Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic, progressive invasiveness with 0.5–1% incidence1,2. The underlying pathogenesis is attributed to the abnormal proliferation of autoreactive CD4+ and CD8+ T cells, manifested by an increase in the proportion of CD4+IFN-γ+ and CD4+IL-17A+ T cells, and the reduced number of CD4+IL-4+ and CD4+CD25+FoxP3+ T cells. Therefore, the secretion of inflammatory cytokines is increased, and an excessive inflammatory reaction destroys the native balance and tolerance function of the body's immune system. Moreover, the helper T lymphocyte (Th) 1 cells that penetrate the joint aggravate the inflammatory response and joint damage. Therefore, the inhibition of excessive inflammatory response and restoration of immune tolerance and immune balance are key to the treatment of RA3,4.
The iNKT cells have both NK cell and T cell functions and characteristics. The iNKT cells harbor a distinct, invariant T cell receptor (TCR) α-chain with limited TCR β-chain repertoires5 and recognize the glycolipid antigen presented by the major histocompatibility complex (MHC) class I molecule CD1d on the surface of the antigen-presenting cells. Mitsuo et al.6 detected a large number of iNKT cells and functional defects in many autoimmune diseases, including RA. Aurore et al.7 demonstrated that iNKT cells have a positive effect on maintaining autoimmune tolerance and that when the number and function of iNKT cells are restored, the disease is alleviated. In addition, Miellot-Gafsou et al.8 found that iNKT cells not only abrogated the disease but also increased the progression of the disease. These contradictory results suggest that iNKT cells are heterogeneous T cells, and the function of different subsets may be reversed. In a clinical study of RA, the frequency of iNKT cells correlated with the score of the disease activity9. The results also confirmed that the frequency of iNKT was decreased in RA patients, the number of CD4+IFN-γ+ T cell subsets increased, and the secretory levels of inflammatory cytokines IFN-γ and TNF-α increased10,11. In addition, Sharif et al.12 investigated type 1 diabetes (T1D) and found that selective infusion of iNKT cells upregulated the expression of the inflammatory cytokine IL-4, maintained immune tolerance, and prevented the development of type 1 diabetes. Therefore, adoptive infusion of specific iNKT cells or targeted activation of iNKT cells increases the level of iNKT cells in RA patients, which can be a breakthrough in RA treatment.
Cellular immunotherapy is currently of great interest and has been widely used in cancer therapy. However, iNKT cells are rare, heterogeneous immunoregulatory cells (only 0.3% of the total number of PBMCs)13, which limits potential clinical applications. These cells are mainly divided into three subpopulations: 1) iNKT1 cells, which have a high expression of promyelocytic leukemia zinc-finger protein (PLZF) and T-box transcription factor (T-bet); 2) iNKT2 cells with intermediate expression of PLZF and GATA binding protein 3 (GATA3); 3) iNKT17 cells with low expression of PLZF and retinoid-related orphan nuclear receptor (ROR)-γt that secrete IFN-γ, IL-4, and IL-1714. Activated iNKT cells secrete Th1, Th2, and Th17-like cytokines, which determine the different immunomodulatory effects of iNKT cells15. The immunomodulatory and immunotherapeutic effects of specific activation of various subpopulations of iNKT cells are different. Therefore, the selection of specific phenotypes of iNKT cells (mainly iNKT2) with anti-inflammatory functions to regulate the immune response of the body can correct the immune imbalance and immune disorders in RA.
The establishment of an ideal animal model is of great significance for the treatment and study of RA pathogenesis. Presently, the most commonly used and mature animal models include collagen-induced arthritis, adjuvant arthritis, zymosan-induced arthritis, and polysaccharide-induced arthritis16–17. However, there is no model that can fully simulate all the features of human RA. Type II collagen-induced arthritis (CIA) is a classic arthritis model. The CIA is induced by immunization of mice with type II collagen-specific monoclonal antibodies, reflecting the antibody dependence of this disease model. Benurs et al. described a model with a systemic immune response to glucose-6-phosphate isomerase (G6PI), which induces peripheral symmetric polyarthritis in susceptible mouse strains18,19. In this model, the development of arthritis depends on T cells, B cells, and innate immunity18,19,20. Horikoshi21 found that RA models resulting from immunization of DBA/1 mice with G6PI polypeptide fragments are more similar to human RA in terms of CD4+ T cells and cytokines (i.e., IL-6 and TNF-α) than the CIA models. In order to increase the stimulating effect on the TCR recognition site, the mixed polypeptide fragments of G6PI (hGPI325-339 and hGPI469-483) were used to immunize DBA/1 mice to construct the RA mouse model. The success rate of this approach can high because hGPI325-339 and hGPI469-483 are immunodominant for I-A q-restricted T cell responses. Therefore, this model can simulate the overproliferation of CD4+ T cells and iNKT cell defects in RA patients22. The basic research of RA immunopathology laid the foundation for our further in-depth investigation.