Discussion
Activation of NLRP3 is a tightly regulated process and a key step in
autoimmunity of CNS. In this study, we provide three advances that
broaden our understanding of NLRP3-associated neuroinflammation. First,
by using Tslpr-/- mice, we demonstrated that
TSLP signaling regulates neuroinflammation and paralysis of mouse.
Second, we show that TSLPR signals via JAK2 to activate NLRP3-associated
neuroinflammation in EAE settings. Third, we show that patients with
anti-NMDAR encephalitis have increased levels of NLRP3 and IL-1β in CSF.
Taken together, the current study identifies novel functions of
TSLP-dominant JAK pathways upstream of NLRP3 represent promising targets
for the treatment of autoimmune disorders.
Generally, Th17 response in autoimmune disorders has been believed to
cause neurons death or inflammatory response 34, 35.
Besides, previous studies also have shown that type 3 innate lymphoid
cell (ILC3) produce IL-17 in autoimmune disorder such as ankylosing
spondylitis 36 and that ILC3 can maintains
neuroinflammation by supporting T cell survival 37.
Thus, both neuroinflammation and demyelination were largely believed to
be mediated by Th17- and ILC3- related responses. We and other have
recently shown that TSLP primes dendritic cells (DCs) maturation21, 38 and that DCs determines status of activation
and survival of ILC3 39. Based on our new data
demonstrating that brain tissue TSLPR signaling critically mediates
neuroinflammation, we speculate that innate immune cells, such as ILC3s,
play important roles in promoting autoimmune inflammation-associated
demyelination, in addition to Th17 cells. Identification of the precise
role of TSLP in Th17 cells- and ILC3-mediated neuroinflammation and
demyelination requires further investigation.
Despite previous observation that TSLP is reported to be critical for
regulatory T cells formation in autoimmune disease, we found that, in
contrast to decreased IL-7R expression on conventional T cells and
reduced Treg function of MS in previous study 40,
deletion of Tslpr efficiently alleviated neuroinflammation
displayed by decreased CD4+ cells infiltration and
restored myelin expression, which coincides with one previous study
showing ameliorated EAE symptoms accompanied by reduced inflammatory
infiltrates in the brain of Tslp -/- mice41. In fact, the epithelial cell-derived cytokine
TSLP, GM-CSF, and IL-25 have been shown to be master initiators of type
3 inflammation via their effects on a variety of cells including Th17,
ILC3, and mast cells 42, 43, 44. These cytokines are
believed to rapidly bind to membrane receptor in order to generate
innate immune response and therefore prime adaptive immune cells.
Strikingly, two recent studies have demonstrated that TSLP directly
activates neurons 45, 46. Additionally, our current
data highly regulated TSLP in the brain in the settings of EAE coincides
with previous study 47 and go beyond by demonstrating
that TSLP receptor signals by phosphorylation of JAK2 to activate
NLRP3-mediated inflammation in response to MOG35-55administration. Thus, we speculate that TSLP cytokine may act as master
regulator of neuroinflammation in immune cells of brain.
In immune cells, cytokine signaling by the JAK-STAT pathway causes
transcriptional changes to promote cellular activation. However,
although JAK inhibitor has been reported to be an alternative
immunotherapy in patients with autoimmune disorders such as
neuromyelitis optica 48, our data indicated that
neuroinflammation to MOG35-55 injection with additional JAK inhibition
by ruxolitinib failed to induce typical neuroinflammation as observed in
EAE mice brain. Also, these decreased neuroinflammation was accompanied
by restored expression of myelin basic protein after treatment with
ruxolitinib. Thus, we predict that alterations in classic JAK-mediated
NLRP3 inflammasome activation is sufficient to explain how
neuroinflammation and demyelination occur. One previous study has shown
that JAK1 mediates sensory neuronal responsiveness which can be enhanced
by cytokines such as IL-4 18. Our data coincides with
this study and further demonstrates that JAK proteins have novel
functions in neurons and regulates meylination/demyelination balance at
least by NLRP3-mediated pathways. However, we note that such alteration
of myelin basic protein expression does not exclude the role of JAK2 or
other pathways in modulating transcription or other post-transcription
of myelin basic protein within CNS. Future studies will be required to
better understand how changes of JAK-STAT pathway impact myelin
expression and neuroinflammation in autoimmune disorders.
Clinical application for ruxolitinib, an non-selective JAK1/2 inhibitor,
have been reported to induce improvement of neurologic disability in
neuromyelitis optica 48. In fact, significant clinical
efficacy of symptoms in autoimmune disorders has been observed in
clinical trials employing other JAK inhibitor such as tofacitinib and
barcitinib 49, 50. Previously, the changes in
neuroinflammation observed with JAK inhibition have been attributed to
the anti-inflammatory role of Th17 response 35.
Recently, study from others demonstrate that transient receptor
potential (TRP) plays a critical role in EAE by mediating axonal and
neuronal degeneration 51 and that JAK-STAT pathway
determines TRP expression 52, 53, which indicates
involvement of TRP in JAK pathway-mediated neuroinflammation. Based on
our and others’ studies, we speculate that the improvement of
neuroinflammation in EAE mice treated with ruxolitinib may be mediated,
at least in part, by disruption of these signals in the CNS and that
these therapies may alleviate neuroinflammation in autoimmune disorders.
Strikingly, recent studies published experimental and clinical evidence
for evobrutinib, the first Bruton’s tyrosine kinase (BTK) inhibiting
molecule being developed, also described reduced disease severity in
clinical and mouse model of multiple sclerosis (MS) by impairment of
encephalitogenic T cells 54, 55. Given our current
study demonstrating a direct role of JAK in neuroinflammation, whether
combinations of TSLP-, JAK-, and BTK-blockade can lead to synergistic
therapeutic improvements of neuroinflammation in autoimmune disorders
such as MS demands further investigation.
In conclusion, our data establish and highlight the capability of
TSLPR-JAK signaling inhibition to control disease-driving
neuroinflamamtion of NLRP3 in inflammatory CNS demyelination. This is
demonstrated here for ruxolitinib, a non-selective JAK inhibitor
clinically tested in several autoimmune diseases, and the elucidated
immunological effects may similar apply to other JAK inhibitors in
clinical development. Based on this, the mechanistic data provided here
will be instrumental in facilitate how this molecule is integrated into
the current autoimmune disorder treatment.