CD27+ B cells in human lymphatic organs: re-evaluating the splenic marginal zone

Human IgM-expressing memory B cells

A hallmark of T cell dependent (TD) humoral immune responses is the generation of long-lived memory B cells. The generation of these cells occurs primarily in the germinal center (GC) reaction, where antigen-activated B cells undergo affinity maturation as a major consequence of the combined processes of proliferation, somatic hypermutation of their immunoglobulin V (IgV) region genes, and selection for improved affinity of their B-cell antigen receptors. As many B cells also undergo class-switching to IgG or IgA in these TD responses, there was traditionally a focus on class-switched memory B cells in both murine and human studies on memory B cells. However, it has become clear that there is also a large subset of IgM-expressing memory B cells, which have important phenotypic and functional similarities but also differences to class-switched memory B cells. There is an ongoing discussion about the origin of distinct subsets of human IgM+ B cells with somatically mutated IgV genes. We argue here that the vast majority of human IgM-expressing B cells with somatically mutated IgV genes in adults is indeed derived from GC reactions, even though a generation of some mostly lowly mutated IgM+ B cells from other differentiation pathways, mainly in early life, may exist.

B cells in human lymphoid structures

Most B cells in the human body are present in tissues where they support immune responses to pathogens, vaccines, autoantigens, and tumours. Despite their clear importance, they are very difficult to study and there are many areas of uncertainty that are difficult to resolve because of limited tissue access. In this review, we consider the zonal structure of lymphoid tissues, the B cell subsets they contain, and how these are regulated. We also discuss the impact that methods of deep interrogation have made on our current knowledge base, especially with respect to studies of cells from dissociated tissues. We discuss in some detail the controversial B cells with marginal zone distribution that some consider being archived memory B cells. We anticipate that more we understand of B cells in tissues and the niches they create, the more opportunities will be identified to harness their potential for therapeutic benefit.

Systematic memory B cell archiving and random display shape the human splenic marginal zone throughout life

Human memory B cells (MBCs) are generated and diversified in secondary lymphoid tissues throughout the organism. A paired immunoglobulin (Ig)-gene repertoire analysis of peripheral blood (PB) and splenic MBCs from infant, adult, and elderly humans revealed that throughout life, circulating MBCs are comprehensively archived in the spleen. Archive MBC clones are systematically preserved and uncoupled from class-switching. Clonality in the spleen increases steadily, but boosts at midlife, thereby outcompeting small clones. The splenic marginal zone (sMZ) represents a primed MBC compartment, generated from a stochastic exchange within the archive memory pool. This is supported by functional assays, showing that PB and splenic CD21⁺ MBCs acquire transient CD21^high expression upon NOTCH2-stimulation. Our study provides insight that the human MBC system in PB and spleen is composed of three interwoven compartments: the dynamic relationship of circulating, archive, and its subset of primed (sMZ) memory changes with age, thereby contributing to immune aging.

Human gut-associated lymphoid tissues (GALT); diversity, structure, and function

Gut-associated lymphoid tissues (GALT) are the key antigen sampling and adaptive immune inductive sites within the intestinal wall. Human GALT includes the multi-follicular Peyer's patches of the ileum, the vermiform appendix, and the numerous isolated lymphoid follicles (ILF) which are distributed along the length of the intestine. Our current understanding of GALT diversity and function derives primarily from studies in mice, and the relevance of many of these findings to human GALT remains unclear. Here we review our current understanding of human GALT diversity, structure, and composition as well as their potential for regulating intestinal immune responses during homeostasis and inflammatory bowel disease (IBD). Finally, we outline some key remaining questions regarding human GALT, the answers to which will advance our understanding of intestinal immune responses and provide potential opportunities to improve the treatment of intestinal diseases.

Contributions of Major Cell Populations to Sjögren's Syndrome

Sjögren’s syndrome (SS) is a female dominated autoimmune disease characterized by lymphocytic infiltration into salivary and lacrimal glands and subsequent exocrine glandular dysfunction. SS also may exhibit a broad array of extraglandular manifestations including an elevated incidence of non-Hodgkin’s B cell lymphoma. The etiology of SS remains poorly understood, yet progress has been made in identifying progressive stages of disease using preclinical mouse models. The roles played by immune cell subtypes within these stages of disease are becoming increasingly well understood, though significant gaps in knowledge still remain. There is evidence for distinct involvement from both innate and adaptive immune cells, where cells of the innate immune system establish a proinflammatory environment characterized by a type I interferon (IFN) signature that facilitates propagation of the disease by further activating T and B cell subsets to generate autoantibodies and participate in glandular destruction. This review will discuss the evidence for participation in disease pathogenesis by various classes of immune cells and glandular epithelial cells based upon data from both preclinical mouse models and human patients. Further examination of the contributions of glandular and immune cell subtypes to SS will be necessary to identify additional therapeutic targets that may lead to better management of the disease.

The geography of memory B cell reactivation in vaccine-induced immunity and in autoimmune disease relapses

Memory B cells (Bmem) provide an active second layer of defense against re-infection by pathogens that have bypassed the passive first layer provided by neutralizing antibodies. Here, we review recent progress in our understanding of Bmem heterogeneity in terms of their origin (germinal center-dependent vs center-independent), phenotype (canonical vs atypical vs age-associated B cells), trafficking (recirculating vs tissue-resident), and fate (plasma cell vs germinal center differentiation). The development of transgenic models and intravital imaging technologies has made it possible to track the cellular dynamics of Bmem reactivation by antigen, their interactions with follicular memory T cells, and differentiation into plasma cells in subcapsular proliferative foci in the lymph nodes of immune animals. Such in situ studies have reinforced the importance of geography in shaping the outcome of the secondary antibody response. We also review the evidence for Bmem reactivation and differentiation into short-lived plasma cells in the pathogenesis of disease flares in relapsing-remitting autoimmune diseases. Elucidating the mechanisms that control the Bmem fate decision to differentiate into plasma cells or germinal center B cells will aid future efforts to more precisely engineer fit-for-purpose vaccines as well as to treat antibody-mediated autoimmune diseases.

Human CD27+ memory B cells colonize a superficial follicular zone in the palatine tonsils with similarities to the spleen. A multicolor immunofluorescence study of lymphoid tissue

BACKGROUND

Memory B cell (mBC) induction and maintenance is one of the keys to long-term protective humoral immunity. MBCs are fundamental to successful medical interventions such as vaccinations and therapy in autoimmunity. However, their lifestyle and anatomic residence remain enigmatic in humans. Extrapolation from animal studies serves as a conceptual basis but might be misleading due to major anatomical distinctions between species.

METHODS AND FINDINGS

Multicolor immunofluorescence stainings on fixed and unfixed frozen tissue sections were established using primary antibodies coupled to haptens and secondary signal amplification. The simultaneous detection of five different fluorescence signals enabled the localization and characterization of human CD27+CD20+Ki67- mBCs for the first time within one section using laser scanning microscopy. As a result, human tonsillar mBCs were initially identified within their complex microenvironment and their relative location to naïve B cells, plasma cells and T cells could be directly studied and compared to the human splenic mBC niche. In all investigated tonsils (n = 15), mBCs appeared to be not only located in a so far subepithelial defined area but were also follicle associated with a previous undescribed gradual decline towards the follicular mantle comparable to human spleen. However, mBC areas around secondary follicles with large germinal centers (GCs) in tonsils showed interruptions and a general widening towards the epithelium while in spleen the mBC-containing marginal zones (MZ) around smaller GCs were relatively broad and symmetrical. Considerably fewer IgM+IgD+/- pre-switch compared to IgA+ or IgG+ post-switch mBCs were detected in tonsils in contrast to spleen.

CONCLUSIONS

This study extends existing insights into the anatomic residence of human mBCs showing structural similarities of the superficial follicular area in human spleen and tonsil. Our data support the debate of renaming the human splenic MZ to 'superficial zone' in order to be aware of the differences in rodents and, moreover, to consider this term equally for the human palatine tonsil.

Plasma cells, plasmablasts, and AID+/CD30+ B lymphoblasts inside and outside germinal centres: details of the basal light zone and the outer zone in human palatine tonsils

Plasma cells (PCs) in human palatine tonsils are predominantly located in the germinal centres (GCs), in the subepithelial space and near the deep connective tissue septa surrounding each crypt. We analysed the location, phenotype, and proliferation of GC PCs by immunohistology comparing them to PCs in the other two locations. Most PCs in GCs were strongly positive for CD38, CD138, CD27, IRF4, and intracellular (ic) IgG. They often accumulated in the basal light zone, but could also be found scattered in the entire light zone. In addition, rows of PCs occurred at the surface of the GC bordering the mantle zone, i.e., in the outer zone, and at the surface of the dark zone. The latter cells were often continuous with PCs in the extrafollicular area. The vast majority of GC PCs were negative for Ki-67. Only a few Ki-67⁺ plasmablasts, predominantly icIgG⁺ or icIgM⁺, were found inside GCs. In certain GCs PCs accumulated around capillaries and the adjacent perikarya of follicular dendritic cells (FDCs). Newly formed PCs might migrate from the basal to the superficial part of the light zone and then back to the dark zone surface to leave the GC. This guarantees an even distribution of secreted Ig for exchange with immune complexes on FDCs. The surface of the dark zone may also be an exit site for Ki-67⁺CD30⁺ B lymphoblasts, which seed perifollicular and extrafollicular sites. We speculate that these cells tend to downmodulate CD20 and activation-induced deaminase and further up-regulate CD30 when developing into pre-plasmablasts.

Dissecting Integrin Expression and Function on Memory B Cells in Mice and Humans in Autoimmunity

Immunological memory ensures life-long protection against previously encountered pathogens, and in mice and humans the spleen is an important reservoir for long-lived memory B cells (MBCs). It is well-established that integrins play several crucial roles in lymphocyte survival and trafficking, but their involvement in the retention of MBCs in secondary lymphoid organs, and differences between B cell subsets in their adhesion capacity to ICAM-1 and/or VCAM-1 have not yet been confirmed. Here, we use an autoimmune mouse model, where MBCs are abundant, to show that the highest levels of LFA-1 and VLA-4 amongst B cells are found on MBCs. In vivo blockade of VLA-4 alone or in combination with LFA-1, but not LFA-1 alone, causes a release of MBCs from the spleen into the blood stream. In humans, we find that in peripheral blood, spleens, and tonsils from healthy donors the highest expression levels of the integrins LFA-1 and VLA-4 are also found on MBCs. Consistent with this, we found MBCs to have a higher capacity to adhere to ICAM-1 and VCAM-1 than naïve B cells. In patients with the autoimmune disease rheumatoid arthritis, it is the MBCs that have the highest levels of LFA-1 and VLA-4; moreover, compared with healthy donors, naïve B and MBCs of patients receiving anti-TNF medication have enhanced levels of the active form of LFA-1. Commensurate levels of the active αL subunit can be induced on B cells from healthy donors by exposure to the integrin ligands. Thus, our findings establish the selective use of the integrins LFA-1 and VLA-4 in the localization and adhesion of MBCs in both mice and humans.

Human Extrafollicular CD4+ Th Cells Help Memory B Cells Produce Igs

Follicular helper T (Tfh) cells are necessary for germinal center B cell maturation during primary immune responses; however, the T cells that promote humoral recall responses via memory B cells are less well defined. In this article, we characterize a human tonsillar CD4⁺ T cell subset with this function. These cells are similar to Tfh cells in terms of expression of the chemokine receptor CXCR5 and the inhibitory receptor PD-1, IL-21 secretion, and expression of the transcription factor BCL6; however, unlike Tfh cells that are located within the B cell follicle and germinal center, they reside at the border of the T cell zone and the B cell follicle in proximity to memory B cells, a position dictated by their unique chemokine receptor expression. They promote memory B cells to produce Abs via CD40L, IL-10, and IL-21. Our results reveal a unique extrafollicular CD4⁺ T cell subset in human tonsils, which specialize in promoting T cell-dependent humoral recall responses.

Capillary networks and follicular marginal zones in human spleens. Three-dimensional models based on immunostained serial sections

We have reconstructed small parts of capillary networks in the human splenic white pulp using serial sections immunostained for CD34 alone or for CD34 and CD271. The three-dimensional (3D) models show three types of interconnected networks: a network with very few long capillaries inside the white pulp originating from central arteries, a denser network surrounding follicles plus periarterial T-cell regions and a network in the red pulp. Capillaries of the perifollicular network and the red pulp network have open ends. Perifollicular capillaries form an arrangement similar to a basketball net located in the outer marginal zone. The marginal zone is defined by MAdCAM-1+ marginal reticular stromal cells. Perifollicular capillaries are connected to red pulp capillaries surrounded by CD271+ stromal capillary sheath cells. The scarcity of capillaries inside the splenic white pulp is astonishing, as non-polarised germinal centres with proliferating B-cells occur in adult human spleens. We suggest that specialized stromal marginal reticular cells form a barrier inside the splenic marginal zone, which together with the scarcity of capillaries guarantees the maintenance of gradients necessary for positioning of migratory B- and T-lymphocytes in the human splenic white pulp.

Three-dimensional reconstruction of serial sections for analysis of the microvasculature of the white pulp and the marginal zone in the human spleen

Although a number of papers have given useful information on splenic microcirculation by light and/or scanning electron microscopy, controversies remain as to the vascular arrangement, especially in the human spleen. The present study re-examined the microvasculature of the human spleen using a three-dimensional reconstruction of immunohistochemically stained tissue sections, and showed that the central artery does not directly issue follicular arteries in the human spleen; follicular arteries are derived from penicillar arteries outside the follicle and end in the white pulp. We found that the splenic follicle is surrounded by an elaborate system of anastomosed capillaries in both the marginal zone and the superficial layer of the white pulp. Most of these capillaries are also branches of the penicillar arterioles that are issued from the central artery in the same, or a different, white pulp system. Because the endothelia of these capillaries are widely open in the marginal zone, this vascular network may play a major role in supplying blood to the marginal zone. The accumulation of sialoadhesin-positive macrophages was also observed around the vascular network, suggesting an important role for this structure as the front line of immune response.

Human spleen microanatomy: why mice do not suffice

The microanatomical structure of the spleen has been primarily described in mice and rats. This leads to terminological problems with respect to humans and their species-specific splenic microstructure. In mice, rats and humans the spleen consists of the white pulp embedded in the red pulp. In the white pulp, T and B lymphocytes form accumulations, the periarteriolar lymphatic sheaths and the follicles, located around intermediate-sized arterial vessels, the central arteries. The red pulp is a reticular connective tissue containing all types of blood cells. The spleen of mice and rats exhibits an additional well-delineated B-cell compartment, the marginal zone, between white and red pulp. This area is, however, absent in human spleen. Human splenic secondary follicles comprise three zones: a germinal centre, a mantle zone and a superficial zone. In humans, arterioles and sheathed capillaries in the red pulp are surrounded by lymphocytes, especially by B cells. Human sheathed capillaries are related to the splenic ellipsoids of most other vertebrates. Such vessels are lacking in rats or mice, which form an evolutionary exception. Capillary sheaths are composed of endothelial cells, pericytes, special stromal sheath cells, macrophages and B lymphocytes. Human spleens most probably host a totally open circulation system, as connections from capillaries to sinuses were not found in the red pulp. Three stromal cell types of different phenotype and location occur in the human white pulp. Splenic white and red pulp structure is reviewed in rats, mice and humans to encourage further investigations on lymphocyte recirculation through the spleen.

A new method for estimating species age supports the coexistence of malaria parasites and their Mammalian hosts

Species in the genus Plasmodium cause malaria in humans and infect a variety of mammals and other vertebrates. Currently, estimated ages for several mammalian Plasmodium parasites differ by as much as one order of magnitude, an inaccuracy that frustrates reliable estimation of evolutionary rates of disease-related traits. We developed a novel statistical approach to dating the relative age of evolutionary lineages, based on Total Least Squares regression. We validated this lineage dating approach by applying it to the genus Drosophila. Using data from the Drosophila 12 Genomes project, our approach accurately reconstructs the age of well-established Drosophila clades, including the speciation event that led to the subgenera Drosophila and Sophophora, and age of the melanogaster species subgroup. We applied this approach to hundreds of loci from seven mammalian Plasmodium species. We demonstrate the existence of a molecular clock specific to individual Plasmodium proteins, and estimate the relative age of mammalian-infecting Plasmodium. These analyses indicate that: 1) the split between the human parasite Plasmodium vivax and P. knowlesi, from Old World monkeys, occurred 6.1 times earlier than that between P. falciparum and P. reichenowi, parasites of humans and chimpanzees, respectively; and 2) mammalian Plasmodium parasites originated 22 times earlier than the split between P. falciparum and P. reichenowi. Calibrating the absolute divergence times for Plasmodium with eukaryotic substitution rates, we show that the split between P. falciparum and P. reichenowi occurred 3.0-5.5 Ma, and that mammalian Plasmodium parasites originated over 64 Ma. Our results indicate that mammalian-infecting Plasmodium evolved contemporaneously with their hosts, with little evidence for parasite host-switching on an evolutionary scale, and provide a solid timeframe within which to place the evolution of new Plasmodium species.

Heterogeneity of stromal cells in the human splenic white pulp. Fibroblastic reticulum cells, follicular dendritic cells and a third superficial stromal cell type

At least three phenotypically and morphologically distinguishable types of branched stromal cells are revealed in the human splenic white pulp by subtractive immunohistological double-staining. CD271 is expressed in fibroblastic reticulum cells of T-cell zones and in follicular dendritic cells of follicles. In addition, there is a third CD2711- and CD271+/) stromal cell population surrounding T-cell zones and follicles. At the surface of follicles the third population consists of individually variable partially overlapping shells of stromal cells exhibiting CD90 (Thy-1), MAdCAM-1, CD105 (endoglin), CD141 (thrombomodulin) and smooth muscle α-actin (SMA) with expression of CD90 characterizing the broadest shell and SMA the smallest. In addition, CXCL12, CXCL13 and CCL21 are also present in third-population stromal cells and/or along fibres. Not only CD27+ and switched B lymphocytes, but also scattered IgD++ B lymphocytes and variable numbers of CD4+ T lymphocytes often occur close to the third stromal cell population or one of its subpopulations at the surface of the follicles. In contrast to human lymph nodes, neither podoplanin nor RANKL (CD254) were detected in adult human splenic white pulp stromal cells. The superficial stromal cells of the human splenic white pulp belong to a widespread cell type, which is also found at the surface of red pulp arterioles surrounded by a mixed T-cell/B-cell population. Superficial white pulp stromal cells differ from fibroblastic reticulum cells and follicular dendritic cells not only in humans, but apparently also in mice and perhaps in rats. However, the phenotype of white pulp stromal cells is species-specific and more heterogeneous than described so far.

Differences in mouse and human nonmemory B cell pools

Identifying cross-species similarities and differences in immune development and function is critical for maximizing the translational potential of animal models. Coexpression of CD21 and CD24 distinguishes transitional and mature B cell subsets in mice. In this study, we validate these markers for identifying analogous subsets in humans and use them to compare the nonmemory B cell pools in mice and humans, across tissues, and during fetal/neonatal and adult life. Among human CD19(+)IgM(+) B cells, the CD21/CD24 schema identifies distinct populations that correspond to transitional 1 (T1), transitional 2 (T2), follicular mature, and marginal zone subsets identified in mice. Markers specific to human B cell development validate the identity of marginal zone cells and the maturation status of human CD21/CD24 nonmemory B cell subsets. A comparison of the nonmemory B cell pools in bone marrow, blood, and spleen in mice and humans shows that transitional B cells comprise a much smaller fraction in adult humans than mice. T1 cells are a major contributor to the nonmemory B cell pool in mouse bone marrow, in which their frequency is more than twice that in humans. Conversely, in spleen, the T1:T2 ratio shows that T2 cells are proportionally ∼ 8-fold higher in humans than in mice. Despite the relatively small contribution of transitional B cells to the human nonmemory pool, the number of naive follicular mature cells produced per transitional B cell is 3- to 6-fold higher across tissues than in mice. These data suggest differing dynamics or mechanisms produce the nonmemory B cell compartments in mice and humans.

B lymphocyte compartments in the human splenic red pulp: capillary sheaths and periarteriolar regions

The microvasculature of human spleens is still incompletely understood. Two enigmatic types of red pulp microvessels, penicillar arterioles and sheathed capillaries, have already been described in the nineteenth century without gaining much attention afterwards. We performed a detailed analysis of sheathed capillaries to clarify the cellular composition of their sheaths by immunohistological double-staining experiments. Capillary sheaths comprise three different cell types, namely specialized cuboidal CD271(++) inner sheath cells surrounded by CD271(-) macrophages and accumulations of B lymphocytes. The CD271(++) inner sheath cells express the chemokine CXCL13 in a unique single dot pattern. Sheath-associated B lymphocytes consist of IgM(+), IgD(++), and of "switched" cells. T lymphocytes do not accumulate in pericapillary sheaths. The predominant sheath-associated macrophage population is CD163(-)CD68(+) and thus differs from the majority of red pulp macrophages. The sheath-associated macrophages strongly express CD169 only in perifollicular sheaths, but not in sheaths located deeper in the red pulp. IgM(+), IgD(++), and "switched" B cells are also closely associated with red pulp arterioles characterized by the expression of smooth muscle actin in muscle cells and in branched periarteriolar stromal cells. Capillary sheaths are observed in a post-arteriolar position and appear to be of limited length. We suggest to change the term "Vagina periarteriolaris makrophagocytica" of the international histological and embryological terminologies to "Vagina pericapillaris."

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

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.

The open microcirculation in human spleens: a three-dimensional approach

It has long been debated whether the red pulp of human spleens harbors an open or a closed microcirculation or both. To solve this issue, the authors differentially stained the endothelium in red pulp arterial microvessels and in venous sinuses using brightfield and fluorescence immunohistology with reagents against CD34 and CD141. Three-dimensional models of red pulp arterial microvessels and sinuses were derived from serial double-stained paraffin sections with the help of license-free open-access software. In each model, arterial microvascular ends were traced and verified by reference to the original serial sections. In total, 142 ends were analyzed in the specimens of three individuals. None of these ends was connected to a sinus, suggesting that the human splenic red pulp harbors an entirely open circulatory system. Thus, the spleen is the only human organ where blood passes through spaces not lined by endothelia or other barrier-forming cells.

beta1 integrin expression increases susceptibility of memory B cells to Epstein-Barr virus infection

Epstein-Barr virus (EBV) uses nasal mucosa-associated lymphoid tissue (NALT) as a portal of entry to establish life-long persistence in memory B cells. We previously showed that naïve and memory B cells from NALT are equally susceptible to EBV infection. Here we show that memory B cells from NALT are significantly more susceptible to EBV infection than those from remote lymphatic organs. We identify beta(1) integrin, which is expressed the most by naïve B cells of distinct lymphoid origin and by memory B cells from NALT, as a mediator of increased susceptibility to infection by EBV. Furthermore, we show that BMRF-2-beta(1) integrin interaction and the downstream signal transduction pathway are critical for postbinding events. An increase of beta(1) integrin expression in peripheral blood memory B cells provoked by CD40 stimulation plus B-cell receptor cross-linking increased the susceptibility of non-NALT memory B cells to EBV infection. Thus, EBV seems to utilize the increased activation status of memory B cells residing in the NALT to establish and ensure persistence.

CD34+ cord blood cell-transplanted Rag2-/- gamma(c)-/- mice as a model for Epstein-Barr virus infection

Recent studies suggest that Epstein-Barr virus (EBV) can infect naïve B cells, driving them to differentiate into resting memory B cells via the germinal center reaction. This hypothesis has been inferred from parallels with the biology of normal B cells but has never been proven experimentally. Rag2(-/-) gamma(c)(-/-) mice that were transplanted with human CD34(+) cord blood cells as newborns were recently shown to develop human B, T, and dendritic cells, constituting lymphoid organs in situ. Here we used this model to better define the strategy of EBV infection of human B cells in vivo and to compare this model system with different conditions of EBV infection in humans. Our results support the model of EBV persistence in vivo in cases that were characterized by follicular hyperplasia and a relatively normal CD4(+) and CD8(+) T-cell distribution. Intriguingly, in cases that were characterized by nodular and diffuse proliferation with a preponderance of CD8(+) T cells, similar to infectious mononucleosis, EBV still infects naïve B cells but also induces clonal expansion and ongoing somatic mutations without germinal center reactions. Our results reveal different strategies of EBV infection in B cells that possibly result from variations in the host immune response. Future experiments might allow understanding of the mechanisms responsible for persistent EBV infection and provide targets for more highly tailored therapeutic interventions.

A splenic marginal zone-like peripheral blood CD27+B220- B cell population is preferentially depleted in HIV type 1-infected individuals

Peripheral blood CD27(+) B cells are reduced in HIV-1-infected individuals. In healthy individuals, the human peripheral blood CD27(+) B cell pool consists of two subsets defined by the expression, or lack thereof, of the CD45 isoform B220. We investigated the presence of circulating B220(+) and B220(-) memory B cells in HIV(+) individuals and found that the reduction in CD27(+) memory B cells occurs primarily among CD27(+)B220(-) B cells. Studies conducted using healthy controls indicate that CD27(+)B220(-) B cells have a splenic marginal zone like the immunophenotype IgM(hi)IgD(lo)CD21(+)CD23(-), express TLR9, and proliferate and secrete IgG and IgM in response to B cell-specific ODN. CD27(+)B220(+) B cells have the immunophenotype IgM(lo)IgD(hi)CD21(+)CD23(+), express activation-induced cytidine deaminase, and proliferate in response to SAC but do not secrete immunoglobulin. The AICD expression, along with CD86 expression, by CD27(+)B220(+) suggests these cells are of germinal center origin. The preferential depletion of CD27(+)B220(-) B cells mirrors alterations in spleen morphology and resident B cell populations due to HIV infection reported by other investigators and may play an important role in the defective B cell immunity against T-independent pathogens such as pneumococcus observed in HIV-1-infected individuals.

Phenotypic and functional heterogeneity of human memory B cells

Memory B cells are more heterogeneous than previously thought. Given that B cells play powerful antibody-independent effector functions, it seems reasonable to assume division of labor between distinct memory B cells subpopulations in both protective and pathogenic immune responses. Here we review the information emerging regarding the heterogeneity of human memory B cells. A better understanding of this topic should greatly improve our ability to target specific B cell subsets either in vaccine responses or in autoimmune diseases and organ rejection among other pathological conditions where B cells play central pathogenic roles.

DEC-205/CD205+ dendritic cells are abundant in the white pulp of the human spleen, including the border region between the red and white pulp

The distribution of dendritic cells (DCs) and macrophages in the human spleen has received less attention than that of lymphocytes. Here we have addressed this problem with the human DEC-205/CD205 marker ('DEC'), which is an endocytic receptor on DCs that mediates efficient presentation of antigens. DEC was abundant on dendritic profiles in the white pulp but absent from the red pulp, the latter defined with antibodies to two antigens, mannose receptor/CD206 on sinusoidal lining cells, and macrosialin/CD68 on macrophages. Double staining with anti-DEC and anti-CD3 showed the expected concentration of DEC+ cells in the relatively small T-cell areas of the human spleen. DEC+ cells were also found in other regions of the white pulp. In all regions, the DEC+ cells were positive for major histocompatibility complex (MHC) class II and the CD11c integrin but largely immature, with low expression of B7-2/CD86 costimulator and DC-lysosome-associated membrane protein (LAMP)/CD208. When we concentrated on the perifollicular region between the red pulp and the marginal zone, we found macrophages that stained with antibodies to sialoadhesin/CD169 and DC-specific ICAM-3 grabbing non-integrin (SIGN)/CD209, and just inside these cells were DEC+ profiles. The DEC+ DCs were intertwined with cells that stained for the vascular addressin mucosal addressin cell adhesion molecule (MAdCAM). Therefore, anti-DEC-205/CD205 antibodies are useful for identifying DCs in human splenic white pulp and its border region with the red pulp.

Phenotypic differences between red pulp capillary and sinusoidal endothelia help localizing the open splenic circulation in humans

The distribution of capillaries, sinuses and larger vessels was investigated by immunohistology in paraffin sections of 12 adult human spleens using a panel of antibodies. Double staining for CD34 and CD141 (thrombomodulin) revealed that capillary endothelia in the cords of the splenic red pulp and at the surface of follicles were CD34(+)CD141(-), while red pulp sinus endothelia had the phenotype CD34(-)CD141(+). Only in the direct vicinity of splenic follicles did sinus endothelial cells exhibit both antigens. Thus, splenic sinuses do not replace conventional capillaries, but exist in addition to such vessels. The endothelium in arterioles, venules and larger arteries and veins was uniformly CD34(+)CD141(+). Anti-CD34 and anti-CD141 both additionally reacted with different types of splenic stromal cells. Differential staining of capillaries and sinuses may permit a three-dimensional reconstruction of serial sections to unequivocally delineate the "open" and "closed" splenic circulation in humans.

Fetal and early post-natal development of the human spleen: from primordial arterial B cell lobules to a non-segmented organ

Immunohistological analysis of 31 human spleens from the 11th week of gestation to the early postnatal period suggested that fetal organ development may be preliminarily divided into four stages. At stage 0 the organ anlage contained erythrocyte precursors, few macrophages and almost no lymphocytes. Fetal spleens of stage I exhibited arterial vascular lobules and lymphocytes just began colonizing the organ. At stage II, B and T lymphocytes formed periarteriolar clusters. B cell clusters predominated, because B cells aggregated around the more peripheral branches of splenic arterioles, while T cells occupied the more centrally located parts of the vessels. The vascular lobules of stage I and II consisted of central arterioles surrounded by B cells, capillaries and peripheral venules. The lobular architecture slowly dissolved at late stage II when sinuses grew out from the peripheral venules into the centre of the lobule. Interestingly, the B cell accumulations around peripheral arterioles did not represent the precursors of follicles, but apparently persisted as periarteriolar B cell clusters in the adult splenic red pulp, while follicles containing FDCs developed at late stage II from B cells in direct contact to T cell clusters around larger arterial vessels. At stage III before birth the lobular architecture was no longer recognized. The chemokine CXCL13 was already present in vascular smooth muscle and adjacent stromal cells at stage I before B cells immigrated. CCL21, on the contrary, was only demonstrated in fibroblast-like cells supporting T cell clusters from stage II onwards.

The splenic marginal zone in humans and rodents: an enigmatic compartment and its inhabitants

The role of the spleen in B memory cell development and maintenance is attracting increased attention. Studies in mice and rats have indicated that memory functions are associated with large B cells residing in the marginal zone (MZ) of the spleen. Although the cellular composition of the MZ is relatively well known in these species, controversies exist about the function of MZ B cells, their dependence on the presence of the spleen and the stage at which their development branches from that of recirculating follicular B cells. Additional confusion has arisen with respect to MZ B cells in humans, because the microscopic anatomy of the human splenic MZ differs decisively from that of rodents. Several recent publications indicate that the functional and migratory properties of human MZ B cells may be species-specific. The hypothesis derived from these publications and from our immunohistological observations implies that at least a major number of human splenic CD27(+) MZ B cells are migratory. Phenotypic data suggest a recirculation pathway between the spleen and mucosal tissues in humans.