Antigen presentation to B cells

Abstract

B cells are capable of mounting responses to a bewildering range of potentially pathogenic antigens through the production of high-affinity antibodies and the establishment of immunological memory. Thus, regulated B-cell activation is critical for protection against a variety of bacterial and viral infections, as well as cancers. Here, we discuss a number of recent imaging studies that have provided new insights into the variety of mechanisms by which B-cell activation is initiated in the lymph node in vivo.

Figure 1.. B-cell activation in the lymph node

Lymphatic fluid containing antigens (red) enters the lymph node (centre) through afferent vessels and moves around the subcapsular sinus (SCS; brown), gaining access to B cells through a variety of recently elucidated mechanisms. (A) Smaller antigens are moved through the follicular conduit network and can gain access to cognate follicular B cells. (B-D) Larger antigens, such as immune complexes, are excluded from the conduit network and can be presented on the surface of (B) SCS macrophages (light brown) or (C) follicular dendritic cells (dark blue) to cognate B cells in the follicles (grey). Alternatively, (D) extrafollicular dendritic cells (orange) may present antigens to cognate B cells as they arrive in the node through the high endothelial vessels (HEV; red). The SCS macrophages, shown in (B), have been the particular focus of a number of recent studies. These macrophages accumulate larger antigens potentially through a variety of cell surface receptors according to the nature of the antigen itself. Accumulated antigen might either move along the cell surface or enter intracellular vesicles and be recycled intact to the cell surface for presentation to cognate B cells through the B-cell receptor (BCR), or to non-cognate B cells through complement receptors. In addition, antigen may enter into processing compartments such as lysosomes for processing and presentation to CD8⁺ T cells or, in the case of lipid antigens, to invariant natural killer (iNKT) cells. MHC I, multi histocompatibility complex class I; TCR, T-cell receptor.