Revisiting the B-cell compartment in mouse and humans: more than one B-cell subset exists in the marginal zone and beyond

Abstract

The immunological roles of B-cells are being revealed as increasingly complex by functions that are largely beyond their commitment to differentiate into plasma cells and produce antibodies, the key molecular protagonists of innate immunity, and also by their compartmentalisation, a more recently acknowledged property of this immune cell category. For decades, B-cells have been recognised by their expression of an immunoglobulin that serves the function of an antigen receptor, which mediates intracellular signalling assisted by companion molecules. As such, B-cells were considered simple in their functioning compared to the other major type of immune cell, the T-lymphocytes, which comprise conventional T-lymphocyte subsets with seminal roles in homeostasis and pathology, and non-conventional T-lymphocyte subsets for which increasing knowledge is accumulating. Since the discovery that the B-cell family included two distinct categories - the non-conventional, or extrafollicular, B1 cells, that have mainly been characterised in the mouse; and the conventional, or lymph node type, B2 cells - plus the detailed description of the main B-cell regulator, FcγRIIb, and the function of CD40(+) antigen presenting cells as committed/memory B-cells, progress in B-cell physiology has been slower than in other areas of immunology. Cellular and molecular tools have enabled the revival of innate immunity by allowing almost all aspects of cellular immunology to be re-visited. As such, B-cells were found to express "Pathogen Recognition Receptors" such as TLRs, and use them in concert with B-cell signalling during innate and adaptive immunity. An era of B-cell phenotypic and functional analysis thus began that encompassed the study of B-cell microanatomy principally in the lymph nodes, spleen and mucosae. The novel discovery of the differential localisation of B-cells with distinct phenotypes and functions revealed the compartmentalisation of B-cells. This review thus aims to describe novel findings regarding the B-cell compartments found in the mouse as a model organism, and in human physiology and pathology. It must be emphasised that some differences are noticeable between the mouse and human systems, thus increasing the complexity of B-cell compartmentalisation. Special attention will be given to the (lymph node and spleen) marginal zones, which represent major crossroads for B-cell types and functions and a challenge for understanding better the role of B-cell specificities in innate and adaptive immunology.

Figure 1

Organisation of the follicular and MZ B-cell compartments in the human spleen. (A) Schematic representation of the various T- and B-cell areas in the human spleen. PALS: periarteriolar lymphatic sheath (T-cell zone). (B) Staining of paraffin-embedded sections of human spleen with CD20 mAb revealed B-cell follicles (BZ) and a ring of B-cells separating the T-cell zone (TZ) from the red pulp (RP) (original magnification x10). (C) Sections of human spleen were simultaneously stained with PAX5, CD3 and ASM (alpha smooth muscle actin) mAbs. The network of fibroblast-like cells stained with the anti-ASM mAb (blue) subdivides the outer (OMZ) from the inner marginal zone around B-cell follicles (PAX5+, green) and separates the T-cell zone (CD3+, red) from the RP (left panels) (original magnification x10). In the upper left panel, a germinal centre (GC) is visible within the B-cell follicle. General tissue organisation is shown by DAPI staining of nuclei (right panels).

Figure 2

Becoming a follicular or a MZ B-cell?. Left panel: Strong signalling through BcR activates Bruton's tyrosine kinase (BTK), which in turn activates the canonical nuclear factor-kB (NF-kB) signalling pathway and prevents the cleavage of Notch2. BAFF-BAFF-R interactions deliver survival signals through NF-KB activation. Right panel: Notch2 can interact with its ligand, Delta-Like 1 (DL1), specifically expressed by the endothelial cells of red pulp venules in mice. This interaction initiates the cleavage of Notch2, which is not inhibited by weak BcR signalling. The intracellular domain of Notch2 enters into the nucleus where it interacts with Mastermind-like 1 (MAML1) and RBP-J transcription factors. This transcriptional complex induces the commitment of B-cells towards MZ B-cells. BAFF-BAFF-R interactions deliver survival signals through canonical NF-KB activation.

Figure 3

MZ B-cells shuttle between the MZ and follicles and transport Ag and pathogens to follicular DC. In steady-state conditions, strong expression of LFA1 and α4β1 integrins and receptors 1 and 3 of Sphingosine 1-Phosphate (S1P) on MZ B-cells, together with high levels of S1P in blood, contributes to the retention of MZ B-cells within the MZ. Type I IFN produced in response to blood-borne pathogens inactivates S1P1 and 3, allowing MZ B-cells to migrate in response to CXCL13, which is highly expressed in follicles. During this relocation, MZ B-cells can transport immune complexes bound to non-BcR receptors and deliver them to follicular DC (FDC). Once on FDC, these import Ags participate in the adaptive Ab response [56,83]. Rapid ligand-induced desensitisation of CXCR5 authorizes MZ B-cells to return to the MZ. Overproduction of BAFF, which preferentially increases the chemotaxis of CD27+ MZ and memory B-cells to CXCL13 might also impair this shuttling and lead to prolonged sequestration of MZ B-cells within follicles [86]. Such a mechanism would be at work during acute infection by SIV, where it might favour sequestration of activated B-cells within follicles [82].

Figure 4

MZ B-cells at the crossroad between BcR-dependent and CD1d-dependent B-cell responses to lipid antigens. Through the expression of LDL-R, MZ B-cells capture and internalize aliprotein E (ApoE)-bound lipid Ags. Dendritic cells and macrophages in tissues secrete ApoE, which is present at low levels in human serum. ApoE-lipid Ag complexes are directed into the endosomal-lysosomal pathway and charged onto CD1d molecules. Exogenous lipids presented by CD1d interact with the invariant TCR of iNKT. These cognate interactions activate iNKT, which produces cytokines, and provide “innate help” to MZ B-cells. Because this internalisation pathway is independent on BcR, it might enhance humoral responses or induce pathogenic Abs [120]. The LDL-R-CD1d-dependent pathway for lipid Ag uptake by B-cells nevertheless provides a mechanism for the adjuvant effects of αGalCer [116]. Other studies suggest that αGalCer is routed to the endosomal-lysosomal pathway and charged onto CD1d molecules after BcR-mediated uptake of protein Ags linked to αGalCer [117], while BcR-mediated stimulation of human B-cells rapidly down-modulates CD1d expression [118].