Discussion
It is widely recognized that MDSCs have the ability to inhibit the expansion and activation of Tregs in various pathological conditions ( Hegde, Leader & Merad, 2021 ). A sufficient understanding about the role of MDSC subsets and potential relationships between MDSC subsets and Tregs in COPD has not yet been achieved. In this study, we observed significantly elevated percentage of circulating M-MDSCs displayed a high degree of positive correlation with suppressive Tregs in COPD, accompanied by a significantly increased expression of PD-1/PD-L1 axis on specific immune cells.

MDSCs have been extensively studied in patients with cancer, but there is a lack of research on their role in non-malignant conditions, particularly in COPD. Some researchers reported that circulating MDSCs were up-regulated in COPD ( Scrimini et al., 2013 ; Kalathil et al., 2014 ; Deshane et al., 2015 ; Scrimini et al., 2015 ; Wu et al., 2015 ). However, another study reported that COPD was not associated with perturbations in the percentages or the function of MDSCs in peripheral blood ( Tan et al., 2014 ). Furthermore, there is limited investigation into the subsets of MDSCs in COPD patients. In our study PBMCs from COPD contained both an elevated frequency of G-MDSCs and M-MDSCs compared to controls group, which is in line with previous on MDSCs ( Scrimini et al., 2013 ; Kalathil et al., 2014 ; Deshane et al., 2015 ; Scrimini et al., 2015 ; Wu et al., 2015 ).

Treg, a subpopulation of CD4 + T cells with immunosuppressive function, plays a crucial role in the suppression of inflammatory pathology. Previous studies have yielded conflicting results regarding the specific role of Treg in COPD. Chiappori et al. (2016) demonstrated lower numbers of circulating Treg cells in COPD patients. According to another study, smokers with COPD also have significantly fewer Treg cells in their lungs ( Lee et al., 2007 ). Conversely, accumulating evidence shows that the frequency of Tregs is increased both in mice exposed to smoke ( Gong et al., 2017 ) and COPD patients ( Plumb et al., 2009 ; Kalathil et al., 2014 ). Consistent with these findings, our study revealed that the proportion of circulating Treg cells in COPD patients was increased compared with the control groups ( Kalathil et al., 2014 ) and showed an positive correlation with the percentage of M-MDSCs, consistent with studies that suggest MDSCs may be crucial to Treg expansion ( Kalathil et al., 2014 ).

Previous research has established the critical role of CTLA-4 in the development and suppressive function of Tregs ( Zheng et al., 2006 ; Wing et al., 2008 ) and CTLA-4 + Tregs have increased suppressive capacity ( Read, Malmström & Powrie, 2000 ; Núñez et al., 2020 ). Our result indicate that there is an increased expression of CTLA-4 on Tregs in COPD patients, and a positive correlation between the percentage of M-MDSCs and the expression of CTLA-4 on Tregs. This suggests that MDSCs may also play a role in modulating the suppressive function of Tregs.

The PD-1/PD-L1(L2) signaling is known to regulate immune tolerance in many immune-mediated diseases, including cancer, autoimmune diseases, and, as emphasized recently, in chronic inflammation and regulates T cell activation negatively and regulates the generation and function of Treg ( Adamczyk & Krasowska, 2021 ; Filippone et al., 2022 ). PD-1 is expressed on Tregs and partially proved to represse Treg suppressive function ( Kumagai et al., 2020 ; Lowther et al., 2016 ) and high PD-1 expression on Treg cells indicates enhanced Treg function ( Asano et al., 2017 ). Previous research has demonstrated that treatment with anti-PD-1 can mitigate lung damage and neutrophilic inflammation ( Ritzmann et al., 2021 ) and acute exacerbations of COPD ( Tan et al., 2018 ), suggesting the involvement of the PD-1 axis in the pathogenesis of COPD. McKendry et al. (2016) discovered that the expression of PD-1 on lung CD8 + T cells was elevated in individuals with COPD compared to controls. Similarly, our study revealed a higher level of PD-1 expression on CD4 + T cells and Tregs, but not on CD8 + T cells, in GOLD stage I/II and stage III/IV COPD patients compared to controls. These findings align with previous research.

Based on previous research, it has been observed that, MDSCs in tumor-bearing mice may exhibit increased expression of PD-L1 under hypoxia ( Noman et al., 2014 ). Furthermore, chronic inflammation induced by LPS has been found to up-regulate the PD-1/PD-L1 axis and lead to the accumulation of MDSCs ( Liu et al., 2021 ). Additionally, a study conducted by Rui et al. (2022) revealed that exposure to cigarette smoke results in the upregulation of PD-L1 in rats with COPD. There is limited information about PD-L1/PD-L2 expression in MDSC subsets in COPD as far as we know. In line with these results, we found the up-regulationof PD-L1 but not PD-L2 on M-MDSCs subsets in COPD patient peripheral blood ( Kalathil et al., 2014 ; Liu et al., 2021 ). Interestingly, a notable decrease in the expression of PD-L1 by G-MDSCs was observed in patients with COPD. This finding leads us to suspect that the functionality of G-MDSCs may be more severely impaired in individuals with COPD.

Smoke is the most important risk factor for COPD. However, the relationship between smoking and MDSC subset frequency in COPD remains poorly understood. Worth mentioning, pack-years of smoking were negatively correlated with the percentages of G-MDSCs cells in our study and imply an inducible decrease in the number of G-MDSCs by smoking. 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. The conflicting conclusions could potentially be attributed to the varying roles of the PD-1/PD-L1 axis in different conditions.

As in other studies, the present work also has some limitations. Firstly, the present study is descriptive in nature and confirmation of this explanation will requires further experiments in vitro . Secondly, despite our interesting findings, the relatively small sample size and single center design may restrict the generalizability of our results. Additionally, certain risk factors such as physical exercise and dietary intake were not taken into account, potentially influencing the relationship between pack year of cigarette smoking and our findings. Fourthly, the fixation and membrane rupture of cells are required for the detection of Foxp3, which may lead to the destruction and death of lymphocytes. Therefore, Treg cells were characterized by CD3 + CD4 + CD25 + CD127 −/low in our study. If FMO controls is used as negative controls, in place of isotype controls, the results of our study would be more accurate. Therefore, caution should be exercised when interpreting the association between these variables.