Background Smoking is the most extensively studied environmental risk factor of rheumatoid arthritis (RA). We have recently shown that smoking is associated with high serum levels of survivin (Svensson, Hafström et al. 2014). Survivin is a novel biomarker for RA that can predict severe joint destruction as well as treatment outcome for RA patients (Levitski et al, 2015). We hypothesized cytotoxic activity to be the reason for the increased serum levels of survivin observed in RA patients and we wished to further investigate possible molecular mechanisms behind cytotoxicity in RA.
Objectives We have studied the effect of nicotine on cytotoxic activity of T lymphocytes during RA paying special attention to the expression of PD-1, a receptor that inhibits activation of T cells.
Methods We used female Balb/c mice that received 0.03% nicotine in drinking water or regular tap water as controls, after 18 days mice were immunized with collagen and subsequently developed arthritis (CIA). At the end of experiment bone marrow, lymph nodes and serum were collected and analyzed using flow cytometry (FACS), qPCR and ELISA. To confirm nicotine-dependent nature of experimental findings, we analyzed blood samples of female RA patients and healthy women with known smoking status.
Results The proportion of CD8+ cells was 1.9 times larger in nicotine treated mice compared to control (p=0.00020). Meanwhile, the nicotine treated mice had a 2.3 times less PD-1 expressing CD8+ cells in the bone marrow (p=0.032), but PD-1 expression in the lymph nodes was not affected by nicotine. This resulted in an accumulation of the CD8+PD-1- population in the bone marrow of the nicotine treated mice (p=0.0074). The nicotine treated mice had higher levels of serum survivin (0.0 vs 0.29 ng/ml, p=0.017) which also correlated to the size of CD8+PD-1- population in the bone marrow (r=0.52, p=0.024). The decrease in PD-1 expression of CD8+ T cells caused by nicotine was confirmed in smoking women. Smokers had a 1.7 times less PD-1 positive CD8+ T cells compared to non-smokers (p=0.041). In patients, the serum levels of survivin correlated to the expression of the cytotoxic marker CD107 on CD8+ T cells (r=0.52, p=0.047).
Conclusions Nicotine exposure supports cytotoxic phenotype of CD8+ T cells characterized by high CD107 and low PD-1 expression in experimental arthritis and RA patients. Increased cytotoxic activity of CD8+ T cells is a plausible mechanism for the enrichment of survivin in serum in RA patients. See graphical abstract.
Svensson B, Hafström I, Erlandsson MC, Forslind K, Bokarewa MI. Smoking in combination with antibodies to cyclic citrullinated peptides is associated with persistently high levels of survivin in early rheumatoid arthritis: a prospective cohort study. Arthritis Res Ther. 2014;16(1):R12. doi: 10.1186/ar4438.
Levitsky A, Erlandsson MC, van Vollenhoven RF, Bokarewa MI, Serum survivin predicts responses to treatment in active rheumatoid arthritis: a post hoc analysis from the SWEFOT trial. BMC Med. 2015;13:247. doi: 10.1186/s12916-015-0485-2.
Disclosure of Interest None declared