Memory B cells generated in T cell-dependent antibody responses colonise the splenic marginal zone

T cell-dependent IgM memory B cells generated during bacterial infection are required for IgG responses to antigen challenge

Immunological memory has long considered to be harbored in B cells that express high-affinity class-switched IgG. IgM-positive memory B cells can also be generated following immunization, although their physiological role has been unclear. In this study, we show that bacterial infection elicited a relatively large population of IgM memory B cells that were uniquely identified by their surface expression of CD11c, CD73, and programmed death-ligand 2. The cells lacked expression of cell surface markers typically expressed by germinal center B cells, were CD138 negative, and did not secrete Ab ex vivo. The population was also largely quiescent and accumulated somatic mutations. The IgM memory B cells were located in the region of the splenic marginal zone and were not detected in blood or other secondary lymphoid organs. Generation of the memory cells was CD4 T cell dependent and required IL-21R signaling. In vivo depletion of the IgM memory B cells abrogated the IgG recall responses to specific Ag challenge, demonstrating that the cell population was required for humoral memory, and underwent class-switch recombination following Ag encounter. Our findings demonstrate that T cell-dependent IgM memory B cells can be elicited at high frequency and can play an important role in maintaining long-term immunity during bacterial infection.

Intrinsic properties of human and murine memory B cells

The central question of how the immune system responds in a qualitatively and quantitatively better way upon re-exposure to a pathogen is largely unanswered. Both the increased frequency of antigen-specific memory cells and the intrinsic properties that memory cells acquire after antigen experience could contribute to the faster and more robust responses seen after repeated exposure to antigen. In the case of the memory B-cell response, it has been difficult to discern the individual contributions of these two effects. However, because of recent advances in identifying memory B cells, there is an increasing understanding of the intrinsic properties of these cells. The current insights into the unique properties of memory B cells and the progress that has been made in understanding how these affect secondary responses in both the human and the mouse systems are discussed. In addition, we compare the various advantages and disadvantages inherent in each of these systems, in terms of studying the intrinsic properties of memory B cells, and introduce the details of the system that we have developed using conventional heavy chain transgenic (Tgic) mice, which addresses some of the drawbacks of traditional memory models.

Development and selection of marginal zone B cells

It is now clear that functionally distinct subsets of mature peripheral B cells exist. Of these subsets, marginal zone (MZ) B cells in the spleen are strategically positioned at the blood-lymphoid interface and are programmed to initiate a fast and intense antibody response to blood-borne viral and bacterial agents. Their ability to respond vigorously to antigen and polyclonal activators make MZ B cells key players in the early response to pathogens in the bloodstream. The specialized functions of these innate-like lymphocytes bridge the gap between the early innate immune response and the slower adaptive antibody response, affected mainly by the more prolific follicular B cells. MZ B cells, like B1 cells, are important not only to combat infections but also in the maintenance of host homeostasis. Here we discuss some aspects of MZ B-cell selection and function in health and disease.

The dual function of the splenic marginal zone: essential for initiation of anti-TI-2 responses but also vital in the general first-line defense against blood-borne antigens

The splenic marginal zone (S-MZ) is especially well equipped for rapid humoral responses and is unique in its ability to initiate an immune response to encapsulated bacteria (T-cell independent type 2 (TI-2) antigens). Because of the rapid spreading through the blood, infections with blood-borne bacteria form a major health risk. To cope with blood-borne antigens, a system is needed that can respond rapidly to a great diversity of organisms. Because of a number of unique features, S-MZ B cells can respond rapid and efficient to all sorts of blood-borne antigens. These unique features include a low blood flow microenvironment, low threshold for activation, high expression of complement receptor 2 (CR2, CD21) and multireactivity. Because of the unique high expression of CD21 in a low flow compartment, S-MZ B cells can bind and respond to TI-2 antigens even with relatively low-avid B cell receptors. Although TI-2 antigens are in general poorly opsonized by classic opsonins, a particular characteristic of these antigens is their ability to bind very rapidly to complement fragment C3d without the necessity of previous immunoglobulin binding. TI-2 primed S-MZ B cells, already by first passage through the germinal centre, will meet antigen-C3d complexes bound to follicular dendritic cells, allowing unique immediate isotype switching. This explains that the primary humoral response to TI-2 antigens is unique in its characterization by a rapid increase in IgM concurrent with IgG antibody levels.

NKX2.3 is required for MAdCAM-1 expression and homing of lymphocytes in spleen and mucosa-associated lymphoid tissue

Targeted disruption of the transcription factor NKX2.3 gene in mice results in anatomical defects of intestine and secondary lymphoid organs. Here, we report that spleen and Peyer's patches of NKX2. 3-deficient mice are considerably reduced in size and lack the ordered tissue architecture. T and B cells are misplaced within the spleen and mesenteric lymph nodes and fail to segregate into the appropriate T and B cell areas. Furthermore, splenic marginal zones, characterized by specific B cells and various types of macrophage-derived cells around the marginal sinus, are absent in mutants. Homozygous NKX2.3 mutants lack the mucosal addressin cell adhesion molecule-1 (MAdCAM-1) that is normally expressed in mucosa-associated lymphoid tissue (MALT) and spleen. We provide evidence that NKX2.3 can activate MAdCAM-1 transcription directly, suggesting that MAdCAM-1 is at least partly responsible for the migration and homing defects of lymphocytes and macrophages in mutants. Therefore, expression of MAdCAM-1 seems to be required for building functional structures in spleen and MALT, a prerequisite for unimpaired migration and segregation of B and T cells to and within these organs.

Development and maturation of secondary lymphoid tissues

The secondary lymphoid tissues are located at strategic sites where foreign antigens can be efficiently brought together with immune system regulatory and effector cells. The organized structure of the secondary lymphoid tissues is thought to enhance the sensitivity of antigen recognition and to support proper regulation of the activation and maturation of the antigen-responsive lymphoid cells. Although a substantial amount is known about the cellular elements that compose the lymphoid and nonlymphoid components of the secondary lymphoid tissues, information concerning the signals that control the development of the tissues and that maintain the organized tissue microenvironment remain undefined. Studies over the past few years have identified lymphotoxin as a critical signaling molecule not only for the organogenesis of secondary lymphoid tissues but for the maintenance of aspects of their microarchitecture as well. Additional signaling molecules that contribute to the formation of normal lymphoid tissue structure are being identified at an accelerating pace. Analyses of mouse strains with congenital defects in different aspects of secondary lymphoid tissue development are beginning to clarify the role of these tissues in immune responses and host defense. This review focuses on studies defining recently identified crucial signals for the biogenesis of secondary lymphoid organs and for the maintenance of their proper microarchitecture. It also discusses new insights into how the structure of these tissues supports effective immune responses.