Antinephrin Antibodies in IgAN: Toward a Phenotype-Driven, Personalized Approach

See Clinical Research on Article 106640

IgA nephropathy (IgAN) is classically regarded as a mesangiopathy; however, a minority of patients, approximately 5% to 15%, present with nephrotic-range proteinuria or frank nephrotic syndrome, a presentation that has long resisted a single mechanistic explanation. In the article by Si et al.,¹ the authors ask whether circulating antinephrin autoantibodies, recently established as drivers of podocyte injury in minimal change disease (MCD) and primary focal segmental glomerulosclerosis,² can mark a definable subset of nephrotic IgAN. Among 234 adults with biopsy-proven IgAN and nephrotic-range proteinuria, overall seropositivity was low, at 8%; however, it increased to 35% among the 17 patients with biopsy-confirmed coexisting MCD versus only 6% in those without, and reached 14% within the nephrotic syndrome subgroup. Seropositive patients had higher proteinuria, lower serum albumin, less hematuria, and milder chronic lesions, together with higher complete remission rates and no composite renal end points during follow-up. These observations raise 4 practical questions.

Why do Antinephrin Antibodies Matter in IgAN?

The detection of antinephrin antibodies in IgAN is conceptually important precisely because it loosens the assumption that antinephrin autoimmunity is the signature of a single disease. The antibodies appear across biologically distinct contexts, IgAN with and without MCD, suggesting that they report on a shared final common pathway of podocyte injury rather than on IgAN itself. This podocyte-centric reading aligns with evidence that ultrastructural podocyte injury, including foot-process effacement, microvillous transformation, and ultimately podocyte depletion, is a key determinant of progression in IgAN.³ Seropositivity may therefore identify the patient in whom slit-diaphragm autoimmunity, rather than immune-complex–driven inflammation, dominates the nephrotic picture.

Who Should be Tested?

The data point toward a recognizable candidate: the patient with IgAN with nephrotic-range proteinuria or nephrotic syndrome, scant hematuria, and an M0/S0/T0 Oxford biopsy, a constellation difficult to reconcile with inflammatory glomerulonephritis and instead evocative of a podocytopathy. Such cases overlap with the MCD-IgAN dual glomerulopathy described by Herlitz et al.,⁴ and a positive antinephrin result would strengthen that diagnosis even when electron microscopy is equivocal, because subtle or resolving foot-process effacement is easily underread.

How Should a Seropositive Patient be Managed?

The analogy to membranous nephropathy is difficult to resist. Just as antiphospholipase A2 receptor antibodies transformed membranous nephropathy into a serologically defined, monitorable disease,⁵ antinephrin positivity may flag a steroid-responsive, podocytopathy-leaning phenotype warranting earlier corticosteroid therapy, as is already recommended for biopsy-proven MCD-IgAN. Because the antibody is B-cell–derived, it offers a mechanistic rationale for B-cell–targeted therapy in relapsing or steroid-dependent cases. The stakes of this distinction were underscored by a recent multicenter cohort of IgAN with diffuse foot-process effacement, in which MCD-like cases remitted far more often than non–MCD-like cases, and the latter, but not the former, derived clear benefit from adding immunosuppression to corticosteroids.⁶ The high remission rates reported by Si et al.¹ are encouraging, but must be read with caution: seropositive patients were also far more likely to receive glucocorticoids; so treatment-selection bias cannot be excluded, and the multivariable model showed only a nonsignificant trend toward higher complete remission.

How Should Such a Patient be Followed-Up With?

If clinical severity tracks antibody level, as the authors and prior MCD and focal segmental glomerulosclerosis series suggest, then serial antinephrin measurement could extend the concept of immunologic remission to nephrotic IgAN, with falling titers anticipating clinical response and rising titers heralding relapse. This remains a hypothesis, because posttreatment antibody dynamics were not assessed in the present cohort.

Discussion

The findings expand the results of 2 other recent reports. Ying et al.⁷ found high antinephrin positivity in pediatric IgAN-MCD but none in typical proliferative IgAN-nephrotic syndrome, and Yang et al.⁸ described an IgAN-MCD overlap defined by tissue IgG-nephrin colocalization without measuring circulating antibody. Si et al.¹ extend both by showing, in a large adult cohort with long-term outcomes, that seropositivity enriches for a podocytopathy-like phenotype not only in histologically confirmed MCD-IgAN but also in a subset of nephrotic IgAN lacking overt podocytopathy.

Several caveats temper enthusiasm. The seropositive group was small (n = 18), constraining subgroup comparisons that are most clinically tantalizing. No tissue colocalization, functional assay, or longitudinal serology was available, so a pathogenic role cannot be confirmed; the antibodies may yet prove to be an epiphenomenon of podocyte stress or heavy proteinuria. Antinephrin is only one of an expanding family of antislit-diaphragm autoantibodies, with antipodocin and anti-Kirrel1 now described in podocytopathies and potentially accounting for seronegative cases.⁹ The discriminating histology, moreover, is one of chronicity rather than quiescence: endocapillary hypercellularity (E1) was present in approximately 60% of seropositive patients and crescents in nearly half, so the milder M0/S0/T0 profile should not be mistaken for an absence of active inflammation.¹ Finally, the assay is not standardized and the cohort derives from a single center.

None of these points diminish the central message. Si et al.¹ show that a simple serologic test can begin to parse the heterogeneity of nephrotic IgAN, separating an immune podocytopathy from inflammation-dominant disease. The framework proposed in Table 1 is a plausible first draft of how antinephrin testing might enter practice. What it now requires is prospective, multicenter validation with standardized assays and serial sampling, the same path that turned a single autoantibody into the cornerstone of personalized care in membranous nephropathy. If that path holds, antinephrin antibodies may mark a genuine step toward individualized therapy in IgAN.

Table 1.

A proposed framework for antinephrin antibody testing in IgA nephropathy with nephrotic-range proteinuria or nephrotic syndrome

Clinical question	Proposed approach in IgAN with nephrotic-range proteinuria or nephrotic syndrome
Why test?	To identify an immune podocytopathy phenotype within heterogeneous nephrotic IgAN and distinguish slit-diaphragm injury from immune-complex–driven inflammation.
Whom to test?	IgAN with nephrotic-range proteinuria or nephrotic syndrome, minimal hematuria, and M0/S0/T0 Oxford lesions; particularly useful when electron-microscopic foot-process effacement is equivocal.
How to manage a positive result?	Supports an MCD-IgAN/podocytopathy overlap; consider earlier corticosteroid therapy, as for MCD. Weigh the risk that false-positive results prompt unnecessary immunosuppression.
How to follow-up?	Serial antibody measurement as a candidate biomarker of immunologic remission and impending relapse, by analogy with anti-PLA2R in membranous nephropathy (requires prospective validation).

FSGS, focal segmental glomerulosclerosis; IgAN, IgA nephropathy; MCD, minimal change disease; PLA2R, phospholipase A2 receptor.

Disclosure

The author declared no competing interests.