Latent Tubulointerstitial Nephritis with IgM-positive Plasma Cells Uncovered in a Case of IgA Nephropathy and Primary Biliary Cholangitis: Clinical and Histological Insights

Patients with anti-mitochondrial antibody positivity rarely develop tubulointerstitial nephritis with IgM-positive plasma cells (IgMPC-TIN). A previous report observed elevated serum IgM and urinary β2 microglobulin levels in all cases of IgMPC-TIN. This case involved IgA nephropathy with latent coexisting IgMPC-TIN; however, no such changes were observed. These results suggest that some patients with primary biliary cholangitis may have latent IgMPC-TIN without specific urine or blood results. Furthermore, the patient simultaneously developed IgA nephropathy and IgMPC-TIN, suggesting that the triggers and exacerbating factors for both conditions share a common underlying cause.