**5. Conclusions**

The elevation in serum Gd-IgA1 levels and immunodeposits enriched for Gd-IgA1 glycoforms are characteristic features of IgAN. To clarify the characteristics of diseasespecific IgA1, many studies have attempted to use lectin assays, monoclonal antibodies, mass spectrometry, and genetic and other types of analyses. However, the detailed characterization of *O*-glycoforms of IgA1 associated with IgAN is still to be performed. In addition, the effect of different *O*-glycoforms in IgA1 HR on the formation of immune complexes, mesangial deposition, and glomerular injury has not been fully elucidated. Table 1 summarizes what we know and what we do not know about glycosylation of IgA1 in IgAN.


**Table 1.** What we know and what we do not know about *O*-glycosylation of IgA1 in IgAN. **Table 1.** *Cont.*


Recent genetic studies have shown that *O*-glycosylation of IgA1 is influenced by genetic variants of *O*-glycosyltransferase, and racial differences in variants have also been noted. Therefore, racial factors should be considered when investigating diseasespecific *O*-glycoforms. By clarifying the disease-specific *O*-glycoform(s), the mechanism underlying aberrant glycosylation, and the effects of glycosylation changes, it is hoped that the elucidation of the etiology and development of diagnostic markers and new therapies for this disease will be advanced.

**Author Contributions:** Conceptualization, Y.O. and K.T.; validation, K.T.; writing—original draft preparation, Y.O. and K.T.; writing—review and editing, Y.O., M.B.R., J.N. and K.T.; visualization, Y.O. and K.T.; supervision, M.B.R. and J.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was funded by the JSPS KAKENHI (grant numbers 19K08691, 19K08715, 20K22915) and by Aichi Jinzou Foundation. J.N. and M.B.R. are supported in part by NIH grants DK078244, DK082753, AI149431, and GM098539.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

**Conflicts of Interest:** M.B.R. and J.N. are co-founders and co-owners of and consultants for Reliant Glycosciences, LLC. M.B.R. and J.N. are co-inventors on US patent application 14/318,082 (assigned to UAB Research Foundation).
