For whom the disc tolls: intervertebral disc degeneration, back pain and toll-like receptors

Intervertebral disc (IVD) degeneration is characterised by catabolic and inflammatory processes that contribute largely to tissue degradation and chronic back pain. The disc cells are responsible for the pathological production of pro-inflammatory cytokines and catabolic enzymes leading to degeneration. However, this phenotypical change is poorly understood. Growing evidence in animal and human studies implicates Toll-like receptors (TLR) and their activation through danger-associated alarmins, found increasingly in degenerating IVDs. TLR signalling results in the release of pro-inflammatory cytokines and proteolytic enzymes that can directly cause IVD degeneration and back pain. This review aims to summarise the current literature on TLR activation in IVD degeneration and discuss potential treatment modalities to alleviate the inflammatory phenotype of disc cells in order to arrest IVD degeneration and back pain.

Sequential targeting of interferon pathways for increased host resistance to bacterial superinfection during influenza

Bacterial co-infections represent a major clinical complication of influenza. Host-derived interferon (IFN) increases susceptibility to bacterial infections following influenza, but the relative roles of type-I versus type-II IFN remain poorly understood. We have used novel mouse models of co-infection in which colonizing pneumococci were inoculated into the upper respiratory tract; subsequent sublethal influenza virus infection caused the bacteria to enter the lungs and mediate lethal disease. Compared to wild-type mice or mice deficient in only one pathway, mice lacking both IFN pathways demonstrated the least amount of lung tissue damage and mortality following pneumococcal-influenza virus superinfection. Therapeutic neutralization of both type-I and type-II IFN pathways similarly provided optimal protection to co-infected wild-type mice. The most effective treatment regimen was staggered neutralization of the type-I IFN pathway early during co-infection combined with later neutralization of type-II IFN, which was consistent with the expression and reported activities of these IFNs during superinfection. These results are the first to directly compare the activities of type-I and type-II IFN during superinfection and provide new insights into potential host-directed targets for treatment of secondary bacterial infections during influenza.

Bacterial co-infections represent a common and challenging clinical complication of influenza. Type-I and type-II interferon (IFN) pathways enhance susceptibility to influenza-pneumococcal co-infection, leading to increased lung pathology and mortality. However, the comparative importance of type-I versus type-II IFN remains unclear. We have used two novel mouse models of co-infection in which pneumococci were inoculated into the upper respiratory tract followed two days later by influenza virus infection. Virus co-infection caused IFN-dependent inflammation that facilitated spreading of the colonizing bacteria into the lungs, followed by tissue damage and death. In this pneumococcal-influenza virus superinfection model, mice lacking both type-I and type-II IFN pathways demonstrated minimal lung pathology and increased survival compared to wild-type mice and mice deficient in only one pathway. Therapeutic neutralization of both type-I and type-II IFN pathways similarly provided optimal protection to superinfected wild-type mice. The most effective treatment regimen involved neutralization of the type-I IFN pathway early during co-infection combined with later neutralization of the type-II IFN pathway. These results provide new insights into potential host-directed therapy for management of bacterial-viral superinfections.

Defective anti-polysaccharide IgG vaccine responses in IgA deficient mice

We report that IgA^-/- mice exhibit specific defects in IgG antibody responses to various polysaccharide vaccines (Francisella tularensis LPS and Pneumovax), but not protein vaccines such as Fluzone. This defect further included responses to polysaccharide-protein conjugate vaccines (Prevnar and Haemophilus influenzae type b-tetanus toxoid vaccine). In agreement with these findings, IgA^-/- mice were protected from pathogen challenge with protein- but not polysaccharide-based vaccines. Interestingly, after immunization with live bacteria, IgA^+/+ and IgA^-/- mice were both resistant to lethal challenge and their IgG anti-polysaccharide antibody responses were comparable. Immunization with live bacteria, but not purified polysaccharide, induced production of serum B cell-activating factor (BAFF), a cytokine important for IgG class switching; supplementing IgA^-/- cell cultures with BAFF enhanced in vitro polyclonal IgG production. Taken together, these findings show that IgA deficiency impairs IgG class switching following vaccination with polysaccharide antigens and that live bacterial immunization can overcome this defect. Since IgA deficient patients also often show defects in antibody responses following immunization with polysaccharide vaccines, our findings could have relevance to the clinical management of this population.

T cell-dependent IgM memory B cells generated during bacterial infection are required for IgG responses to antigen challenge

Immunological memory has long considered to be harbored in B cells that express high-affinity class-switched IgG. IgM-positive memory B cells can also be generated following immunization, although their physiological role has been unclear. In this study, we show that bacterial infection elicited a relatively large population of IgM memory B cells that were uniquely identified by their surface expression of CD11c, CD73, and programmed death-ligand 2. The cells lacked expression of cell surface markers typically expressed by germinal center B cells, were CD138 negative, and did not secrete Ab ex vivo. The population was also largely quiescent and accumulated somatic mutations. The IgM memory B cells were located in the region of the splenic marginal zone and were not detected in blood or other secondary lymphoid organs. Generation of the memory cells was CD4 T cell dependent and required IL-21R signaling. In vivo depletion of the IgM memory B cells abrogated the IgG recall responses to specific Ag challenge, demonstrating that the cell population was required for humoral memory, and underwent class-switch recombination following Ag encounter. Our findings demonstrate that T cell-dependent IgM memory B cells can be elicited at high frequency and can play an important role in maintaining long-term immunity during bacterial infection.

Influence of environmental enrichment and depleted uranium on behaviour, cholesterol and acetylcholine in apolipoprotein E-deficient mice

Alzheimer's disease is associated with genetic risk factors, of which the apolipoprotein E (ApoE) is the most prevalent, and is affected by environmental factors that include education early in life and exposure to metals. The industrial and military use of depleted uranium (DU) resulted in an increase of its deposition in some areas and led to a possible environmental factor. The present study aims to ascertain the effects on the behaviour and the metabolism of cholesterol and acetylcholine of ApoE-/- mice exposed to enriched environment (EE) and exposed to DU (20 mg/L) for 14 weeks. Here we show that ApoE-/- mice were unaffected by the EE and their learning and memory were similar to those of the non-enriched ApoE-/- mice. ApoE-/- mice showed a significant decrease in total (-16 %) and free (-16 %) cholesterol in the entorhinal cortex in comparison to control wild-type mice. Whatever the housing conditions, the exposure to DU of ApoE-/- mice impaired working memory, but had no effect on anxiety-like behaviour, in comparison to control ApoE-/- mice. The exposure of ApoE-/- mice to DU also induced a trend toward higher total cholesterol content in the cerebral cortex (+15 %) compared to control ApoE-/- mice. In conclusion, these results demonstrate that enriched environment does not ameliorate neurobehaviour in ApoE-/- mice and that ApoE mutation induced specific effects on the brain cholesterol. These findings also suggested that DU exposure could modify the pathology in this ApoE model, with no influence of housing conditions.

The omentum is a site of IgM production during T cell-independent bacterial infection in secondary lymphoid organ-deficient mice (43.6)

Infection of mice with the bacterium Ehrlichia muris results in a T cell-independent (TI) splenic IgM response, and this antibody is secreted largely by a population of CD11c-expressing B cell plasmablasts in the spleen. However, we found that the TI IgM is maintained in the absence of a spleen; infection of splenectomized mice resulted in a modest decrease in antigen-specific serum IgM, indicating that splenic B cells are not required for the generation of the protective antibody response. To identify the source of the IgM-secreting cells, we infected secondary lymphoid organ (SLO)-deficient mice, which lack both spleen and lymph nodes. SLO-deficient mice were similarly able to produce ehrlichia-specific IgM, and both splenectomized and SLO-deficient mice were protected against challenge infection with a lethal ehrlichial species. These findings led us to investigate the omentum as a source of IgM-secreting cells during infection. We identified the omentum as a large reservoir of antigen-specific IgM-secreting cells; these B cells expressed CD11c, but lacked expression of CD138. CD11c-expressing B cells were elicited by intraperitoneal, but not intravenous infection, revealing a route-dependent response to peritoneal antigen. Our data reveal the omentum as an important site of TI IgM production against peritoneal antigens, in the absence of conventional secondary lymphoid organs.

IgM memory B cells generated during bacterial infection are required for secondary IgG responses to antigenic challenge (43.2)

Immunological memory was long considered to be harbored in B cells that express high affinity class-switched IgG. More recently, IgM memory B cells have been identified following immunization with nominal antigens and adjuvants. Based on the expression of CD11c, we have identified a large population of such cells using a natural model of infection by the bacterium Ehrlichia muris. These CD11c+ IgM memory cells exhibit phenotypic characteristics of memory B cells, including expression of CD73, CD11b, and PD-L2. In addition, these CD11c+ IgM memory cells lack expression of CD138, are largely quiescent, and have accumulated somatic mutations. Although these cells did not proliferate or secrete antibody ex vivo, they produced antigen-specific IgM upon in vitro stimulation with mitogens. Furthermore, in vivo depletion abrogated the IgG recall response to specific antigenic challenge. Our findings are consistent with previous data demonstrating that IgM memory cells undergo class switch recombination and affinity maturation following re-encounter with cognate antigen. We propose that these CD11c+ IgM memory B cells are responsible for the IgG produced following secondary challenge in ehrlichial infection, and likely function to provide greater flexibility to variant antigenic challenges.

BAFF neutralization blocks T cell-independent IgM production (70.3)

Infection of mice with the intracellular bacterium Ehrlichia muris induces a protective T cell-independent (TI) IgM response. The TI IgM produced during acute infection is secreted almost exclusively by a population of CD11c-expressing B cell plasmablasts in the spleen. We investigated the role of B cell activating factor of the TNF family (BAFF) on the generation of the IgM-secreting plasmablasts. To this end, we neutralized BAFF in vivo using a monoclonal antibody directed against murine BAFF. Blockade of BAFF in infected mice did not affect the generation of splenic plasmablasts; however, treatment nearly eliminated the number of antigen-specific IgM-secreting cells, serum IgM titers were reduced, and the plasmablasts failed to express cell-surface CD138. Analysis of mRNA expression in FACS-sorted plasmablasts from anti-BAFF and isotype control-treated mice revealed no differences in transcript levels of prdm1 and xbp1, genes encoding the transcription factors Blimp-1 and XBP-1, which are essential for plasma cell differentiation. Thus, our data indicate that BAFF signaling regulates a checkpoint in plasmablast differentiation, downstream of Blimp-1 and XBP-1 transcription, but prior to CD138 expression.

Influence of depleted uranium on hepatic cholesterol metabolism in apolipoprotein E-deficient mice

Depleted uranium (DU) is uranium with a lower content of the fissile isotope U-235 than natural uranium. It is a radioelement and a waste product from the enrichment process of natural uranium. Because of its very high density, it is used in the civil industry and for military purposes. DU exposure can affect many vital systems in the human body, because in addition to being weakly radioactive, uranium is a toxic metal. It should be emphasized that, to be exposed to radiation from DU, you have to eat, drink, or breathe it, or get it on your skin. This particular study is focusing on the health effects of DU for the cholesterol metabolism. Previous studies on the same issue have shown that the cholesterol metabolism was modulated at molecular level in the liver of laboratory rodents contaminated for nine months with DU. However, this modulation was not correlated with some effects at organs or body levels. It was therefore decided to use a "pathological model" such as hypercholesterolemic apolipoprotein E-deficient laboratory mice in order to try to clarify the situation. The purpose of the present study is to assess the effects of a chronic ingestion (during 3 months) of a low level DU-supplemented water (20 mg L(-1)) on the above mentioned mice in order to determine a possible contamination effect. Afterwards the cholesterol metabolism was studied in the liver especially focused on the gene expressions of cholesterol-catabolising enzymes (CYP7A1, CYP27A1 and CYP7B1), as well as those of associated nuclear receptors (LXRα, FXR, PPARα, and SREBP 2). In addition, mRNA levels of other enzymes of interest were measured (ACAT 2, as well as HMGCoA Reductase and HMGCoA Synthase). The gene expression study was completed with SRB1 and LDLr, apolipoproteins A1 and B and membrane transporters ABC A1, ABC G5. The major effect induced by a low level of DU contamination in apo-E deficient mice was a decrease in hepatic gene expression of the enzyme CYP7B1 (-23%) and nuclear receptors LXRα (-24%), RXR (-32%), HNF4α (-21%) when compared to unexposed ones. These modifications on cholesterol metabolism did not lead to increased disturbances that are specific for apolipoprotein E-deficient mice, suggesting that chronic DU exposure did not worsen the pathology in this experimental model. In conclusion, the results of this study indicate that even for a sensitive pathologic model the exposure to a low dose of DU has no relevant impact. The results confirm the results of our first study carried out on healthy laboratory rodents where a sub-chronic contamination with low dose DU did not affect in vivo the metabolism of cholesterol.

Identification of a putative IgM memory B cell population during bacterial infection (99.31)

Humoral immunity and B cell memory are essential components of the adaptive immune response. These elements of immunity have remained largely unexplored in intracellular bacterial infections. Ehrlichia muris is an obligate intracellular bacterium that generates a chronic infection in immunocompetent mice. Chronic infection with E. muris is characterized by long-term IgM production that confers antibody-mediated protection against virulent ehrlichial challenge. We have identified a novel CD19+ B cell population in the spleens of chronically infected mice based on the expression of the cell surface markers CD11c, CD73, and PD-L2. As determined by fluorescence microscopy, these cells are localized in germinal centers of the spleen. Upon stimulation with LPS in vitro, the CD11c+/CD73+/PD-L2+ B cells proliferated and produced antigen specific IgM. BrdU incorporation studies revealed that the population is largely quiescent, and new cells were not recruited to this population during chronic infection. We propose that the CD11c+/CD73+/PD-L2+ B cells we have identified during chronic infection, are long-lived effector/memory cells responsible for the maintenance of long-term immunity during chronic ehrlichial infection.

IgM production by bone marrow plasmablasts contributes to long-term protection against intracellular bacterial infection

IgM responses are well known to occur early postinfection and tend to be short-lived, which has suggested that this Ig does not significantly contribute to long-term immunity. In this study, we demonstrate that chronic infection with the intracellular bacterium Ehrlichia muris elicits a protective, long-term IgM response. Moreover, we identified a population of CD138(high)IgM(high) B cells responsible for Ag-specific IgM production in the bone marrow. The IgM-secreting cells, which exhibited characteristics of both plasmablasts and plasma cells, contributed to protection against fatal ehrlichial challenge. Mice deficient in activation-induced cytidine deaminase, which produce only IgM, were protected against fatal ehrlichial challenge infection. The IgM-secreting cells that we have identified were maintained in the bone marrow in the absence of chronic infection, as antibiotic-treated mice remained protected against challenge infection. Our studies identify a cell population that is responsible for the IgM production in the bone marrow, and they highlight a novel role for IgM in the maintenance of long-term immunity during intracellular bacterial infection.

In mice, tuberculosis progression is associated with intensive inflammatory response and the accumulation of Gr-1 cells in the lungs

Background

Infection with Mycobacterium tuberculosis (Mtb) results in different clinical outcomes ranging from asymptomatic containment to rapidly progressing tuberculosis (TB). The mechanisms controlling TB progression in immunologically-competent hosts remain unclear.

Methodology/Principal Findings

To address these mechanisms, we analyzed TB progression in a panel of genetically heterogeneous (A/SnxI/St) F2 mice, originating from TB-highly-susceptible I/St and more resistant A/Sn mice. In F2 mice the rates of TB progression differed. In mice that did not reach terminal stage of infection, TB progression did not correlate with lung Mtb loads. Nor was TB progression correlated with lung expression of factors involved in antibacterial immunity, such as iNOS, IFN-γ, or IL-12p40. The major characteristics of progressing TB was high lung expression of the inflammation-related factors IL-1β, IL-6, IL-11 (p<0.0003); CCL3, CCL4, CXCL2 (p<0.002); MMP-8 (p<0.0001). The major predictors of TB progression were high expressions of IL-1β and IL-11. TNF-α had both protective and harmful effects. Factors associated with TB progression were expressed mainly by macrophages (F4-80⁺ cells) and granulocytes (Gr-1^hi/Ly-6G^hi cells). Macrophages and granulocytes from I/St and A/Sn parental strains exhibited intrinsic differences in the expression of inflammatory factors, suggesting that genetically determined peculiarities of phagocytes transcriptional response could account for the peculiarities of gene expression in the infected lungs. Another characteristic feature of progressing TB was the accumulation in the infected lungs of Gr-1^dim cells that could contribute to TB progression.

Conclusions/Significance

In a population of immunocompetent hosts, the outcome of TB depends on quantitatively- and genetically-controlled differences in the intensity of inflammatory responses, rather than being a direct consequence of mycobacterial colonization. Local accumulation of Gr-1^dim cells is a newly identified feature of progressing TB. High expression of IL-1β and IL-11 are potential risk factors for TB progression and possible targets for TB immunomodulation.

Cholesterol 7alpha-hydroxylase (CYP7A1) activity is modified after chronic ingestion of depleted uranium in the rat

Depleted uranium (DU) is a radioactive heavy metal derived from the nuclear energy production. Its wide use in civilian and military items increases the risk of its environmental dissemination, and thus the risk of internal contamination of populations living in such contaminated territories. Previous studies have shown that vitamin D and cerebral cholesterol metabolisms were affected following chronic ingestion of DU. Even more than the brain, the liver is a crucial organ in cholesterol homeostasis since it regulates cholesterol distribution and elimination at body level. The aim of this work was to assess the impact of a low-level chronic ingestion of DU on hepatic cholesterol metabolism. Rats were contaminated with DU in their drinking water at a concentration of 40mg/l for 9 months. The major effect induced by DU was a decrease of CYP7A1 specific activity (-60%) correlated with a matching decrease of its product 7alpha-hydroxycholesterol in the plasma. Hepatic gene expression of transporters ABC A1, ABC G5, ABC G8 and of nuclear receptor RXR was increased, whereas that of catabolism enzyme CYP7B1 was decreased. Thus, after a chronic ingestion of DU, rats experience a modulation of cholesterol catabolism but overcome it, since their cholesterolemia is preserved and no pathology is declared.

Impaired germinal center responses and suppression of local IgG production during intracellular bacterial infection

Germinal centers (GCs) are specialized microenvironments in secondary lymphoid organs that facilitate the development of high-affinity, isotype-switched Abs, and immunological memory; consequently, many infections require GC-derived IgG for pathogen clearance. Although Ehrlichia muris infection elicits a robust expansion of splenic, IgM-secreting plasmablasts, we detected only very low frequencies of isotype-switched IgG-secreting cells in mouse spleens, until at least 3 wk postinfection. Instead, Ag-specific IgG was produced in lymph nodes, where it required CD4 T cell help. Consistent with these findings, organized GCs and phenotypically defined splenic GC B cells were found in lymph nodes, but not spleens. Ehrlichial infection also inhibited spleen IgG responses against a coadministered T cell-dependent Ag, hapten 4-hydroxy-3-nitrophenyl acetyl (NP)-conjugated chicken gamma globulin in alum. NP-specific B cells failed to undergo expansion and differentiation into GC B cells in the spleen, Ab titers were reduced, and splenic IgG production was inhibited nearly 10-fold when the Ag was administered during infection. Our data provide a mechanism whereby an intracellular bacterial infection can compromise local immunity to coinfecting pathogens or antigenic challenge.

In vitro study of the interaction between neptunium ions and aposerumtransferrin by absorption spectrophotometry and ultrafiltration: the case of Np(V)

The interaction between Np(V) and human aposerumtransferrin (apoTf) was studied in vitro under physiological conditions (37°C, pH 7.4, [NaCl]=0.15 M, [HEPES]=5×10-2 M) by UV-visible and near-IR absorption spectrophotometry and by ultrafiltration. It was found that Np(V) was bound in small fraction (<12%) to apoTf, and that the role of carbonate and citrate anions in binding is more competitive than synergistic. The complexes NpO2CO3 - and NpO2Cit2- tend to form rather than the Np(V)-apoTf complex. Np(V) binding on apoTf appears to be pH-dependent and reversible. Displacement experiments with FeIIINTA showed non-specific binding, suggesting a weak interaction between Np(V) and apoTf different from the reactions involving transferrin-specific sites with other metallic ions, such as Fe(III). The low overall charge, the size and the geometry of the linear di-oxocation NpO2 + could account for the weak interaction between Np(V) and human transferrin.

IgM in microbial infections: taken for granted?

Much has been learned about the structure, function, and production of IgM, since the antibody's initial characterization. It is widely accepted that IgM provides a first line of defense during microbial infections, prior to the generation of adaptive, high-affinity IgG responses that are important for long-lived immunity and immunological memory. Although IgM responses are commonly used as a measure of exposure to infectious diseases, it is perhaps surprising that the role of and requirement for IgM in many microbial infections has not been well explored in vivo. This is in part due to the lack of capabilities, until relatively recently, to evaluate the requirement for IgM in the absence of coincident IgG responses. Such evaluations are now possible, using gene-targeted mouse strains that produce only IgM, or isotype-switched IgG. A number of studies have revealed that IgM, produced either innately, or in response to antigen challenge, plays an important and perhaps under appreciated role in many microbial infections. Moreover, the characterization of the roles of various B cell subsets, in the production of IgM, and in host defense, has revealed important and divergent roles for B-1a and B-1b cells. This review will highlight studies in which IgM, in its own right, has been found to play an important role, not only in early immunity, but also in long-term protection, against a variety of microbial pathogens. Observations that long-lived IgM responses can be generated in vivo suggest that it may be feasible to target IgM production as part of vaccination strategies.

Interferon gamma-dependent alterations in the hematopoietic stem cell pool during infection (87.4)

Infection with Ehrlichia muris induces blood cytopenias and a decrease in bone marrow (BM) cellularity and function. Here we addressed whether infection altered hematopoietic stem cells (HSCs) and progenitor cells. We observed changes in the frequency of BM HSCs by day 8 post-infection; lineage-negative (Lin-), c-Kit-, Sca-1+ progenitor cells were absent, and the Lin- population was composed entirely of c-Kit+, Sca-1+ cells (LSK). In addition, common myeloid progenitors (Lin-, Sca-1-, c-Kit+, CD127-) were diminished and common lymphoid progenitors (LSK, CD127+) were increased. Competitive reconstitution experiments revealed that LSK cells isolated from infected mice gave rise to predominantly B lymphocytes, suggesting that infection favored lymphopoiesis. We next stained Lin- cells for expression of other markers for HSCs (CD150, CD48, and CD244) which revealed that the LSK population were likely true long-term reconstituting HSCs (CD150+, CD48-, CD244-) and multipotential progenitors (CD150-, CD244+, CD48-). We examined several knockout strains in our infection model and found that interferon gamma (IFNg)-deficient mice exhibited reduced CLPs after infection, suggesting that IFNg contributes to alterations in the HSC pool. Current research is aimed at understanding the specific requirement of IFNg in processes that govern HSC self-renewal, differentiation, and mobilization.

Protective CD4 T cell-independent IgM responses impairs the production of isotype-switched antibodies during intracellular bacterial infection (133.14)

Ehrlichia muris, an intracellular tick-borne pathogen, generates a CD4 T cell independent (TI) protective response in C57BL/6 mice. At peak infection, the response is accompanied by the expansion splenic extrafollicular CD11clo-expressing plasmablasts that produce antigen-specific IgM. Using secretory IgM (sIgM)-deficient mice and activation-induced adenosine deaminase (AID)-deficient mice, which are unable to undergo antibody class-switching, we demonstrated that IgG is dispensable for immunity and unmutated IgM was sufficient for protection to challenge infection with a highly virulent ehrlichia. Furthermore, while IgM-secreting cells were abundant in the spleen, prior to day 21, IgG was largely absent and instead produced in infected LNs. Thus, the robust IgM response impaired the formation of germinal centers and production IgG-secreting cells in the spleen. These findings, in turn, suggested that ehrlichial infection might suppress the spleen's ability to produce isotype-switched antibodies to blood borne antigens or pathogens. Indeed, E. muris infected-mice immunized with NP-CGG exhibited an impaired NP-specific IgG response. Thus, our studies demonstrate a major protective role for CD4 T cell-independent IgM during intracellular bacterial infection, and suggest a mechanism whereby a T cell-independent response may suppress T-dependent responses during co-infecting pathogens.

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.

T-Cell-independent humoral immunity is sufficient for protection against fatal intracellular ehrlichia infection

Although humoral immunity has been shown to contribute to host defense during intracellular bacterial infections, its role has generally been ancillary. Instead, CD4 T cells are often considered to play the dominant role in protective immunity via their production of type I cytokines. Our studies of highly pathogenic Ehrlichia bacteria isolated from Ixodes ovatus (IOE) reveal, however, that this paradigm is not always correct. Immunity to IOE infection can be induced by infection with a closely related weakly pathogenic ehrlichia, Ehrlichia muris. Type I cytokines (i.e., gamma interferon, tumor necrosis factor alpha, and interleukin-12) were not necessary for E. muris-induced immunity. In contrast, humoral immunity was essential, as shown by the fact that E. muris-infected B-cell-deficient mice were not protected from IOE challenge and because E. muris immunization was effective in CD4-, CD8-, and major histocompatibility complex (MHC) class II-deficient mice. Immunity was unlikely due to nonspecific inflammation, as prior infection with Listeria monocytogenes did not induce immunity to IOE. Antisera from both wild-type and MHC-II-deficient mice provided at least partial resistance to challenge infection, and protection could also be achieved following transfer of total, but not B-cell-depleted, splenocytes obtained from E. muris-immunized mice. The titers of class-switched antibodies in immunized CD4 T-cell- and MHC class II-deficient mice, although lower than those observed in immunized wild-type mice, were significant, indicating that E. muris can induce class switch recombination in the absence of classical T-cell-mediated help. These studies highlight a major protective role for classical T-cell-independent humoral immunity during an intracellular bacterial infection.

Selective loss of early suppression in the dentate gyrus precedes kainic acid induced electrographic seizures

The role of inhibitory and facilitatory processes in the induction of seizures was studied in a kainic acid (KA) model of epilepsy. The dentate gyrus (DG) response to paired-pulse stimulation of the perforant path (PP) was monitored prior to and immediately following the initial KA induced afterdischarge (AD) in rats chronically prepared with stimulation recording electrodes. The subjects received a 1-h program of stimulation consisting of repeated sequences of pulse pairs at a short (20-30 ms), intermediate (45-90 ms), and long (200-300 ms) interpulse interval (IPIs). The stimulation program was administered both under control conditions and immediately following systemic injection of KA. During the control condition, stable suppression of population spike measures was obtained at the short (early phase) and long (late phase) IPIs, while facilitation was observed at the intermediate IPI. Administration of KA resulted in a progressive loss of suppression prior to the initial AD at the short IPI; neither facilitation nor the late phase of suppression were significantly affected. The early phase decreased further following the initial discharge. Since the early phase most likely reflects recurrent inhibition, these results provide evidence that inhibitory loss precedes the occurrence of KA induced AD, and that this inhibitory loss is increased further following the initial evoked AD. A use-dependent disinhibition is one possible explanation for the change in responsiveness that precedes the AD. This disinhibition could result from a depressed response at GABA-A receptors, an increased responsiveness at GABA-B receptors or possibly both.

Electrical stimulation of the septal region of aged rats improves performance in an open-field maze

Memory deficits of age and disease may result from dysfunction of septohippocampal structures. Electrical brain stimulation might ameliorate these memory deficits. We show here that septal stimulation of very old rats leads to a marked and progressive improvement in performance in an open-field maze task. Unilateral stimulation of the perforant path is less effective. The frequency of stimulation is important: stimulation at 5 Hz and 50 Hz is effective, whereas stimulation at 0.5 Hz is less effective (though still significantly better than control). Hippocampal (dentate) EEG does not change significantly with septal stimulation frequency. These results may bear on the memory deficit of old age in humans. The results may also bear on the memory deficits seen in human disease states such as Alzheimer's disease.

Development of spontaneous seizures over extended electrical kindling. II. Persistence of dentate inhibitory suppression

The effect of an extended program of perforant path or amygdala kindling on paired-pulse suppression in the dentate gyrus was studied in male hooded rats. Repeated kindling stimulations were delivered twice or three times daily until either 300 stimuli had been delivered or generalized convulsions had been observed to occur spontaneously. Paired-pulse suppression was monitored prior to and over the course of kindling using a standard variable interval paradigm. We also used a variable intensity paradigm in which the intensity of the conditioning pulse was varied while the test pulse intensity was fixed at 600 microA and the interpulse interval was fixed at 30 ms. Both procedures revealed progressive increases in paired-pulse suppression which persisted over the course of kindling. This increased inhibition also persisted in animals which developed spontaneous seizures. The variable intensity paired-pulse procedure also allowed us to monitor facilitation effects which were relatively uncontaminated by recurrent inhibition (when the conditioning pulse intensity was low). Kindling was found to increase paired-pulse facilitation. With the standard variable interval paradigm, these increases in facilitation masked the increases in suppression.

Hippocampal stimulation of fornical-lesioned rats improves working memory

Intrinsic rhythmic electrical activity in the brain, such as the hippocampal theta rhythm, might serve important roles in normal cognition. Lesions to the medial septal nuclei, or to the fimbria/fornix, disrupt the hippocampal theta rhythm and lead to memory impairment. We have superimposed an artificial stimulating rhythm to the hippocampus of rats with prior lesion of the fornix, during testing in the Morris water maze. This intervention improves performance in a test of working memory, and lends support to the view that intrinsic rhythmic activity may play an important role in normal physiology, and in certain disease states.

Changes in inhibitory processes in the hippocampus following recurrent seizures induced by systemic administration of kainic acid

Rats were chronically prepared with stimulation electrodes in the angular bundle and recording electrodes in the dentate gyrus under electrophysiological guidance. Following testing of dentate gyrus field potentials, the animals were given a single injection of kainic acid which caused repeated seizures and led to status epilepticus. The seizures were stopped by administration of a barbiturate anesthetic after 60 min. Changes in inhibition during seizure development were monitored by administering pulse pairs at regular intervals. The results revealed a progressive kainic acid-induced loss in inhibition that preceded the occurrence of seizures. This breakdown of inhibition was transient, and generally disappeared within 24 h. Over subsequent testing, recurrent inhibition, as measured by the double pulse test, increased beyond baseline levels. This increase persisted for at least one month and was restricted to the early phase of inhibition with a conditioning/test pulse interval of less than 50 ms. A later phase of inhibition, measured at interpulse intervals between 200 and 300 ms, showed a transient decrease which lasted about a week. These results contrast with previous reports of a long-term period of hyperexcitability following recurrent seizures. Procedural differences which might account for such discrepancies are discussed.

Effects of diazepam on hippocampal excitability in the rat: action in the dentate area

Acute and chronic experiments were performed on rats to examine the effects of diazepam (Valium) on recurrent inhibition in the monosynaptic perforant path-dentate synapse of the dentate area of the dorsal hippocampus. Evidence was obtained which indicated that diazepam facilitated a presumably GABA mediated post synaptic recurrent inhibition in both acute and chronic preparations at 1 and 2 mg/kg doses (IP). Acute studies also revealed that diazepam prolonged recurrent inhibition, possibly by lengthening the IPSP. An effect of the drug on cholinergically mediated positive feedback from the septum could not be ruled out, however. Taken together, this study extends the evidence that diazepam acts centrally on GABA mediated inhibition in the limbic system. Furthermore, the limbic action of diazepam revealed here suggests a mechanism for the anticonvulsant properties of diazepam in epilepsy involving subcortical (limbic) circuitry.

The effect of procaine hydrochloride and diazepam, separately or in combination, on cortico-generalized kindled seizures

The effects of cortically kindled seizure responses of procaine hydrochloride, diazepam and combinations of these two drugs were tested in this study. The cortex was stimulated until seizure responses developed past the focal stage (accompanied primarily by brief tonic convulsions) to the cortico-generalized stage (accompanied, typically, by an early brief tonus followed by a longer clonic seizure that is characteristic of subcortically triggered seizures). Diazepam was found to block the generalized component of the cortico-generalized electrographic and motor seizure leaving the tonus only slightly suppressed. Procaine blocked the tonus leaving the clonic seizure and discharge that is characteristic of the generalized response relatively intact. Combinations of half doses of the two drugs completely blocked all electrographic and motor seizure responses in about half the animals. The remaining animals had a very brief discharge with no convulsive responses.

The effects of atropine and reserpine on cortical kindling in the rat

The effects on cortical kindling of atropine (a muscarinic, cholinergic blocking agent) and reserpine (a depleter of catecholamines and 5 hydroxytryptamine) were tested in this study. Atropine, which had previously been found to retard amygdaloid kindling, had similar but somewhat weaker effects on cortical kindling. Reserpine also had similar effects on cortical kindling compared to subcortical kindling in that it potentiated seizure responses.

Effects of midbrain raphe lesions or systemic p-chlorophenylalanine on the development of kindled seizures in rats

Previous research has suggested that brain serotonin (5-hydroxytryptamine or 5-HT) neurons inhibit epileptiform seizure activity. To test further this possibility, experiments were performed to determine if brain 5-HT depletion would enhance the occurrence and/or magnitude of seizures "kindled" from the amygdala or neocortex of rats. Two modes of 5-HT depletion were used: (1) radiofrequency heat lesions of the midbrain dorsal and median raphe nuclei, and (2) systemic injection of the 5-HT synthesis inhibitor, p-chlorophenylalanine (pCPA). Both modes of 5-HT depletion reliably enhanced the strength of motor convulsions kindled from the cortex. Systemic pCPA also reduced the duration of after-discharges (ADs) in cortically-stimulated rats. However, pCPA reduced rather than enhanced convulsions kindled from the amygdala. In contrast to this, raphe lesions appeared to sensitize rats to the effects of amygdaloid kindling, i.e., lesions lowered AD thresholds, AD durations and number of ADs to elicit motor convulsions. Viewed together, these data support the hypothesis that 5-HT neurons can serve to inhibit seizures. However, the lack of robustness across parameters of epileptogenesis as well as discrepant findings related to 5-HT depletion mode additionally suggest that kindled seizures affect other neuronal populations in addition to those under serotonergic influence.

Kindling: the first decade

The kindling phenomenon is a progressive increase in the strength of epileptiform activity evoked by spaced (in time) and repeated electrical stimulation of certain brain structures. The work that has been done on the kindling phenomenon is reviewed, with an emphasis on those studies that deal with underlying mechanisms. Based on the work that has been done thus far, it is clear that the kindling effect is not due to any type of gross tissue damage. It is also clear that at least some of the effects are due to changes at the synapse and that these changes are widely distributed in the brain. The changes might be due to an increasing efficacy at excitatory synapses or a decreased effectiveness at inhibitory synapses, or both. The long term post-tetanic potentiation data and some preliminary electron microscopic studies support the former mechanism, whereas the depletions of catecholamines in kindled tissue support the latter. In addition to these transynaptic changes, there may be other changes that occur at the site of the stimulating electrode, and these changes may be based on a different mechanism. These ideas and the relevant data are discussed.

Afterdischarge thresholds and kindling rates in dorsal and ventral hippocampus and dentate gyrus

Electrodes were implanted to dorsal hippocampus (CA1), ventral CA1, DOrsal dentate gyrus or ventral dentate gyrus. Epileptiform afterdischarge (AD) thresholds were lower in dorsal areas than in ventral areas. Dorsal areas, however, required a greater number of stimulations to develop ("kindle") a fully generalized convulsion than did ventral areas. Thresholds and kindling rates in the dentate gyrus were intermediate between dorsal and ventral CA1, except for the ventral dentate which had higher AD thresholds than ventral CA1. Secondary sites within the hippocampus subsequently kindled within a few stimulations following completion of kindling in the primary site, regardless of which hippocampal area served as the primary site.

Kindling, unit discharge patterns and neural plasticity

Two approaches to the study of the kindling phenomenon were discussed: 1) an attempt to identify the pattern of neural activity required to produce the changes underlying kindling and 2) an investigation into the nature of those changes. Three experiments were reported that used the neocortical transcallosal system as a monosynaptic model system in which to study possible synaptic mechanisms of the kindling effect. Experiment I showed an increase in the transcallosal evoked potential following neocortical kindling. Experiment II showed an increase in the strength of the transcallosal evoked cell discharge following neocortical kindling. Experiment III reported the results of an histological examination of neocortical tissue in kindled and non-kindled animals using the Golgi-Cox technique. Spine density, spine dimension and branching were measured for pyramidal cell apical dendrites. No differences were found between primary and secondary (contralateral) foci or between kindled and non-kindled animals.

Post-activation potentiation and the kindling phenomenon

Potentials evoked in the hippocampus and preoptic region of rats by single biphasic pulses applied to the amygdala were compared during recruiting, after post-tetanic potentiation (PTP) and after amygdaloid kindling. The same components were enhanced temporarily by recruiting and PTP as were enhanced permanently by kindling. Trains of tetanic stimulation with parameters which partially mimicked the cellular discharge parameters during an amygdaloid afterdischarge (AD) were applied to the amygdala at a frequency of 1 per 5 sec continuously for 2.5 h or for 15 min a day for 10 days to produce a total of 1800 trains. Amygdaloid kindling rates were then measured and compared with control groups. Animals pretreated with tetanic stimulation required significantly fewer ADs to develop maximal seizures. Further experiments showed that tetanic stimulation, but not recruiting stimulation (10 c/sec), low frequency stimulation (1 c/sec), or handling, would produce a permanent change in potentials evoked in secondary sites by single pulses applied to the amygdala. This change in evoked potential amplitude was significant but smaller than that produced by kindling. Also tetanic stimulation, but not recruiting or single pulses, facilitated subsequent kindling.

Effects of procaine hydrochloride, diazepam, and diphenylhydantoin on seizure development in cortical and subcortical structures in rats

Procaine HCl and diphenylhydantoin (DPH) increased the duration and propagation of epileptiform afterdischarges (ADs) produced by electrical stimulation of the amygdala in rats. Procaine and DPH also increased the rate of seizure development (kindling) produced by repeated stimulation of the amygdala. Procaine and to a limited extentDPH would themselves act as convulsants in well kindled subjects. Diazepam, on the other hand, retarded or blocked amygdaloid kindling. Diazepam trigered a high frequency (20-30 c/sec) rhtthm in the amygdala, hippocampus and preoptic area. None of these drugs had any significant effect on potentials evoked in secondary limbic sites by single electrical pulses applied to the amygdala. Also, none of these drugs had any effect on recruiting or post-tetanic potentiation (PTP) in secondary sites produced by amygdala stimulation and none of the drugs had any effect on amygdaloid AD thresholds. The effects of these drugs on the responses evoked by anterior neocortex stimulation were quite different. Diazepam had no effect on any of the characteristics of the discharge or convulsion even at twice the dose levels used for the amygdala group. Procaine and DPH, however, blocked not only the eonvulsion but the AD as well. Eighty percent of the procaine- and DPH-treated rats failed to respond with neocortical AD even at current levels as high as 2000 muA. The few cortically stimulated subjects that did respond with an AD showed a subcortical rather than a neocortical seizure response. DPH had no effect on recruiting or PTP of the transcallosal response. Both procaine and DPH produced a weak but significant increase in the amplitude of the transcallosal evoked potential, while diazepam produce a weak decrement in that response.