Dynamic recruitment of inhibitory complexes by CD25 controls B-cell development and selection

Initiation of BCR signaling, and subsequent antigen-encounter in germinal centers are both marked by sharp increases of CD25 surface-expression. While CD25 is known as an IL2-receptor chain on T- and NK-cells, the significance of its expression on B cells was unclear.

Our genetic studies revealed that, rather than functioning as an IL2-receptor chain, CD25 expressed on B-cells assembled an inhibitory complex including PKCd, ITIM-bearing receptors and inhibitory phosphatases for feedback control of BCR-signaling. Conditional CD25-deletion resulted in clonal expansion and autoimmunity.

Mechanistic studies revealed that BCR engagement induced PKCd-mediated phosphorylation of CD25, which anchored CD25 on cell surface in proximity to the BCR complex. Moreover, CD25 recruited inhibitory receptors CD22, that bear ITIM-motifs for the activation of inhibitory phosphatases (e.g., SHP1). The positively charged tail of CD25 engages negatively charged residues in the cytoplasmic tails of the BCR signaling chain CD79B and the inhibitory CD22-ITIM. Molecular dynamics simulations, Co-IP and TIRF microscopy results revealed that the ternary complex between CD25, CD79B and CD22 enables recruitment and activation of the phosphatase SHP1 within reach of its substrate. Thereby, the SH2-domain of SHP1 engages the CD22-ITIM motif, while its phosphatase domain interacts with the CD79B-ITAM to terminate BCR-signaling. This complex, while engaging and activating SHP1, is held together by CD25.