The Role of Glomerular and Serum Expression of Lymphocyte Activating Factors BAFF and APRIL in Patient with Membranous and IgA Nephropathies

Hyperactivation of B lymphocytes is fundamental to many autoimmune diseases, including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and Sjögren's syndrome (SS). It also plays a key role in the development of various types of glomerulonephritis (GN), such as lupus nephritis (LN), IgA nephropathy (IgAN), and minimal change disease (Oniszczuk et al. 2021; Cai et al. 2023; Petrou et al. 2023). The immune response is divided into innate and adaptive immunity. The innate response is non-specific and includes mechanisms such as phagocytosis by granulocytes and monocytes, as well as the natural killer cell response. In contrast, the adaptive response is specific and generates a reaction tailored to a recognized antigen. The adaptive immune response requires precise coordination between T and B lymphocytes and is conventionally divided into cell-mediated and humoral immunity.

The humoral response, which leads to antibody production, is initiated either by antigenic or non-antigenic stimulation. Antigenic stimulation is mediated via the B-cell receptor (BCR) and its co-receptors, whereas non-antigenic stimulation occurs through mechanisms such as complement activation, Toll-like receptors (TLRs), and other receptors expressed on B cells (Figure 1).

Fig 1.

The overall view on the roles of BAFF and APRIL in B lymphocyte life-cycle. APRIL, A PRoliferation-Inducing Ligand; BAFF, B-cell-activating factor; BAFF-R, BAFF receptor; BCMA, B-cell maturation antigen; BCR, B-cell receptor; TACI, Transmembrane activator and calcium-modulator and cyclophilin ligand interactor.

The process of antibody production by B lymphocytes is regulated by numerous factors. Classical examples include BAFF and APRIL, which are synthesized by myeloid cells such as monocytes, macrophages, and dendritic cells (Mathur et al. 2023; Cheung et al. 2024). BAFF exists in both transmembrane and soluble forms, the latter generated via proteolytic cleavage, whereas APRIL is initially membrane-bound and later circulates as a trimeric molecule. These cytokines interact with three receptors belonging to the tumor necrosis factor (TNF) receptor family:

BAFF receptor (BAFF-R): Expressed on immature (transitional) and mature B cells, it promotes B-cell survival and maturation.

Once B lymphocytes are activated by antigens via the BCR, sustained activation is achieved through co-stimulation by T cells (via the CD40 ligand) and BAFF-R, which promotes B-cell maturation, survival, and proliferation. Mature B cells are further stimulated by the TACI receptor, which facilitates antibody class switching and interacts with both BAFF and APRIL. Additionally, plasma cells express BCMA receptors, which enhance their survival.

BAFF and APRIL are primarily produced by myeloid cells, while their receptors are localized on B lymphocytes. Additionally, BAFF-R is expressed on the cell membrane of human mesangial cells. Activation of these receptors leads to mesangial cell proliferation, whereas blocking BAFF/BAFF-R binding inhibits this effect (Zheng et al. 2015). Since mesangial proliferation is an unfavorable prognostic factor in most types of GN, local upregulation of BAFF production may have a detrimental effect on the progression of chronic kidney disease. Evidence also suggests that BAFF levels are elevated either systemically or locally in certain types of GN, such as IgAN and LN (Zheng et al. 2015). In the glomeruli of patients with proliferative LN, APRIL and BAFF mRNA levels were found to be 12-fold and 30-fold higher, respectively, compared to pretransplant biopsies of living donors. Subsequent immunohistochemical staining demonstrated significant APRIL protein expression in 11 of 21 biopsies, localized in the tubulointerstitium, parietal epithelial cells, and at the vascular pole of the glomeruli. In contrast, biopsies from membranous nephritis patients were mostly negative for APRIL. Despite the increased mRNA expression, BAFF protein was detected only in inflammatory infiltrates in the tubulointerstitium, co-expressed with BCMA and TACI (Neusser et al. 2011). Figure 2 illustrates the distribution of B-lymphocyte-activating receptors as reported in various studies. Notably, some discrepancies—such as the presence of BAFF-R receptors in healthy controls and their absence in GN patients—are likely due to methodological differences (e.g., immunohistochemistry vs. mRNA expression analysis) or small patient sample sizes. Immunohistochemical staining is less precise but can be performed on archived biopsy tissues, whereas mRNA expression analysis is more objective but requires specialized tissue preparation and greater financial resources.

Fig 2.

The expression of B-cell activation receptors and molecules reported in various studies. Underlined data come from mRNA analysis. *Identified both by mRNA and immunohistochemistry analysis (Zheng et al. 2015; Han et al. 2018; Cao et al. 2020; Forero-Delgadillo et al. 2022; Marín-Rosales et al. 2022). Both immunohistochemistry staining and mRNA expression were used in independent research, which probably influence incoherent results. APRIL, A PRoliferation-Inducing Ligand; BAFF, B-cell-activating factor; BAFF-R, BAFF receptor; BCMA, B-cell maturation antigen; TACI, Transmembrane activator and calcium-modulator and cyclophilin ligand interactor.

To our knowledge, no direct comparison of BCR expression in IgAN, LN, and membranous nephropathy (MN) has been published. This underlines the need for further studies, particularly as more advanced tissue examination methods become available. The future of receptor expression research may lie in spatial transcriptomics, a technique capable of identifying active genes in samples as small as three glomeruli. For instance, the molecular profiles of patients with IgAN and mesangial proliferation have been shown to exhibit significantly higher expression of genes involved in adhesion molecules, vascular development, and extracellular matrix formation compared to controls with non-proliferative forms of IgAN.

In B-cell-related diseases, several strategies have been explored. One key approach involves targeting B lymphocytes by eliminating cells expressing the CD20 marker. RTX, a chimeric monoclonal antibody, has been widely used in various antibody-mediated conditions. In the European Union, RTX is registered for the treatment of non-Hodgkin lymphoma, chronic lymphocytic leukemia (CLL), RA, granulomatosis with polyangiitis, microscopic polyangiitis, and pemphigus vulgaris (Union Register of Medicinal Products – Public Health – European Commission 2024). It is also the first-line treatment in high-risk PLA2R-positive MN. Despite its ability to lower dsDNA antibody levels, RTX failed to meet primary endpoints in two major trials: EXPLORER, which focused on non-renal SLE and LUNAR, which evaluated its effectiveness in LN. This failure may be attributed to inadequate depletion of CD19+ B cells or the high variability of pathogenic mechanisms involved in SLE, such as autoreactive B cells, alterations in TLR receptor function, differences in the interferon (IFN)-α pathway, or T-cell dysfunction (Mo et al. 2024). Tailoring treatment to the dominant mechanism in a specific patient (for example, by measuring BAFF levels) could, in theory, bring better results. So far, RTX is recommended by the European Alliance of Associations for Rheumatology and the American College of Rheumatology for refractory lupus, based mostly on observational data (Piantoni and Korsten 2022). Unfortunately, RTX has not proven efficacious in IgAN. It failed to reduce proteinuria, serum levels of Gd-IgA1, or antibodies against Gd-IgA1. Moreover, no significant difference in eGFR was observed. Whether these outcomes are due to the activity of plasma cells escaping RTX-induced cytotoxicity or the involvement of the mucosal immune system remains unknown (Lafayette et al. 2017).

The second- and third-generation anti-CD20 antibodies, including humanized (ocrelizumab, obinutuzumab) and fully human (ofatumumab) antibodies, may prove more effective than RTX, a first-generation anti-CD20 antibody (Roos et al. 2023). These newer generations are glycoengineered to enhance binding to immune effector cells (e.g., natural killer cells), as well as improve direct or Ab-dependent cellular cytotoxicity and complement-dependent toxicity. RTX is a chimeric murine-human antibody, which can trigger anti-drug antibodies (ADA), reducing its efficacy over time. Humanized or fully human antibodies decrease the likelihood of ADA development, potentially leading to less resistance and sustained efficacy. Newer agents may allow dose adjustments or subcutaneous administration (e.g., ofatumumab), which can help in achieving more consistent drug levels.

For example, in a head-to-head comparison with RTX in multiple sclerosis, patients treated with ocrelizumab had a lower rate of relapses than those treated with RTX. In a newly published study, obinutuzumab demonstrated higher complete renal response (CRR) rates (defined as urine protein-creatinine ratio [UPCR] <0.5, no worsening of baseline serum creatinine by >15%, and inactive urinary sediment) than standard therapy alone in class III and IV LN. CRR at week 76 was achieved by 42.7% vs. 30.9% in the placebo + standard therapy group, and a UPCR <0.8 was seen in 55.5% vs. 41.9% (Furie et al. 2025). In a previous trial, the results were similar, with 35% vs. 23% of patients achieving CRR (Furie et al. 2022a). In the 2025 study, serious adverse events occurred in 32.4% of patients in the obinutuzumab group and in 18.2% in the placebo group. The most frequent serious adverse events were infections, including Covid-19, urinary tract infection, pneumonia, and gastroenteritis. The authors concluded that obinutuzumab presented a good safety profile, but the decision to double the risk of serious adverse events in order to increase the chance of CRR by approximately 10% remains a difficult one. Alternative strategies for targeting B cells beyond anti-CD20 therapies may be necessary.

Belimumab, a monoclonal antibody targeting the soluble form of BAFF, is the first biological drug approved for SLE. A comprehensive review of published clinical trials (real-world effectiveness) reported a reduction in disease activity, measured by the SELENA-SLEDAI score, 10.1–4.4 points (Huang et al. 2022). Belimumab is also the only biological agent registered for the treatment of LN; however, the results are not groundbreaking. A CRR and an eGFR no more than 10% below the open-label baseline value or ≥90 mL/min per 1.73 m² was achieved in only 30% of patients compared to 20% in the placebo group after 104 weeks of treatment (Furie et al. 2020). In a two-year extension of this trial, the proportion of patients with CRR increased from 36% to 48% in the placebo-to-belimumab group and from 48% to 62% in the belimumab-to-belimumab group (Furie et al. 2022b). Even though belimumab did not prove to be a groundbreaking medication in LN and is recommended by EULAR in induction or maintenance therapy of LN only in combination with other agents (mycophenolate, low-dose cyclophosphamide, or azathioprine), its strength may lie in its steroid-sparing effect (Worley et al. 2023). No other biological agent has been registered for the treatment of LN so far.

As B-cell activation is dependent on BAFF, TACI, and the BCR receptor, dual inhibition should, in theory, offer improved efficacy. Atacicept is a recombinant human TACI-immunoglobulin fusion protein that binds to both BAFF and APRIL. In a study evaluating atacicept in SLE, atacicept reduced the incidence of severe flares, disease activity as measured by various indices, and anti-dsDNA antibody levels. However, it did not significantly increase the proportion of patients achieving a corticosteroid dosage reduction to ≤7.5 mg/day. No information regarding proteinuria and eGFR was available (Wallace et al. 2021). Unfortunately, a new study on atacicept in patients with active LN (COMPASS, NCT05609812), has been suspended due to business reasons.

Both belimumab and atacicept have relatively good safety profiles. 74.5% of belimumab-treated patients experienced an infection during 76 weeks of study, compared to 69% on placebo, with a similar rate of serious infections (7% vs. 5.8%, respectively). Hypogammaglobulinemia occurred in only 1 of 544 patients (Furie et al. 2011). Infections with atacicept were reported in 46.4% of patients compared to 43.6% in the placebo group, while hypogammaglobulinemia occurred in only 6 of 1085 patients (Gordon et al. 2019). This is significantly lower than the rate reported with RTX, which could be explained by the fact that BAFF and APRIL inhibitors reduce B-cell function and antibody production more selectively, without directly depleting B cells through complement activation or antibody-dependent cellular cytotoxicity (Benson et al. 2008). The authors' selection of biological agents targeting BAFF and APRIL, along with their potential applications, is presented in Table 1.

Except for rapidly progressive cases with a high percentage of proliferating parietal epithelial cells on kidney biopsy (crescents), the depletion of CD20 cells has not found widespread application in the treatment of IgAN (Zhuang et al. 2023). As GWAS research in IgAN has identified a gene pair encoding APRIL and its receptor as susceptibility loci, most efforts have been focused on targeting this pathway. Sibeprenlimab, a humanized monoclonal antibody that binds to and neutralizes APRIL, demonstrated up to 62.0 ± 5.7% reduction in the 24-h UPCR at 12 months (Mathur et al. 2024). Zigakibart, another humanized monoclonal antibody that blocks APRIL, produced clinically meaningful reductions in proteinuria, with 7 out of 8 patients achieving a >50% decrease in UPCR from baseline at week 52 (Barratt et al. 2023). Dual blockade by telitacicept, a fusion protein combining the TACI with the fragment crystallizable portion of IgG, which neutralizes both BAFF and APRIL, also shows promise. In higher doses (240 mg), telitacicept reduced mean proteinuria in IgAN by 49% from baseline compared to placebo, with the effect being dose-dependent. In smaller doses (160 mg), proteinuria reduction reached only 25%. Importantly, eGFR remained stable, and adverse events were similar in both groups (Lv et al. 2023). Likewise, atacicept resulted in a 25% reduction of UPCR vs. placebo at 24 weeks, with stabilization of eGFR at week 36. This was associated with a mean geometric difference of 11% between groups, corresponding to approximately 5.7 mL/min/1.73 m² (Lafayette et al. 2024). Interestingly, no studies have reported on the effectiveness of sole BAFF blockade by belimumab in IgAN. Results from the BRIGHT-SC study (estimated primary completion in October 2016), investigating the impact of blisibimod, a BAFF inhibitor, on proteinuria in IgAN, were never published.

CAR-T cells are a cell-based therapy initially developed for cancer and recently showing preliminary efficacy in autoimmune diseases. In this approach, a patient's T cells are collected from the blood through leukapheresis and genetically modified to express a specialized receptor known as CAR. This receptor is designed to recognize a specific antigen, subsequently activating the T cells to destroy the target cells. While originally used in cancer treatment, CAR-T cells can be engineered to target various antigens, including BAFF-R. However, the application of CAR-T therapy is currently restricted to the most severe conditions, such as hematological malignancies and refractory, life-threatening autoimmune diseases, due to potentially life-threatening side effects associated with this treatment. One major complication of CAR-T cell therapy is cytokine release syndrome (CRS), a condition characterized by a cytokine surge that can occur shortly after the infusion of CAR-T cells. CRS can manifest as fever, hypoxia, hypotension, and life-threatening multi-organ failure. The incidence of CRS can vary, occurring in 40%–100% of patients, depending on the type of CAR-T cells used (Xiao et al. 2021). Additionally, CAR-T cell therapy may be complicated by the development of T-cell lymphoma. Retroviral or lentiviral vectors, which are used to insert genetic material into T cells, may, in rare cases, disrupt tumor suppressor genes or activate oncogenes, leading to the development of T-cell malignancies. Lastly, CAR-T therapy requires conditioning with fludarabine and cyclophosphamide, which are associated with hematological toxicity, increased susceptibility to infections, and an elevated risk of malignancies. With these precautions in mind, CAR-T cells might find use in certain autoimmune conditions.

The first trials targeting B lymphocytes in autoimmune diseases have already been published, and the results, especially in SLE, are promising. In one case, a 20-year-old woman with severe and refractory SLE, who was unresponsive to both RTX and belimumab, achieved clinical remission after receiving an infusion of autologous CD19 CAR T cells. Her SLEDAI score decreased from 16 to 0 (Mougiakakos et al. 2021). In a case series involving five SLE patients with active, refractory disease, anti-CD19 CAR-T cell therapy resulted in a significant improvement in clinical symptoms and normalization of laboratory parameters, including the seroconversion of anti-dsDNA antibodies (Mackensen et al. 2022). A Phase 1 clinical trial treated 12 patients with refractory SLE using CD19/BCMA CAR-T cells with a median follow-up of 118.5 days (range: 45–524 days). Despite all patients developing CRS and one-third experiencing infections, all patients recovered after treatment. The SLEDAI-2K score decreased from a mean of 18.3 to 1.5 (Feng et al. 2023). In a study by Wang et al. (2024) 13 LN patients were treated with BCMA and CD19-directed CAR-T cells, resulting in a reduction of the mean SLEDAI score from 10.6 to 2.7 after 3 months, with a significant improvement in renal function in 10 out of 13 patients. The therapy was well tolerated, with only mild CRS observed (Wang et al. 2024).

As stated previously, BAFF binds the three receptors: BAFF-R, BCMA, and TACI. In case of a disease relapse due to the loss of the CD19 antigen, as sometimes occurs in leukemia, BAFF-CAR-T cells could offer additional therapeutic benefits, as almost all B-cell cancers are reported to express at least one of these receptors (Wong et al. 2022). BAFF CAR-T cells can successfully bind to each of the three BAFF-Rs and effectively destroy B-cell cancers, including mantle cell lymphoma, multiple myeloma, and acute lymphoblastic leukemia (ALL) in vitro and in vivo in xenograft models (Wong et al. 2022). Additionally, a BAFF-R CAR against CLL cells (MC10029) was tested in mice, resulting in 100% survival after 84 days (Luo et al. 2023). In another study, a dual CAR construct targeting both CD19 and BAFF-R achieved complete eradication of ALL cells in a mouse model, showing an advantage over monospecific CD19 or BAFF CAR-T therapies, where relapse occurred (Wang et al. 2022). Table 2 presents ongoing trials of CAR-T cells targeting the BAFF molecule. However, to date, no APRIL-targeting CAR-T lymphocytes have been developed.

CAR-T cells primarily affect circulating cells, such as B lymphocytes, and are capable of active penetration into tissues to some extent, an advantage over monoclonal antibodies, which do that solely by diffusion. CAR-T cells are able to penetrate lymph nodes and induce deep-tissue depletion of B cells. Whether this is sufficient to affect the mesangial cells is yet unknown. The majority of kidney injury after CAR-T cell therapy stems from CRS, and cases of glomerular injury are extremely rare. The expression of BAFF, along with IFN-α and IL-12, on renal immune cells is significantly higher in patients with proliferative LN compared to those with IgAN, LN class II, or healthy controls. Notably, the local increase in IFN-α and IL-12 correlates with the presence of active lesions, a poorer prognosis (for IFN-α), and a higher tubular atrophy score (for IL-12). In contrast, BAFF predominance is associated with a lower modified National Institute of Health total index and activity index, fewer cases of class IV LN, and a better overall prognosis. However, this could reflect the fact that BAFF expression tends to occur in the earlier stages of the disease (Nawata et al. 2023).

Currently, no trials involving CAR-T cells have been carried out or are in planning for IgAN and MN. The high risks associated with CAR-T therapy, combined with the relatively low risk of end-stage kidney failure and low mortality rates in these conditions, make CAR-T a less attractive option for these diseases. However, the further development of CAR-T cells targeting specific populations of B cells in tissues could provide a highly desirable treatment approach. If such therapies become feasible, and based on the efficacy of current biological treatments, anti-BAFF therapies may offer better outcomes for MN, while anti-APRIL therapies could be more suitable for IgAN.

There is still much to discover about the activation of B lymphocytes and the survival of plasma cells in kidney diseases. Further studies are needed to understand the role of plasmablasts in various GN types, the involvement of BAFF-Rs in kidney tissue, and the differences between MN with and without PLA2R antibodies. Gaining deeper insights into these processes will enhance our understanding of incomplete remission or persistent disease activity despite aggressive treatment. In the future, a combination of agents targeting plasma and B cells—including CAR-T cells—along with complement system inhibitors might be indicated for patients with particularly resilient types of GN. Agents that inhibit both BAFF and APRIL, such as atacicept or telitacicept, should theoretically be more effective than belimumab by preventing B-cell activation via alternative receptors. No evidence currently exists supporting the efficacy of belimumab in IgAN, whereas both telitacicept and atacicept have been reported to reduce proteinuria in IgAN. However, these results require validation through head-to-head trials, as not all reports are entirely optimistic. For instance, an observational study comparing LN patients treated with belimumab or telitacicept found no significant difference in kidney parameters after 52 weeks of follow-up (Jin et al. 2025). This suggests that sole B-cell targeting may not be effective in LN. Measuring B-cell activity, such as BAFF levels, could help identify patients who would most benefit from B-cell depletion. While CAR-T cells offer a powerful option for refractory or life-threatening diseases, their use remains limited due to safety concerns. Biological drugs, on the other hand, present a safer option for autoimmune conditions with a moderate risk of end-organ damage, such as IgAN or MN.