SPDL1 and rheumatoid arthritis: Matsuda et al. (2018) revealed that smoking suppresses PD-1 expression and limited serum sPD-L1 in rheumatoid arthritis (RA) patients (Luo et al., 2018). It has, however, been suggested an inducible expression of PD-L1 by smoking (Calles et al., 2015; Psomas et al., 2019). Our findings indicate a negative correlation between pack-years of cigarette smoking and the expression of PD-1 on Tregs and CD4+ T cells.