Integrin-associated protein/CD47 regulates motile activity in human B-cell lines through CDC42

Advances in Anti-Tumor Treatments Targeting the CD47/SIRPα Axis

CD47 is an immunoglobulin that is overexpressed on the surface of many types of cancer cells. CD47 forms a signaling complex with signal-regulatory protein α (SIRPα), enabling the escape of these cancer cells from macrophage-mediated phagocytosis. In recent years, CD47 has been shown to be highly expressed by various types of solid tumors and to be associated with poor patient prognosis in various types of cancer. A growing number of studies have since demonstrated that inhibiting the CD47-SIRPα signaling pathway promotes the adaptive immune response and enhances the phagocytosis of tumor cells by macrophages. Improved understanding in this field of research could lead to the development of novel and effective anti-tumor treatments that act through the inhibition of CD47 signaling in cancer cells. In this review, we describe the structure and function of CD47, provide an overview of studies that have aimed to inhibit CD47-dependent avoidance of macrophage-mediated phagocytosis by tumor cells, and assess the potential and challenges for targeting the CD47-SIRPα signaling pathway in anti-cancer therapy.

Congenital Defects in Actin Dynamics of Germinal Center B Cells

The germinal center (GC) is a transient anatomical structure formed during the adaptive immune response that leads to antibody affinity maturation and serological memory. Recent works using two-photon microscopy reveals that the GC is a highly dynamic structure and GC B cells are highly motile. An efficient selection of high affinity B cells clones within the GC crucially relies on the interplay of proliferation, genome editing, cell-cell interaction, and migration. All these processes require actin cytoskeleton rearrangement to be well-coordinated. Dysregulated actin dynamics may impede on multiple stages during B cell affinity maturation, which could lead to aberrant GC response and result in autoimmunity and B cell malignancy. This review mainly focuses on the recent works that investigate the role of actin regulators during the GC response.

Rho GTPase Cdc42 is essential for B-lymphocyte development and activation

Cdc42 is a member of the Rho GTPase family that has been implicated in several cell functions including proliferation and migration, but its physiologic role needs to be dissected in each cell type. We achieved B-cell and hematopoietic stem cell deletion of Cdc42 by conditional gene targeting in mice. Deletion of Cdc42 from proB/preB-cell stage significantly blocked B-cell development at T1 and later stages, resulting in reduced mature B-cell populations and reduced antigen-specific immunoglobulin M (IgM), IgG1, and IgG3 production. The Cdc42(-/-) B cells, themselves, were abnormal with impaired proliferation and survival. The mutant B cells were further characterized by a B-cell receptor (BCR) signaling defect with increased Erk and decreased Akt activation, as well as a defect in BCR-mediated B-cell-activating factor (BAFF) receptor up-regulation and subsequent BAFF receptor signaling in mature resting B cells. Surprisingly, Cdc42 was dispensable for stromal cell-derived factor-1alpha (SDF-1alpha)- or B-lymphocyte chemoattractant (BLC)-induced B-cell migration. Finally, loss of Cdc42 from hematopoietic stem cells did not alter common lymphoid progenitor production but severely reduced proB/preB- and immature B-cell populations, indicating that Cdc42 is also involved in B-cell precursor differentiation. These results reveal multifaceted roles of Cdc42 in B-cell development and activation.

Mechanisms of CD47-induced caspase-independent cell death in normal and leukemic cells: link between phosphatidylserine exposure and cytoskeleton organization

Dying cells, apoptotic or necrotic, are swiftly eliminated by professional phagocytes. We previously reported that CD47 engagement by CD47 mAb or thrombospondin induced caspase-independent cell death of chronic lymphocytic leukemic B cells (B-CLL). Here we show that human immature dendritic cells (iDCs) phagocytosed the CD47 mAb-killed leukemic cells in the absence of caspases 3, 7, 8, and 9 activation in the malignant lymphocytes. Yet the dead cells displayed the cytoplasmic features of apoptosis, including cell shrinkage, phosphatidylserine exposure, and decreased mitochondrial transmembrane potential (DeltaPsim). CD47 mAb-induced cell death also occurred in normal resting and activated lymphocytes, with B-CLL cells demonstrating the highest susceptibility. Importantly, iDCs and CD34(+) progenitors were resistant. Structure-function studies in cell lines transfected with various CD47 chimeras demonstrated that killing exclusively required the extracellular and transmembrane domains of the CD47 molecule. Cytochalasin D, an inhibitor of actin polymerization, and antimycin A, an inhibitor of mitochondrial electron transfer, completely suppressed CD47-induced phosphatidylserine exposure. Interestingly, CD47 ligation failed to induce cell death in mononuclear cells isolated from Wiskott-Aldrich syndrome (WAS) patients, suggesting the involvement of Cdc42/WAS protein (WASP) signaling pathway. We propose that CD47-induced caspase-independent cell death be mediated by cytoskeleton reorganization. This form of cell death may be relevant to maintenance of homeostasis and as such might be explored for the development of future therapeutic approaches in lymphoid malignancies.