Dynamic phosphatase-recruitment controls B-cell selection and oncogenic signaling

Abstract

Initiation of B-cell receptor (BCR)¹ signaling, and subsequent antigen-encounter in germinal centers^2,3 represent milestones of B-lymphocyte development that are both marked by sharp increases of CD25 surface-expression. Oncogenic signaling in B-cell leukemia (B-ALL)⁴ and lymphoma⁵ also induced CD25-surface expression. While CD25 is known as an IL2-receptor chain on T- and NK-cells^6–9, the significance of its expression on B-cells was unclear. Our experiments based on genetic mouse models and engineered patient-derived xenografts revealed that, rather than functioning as an IL2-receptor chain, CD25 expressed on B-cells assembled an inhibitory complex including PKCδ and SHIP1 and SHP1 phosphatases for feedback control of BCR-signaling or its oncogenic mimics.

Recapitulating phenotypes of genetic ablation of PKCδ¹⁰–¹², SHIP1^13,14 and SHP1^{14, 15,16}, conditional CD25-deletion decimated early B-cell subsets but expanded mature B-cell populations and induced autoimmunity. In B-cell malignancies arising from early (B-ALL) and late (lymphoma) stages of B-cell development, CD25-loss induced cell death in the former and accelerated proliferation in the latter. Clinical outcome annotations mirrored opposite effects of CD25-deletion: high CD25 expression levels predicted poor clinical outcomes for patients with B-ALL, in contrast to favorable outcomes for lymphoma-patients. Biochemical and interactome studies revealed a critical role of CD25 in BCR-feedback regulation: BCR-signaling induced PKCδ-mediated phosphorylation of CD25 on its cytoplasmic tail (S²⁶⁸). Genetic rescue experiments identified CD25-S²⁶⁸ tail-phosphorylation as central structural requirement to recruit SHIP1 and SHP1 phosphatases to curb BCR-signaling. A single point mutation CD25^S268A abolished recruitment and activation of SHIP1 and SHP1 to limit duration and strength of BCR-signaling. Loss of phosphatase-function, autonomous BCR-signaling and Ca²⁺-oscillations induced anergy and negative selection during early B-cell development, as opposed to excessive proliferation and autoantibody production in mature B-cells. These findings highlight the previously unrecognized role of CD25 in assembling inhibitory phosphatases to control oncogenic signaling in B-cell malignancies and negative selection to prevent autoimmune disease.