Mesangial Deposition Can Strongly Involve Innate-Like IgA Molecules Lacking Affinity Maturation

Significance Statement IgA nephropathy (IgAN) is characterized by IgA glomerular mesangial deposition, but its pathogenesis remains unclear. Using humanized transgenic mouse models, the authors explored whether a hypogalactosylated hinge region (found in most mesangial IgA1 in human IgAN) is required for IgA deposition, demonstrating that hinge hypoglycosylation was not mandatory for deposition. To investigate whether low-affinity IgA produced by innate-like B cells might also yield mesangial deposits, they compared mice able to produce high-affinity mature IgA antibodies with mice lacking affinity maturation. They found that the low-affinity IgA can deposit in the mesangium and activate complement, that it is especially prone to induce glomerular cell thickening, and that it can initiate nephrotoxicity. These findings offer a new perspective regarding glomerular IgA deposits involving innate-like antibody responses. Background IgA nephropathy (IgAN) often follows infections and features IgA mesangial deposition. Polymeric IgA deposits in the mesangium seem to have varied pathogenic potential, but understanding their pathogenicity remains a challenge. Most mesangial IgA1 in human IgAN has a hypogalactosylated hinge region, but it is unclear whether this is required for IgA deposition. Another important question is the role of adaptive IgA responses and high-affinity mature IgA antibodies and whether low-affinity IgA produced by innate-like B cells might also yield mesangial deposits. Methods To explore the effects of specific qualitative variations in IgA and whether altered affinity maturation can influence IgA mesangial deposition and activate complement, we used several transgenic human IgA1-producing models with IgA deposition, including one lacking the DNA-editing enzyme activation-induced cytidine deaminase (AID), which is required in affinity maturation. Also, to explore the potential role of the IgA receptor CD89 in glomerular inflammation, we used a model that expresses CD89 in a pattern observed in humans. Results We found that human IgA induced glomerular damage independent of CD89. When comparing mice able to produce high-affinity IgA antibodies with mice lacking AID-enabled Ig affinity maturation, we found that IgA deposition and complement activation significantly increased and led to IgAN pathogenesis, although without significant proteinuria or hematuria. We also observed that hinge hypoglycosylation was not mandatory for IgA deposition. Conclusions In a mouse model of IgAN, compared with high-affinity IgA, low-affinity innate-like IgA, formed in the absence of normal antigen-driven maturation, was more readily involved in IgA glomerular deposition with pathogenic effects.