Result
A total of 54 COPD patients (25 in stages I–II and 29 in stages III–IV) and 24 healthy individuals participated in the study from September 9, 2021, to May 1, 2022. The clinical features of all the participants are provided in Table 1 . In comparison with healthy subjects, COPD patients had no significant differences in age or smoking history. The sex ratio was unequal mostly because the incidence rate of COPD is higher among males than females in China.

Notes:

The data are presented as mean ± standard error of mean (SEM) or mean (range).

Abbreviations: COPD, patients with stable chronic obstructive pulmonary disease; FEV 1, forced expiratory volume in one second; FVC, forced vital capacity.

To assess the role of MDSCs, we measured the proportion of two different subsets of MDSCs (M-MDSC and G-MDSC). The results depicted in Figure 1 demonstrated a significantly higher proportion of M-MDSC in the peripheral blood of COPD patients compared to controls controls (9.02 ± 0.63 versus 16.33 ± 1.12, p < 0.0001) ( Figs.1C and 1D ). Additionally a greater diversity of G-MDSC percentages was observed in the peripheral blood of COPD patients compared with controls (1.68 ± 0.14 vs . 2.67 ± 0.21, p < 0.01) ( Figs.1E and 1F ).

To investigate whether MDSC recruitment promotes Treg expansion in patients with COPD, we proceeded to analyze the frequency of circulating Tregs in their blood. As shown in Figure 2B , there was a significantly increased proportion of Tregs (7.26 ± 0.29 versus 9.07 ± 0.38, p < 0.01) in COPD patients peripheral blood in comparison with controls ( Figure 2B ). Furthermore, the frequencies of Tregs were found to be elevated in COPD patients in GOLD stage I/II or stage III/IV on Tregs compared with matched healthy subjects ( Figure 2C ).

It has been established that CTLA-4 is essential for Treg-mediated immunosuppression and Tregs expressing CTLA-4 demonstrate enhanced immunosuppressive properties. The expression of CTLA-4 (11.96 ± 0.61 versus 15.74 ± 0.77, p < 0.01) was significantly elevated on Tregs of COPD patients ( Figure 2D ), indicating a heightened suppressive function of Tregs in COPD. Furthermore, the expression of CTLA-4 were upregulated in COPD patients in GOLD stage I/II or stage III/IV on Tregs compared with matched healthy individuals ( Figure 2E ). However, no significant difference in CTLA-4 expression on Tregs was observed between GOLD stages I/II and stages III/IV COPD patients ( Figure 2E ).

Although the role of MDSCs-mediated T-cell suppression under neoplastic disease has been extensively studied, their function and mechanism have not been fully elucidated in COPD. MDSCs exerted their immunosuppressive activities by expression or secretion of mediators or molecules, including PD-L1/PD-L2, which can lead to T cell exhaustion. In this study, we investigated the expression of PD-L1 and PD-L2 on different subsets of MDSCs using anti-CD274 (PD-L1) and anti-CD273 (PD-L2) antibodies. We detected PD-L1/PD-L2 expression on MDSC subsets. There was increased upregulation of PD-L1 on M-MDSCs in COPD patients compared to the control group (68.07 ± 4.67 versus 89.27 ± 1.93, p < 0.0001, Figs. 3B and 3F ). In contrast, significantly decreased expression of PD-L1 by G-MDSCs was seen in COPD patients blood compared with control group (70.59 ± 4.48 versus 53.74 ± 2.68, p < 0.001, Figs. 3D and 3H ). There was no statistically significant difference observed in the expression level of PD-L2 between M-MDSC ( Figs. 3A and 3E ) or G-MDSCs ( Figs. 3C and 3J ) between the COPD and control groups.

The expression of PD-1, the receptor of PD-L1/L2, was assessed on CD4 + T cells, CD8 + T cells and Treg cells. Compared to the controls, the expression pattern of PD-1 was higher on CD4 + T cells (18.23 ± 1.08 versus 23.87 ± 1.23, p < 0.001; Figure 3I ) and Tregs (20.02 ± 1.06 versus 23.84 ± 1.14, p < 0.05; Figure 3J ), but not on CD8 + Tcells (data not shown). Next, subgroup analysis was performed based on the degree of airflow obstruction according to the Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines. It is noteworthy that COPD patients in GOLD stages I/II or stages III/IV exhibited a significant upregulation of PD-1 expression Ion CD4 + Tcells, but not CD8 + Tcells, compared to the control group ( Figure 3K ). The expression of PD-1 on Treg showed a slight increase in patients with COPD in stages III/IV compared to those in stages I/II, although this difference was not statistically significant (21.53 ± 1.55 versus 25.83 ± 1.58, p = 0.0597; Figure 3L ).

The percentages of M-MDSCs cells were found to be positively correlated with the expression of PD-1 on CD4 + T cells (r = 0.42, p = 0.001; Figure 4A ) and CD8 + T cells (r = 0.36, p = 0.008) ( Figure 4B ). Furthermore, upon further analysis, it was discovered that there was a positive correlation between the percentages of M-MDCS and the expression of PD-1 (r = 0.51, p < 0.0001; Figure 4C ) and CTLA-4 (r = 0.42, p = 0.0014; Figure 4D ) on Tregs. We also found a positive relationship between the percentages of M-MDSCs and Treg cells in COPD patients (r = 0.315, p = 0.02; Figure 4E ).

Notably, the percentages of G-MDSCs cells were positively related to the percentages of M-MDSCs (r = 0.39, p = 0.0037) ( Figure 4F ). Our findings further demonstrated that the increased percentages of G-MDSCs was associated with the increased expression of PD-1 on CD4 + T cells (r = 0.299, p = 0.028; Figure 4G ) and PD-1 on Treg (r = 0.297, p = 0.028; Figure 4H ). However, the proportions of G-MDSC or M-MDSC failed to significantly correlate with lung function in COPD patients.

The risk of COPD increases with pack-years of smoking. In our study, we observed a negative correlation between the number of pack-years of smoking and the expression of PD-1 on CD4 + T cells (r = −0.33, p = 0.04; Figure 4I ), Tregs (r = −0.37, p = 0.02; Figure 4J ), and percentages of G-MDSCs cells (r = −0.35, p = 0.03; Figure 4K ) in COPD patients, but not the percentages of M-MDSCs, PD-L1 expression on MDSCs subsets and CTLA-4 expression on Treg.