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(1/1118) Lymph node germinal centers form in the absence of follicular dendritic cell networks.

Follicular dendritic cell networks are said to be pivotal to both the formation of germinal centers (GCs) and their functions in generating antigen-specific antibody affinity maturation and B cell memory. We report that lymphotoxin beta-deficient mice form GC cell clusters in the gross anatomical location expected of GCs, despite the complete absence of follicular dendritic cell networks. Furthermore, antigen-specific GC generation was at first relatively normal, but these GCs then rapidly regressed and GC-phase antibody affinity maturation was reduced. Lymphotoxin beta-deficient mice also showed substantial B cell memory in their mesenteric lymph nodes. This memory antibody response was of relatively low affinity for antigen at week 4 after challenge, but by week 10 after challenge was comparable to wild-type, indicating that affinity maturation had failed in the GC phase but developed later.  (+info)

(2/1118) Minimal cross-linking and epitope requirements for CD40-dependent suppression of apoptosis contrast with those for promotion of the cell cycle and homotypic adhesions in human B cells.

Eight different CD40 mAb shared with soluble trimeric CD40 ligand (sCD40LT) the capacity to rescue germinal center (GC) B cells from spontaneous apoptosis and to suppress antigen receptor-driven apoptosis in group I Burkitt's lymphoma cells. Three mAb (G28-5, M2 and M3) mimicked sCD40LT in its ability to promote strong homotypic adhesion in resting B cells, whereas others (EA5, BL-OGY/C4 and 5C3) failed to stimulate strong clustering. Binding studies revealed that only those mAb that promoted strong B cell clustering bound at, or near to, the CD40L binding site. While all eight mAb and sCD40LT were capable of synergizing with IL-4 or phorbol ester for promoting DNA synthesis in resting B cells, co-stimulus-independent activation of the cells into cycle through CD40 related directly to the extent of receptor cross-linking. Thus, mAb which bound outside the CD40L binding site synergized with sCD40LT for promoting DNA synthesis; maximal levels of stimulation were achieved by presenting any of the mAb on CD32 transfectants in the absence of sCD40LT or by cross-linking bound sCD40LT with a second antibody. Monomeric sCD40L, which was able to promote rescue of GC B cells from apoptosis, was unable to drive resting B cells into cycle. These studies demonstrate that CD40-dependent rescue of human B cells from apoptosis requires minimal cross-linking and is essentially epitope independent, whereas the requirements for promoting cell cycle progression and homotypic adhesion are more stringent. Possible mechanisms underlying these differences and their physiological significance are discussed.  (+info)

(3/1118) Cathepsin S required for normal MHC class II peptide loading and germinal center development.

Major histocompatibility complex (MHC) class II molecules acquire antigenic peptides after degradation of the invariant chain (Ii), an MHC class II-associated protein that otherwise blocks peptide binding. Antigen-presenting cells of mice that lack the protease cathepsin S fail to process Ii beyond a 10 kDa fragment, resulting in delayed peptide loading and accumulation of cell surface MHC class II/10 kDa Ii complexes. Although cathepsin S-deficient mice have normal numbers of B and T cells and normal IgE responses, they show markedly impaired antibody class switching to IgG2a and IgG3. These results indicate cathepsin S is a major Ii-processing enzyme in splenocytes and dendritic cells. Its role in humoral immunity critically depends on how antigens access the immune system.  (+info)

(4/1118) Plasma cell development in synovial germinal centers in patients with rheumatoid and reactive arthritis.

Plasma cells are found surrounding the inflammatory infiltrates of macrophages, T, and B cells in the synovial tissue of patients with rheumatoid and reactive arthritis. This characteristic arrangement suggests that in the synovial tissue CD20+ B cells differentiate into plasma cells. To examine clonal relationships, we have used micromanipulation to separately isolate CD20+ B cells and plasma cells from single infiltrates. DNA was extracted, and from both populations the VH/VL gene repertoires was determined. The data show that in the inflamed synovial tissue activated B cells are clonally expanded. During proliferation in the network of follicular dendritic cells, V gene variants are generated by the hypermutation mechanism. Surprisingly, we do not find identical rearrangements between CD20+ B cells and plasma cells. Nevertheless, the finding of clonally related plasma cells within single infiltrates suggests that these cells underwent terminal differentiation in the synovial tissue. These results indicate that B cell differentiation in the synovial tissue is a dynamic process. Whereas CD20+ B cells may turnover rapidly, plasma cells may well be long lived and thus accumulate in the synovial tissue. The analysis of individual B cells recovered from synovial tissue opens a new way to determine the specificity of those cells that take part in the local immune reaction. This will provide new insights into the pathogenesis of chronic inflammatory diseases like rheumatoid or reactive arthritis.  (+info)

(5/1118) Segmented filamentous bacteria are potent stimuli of a physiologically normal state of the murine gut mucosal immune system.

Segmented filamentous bacteria (SFB) are autochthonous bacteria inhabiting the intestinal tracts of many species, including humans. We studied the effect of SFB on the mucosal immune system by monoassociating formerly germfree C3H/HeN mice with SFB. At various time points during 190 days of colonization, fragment cultures of small intestine and Peyer's patches (PP) were analyzed for total immunoglobulin A (IgA) and SFB-specific IgA production. Also, phenotypic changes indicating germinal center reactions (GCRs) and the activation of CD4(+) T cells in PP were determined by using fluorescence-activated cell sorter analyses. A second group of SFB-monoassociated mice was colonized with a gram-negative commensal, Morganella morganii, to determine if the mucosal immune system was again stimulated and to evaluate the effect of prior colonization with SFB on the ability of M. morganii to translocate to the spleen and mesenteric lymph nodes. We found that SFB stimulated GCRs in PP from day 6 after monoassociation, that GCRs only gradually waned over the entire length of colonization, that natural IgA production was increased to levels 24 to 63% of that of conventionally reared mice, and that SFB-specific IgA was produced but accounted for less than 1.4% of total IgA. Also, the proportion of CD4(+), CD45RBlow T cells, indicative of activated cells, gradually increased in the PP to the level found in conventionally reared mice. Secondary colonization with M. morganii was able to stimulate GCRs anew, leading to a specific IgA antibody response. Previous stimulation of mucosal immunity by SFB did not prevent the translocation of M. morganii in the double-colonized mice. Our findings generally indicate that SFB are one of the single most potent microbial stimuli of the gut mucosal immune system.  (+info)

(6/1118) Nonimmunoglobulin gene hypermutation in germinal center B cells.

Somatic hypermutation is the most critical mechanism underlying the diversification of Ig genes. Although mutation occurs specifically in B cells during the germinal center reaction, it remains a matter of debate whether the mutation machinery also targets non-Ig genes. We have studied mutations in the 5' noncoding region of the Bcl6 gene in different subtypes of lymphomas. We found frequent hypermutation in follicular lymphoma (25 of 59 = 42%) (germinal center cell origin) and mucosa-associated lymphoid tissue (MALT) lymphoma (19 of 45 = 42%) (postgerminal center), but only occasionally in mantle cell lymphoma (1 of 21 = 4.8%) (pregerminal center). Most mutations were outside the motifs potentially important for transcription, suggesting they were not important in lymphomagenesis but may, like Ig mutation, represent an inherent feature of the lymphoma precursor cells. Therefore, we investigated their normal cell counterparts microdissected from a reactive tonsil. Bcl6 mutation was found in 13 of 24 (54%) clones from the germinal centre but only in 1 of 24 (4%) clones from the naive B cells of the mantle zone. The frequency, distribution, and nature of these mutations were similar to those resulting from the Ig hypermutation process. The results show unequivocal evidence of non-Ig gene hypermutation in germinal center B cells and provide fresh insights into the process of hypermutation and lymphomagenesis.  (+info)

(7/1118) Human immunodeficiency virus-associated Hodgkin's disease derives from post-germinal center B cells.

Human immunodeficiency virus-associated Hodgkin's disease (HIV-HD) displays several peculiarities when compared with HD of the general population. These include overrepresentation of clinically aggressive histologic types and frequent infection of Reed-Sternberg (RS) cells by Epstein-Barr virus (EBV). Recently, we have reported that the histogenesis of HD of the general population may be assessed by monitoring the expression pattern of BCL-6, a transcription factor expressed in germinal center (GC) B cells, and of CD138/syndecan-1 (syn-1), a proteoglycan associated with post-GC, terminal B-cell differentiation. In this study, we have applied these two markers to the study of HIV-HD histogenesis and correlated their expression status to the virologic features of this disease. We have found that RS cells of all histologic categories of HIV-HD consistently display the BCL-6(-)/syn-1(+) phenotype and thus reflect post-GC B cells. Although BCL-6(-)/syn-1(+) RS cells of HIV-HD express CD40, they are not surrounded by CD40 ligand-positive (CD40L+) reactive T lymphocytes, which, in HD of the general population, are thought to regulate the disease phenotype through CD40/CD40L interactions. Conversely, RS cells of virtually all HIV-HD express the EBV-encoded latent membrane protein 1 (LMP1), which, being functionally homologous to CD40, may contribute, at least in part, to the modulation of the HIV-HD phenotype.  (+info)

(8/1118) Toward a role of dendritic cells in the germinal center reaction: triggering of B cell proliferation and isotype switching.

We have reported previously that in vitro generated dendritic cells (DC) can directly regulate B cell responses. Recently, germinal center DC (GCDC) were identified within B cell follicles. Due to their particular localization, we have tested in the present study whether GCDC could contribute to key events characteristic of the GC reaction. Our present results demonstrate that 1) ex vivo GCDC induce a dramatic GC B cell expansion upon CD40 and IL-2 activation and drive plasma cell differentiation, 2) this property is shared by GCDC and blood DC, but not by Langerhans cells, 3) IL-12 production by GCDC is critical in GC B cell expansion and differentiation, and 4) importantly, GCDC also induce IL-10-independent isotype switching toward IgG1. These observations support the novel concept that GCDC directly contribute to the germinal center reaction.  (+info)