Abstract
The ability of B cells to produce high-affinity antibodies and to establish immunological memory in response to a wide range of pathogenic antigens is an essential part of the adaptive immune response. The initial step that triggers a humoral immune response involves the acquisition of antigens by B cells via their surface immunoglobulin, the B cell receptor (BCR). BCR-engaged antigens are transported into specialized lysosomal compartments where proteolysis and production of MHC class II-peptide complexes occur, a process referred to as antigen processing. Expression of MHC class II complexes at the B cell surface allows them to interact with T cells and to receive their help to become fully activated. In this review, we describe how B cells rely on conserved cell polarity mechanisms to coordinate local proteolytic secretion and mechanical forces at the B cell synapse enabling them to efficiently acquire and present extracellular antigens. We foresee that the mechanisms that dictate B cell activation can be used to tune B cell responses in the context of autoimmune diseases and cancer.
Highlights
B lymphocytes are tailored to mount antibody responses upon recognition of foreign antigens and display critical roles as antigen-presenting cells that can shape immune responses
A highly organized, yet dynamic structure is formed: two concentric regions referred to as the central supramolecular activation cluster, where B cell receptor (BCR) are concentrated and the peripheral SMAC that contains adhesion molecules such as LFA-1 bound to its ligand ICAM-1 [12, 18] (Figure 1, inset)
The current view on how B cells acquire antigens has rapidly evolved from classical receptor-mediated endocytosis toward the formation of a dynamic platform, the immune synapse. This process involves the coordination of complex cellular pathways that regulate local secretion of proteases as well as mechanical forces used for extraction of extracellular antigens
Summary
B lymphocytes are tailored to mount antibody responses upon recognition of foreign antigens and display critical roles as antigen-presenting cells that can shape immune responses. A highly organized, yet dynamic structure is formed: two concentric regions referred to as the central supramolecular activation cluster (cSMAC), where BCRs are concentrated and the peripheral SMAC (pSMAC) that contains adhesion molecules such as LFA-1 bound to its ligand ICAM-1 [12, 18] (Figure 1, inset) This characteristic arrangement of cell surface receptors was originally observed in CD4+ T cells that establish immune synapses upon recognition of MHC class II-peptide complexes displayed by antigen-presenting cells [18, 19]. The presence of co-stimulatory signals, such as the engagement of adhesion molecules during synapse formation, can determine how antigens are extracted from the surface of presenting cells Activation of both B and T cells are fine-tuned by interactions between adhesion molecules expressed on antigenpresenting cells, such as ICAM-1 and LFA-1 [24]. The ability of B cells to coordinate vesicle trafficking at the immune synapse relies on conserved polarity proteins and is critical for B cells to become fully competent as antigen-presenting cells
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