Figure. Colocalization of macrophages and mineral within a Randall’s plaque. A, Alizarin red staining of Randall’s plaque tissue identifying areas of calcification. B, Immunofluorescent staining of same tissue using CD86 (M1 marker) and CD163 (M2 marker).

Randall’s plaques lie at the heart of calcium oxalate kidney stone formation, yet how these plaques form at the tip of the renal papilla has been a century-old puzzle. Recent evidence suggests there may be parallels between Randall’s plaque formation and atherosclerosis—mineralization that occurs within the wall of blood vessels—regarding the role of macrophages, an omnipresent immune cell type with diverse biological functions. It has been hypothesized that different kinds of macrophages (M1 or M2 polarized) can interact with the papillary microenvironment to either promote or inhibit mineralization. Too much of the “wrong” kind of macrophage might cause Randall’s plaques to form.

To better understand the nature of these macrophages, we performed snRNAseq (single nucleus RNA sequencing) of human Randall’s plaque tissue, which we obtained via endoscopic biopsy during kidney stone procedures. This powerful sequencing technology is used to profile and compare gene expression cell by cell. In our study, it allowed us to identify the macrophage populations and determine what makes those associated Randall’s plaques different from other macrophages in the kidney. We found that rather than being derived from circulating monocytes, which would be typical for an inflammatory process, Randall’s plaque macrophages are tissue resident with an embryonic origin.

This result is highly surprising. For one, this adds an interesting twist in the debate about how inflammation influences Randall’s plaque formation. Studies of monocyte-derived macrophages in mouse models have demonstrated that pro-inflammatory (M1) polarization promotes renal mineralization more so than anti-inflammatory (M2) polarization. Tissue resident macrophages are more M2-like in nature, however, and our snRNAseq data show that they express some M2-associated surface markers, which we confirmed with immunofluorescent staining (see Figure). Since they are not monocyte derived, they may be involved in an entirely separate activation pathway. Regardless, they do produce osteopontin, osteonectin, and collagen, genes that have been implicated in biomineral formation.

Another surprising aspect of our findings is that these macrophages are not normally found at the papillary tip. In the landmark study that comprehensively mapped the immune cells in the kidney from embryonic development to adulthood, tissue resident macrophages were located exclusively in the cortex.¹ Understanding how these macrophages relocate from the cortex to the papilla may unlock clues about their physiological role and how this relates to Randall’s plaque formation. Interestingly, our preliminary analyses suggest chemokine signaling from the loops of Henle may be responsible for attracting these cells to the papillary tip. One possible mechanism could be that chronic cellular stress at the loop of Henle recruits tissue resident macrophages to promote healing, and Randall’s plaques are a byproduct of their activity in this location due to the high solute concentrations.

Our results call for new functional studies to understand how these macrophages influence biomineralization in the kidney. While our story generates more questions than answers, identifying this unexpected culprit was a critical step in solving this age-old mystery. Ultimately, unraveling the mechanism of Randall’s plaque formation would enhance our understanding of kidney stone disease and highlight new targets for stone prevention therapy.