Therapeutic agents targeting the tumor necrosis factor (TNF) superfamily cytokines B-cell activating factor (BAFF, BLyS) and/or A PRoliferation Inducing Ligand (APRIL) have demonstrated clinical effectiveness in multiple autoimmune diseases, such as systemic lupus erythematosus, lupus nephritis, and immunoglobulin A nephropathy (IgAN). However, their clinical utility can often be limited by incomplete and/or prolonged times to clinical response and inconvenient dosing regimens, which may be improved by more potent dual inhibition of both cytokines. Povetacicept (ALPN-303; TACI vTD-Fc) is a crystallizable fragment (Fc) fusion protein of an engineered transmembrane activator and CAML interactor (TACI) domain which mediates more potent inhibitory activity than wild-type TACI-Fc or BAFF- or APRIL-specific antibodies and demonstrates superior pharmacokinetic and pharmacodynamic activity in multiple preclinical disease models. In this first-in-human study in healthy adults, povetacicept was well-tolerated as single ascending doses of up to 960 mg administered intravenously or subcutaneously. Dose-dependent pharmacokinetics were observed. Coverage of BAFF and APRIL was observed for 2-3 weeks and ≥4 weeks after doses of 80 mg and ≥240 mg, respectively. Maximal pharmacodynamic effects were observed at dose levels ≥80 mg for a single dose, associated with on-target reductions in antibody-secreting cells as well as in all circulating immunoglobulin isotypes, including the IgAN disease-related biomarker galactose-deficient-immunoglobulin A1 (Gd-IgA1), and were superior to results reported for wild-type TACI-Fc. These data strongly support further development of povetacicept for the treatment of B-cell-mediated automimmune diseases.