4.Discussion
pSS is a systemic autoimmune disease.In addition to dry symptoms caused by gland dysfunction, there are also extraglandular manifestations, which can be life-threatening in severe cases, at the same time, these symptoms such as dryness, fatigue and pain often adversely affect the quality of life in patients[16].At present, the therapy of pSS is mainly based on clinical experience and symptomatic treatment, such as alleviating dryness and applying total glucosides of paeony(TGP), hydroxychloroquine(HCQ) and leflunomide(LEF) to regulate immunity[2, 17].Biological therapy of pSS has a valuable application prospect, but its effectiveness and safety are still controversial. Therefore, it is essential to explore effective potential therapeutic targets for immunotherapy of pSS. NK cells are congenital lymphocytes, which can kill their target cells through perforin, granzyme or death-inducing receptors. At present, more and more evidence reveals the role of NK cells in autoimmune diseases such as systemic sclerosis(SSc),systemic lupus erythematosus(SLE), PBC and pSS[6, 18-21].Although T cells and B cells are dominant in the pathogenesis of pSS, innate immune cells such as NK cells also play an important role in pSS.
In this study,we observed that the proportion of CD56+NK cells in peripheral blood of pSS patients(including active and inactive patients) was significantly lower than that of HCs. In recent years, it has also been reported that the frequency and absolute number of CD3-CD56+NK cells in peripheral blood of pSS patients decreased significantly, and the ratio of CD56bright NK to CD56dim NK in peripheral blood may have relatively specific diagnostic value for pSS[6]. It was speculated that the reduction of NK cells in peripheral blood of pSS patients may be due to augment homing of cytotoxic cells to exocrine glands, which trigger and maintain tissue inflammation by producing Th1 cytokines and cytotoxic mediators[6].Another study found that the absolute number of NK cells in pSS patients with renal tubular acidosis(RTA)was significantly lower than that in patients without RTA[4].The above studies indicate that NK cells are involved in the disease development of pSS. Similarly, previous studies found that NK cells in peripheral circulation decreased in SSc, especially in patients with organ involvement,and speculated that the decrease of NK cells proportion in peripheral blood may be due to the infiltration of NK cells into the involved tissues[22].However, the reasons for the decrease of NK cells in pSS and the mechanism of NK cells affecting the development of pSS diseases need further study.
CD161 is a type C lectin-like type II transmembrane protein, which is mainly expressed on the surface of most natural killer cells and circulating memory T cells[23, 24].Human CD161 binds to its ligand LLT1, and inhibits the activity and function of NK cells, but it is still controversial whether it inhibits or activates T cells[25].In pSS patients, there are few researches on CD161.There were two studies on CD161 expression on T cells in peripheral blood of pSS and its clinical relevance with diseases. Zhao et al. found that the expression of CD161 on CD4+T cells of pSS patients was higher than that in HCs, and the retinoic acid receptor-related orphan nuclear receptor (ROR)-γ frequency on CD161 CD4+T cells in peripheral blood increased, which was positively correlated with anti-SSA/SSB autoantibodies and hypergammaglobulinemia[11].Another study reported that compared with HCs, CD4+CD25+CD161+T cell subsets significantly increased in the peripheral blood of pSS patients, and the proportion of IL-17-producing cells in CD161+ T cell was higher than that in CD161-T cell, and CD4+CD161+T cells in peripheral circulation were related to the activity and severity of pSS disease[12].These studies indicated that CD161 played an important role in the pathogenesis of pSS and may be a potential therapeutic target for pSS.
In this study, we found that the proportion of CD161 on the surface of CD56+NK cells in peripheral blood decreased significantly compared with HCs.Similar results were also reflected in SLE.Two studies were reported that the expression of CD161 decreased on the surface of NK cells in peripheral circulation of SLE patients, and suggested that CD161+NK cells were involved in the pathogenesis of SLE[26, 27]. The receptors on the surface of NK cells, including activated receptors and inhibitory receptors, regulate the function of NK cells through balancing signal transmission. CD161, as an inhibitory receptor on the surface of NK cells, inhibits the transmission of cytotoxic functional signals of NK cells[9].Therefore, we speculated that the decease of CD161 expression on NK cells in peripheral blood of pSS patients to weaken the inhibition of function on NK cells, which led to the enhancement of cytotoxicity and the increase release of cytokine.Likewise, it was reported that the frequency of circulating CD56+CD161+ NK Cells decreased in human visceral leishmaniasis[28].All the above studies reflected that CD161 involved in the pathogenesis of autoimmune diseases and infectious diseases by mediating the function of NK cells.
We further observed that the proportion of CD161+CD56+NK cells was associated with the clinical characteristics and laboratory parameters in pSS.The CD161+CD56+NK cells proportion was significantly lower in pSS patients with decayed tooth, fatigue, arthralgia,skin involvement, PBC and ILD than that in patients without above features.Furthermore, we found that the proportion of CD161+CD56+NK cells in peripheral blood of active patients (ESSDAI>5) reduced obviously compared with that in inactive pSS patients.Further clinical correlation analysis showed that the proportion of CD161+CD56+NK cells was negatively correlated with disease activity and severity of pSS.These results suggested that the decrease of CD161+CD56+NK cells may contribute to the progression of pSS.Lenart M. et al. found that activation of the LLT1-CD161 axis can inhibit granzyme B and interferon-γ(IFN-γ) production by NK cells and hamper the function of NK cells[29].CD161 is expressed in the early stage of NK cell development,and in the peripheral circulation, the crosslinking of CD161 leads to upregulate the expression of IFN-γ and inhibits the cytotoxicity of NK cells[30, 31].Another study showed that CD161 on NK cells combined with its ligand on target cells and inhibited NK cytotoxicity by activating acidic sphingomyelinase[32].Thus, we speculated that the decrease of CD161 may affect the function of CD56+NK cells through some mechanism, leading to enhanced cytotoxicity and increased secretion of inflammatory cytokines,and aggravating the progress of pSS disease.
One of the features in pSS is the production of autoantibodies and increase of immunoglobulins in patients after overactivation of B cells.[33].In our study, we also found that the proportion of CD161+CD56+NK cells in peripheral circulation decreased significantly in pSS patients with anti-SSA/Ro60 positive,anti-SSB positive and high IgG.It has been shown that cytokines produced by NK cells, such as IFN-γ can promote the activation of B cells and enhance the production of immunoglobulin[34].Rosen DB et al. reported that CD161 interacted with LLT1 expressed on activated B cells, regulating the crosstalk between NK cells and B cells[30].Early studies have also confirmed that NK cells can enhance the proliferation of B cells[35].Another research showed that human invariant NKT cell could directly help autologous B lymphocytes, induce the proliferation of naive and memory B cells,produce immunoglobulin and antibodies in vitro[36].Therefore, we speculated that CD161, as an inhibitory receptor of NK cells, decreased on the surface of CD56+NK cells in pSS patients,which weakened the inhibition on NK cell function, and led to the increased secretion of cytokines such as INF-γ, promoted the activation and proliferation of B cells, and produced more autoantibodies and immunoglobulins in pSS patients.
However, there still exist some limitations in our research.According to the expression of CD56, human NK cells can be divided into CD56bright and CD56dim subsets.Our study didn’t deeply analyze the difference of CD161 expression on CD56bright and CD56dim subsets.On the other hand,this research was a cross-sectional and observational study, and the number of participants recruited was small.We only analyzed the clinical correlation between the proportion of CD161+CD56+NK cells and pSS, and it was not clear how CD161 mediated the function of NK cells to participate in the pathogenesis of pSS.It needs our further study in the later stage.Finally,it is generally believed that CD56+ NK cell subsets in salivary glands of pSS patients are more appropriate to reflect the lesions in glands,but,this study was lack of the histopathological verification of target tissues such as salivary gland tissues.Next, we will further explore how CD161 mediates the function of NK cell to involve in the pathogenesis of pSS from the above aspects.