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.