Rare cases of immunoglobulin G (IgG)-dominant immune complex–mediated glomerulonephritis demonstrate immunoglobulin subclass restriction without light chain restriction. Some of these cases may represent proliferative glomerulonephritis with monotypic immunoglobulin deposits (PGNMID) in which monotypic immunoglobulin is obscured by coexisting polytypic immunoglobulin. However, rigorous demonstration of this possibility is lacking to date. Here, we describe a case of IgG3-restricted immune complex–mediated glomerulonephritis without light chain restriction that apparently “transformed” into IgG3κ-PGNMID in a subsequent biopsy. We demonstrate, using several ancillary techniques, including use of the newly described antibodies directed against the conformational epitope at the junctions of heavy and light chains (HLC-IF), that the first biopsy likely represents IgG3κ-PGNMID in which monotypic IgG3κ was hidden by polytypic IgM. This case underscores the need to consider PGNMID in a differential diagnosis of IgG-dominant immune complex–mediated glomerulonephritis without light chain restriction and highlights the potential utility of IgG subclass staining and HLC-IF in such cases to detect monotypic immunoglobulin that may be obscured by coexisting IgM and/or IgA deposits.

免疫球蛋白 G (IgG) 占主导地位的免疫复合物介导的肾小球肾炎的罕见病例表现出免疫球蛋白亚类限制而没有轻链限制。其中一些病例可能代表增生性肾小球肾炎伴单型免疫球蛋白沉积物 (PGNMID)，其中单型免疫球蛋白被共存的多型免疫球蛋白所掩盖。然而，迄今为止，缺乏对这种可能性的严格证明。在这里，我们描述了一个没有轻链限制的 IgG3 限制性免疫复合物介导的肾小球肾炎病例，该病例在随后的活检中明显“转化”为 IgG3κ-PGNMID。我们证明，使用几种辅助技术，包括使用新描述的针对重链和轻链连接处的构象表位的抗体 (HLC-IF)，第一次活检可能代表 IgG3κ-PGNMID，其中单型 IgG3κ 被多型 IgM 隐藏。该病例强调了在无轻链限制的 IgG 显性免疫复合物介导的肾小球肾炎的鉴别诊断中需要考虑 PGNMID，并强调了 IgG 亚类染色和 HLC-IF 在此类病例中检测单型免疫球蛋白的潜在用途共存的 IgM 和/或 IgA 沉积物。