How is the KM55 immunohistochemical assay used in diagnosing and monitoring IgA nephropathy?

KM55 in IgA Nephropathy

KM55 is a monoclonal antibody that specifically detects galactose-deficient IgA1 (Gd-IgA1) in glomerular deposits and serves as a powerful diagnostic tool to distinguish primary IgA nephropathy from other IgA-containing glomerular diseases, particularly lupus nephritis and IgA-variant monoclonal gammopathy of renal significance. 1, 2

What KM55 Detects and Why It Matters

• KM55 recognizes a specific epitope (PST[GalNAc]PP motif) on the hinge region of galactose-deficient IgA1, which is the pathogenic form of IgA central to primary IgA nephropathy pathogenesis 3
• Galactose-deficient IgA1 is mucosal-derived and represents the key effector molecule in the multi-hit pathogenesis of IgA nephropathy, making its detection clinically meaningful 1, 2
• The antibody can be used both for serum measurements via ELISA and for immunofluorescence staining of renal biopsy specimens 1

Diagnostic Applications in Renal Biopsy

Primary Diagnostic Use: Distinguishing IgA Nephropathy from Other IgA-Containing Diseases

When immunofluorescence shows IgA-dominant or co-dominant staining, KM55 immunostaining definitively separates mucosal-derived IgA diseases from systemic IgA deposition:

• Primary IgA nephropathy shows strong (2-3+) granular KM55 staining that co-localizes with IgA deposits in the mesangium 2, 4
• Henoch-Schönlein purpura (HSP) also shows strong KM55 positivity (2-3+), confirming shared pathogenesis with primary IgA nephropathy 2
• Secondary IgA nephropathy (including chronic liver disease-related) demonstrates 2-3+ KM55 staining, suggesting mucosal-derived pathogenic IgA 2, 4

Critical Negative Results That Change Diagnosis

• Lupus nephritis with IgA deposition shows negative or minimal KM55 staining despite prominent IgA on routine immunofluorescence, indicating systemic rather than mucosal IgA origin 2, 5
• IgA-dominant/co-dominant membranoproliferative glomerulonephritis is KM55-negative, distinguishing it from IgA nephropathy with membranoproliferative pattern 2
• IgA-variant monoclonal gammopathy of renal significance (IgA-MGRS) shows negative or weak KM55 staining, as monoclonal IgA lacks the galactose-deficient glycosylation pattern 4

Quantitative Approach Using KM55/IgA Ratio

A quantified KM55/IgA intensity ratio provides objective cut-off values for diagnostic classification:

• KM55/IgA ratio ≥0.78 distinguishes primary IgA nephropathy from secondary IgA nephropathy with optimal sensitivity and specificity 4
• KM55/IgA ratio ≥0.21 separates monotypic IgA nephropathy from IgA-MGRS, where the latter shows minimal galactose-deficient IgA1 4
• Primary IgA nephropathy consistently shows the highest KM55/IgA ratio among all IgA-containing glomerular diseases 4

Practical Clinical Algorithm for Using KM55

Step 1: Identify IgA-Dominant or Co-Dominant Deposition on Routine Immunofluorescence

When routine immunofluorescence shows mesangial IgA as dominant or co-dominant immunoglobulin, proceed to KM55 staining 2, 5

Step 2: Perform KM55 Immunofluorescence on Paraffin-Embedded Tissue

• KM55 can be performed on formalin-fixed paraffin-embedded tissue sections, making it widely applicable even when frozen tissue is limited 1, 2
• Assess both the intensity (0 to 3+) and pattern (granular mesangial) of KM55 staining 2, 4
• Compare KM55 distribution with IgA distribution to confirm co-localization 1, 5

Step 3: Interpret Results Based on Clinical Context

Strong KM55 positivity (2-3+) with mesangial co-localization:

• Confirms primary IgA nephropathy, HSP, or secondary IgA nephropathy (including liver disease-related) 2, 4
• Proceed with management according to KDIGO 2021 guidelines for IgA nephropathy 6

Negative or weak KM55 staining despite prominent IgA:

• In patients with systemic lupus erythematosus: diagnose lupus nephritis with atypical immunofluorescence rather than IgA nephropathy 5
• In patients with monoclonal protein: diagnose IgA-MGRS and evaluate for underlying plasma cell dyscrasia 4
• In IgA-dominant MPGN pattern: classify as MPGN rather than IgA nephropathy with MPGN features 2

Step 4: Calculate KM55/IgA Ratio for Borderline Cases

• Quantify fluorescence intensity of both KM55 and IgA staining using standardized imaging software 4
• Apply cut-off of 0.78 to distinguish primary from secondary IgA nephropathy when clinical features are ambiguous 4
• Apply cut-off of 0.21 to distinguish monotypic IgA nephropathy from IgA-MGRS when monoclonal protein is detected 4

Resolving Diagnostic Dilemmas with Confounding Histology

KM55 is particularly valuable when histologic patterns overlap between different disease entities:

• Membranoproliferative pattern with IgA deposition: Strong KM55 positivity indicates IgA nephropathy with secondary MPGN pattern; negative KM55 indicates IgA-dominant MPGN 2
• Diffuse endocapillary proliferative pattern with IgA: KM55 positivity confirms infection-triggered IgA nephropathy or HSP; negative KM55 suggests lupus nephritis or infection-related glomerulonephritis 2
• "Full house" immunofluorescence pattern in SLE patient: Negative KM55 confirms lupus nephritis despite IgA dominance, excluding concurrent IgA nephropathy 5

Integration with Serum Testing

• Serum Gd-IgA1 levels measured by KM55 ELISA correlate with disease activity and are elevated in IgA nephropathy compared to other renal diseases 1
• The KM55 ELISA method is more robust than lectin-based assays (HAA lectin) for measuring serum Gd-IgA1, though results correlate well (R=0.75) 1
• Serum testing complements but does not replace tissue diagnosis, as glomerular KM55 staining provides definitive pathologic classification 1, 2

Common Pitfalls and How to Avoid Them

• Do not diagnose IgA nephropathy in an SLE patient based solely on IgA-dominant immunofluorescence without KM55 confirmation, as lupus nephritis can present with atypical IgA-dominant patterns 5
• Always perform KM55 staining when evaluating patients over age 50 with IgA deposition, as IgA-MGRS becomes more prevalent and requires different management than primary IgA nephropathy 4
• Ensure adequate tissue processing and antibody standardization, as KM55 staining intensity must be reproducible for quantitative ratio calculations 4
• Recognize that negative KM55 in the setting of IgA deposition is diagnostically informative, not a failed test—it actively excludes mucosal-derived pathogenic IgA 2, 5