The bone marrow and the thymus are the sites of primary, antigen- independent lymphocyte development and are distinguished from secondary lymphoid organs that include the spleen, lymph nodes, and epithelial and mucosa-associated lymphoid tissues such as Peyer patches in the small intestine. Secondary lymphoid tissues are the sites of B-cell maturation and the initiation of humoral immune responses.
Anatomic Organization of Secondary Lymphoid Tissues
The spleen is divided into white pulp and red pulp (Fig. 1). Red pulp functions as a site of extramedullary hematopoiesis early in fetal life and is a storage site for iron, erythrocytes, and platelets throughout life. White pulp is composed mainly of lymphoid cells and includes three regions referred to as the periarteriolar lymphoid sheath (PALS), the marginal zone (MZ), and primary and secondary follicles (see Fig. 1). In mice, the PALS contains abundant T cells and is found surrounding the central artery, but few T cells are present in this region in humans. The MZ is located at the outer portion of the white pulp.
Fig1. ORGANIZATION OF B CELLS IN SECONDARY LYMPHOID ORGANS. (A) The spleen consists of the red pulp and white pulp. The red pulp is the site of myeloid cells which ingest and remove opsonized antigens and damaged red blood cells from the circulation. (B) The white pulp consists of lymphoid cells, with a periarteriolar T-cell sheath (PALS), a marginal zone, and mixed B- and T-cell follicles. (C) Lymph nodes are surrounded by a capsule. Afferent lymphatics draining tissues enter the node on the convex side into the capsule. Fluid and cells drain through the node and collect in the medullary sinus, where the fluid leaves the node through efferent lymphatics to rejoin the lymphatic circulation. The outer rim of the node is called the cortex and contains primary follicles composed of naïve, nonproliferating B cells that have not encountered antigens and secondary follicles with proliferating B cells in the germinal center. (D) A section of lymph node showing a high magnification of a follicle. The schematic shows the regions of the follicle.
The spleen lacks afferent lymphatics and samples foreign antigens in blood rather than lymphatic fluid. Unlike other organs that have a closed vascular circulation in which blood flows from arteries to veins through capillary beds, branches of the splenic artery penetrate the white pulp, forming an open sinusoidal network termed the marginal sinuses. This architecture ensures the interposing of blood, and blood borne antigens, with the lymphoid areas of the white pulp. From the marginal sinuses, blood filters through the white pulp regions of the spleen and encounters resident B and T cells. Blood is drained via branches of the splenic vein, but an efferent lymphatic circulation also collects and drains the spleen. Beyond the white pulp, the splenic artery sends additional branches into the red pulp for further filtration and subsequent blood antigen surveillance that is accomplished by macrophages (see Fig. 1).
In contrast to the spleen which samples blood-borne antigens, fluid and cells gain entry to the convex surface of lymph nodes via afferent lymphatic vessels that drain into the subcapsular sinus. Lymphatic fluid in the subcapsular sinus then courses into the trabecular sinus network that runs perpendicular to the capsule through an area called the cortex. The cortex is composed of follicles and interfollicular zones. Follicles consist mainly of B cells, some T cells, and antigen presenting cells such as macrophages and follicular dendritic cells while interfollicular zones consist mainly of T cells and additional antigen presenting cells. These zones are separate but contiguous compartments where B cells and T cells initially encounter antigen. The net effect of antigenic exposure is the proliferation of antigen-specific lymphocytes (see Fig. 1).
In addition to lymph nodes and spleen, mucosa-associated lymphatic tissue (MALT) is a critical part of the secondary lymphoid system. As the name implies, the MALT is in physical proximity with the mucosa (i.e., the epithelium and associated connective tissue that line the surfaces of the body) and is found at sites where antigens most commonly breach these epithelial barriers in the gastrointestinal, respiratory, and genitourinary tracts. In some tissues, MALT forms relatively large structures that can be clearly distinguished histologically, such as the Peyer patches in the ileum and in the lymphoid tissue under the epithelium of the appendix. In these sites, perhaps because of the constant stimulation by microbial pathogens in the intestine, the MALT resembles lymphatic tissue in the spleen and lymph nodes, with well-demarcated primary and secondary follicles that contain primarily B cells and intervening T-cell-rich zones.
Transitional B Cells
Even though they express a BCR, newly produced B cells are functionally immature, and they complete their maturation in secondary lymphoid tissues soon after their arrival from the bone marrow. These events are well defined in the mouse spleen where newly generated B cells mature through transitional stages of development before generating MZ or follicular (FO) B cells.
The most immature transitional cells, referred to as transitional 1 (T1) B cells, localize at the outer edge of the PALS. T1 B cells give rise to a more mature population of B cells, referred to as transitional 2 (T2) cells. The T1 and T2 population can be phenotypically distinguished, respond differentially to developmental stimuli, and undergo a considerable degree of selection during the T1 to T2 transition. For example, T1 cells with BCR specificities for blood-borne self-antigens are deleted by negative selection. Positive selection via BCR signaling must occur, and if it does not, the T2 cells will die by neglect. The survival of T2 cells, but not T1 cells, is also dependent on the B-cell growth factor BAFF (BLyS, TALL-1, THANK, zTNF4), which is produced by the splenic microenvironment. A fraction of T2 cells are no longer in Go phase of the cell cycle, suggesting they are in a more activated state than is the case for T1 cells. An additional population, transitional 3 (T3) B cells, has also been described and proposed to be either an anergic population or a stage of development between T2 and naïve B cells.
Whether transitional cells mature into MZ or FO B cells is influenced by the extrinsic signals they receive. Weak signaling through the BCR along with engagement of the Notch2 receptor promotes entry into the MZ B-cell compartment. There is a marked depletion of MZ B cells when the Notch2 pathway is blocked. Self-reactive B cells are enriched within the MZ population, suggesting that weak self-antigens may play an important role in their generation. This feature may permit them to respond rapidly to cross-reactive epitopes on pathogens. BCR signals, along with activation of the alternative NF-κB pathway, are required for T2 cells to mature into an FO B cell. It is estimated based on murine studies that only 1% to 3% of splenic transitional B cells develop into mature, naïve B cells.
In addition to MZ and FO B cells, an additional population of B regulatory (Breg) cells exists. Bregs have been identified by their pro duction of IL-10 but they also function via IL-10 independent mechanisms. The developmental origin(s) of these cells and their role in the regulation of immune responses is an area of active investigation.
Immune Responses in Secondary Lymphoid Tissues
A feature of the spleen that distinguishes it from other secondary lymphoid tissues is the presence of an MZ, and this is the site where MZ B cells localize along with macrophages and dendritic cells. MZ B cells play a role in T-independent responses elicited by polymeric antigens, such as polysaccharides, that are composed of repetitive antigenic epitopes. On antigen binding, MZ B cells undergo rapid proliferation and maturation into plasma cells that secrete low-affinity IgM and IgG3 antibodies that provide a first line of defense. The rapid response of MZ B cells to antigen has led to the idea that this effector population, similar to B-1 B cells, constitutes a key element of the innate immune response to bacterial and other selected pathogens. Human MZ B cells are heterogeneous and include a large pro portion of CD27+ IgM+ unswitched memory B cells with somatically mutated Ig heavy chains. The origin of this cell population is unclear but is presumed to be antigen driven yet may not require T-cell help. Although MZ B cells in rodents appear to be a static, nonrecirculating population, cells with a CD27+ IgM+ MZ phenotype are clearly present in human peripheral blood as well as other lymphatic tissues. However, it remains unclear if these circulating memory cells can reenter the splenic MZ. The poor response of infants to some types of T-independent antigens correlates with the fact that the MZ is not fully formed until the age of 1 to 2 years. In addition, splenectomized individuals are more susceptible to infection with some bacteria, owing to the deficient antibody response to capsular polysaccharides.
FO B cells circulate and take up residence in follicles found in lymphoid tissues that include lymph nodes, intestinal Peyer patches, and the spleen following their generation. The B cells in primary follicles are mature FO cells that have not yet encountered antigen and initiated an immune response. Although some B cells in the MZ can respond to T-dependent antigens, most B cells that do so are FO B cells. After their binding of a T-dependent antigen (as soluble antigen, indirectly via presentation by a local antigen presenting cell, or as an immune complex) mature, naïve B cells in primary follicles undergo a blastogenic response. Some of these cells will immediately mature into plasma cells that secrete low-affinity IgM to provide a rapid initial response to infection. In response to T-cell help, however, other B cells undergo further proliferation and differentiation forming secondary follicles. The histologic appearance of the follicle changes as these events evolve. The nonresponsive B cells form an outer mantle zone surrounding the proliferating, antigen-responsive B cells in a central germinal center. Germinal center B cells are shielded from soluble antigens and are exposed only to a unique set of antigens presented by follicular dendritic cells.
Germinal Centers
Germinal centers are classically divided into two compartments, denoted as the dark and light zones based on their appearance under light microscopy (see Fig. 1). Dark zones are located adjacent to the T-cell areas and contain a high density of proliferating B cells termed centroblasts, which are large cells with a high nuclear:cytoplasmic ratio that do not express surface BCR. The light zone has a lower cellular density due to the presence of an extensive loose network of follicular dendritic cells, and this imparts the “light” appearance. B cells in the light zone are termed centrocytes, which in contrast to the centroblasts, are small B cells expressing surface BCR.
Within the germinal center, the series of sequential shuttling and reentry of B cells into the dark and light zones is termed the germinal center reaction or the B-cell selection process. The cyclic reentry model proposes that centroblasts in the dark zone undergo cell division, class switching, and somatic hypermutation, discussed in the next section. Next, they exit the cell cycle and migrate into the light zone to interact with antigen-presenting follicular dendritic cells and T follicular helper cells (Tfhs). In light zones, B cells with increased affinity for antigen are preferentially selected for survival by receiving vital signals from Tfh cells; in contrast, B cells with impaired or absent antigen binding undergo apoptosis and clearance by resident macrophages, known as tingible body macrophages.
Selected centrocytes in the light zone are thought to return to the dark zone to undergo further rounds of proliferation, affinity maturation, and selection to improve the affinity of B-cell repertoire. Positively selected germinal center B cells eventually leave the germinal center, differentiating into memory B cells or plasma cells possessing somatically mutated Ig genes that encode for a high-affinity BCR. The modified capability of antigen-selected memory B cells to generate a fast, highly specific humoral immune response upon a second encounter with the same pathogen forms the mechanistic basis of humoral memory. Plasma cells are long-lived cells that take up residence in the BM and spleen, and are responsible for maintaining high levels of Ig seen in the serum.
Immunoglobulin Class Switching and Affinity Maturation
Germinal center B cells undergo three distinct modifications to increase the affinity of the Ig for antigen. Ig class switching involves deletions of germline DNA resulting in religation of the VDJ com plex to downstream heavy chain C region genes, such as γ3, γ1 γ2b, γ2a, ε, and α. These DNA deletions are believed to occur at or near nucleotide sequences called switch regions that are located in the intron 5′ to each CH exon. These class-switching events are highly regulated, secondary-differentiation events potentiated by helper T cells and their secreted products. Note that Ig class switching is dis tinct from the previously discussed differential splicing events that allows the newly produced B cell to express the same VDJ complex associated with either the μ or δ heavy chain C regions.
B cells may also undergo receptor editing. Receptor editing usually involves modifications of the existing light chain in which an upstream V region segment joins to a downstream J region gene. As a result, the genetic region encoding the originally expressed light chain is deleted. RAG-1 and RAG-2 expression are required for this process to occur. It has been proposed that B cells in germinal centers might reactivate Rag gene expression to mediate events such as receptor editing. However, that this occurs has been questioned. Instead, receptor editing in splenic B cells may be limited to a small subset of recent immature BM immigrants that enter germinal centers before their Rag expression has been extinguished.
Somatic hypermutation provides the primary means to increase antibody affinity. During this process, single-nucleotide exchanges, deletions, and mutations are introduced into the genes encoding the antibody-binding regions of the Ig receptor. A B-cell–specific gene that encodes activation-induced cytidine deaminase (AID), which is expressed in germinal center B cells, has been identified. AID is a putative RNA-editing enzyme that acts as a cytidine deaminase and has been shown to be indispensable for somatic hypermutation and class switch recombination.
As discussed in the previous section, these events are dependent on signals delivered to the antigen-responsive B cells by antigen-specific Tfh lymphocytes that migrate into the germinal center from the PALS or the T-cell zone within lymph nodes. Tfh cells mediate their effects on B cells through the secretion of cytokines as well as through direct intercellular contacts, and these stimuli result in B-cell growth, differentiation, and Ig class switching. For example, CD40 is a T-cell surface glycoprotein encoded by a member of the tumor necrosis gene family, and its ligand is expressed on B cells. CD40 ligand knock out mice do not form germinal centers, and humans who do not express CD40 ligand have X-linked hyper IgM immunodeficiency. Additional key T-cell costimulatory signals include cytokines such as IL-4, IL-10, IL-21, and interferon γ secreted by T cells in response to their activation via the “inducible costimulator” ICOS. Humans lacking expression of ICOS on T cells have adult-onset common variable immune deficiency, leading to a severe deficit in generation of class-switched and memory B cells.
There are two unintended consequences of affinity maturation. One is that autoreactive clones may be inadvertently generated. The other is the development of B-cell lymphoma. Lymphomagenesis results in part from the fact that vigorous B-cell proliferation, combined with the changes at the DNA level that lead to molecular alterations, may promote malignant transformation. Numerous studies have assigned B-cell lymphomas to each of the normal B-cell counterparts. Events that limit differentiation of immature or activated mature B cells can also promote malig nant transformation. Importantly, many genetic lesions leading to B-cell lymphoma directly impact the BCR signaling cascade, and an improved understanding of these signaling events may lead to new therapies. These include application of Btk-specific small molecule inhibitors in a broad range of human lymphomas.
الاكثر قراءة في المناعة
اخر الاخبار
اخبار العتبة العباسية المقدسة
