Rethinking the Benign Nature of Class II Lupus Nephritis

Class II lupus nephritis (LN) has long been viewed as a harmless form of the disease, yet a recent study has revealed that up to 50% of patients with this condition can experience a worsening of their symptoms. The research team aimed to investigate the molecular landscape of Class II LN and identify any cellular or transcriptional factors that could be linked to kidney outcomes.

To do this, they conducted single-cell RNA sequencing on kidney biopsies obtained from fifteen Class II LN patients, eight of whom had developed the condition de novo while the other seven had previously suffered from either proliferative or membranous LN.

The researchers also compared their findings with 155 samples from patients with other forms of LN, as well as 30 healthy controls, which were part of the Accelerating Medicines Partnership in SLE. By correlating the data with the patients' 52-week renal response, they were able to support their results with urinary proteomics.

Despite the minimal histological changes observed in Class II LN, the study found that the immune cell expansion within these biopsies was comparable to that seen in more severe forms of LN, such as proliferative and membranous LN. This expansion included CCL3+ CCL4+ intermediate monocytes producing TNF, FOLR2+ SIGLEC1+ macrophages producing TGF-β and PDGF, and CD69+ CD8+ effector memory T cells producing IFN-γ.

Additionally, fibroblasts and myofibroblasts were significantly expanded, and their transcriptional programs displayed divergent patterns. The pro-fibrotic and inflammatory myofibroblast modules were associated with worse 52-week outcomes, while interferon-responsive and matrix-remodeling fibroblast programs were linked to improvement in kidney function.

The researchers discovered that both pathogenic and protective fibroblast populations received overlapping immune-derived signals, indicating that the fibroblast transcriptional state rather than the identity of incoming signals determined the stromal response. Furthermore, they identified baseline chronicity index as the clinical parameter that most strongly correlated with the patients' outcomes. Finally, urinary proteomics aligned Class II LN more closely with proliferative rather than membranous disease.

In conclusion, the study highlights the substantial molecular activity present in Class II LN, which is not typically captured by routine histological analysis. The researchers suggest that fibroblast transcriptional programs and baseline chronicity index may serve as candidate correlates of kidney outcomes in Class II LN patients and advocate for further validation in larger, prospectively followed cohorts.