The gut is where kidney disease begins - and now researchers can see exactly which cells are responsible

Firefighting in the wrong organ

IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis worldwide. It destroys kidneys. And for decades, virtually all treatments have focused on suppressing inflammation in the kidneys - essentially hosing down the fire while the arsonist operates freely elsewhere.

The arsonist, researchers have long suspected, lives in the gut. The "gut-kidney axis" hypothesis proposes that IgAN begins in the intestinal mucosa, where an abnormal immune response churns out a defective form of the antibody IgA1 - specifically, galactose-deficient IgA1 (Gd-IgA1). These malformed antibodies circulate to the kidneys, deposit in the glomeruli, and trigger the inflammation that gradually destroys renal function.

The hypothesis made biological sense. But proving it required looking inside the gut of IgAN patients at single-cell resolution - something that was technically and logistically difficult until recently. A team from Peking University First Hospital and Yunnan University has now done exactly that, and the results, published in hLife, point to a specific and potentially druggable target.

Mapping the intestinal immune landscape

The researchers, led by Professors Hong Zhang, Yuemiao Zhang, and Zijie Zhang, performed single-cell RNA sequencing on biopsy samples from the terminal ileum - the last section of the small intestine - comparing IgAN patients with healthy controls.

The gut immune system in IgAN patients looked markedly different. Plasma cells - the antibody factories of the immune system - were significantly expanded in the intestinal lamina propria. More specifically, the expanded population consisted primarily of IgA-producing plasma cells that expressed high levels of B-cell maturation antigen (BCMA), a surface receptor that promotes plasma cell survival and sustains antibody production.

In other words: the gut of IgAN patients is not just passively leaking bad antibodies. It has actively remodeled its immune architecture, expanding a specific population of long-lived plasma cells that are tooled up to produce the very antibodies causing kidney damage - and BCMA is the switch keeping those cells alive.

Clinical correlation and a primate proof of concept

The team went beyond description. They found that the abundance of BCMA-positive plasma cells in patients' gut tissue correlated with clinical severity markers: higher proteinuria levels and faster kidney function decline. That correlation strengthened the case that these cells are not bystanders but active drivers of disease progression.

Then came the therapeutic test. In cynomolgus monkeys, the researchers administered Cizutamig, a bispecific antibody designed to target BCMA. The treatment effectively depleted B cells and produced a deep, sustained reduction in serum IgA levels - significantly more than its effect on other immunoglobulin types. The specificity of the IgA reduction is important: it suggests the therapy preferentially hits the IgA-producing plasma cell population rather than broadly suppressing humoral immunity.

From atlas to intervention

The significance of this work is its directness. Previous studies inferred the gut's role in IgAN through indirect evidence - serum biomarkers, tonsil biopsies, genetic associations. This study visualizes the actual immune cells in the actual tissue at single-cell resolution, names the receptor keeping them alive, and demonstrates in a primate model that targeting that receptor depletes the relevant cells and drops IgA levels.

A clinical trial (NCT07135219) evaluating BCMA-directed treatment in IgAN patients is already underway. If the human results mirror the primate data, it would represent a shift from managing downstream kidney inflammation to interrupting disease at its source.

What is not yet known

This is still early. The single-cell data comes from a limited number of patient biopsies, and intestinal tissue sampling is invasive, making large-scale validation challenging. The primate study demonstrates that BCMA targeting can reduce IgA, but primates do not have IgA nephropathy - so whether IgA reduction alone is sufficient to halt kidney damage in human patients remains an open question.

There is also the issue of immune suppression. BCMA is expressed on plasma cells broadly, not just the pathogenic ones. Long-term BCMA-targeted therapy could impair mucosal immunity and increase infection risk, a trade-off that will need careful monitoring in clinical trials.

The work was conducted at the Renal Division of Peking University First Hospital and the School of Life Sciences at Yunnan University, supported by the National Natural Science Foundation of China and the Beijing Nova Programme.