Extrafollicular plasmablast B cells play a key role in carrying retroviral infection to peripheral organs

Early IgE Production Is Linked with Extrafollicular B- and T-Cell Activation in Low-Dose Allergy Model

Despite its paramount importance, the predominant association of early IgE production with harmless antigens, via germinal-center B- and T-cell subpopulations or extrafollicular activation, remains unresolved. The aim of this work was to clarify whether the reinforced IgE production following the subcutaneous immunization of BALB/c mice with low antigen doses in withers adipose tissue might be linked with intensified extrafollicular or germinal-center responses. The mice were immunized three times a week for 4 weeks in the withers region, which is enriched in subcutaneous fat and tissue-associated B cells, with high and low OVA doses and via the intraperitoneal route for comparison. During long-term immunization with both low and high antigen doses in the withers region, but not via the intraperitoneal route, we observed a significant accumulation of B220-CD1d-CD5-CD19+ B-2 extrafollicular plasmablasts in the subcutaneous fat and regional lymph nodes but not in the intraperitoneal fat. Only low antigen doses induced a significant accumulation of CXCR4+ CXCR5- CD4+ extrafollicular T helpers in the withers adipose tissue but not in the regional lymph nodes or abdominal fat. Only in subcutaneous fat was there a combination of extrafollicular helper accumulation. In conclusion, extrafollicular B- and T-cell activation are necessary for early IgE class switching.

Mycobacterium tuberculosis-specific plasmablast levels are differentially modulated in tuberculosis infection and disease

Background

While T cell responses to Mycobacterium tuberculosis (Mtb) have been extensively studied, the role of B-cells and antibodies are less well characterised. The aim of this study was to assess levels of Mtb-specific IgG + plasmablasts across the Mtb infection spectrum.

Methods

Patients with active TB were analysed at baseline and 6 months of therapy (n = 20).Their exposed household contacts (HHC) included individuals with latent TB infection (LTBI; n = 20); evident at baseline; individuals with a negative Tuberculin Skin Test (TST) at baseline who became; positive at 6 months (converters; n = 11) and those who remained negative (non-converters; n = 10). An e x-vivo B-cell ELISPOT was performed to analyse plasmablast responses.

Results

Frequencies of ESAT-6/CFP-10 (EC)- but not Whole Cell Lysate (WCL)-specific plasmablasts were significantly higher in patients with active TB pre-treatment compared to post-treatment (p = 0.002) and compared to HHC with LTBI (p < 0.0001). Conversely, total IgG + plasmablasts were significantly decreased in TB patients at baseline. No difference was seen in levels of plasmablasts between TST converters and non-converters at baseline.

Conclusions

We show that EC-specific plasmablast levels are differentially modulated during TB infection and disease, with levels highest during active TB. These data provide new insights into TB biomarker development and avenues for novel immune interventions.

Plasmacytoid Dendritic Cells and Type I Interferon Promote Extrafollicular B Cell Responses to Extracellular Self-DNA

Class-switched antibodies to double-stranded DNA (dsDNA) are prevalent and pathogenic in systemic lupus erythematosus (SLE), yet mechanisms of their development remain poorly understood. Humans and mice lacking secreted DNase DNASE1L3 develop rapid anti-dsDNA antibody responses and SLE-like disease. We report that anti-DNA responses in Dnase1l3-/- mice require CD40L-mediated T cell help, but proceed independently of germinal center formation via short-lived antibody-forming cells (AFCs) localized to extrafollicular regions. Type I interferon (IFN-I) signaling and IFN-I-producing plasmacytoid dendritic cells (pDCs) facilitate the differentiation of DNA-reactive AFCs in vivo and in vitro and are required for downstream manifestations of autoimmunity. Moreover, the endosomal DNA sensor TLR9 promotes anti-dsDNA responses and SLE-like disease in Dnase1l3-/- mice redundantly with another nucleic acid-sensing receptor, TLR7. These results establish extrafollicular B cell differentiation into short-lived AFCs as a key mechanism of anti-DNA autoreactivity and reveal a major contribution of pDCs, endosomal Toll-like receptors (TLRs), and IFN-I to this pathway.

Extrafollicular responses in humans and SLE

Chronic autoimmune diseases, and in particular Systemic Lupus Erythematosus (SLE), are endowed with a long-standing autoreactive B-cell compartment that is presumed to reactivate periodically leading to the generation of new bursts of pathogenic antibody-secreting cells (ASC). Moreover, pathogenic autoantibodies are typically characterized by a high load of somatic hypermutation and in some cases are highly stable even in the context of prolonged B-cell depletion. Long-lived, highly mutated antibodies are typically generated through T-cell-dependent germinal center (GC) reactions. Accordingly, an important role for GC reactions in the generation of pathogenic autoreactivity has been postulated in SLE. Nevertheless, pathogenic autoantibodies and autoimmune disease can be generated through B-cell extrafollicular (EF) reactions in multiple mouse models and human SLE flares are characterized by the expansion of naive-derived activated effector B cells of extrafollicular phenotype. In this review, we will discuss the properties of the EF B-cell pathway, its relationship to other effector B-cell populations, its role in autoimmune diseases, and its contribution to human SLE. Furthermore, we discuss the relationship of EF B cells with Age-Associated B cells (ABCs), a TLR-7-driven B-cell population that mediates murine autoimmune and antiviral responses.

Apoptosis Susceptibility Prolongs the Lack of Memory B Cells in Acute Leukemic Patients After Allogeneic Hematopoietic Stem Cell Transplantation

Long-term survival after allogeneic hematopoietic stem cell transplantation requires intact immunosurveillance, which is hampered by lymphoid organ damage associated with conditioning therapy, graft-versus-host disease, and immunosuppression. Our study aimed to identify the mechanisms contributing to sustained low memory B cell numbers after transplantation. Peripheral B and T cell subset recovery and functional marker expression were investigated in 35 acute leukemic patients up to 1 year after transplantation. Apoptosis of B cells after CD40/TLR-9, CD40/BCR, and CD40/BCR/TLR-9-dependent stimulation and drug efflux capacity were analyzed. One half of the patients suffered from infections after day 180. All patients had strongly diminished CD27(+) memory B cells despite already normalized total B cell numbers and fully recovered CD27(-)IgD(-) memory B cells, putatively of extra-follicular origin. Circulating memory follicular helper T cells were reduced in the majority of patients as well. Naïve B cells exhibited a decreased expression of CXCR5, which mediates follicular B cell entry. Additionally, a lower HLA-DR expression was found on naïve B cells, impairing antigen presentation. Upon CD40/TLR-9-dependent activation, B cells underwent significantly increased apoptosis paralleled by an aberrant up-regulation of Fas-L on activated T cells and Fas on resting B cells. Significantly increased B cell apoptosis was also observed after CD40/BCR and CD40/BCR/TLR-9-dependent activation. Drug efflux capacity of naïve B cells was diminished in cyclosporin A-treated patients, additionally contributing to an apoptosis-prone phenotype. We conclude that B cell survival and migration and T cell communication defects are contributing candidates for an impaired germinal center formation of memory B cells after allogeneic hematopoietic stem cell transplantation. Follow-up studies should evaluate effectiveness of revaccinations on the cellular level and should address the long-term sequelae of B cell defects after transplantation.

Shp1 signalling is required to establish the long-lived bone marrow plasma cell pool

Germline or B-cell-specific loss of Ptpn6 gene encoding the Shp1 protein tyrosine phosphatase leads to skewed B lymphopoiesis and systemic autoimmunity. Here, to study its role in B-cell terminal differentiation, we generated Ptpn6^f/fAicda^Cre/+ mice with Shp1 ablated only in activated B cells. We show that Ptpn6^f/fAicda^Cre/+ mice have normal B-cell development but exhibit defective class-switched primary and recalled antibody response to a T-cell-dependent antigen. Germinal centres are present but do not persist and memory B cells are not formed. Interestingly, Shp1-deficient plasma cells are generated in the spleen but do not contribute to the bone marrow long-lived pool. Plasma cells lacking Shp1 exhibit aberrant α4β1 integrin activation due to dysregulated Src- and PI3-kinase signalling and manifest attenuated migration in vitro and defective bone marrow homing when reconstituted in vivo. Interrupting α4β1–VCAM-1 interaction rectifies this defect. These data suggest that Shp1 signalling is required for the establishment of a life-long protective humoral immunity.

SHP-1 signalling is required for the normal development of B lymphocytes but its role in the terminal differentiation of these cells has not been fully established. Here, the authors show that SHP-1 ablation impairs the establishment of long-lived bone marrow-resident plasma cells due to aberrant integrin activation.

Vaccine Development as a Means to Control Dengue Virus Pathogenesis: Do We Know Enough?

Dengue virus (DENV) is a mosquito-transmitted RNA virus responsible for 390 million infections each year and significant morbidity and mortality throughout tropical and subtropical regions of the world. Efforts to develop a DENV vaccine span 70 years and include the work of luminaries of the virus vaccine field. Although vaccines have been used to reduce the global health burden of other flaviviruses, the unique requirement for a single vaccine to protect against four different groups of dengue viruses, and the link between secondary infections and DENV disease pathogenesis, has limited success to date. In this review, we discuss several promising DENV vaccine candidates in clinical trials and assess how recent advances in understanding of DENV biology and immunity may expedite efforts toward the development of safe and effective vaccines.

Immune System Cells in Healthy Ferrets

The ferret has emerged as an excellent animal model to characterize several physiologic and pathologic conditions. The distribution and characterization of different types of immune system cells were studied in healthy ferret tissues. Eight primary antibodies were tested for immunohistochemistry in formalin-fixed tissues: anti-CD3, anti-CD79α, anti-CD20, anti-HLA-DR, anti-lysozyme, anti-CD163, anti-SWC3, and anti-Mac387. The anti-CD3 antibody labeled T cells mainly in interfollicular and paracortical areas of lymph nodes, cortex and thymic medulla, and periarteriolar lymphoid sheaths in the spleen. The anti-CD79α and anti-CD20 antibodies immunolabeled B cells located in lymphoid follicles at lymph nodes, spleen, and Peyer patches. The CD79α and CD20 antibodies also labeled cells with nonlymphoid morphology in atypical B-cell locations. The anti-HLA-DR antibody labeled macrophages, some populations of B and T lymphocytes, and different populations of dendritic cells in lymph nodes, Peyer patches, spleen, and thymus. The anti-lysozyme antibody immunolabeled macrophages in the liver, lymph nodes, spleen, and thymus. The Mac-387, CD163, and SWC3 antibodies did not show any positive reaction in formalin-fixed or frozen tissues. To elucidate the origin of the uncommon CD79α/CD20 positive cells, a double immunohistochemistry was carried out using the anti-HLA-DR + the anti-CD79α, the anti-HLA-DR + the anti-CD20, and the anti-lysozyme + the anti-CD79α antibodies. Double labeling was mainly observed when the anti-HLA-DR + the anti-CD79α antibodies were combined. The immunohistologic characterization and distribution of these immune system cells in healthy ferret tissues should be of value in future comparative studies of diseases in ferrets.

Tissue specific heterogeneity in effector immune cell response

Post pathogen invasion, migration of effector T-cell subsets to specific tissue locations is of prime importance for generation of robust immune response. Effector T cells are imprinted with distinct “homing codes” (adhesion molecules and chemokine receptors) during activation which regulate their targeted trafficking to specific tissues. Internal cues in the lymph node microenvironment along with external stimuli from food (vitamin A) and sunlight (vitamin D₃) prime dendritic cells, imprinting them to play centre stage in the induction of tissue tropism in effector T cells. B cells as well, in a manner similar to effector T cells, exhibit tissue-tropic migration. In this review, we have focused on the factors regulating the generation and migration of effector T cells to various tissues along with giving an overview of tissue tropism in B cells.

The tetraspanin CD37 orchestrates the α(4)β(1) integrin-Akt signaling axis and supports long-lived plasma cell survival

Signaling by the serine and threonine kinase Akt (also known as protein kinase B), a pathway that is common to all eukaryotic cells, is central to cell survival, proliferation, and gene induction. We sought to elucidate the mechanisms underlying regulation of the kinase activity of Akt in the immune system. We found that the four-transmembrane protein CD37 was essential for B cell survival and long-lived protective immunity. CD37-deficient (Cd37(-/-)) mice had reduced numbers of immunoglobulin G (IgG)-secreting plasma cells in lymphoid organs compared to those in wild-type mice, which we attributed to increased apoptosis of plasma cells in the germinal centers of the spleen, areas in which B cells proliferate and are selected. CD37 was required for the survival of IgG-secreting plasma cells in response to binding of vascular cell adhesion molecule 1 to the α(4)β(1) integrin. Impaired α(4)β(1) integrin-dependent Akt signaling in Cd37(-/-) IgG-secreting plasma cells was the underlying cause responsible for impaired cell survival. CD37 was required for the mobility and clustering of α(4)β(1) integrins in the plasma membrane, thus regulating the membrane distribution of α(4)β(1) integrin necessary for activation of the Akt survival pathway in the immune system.

B cell-specific S1PR1 deficiency blocks prion dissemination between secondary lymphoid organs

Many prion diseases are peripherally acquired (e.g., orally or via lesions to skin or mucous membranes). After peripheral exposure, prions replicate first upon follicular dendritic cells (FDC) in the draining lymphoid tissue before infecting the brain. However, after replication upon FDC within the draining lymphoid tissue, prions are subsequently propagated to most nondraining secondary lymphoid organs (SLO), including the spleen, by a previously underdetermined mechanism. The germinal centers in which FDC are situated produce a population of B cells that can recirculate between SLO. Therefore, we reasoned that B cells were ideal candidates by which prion dissemination between SLO may occur. Sphingosine 1-phosphate receptor (S1PR)1 stimulation controls the egress of T and B cells from SLO. S1PR1 signaling blockade sequesters lymphocytes within SLO, resulting in lymphopenia in the blood and lymph. We show that, in mice treated with the S1PR modulator FTY720 or with S1PR1 deficiency restricted to B cells, the dissemination of prions from the draining lymph node to nondraining SLO is blocked. These data suggest that B cells interacting with and acquiring surface proteins from FDC and recirculating between SLO via the blood and lymph mediate the initial propagation of prions from the draining lymphoid tissue to peripheral tissues.

The impact of perinatal immune development on mucosal homeostasis and chronic inflammation

The mucosal surfaces of the gut and airways have important barrier functions and regulate the induction of immunological tolerance. The rapidly increasing incidence of chronic inflammatory disorders of these surfaces, such as inflammatory bowel disease and asthma, indicates that the immune functions of these mucosae are becoming disrupted in humans. Recent data indicate that events in prenatal and neonatal life orchestrate mucosal homeostasis. Several environmental factors promote the perinatal programming of the immune system, including colonization of the gut and airways by commensal microorganisms. These complex microbial-host interactions operate in a delicate temporal and spatial manner and have an important role in the induction of homeostatic mechanisms.

Alphavirus-induced encephalomyelitis: antibody-secreting cells and viral clearance from the nervous system

Sindbis virus (SINV) infection of the central nervous system (CNS) provides a model for understanding the role of the immune response in recovery from alphavirus infection of neurons. Virus clearance occurred in three phases: clearance of infectious virus (days 3 to 7), clearance of viral RNA (days 8 to 60), and maintenance of low levels of viral RNA (>day 60). The antiviral immune response was initiated in the cervical lymph nodes with rapid extrafollicular production of plasmablasts secreting IgM, followed by germinal center production of IgG-secreting and memory B cells. The earliest inflammatory cells to enter the brain were CD8(+) T cells, followed by CD4(+) T cells and CD19(+) B cells. During the clearance of infectious virus, effector lymphocytes in the CNS were primarily CD8(+) T cells and IgM antibody-secreting cells (ASCs). During the clearance of viral RNA, there were more CD4(+) than CD8(+) T cells, and B cells included IgG and IgA ASCs. At late times after infection, ASCs in the CNS were primarily CD19(+) CD38(+) CD138(-) Blimp-1(+) plasmablasts, with few fully differentiated CD38(-) CD138(+) Blimp-1(+) plasma cells. CD19(+) CD38(+) surface Ig(+) memory B cells were also present. The level of antibody to SINV increased in the brain over time, and the proportion of SINV-specific ASCs increased from 15% of total ASCs at day 14 to 90% at 4 to 6 months, suggesting specific retention in the CNS during viral RNA persistence. B cells in the CNS continued to differentiate, as evidenced by accumulation of IgA ASCs not present in peripheral lymphoid tissue and downregulation of major histocompatibility complex (MHC) class II expression on plasmablasts. However, there was no evidence of germinal center activity or IgG avidity maturation within the CNS.

Increases in IgA(+) B cells in Peyer's patches during milk-borne mouse mammary tumor virus infection are influenced by Toll-like receptor 4 and are completely dependent on the superantigen response

Mouse mammary tumor virus (MMTV) is a milk-borne betaretrovirus that has developed strategies to exploit and subvert the host immune system. Although mammary glands are the final target of infection, Peyer's patches (PP) are the entry site of the virus. Herein, we show that the infection induces increases in the number of PP IgA(+) B cells and higher expression of the α circular transcript, which is a specific marker of the switch to IgA. In addition, IgA(+) B-cell increases correlated with higher levels of cytokines related to IgA class switching, such as interleukin (IL)-5 and IL-6. Of interest, the increases in IgA(+) B cells were lower in Toll-like receptor 4-deficient mice and were completely dependent on the presence of superantigen-reactive T cells. Our results point to a novel mechanism involved in MMTV infection and suggest that IgA(+) B cells may play an important role in carrying the virus to the mammary glands.

Rev and Rex proteins of human complex retroviruses function with the MMTV Rem-responsive element

Background

Mouse mammary tumor virus (MMTV) encodes the Rem protein, an HIV Rev-like protein that enhances nuclear export of unspliced viral RNA in rodent cells. We have shown that Rem is expressed from a doubly spliced RNA, typical of complex retroviruses. Several recent reports indicate that MMTV can infect human cells, suggesting that MMTV might interact with human retroviruses, such as human immunodeficiency virus (HIV), human T-cell leukemia virus (HTLV), and human endogenous retrovirus type K (HERV-K). In this report, we test whether the export/regulatory proteins of human complex retroviruses will increase expression from vectors containing the Rem-responsive element (RmRE).

Results

MMTV Rem, HIV Rev, and HTLV Rex proteins, but not HERV-K Rec, enhanced expression from an MMTV-based reporter plasmid in human T cells, and this activity was dependent on the RmRE. No RmRE-dependent reporter gene expression was detectable using Rev, Rex, or Rec in HC11 mouse mammary cells. Cell fractionation and RNA quantitation experiments suggested that the regulatory proteins did not affect RNA stability or nuclear export in the MMTV reporter system. Rem had no demonstrable activity on export elements from HIV, HTLV, or HERV-K. Similar to the Rem-specific activity in rodent cells, the RmRE-dependent functions of Rem, Rev, or Rex in human cells were inhibited by a dominant-negative truncated nucleoporin that acts in the Crm1 pathway of RNA and protein export.

Conclusion

These data argue that many retroviral regulatory proteins recognize similar complex RNA structures, which may depend on the presence of cell-type specific proteins. Retroviral protein activity on the RmRE appears to affect a post-export function of the reporter RNA. Our results provide additional evidence that MMTV is a complex retrovirus with the potential for viral interactions in human cells.

CD11c expression identifies a population of extrafollicular antigen-specific splenic plasmablasts responsible for CD4 T-independent antibody responses during intracellular bacterial infection

Although T-independent immunity is known to be generated against bacterial capsular and cell wall polysaccharides expressed by a number of bacterial pathogens, it has not been studied in depth during intracellular bacterial infections. Our previous study demonstrated that Ehrlichia muris, an obligate intracellular tick-borne pathogen, generates protective classical TI responses in CD4 T cell-deficient C57BL/6 mice. We found that E. muris T-independent immunity is accompanied by the expansion of a very large extrafollicular spleen population of CD11c(low)-expressing plasmablasts that exhibit characteristics of both B-1 and marginal zone B cells. The plasmablasts comprised up to 15% of the total spleen lymphocytes and approximately 70% of total spleen IgM(high)IgD(low) cells during peak infection in both wild-type and MHC class II-deficient mice. The CD11c(low) cells exhibited low surface expression of B220, CD19, and CD1d, high expression of CD11b, CD43, but did not express CD5. Approximately 50% of the CD11c(low) cells also expressed CD138. In addition to CD11b and CD11c, the plasmablasts expressed the beta(1) (CD29) and alpha4 (CD49d) integrins, as well as the chemokine receptor CXCR4, molecules which may play roles in localizing the B cells extrafollicular region of the spleen. During peak infection, the CD11c(low) cells accounted for the majority of the IgM-producing splenic B cells and nearly all of the E. muris outer membrane protein-specific IgM-secreting cells. Thus, during this intracellular bacterial infection, CD11c expression identifies a population of Ag-specific spleen plasmablasts responsible for T-independent Ab production.

Anti-chromatin antibodies drive in vivo antigen-specific activation and somatic hypermutation of rheumatoid factor B cells at extrafollicular sites

A dominant type of spontaneous autoreactive B cell activation in murine lupus is the extrafollicular generation of plasmablasts. The factors governing such activation have been difficult to identify due to the stochastic onset and chronic nature of the response. Thus, the ability to induce a similar autoreactive B cell response with a known autoantigen in vivo would be a powerful tool in deciphering how autoimmune responses are initiated. We report here the establishment and characterization of a system to initiate autoreactive extrafollicular B cell responses, using IgG anti-chromatin antibodies, that closely mirrors the spontaneous response. We demonstrate that exogenously administered anti-chromatin antibody, presumably by forming immune complexes with released nuclear material, drives activation of rheumatoid factor B cells in AM14 Tg mice. Anti-chromatin elicits autoreactive B cell activation and development into antibody-forming cells at the T zone/red pulp border. Plasmablast generation occurs equally in BALB/c, MRL/+ and MRL/lpr mice, indicating that an autoimmune-prone genetic background is not required for the induced response. Importantly, infused IgG anti-chromatin induces somatic hypermutation in the absence of a GC response, thus proving the extrafollicular somatic hypermutation pathway. This system provides a window on the initiation of an autoantibody response and reveals authentic initiators of it.

Lymphocyte homing to the gut: attraction, adhesion, and commitment

Lymphocytes continuously migrate from the blood into the intestine. Naive lymphocytes leave the blood through high endothelial venules in Peyer's patches. During the multistep extravasation cascade, they sequentially roll on, firmly adhere to, and transmigrate through the endothelial layer using multiple adhesion molecules and chemotactic signals. In the organized lymphoid tissues of the gut, lymphocytes can become activated, if they meet their cognate antigens transported to Peyer's patches through the gut epithelium. During activation and proliferation, the lymphocytes become imprinted by the local dendritic cells, so that after returning to systemic circulation via the efferent lymphatic vasculature, they preferentially home to lamina propria of the gut to execute their effector functions. In inflammation, the recirculation routes of lymphocytes are altered, and these may explain the pathogenesis of certain extra-intestinal manifestations of gut infections and inflammatory bowel diseases. The increased knowledge on the mechanisms that regulate lymphocyte homing and imprinting has clear applicability in designing more effective vaccination regimens. A detailed understanding of the mucosal homing has recently led to the development of the first successful anti-adhesive therapeutics in human.

Immunohistochemistry of the B-Cell Component in Lower Lip Salivary Glands of Sjogren's Syndrome and Healthy Subjects

Serial sections of lower lip salivary gland (LSG) biopsies were examined by immunohistochemistry, using a battery of B- and partly T-related antibodies (CD5, CD20, CD21, CD27, CD38, CD45RO, CD79a, Bcl-2 and Bcl-6) in different groups of subjects: healthy controls and clinically verified smoking or nonsmoking cases of primary Sjögren's syndrome (SS). The purpose was to characterize the B-cell pattern of the lymphocytic foci and of the tiny perivascular infiltrates preceding the development of foci. Hyperplastic tonsil was used as stain control. In normal LSG, widely dispersed CD38+ and CD79a+ as well as some CD5+ cells are a normal constituent, with lack of staining with the other antibodies. In SS/LSG, the lymphocytic foci showed staining with all the antibodies, with variable degrees of overlapping or nonoverlapping. In SS/LSG of nonsmokers, CD20+ B cells make up a prominent part of the fully developed periductal lymphocytic foci, not overlapping with CD45RO. Also, CD20+ B cells did not overlap in the infiltrates with colocalized CD27+/CD38+ cells. CD20+ B cells and CD45RO+ T cells also occur as minute infiltrates perivascularly in areas of no foci in SS/LSG as well as in SS smokers lacking the typical foci. Smokers lack foci, but tiny infiltrates express CD20 as well CD45R0. Our findings suggest that CD20+ B cells and CD45RO+ T cells are early immigrants in the LSG of SS of smokers as well as nonsmokers and that another subgroup of CD27+/CD38+ B cells gradually mix with the first two to form the characteristic foci in SS/LSG. The simultaneous demonstration of CD20+ and CD27+ B cells in SS/LSG may constitute a significant diagnostic tool. Further, the findings suggest that the early immigrating lymphocytes may have been primed at a site remote from the glands before arriving via the blood to the gland tissue.

Plasmablast and Plasma Cell Production and Distribution in Trout Immune Tissues

These studies describe the in vitro and ex vivo generation of plasmablasts and plasma cells in trout (Oncorhynchus mykiss) peripheral blood and splenic and anterior kidney tissues. Cells were derived either from naive trout and cultured with the polyclonal activator, Escherichia coli LPS, or from trout that had been immunized with trinitrophenyl-keyhole limpet hemocyanin. Hydroxyurea was used to resolve populations of replicating (plasmablast) and nonreplicating (plasma cell) Ab-secreting cells (ASC). Complete inhibition of Ig secretion was only observed within the PBL. Both anterior kidney and splenic lymphocytes possessed a subset of ASCs that were hydroxyurea resistant. Thus, in vitro production of plasma cells appears to be restricted to the latter two tissues, whereas peripheral blood is exclusively restricted to the production of plasmablasts. After immunization with trinitrophenyl-keyhole limpet hemocyanin, specific ASC could be isolated from all immune organs; however, the anterior kidney contained 98% of all ASC. Late in the response (>10 wk), anterior kidney ASC secreted specific Ab for at least 15 days in culture, indicating that they were long-lived plasma cells. Cells from spleen and peripheral blood lost all capacity to secrete specific Ab in the absence of Ag. Late in the Ab response, high serum titer levels are solely the result of Ig secretion from anterior kidney plasma cells.

Plasma-cell homing

Recent studies indicate that chemoattractant cytokines (chemokines), together with tissue-specific adhesion molecules, coordinate the migration of antibody-secreting cells (ASCs) from their sites of antigen-driven differentiation in lymphoid tissues to target effector tissues. Developing ASCs downregulate the expression of receptors for lymphoid tissue chemokines and selectively upregulate the expression of chemokine receptors that might target the migration of IgA ASCs to mucosal surfaces, IgG ASCs to sites of tissue inflammation and both types of ASC to the bone marrow - an important site for serum antibody production. By directing plasma-cell homing, chemokines might help to determine the character and efficiency of mucosal, inflammatory and systemic antibody responses.

Passive immunization with neutralizing antibodies interrupts the mouse mammary tumor virus life cycle

Mouse mammary tumor virus (MMTV) infects the host via mucosal surfaces and exploits the host immune system for systemic spread and chronic infection. We have tested a neutralizing rat monoclonal antibody specific for the retroviral envelope glycoprotein gp52 for its efficiency in preventing acute and chronic mucosal and systemic infection. The antibody completely inhibits the superantigen response and chronic viral infection following systemic or nasal infection. Surprisingly however, the antibody only partially inhibits the early infection of antigen-presenting cells in the draining lymph node. Despite this initially inefficient protection from infection, superantigen-specific B- and T-cell responses and systemic viral spread are abolished, leading to complete clearance of the retroviral infection and hence interruption of the viral life cycle. In conclusion, systemic neutralizing monoclonal antibodies can provide an efficient protection against chronic retroviral amplification and persistence.

Homing of antibody secreting cells

As activated B cells differentiate into plasma cells they complete a final series of migration steps that take them to locations where they can efficiently carry out their effector function, secreting immunoglobulin (Ig) M or IgG into circulation or releasing dimeric IgA adjacent to the epithelium for transcytosis. Recent experiments have established a key role for chemokines in directing antibody secreting cell (ASC) movement within the secondary lymphoid organs where they are generated, as well as in guiding the cells to the bone marrow or mucosal surfaces. This review discusses the chemokines involved in directing ASC movements, particularly focusing on the role of CXCR4 and CXCL12/SDF1. The function of CCR9 and CCR10 in IgA ASC homing and contributions made by integrins and lectins are also discussed.

The role of neutralizing antibodies for mouse mammary tumor virus transmission and mammary cancer development

Mouse mammary tumor virus (MMTV) infection establishes chronic germinal centers and a lifelong neutralizing Ab response. We show that removal of the draining lymph node after establishment of the germinal center reaction led to complete loss of neutralizing Abs despite comparable infection levels in peripheral lymphocytes. Importantly, in the absence of neutralization, only the exocrine organs mammary gland, salivary gland, pancreas, and skin showed strikingly increased infection, resulting in accelerated mammary tumor development. Induction of stronger neutralization did not influence chronic infection levels of peripheral lymphoid organs but strongly inhibited mammary gland infection and virus transmission to the next generation. Taken together, we provide evidence that a tight equilibrium in virus neutralization allows limited infection of exocrine organs and controls cancer development in susceptible mouse strains. These experiments show that a strong neutralizing Ab response induced after infection is not able to control lymphoid MMTV infection. Strong neutralization, however, is capable of blocking amplification of mammary gland infection, tumor development, and virus transmission to the next generation. The results also indicate a role of neutralization in natural resistance to MMTV infection.

CD4+CD3- cells induce Peyer's patch development: role of alpha4beta1 integrin activation by CXCR5

CD4+CD3- cells are the predominant hematopoietic cells found in mouse fetal intestine. We prove their role as Peyer's patch (PP)-inducing cells by transfer into neonatal PP-deficient mice. To test the requirement of chemokines and adhesion molecules in induction of PP, we studied mice deficient in CXCR5 and/or alpha4beta1 integrin-mediated adhesion. CXCR5-/- mice have CD4+CD3- cells, which are inefficient in inducing PP formation. We show here that CXCR5/CXCL13 signaling activates alpha4beta1 integrin on CD4+CD3- cells. Blocking of beta1 integrin or VCAM-1, the ligand of alpha4beta1 integrin, inhibits PP formation. This study demonstrates the link between chemokine receptors and adhesion molecules that regulates stromal/hematopoietic cell interaction leading to PP formation.

Chemokines and the tissue-specific migration of lymphocytes

Tissue-selective trafficking of memory and effector T and B lymphocytes is mediated by unique combinations of adhesion molecules and chemokines. The discovery of several related epithelial-expressed chemokines (TECK/CCL25 in small intestine, CTACK/CCL27 in skin, and MEC/CCL28 in diverse mucosal sites) now highlights an important role for epithelial cells in controlling homeostatic lymphocyte trafficking, including the localization of cutaneous and intestinal memory T cells, and of IgA plasma cells. Constitutively expressed epithelial chemokines may help determine the character of local immune responses and contribute to the systemic organization of the immune system.