PD-1 is an inhibitory receptor of the CD28 family that is inducibly expressed on T cells, B cells, and NK cells, and is also expressed in subsets of myeloid cells (13). Its ligands, programmed death-ligand 1 (PD-L1) and programmed death-ligand 2 (PD-L2), show distinct but complementary patterns of expression. PD-L1 is broadly expressed at low levels on APCs and on a broad range of non-hematopoietic tissues such as endothelial and epithelial cells. Its expression is strongly induced by inflammatory cytokines, particularly type I and type II interferons. PD-L2 expression is more restricted, largely confined to dendritic cells and macrophages, and is upregulated by IL-4 and GM-CSF (14).

Upon ligand binding, the cytoplasmic tail of PD-1 recruits the phosphatases SHP-1 and SHP-2 through its immunoreceptor tyrosine-based inhibitory motif (ITIM) and switch motif (ITSM). SHP-2 dephosphorylates key signaling molecules such as CD3ζ and ZAP70, which leads to reduced TCR activation. This also inhibits PI3K signaling, leading to reduced interleukin-2 (IL-2) production and impaired metabolic activity (15). The resulting IL-2 deficiency promotes anergy in both CD4⁺ and CD8⁺ T cells (16). Inhibitory signals mediated by PD-1 and its ligands (PD-L1/PD-L2) contribute to the regulation of central and peripheral tolerance through various mechanisms. In the thymus, PD-1 expression is initiated in CD4 − CD8 − double-negative thymocytes as they undergo TCRβ rearrangement, where it fine-tunes signaling thresholds for positive selection, thereby limiting the expansion of double-positive thymocytes and contributing to central tolerance (17). At the peripheral level, PD-1 signaling depends on the differentiation state of the T cell: in naïve T cells, PD-1 ligation increases the activation threshold. This restricts proliferation and differentiation after antigen stimulation and thereby contributes to the maintenance of peripheral tolerance (13); in effector and memory T cells, sustained PD-1 signaling suppresses cytokine secretion and cytotoxic activity and contributes to the establishment of an exhausted phenotype during chronic antigen exposure (18). Moreover, PD-L1 also contributes to tolerance by promoting the differentiation of naïve CD4⁺ T cells into Foxp3⁺ induced regulatory T cells (iTreg), highlighting its essential role in peripheral tolerance (19).

Dysregulation of checkpoint signaling, particularly within the PD-1/PD-L1/PD-L2 axis, provides a mechanistic basis for the development of autoimmunity. Under physiological conditions, PD-1 signaling attenuates TCR- and CD28-driven pathways, limiting the expansion of autoreactive clones and maintaining peripheral tolerance. In systemic autoimmunity, when this inhibitory input is impaired—through genetic deficiency, altered ligand expression, or interference by soluble isoforms—CD4⁺ and CD8⁺ T cells exhibit sustained activation. This results in increased production of proinflammatory cytokines such as TNF-α, IFN-α, and IFN-γ, which promote the differentiation of autoreactive B cells and the persistence of long-lived memory clones. T follicular helper (Tfh) and T peripheral helper (Tph) cells, which frequently express high levels of PD-1, provide signals that promote B-cell activation and differentiation, thereby enhancing germinal center responses and driving the production of autoantibodies (20).

In AAV specifically, monocytes from patients exhibit reduced PD-L1 expression, indicating impaired inhibitory signaling from APC to T cells (21). In parallel, expansion of PD-1^hi CXCR5⁻ Tph subsets has been reported, providing augmented signals for B-cell activation and differentiation and thereby supporting autoreactive responses (22). In renal tissue, both PD-1 and PD-L1 expression are decreased, indicating a local loss of checkpoint control (23). Moreover, recent single-cell transcriptomic analyses have identified neutrophil subsets highly responsive to IFN-γ and TNF-α, cytokines that are typically elevated when checkpoint restraint is diminished. These neutrophils exhibit increased MPO and FcγR expression and are particularly susceptible to ANCA-induced activation and NET formation, linking checkpoint dysfunction to T-cell activation and neutrophil-driven vascular injury (24). In addition, Slot et al. (25) described genetic polymorphisms on PD-1 and CTLA4 than may also lead to T-cell hyperreactivity and contribute to the pathogenesis of AAV.

Similar alterations are reported in other autoimmune diseases. In systemic lupus erythematosus (26) and rheumatoid arthritis (27), circulating T cells often show increased PD-1 expression, which paradoxically associates with sustained activation and autoantibody production. In contrast, patients with type 1 diabetes or Sjögren’s syndrome, the frequency of PD-1⁺ T lymphocytes were found to be significantly decreased (27, 28). Despite these apparent differences, both scenarios reflect a common defect: inhibitory signaling through PD-1/PD-L1 is insufficient to maintain tolerance. Checkpoint knockout models clearly demonstrate their essential role in tolerance. PD-1–deficient mice develop chronic autoimmune disease, including lupus-like glomerulonephritis and arthritis (29), demonstrating that loss of this pathway lowers the threshold for lymphocyte activation and drives organ-specific autoimmunity (30).

The development of ANCA-associated vasculitis following immune checkpoint inhibitor therapy provides compelling evidence for the involvement of the PD-1 axis in disease pathogenesis (31). Understanding the precise role of the PD-1 axis in AAV could open avenues for targeted immunomodulatory strategies, potentially allowing the identification of patients at risk for relapse or those who may benefit from therapies such as rituximab.

CD28 is a central co-stimulatory receptor of the CD28 family and an essential regulator of naïve T-cell activation. It is constitutively expressed on most CD4⁺ T cells and on a subset of CD8⁺ T cells. Through binding to the B7 ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells, CD28 provides the co-stimulatory signals required for effective T-cell proliferation, survival, and cytokine production. This interaction is tightly controlled by the inhibitory receptor CTLA-4, which shares structural homology with CD28 and displays higher affinity for CD80 and CD86. By competing for these ligands, CTLA-4 restricts CD28-mediated activation and contributes to the regulation of T-cell responses (32). The balance between these activating and inhibitory inputs determines whether antigen-engaged T cells proceed toward effector differentiation or remain functionally restrained.

In ANCA-associated vasculitis, components of the CD28 pathway show disease-related modulation. The proportion of circulating CD4⁺CD28⁺ T cells is significantly higher in patients with low BVAS compared with those with high BVAS, suggesting that loss or down-modulation of CD28 expression may be associated with more active inflammatory disease (33). Consistent with functional relevance in AAV, T cells from patients with active granulomatosis with polyangiitis display enhanced proliferative responses following CD2/CD28 co-stimulation compared with healthy controls (34), supporting a role for heightened CD28-dependent signaling in pathogenic T-cell activation.

Evidence from other forms of vasculitis further reinforces the relevance of CD28-mediated co-stimulation in vascular inflammation. In a human artery–severe combined immunodeficiency mouse chimera model of giant cell arteritis, blockade of CD28 signaling disrupted T-cell metabolic fitness and markedly attenuated vessel wall inflammation and remodeling (35). Similarly, in Takayasu’s arteritis, active disease is associated with increased CD28 mRNA expression compared with inactive stages (36).

Finally, alterations of the CD28 axis have also been described in other systemic autoimmune diseases, providing additional context for its pathogenic relevance. In systemic lupus erythematosus, active disease is characterized by reduced proportions of CD4⁺CD28⁺ T cells together with elevated circulating levels of soluble CD28, which show a positive correlation with clinical disease activity and are associated with organ involvement, including lupus nephritis (37). These observations parallel the CD28 dysregulation observed in AAV and other vasculitis, supporting the concept that imbalance within the CD28/CTLA-4 pathway represents a shared mechanism of pathological T-cell activation across systemic autoimmune disorders.

CTLA-4 is an inhibitory receptor of the CD28 family that regulates early T-cell activation by competing with CD28 for the shared ligands CD80 and CD86 on antigen-presenting cells. Because CTLA-4 binds these ligands with higher affinity, it dampens co-stimulatory signaling, promotes peripheral tolerance, and supports the suppressive function of regulatory T cells. CTLA-4 also shapes helper T-cell differentiation by favoring Th1-driven responses while limiting Th2-associated cytokines—an effect that counters CD28-mediated activation (38). Given the predominance of Th1-type immunity in several autoimmune diseases, the balance between CD28 and CTLA-4 signaling is considered highly relevant to disease pathogenesis.

Alterations in both membrane-bound and soluble forms of CTLA-4 have been reported in AAV. At the cellular level, CTLA-4 expression is increased on circulating CD4⁺ T cells during active disease, and higher expression levels correlate with more severe clinical manifestations. However, despite this elevated baseline expression, T cells from GPA patients exhibit a reduced ability to further upregulate CTLA-4 following polyclonal stimulation, indicating an impaired capacity to mount appropriate inhibitory responses (39). Soluble components of this pathway are also disrupted: serum sCTLA-4 concentrations are lower in AAV patients than in healthy controls and decline further during remission, paralleling decreases in BVAS (33). These findings suggest that dysregulated CTLA-4 signaling—characterized by inadequate inducible inhibition and altered soluble mediator levels—may contribute to excessive T-cell activation and the loss of immune homeostasis in AAV.

The inducible T-cell co-stimulator (ICOS, CD278) is a member of the CD28/CTLA-4 co-stimulatory receptor family that regulates T-cell activation and T–B cell collaboration. ICOS is inducibly expressed on activated CD4⁺ and CD8⁺ T cells, particularly on Tfh and Treg, where it promotes cytokine production, T-cell proliferation, and B-cell response. Its ligand, ICOSL (B7-H2, CD275), belongs to the B7 superfamily and is expressed on antigen-presenting cells, including B cells, monocytes, dendritic cells, and some non-hematopoietic cells such as endothelial and epithelial cells (40). The ICOS–ICOSL interaction activates downstream PI3K signaling, supporting germinal center formation, antibody production, and maintenance of long-lived plasma and memory cells (41). Given its central role in T–B collaboration, alterations in the ICOS pathway have been investigated as indicators of humoral immune activation in AAV.

Dysfunctional ICOS–ICOSL signaling is a well-recognized feature across multiple autoimmune diseases, providing mechanistic context for its involvement in AAV. In AAV, alterations in the ICOS–ICOSL co-stimulatory pathway have been documented at both cellular and soluble levels. Patients with MPO-AAV display an increased frequency of ICOS⁺ Tfh cells and a higher ICOS⁺/PD-1⁺ Tfh ratio compared with healthy controls (42), indicating an expansion of activated Tfh subsets capable of driving ANCA production through sustained B-cell stimulation. Consistent with these cellular abnormalities, serum concentrations of soluble ICOS (sICOS) are significantly elevated in AAV relative to controls (33), suggesting that systemic enhancement of ICOS signaling may contribute to dysregulated humoral immunity.

In RA, overexpression of ICOS and ICOSL within the synovium and in circulating immune cells promotes T-cell activation and Tfh-mediated B-cell responses. Increased frequencies of CD19⁺ICOSL⁺ B cells correlate with clinical disease activity and histopathologic damage (43), while therapeutic blockade of this pathway ameliorates inflammation and joint destruction in murine arthritis models (44). Similarly, in systemic lupus erythematosus (SLE), ICOS expression is significantly increased on both CD4⁺CD45RO⁺ and CD8⁺CD45RO⁺ T cells, with even higher levels observed in patients with lupus nephritis (45). Functional studies using a high-affinity anti-ICOS monoclonal antibody (JTA009) demonstrate that ICOS engagement enhances T-cell proliferation and cytokine secretion—including IFN-γ, IL-4, and IL-10—and promotes IgG anti–double-stranded DNA antibody production by autologous B cells (46).

Together, these findings highlight how exaggerated ICOS signaling supports aberrant T–B cell cooperation and amplifies humoral autoimmunity in AAV and likely contributes to the expansion of pathogenic Tfh cells, persistent B-cell activation, and the generation of ANCAs, underscoring ICOS–ICOSL signaling as a relevant pathway in disease pathogenesis.

The CD40–CD40L interaction constitutes a major co-stimulatory pathway that regulates communication between T cells, B cells, and antigen-presenting or endothelial cells. CD40 is a member of the tumor necrosis factor receptor (TNFR) superfamily expressed on B cells, dendritic cells, macrophages, and various non-hematopoietic cells, whereas its ligand CD40L (CD154) is transiently expressed on activated CD4⁺ T cells and platelets. Engagement of CD40 by CD40L triggers downstream NF-κB signaling, induces cytokine release, promotes B-cell differentiation, immunoglobulin class switching, and upregulation of adhesion molecules, ultimately sustaining inflammatory and autoimmune responses (47).

In AAV, both membrane-bound and soluble CD40L (sCD40L) are elevated compared with healthy controls (48), highlighting a state of heightened costimulatory signaling. In contrast, serum concentrations of soluble CD40 (sCD40) did not differ significantly from controls but displayed strong correlations with indices of disease severity and renal dysfunction (33). As the CD40–CD40L axis is essential for T–B cell collaboration, germinal center formation, and the generation of high-affinity, in the context of AAV increased CD40L availability may potentiate aberrant B-cell activation and promote the production of ANCAs, a central driver of disease. Enhanced CD40 signaling can also amplify T-cell activation and inflammatory cytokine release, strengthening the proinflammatory milieu that contributes to endothelial injury and necrotizing vasculitis.

Findings from other autoimmune diseases provide strong mechanistic support for the pathogenic role of this pathway. In SLE, CD40L is overexpressed on immune cells (49), serum sCD40L levels correlate with disease activity and renal involvement (50), and transgenic overexpression of CD40L induces a lupus-like phenotype in mice (51), whereas blockade of the CD40–CD40L interaction reduces autoantibody production and glomerular inflammation (49). Similar patterns are observed in MS, where CD40 activation within B cells and the CNS promotes inflammation and demyelination, and inhibition of this pathway ameliorates disease in EAE models (52). These cross-disease observations reinforce the concept that sustained CD40–CD40L signaling drives chronic autoimmunity.

The OX40–OX40L interaction represents a co-stimulatory signaling pathway that regulates T-cell activation, proliferation, and survival. OX40 (CD134), a member of the TNF receptor superfamily, is transiently expressed on activated CD4⁺ and CD8⁺ T cells, while its ligand OX40L (CD252, TNFSF4) is expressed on dendritic cells, B cells, activated T cells, mast cells, Langerhans cells, and vascular endothelial cells (12). Engagement of OX40 by trimeric OX40L enhances Th-cell polarization, sustains the function of regulatory and memory T cells, and promotes adhesion of activated T cells to the endothelium, thereby contributing to chronic inflammatory responses (53).

In AAV, increased expression of OX40 and an expansion of OX40⁺ T cells have been identified in both peripheral blood and inflamed tissues, where these cells predominantly produce TNF-α (54, 55). Although quantitative data on soluble OX40L in AAV remain limited, the available evidence suggests that activation of the OX40–OX40L pathway parallels disease activity and may contribute to the persistence of chronic T-cell responses that support ongoing vascular inflammation.

Beyond AAV, the involvement of the OX40–OX40L axis in other systemic autoimmune disorders further highlights its pathogenic potential. In RA, OX40 and OX40L are overexpressed in inflamed synovial tissue and circulating immune cells, where they enhance effector T-cell activation, promote Tfh differentiation, and sustain autoantibody production (56–58). Consistent with these cellular findings, soluble OX40L (sOX40L) is detectable in early RA and correlates with autoantibody positivity, supporting its value as a serological marker of humoral immune activation (57). Experimental blockade of the OX40–OX40L interaction in murine arthritis models markedly reduces inflammation and joint destruction, confirming its central role in disease pathogenesis (56). Similar mechanisms appear operative in SLE, where OX40L expression is increased on B cells and antigen-presenting cells, promoting Tfh differentiation and augmenting autoantibody production. Inhibition of OX40L in murine lupus models reduces disease severity and nephritis, reinforcing the contribution of this pathway to aberrant T–B cell collaboration and chronic autoimmunity (59, 60).

Taken together, these observations support the concept that heightened OX40–OX40L signaling contributes to the expansion of pathogenic T-cell populations, sustained B-cell help, and chronic inflammation in AAV, mirroring its established pathogenic roles in other autoimmune diseases.

Lymphocyte activation gene-3 (LAG-3, CD223) is an inhibitory receptor structurally related to CD4 that binds MHC class II with higher affinity and modulates T-cell activation, proliferation, and effector function. LAG-3 contributes to the control of CD4⁺ and CD8⁺ T-cell responses, supports regulatory T-cell activity, and helps maintain immune homeostasis during persistent antigen exposure (61).

In AAV, LAG-3 remains a relatively underexplored immune checkpoint, and current evidence is largely restricted to its soluble form. Serum concentrations of soluble LAG-3 (sLAG-3) are elevated in patients compared with healthy controls (33), mirroring the increases reported for other inhibitory checkpoint molecules during active disease. These findings suggest that alterations in LAG-3 signaling may participate in the broader dysregulation of T-cell inhibitory pathways characteristic of AAV, although further studies are needed to clarify its mechanistic role.

T-cell immunoglobulin and mucin-domain containing protein-3 (TIM-3) is a key inhibitory receptor expressed on a wide range of immune cells, including Th1 lymphocytes, cytotoxic T cells, Treg, and innate immune populations (61). Through interactions with ligands such as galectin-9, TIM-3 plays a critical role in limiting Th1-mediated inflammation and modulating myeloid cell activation.

In AAV, both cellular and soluble components of the TIM-3 pathway are altered in a disease-associated manner. At the cellular level, MPO-AAV patients display markedly reduced TIM-3 expression on dendritic cells. Notably, experimental blockade of TIM-3 further enhances NET-mediated dendritic cell cytokine production, indicating that loss of TIM-3 function facilitates NET-driven inflammation (62). These observations suggest that impaired TIM-3 signaling contributes to the amplification of innate and adaptive immune responses in AAV, promoting vascular inflammation and tissue damage.

B- and T-lymphocyte attenuator (BTLA) is an inhibitory receptor expressed on T cells, B cells, and dendritic cells, which interacts with its ligand, herpesvirus entry mediator (HVEM), to deliver negative regulatory signals that restrain immune activation. BTLA signaling is essential for maintaining peripheral tolerance, limiting T-cell proliferation, and dampening proinflammatory cytokine production (63).

Evidence on BTLA as a co-inhibitory pathway in human autoimmune diseases remains limited, and its pathogenic relevance is yet to be fully established. However, impaired BTLA–HVEM signaling has been hypothesized to diminish inhibitory checkpoint control, thereby facilitating sustained T- and B-cell activation. Available data in SLE support this concept, as reduced BTLA levels have been reported in patients and have been associated with disease progression (64). In AAV specifically, Werner et al. (65) reported reduced BTLA expression in circulating CD3⁺CD4⁻CD8⁻ double-negative T cells from AAV patients in remission compared with healthy controls, and this reduction was associated with disease activity and relapse rate. Moreover, BTLA stimulation using agonistic antibodies suppresses T-cell proliferation and decreases Th17 activity in both patients and controls, supporting a potential therapeutic role for BTLA pathway modulation in AAV. Notably, recent renal transcriptomic profiling studies have reported upregulation of BTLA in affected kidney tissue compared with healthy controls (66). In addition, soluble BTLA (sBTLA) levels were reported to significantly decrease with declining BVAS in AAV patients, supporting its potential utility as a dynamic marker of disease activity (33).

These findings suggest that BTLA dysregulation may contribute to disease pathogenesis in AAV. Further studies are needed to characterize both cellular and soluble components of the BTLA axis and to explore its potential as a therapeutic target.

CD27 is a co-stimulatory receptor of the tumor necrosis factor (TNF) receptor superfamily, expressed primarily on T cells, memory B cells, and subsets of natural killer cells. Its interaction with CD70 provides critical signals for T-cell activation, B-cell differentiation, and the generation of long-lived plasma cells. CD27–CD70 signaling is essential for effective adaptive immune responses, but sustained or dysregulated activation can contribute to chronic inflammation and autoimmunity (67).

In AAV, alterations in the CD27 axis have been less extensively studied, but available evidence suggests a potential role in disease pathogenesis. Increased frequencies of CD27⁺ T cells and CD27⁺ memory B cells could facilitate enhanced B-cell help and support ANCA production, thereby supporting persistent humoral autoimmunity, as circulating CD27^hi CD38^hi B cells have been identified as direct precursors of autoantibody-producing plasma cells. Focusing on B cells, Wang et al. described a significant increase in CD27^hi CD38^hi B cells during the acute phase of the disease, which additionally correlated with renal involvement (68). Although data remain limited, recent renal transcriptomic profiling studies further support the relevance of this pathway at the target organ level, reporting upregulation of CD27 in affected kidney tissue compared with healthy (66). In parallel, serum concentrations of soluble CD27 (sCD27) are elevated in patients compared with healthy controls (33). Investigation of both cellular and soluble components of this pathway may provide insights into disease activity and identify novel therapeutic opportunities.

Multiple studies have reported dysregulation of the PD-1 axis in AAV. As described before, at the cellular level, active AAV is characterized by increased PD-1 expression on circulating Tph cells involved in extrafollicular B-cell activation, with PD-1^hi Tph frequencies decreasing after immunosuppressive therapy (22). In parallel, a reduced availability of PD-1 ligands is observed in myeloid cells: circulating monocytes and neutrophils show diminished PD-L1 expression, which correlates with higher ANCA titers and greater disease severity (21). Importantly, reduced PD-L1 expression—particularly on neutrophils—can persist even during clinical remission, indicating incomplete restoration of immune checkpoint regulation (23). Consistent with these systemic findings, renal biopsies from patients with pauci-immune glomerulonephritis reveal a profound disruption of the PD-1/PD-L1 inhibitory axis. Both tubular and glomerular compartments show marked loss of PD-L1 expression (23), generating a renal microenvironment with limited inhibitory signaling that may favor the infiltration and activation of CD4⁺ T cells, B cells, neutrophils, and macrophages. In parallel, tubulointerstitial PD-1 expression is reduced in renal AAV and correlates with the presence of active glomerular and interstitial lesions (69), supporting a role for local checkpoint dysregulation in sustaining renal inflammation.

Soluble elements of this pathway also exhibit disease-associated variation, as serum sPD-1 concentrations are elevated in active AAV and correlate with BVAS and systemic inflammatory markers (33). In addition, it has been described that sPD-L1 increases during active disease and decreases with treatment, whereas sPD-L2 shows the opposite pattern, lower during activity and higher in remission (33). Extending these observations to the urinary compartment, urinary PD-1 and PD-L2 concentrations are reduced in patients with AAV compared with healthy controls, reaching their lowest levels during acute disease and partially recovering during remission, suggesting that local renal checkpoint dynamics are also reflected in urine biomarkers (70).

Taken together, these findings highlight a broad breakdown of PD-1–mediated inhibitory signaling in AAV, with increased PD-1 on pathogenic T-cell subsets and reduced PD-L1 across immune and renal compartments, a pattern that may serve as a useful indicator of immunologic activity and disease flare. Comparable alterations within the PD-1 axis have also been described in SLE, underscoring that the impairment of this axe represents a recurrent immunological disturbance across systemic autoimmune diseases (71–73).

Increased levels of several immune checkpoint molecules are associated with the presence and severity of disease activity. Serum concentrations of soluble CD28 (sCD28) are elevated compared with healthy controls and correlate with BVAS and systemic inflammatory indices (33). In contrast, serum concentrations of soluble CD40 (sCD40) are not significantly different from controls but show strong correlations with indices of disease severity and renal dysfunction (33). sTIM-3 levels correlate strongly with BVAS and renal parameters and decrease with treatment, confirming its value as a reliable and dynamic marker (33). Taken together, these findings indicate that circulating checkpoint molecules—particularly sCD28 and sTIM-3—may serve as useful biomarkers for assessing disease activity and organ involvement in AAV.

The evaluation of checkpoint molecule levels may be useful in assessing the response to rituximab. In the study conducted by Gamreith et al. (74) among patients receiving rituximab induction therapy the combination of lower sLAG-3 and higher sCD27 predicted therapy failure. In another study, Miyazaki et al. (75) reported that patients with a higher proportion of CD27⁻ B cells are at greater risk of poor remission, but treatment with rituximab can significantly improve their clinical outcomes compared to conventional therapy. This highlights CD27⁻ B cells as a potential biomarker for identifying patients who may benefit most from rituximab.

Some authors have also explored the role of checkpoint molecules as markers of relapse risk. In patients treated with rituximab, a multimarker strategy combining sTIM-3, sCD27, and sBTLA4 showed that lower levels of these molecules predicted relapse (74). In the same line, frequently relapsing EGPA has been associated with increased percentages of CD80⁺ B cells during active disease (76), indicating enhanced ligand availability in flare conditions.