NKT cells constitute a unique subset of lymphocytes that co-express TCRs and natural killer cell markers, functioning as a crucial bridge between innate and adaptive immunity (26). Based on TCR composition and antigen specificity, NKT cells are classified into three major subsets: type I NKT cells (invariant NKT, iNKT), type II NKT cells, and NKT-like cells (27). Type I NKT cells express a semi-invariant TCR—Vα14-Jα18 paired with Vβ8.2 in mice and Vα24-Jα18 paired with Vβ11 in humans—and recognize glycolipid antigens presented by the non-polymorphic CD1d molecule, with α-galactosylceramide (α-GalCer) serving as the prototypical ligand (28). During thymic development, iNKT cells undergo agonist selection through strong TCR engagement with CD1d-expressing double-positive cortical thymocytes, unlike conventional T cells that interact with thymic epithelial cells. This selection induces the expression of promyelocytic leukemia zinc finger (PLZF), a master regulator that programs their innate-like phenotype (25). Subsequent lineage commitment into iNKT1, iNKT2, or iNKT17 subsets is directed by transcription factors T-bet, GATA3, and RORγt, respectively, under the influence of local cytokines (29). In contrast, type II NKT cells exhibit diverse TCR repertoires and are capable of recognizing a broader spectrum of endogenous and microbial-derived lipid antigens, although their precise biological roles remain poorly defined due to the absence of specific surface markers (12). NKT-like cells, although expressing NK-associated markers such as CD161 and CD56, display highly variable TCRs and do not depend on CD1d-mediated antigen presentation (30). Upon activation, NKT cells rapidly secrete a broad array of cytokines, including IFN-γ, IL-4, IL-10, and IL-17, thereby exerting widespread regulatory effects on dendritic cells, B cells, conventional T cells, macrophages, and NK cells (31). Notably, the functional outcomes of NKT cell activation are context-dependent, with cytokine milieu and antigen presentation influencing whether they assume pro-inflammatory or immunoregulatory roles, thereby highlighting their versatility in immune modulation (32, 33). In mice models, iNKT cells are further categorized into functional subsets according to transcription factor expression and cytokine profiles: iNKT1 (IFN-γ⁺, T-bet⁺), iNKT2 (IL-4⁺, GATA3⁺), iNKT17 (IL-17⁺, RORγt⁺), and iNKT10 (IL-10⁺) (34, 35). These subsets exhibit tissue-specific distributions. Hogquist et al. have demonstrated that iNKT1 cells are prevalent in the liver, whereas iNKT10 cells are enriched in adipose tissue, where they may contribute to the regulation of metabolic homeostasis (36). Although similar functional polarization has been proposed in humans, definitive counterparts to murine iNKT1, iNKT2, and iNKT17 subsets have yet to be conclusively identified (37).

MAIT cells represent a distinct subset of unconventional T lymphocytes, defined by the expression of a semi-invariant TCR. In humans, the canonical MAIT TCR typically comprises the TRAV1–2 gene segment paired with TRAJ33, TRAJ12, or TRAJ20, and is most frequently associated with TCRβ chains TRBV6 or TRBV20 (38). Unlike conventional T cells, which recognize peptide antigens presented by classical MHC molecules, MAIT cells are restricted by the monomorphic MHC class I-related molecule MR1. They recognize riboflavin (vitamin B2) metabolite derivatives synthesized by a broad spectrum of bacteria and fungi, with 5-OP-RU (5-(2-oxopropylideneamino)-6-D-ribityllumazine) being the most potent and well-characterized ligand (39). MAIT cells are highly enriched in mucosal tissues, including the lungs, liver, and gastrointestinal tract, accounting for up to 20–45% of T cells in the liver and 1–10% in the intestinal lamina propria. They also account for 1–10% of circulating T cells in peripheral blood, though this frequency varies with age, sex, and microbiome exposure (40). Phenotypically, MAIT cells display a memory-like and activated phenotype, even under steady-state conditions. They are characterized by high expression of CD161, CD26, and transcription factors such as PLZF, T-bet, and RORγt, reflecting their rapid effector potential and innate-like properties (41). Functionally, MAIT cells can rapidly produce pro-inflammatory cytokines—including IFN-γ, TNF-α, and IL-17—either through TCR-mediated recognition of microbial ligands or via cytokine-driven activation, particularly in response to IL-12, IL-18, and IL-7 (42). In addition to cytokine secretion, MAIT cells also exhibit cytotoxic activity. They release granzyme B and perforin and express surface markers such as CD107a during degranulation, thereby enabling the direct killing of infected or stressed cells (43). Park et al. discovered that MAIT cell migration and tissue homing are regulated by chemokine receptors such as CCR5, CCR6, CCR9, and CXCR6, facilitating their recruitment to mucosal and inflamed tissues (44). MAIT cell development is thymus-dependent and proceeds through three major developmental stages. Their positive selection in the thymus requires MR1 expression and riboflavin-derived ligands from commensal microbes. After thymic exit, microbial exposure in mucosal sites drives peripheral expansion and maturation. This environmental stimulation activates transcriptional programs involving PLZF, T-bet, and RORγt, shaped by IL-7, IL-12, IL-18, and IL-23 signaling (42, 43, 45). This maturation is regulated by commensal microbial exposure—particularly riboflavin-derived ligands—and transcriptional programs involving PLZF, RORγt, and T-bet. These transcriptional programs drive the differentiation of MAIT cells into functionally distinct subsets such as MAIT1 (IFN-γ⁺), MAIT17 (IL-17⁺), and intermediate states like MAIT2, which may function as progenitors or exhibit plasticity during inflammation (44, 46). In addition to classical MAIT1 and MAIT17 subsets, recent studies have identified a functionally distinct population of CD161^- MAIT cells that emerges during chronic inflammation. While CD161⁺ MAIT cells exhibit tissue-homing and antimicrobial properties, the CD161^- subset displays a more pronounced pro-inflammatory phenotype, characterized by heightened TNF-α and GM-CSF secretion and reduced cytolytic activity (47, 48). This phenotypic shift is thought to result from persistent TCR-independent stimulation, such as chronic exposure to IL-7, IL-15, or inflammatory cytokines in the synovium. Moreover, these cells exhibit altered MR1 responsiveness and may represent a partially exhausted yet pathologically active subset. In RA, the accumulation of CD161^- MAIT cells could therefore contribute to sustained inflammation and tissue injury, highlighting the importance of dissecting MAIT cell heterogeneity in both circulation and synovial tissue (49).

γδ T cells constitute a distinct subset within the T lymphocyte lineage, characterized by expression of a TCR composed of γ and δ chains, in contrast to conventional αβ T cells (50). Functioning at the interface of innate and adaptive immunity, γδ T cells rapidly respond to cellular stress signals and contribute to immune surveillance, microbial defense, and the maintenance of tissue homeostasis. In humans, γδ T cells are primarily classified into two major subsets based on the variable region of the δ chain: Vδ1⁺ and Vδ2⁺ cells (51). Additionally, a less common Vδ3⁺ subset exists, predominantly in the liver (52). Their thymic development involves lineage imprinting determined by TCR usage (e.g., Vδ1⁺ vs Vγ9Vδ2⁺), Notch signals, and cytokines. Vγ9Vδ2⁺ cells—dominant in peripheral blood—are programmed for IL-17 or IFN-γ production and recognize phosphoantigens via BTN3A1/BTN2A1 complexes (53, 54). Vδ1⁺ T cells are predominantly localized in mucosal and epithelial tissues, including the intestine, skin, and lungs. These cells exhibit broad ligand recognition, including CD1d and endothelial protein C receptor (EPCR), and demonstrate pronounced tissue tropism and functional plasticity (55). In contrast, Vδ2⁺ T cells—typically paired with the Vγ9 chain (Vγ9Vδ2⁺)—represent the predominant γδ T cell population in adult peripheral blood (53). These cells recognize small phosphorylated metabolites (phosphoantigens, pAgs), such as HMBPP—a microbial intermediate produced via the non-mevalonate isoprenoid biosynthesis pathway. Binding of phosphoantigens induces conformational changes in BTN3A1, thereby triggering TCR-dependent activation of Vγ9Vδ2⁺ cells (56). Based on cytokine secretion profiles, γδ T cells can be further subdivided into functional subsets: γδT1 cells, which primarily produce IFN-γ and are involved in antiviral and antitumor responses; and γδT17 cells, which secrete IL-17 and contribute to inflammatory processes and tissue remodeling (57). Upon activation, γδ T cells promptly release effector molecules, including IFN-γ, TNF-α, IL-17, perforin, and granzyme B, facilitating direct cytotoxicity against infected or transformed cells (58). Under inflammatory conditions, they may upregulate antigen-presenting molecules (e.g., HLA-DR) and co-stimulatory molecules (e.g., CD80, CD86), acquiring antigen-presenting cell (APC)-like properties that support αβ T cell activation (59). Importantly, γδ T cells exhibit memory-like features. In particular, Vγ9Vδ2⁺ cells can undergo clonal expansion in response to antigenic stimulation, giving rise to long-lived effector memory populations with the capacity for rapid recall responses, enhanced cytokine production, and durable immune protection (58).

NKT cells, which integrate characteristics of both innate and adaptive immunity, perform multifaceted roles in the immunopathogenesis of RA ( Figure 1 ). Linsen et al. observed depleted NKT cells in RA peripheral blood, but functional Th0-like NKT cells persisted in synovial fluid, suggesting they may locally regulate immune responses and contain joint inflammation (60). Functionally, NKT cells exhibit both pro-inflammatory and anti-inflammatory properties. They contribute to inflammatory processes by producing IFN-γ and TNF-α, which enhance macrophage activation and T cell responses. Conversely, they can exert anti-inflammatory effects through the secretion of IL-10 and TGF-β. In RA, NKT cells in synovial fluid predominantly exhibit a pro-inflammatory phenotype, with elevated levels of IFN-γ and reduced IL-10 expression, indicative of a disease-promoting functional shift (61). Coppieters et al. confirmed that beyond cytokine-driven effects, NKT cells also contribute to synovial pathology and joint destruction through multiple effector mechanisms. They interact with fibroblast-like synoviocytes (FLS), promoting their proliferation and stimulating the release of inflammatory mediators such as IL-6 and IL-8. TNF-α released by activated NKT cells binds to TNFR1 on FLSs, activating canonical NF-κB and MAPK signaling cascades (notably p38 and JNK), which leads to increased FLS proliferation and elevated secretion of IL-6 and IL-8. These cytokines further amplify local inflammation and contribute to recruitment of neutrophils and osteoclast precursors. In parallel, TNF-α and IFN-γ upregulate RANKL expression on FLSs and osteoblasts, promoting interaction with RANK on osteoclast progenitors. This triggers TRAF6-dependent signaling and activation of NFATc1, the master transcription factor for osteoclastogenesis, thereby accelerating bone resorption in RA joints (62–65). Furthermore, NKT cells promote osteoclastogenesis by inducing the expression of osteoclast-associated genes—including tartrate-resistant acid phosphatase (TRAP), calcitonin receptor (CTR), and cathepsin K—thereby facilitating bone resorption and joint erosion (62). In addition, NKT cells may influence RA progression through their cytolytic activity. Although the relative proportions of NK and NKT cells may remain unchanged in RA patients, their cytotoxic potential—evidenced by upregulated expression of perforin and granzyme B—is enhanced and correlates with disease severity (63). This suggests that it is not merely their presence, but rather their effector function, that contributes to tissue damage. CD1d, the critical antigen-presenting molecule for NKT cell activation, exists in both membrane-bound and soluble forms. In RA, circulating levels of soluble CD1d (sCD1d) are markedly reduced, impairing peripheral NKT cell activation and IFN-γ production, thereby contributing to immune dysregulation (64). Both experimental and clinical data support the therapeutic activation of NKT cells using glycolipid antigens such as α-GalCer (KRN7000), which can suppress autoimmune inflammation by skewing immune responses toward Th2 or regulatory phenotypes. However, RA patients exhibit heterogeneous responses to α-GalCer stimulation: synovial NKT cells retain partial functionality, whereas peripheral NKT cells often show impaired expansion and diminished IFN-γ secretion (60). Notably, Jin et al. observed that treatment with α-GalCer in mice with collagen-induced arthritis (CIA) models mitigates disease severity, reduces osteoclast activity, and lowers pro-inflammatory cytokine levels—likely via IFN-γ–mediated mechanisms (65). Recent evidence also implicates NKT cells in the regulation of CD4⁺ T cell subset differentiation. NKT-derived cytokines—including IL-4, IL-12, and IFN-γ—significantly influence the polarization of naïve CD4⁺ T cells into Th1, Th2, Th17, or Treg lineages, thereby indirectly shaping the adaptive immune landscape in RA (66). Single-cell transcriptomic profiling of RA synovial tissues has revealed a significant reduction in NKT cell signatures compared to osteoarthritic tissues, underscoring their potential as biomarkers and therapeutic targets within the inflamed synovial microenvironment (67). Overall, NKT cells serve as a crucial link between immune dysregulation, synovial hyperplasia, and bone erosion, and represent promising candidates for immunomodulatory therapies in RA. However, clinical and transcriptomic data reveal a reduction of iNKT cell populations in both peripheral blood and synovial tissues of RA patients. Reduced CD1d expression on antigen-presenting cells may limit iNKT activation and impair IFN-γ production, disrupting the balance between pro-inflammatory and regulatory responses (68, 69). This reduction could result from chronic inflammatory feedback, insufficient survival signals, or persistent antigen exposure that promotes NKT cell exhaustion or egress. Notably, single-cell transcriptomic analyses have failed to detect robust iNKT signatures in inflamed synovium, despite their known capacity for local immunoregulation (70). These findings raise the possibility that iNKT cells exert their protective functions primarily in early RA stages but are numerically and functionally depleted during disease progression. Therefore, their loss may reflect a breakdown of innate regulatory networks, allowing unchecked Th17-driven inflammation and tissue destruction to predominate.

MAIT cells, defined by their recognition of microbial-derived vitamin B2 metabolites via MR1, are increasingly implicated in RA pathogenesis ( Figure 2 ) (19, 71). Zhao et al. demonstrated that, although the frequency of peripheral MAIT cells in RA patients was comparable to that in healthy controls, these cells exhibited distinct phenotypic alterations, including markedly reduced CD161 expression and a skewing toward the CD4⁺ subset (72). Functionally, MAIT cells displayed hyporesponsiveness, as indicated by impaired CD25 and CD69 upregulation upon E. coli stimulation, suggestive of chronic activation and functional exhaustion. Importantly, CD161 expression levels were inversely correlated with disease activity, underscoring their potential relevance to RA pathogenesis (72). Furthermore, MAIT cells are enriched in the synovial fluid of RA patients, suggesting tissue recruitment (46). Chiba et al. reported that pro-inflammatory cytokines such as TNF-α and IL-1β upregulate adhesion molecules (e.g., E-selectin) and chemokines (e.g., CCL20), facilitating MAIT cell migration through interactions involving Sialyl Lewis X (SLeX) and CCR6. This supports a model whereby systemic inflammation and vascular activation promote MAIT infiltration into inflamed joints (15, 73). Once within synovial tissues, MAIT cells exhibit heightened activation and cytokine output, notably IL-17 and TNF-α, which contribute to FLS activation and joint damage (46, 71). IL-17 and TNF-α produced by MAIT cells synergistically activate FLSs through NF-κB and STAT3 signaling pathways, leading to increased expression of IL-6, IL-8, and matrix metalloproteinases (MMPs), as well as RANKL, which further drives osteoclastogenesis and cartilage degradation (46, 71, 73). Additionally, MAIT cells mediate osteoblast lysis via granzyme B and perforin release. These cytolytic molecules disrupt mitochondrial integrity, induce cytochrome c release, and activate caspase-dependent apoptotic pathways, thereby contributing to bone loss in RA (71). At the molecular level, MAIT cells promote B cell activation and autoantibody production through direct cell–cell interactions involving CD40L–CD40 signaling and by secreting IL-21, which enhances plasmablast differentiation and IgG class switching (46, 74). They may also exert direct cytotoxicity via granzyme B and perforin pathways, and upregulate NKG2D, further linking them to cytolytic effector functions (44). Notably, in murine CIA models, MR1-deficient mice display attenuated arthritis severity, while adoptive transfer of MAIT cells restores disease intensity—highlighting their effector role independent of TCR signaling and likely mediated via IL-23/IL-1β stimulation (74). These findings align with human data linking MAIT cell-derived IL-17 to joint pathology (73). Furthermore, Li et al. revealed that cross-talk between MAIT cells and B cells may promote autoantibody production. MAIT cells have been shown to increase plasmablasts and Ig production in vitro, and may contribute to the humoral autoimmune responses characteristic of RA (46). The dual roles of MAIT cells—protective in some tissues, pathogenic in joints—highlight the complexity of their involvement, which appears dependent on localization, activation context, and disease phase. Although MAIT cells have been implicated in pathogenic inflammation within the joints of RA patients, they also exhibit protective functions in other tissue contexts—particularly at mucosal barriers. In the liver and gut, MAIT cells contribute to epithelial integrity, produce tissue-repair cytokines such as IL-17A and amphiregulin, and help maintain barrier homeostasis following microbial challenge (48). Their rapid response to bacterial metabolites presented via MR1 allows them to limit pathogen dissemination without triggering excessive inflammation. Notably, MAIT cells can produce IL-22 and GM-CSF in the context of commensal-derived antigens, which further supports epithelial regeneration and antimicrobial peptide production (75). In models of intestinal injury (e.g., DSS-induced colitis), MAIT cells were shown to limit barrier breakdown and accelerate recovery through cytokine-driven repair programs, a function partly dependent on IL-17A signaling (76). Similarly, in lung infection models, MAIT cells facilitated pathogen clearance while promoting epithelial regeneration—highlighting their role in host protection without overt tissue damage (77). Taken together, these findings suggest that MAIT cell functional polarization is tissue- and context-dependent. While they may adopt a pathogenic phenotype in inflamed joints—particularly under chronic IL-7/IL-15 stimulation or microbial dysbiosis—they retain the capacity for immunoregulation and tissue protection in non-joint sites. Understanding the signals that drive this dichotomy is critical for therapeutic targeting.

γδ T cells are a critical subset of unconventional T lymphocytes that bridge innate and adaptive immunity and have been identified as key regulators in the multifactorial pathology of RA ( Figure 3 ). However, the reported frequency of γδ T cells in both peripheral blood and synovial fluid remains controversial. While some studies report increased levels, others observe reductions in blood, possibly due to migration into inflamed joints (78–80). These discrepancies likely stem from methodological differences (e.g., flow cytometry panels, subset gating), patient heterogeneity (e.g., disease stage, serostatus), tissue compartment analyzed (blood vs. synovium), and treatment exposure (80–84). This variability highlights the need for standardized protocols and subset-specific analysis to clarify the role of γδ T cells in RA pathogenesis. Although they do not constitute the predominant T cell population in synovial tissue, γδ T cells exert disproportionate influence due to their rapid cytokine secretion, antigen-presenting capabilities, and extensive interactions with other immune cells—playing a potent “small-in-number, strong-in-function” role in the RA inflammatory microenvironment (81). Distinct γδ T cell subsets exert different functions in RA. Vδ1⁺ T cells are mainly distributed in epithelial and mucosal tissues and may possess immunoregulatory properties in RA (82). In contrast, Vδ2⁺ T cells are markedly expanded in peripheral blood and synovial compartments of RA patients, displaying a highly pro-inflammatory phenotype and producing large amounts of IL-17, IFN-γ, and TNF-α, all of which contribute to synovitis and bone erosion (80, 81, 83). γδ T cells promote bone destruction through multiple mechanisms. IL-17 secreted by γδ T cells induces RANKL expression on synovial fibroblasts and osteoblasts. In addition, TNF-α released by γδ or NKT cells binds to TNFR1 on FLSs and osteoblasts, activating the canonical NF-κB and p38 MAPK signaling pathways. These pathways upregulate RANKL transcription via nuclear RelA (p65) translocation, enhancing osteoclastogenic potential and contributing to bone resorption in RA (65, 80). However, under certain conditions, such as CD3/CD28 co-stimulation, γδ T cells produce high levels of IFN-γ, which can inhibit osteoclastogenesis, indicating that γδ T cells may possess environmentally dependent protective effects (59). The migratory potential of γδ T cells is another crucial factor in RA pathogenesis. Mo et al. showed that Vδ2⁺ T cells in RA patients express elevated levels of chemokine receptors CCR5 and CXCR3, upregulated via the TNF-α/NF-κB pathway. Specifically, TNF-α stimulation activates the canonical NF-κB cascade in Vδ2⁺ cells through TNFR1, leading to IκBα degradation and RelA (p65) nuclear translocation, which promotes CCR5 and CXCR3 gene transcription. Anti-TNF-α therapies can downregulate these receptors and partially restore peripheral γδ T cell levels, highlighting their potential as therapeutic targets (80). Beyond classic inflammatory pathways, γδ T cells are closely associated with neutrophil function. Bouchareychas et al. demonstrated that γδ T cells regulate the production of IL-27 in neutrophils. Blockade of γδ T cells leads to increased IL-27 production, which in turn suppresses IL-23-induced arthritis. Blockade of γδ T cells leads to increased IL-27 production, which in turn suppresses IL-23-induced arthritis. Mechanistically, IL-27 inhibits STAT3 phosphorylation in IL-23-responsive cells such as Th17 and γδ T17 cells, reducing IL-17 production and limiting downstream inflammatory amplification. This establishes a negative feedback loop between γδ T cells, neutrophils, and IL-27 (83, 85). In addition, γδ T cells exhibit memory-like immune features. For instance, Vγ9Vδ2⁺ T cells in RA can undergo clonal expansion and respond rapidly to antigen re-stimulation, releasing high levels of inflammatory cytokines. This property sustains chronic inflammation and plays a pivotal role in the persistent phase of RA (83). Interestingly, γδ T cells may also exert regulatory or protective functions. For instance, Su et al. elucidated that treatment of RA patients with ⁹⁹Tc-methylene diphosphonate (⁹⁹Tc-MDP) significantly increases γδ T cells and Tregs, alongside decreased TNF-α and IL-6 levels and elevated TGF-β expression, indicating a potential role in promoting immune tolerance under specific conditions (84). Moreover, in rat adjuvant arthritis models, depletion of γδ T cells exacerbates joint destruction, further supporting their stage-dependent dual role in RA (86). In RA synovial fluid, γδ T cells also exhibit activation-dependent phenotypic changes, including downregulation of CD16 (FcγRIII) and upregulation of HLA-DR, suggesting a role in antigen presentation and further amplification of immune responses (81). In summary, γδ T cells play multifaceted roles in RA, functioning as both drivers of pathogenic inflammation and potential immune regulators depending on microenvironmental cues ( Table 2 ).