Three AD datasets were screened to identify 25 DEGs, of which FGL2 and SCL11A1 were verified to be associated with immuno-inflammatory responses in TAAD.
AD is associated with a high immune-inflammation level which destroys and weakens the normal function of the aortic wall leading to rupture. Immune cells, including neutrophils, NK, and B cells, are significantly increased in the peripheral blood of AAD patients [7]. Moreover, macrophages and activated B and T lymphocytes were proved to accumulate inside the aortic wall around the vasa vasorum and at the edge of the ruptured media [5–6]. In addition, upregulation of pro-inflammatory cytokines and genes has been described in patients with type-A Stanford AD [8].
In this study, γδT cells, a subset of the T cell family, were relatively less expressed, whereas neutrophils were highly expressed in AD. γδT cells are proven to enrich epithelial and mucosal surfaces [9]. Many studies exhibited γδT cells with immune function secreting various cytokines (IL-17 and IFN-γ), which were highly expressed in AD, influencing immune cell recruitment [10–12]. This might result in high expression of γδT cells in AD; however, our study showed the opposite consequence. The vascular endothelium enriched with γδT cells in the ruptured stage of AD may be missing by high-pressure blood flow flushing, therefore explaining why AD samples may have no infiltrated γδT cells in the intima.
The neutrophil results were consistent with other studies [13–15]. Tissue damage in AD can induce local inflammation and secrete damage-related molecular patterns (DAMP), including proteases and cytokines, thereby promoting early neutrophil recruitment and activation [16]. Neutrophil hyperactivity can lead to tissue damage in severe inflammation, explaining the high expression of neutrophils in AD.
Fibrinogen-like2 (FGL2) is a member of the fibrinogen-related protein superfamily of proteins and has an immune suppressive effect on adaptive immunity by inhibiting DC maturation, downregulating T cell function, and inducing B cell apoptosis [17]. In AD, early neutrophil recruitment and activation can recruit various inflammatory cells and cytokines, leading to a high state of the immune response. However, regulatory T cells (Tregs), the main source of FGL2, which exerts immune suppressive activity in many diseases, were less expressed in AD, reflecting inhibited immune suppression by adaptation. In line with the present study, there are other reports of decreased Treg, Th2, and their transcription factor levels in AD patients [12]. It is hypothesized that due to the excessive-high state of immune inflammation by neutrophils in AD, the immune suppressive Tregs are inhibited, resulting in the low level of FGL2. However, FGL2 is negatively correlated with IFN-γ and IL-17 levels, which are high in AD [12, 18]. FGL2 is an immunosuppressor that reduces the production of IFNγ and IL-17, which are partly derived from γδT cells [19]. Despite the low expression of γδT cells in AD in this study due to the lost endothelium, γδT cell infiltration and cytokine recruitment cannot be ruled out.
Solute carrier family 11 members a1 protein (SLC11A1), formerly known as NRAMP1 (natural resistance-associated macrophage protein 1), exerts pleiotropic effects on immune-inflammatory functions, including enhanced antigen presentation to T cells, overexpression of MHC class II, increased production of pro-inflammatory cytokines and upregulation of KC, one chemotactic for neutrophils [20–22]. Alleles 2 and 3 determine the level of SLC11A1, with allele 2 driving high SLC11A1 expression and allele 3 driving low expression [23]. This sequence-dependent modulation of gene expression is influenced by pro-inflammatory cytokines [24]. Moreover, SLC11A1 is expressed in macrophages and neutrophils and can regulate macrophage and neutrophil activities in arthritis [25]. In addition, SLC11A1 is characterized by increased secretion of pro-inflammatory cytokines, including IFN-γ and TNF-α, and the infiltration of neutrophils and macrophages in colitis [26]. SLC11A1 transfection into a human γδ T cell-like line rendered the cells more prone to activation, and knockout SLC11A1 mice express fewer IFN-γ compared to wild type [27]. SLC11A1 might enhance IFN-γ expression through γδ T cells, so it was hypothesized that in AD, neutrophil infiltration in the early stage triggers an inflammatory cascade that stimulates allele 3 expression resulting in high SLC11A1 expression, thereby increasing the production of inflammatory chemokines and neutrophil and macrophage infiltration, generating a vicious circle inducing more severe immune-inflammatory damage. SLC11A1 enhanced IFN-γ expression might be attributed to γδ T cells.
The present study has several limitations. First, the datasets and clinical tissues were insufficient, and although two specifically expressed genes were identified, further studies are required to verify the findings. Second, the protein expression was undetected and unverified, so the specific mechanisms involved in the immune-inflammatory response in AD require further study. However, this study used GEO common databases to screen DEGs, which saved clinical time and resources, then clinical samples were used to verify the results. These results which contain multiple databases in multiple centers are credible. Moreover, we targeted the immune-inflammatory response to set the foundation for further mechanistic studies of FGL2 and SLC11A1 in AD.