From gut to glomerulus: the mucosa–Gd-IgA1–complement axis as an integrated treatment roadmap for IgA vasculitis nephritis, and the limits of extrapolation from IgA nephropathy

IgA vasculitis (IgAV), the commonest systemic small-vessel vasculitis, has a long-term prognosis decided almost entirely by its nephritis, a burden carried disproportionately by adults rather than children. Yet mechanistic understanding remains piecemeal, leaning heavily on evidence extrapolated from IgA nephropathy (IgAN). We therefore separate what the two diseases share from what distinguishes them and state which histological, prognostic and therapeutic evidence transfers legitimately. We read IgAV nephritis as a single druggable axis running from gut to glomerulus through four connected nodes. It begins at the mucosa, where microbial signals acting on a leaky barrier drive production of galactose-deficient IgA1 (Gd-IgA1) that the four-hit sequence assembles into pathogenic immune complexes. Although the glycosylation defect is genetically shared with IgAN, its reach differs, mapping onto the kidney rather than the vasculitis. The deposit is then enlarged by the alternative and lectin pathways, where IgAV unusually supplies its own direct evidence across skin, kidney and urine, before age-modified injury develops in both organs. Viewed through the axis, the complement and injury markers it sheds can be specified, node by node, as a candidate, hypothesis-generating non-invasive panel that pairs complement fragments, Gd-IgA1, urinary IgA and tubular-injury markers with histology and is offered to be tested rather than used, whilst agents in trials that target a proliferation-inducing ligand (APRIL), B-cell activating factor (BAFF) or complement map onto defined nodes under the dual-driver logic of the 2025 Kidney Disease: Improving Global Outcomes (KDIGO) guideline. The central caveat is that, apart from rituximab, non-randomised telitacicept data, negative randomised trials of corticosteroids and cyclophosphamide, and vasculitis-specific guidance, almost every result for the agents mapped here remains borrowed from IgAN and unproven in IgAV. The axis nonetheless offers something concrete: a route from scattered literature to mechanism-matched targets and a priority for age-stratified trials built for IgAV, with endpoints suited to systemic disease.