DISCUSSION
The transverse expression from innate to adaptive immune lymphocytes
makes NKG2D a remarkable activating receptor. Dysregulation of the
NKG2D/NKG2DL pathway may lead to inadequate NK and T cell activation,
and contributes to initiating or maintaining an inflammatory cascade,
resulting in self-reactivity [16]. In this study, we explored the
roles of NKG2D in the pathogenesis of different subgroups of JIA and its
regulatory roles in the crosstalk between CD4+ T cells
and DCs in JIA patients.
NKG2D expression on CD4+ cells in the main three
subgroups of active JIA patients was significantly increased. In
addition, NKG2DL was markedly increased compared the levels in healthy
controls. The findings indicate a role for NKG2D in the activation of
CD4+ T cells in JIA patients. This indication is
consistent with the previously reported associations between
NKG2D/NKG2DL polymorphisms and rheumatoid arthritis. In particular,
genetic variation within the NKG2D gene may be involved in disease
susceptibility, characteristics, severity, and efficacy of anti-tumor
necrosis factor therapy [17-23]. Thus, NKG2D may be a potential
biomarker of JIA disease activity.
The
marked variation in clinical features among subtypes of JIA is caused by
the different immunological characteristics. Activation of
CD4+NKG2D+ T cells and NKG2DL were
greater in paJIA and poJIA patients than in soJIA patients. Joint
disease of articular JIA usually overshadows the general symptomatology.
It is generally thought that activation of autoreactive
CD4+ T cells is a key manifestation in the
pathogenesis of arthritis [24-27]. Dysregulation of local
CD4+NKG2D+ T cells results in
autoreactive T cell and arthritis. Systemic JIA is mainly characterized
as an auto-inflammatory disease and is the most distinct clinical
subtype of JIA with its marked extra-articular feature of spiking
fevers, which can easily lead to misdiagnosis at the early stage. An
NKG2D-dependent regulatory role of NK cells in systemic JIA has been
described [3]. We assayed the expression of NKG2DL on DCs, which are
the strongest antigen present cells. Articular JIA patients displayed
higher levels of MICA and MICB on DCs than healthy individuals and
systemic JIA patients. The different roles of NKG2D in various immune
cells might contribute to the marked clinical variation among subsets of
JIA.
The influence of the regulation of NKG2D on interaction between
CD4+T cells and DCs were explored by using blood
samples from arthritic JIA patients and healthy controls. NKG2D
modulated subgroups of Th1 and Th17, with declines in the Th2 and Treg
subgroups by regulation of the secretions of IFN-γ, IL-17, and IL-4 and
expressions of transcription factors RORc, Foxp3, T-bet and GATA3 in JIA
patients. In addition, NKG2D also promoted higher production of the IL12
proinflammatory cytokine and reduced productions of the TGF-β and IL-10
anti-inflammatory cytokines by CD4+T cells of JIA.
These findings indicate the role of the NKG2D-mediated signaling pathway
in CD4+ T cell inflammation in JIA pathogenesis.
Furthermore, NKG2D had a stronger regulatory effect on
CD4+ T cells in JIA patients than in healthy controls.
NKG2D typically is not expressed, or is expressed at a low level on
healthy CD4+ T cells. NKG2DL is termed a
“stress-induced ligand” based on the expression after autoimmune
stimulation. Marked overproduction of NKG2DL was observed in local
synovial fibroblasts and synovial fluid of rheumatic arthritis patients
[28], accounting for the powerful regulation of NKG2D on unhealthy
cells of JIA patients. NKG2D/NKG2DL act as a powerful costimulatory
signal in the autoimmune condition, and lead to inflammation induced by
antigen-specific CD4+ T cells [29]. The collective
findings implicate NKG2D as a potential target for the amelioration of
Th1- and Th17-mediated chronic inflammatory diseases in JIA.
The bidirectional crosstalk between CD4+ T cells and
DCs lead to mutual activation, maturation of DCs, and differentiation of
CD4+ T cells. Accordingly, we investigated the effects
of genetically NKG2D modified CD4+ T cells on DCs
during interactions between the cell types. NKG2D indirectly lowered the
productions of TGF-β and IL-10, but not IL-12, by DCs in healthy
individuals, and lowered the secretion of IL-10 from DCs in JIA
patients. The indirect regulations of NKG2D on DCs is more potent in
healthy individuals than in JIA patients, and relies on
anti-inflammatory IL-10. IL-10 suppresses the activation of T cells,
induces differentiation of T regulatory cells, and also blocks the
maturation process and down-regulates costimulatory molecules of DCs.
These actions subsequently limit the autoimmune response [30,31]. In
addition, NKG2D indirectly prompted the expression of the CD83 DC
maturation marker in both healthy and JIA individuals, but had no
effects on the expression of the costimulatory molecules CD80 and CD86,
and HLA-DR on DCs. MICA and MICB expressions on DCs were also increased
dramatically by NKG2D indirectly in both JIA and healthy controls. The
collective findings indicate that NKG2D indirectly regulates the
activation and maturation of DCs through their crosstalk with
CD4+ T cells.
In conclusion, CD4+NKG2D+ T cells
and NKG2D ligands MICA and MICB are increased dramatically in JIA
patients, particularly paJIA and poJIA patients. NKG2D directly
regulates the differentiation and activation of CD4+ T
cells, and indirectly regulates the maturation and activation of DCs.
The stronger regulations of NKG2D on CD4+ T cells of
JIA implicates targeting CD4+ NKG2D+T cells as a potential therapy to treat the inflammation of JIA.