Revisiting the CD40-CD40L axis: from mechanistic insight to therapeutic renewal in autoimmune disease

Abstract

Purpose of review

CD40 ligand (CD40L, CD154) is a costimulatory molecule required for adaptive immune responses. It arms CD4⁺ T cells to provide critical “help” to B cells, dendritic cells, and other antigen-presenting cells through interactions with CD40. Enhanced CD40L signaling promotes autoreactive B cell activation, germinal center hyperactivity, and autoantibody production, processes central to autoimmune pathogenesis. Recognition of the pathway’s importance in autoimmunity prompted clinical trials of anti-CD40L monoclonal antibodies in lupus, but early enthusiasm faded after unexpected thrombotic events. These first generation anti-CD40L antibodies were later found to trigger platelet activation through an Fc-mediated mechanism and led to re-engineering of antagonist therapeutic proteins.

Recent findings

Preclinical studies confirm that CD40L inhibition ameliorates disease across diverse autoimmune models by restraining aberrant B-cell and T-cell responses. Novel Fc-silent anti-CD40L antibodies, nonantibody CD40L antagonists, and anti-CD40 antibodies have since been developed to overcome prior safety concerns. These new-generation CD40L and CD40 antagonists have shown promising results in phase 2 trials spanning multiple autoimmune settings, renewing interest in therapeutic blockade of this pathway.

Summary

Next-generation CD40L and CD40-directed therapies are redefining costimulatory blockade in autoimmunity. Integrating preclinical discoveries with clinical translation offers new opportunities to optimize treatment and transform management of B cell-driven autoimmune disease.

INTRODUCTION

Often described as the field’s “holy grail”, the central challenge of rheumatology is how to suppress autoreactivity, or induce immune tolerance, in patients with autoimmune disease while preserving the mechanisms essential for effective host defense. However, many therapies in the rheumatologists’ toolbox rely on broad immunosuppression to achieve disease control. Although effective, these traditional approaches compromise protective immunity, increasing susceptibility to infections, impairing vaccine responses, and in some cases increasing the risk for malignancies. A road to advance the therapeutic paradigm can be found in selectively interrupting the aberrant signaling mechanisms controlling the key pathogenic drivers, most relevantly the autoreactive lymphocytes.

Generation of successful adaptive immunity requires multiple molecular signals to ensure the activation and expansion of antigen-specific lymphocytes. Costimulatory signals act in concert with antigen receptor engagement to enhance both the strength and specificity of the response. Among these costimulatory pathways, the CD40-CD40L signaling axis stands out for its central role in coordinating key aspects of adaptive immunity. CD40L, which is transiently expressed on activated T cells, plays an instrumental role in promoting the activation of B cells, dendritic cells, and stromal cells [1]. In the setting of autoimmunity, dysregulated CD40-CD40L signaling drives the persistence of autoreactive B cells, promotes autoantibody production, and amplifies inflammatory cascades by stimulating antigen-presenting cell (APC) function [2,3]. Thus, the CD40-CD40L axis is a central hub for cross talk between adaptive and innate immune responses. In this review, we examine how this signaling axis influences normal host defense and drives pathogenic responses in the context of autoimmunity. We focus on the CD40L-CD40 pathway as a compelling target for emerging therapies in autoimmunity and highlight both the opportunities and hurdles in targeting this pathway for therapeutic benefit.

CD40L (CD154): A NEXUS OF INNATE AND ADAPTIVE IMMUNE SIGNALING

The CD40-CD40L signaling axis mediates T cell “help” to B cells, APCs such as dendritic cells, and stromal cells. While CD40L expression is low to absent on resting T cells, CD40L is rapidly expressed and transiently upregulated on activated CD4⁺ T cells in response to T cell receptor (TCR) engagement (Signal 1), additional co-stimulatory cues (Signal 2), and cytokine signals (Signal 3) [4–6]. CD40L is also expressed on activated platelets [7] and, to a lesser extent, on other leukocytes and nontraditional immune subsets. In contrast to CD40L, CD40 expression is relatively widespread on both hematopoietic and nonhematopoietic cells. CD40 is expressed on B cells and APCs as well as on a range of stromal cells [8]. While CD40L operates primarily as a membranebound protein, it also exists in a soluble form with comparable bioactivity [9,10].

Initially characterized by Clark and Ledbetter as a B cell surface molecule [11], CD40 has emerged as a key player in the generation of humoral immunity, including clonal B cell expansion, B cell survival, germinal center maintenance, class-switch recombination, and affinity maturation [6,12] (Fig. 1). When CD40 engages CD40L, it undergoes conformational changes that facilitate recruitment of TNF receptor-associated factors (TRAFs) to its cytoplasmic tail, in turn triggering downstream signaling cascades that result in activation of nuclear factor κB (NF-κB), phosphoinositide 3-kinase (PI3K), and the mitogen-activated protein kinases (MAPK) [13,14]. The necessity of the CD40-CD40L pathway for B cell activation and germinal center responses is supported by the fact that CD40L signaling is required for T follicular helper cell differentiation [15]. In dendritic cells (DCs), CD40L-mediated CD40 activation induces cellular maturation, increases the production of T-cell activating cytokines, and upregulates expression of both MHC class I and II molecules and costimulatory molecules such as CD80 and CD86, enabling effective priming of naive T cells [16]. Patients with CD40L deficiency that develop X-linked hyper-IgM syndrome demonstrate profound defects in class-switch recombination and memory B cell responses [17,18] and experience recurrent infections and have impaired antibody responses. Similarly, CD40 or CD40L-deficient mice exhibit defective T cell-dependent antibody production and lack mature germinal centers [19,20].

FIGURE 1.

The CD40-CD40L signaling axis coordinates adaptive immune activation by linking CD4⁺ T cells with B cells, antigen-presenting cells (APCs), platelets, plasmacytoid dendritic cells, and stromal targets. Engagement of CD40 by CD40L drives lymphocyte activation, cytokine release, and immune-stromal crosstalk central to autoimmune pathology. Therapeutic CD40L blockade interrupts this costimulatory network, dampening aberrant immune activation while preserving immune homeostasis. IFN, interferon; MHC, major histocompatibility complex; TCR, T cell receptor. Created in BioRender. Bracken, S. (2026) https://BioRender.com/hkwbp6t.

CD40 signaling also enhances type I interferon (IFNα) production by plasmacytoid dendritic cells [21], promotes cytokine release and tissue-remodeling responses by monocytes, macrophages, and fibroblasts, and induces endothelial production of inflammatory mediators and adhesion molecules that facilitate leukocyte recruitment [22,23]. As noted earlier, CD40L expression on platelets contributes to platelet activation and thrombus formation [24] and plays a key role in sustaining immune responses. Through their ability to form aggregates with CD40-expressing leukocytes, CD40L-positive platelets have been shown to direct leukocyte trafficking to sites of inflammation [25]. Furthermore, plasmacytoid dendritic cells engaged by platelet-leukocyte aggregates further amplify type I interferon production through CD40-CD40L signaling [26].

CD40L IN THE DEVELOPMENT OF AUTOIMMUNE DISEASE

Upon activation through the B-cell receptor (BCR), B cells rely on additional co-stimulatory signals to avoid apoptosis, a safeguard that normally prevents the survival of autoreactive clones [27]. Accordingly, CD40 and CD40L interactions are tightly regulated to prevent inappropriate lymphocyte activation. However, dysregulation of this axis is a hallmark of multiple autoimmune diseases and drives pathogenic lymphocyte activation, autoantibody production, and tissue inflammation [28].

Most interestingly, CD40L expression is not strictly confined to activated T cells. Studies in patients with systemic lupus erythematosus (SLE) have shown that B cells aberrantly upregulate CD40L, reaching expression levels comparable to those observed on activated T cells [2]. Hyperexpression of CD40L on B cells correlates with disease activity and supports autocrine B cell activation by circumventing the requirement for T cell help, further destabilizing immune tolerance. Transgenic mice overexpressing CD40L on B cells develop a lupus-like phenotype characterized by spontaneous germinal center formation, hypergammaglobulinemia, and immune complex-mediated organ damage, demonstrating that dysregulated CD40L expression is sufficient to trigger systemic autoimmunity [29]. Overexpression of CD40L in murine epidermal cells results in chronic inflammation and features of systemic autoimmunity [30]. In the tight skin (TSK) mouse model of systemic sclerosis, blockade of CD40L markedly attenuates hypodermal thickening, B cell activation, and autoantibody production, directly linking CD40L signaling to the pathogenesis of immune-mediated fibrosis [31]. Moreover, anti-CD40L therapy prevents or reverses autoimmune pathology in murine models of Sjögren disease, autoimmune thyroiditis, and type 1 diabetes and has been shown to eliminate spontaneous germinal centers that sustain organ-specific immune response [32]. Collectively, these studies establish CD40L as a pivotal mediator of both systemic and organ-specific autoimmunity in mice.

Human studies reinforce the concept that CD40L plays a pathogenic role in autoimmune disease. Heightened CD40L expression has been detected in renal biopsies from patients with lupus nephritis and has been shown to correlate with disease severity [33]. In Sjögren disease, CD40L is upregulated in salivary gland mononuclear infiltrates, while increased CD40 is expressed on acinar and ductal epithelial cells [34,35]. Elevated surface CD40L is also consistently observed on circulating T cells in a variety of autoimmune diseases, including SLE, rheumatoid arthritis, and multiple sclerosis [2]. Moreover, soluble CD40L levels often correlate with markers of disease activity, underscoring its potential as both a biomarker of disease activity and contributor to disease pathogenesis [36].

THE RISE, FALL, AND RENEWAL OF CD40L BLOCKADE IN AUTOIMMUNITY

The nonredundant role of the CD40-CD40L axis in immune activation and its broad influence across immune compartments justify this pathway’s emergence as a high-profile therapeutic target. The earliest attempts to block the CD40-CD40L pathway in patients with autoimmunity date back to the early 2000 s, when toralizumab (IDEC-131), a humanized monoclonal antibody against CD40L, was tested in patients with SLE. The trial failed to demonstrate that CD40L blockade could reduce disease activity scores or disease biomarkers beyond that observed in the placebo-treated group [37]. The results from a trial of ruplizumab (BG9588), a humanized anti-CD40L directed monoclonal antibody, were reported soon thereafter in a small group of patients with proliferative lupus nephritis receiving concomitant immune-modulating therapy [38]. Treatment with ruplizumab was associated with a moderate reduction in autoantibody levels (antidsDNA) without any changes in the serum levels of total immunoglobulins or vaccinespecific immunoglobulins, suggesting that anti-CD40L selectively targeted antigen-activated, autoreactive B cells. However, this study lacked a comparator group and was prematurely terminated after multiple participants experienced thromboembolic complications [39]. These thrombotic adverse events were subsequently attributed to anti-CD40L Fc-mediated platelet activation, derailing the field for nearly two decades [40]. The lessons learned from these early studies led to the design of Fc-silent or nonantibody CD40L inhibitors.

Frexalimab, a next-generation anti-CD40L antibody, has shown striking early efficacy in relapsing multiple sclerosis. In a phase 2, double-blind, randomized, placebo-controlled trial, patients receiving frexalimab in the absence of background disease-modifying therapy demonstrated significant reductions in inflammatory brain MRI lesions, a surrogate marker of clinical disease activity, and decreases in biomarkers of inflammation (CXCL13) and neuronal damage [41]. Demonstrable efficacy was observed with both a high-dose intravenous regimen and a low-dose subcutaneous regimen. Additionally, these benefits occurred without any observable thromboembolic events, though a higher percentage of patients receiving frexalimab experienced COVID-19 infection and headache. While long-term effects on disease progression, relapse rates, and safety outcomes remain to be determined in phase 3 studies, frexalimab revived the potential of CD40L inhibition to be a potent, nonlymphocyte-depleting therapeutic strategy.

Recent results from a randomized, placebo-controlled phase 2 trial of the CD40L antagonist dazodalibep (DAZ), a novel nonantibody fusion protein, provide evidence that therapeutic blockade of CD40L can potentially benefit patients with Sjögren disease [42▪]. In this study, DAZ treatment produced statistically significant improvements in both systemic disease activity and patient-reported symptoms. The study was conducted in two patient groups: moderate to high systemic disease activity and high symptoms burden and low disease activity. In the group with moderate to high systemic disease activity, the ESSDAI, a validated tool designed to measure systemic disease activity across 12 domains, declined by a mean of 6.3 points in the DAZ group versus 4.1 points in the placebo group. The ESSPRI, a validated endpoint calculated from the combined ratings from visual analog scales measuring dryness, fatigue, and pain, showed a mean change of −1.8 points in the DAZ group compared with −0.5 in the placebo group. The between-group differences (−2.2 for ESSDAI in group 1, −1.3 for ESSPRI in group 2) were statistically significant and considered to be clinically meaningful [43]. DAZ therapy was also associated with reduced circulating biomarkers of B cell activity, such as CXCL13 and rheumatoid factor. Treatment with DAZ was generally well tolerated in all patients, although a higher incidence of COVID-19 infections was observed in the DAZ group. The progress in the development of other next-generation inhibitors of the CD40L-CD40 pathway in clinical development, including dapirolizumab pegol, iscalimab, and abiprubart, is detailed in Table 1 [44,45,46▪▪,47] and underscores the renewed interest in CD40-CD40L-directed therapy.

Table 1.

Snapshot of recently completed and enrolling CD40L and CD40-targeted clinical trials across autoimmune diseases

Target	Drug (Alternative Name)	Modality	Disease Indication	Latest Phase	Sponsor	Key Outcomes
CD40L	Frexalimab	Fc-engineered mAb	Multiple sclerosis	3	Sanofi	90% reduction in new gadolinium-enhancing T1 lesions; no thromboembolic signal [41]
			Type 1 Diabetes	2		Phase 2b study (FABULINUS, NCT06111586) ongoing; primary endpoint includes preservation of stimulated C-peptide at 52 weeks. No peer-reviewed efficacy data reported to date
CD40L	Dapirolizumab pegol	Fc-free CD40 ligand signaling inhibitor	Systemic lupus erythematosus	3	UCB Biopharma SRL	Numerical BICLA, SLEDAI-2K, and serologic marker improvement but primary dose-response endpoint not met; well-tolerated [44]
CD40L	Dazodalibep (VIB4920)	Non-antibody CD40 ligand antagonist	Sjögren disease	3	Amgen	Met coprimary ESSDAI and ESSPRI improvements; reduced CXCL13 and RF; well tolerated [42]
CD40	Iscalimab (CFZ533)	Humanized anti-CD40 mAb	Myasthenia gravis	2	Novartis	Composite MG score improvement [45]
			Sjögren disease	2		Disease-modifying signal with modest reduction in ESSPRI score [46]
CD40	Abiprubart (KPL-404)	Humanized anti-CD40 IgG4 mAb	Rheumatoid arthritis	2	Kiniksa Pharmaceuticals	Reduction in DAS28-CRP at 12 weeks; no safety signal [47]

BICLA, British Isles Lupus Assessment Group-Based Composite Lupus Assessment; CRP, C-reactive protein; DAS28, Disease Activity Score-28; ESSDAI, EULAR Sjögren’s Syndrome Disease Activity Index; ESSPRI, EULAR Sjögren’s Syndrome Patient Reported Index; MG composite, myasthenia gravis composite; SLEDAI-2K, Systemic Lupus Erythematosus Disease Activity Index 2000.

THERAPEUTIC POSITIONING OF CD40L AND CD40 BLOCKADE IN AUTOIMMUNE DISEASE

The therapeutic roles of CD40L and CD40 blockade must be considered in the context of their specific mechanisms of action and the autoimmune setting. While CD40L blockade offers a compelling strategy to selectively disrupt T-dependent B cell activation, questions remain regarding its efficacy across the spectrum of autoimmune diseases. In particular, it is unclear whether CD40L blockade will suffice in conditions where pathogenic B cells are activated through T-cell independent, TLR-driven mechanisms. Age-associated B cells (ABCs), an extrafollicular subset implicated in SLE, Sjögren disease, systemic sclerosis, and other systemic autoimmune conditions, commonly arise through these TLR-mediated, CD40L-independent mechanisms. Because ABCs expand with age and are strongly linked to autoantibody production and tissue inflammation, their persistence may circumvent the therapeutic impact of CD40L inhibition. This raises the possibility that optimal B cell-directed therapy in some autoimmune diseases will require strategies that target both CD40L-dependent germinal center responses and CD40L-independent extrafollicular B cell activation, including interactions between autoreactive B cells and T peripheral helper (Tph) cells [48,49].

In the broader landscape of B cell-targeted therapies for autoimmune disease, it remains unclear where CD40-CD40L pathway blockade will ultimately fit. Anti-CD20 antibodies effectively deplete circulating B cells but spare most tissue-resident B cells, potentially leading to incomplete suppression of autoantibody production [50▪]. BAFF pathway inhibition attenuates B-cell survival yet provides modest efficacy as monotherapy. By contrast, CD19-directed strategies, including CAR T cells, achieve deep and durable B cell depletion but carry greater risks and logistical barriers. CD40L targeting may offer a potentially distinct advantage. Instead of wholesale depletion of B cells, it interrupts the T cell “help” required by antigen-activated B cell clones and thus preferentially silences autoreactive B cell responses. How the clinical efficacy of CD40-CD40L axis inhibition will compare to that of B cell depletion will require additional studies. It should also be considered these two approaches may be complementary in some settings.

Targeting CD40 as opposed to CD40L could yield similar or somewhat different clinical outcomes. Drawing parallels with the DAZ trial in Sjögren disease, the phase 2b TWINSS trial of iscalimab, a CD40-directed monoclonal antibody, in a similar study population showed statistically significant clinical efficacy for reducing systemic disease activity, as measured by the ESSDAI [46▪▪]. However, patient-reported symptom improvements, as measured by the ESSPRI, were modest and not significantly different from placebo [42▪]. CD40 blockade acts on a broader array of immune cells. However, a possible distinguishing feature of CD40L-directed therapy is that its target, CD40L, mediates not only forward signaling to CD40-bearing cells but also reverse signaling into the CD40L-expressing cell by engagement with CD40 in trans. Recent work has shown that the cytoplasmic tail of CD40L interacts with receptor for activated C kinase 1 (RACK1), linking CD40L to downstream pathways (e.g. PKC, AKT, ERK, STAT1) that enhance T-cell activation and cytokine production [51]. Consequently, CD40L blockade may interrupt activation of B cells and APCs, as well as the CD40L-dependent signals that amplify T-cell effector responses. Although CD40L blockade can transiently reduce vaccine responsiveness and T cell-dependent immunity, such effects are a predictable consequence of its mechanism of action [52]. With continued refinement, this approach may bring the field of rheumatology closer to achieving durable disease control without dangerously sacrificing immune competence.

EMERGING FRONTIERS IN CD40L-TARGETED THERAPY

The ongoing clinical development of CD40 and CD40L-targeted therapies highlights several emerging directions for future study. One important step will be the integration of biomarkers that reflect both target engagement (e.g. soluble CD40L) and downstream immunologic effects (e.g. B cell activation markers, autoantibodies, and key cytokine levels) to guide individual responses to therapy. Incorporating such biomarkers into trial design could refine the clinical use of CD40 and CD40L inhibitors and enable a precision medicine approach.

Because pathogenic B cells may also be activated through CD40L-independent pathways, dual targeting approaches could enhance therapeutic efficacy particularly when nongerminal center responses are dominant aspects of disease pathology. For example, combining CD40L blockade with BAFF antagonism, TLR antagonism, or additional costimulatory signal inhibitors that suppress T cell-driven B cell responses might more comprehensively suppress autoreactive B cell responses. Notably, combination blockade strategies including CTLA4-Ig and emerging CD40L-directed agents are currently being explored in solid organ transplantation to prolong graft survival and reduce alloantibody formation [53]. Sequential strategies may also hold promise: B-cell depletion followed by CD40L blockade could debulk pathogenic B cells and subsequently prevent repopulating B cells from re-entering autoreactive germinal center-dependent pathways. Early experience with sequential B cell-directed therapies further supports the concept that targeting distinct immune pathways can improve patient responses and disease activity markers [54,55].

The expansion of CD40 and CD40L-targeted therapy across diseases is an area of considerable interest. Beyond lupus, multiple sclerosis, and Sjögren’s disease, blockade of this pathway may have utility in systemic sclerosis, myasthenia gravis, autoimmune cytopenias, and even following transplantation, where selective immunomodulation without broad immunosuppression is desirable [45,56–58]. Together, these directions highlight both the breadth of potential applications and the importance of continued mechanistic and clinical investigation to define the role of CD40-CD40L axis inhibition across immune-mediated diseases.

CONCLUSION

Targeting the signaling mechanisms that sustain autoreactive B cell responses is the next frontier for the treatment of autoimmune disease. Among these possibilities, CD40L blockade offers a uniquely attractive strategy through selective disruption of the T cell “help” that sustains pathogenic B cell activation, germinal center maintenance, and autoantibody production. Advances in antibody engineering have addressed earlier safety concerns, reviving the CD40-CD40L pathway as a viable therapeutic target. While early clinical trials show promising biologic activity and improved safety, long-term efficacy and potential for infection require additional investigation. Nevertheless, CD40L-directed therapies should inspire cautious optimism for their potential to bring rheumatology closer to its long-sought goal of achieving precise and durable immune modulation.

KEY POINTS.

• The CD40-CD40L axis delivers essential costimulatory signals linking adaptive and innate immunity across T cells, B cells, APCs, platelets, and stromal cells.

• Dysregulated CD40-CD40L signaling drives autoreactive B-cell activation, germinal center pathology, and tissue inflammation in systemic autoimmune diseases.

• Early anti-CD40L antibodies demonstrated biologic activity but were limited by Fc-mediated platelet activation and thrombotic events.

• Fc-silent and engineered domain antibodies have revived CD40L blockade as a well tolerated, selective, and potent therapeutic approach with promising efficacy across autoimmune diseases.

• CD40L inhibition offers a distinct approach within the landscape of B cell-targeted therapies by interrupting T-cell “help” rather than depleting B cells, enabling more selective suppression of autoreactive responses.