#1035 Differential impacts of APRIL and APRIL/BAFF inhibition on immune populations: implications for IgAN treatment and protective immunity

Abstract Background and Aims APRIL and BAFF are essential cytokines for B cell and plasma cell development and survival. Inhibition of APRIL or APRIL/BAFF has emerged as a promising strategy for the treatment of kidney and other immune-mediated diseases, yet their broader impacts on immune homeostasis remain incompletely understood. Given the importance of B cells in immunity, such inhibition may pose risks of immunosuppression and increase susceptibility to infection. Hence, we characterized the differential effects of APRIL and APRIL/BAFF inhibition on immune cell populations, serological responses, and protective immunity in mouse models. Method Mice were dosed twice per week with anti-APRIL antibody (4540), or APRIL/BAFF inhibitors (TACI-Fc fusion proteins POV and ATA, based on the sequences of povetacicept and atacicept, respectively), or isotype control antibody (MOTA). Serum immunoglobulin levels (IgA, IgM, and IgG) were quantified weekly by ELISA. Comprehensive immunophenotyping was performed on splenocytes harvested 8 weeks after initial dosing to characterize B and T cell populations. Single cell RNA sequencing and spatial transcriptomics were performed on mouse splenocytes. Results Repeated dosing of mice with anti-APRIL mAb 4540 did not significantly reduce B-cell or antibody secreting cell (ASC) subsets, except for splenic germinal center B cells. Conversely, both dual APRIL/BAFF inhibitors led to a reduction of total B cells (up to 89%) and most B cell subsets, including ASCs (Fig. 1A). Follicular B cells were reduced by APRIL/BAFF inhibitors and exhibited lower CD21/35 expression (Fig. 1B), suggesting an abrogation of maturation and proliferation potential. APRIL and APRIL/BAFF inhibition had minimal impact on most T cell populations. APRIL/BAFF dual inhibitors, but not anti-APRIL antibody, led to a reduction in T follicular helper (Tfh) cells, an observation consistent with BAFF promoting development of Tfh cells (Fig. 1A). Single cell and spatial transcriptomic analyses supported these observations across B and T cell compartments with further characterization of cell state and pathway changes. Serology measurements from the same treated mice demonstrated both classes of inhibitors yielded ≥40% reduction in IgA. The APRIL/BAFF inhibitors also reduced IgM and IgG by ≥60% and ≥20%, respectively. APRIL-only inhibition did not reduce IgM or IgG (Fig. 2). Within the mouse, we present the implications of these immune cell changes to protective immunity against respiratory viruses. Conclusion This study demonstrates that administration of both APRIL and APRIL/BAFF inhibitors reduce serum IgA, a desirable effect for kidney diseases like IgA nephropathy, in which elevated levels of aberrantly glycosylated IgA have a pathogenic role. However, dual APRIL/BAFF inhibition leads to significant ablation of most B cell subsets, ASCs, and Tfh cells, whereas APRIL-only inhibition does not substantially alter splenic cell populations. Broad immune cell ablation has implications for immune homeostasis and responses to vaccines and pathogens. Ongoing animal studies will extend these analyses of APRIL and APRIL/BAFF inhibition to multiple vaccine and viral challenge models.