# Chronic Glomerular Thrombotic Microangiopathy in a 72-Year-Old Patient with B-Cell Chronic Lymphocytic Leukemia and IgG Lambda Paraprotein

## TL;DR

A 72-year-old man with leukemia and a paraprotein developed kidney damage, which was linked to the paraprotein acting as a toxin, not to immune system activation.

## Contribution

This case highlights paraprotein-induced glomerular thrombotic microangiopathy unrelated to complement system activation in a leukemia patient.

## Key findings

- Chronic glomerular thrombotic microangiopathy was diagnosed with structural changes in capillary walls.
- Treatment with ibrutinib reduced paraprotein and improved kidney function without affecting classical complement pathway activity.
- The paraprotein, not complement activation, was identified as the likely cause of kidney damage.

## Abstract

The cause of nephrotic–nephritic syndrome and elevated blood pressure values was investigated by renal biopsy in a 72-year-old Caucasian male with B-cell chronic lymphocytic leukemia (B-CLL) and a low level of IgG/lambda paraprotein. Double-contoured glomerular capillaries, glomerular thrombi, interstitial B-CLL infiltrates, and normal-looking arteries and arterioles were observed histologically. The glomerular capillaries displayed nonspecific entrapment of IgM and C3 and pseudolinear C4d positivity immunohistochemically. With electron microscopy, diffusely effaced foot processes, widened and duplicated glomerular basement membrane (BM), mesangial cell interposition, and thickened, non-fenestrated, and serrated endothelial cells located on subendothelial BM layer(s) were seen. The peritubular capillaries lacked any significant BM multilayering. Chronic glomerular thrombotic microangiopathy (TMA) was diagnosed; the C4d positivity result indicated structural remodeling of glomerular capillary walls. Laboratory features of microangiopathic hemolytic anemia were absent. The functional complement assay found selective classical pathway activation and the consumption of early complement components. The components of the alternative pathway were not consumed. A disease-causing variant in the coding region of the complement C2 gene was screened, with negative results. The kidney function gradually deteriorated to stage 4 chronic kidney disease over a period of six months. Second-line treatment with ibrutinib markedly decreased the leukemic symptoms, stopped the production of paraprotein, and eliminated the nephrotic syndrome; the kidney function improved. The decreased activity of the classical pathway remained unchanged. The culprit of glomerular anomalies seemed to be the paraprotein, which acted as a nephrotoxic mediator and triggered glomerular TMA. A hypothetical pathophysiologic explanation of TMA is presented. The paraneoplastic classical pathway activation of complement did not play any role in the development of glomerular TMA.