B-Cell Depletion and the BAFF-APRIL Axis

The target you choose decides which cells you keep. B-lineage surface proteins switch on and off along development, and a depleting antibody can only see what is expressed at the moment it arrives. B220 covers essentially the whole mouse B compartment. CD19 appears early and, critically, is retained on plasmablasts. CD20 appears later than CD19 and is lost as cells become plasma cells — which is the single most consequential fact in this diagram, because it means anti-CD20 spares the long-lived plasma cell that secretes the pathogenic antibody. CD22 is the inhibitory co-receptor of the mature cell. CD38, BCMA and CD138 mark the plasma-cell end of the axis, and are the targets you need if the goal is to remove antibody rather than to remove B cells.

Depletion changes the survival economy. BAFF and APRIL are made by myeloid and stromal cells at a rate that does not fall when B cells disappear. Remove most of the compartment and the surviving cells face a large excess of ligand. BAFF-R is the dominant receptor on naive and memory cells and signals through TRAF3 degradation, which releases NIK (MAP3K14) and switches on the non-canonical NF-κB pathway; the output is BCL-2 and MCL-1, and therefore survival. TACI and BCMA read APRIL as well as BAFF, and BCMA is what keeps the long-lived plasma cell alive in its niche. This is why B-cell return after depletion is not simply repopulation, and why combining depletion with BAFF blockade behaves differently from either alone. A note on reagents: the NIK/MAP3K14 antibody linked here is validated for rat reactivity only.

The differentiation programme runs underneath all of it. Antigen through the BCR engages SYK and BTK — the node BTK inhibitors act on — while CD22 restrains the same signal. CD4⁺ T follicular helper cells supply IL-21, and IL-6 from stroma completes the differentiation signal towards CD138⁺ plasma cells. Inside the cell the order is fixed: IRF4 at high concentration induces Blimp-1 (PRDM1), Blimp-1 shuts down the B-cell programme and licenses XBP1, and XBP1 builds the secretory apparatus. Everything downstream of Blimp-1 is committed — which is precisely why plasma cells are hard to remove.

Three killing mechanisms, three different failure modes. A bound depleting antibody recruits CD16 on NK1.1⁺ cells for ADCC, drives CD11b⁺ macrophage phagocytosis, and fixes C1q to run C3 and the C5b-9 membrane attack complex. ADCC saturates when effector cells are exhausted or their Fc receptors are blocked by circulating IgG; phagocytosis is held back by CD47 signalling into SIRPα; complement is consumed and depletes locally. The readout is not the same as the mechanism: IgM falls quickly after depletion because it comes from short-lived cells, while IgG and anti-dsDNA can persist for months. The In Vivo tie-in: this map has a deep functional-grade layer. Anti-CD20 and anti-CD19 give you two depletion depths; anti-B220 gives the broadest; anti-CD38 and anti-BCMA reach the plasma-cell compartment; and anti-NK1.1, anti-CD11b and anti-CD16/CD32 let you remove one killing mechanism at a time to find out which one your depletion actually depends on. That last set is the experiment most often skipped. For research use only; not for use in diagnostic or therapeutic procedures.