Discussion
In the current study, we developed a method to sort live Th17 cells from
PF of severe endometriosis patients to evaluate the function of
pathological Th17 cells. We are the first to use CXCR3, CCR4, and CCR6
isolate PF Th17 cells from endometriosis patients. In the future, it
would be necessary to detect more surface markers to precisely pinpoint
Th17 cells. Furthermore, detection of the above chemokine receptors
would recognize Th1, Th2, and Th1/Th17 subsets and thus greatly
facilitates research on T cell biology at endometriosis lesions.
Th17 cells are heterogeneous and possess a remarkable degree of
plasticity [16]. Our study indicates
that PF Th17 cells are heterogeneous according to the expression of CD27
and IL-17RE. CD27 is a member of TNF receptor family expressed on
CD4+ T cells at the priming or effector stage, while
its ligand, CD70, is expressed on activated dendritic cells and
lymphocytes [17]. Recent research
suggests its inhibitory effect on Th17 cells through repressing
transcription of IL-17 and CCR6 [15].
IL-17RE forms a receptor complex with IL-17RA to bind to IL-17C to
potentiate the Th17 cell response
[14]. IL-17C is preferentially
secreted by epithelial cells in the lung, skin, and colon
[18]. Interestingly, the Human
Protein Atlas describes high IL-17C expression in the endometrium
(https://www.proteinatlas.org/ENSG00000124391-IL17C/tissue),
strongly suggesting the existence of IL-17C in endometriosis lesions.
Our ongoing research is investigating the levels of IL-17C in PF and
endometriosis tissues. Besides, our data indicate that IL-17RE rather
than CD27 marks a more active Th17 subpopulation, because
IL-17RE+ Th17 cells expressed more Th17-related
cytokines and were more proliferative than IL-17RE-Th17 cells. A possible explanation is that IL-17RE-triggered signaling
overcomes CD27-induced signaling.
Through RNA-Seq, we found that most of the top 20 up-regulated metabolic
genes in IL-17RE+ Th17 cells encode enzymes involved
in OXPHOS and ETC, while most of the down-regulated metabolic genes were
associated with other metabolic pathways. It has been known that
mitochondrial respiration is enhanced during Th17 differentiation and
OXPHOS is essential for Th17 cell pathogenic signature gene expression
[19]. Besides, Th17 cells have
limited capacity to promote glycolysis to react to metabolic stresses
and they rely on OXPHOS for the production of energy and cytokines
[20]. Indeed, we observed higher
pro-inflammatory activity associated with higher ATP and ROS in
IL-17RE+ Th17 cells. This suggests that OXPHOS and ETC
support the pathogenicity of PF IL-17RE+ Th17 cells in
endometriosis. Our data also poses an interesting question: whether the
IL-17C/IL-17RE axis results in the up-regulation of OXPHOS and
ETC-related genes. This could be answered in future studies.
What remains unknown is the origin of the heterogeneous PF Th17 subsets.
Are they a CD4+ T cell population at different
maturation statuses? Or do they arise from different
CD4+ T cell subsets under distinct microenvironment
factors? Lineage tracing of Th17 cells in animal models and single-cell
transcriptome sequencing might give the answer.
In conclusion, this study provides a novel method to detect and isolate
live PF Th17 cells from endometriosis patients and unveils the
functional and metabolic heterogeneity of PF Th17 subsets. Furthermore,
the data of other differentially expressed genes shed light on the
elucidation of other molecular mechanisms that modulate the function of
pathological Th17 cells in endometriosis.