BackgroundFollicular structures resembling germinal centres (GCs) that are characterized by follicular dendritic cell (FDC) networks have long been recognized in chronically inflamed tissues in autoimmune diseases, including the synovium of rheumatoid arthritis (RA). However, it is debated whether these ectopic structures promote autoimmunity and chronic inflammation driving the production of pathogenic autoantibodies. Anti-citrullinated protein/peptide antibodies (ACPA) are highly specific markers of RA, predict a poor prognosis, and have been suggested to be pathogenic. Therefore, the main study objectives were to determine whether ectopic lymphoid structures in RA synovium: (i) express activation-induced cytidine deaminase (AID), the enzyme required for somatic hypermutation and class-switch recombination (CSR) of Ig genes; (ii) support ongoing CSR and ACPA production; and (iii) remain functional in a RA/severe combined immunodeficiency (SCID) chimera model devoid of new immune cell influx into the synovium.Methods and FindingsUsing immunohistochemistry (IHC) and quantitative Taqman real-time PCR (QT-PCR) in synovial tissue from 55 patients with RA, we demonstrated that FDC+ structures invariably expressed AID with a distribution resembling secondary lymphoid organs. Further, AID+/CD21+ follicular structures were surrounded by ACPA+/CD138+ plasma cells, as demonstrated by immune reactivity to citrullinated fibrinogen. Moreover, we identified a novel subset of synovial AID+/CD20+ B cells outside GCs resembling interfollicular large B cells. In order to gain direct functional evidence that AID+ structures support CSR and in situ manufacturing of class-switched ACPA, 34 SCID mice were transplanted with RA synovium and humanely killed at 4 wk for harvesting of transplants and sera. Persistent expression of AID and Iγ-Cμ circular transcripts (identifying ongoing IgM-IgG class-switching) was observed in synovial grafts expressing FDCs/CD21L. Furthermore, synovial mRNA levels of AID were closely associated with circulating human IgG ACPA in mouse sera. Finally, the survival and proliferation of functional B cell niches was associated with persistent overexpression of genes regulating ectopic lymphoneogenesis.ConclusionsOur demonstration that FDC+ follicular units invariably express AID and are surrounded by ACPA-producing plasma cells provides strong evidence that ectopic lymphoid structures in the RA synovium are functional and support autoantibody production. This concept is further confirmed by evidence of sustained AID expression, B cell proliferation, ongoing CSR, and production of human IgG ACPA from GC+ synovial tissue transplanted into SCID mice, independently of new B cell influx from the systemic circulation. These data identify AID as a potential therapeutic target in RA and suggest that survival of functional synovial B cell niches may profoundly influence chronic inflammation, autoimmunity, and response to B cell–depleting therapies.

No abstract

Lymphomas of the gastrointestinal tract, salivary glands, lung and thyroid are grouped together as tumours arising in mucosa-associated lymphoid tissue. The great majority of them are of B-cell origin but distinctive T-cell lymphomas are also recognized in the gastrointestinal tract. These lymphomas tend to remain localized for prolonged periods but, whereas the B-cell group respond favourably to local therapy, the T-cell group are associated with severe morbidity and their overall prognosis is extremely poor. Accepted histological classifications of non-Hodgkin's lymphomas are difficult to apply to these tumours. In this paper their morphological features are reviewed; recent findings based on immunohistochemistry and DNA analysis are presented; and the biological behaviour of these tumours is discussed insofar as they offer insight into mucosal immunological mechanisms.

No abstract

SummaryThe splenic marginal zone (MGZ), which surrounds the mantle zone (MTZ) in human splenic white pulp, contains a phenotypically and morphologically distinct population of B cells. The origin of MGZ B cells is .uncertain. Whereas some experiments in rodents have suggested that they are a distinct cell lineage responsible for the immune response to T-independent type 2 antigens, others have suggested that they are memory B cells derived from a germinal center (GC) response. The progeny of a GC reaction is expected to have rearranged immunoglobulin (Ig) genes that are mutated. The distribution of mutations would be expected to reflect the selection of Ig by its affinity for antigen. We have analyzed rearranged Ig heavy chain variable region (VH) 6 and VH 4.21 genes in MGZ and MTZ B cells microdissected from frozen sections of human spleen to determine whether these genes have the properties of an affinity-selected memory B cell population. MTZ B cells contained germline Ig VH genes, confirming previous reports and providing an internal control for mutational analysis. MGZ B cells contained Ig VH genes that were mutated, showing that these cells had been subjected to a mutational mechanism characteristically active in the GC. The rearranged VH 6 genes showed patterns of mutation indicative of an antigen selection process, whereas the distribution of mutations in VH 4.21 genes was not characteristic of gene selection by conventional T-dependent antigen. Our studies provide the first evidence that the human splenic MGZ is a reservoir of memory B cells.T he marginal zone (MGZ) l of the spleen is a microanatomically defined lymphoid compartment at the border of the white and red pulp. The major cellular components of the MGZ are B cells and macrophages (1-3). In humans and rodents, MGZ B cells are characteristically IgM + , IgD-, and they strongly express CD21 (1, 4-6). They are not in cell cycle and do not express antigens associated with cellular activation (7,8).Two different functions have been ascribed to MGZ B cells in rodents: the antibody response to T-independent type 2 (TI-2) antigens and B cell memory. It has been well documented that an intact splenic MGZ is required for an efficient response to TI-2 antigen (9-11). A number of experiments have suggested that the B cells in the MGZ (12) and the B cells responding to TI-2 antigens (13) differ developmentally from other B cell populations, and they have prompted the suggestion that MGZ B ceUs are of a distinct 1 Abbreviations used in this paper: Fw, framework region; GC, germinal center; MGZ, marginal zone; MTZ, mantle zone; R, replacement; s, silent; TD, T dependent; TI-2, T independent type 2; Vs, heavy chain variable region.lineage. However, in vitro comparison of the ability of mufine MGZ B cells and follicle center B cells to respond to T-dependent (TD), TI-1, and TI-2 antigens have failed to show any major differences, suggesting that responsiveness to TI-2 antigens is due to factor(s) in the microenvironment of the MGZ (14). The role of MGZ macro...

Demonstration of ectopic germinal center-like structures (GC-LSs) in chronically inflamed tissues in patients with autoimmune disorders is a relatively common finding. However, to what extent ectopic lymphoid structures behave as true GC and are able to support class switch recombination (CSR) and somatic hypermutation (SHM) of the Ig genes is still debated. In addition, no information is available on whether CSR and SHM can take place in the absence of GCs at extrafollicular sites in an ectopic lymphoid tissue. In this study, we show that in salivary glands (SGs) of Sjögren’s syndrome (SS) activation-induced cytidine deaminase (AID), the enzyme responsible for CSR and SHM is invariably expressed within follicular dendritic cell (FDC) networks but is not detectable in SGs in the absence of ectopic GC-LSs, suggesting that FDC networks play an essential role in sustaining the Ag-driven B cell proliferation within SS-SGs. We also show that the recently described population of interfollicular large B cells selectively expresses AID outside ectopic GC in the T cell-rich areas of periductal aggregates. Finally, we report that AID retains its exclusive association with numerous, residual GCs in parotid SS-MALT lymphomas, whereas neoplastic marginal zone-like B cells are consistently AID negative. These results strongly support the notion that ectopic lymphoid structures in SS-SGs express the molecular machinery to support local autoantibody production and B cell expansion and may play a crucial role toward lymphomagenesis.

Intestinal damage as a result of allergy and infection is a major cause of morbidity in man and animals (1-3). In intestinal allergy the most commonly seen lesions are small intestinal villous atrophy and crypt cell hyperplasia resulting in malabsorption due to a decreased intestinal absorptive surface and decreased digestive enzyme levels in the epithelial cells. The mechanisms involved in these changes in man are not known, but there is evidence from experimental animal studies that T cells may play an important role in the pathogenesis of the intestinal lesion (4). The enteropathy observed in intestinal graft-versus-host disease and rejection of transplanted intestinal allografts is characterized by a lymphocytic infiltrate, shortening of the villi, lengthening of the crypts and an increased rate of epithelial cell proliferation (5, 6). However, these in vivo models, although useful, do not allow a dissection of the mechanisms involved in the development of enteropathy in human small intestine.We have recently shown (7) that fetal human small intestine becomes increasingly populated with T cells from^-14 wk of gestation. By 19-22 wk the fetal gut epithelium and lamina propria contain many isolated T cells and the lamina propria contains aggregates of T and B cells that are probably early Peyer's patches (8). Small explants of human fetal small intestine can be maintained in organ culture for several weeks with retention of gut structure and normal epithelial cell function (9) . We have thus attempted to directly stimulate mucosal T cells in situ in cultures of human small intestine. Materials and MethodsOrgan Culture of Fetal Human Small Intestine. Small intestine from therapeutically aborted fetuses was placed in a petri dish in serum-free CMRL-1066 medium (Flow Laboratories, Inc ., McLean, VA) modified as described by Autrup et al (10) . The intestine was cut into segments 2-3 mm in length which were then bisected longitudinally to expose the lumen . The explants were then trimmed into pieces 2-3 mm square . Five pieces of tissue were cultured in 7 ml modified CMRL-1066 medium in 5-cm diameter tissue culture dishes (Sterilin ; Scientific Supplies Co . Ltd ., Vine Hill, London) . The cultures were incubated for 72 h at 37°C in a 95% oxygen, 5% CO atmosphere . At the end of the culture period the five explants in each dish were carefully removed from the

No abstract