Background 50–80% of SLE patients develop end-stage renal failure (lupus nephritis (LN)) that is one of major causes of morbidity and mortality. Taking renal biopsies is an invasive procedure not free from risk and it is not applicable to monitor LN activity over time. Therefore, the goal of this study was to detect and define cell-based urinary abnormalities and signatures circumventing a critical intervention. We applied mass cytometry to the multiparametric analysis of urine samples of LN patients.
Methods Urine and heparin anti-coagulated blood samples were simultaneously collected from 7 LN patients and prepared for mass cytometry analysis. All patients had clinically active SLE and had already started an immunosuppressive therapy. Peripheral blood cells were obtained after erythrocyte lysis. Two panels including 54 surface markers in total were used.
Results Mass cytometry allows a comprehensive immunophenotyping of immune cells washed out in urine circumventing problems regarding sample size, autofluorescence-induced artefacts and spectral overlap inherent in conventional flow cytometry. Neutrophils and monocytes/macrophages followed by T cells represent the dominant leukocytes populations in urine samples of SLE patients. The elevated CD8 T cell count was used as biomarker for proliferative LN among SLE patients. Although T cells were below 3% of CD45+ cells, we were able to observe a decreased CD4/CD8 ratio as already described in the literature for LN blood. Additionally, a T cell subpopulation expressing the activation marker CD69 and CD38 was identified in urine but not in blood. Active LN patients show a raised urinary monocyte/macrophage population (CD14+CD36+HLA-DR++).
Conclusion First patient data shown indicate that leukocytes detectable in urine of LN patients are promising biosensors reflecting chronic inflammation in the kidneys. Obviously, the LN histopathologic heterogeneity seems to be better reflected by urinary cell populations than by peripheral blood. Therefore, larger patient cohorts and additional epithelial markers will be analysed to identify robust biosignatures that can be used diagnostically for a classification of lupus nephritis.