The present work has three main findings. First, there are no gender differences in plasma biomarker levels in smokers without COPD with similar clinical characteristics. Second, there are gender differences in some plasma biomarker levels in patients with COPD and similar degree of airflow obstruction. Third, there also gender differences in the strength of the association between selected biomarkers and clinical variables known to be associated with poor outcomes.
Several lines of evidence suggest the presence of gender differences in plasma inflammatory cytokines levels in health 
as well as in disease 
. Several factors have been implicated as possible causes for these differences. The most important factors thought to account for these differences include a difference in the proportion of fat tissue and its distribution 
, and the level of sex hormones 
. These reported differences could influence the way women with respiratory diseases cope with the inflammatory response related to the disease and ultimately relate to the development of lung cancer 
and COPD 
In a well-characterized population of patients with COPD, our group has previously shown that there are important gender differences in the clinical presentation as well as in the survival of men and women with COPD 
. Based on these clinical findings, we planned this cross sectional observational study to test the hypothesis that COPD patients with similar degree of airflow obstruction manifest gender related differences in plasma biomarkers. The biomarkers were selected based on the results of a previous study by our group using high throughput proteomic analysis 
. The final grouping included: A. inflammatory markers (IL-6, IL-16, and IL-8); B. Injury and repair markers (MMP-9, VEGF) and C. markers that are thought to be chemoattractants (MCP-1, PARC).
The first observation in this study is that smokers without COPD with similar clinical characteristics did not show gender differences in the plasma levels of the markers measured. This is an important and novel finding because it suggests that there are no gender differences in those smokers that are able to handle the injurious local and systemic response induced by tobacco consumption once age, BMI, smoking status and co-morbidities are controlled. Perhaps this explains the difference with previous reports in healthy individuals where the volunteers had differences in the baseline characteristics 
In contrast to the findings in non- COPD controls, we observed that in patients with COPD there are statistically significant gender differences in IL-6, IL-16 and VEGF. Interestingly, different associations with important clinical or physiological outcomes were found with the same biomarkers in each sex, supporting a true gender role.
IL-6 is a pro-inflammatory cytokine that provides a link between innate and acquired immunity. Increased levels of IL-6 in exhaled breath condensate, sputum and plasma has been described in patients with COPD. IL-6 is responsible for stimulating hepatocytes to secrete C reactive protein and has been proposed to be responsible for the systemic consequences of COPD (insulin resistance, osteoporosis 
, muscle degradation 
, and depression 
). The plasma IL-6 levels were significantly lower in females with COPD compared with smoking women without COPD (p
0.01). This was not observed in males were the serum level in both groups were similar (p
0.15). In addition, the plasma IL-6 levels in COPD patients was significantly lower in females than males. The clinical significance of the difference is difficult to ascertain because this has not been determined yet and there is variability in the serum level of this biomarker.
IL-16 is an important chemokine that regulates recruitment and activity of CD4 lymphocytes. Little information is available on plasma levels of IL-16 in COPD patients. It has been related to the process of pulmonary vascular remodeling 
. Females in our cohort had higher plasma IL-16 levels. The level of IL-16 was associated with BMI in female COPD patients, but the clinical relevance of this finding is unclear. However, this gender difference should be taken in account when this cytokine is analysed in COPD patients.
VEGF has an important role in regulating the growth of new vessels and vascular leakage, and has been shown to be reduced in the lung and sputum of COPD patients. In rats, blockade of VEGF receptor 2 (VEGF-R2) induces alveolar cell wall apoptosis and the development of emphysema like pathology 
. Previous studies have shown elevated plasma levels in COPD inversely associated with the degree of airway obstruction 
. In the present work we also found elevated levels of VEGF in COPD in comparison with smokers without COPD. We also observed that gender was one of the predictors of its plasma levels. Its levels correlated with the BODE index in both genders and with markers of lung hyperinflation (IC/TLC) and emphysema (DLCO) in men. These observations suggest a different sex response in this important growth factor that has been implicated in the development of emphysema, a phenotype more prevalent in males 
There are some limitations in our study. First, we did not analyze other important plasma markers associated with COPD (CRP, TNF-alpha, surfactant protein D or fibronectin). We chose our panel of plasma markers based on our group's experience, biological plausibility and attempting to represent the multidimensional pathological process of COPD 
. We also acknowledge that not all phenotypes of the disease are represented in this sample of COPD patients and that identification of phenotypes such as emphysema or chronic bronchitis could help in determining a distinct plasma marker profile. However, our main goal was to explore if gender could influence the level of plasma response and not to comprehensively describe the plasma markers of COPD. Although we did not find significant differences in plasma markers levels in smokers without COPD, we acknowledge this could be due to our sample size, which was relatively small, thereby generating a β type II error. However, we still believe that our sample of healthy smokers (40 males and 40 females) with similar age, BMI and comorbidities, is a good sample to represent normal individuals.
In summary, there were no gender differences in selected plasma biomarker levels in smokers without COPD, while in patients with COPD there were gender differences in some biomarker levels, especially those thought to be implicated in the genesis of emphysema. Further, the gender differences in plasma levels are in line with the reported gender specific clinical manifestations of COPD. We acknowledge this to be a pilot study that should be taken as hypothesis generating work. Further studies measuring these and other plasma markers should confirm the relevance of these findings.