*Article* **Galactose-Deficient IgA1 as a Candidate Urinary Marker of IgA Nephropathy**

**Yusuke Fukao 1, Hitoshi Suzuki 1,2,\*, Jin Sug Kim 3, Kyung Hwan Jeong 3, Yuko Makita 1, Toshiki Kano 1, Yoshihito Nihei 1, Maiko Nakayama 1, Mingfeng Lee 1, Rina Kato 1, Jer-Ming Chang 4, Sang Ho Lee 3 and Yusuke Suzuki 1,\***

	- 3 Division of Nephrology, Department of Internal Medicine, College of Medicine, Kyung Hee University, Seoul 130-701, Korea; jinsuk0902@naver.com (J.S.K.); kyunghwan@naver.com (K.H.J.); lshkidney@khu.ac.kr (S.H.L.)
	- 4 Division of Nephrology, Kaohsiung Medical University Hospital, Kaohsiung 80756, Taiwan; jemich@cc.kmu.edu.tw
	- **\*** Correspondence: shitoshi@juntendo.ac.jp (H.S.); yusuke@juntendo.ac.jp (Y.S.)

**Abstract:** In patients with IgA nephropathy (IgAN), circulatory IgA1 and IgA1 in the mesangial deposits contain galactose-deficient IgA1 (Gd-IgA1). Some of the Gd-IgA1 from the glomerular deposits is excreted in the urine and thus urinary Gd-IgA1 may represent a disease-specific marker. We recruited 338 Japanese biopsy-proven IgAN patients and 120 patients with other renal diseases (disease controls). Urine samples collected at the time of renal biopsy were used to measure Gd-IgA1 levels using a specific monoclonal antibody (KM55 mAb). Urinary Gd-IgA1 levels were significantly higher in patients with IgAN than in disease controls. Moreover, urinary Gd-IgA1 was significantly correlated with the severity of the histopathological parameters in IgAN patients. Next, we validated the use of urinary Gd-IgA1 levels in the other Asian cohorts. In the Korean cohort, urinary Gd-IgA1 levels were also higher in patients with IgAN than in disease controls. Even in Japanese patients with IgAN and trace proteinuria (less than 0.3 g/gCr), urinary Gd-IgA1 was detected. Thus, urinary Gd-IgA1 may be an early disease-specific biomarker useful for determining the disease activity of IgAN.

**Keywords:** urinary galactose-deficient IgA1; KM55; IgA nephropathy
