IgA nephropathy (IgAN) is an autoimmune disease characterized by the deposition of galactose-deficient IgA1 (Gd-IgA1)-containing immune complexes in the kidneys. Elevated serum levels of Gd-IgA1, the main autoantigen in IgAN, are associated with mucosal infections and poor renal outcome in IgAN patients, but little is known about the activation of IgA1-secreting cells overproducing this autoantigen. We found that in peripheral blood mononuclear cells (PBMCs), cytokine stimulation elevated Gd-IgA1 production in B cells from IgAN patients but not in those from healthy controls (p < 0.01). These results were replicated in immortalized B cells derived from PBMCs of IgAN patients and healthy controls. Using single-cell transcriptomics, we identified subsets of IgA1-secreting cells from IgAN patients, but not from healthy controls, with decreased expression of C1GALT1 in response to cytokine stimulation. The C1GALT1-encoded glycosyltransferase is responsible for addition of galactose to IgA1 O-glycans, and its reduced activity is associated with elevated serum levels of Gd-IgA1. These newly identified subsets of IgA1-secreting cells with reduced C1GALT1 expression exhibited reduced expression of several genes related to cytokine-mediated signaling, including those encoding phosphatases, such as SOCS1. siRNA knock-down of SOCS1, and the related SOCS3, increased Gd-IgA1 production in cells derived from PBMCs of healthy controls, indicating a role of these regulators in abnormal cytokine signaling and Gd-IgA1 overproduction. These results revealed that specific subsets of IgA1-secreting cells may be responsible for autoantigen production in IgAN due to abnormal regulation of cytokine-mediated signaling, a process that may occur in inflammatory responses in IgAN patients.