Hope or hype? Clinicians’ dilemma in the era of ever-expanding antigens in membranous nephropathy

The year is 2030, and a 45-year-old man with a remote history of non-steroidal anti-inflammatory drugs (NSAID) use is being evaluated for proteinuria of 10 grams per day. Serological workup is negative except for a positive antinuclear antibody. A phospholipase A2 receptor (PLA2R) antibody is negative. A kidney biopsy shows membranous nephropathy (MN). Mass spectrometry was performed for antigen typing, with the antigen type listed within the diagnostic line of the pathology report. A blood sample sent at the time of biopsy was tested for this antigen for serologic testing to provide a baseline autoantibody titer. This antigen is associated with malignancy in one-third of cases, in autoimmune disease in approximately 10% of cases, and is rarely associated with NSAID use and sarcoidosis. How does this information impact the clinical workup? In addition, at this time, chimeric antigen receptor T cell (CAR-T) therapy is available for specific B-cell depletion for PLA2R-expressing cells, but not yet for this antigen. There is an ongoing clinical trial at a nearby center, but only in phase I level. Should the clinician monitor the patient off NSAIDs, treat the patient with non-specific immunosuppression (such as with rituximab or the Ponticelli regimen which will lead to remission in the vast majority of cases), or do they consider the patient for enrollment in a novel targeted clinical trial? The patient may not be eligible if aggressive immunosuppression is used first. The clinician is at a crossroads regarding this patient's potential workup and treatment.

This patient's disease, MN, is a leading cause of nephrotic syndrome, currently characterized by a histologic pattern of immunoglobulin G (IgG) deposition in the subepithelial space [1], but likely representing multiple separate disease states. The identification of the antigenic targets for this IgG deposition has yielded multiple MN subtypes, some of which have unique histopathologic features and disease associations and others that are yet to be elucidated. In just a few years, the landscape of antigens associated with MN has markedly expanded since the initial discovery of the target antigen on the podocyte's surface, the M-type phospholipase A2 receptor, in 2009 by Beck et al. [2]. Following the discovery of PLA2R, Tomas et al. [3] discovered another target antigen, thrombospondin type 1 domain-containing 7A (THSD7A), in 2014. In a span of 4 years from 2018 to 2022, 10 new target antigens were discovered, Exostosin1/Exostosin 2 (EXT1/EXT2), neural epidermal growth factor-like 1 (NELL-1), semaphorin-3B (sema3B), serine protease HTRA1 (HTRA1), protocadherin 7A (PCDH7A), FAT1, netrin G1 (NTNG1), contactin 1 (CNTN1), neural cell adhesion molecule-1 (NCAM1), transforming growth factor beta receptor 3 (TGFBR3), neuron-derived neurotropic factor (NDNF) and proprotein convertase subtilisin/kexin type 6. (PCSK6) [1, 4–6]. Most recently, in 2023, 17 additional putative antigens were described [7, 8], significantly reducing our knowledge gap of targets in the disease. These include seizure-related 6 homolog-like 2 (SEZ6L2), vasorin (VASN), early endosomal antigen 1 (EEA1), macrophage-stimulating 1 (MST1), natriuretic peptide receptor 3 (NPR3), ficolin 3 (FCN3), CD206, cysteine-rich motor neuron protein (CRIM1), leucine-rich repeat transmembrane protein (FLRT3), insulin-degrading enzyme (IDE), reversion-inducing cysteine-rich protein with Kazal motifs (RECK), neuroligin 3 (NLGN3), peptidoglycan recognition protein 1 (PGLYRP1), vascular endothelial growth factor A (VEGFA), sulfatase 1 (SULF1), epidermal growth factor–containing fibulin extracellular matrix protein 2 (EFEMP2) and matrix protein Fraser syndrome 1 (FRAS1). Evidence regarding the pathogenicity, clinicopathologic associations and clinical role of antibodies against each of the aforementioned antigens varies in the literature [1]. As a busy clinician, how does one implement the variable clinicopathological characteristics of each antigen into clinical practice and how do these discoveries impact patient care?

The diagnosis of MN is changing to an antigen-based classification based on the above discoveries. It is impractical to type MN cases for all of the above antigens by immunostaining and a multiplexed approach, such as mass spectrometry, will allow interrogation of all antigen types simultaneously and is currently under development (Fig. 1). An antigen-based classification is in its infancy, however, and further studies will be needed to identify further disease associations, identify circulating autoantibodies against each target and establish whether antibody titer correlates with disease activity for these new antigenic targets. Investigation of specific subgroups within MN can help us better understand the role of genetic, environmental and clinical factors in the development and progression of these separate disease states. Additionally, it opens the door for developing serologic tests for non-invasive disease monitoring, as is currently available for PLA2R and THSD7A-associated MN. Autoantibodies have been identified for many of the newly identified targets in MN, including NELL1, HTRA1, NCAM1, SEMA3B, NTNG1 and CNTN1, demonstrating feasibility for serologic assay development.

Figure 1:

The figure represents the clinical approach for biopsy-proven membranous nephropathy. If PLA2R-positive MN, the patient should undergo risk stratification and management according to the risk assessed. If PLA2R is negative, the workup should be focused on testing other known antigens like THSD7A, NELL-1, etc., and look for the clinical associations for the respective antigens by a panel of immunostains or through mass spectrometry.

The presence and level of autoantibodies associated with MN can provide valuable information for disease monitoring and treatment decisions. However, the question arises of whether nephrologists can make treatment decisions solely based on the positivity and level of these antibodies. In a multicenter retrospective validation study by Bobart et al. [9], a cohort of 79 patients with positive serologies for PLA2R by either enzyme-linked immunosorbent assay or immunofluorescence assay, and preserved kidney function, kidney biopsy did not reveal treatment-changing information. Patients with high levels of PLA2R antibodies are associated with more severe disease presentation (severe proteinuria and hypoalbuminemia) than patients with lower PLA2R antibody levels. Disease prognosis based on antibody levels was studied in a Chinese cohort [10]; the authors concluded that baseline levels of PLA2R antibodies were an independent factor for complete and total remission. High-risk patients with PLA2R antibody levels of >150 RU/ml had higher disease activity and worse prognosis, independently of the immunosuppressive regimen received. While the presence and level of autoantibodies can provide valuable information in MN, treatment decisions should not be based exclusively on these factors. Additionally, a kidney biopsy may still be necessary in certain cases to gather more detailed information about the underlying pathology and guide treatment decisions.

One way that knowledge of the target antigen can help the clinician is through alerting to potential etiologic triggers of disease. EXT1/EXT2-, NCAM1- and TGFBR3-associated MN have been strongly associated with autoimmune disease, and therefore, a thorough rheumatologic work-up for underlying autoimmune disease is indicated [1]. NELL1-associated MN is associated with malignancy in one-third of cases and may be identified in conjunction with use of specific medications, such as lipoic acid or bucillamine [1]. NELL1 is also the target antigen of mercury-associated MN, which can be seen in multiple settings, including use of indigenous medicines, skin whitening creams or in fluorescent light bulb recycling [11]. Without knowledge of this antigenic target, it is highly likely that we could miss one of these unusual and reversible etiologies of disease. Furthermore, the emergence of NDNF as a novel antigen implicated in syphilis-associated MN [1], along with the recent discovery of PCSK6 as an antigen linked to NSAID-associated MN [1], further expands our knowledge in this field.

The robust discovery of a multitude of antigens with different associations and therapeutic implications paves the way for precision medicine. For example, new therapies can focus on targeting and depleting specific cells associated with expressing pathogenic antibodies. CAR-T therapy is a promising new approach currently being studied for the treatment of several autoimmune diseases, including MN. This therapy involves genetically modifying a patient's T cells to express a chimeric antigen receptor that recognizes and targets specific antigens on B cells, with the ultimate goal of eliminating B cells expressing antigen receptors against MN antigens [12]. A recent small study [13] of five patients with systemic lupus erythematosus who had active disease refractory to several immunosuppressive regimens underwent CAR-T therapy for compassionate use. CAR-T therapy in these patients led to profound B-cell depletion, and improvement in SLE markers and clinical symptoms. Remission was achieved in all five patients after 3 months and no relapses occurred during the study period. While CAR-T therapy for patients with lupus would primarily target B cells, one would anticipate antigen-specific CAR-T therapy to evolve for various diseases, including MN [14]. In the era of precision medicine an antigen specific B-cell depleting–targeted CAR-T therapy is currently under investigation in clinical trials, including targeting of specific B cells expressing the desmoglein 3 antigen in the autoimmune disease pemphigus vulgaris [15]. CAR-T therapy is in development for PLA2R-positive MN, with promising data from preclinical studies, but not yet in human trials [16–18]. CAR-T cell therapy is in the early stages of development for the treatment of autoimmune diseases, but holds great promise for the treatment of autoimmune diseases [19], including MN. Limitations of CAR-T therapy include toxicities due to cytokine release and tumor lysis syndrome, electrolyte abnormalities, antigen escape and resistance to CAR-T, targeting similar antigens expressed on normal human cells, and CAR-T trafficking [14, 20].

The rapid evolution and unraveling of the underlying immunological mechanisms underlying the pathophysiology of MN is a double-edged sword. For example, Sethi et al. recently reported that PCSK6 was associated with NSAID induced MN [21]. If careful history taking excludes NSAID use, would a positive PCSK6 antigen change patient management? Moreover, currently mass spectrometry is an expensive technique and not readily available, and may be cost prohibitive especially in developing countries where healthcare resources and infrastructure may be limited. Furthermore, the expanding landscape of MN-related antigens and the complex nature of their associations with various systemic diseases can present challenges for nephrologists in managing cases with multiple abnormal antigens. Keeping up with the rapidly evolving understanding of these antigens and their clinical implications can be demanding, and some nephrologists may not feel comfortable managing such complex cases. In such situations referral to specialized centers with expertise in MN may be beneficial.

In conclusion, the paradigm of our understanding of MN has evolved robustly with the recent discovery of >30 new antigens in only the last 5 years. We have come to learn that different antigens are associated with a variety of diseases, which will continue to be identified through studying larger cohorts encompassing all of these individual subtypes. Antigen-based nomenclature to help guide clinicians individualize the clinical work-up and treatment for patients with MN and help pave the way for a precision medicine–based approach to optimize care and improve the outcomes of patients with MN. This futuristic approach to the care of the 45-year-old gentleman we are treating in 2030 may not actually be too far into the future.

CONFLICT OF INTEREST STATEMENT

K.D.J. is a founder and co-president of the American Society of Onco-Nephrology; reports consultancy agreements with Secretome, George Clinicals, PMV pharmaceuticals and Calliditas. K.D.J. reports honoraria from the American Society of Nephrology, UpToDate.com; reports serving on the editorial boards of the American Journal of Kidney Diseases, CJASN, Clinical Kidney Journal, Journal of Onconephrology, Kidney International and Nephrology Dialysis Transplantation; and reports serving as Editor-in-Chief of ASN Kidney News and section editor for onconephrology for Nephrology Dialysis Transplantation. T.C. has received funding from the National Institutes of Health (grant numbers #R44DK130702-02 and #R43AR081720-0). M.B. and M.H. have nothing to report.