In cRA sPD-1 levels were increased in SF (0.31ng/ml, (0.14 ng/ml -0.73 ng/ml)) compared with plasma (p<0.01). T cell survival in FLS co-cultures was 76% when adding sPD-1 compared with 61% in controls. Stimulation with sPD-1 did not affect proliferation, CD25, CD69 or PD-1 expression by T cells. Co-culture with stimulated T cells increased PD-L1 expression on FLS from 2% to 45%. In co-cultures with TNFα, PD-L1 was expressed on nearly all FLS.

Conclusion:

Plasma levels of sPD-1 were increased in eRA and decreased in response to treatment suggesting sPD-1 to be important for the initial phase of RA. The high concentrations found in SF points to a function of sPD-1 locally in the joint. The close association with IgM-RF and anti-CCP, the increased risk of flare with high sPD-1 in eRA plasma and the ability of sPD-1 to promote T cell survival in co-cultures with FLS support a role for sPD-1 in adaptive immunity and generation of immunological memory. Taken together, our findings suggest that sPD-1 is capable of blocking the PD-L1 binding-site on FLS, inhibiting the effects of the PD-1-induced TCR regulation and thereby increasing the risk of generating auto-reactive cells.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/increased-soluble-pd-1-a-link-between-generation-of-immunological-memory-and-risk-of-disease-flare-in-early-ra/