In our study, DM patients exhibited increased sVCAM-1 level in sera and VLA-4 expression on neutrophils surface compared with healthy controls. To our knowledge, this is the first time to detect VLA-4 expression on the surface of neutrophils in DM patients. In addition, these two indicators were elevated in patients with ILD compared to non-ILD. Patients with DAD patterns had higher VCAM-1 levels in sera and VLA-4 expression on neutrophils than those without DAD.
The migration of leukocytes to sites of injury or infection is tightly regulated by the leukocyte adhesion cascade. In the beginning, rolling of leukocytes is medicated by selectins[23]. Then, leukocyte activation and slow rolling are induced by chemokines which activated the extracellular domains of integrins including LFA-1, Mac-1 and VLA-4[24–26]. Activated integrins binding adhesion molecules contribute to firm adhesion of leukocytes to endothelial cells and lead to trans-endothelial migration. VCAM-1 works with other adhesion molecules to regulate immune surveillance and inflammation. However, when the stimulation is not properly eliminated, this beneficial reaction can lead to chronic and detrimental inflammatory processes, such as rheumatoid arthritis, asthma, and psoriasis. It has been reported that VCAM-1 expression on endothelial cells is activated during inflammatory diseases[27]. Structurally, VCAM-1 contains an extracellular domain with six or seven immunoglobulin (Ig)-like domains, a transmembrane domain and a cytoplasmic domain[27]. The soluble ectodomain of VCAM-1 can be released from the cell surface into the circulation via proteolytic cleavage[4, 28]. The soluble forms of cellular adhesion molecules (sCAMs) were observed to correlate with the endothelial surface expression of CAMs and can be used as potential biomarkers for endothelial activation[29]. In our study, serum levels of sVCAM-1 were significantly increased in DM, especially in DM-ILD and sVCAM-1 levels were significantly negatively correlated with lung function, including FVC, DLCO and PaO2. This result revealed that the pathogenesis of DM-ILD may be associated with endothelial cell damage. In addition, serum sVCAM-1 level positively correlated with D-dimer and fibrinogen confirmed this view.
Soluble VCAM-1 plays a key role in the onset of synovitis in RA, which is accompanied by the infiltration of T cells and monocytes[30, 31]. Multiple factors, including increased production of proinflammatory cytokines, the presence of autoantibodies, and increased oxidative stress activate endothelial cells, leading to increased expression of VCAM-1[32]. sVCAM-1 recruits pathological levels of neutrophils to injury sites and amplifies lung inflammation during acute lung injury[33]. Bronchoalveolar lavage fluid from a patient with DM-ILD revealed neutrophil infiltration[16]. It has been reported that excessive formation of neutrophil extracellular traps (NETs) by neutrophils in DM caused damage to pulmonary vascular endothelial cells and infiltration of inflammatory cells, leading to the occurrence of ILD[34]. Therefore, we hypothesized that the pathogenesis of DM-ILD may be related to VCAM-1 and neutrophil infiltration. Ibbotson et al. first discovered that neutrophil recruitment was depended on the VLA-4/VCAM-1 pathway in human disease[35]. We thus explored VLA-4 expression on surface of neutrophils in patients with DM. Our findings manifested that VLA-4 expression on neutrophils were elevated in DM and significantly negatively correlated with PaO2. Patients with DAD patterns had higher VCAM-1 levels in sera and VLA-4 expression than those without DAD. The results may confirm the hypothesis that neutrophils infiltrated via VCAM-1/VLA-4 pathway in DM-ILD. Besides, in our cohort, 10 patients were diagnosed with CADM among 30 patients. We compared sVCAM-1 level in sera and VLA-4 expression on neutrophils surface in classic DM and CADM and found no significant differences. Due to the small sample size, we did not divide them into subgroups for further comparison.