T-bet+ B cells Dominate the Peritoneal Cavity B Cell Response during Murine Intracellular Bacterial Infection

T-bet⁺ B cells have emerged as a major B cell subset associated with both protective immunity and immunopathogenesis. T-bet is a transcription factor associated with the type I adaptive immune response to intracellular pathogens, driving an effector program characterized by the production of IFN-γ. Murine infection with the intracellular bacterium, Ehrlichia muris, generates protective extrafollicular T cell-independent T-bet⁺ IgM-secreting plasmablasts, as well as T-bet⁺ IgM memory cells. Although T-bet is a signature transcription factor for this subset, it is dispensable for splenic CD11c⁺ memory B cell development, but not for class switching to IgG2c. In addition to the T-bet⁺ plasmablasts found in the spleen, we show that Ab-secreting cells can also be found within the mouse peritoneal cavity; these cells, as well as their CD138^- counterparts, also expressed T-bet. A large fraction of the T-bet⁺ peritoneal B cells detected during early infection were highly proliferative and expressed CXCR3 and CD11b, but, unlike in the spleen, they did not express CD11c. T-bet⁺ CD11b⁺ memory B cells were the dominant B cell population in the peritoneal cavity at 30 d postinfection, and although they expressed high levels of T-bet, they did not require B cell-intrinsic T-bet expression for their generation. Our data uncover a niche for T-bet⁺ B cells within the peritoneal cavity during intracellular bacterial infection, and they identify this site as a reservoir for T-bet⁺ B cell memory.

Adenosine receptor 2a agonists target mouse CD11c+T-bet+ B cells in infection and autoimmunity

CD11c+T-bet+ B cells are recognized as an important component of humoral immunity and autoimmunity. These cells can be distinguished from other B cells by their higher expression of the adenosine receptor 2a. Here we address whether A2A receptor activation can affect CD11c+T-bet+ B cells. We show that administration of the A2A receptor agonist CGS-21680 depletes established CD11c+T-bet+ B cells in ehrlichial-infected mice, in a B cell-intrinsic manner. Agonist treatment similarly depletes CD11c+T-bet+ B cells and CD138+ B cells and reduces anti-nuclear antibodies in lupus-prone mice. Agonist treatment is also associated with reduced kidney pathology and lymphadenopathy. Moreover, A2A receptor stimulation depletes pathogenic lymphocytes and ameliorates disease even after disease onset, highlighting the therapeutic potential of this treatment. This study suggests that targeting the adenosine signaling pathway may provide a method for the treatment of lupus and other autoimmune diseases mediated by T-bet+ B cells.

CD11c+ T-bet+ B Cells Require IL-21 and IFN-γ from Type 1 T Follicular Helper Cells and Intrinsic Bcl-6 Expression but Develop Normally in the Absence of T-bet

CD11c⁺ T-bet⁺ B cells generated during ehrlichial infection require CD4⁺ T cell help and IL-21 signaling for their development, but the exact T cell subset required had not been known. In this study, we show in a mouse model of Ehrlichia muris that type 1 T follicular helper (T_FH1) cells provide help to CD11c⁺ T-bet⁺ B cells via the dual secretion of IL-21 and IFN-γ in a CD40/CD40L-dependent manner. T_FH1 cell help was delivered in two phases: IFN-γ signals were provided early in infection, whereas CD40/CD40L help was provided late in infection. In contrast to T-bet⁺ T cells, T-bet⁺ B cells did not develop in the absence of B cell-intrinsic Bcl-6 but were generated in the absence of T-bet. T-bet-deficient memory B cells were largely indistinguishable from their wild-type counterparts, although they no longer underwent switching to IgG2c. These data suggest that a primary function of T-bet in B cells during ehrlichial infection is to promote appropriate class switching, not lineage specification. Thus, CD11c⁺ memory B cells develop normally without T-bet but require Bcl-6 and specialized help from dual cytokine-producing T_FH1 cells.

T-bet regulates T cell-independent plasmablast responses during intracellular bacterial infection

T-bet+ B cells have emerged as a major B cell subset associated with both protective immunity and autoimmunity. T-bet is considered to be a master transcription factor in type I adaptive immune responses to intracellular pathogens, a response characterized by the production of interferon gamma. Our studies have shown that infection with the intracellular bacterium, E. muris, generates both extrafollicular T cell-independent T-bet+ CD11c+ IgM-secreting plasmablasts (PB), as well as T-bet+ CD11c+ IgM memory cells. Both of these B cell populations play key roles in antigen-specific humoral immunity, although the role of T-bet in their development had not been resolved. Although T-bet is often considered to define lineage in Type I B cells, we found that T-bet was dispensable for memory B cell development. Memory B cells from mice with B cell-specific T-bet deficiency exhibited nearly identical surface marker expression as those of wild-type mice, including CD11c, CD73, PD-L2, CD80, CD38, CD95, and CXCR3. In contrast, T-bet-deficient early CD11c+ splenic PB were significantly reduced in frequency, suggesting that T-bet regulates PB, but not memory cell development or differentiation. As has been reported, antigen-specific IgG2c was no longer the dominant serum antibody isotype in mice lacking B cell T-bet expression. Our data suggest that a critical function of T-bet in B cells is to promote the differentiation of short-lived antibody-secreting PB, and to restrict class switching to protective IgG2c antibodies. Our findings are relevant to how T-bet+ B cells function not only in pathogen-specific immunity, but also in autoimmune diseases, wherein the quality of the humoral response is of critical importance.

T-Bet+ IgM Memory Cells Generate Multi-lineage Effector B Cells

Immunoglobulin M (IgM) memory cells undergo differentiation in germinal centers following antigen challenge, but the full effector cell potential of these cells is unknown. We monitored the differentiation of enhanced yellow fluorescent protein (eYFP)- labeled CD11c⁺ and CD11c^neg T-bet⁺ IgM memory cells after their transfer into naive recipient mice. Following challenge infection, many memory cells differentiated into IgM-producing plasmablasts. Other donor B cells entered germinal centers, down- regulated CD11c, underwent class switch recombination, and became switched memory cells. Yet other donor cells were maintained as IgM memory cells, and these IgM memory cells retained their multi-lineage potential following serial transfer. These findings were corroborated at the molecular level using immune repertoire analyses. Thus, IgM memory cells can differentiate into all effector B cell lineages and undergo self-renewal, properties that are characteristic of stem cells. We propose that these memory cells exist to provide long-term multi-functional immunity and act primarily to maintain the production of protective antibodies.

T-bet⁺ B cells have now been identified in a wide range of immunological contexts. Using a model bacterial infection, Kenderes et al. show that single T-bet⁺ IgM memory cells exhibit multi-lineage potential and can undergo self-renewal, both properties of stem cells.

TNF-α Contributes to Lymphoid Tissue Disorganization and Germinal Center B Cell Suppression during Intracellular Bacterial Infection

Bacterial, parasitic, and viral infections are well-known causes of lymphoid tissue disorganization, although the factors, both host and/or pathogen derived, that mediate these changes are largely unknown. Ehrlichia muris infection in mice causes a loss of germinal center (GC) B cells that is accompanied by the generation of extrafollicular T-bet⁺ CD11c⁺ plasmablasts and IgM memory B cells. We addressed a possible role for TNF-α in this process because this cytokine has been shown to regulate GC development. Ablation of TNF-α during infection resulted in an 8-fold expansion of GL7⁺ CD38^lo CD95⁺ GC B cells, and a 2.5- and 5-fold expansion of CD138⁺ plasmablasts and T-bet⁺ memory cells, respectively. These changes were accompanied by a reduction in splenomegaly, more organized T and B cell zones, and an improved response to Ag challenge. CXCL13, the ligand for CXCR5, was detected at 6-fold higher levels following infection but was much reduced following TNF-α ablation, suggesting that CXCL13 dysregulation also contributes to loss of lymphoid tissue organization. T follicular helper cells, which also underwent expansion in infected TNF-α--deficient mice, may also have contributed to the expansion of T-bet⁺ B cells, as the latter are known to require T cell help. Our findings contrast with previously described roles for TNF-α in GCs and reveal how host-pathogen interactions can induce profound changes in cytokine and chemokine production that can alter lymphoid tissue organization, GC B cell development, and extrafollicular T-bet⁺ B cell generation.

Early derivation of IgM memory cells and bone marrow plasmablasts

IgM memory cells are recognized as an important component of B cell memory in mice and humans. Our studies of B cells elicited in response to ehrlichial infection identified a population of CD11c-positive IgM memory cells, and an IgM bone marrow antibody-secreting cell population. The origin of these cells was unknown, although an early T-independent spleen CD11c- and T-bet-positive IgM plasmablast population precedes both, suggesting a linear relationship. A majority of the IgM memory cells detected after day 30 post-infection, also T-bet-positive, had undergone somatic hypermutation, indicating they expressed activation-induced cytidine deaminase (AID). Therefore, to identify early AID-expressing precursor B cells, we infected an AID-regulated tamoxifen-inducible Cre-recombinase-EYFP reporter strain. Tamoxifen administration led to the labeling of both IgM memory cells and bone marrow ASCs on day 30 and later post-infection. High frequencies of labeled cells were identified on day 30 post-infection, following tamoxifen administration on day 10 post-infection, although IgM memory cells were marked when tamoxifen was administered as early as day 4 post-infection. Transcription of Aicda in the early plasmablasts was not detected in the absence of CD4 T cells, but occurred independently of TLR signaling. Unlike the IgM memory cells, the bone marrow IgM ASCs were elicited independent of T cell help. Moreover, Aicda was constitutively expressed in IgM memory cells, but not in bone marrow ASCs. These studies demonstrate that two distinct long-term IgM-positive B cell populations are generated early in response to infection, but are maintained via separate mechanisms.

Regulation of CD11c+ T-bet+ IgM+ Memory B-cells by TNFα

Our studies of ehrlichial infection have identified a population of CD11c+ T-bet+ IgM memory B-cells that arise within 30 days post-infection. We and others have shown that CD11c+ T-bet+B cells require for their development or function a number of factors, including T-bet, IFNγ, CD4 T-cells, IL-21, and TLRs. Here, we addressed a possible role for TNFα in the generation of CD11c+ T-bet+IgM memory B-cells, because aside from being a major inflammatory cytokine, it can regulate germinal center development, and mediate apoptosis. We observed an 8-fold increase in the number of CD11c+IgM memory B-cells (also CD73+) in TNFα-deficient mice, relative to wild-type mice, on day 16 post-infection. The B cell population persisted at much higher frequency and number on day 30 post-infection. The changes in the IgM memory cell population were associated with a concomitant 10-fold relative increase in the frequency of GL7+ CD38lo CD95+germinal center B cells in TNFα-deficient mice. These latter data contrast with published studies that have reported that TNFα is required for GC development, suggesting that the requirement we have observed for TNFα is context-dependent. These data suggest that there may be a positive relationship between GC development and generation of CD11c+ IgM memory cells, and that TNFα may act to mediate IgM memory cell homeostasis by regulating apoptosis. Ongoing studies will address mechanisms whereby TNFα contributes to the genesis and maintenance of CD11c+ T-bet+IgM memory B-cells.

Identification of secreted and membrane-bound bat immunoglobulin using a Microchiropteran-specific mouse monoclonal antibody

Bat immunity has received increasing attention because some bat species are being decimated by the fungal disease, White Nose Syndrome, while other species are potential reservoirs of zoonotic viruses. Identifying specific immune processes requires new specific tools and reagents. In this study, we describe a new mouse monoclonal antibody (mAb) reactive with Eptesicus fuscus immunoglobulins. The epitope recognized by mAb BT1-4F10 was localized to immunoglobulin light (lambda) chains; hence, the mAb recognized serum immunoglobulins and B lymphocytes. The BT1-4F10 epitope appeared to be restricted to Microchiropteran immunoglobulins and absent from Megachiropteran immunoglobulins. Analyses of sera and other E. fuscus fluids showed that most, if not all, secreted immunoglobulins utilized lambda light chains. Finally, mAb BT1-4F10 permitted the identification of B cell follicles in splenic white pulp. This Microchiropteran-specific mAb has potential utility in seroassays; hence, this reagent may have both basic and practical applications for studying immune process.

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.

MyD88 signaling in CD4 T cells promotes IFN-γ production and hematopoietic progenitor cell expansion in response to intracellular bacterial infection

Hematopoietic stem and progenitor cell (HSPC) phenotype and function can change in response to infectious challenge. These changes can be mediated by cytokines, IFNs, and pathogen-associated molecules, via TLR, and are thought to promote tailored immune responses for particular pathogens. In this study, we investigated the signals that activate HSPCs during ehrlichiosis, a disease characterized by profound hematopoietic dysfunction in both humans and mice. In a mouse model of ehrlichiosis, we observed that infection-induced proliferation of bone marrow HSPCs was dependent on IFN-γ signaling and was partially dependent on MyD88. However, MyD88 was not required in HSPCs for their expansion during infection, because similar frequencies of MyD88-deficient and wild-type HSPCs proliferated in mixed bone marrow chimeric mice. MyD88-deficient mice exhibited low serum and bone marrow concentration of IFN-γ compared with wild-type mice. We next identified CD4 T cells as the primary cells producing IFN-γ in the bone marrow and demonstrated a nonredundant role for CD4-derived IFN-γ in increased HSPCs. Using mixed bone marrow chimeric mice, we identified a requirement for MyD88 in CD4 T cells for increased T-bet expression, optimal IFN-γ production, and CD4 T cell proliferation. Our data demonstrate an essential role for CD4 T cells in mediating HSPC activation in response to bacterial infection and illustrate a novel role for MyD88 signaling in CD4 T cells in this process. These findings further support the idea that IFN-γ production is essential for HSPC activation and hematopoietic responses to infection.

Maintenance of peripheral T cell responses during Mycobacterium tuberculosis infection

Fully functional T cells are necessary for the maintenance of protective immunity during chronic infections. However, activated T cells often undergo apoptosis or exhaustion upon chronic stimulation mediated by Ag or inflammation. T cell attrition can be compensated for by the production of thymus-derived T cells, although the new naive T cells must undergo T cell priming and differentiation under conditions different from those encountered during acute infection. We used a murine model of Mycobacterium tuberculosis infection to address how the activation and differentiation of new thymic emigrants is affected by chronic inflammation, as well as whether the newly developed effector T cells help to maintain peripheral T cell responses. Although new thymic emigrants contributed to the peripheral T cell response early during acute M. tuberculosis infection, the relative contribution of new effector T cells to the peripheral CD4 and CD8 T cell pools declined during chronic infection. The decline in new T cell recruitment was a consequence of quantitative and/or qualitative changes in Ag presentation, because during chronic infection both the priming and expansion of naive T cells were inefficient. Thus, although thymic tolerance is not a major factor that limits protective T cell responses, the chronic environment does not efficiently support naive T cell priming and accumulation during M. tuberculosis infection. These studies support our previous findings that long-term protective T cell responses can be maintained indefinitely in the periphery, but also suggest that the perturbation of homeostasis during chronic inflammatory responses may elicit immune pathology mediated by new T cells.

Antigen-driven induction of polyreactive IgM during intracellular bacterial infection

Polyreactivity is well known as a property of natural IgM produced by B-1 cells. We demonstrate that polyreactive IgM is also generated during infection of mice with Ehrlichia muris, a tick-borne intracellular bacterial pathogen. The polyreactive IgM bound self and foreign Ags, including single-stranded and double-stranded DNA, insulin, thyroglobulin, LPS, influenza virus, and Borrelia burgdorferi. Production of polyreactive IgM during infection was Ag driven, not due to polyclonal B cell activation, as the majority of polyreactive IgM recognized ehrlichial Ag(s), including an immunodominant outer membrane protein. Monoclonal polyreactive IgM derived from T cell-independent spleen plasmablasts, which was germline-encoded, also bound cytoplasmic and nuclear Ags in HEp-2 cells. Polyreactive IgM protected immunocompromised mice against lethal bacterial challenge infection. Serum from human ehrlichiosis patients also contained polyreactive and self-reactive IgM. We propose that polyreactivity increases IgM efficacy during infection but may also exacerbate or mollify the response to foreign and self Ags.

Transient activation of hematopoietic stem and progenitor cells by IFNγ during acute bacterial infection

How hematopoietic stem cells (HSCs) respond to inflammatory signals during infections is not well understood. Our studies have used a murine model of ehrlichiosis, an emerging tick-born disease, to address how infection impacts hematopoietic function. Infection of C57BL/6 mice with the intracellular bacterium, Ehrlichia muris, results in anemia and thrombocytopenia, similar to what is observed in human ehrlichiosis patients. In the mouse, infection promotes myelopoiesis, a process that is critically dependent on interferon gamma (IFNγ) signaling. In the present study, we demonstrate that E. muris infection also drives the transient proliferation and expansion of bone marrow Lin-negative Sca-1⁺ cKit⁺ (LSK) cells, a population of progenitor cells that contains HSCs. Expansion of the LSK population in the bone marrow was associated with a loss of dormant, long-term repopulating HSCs, reduced engraftment, and a bias towards myeloid lineage differentiation within that population. The reduced engraftment and myeloid bias of the infection-induced LSK cells was transient, and was most pronounced on day 8 post-infection. The infection-induced changes were accompanied by an expansion of more differentiated multipotent progenitor cells, and required IFNγ signaling. Thus, in response to inflammatory signals elicited during acute infection, HSCs can undergo a rapid, IFNγ-dependent, transient shift from dormancy to activity, ostensibly, to provide the host with additional or better-armed innate cells for host defense. Similar changes in hematopoietic function likely underlie many different infections of public health importance.

Cutting edge: activation of virus-specific CD4 T cells throughout γ-herpesvirus latency

CD4 T cells are essential for immune control of γ-herpesvirus latency. We previously identified a murine MHC class II-restricted epitope in γ-herpesvirus-68 gp150 (gp150(67-83)I-A(b)) that elicits CD4 T cells that are maintained throughout long-term infection. However, it is unknown whether naive cells can be recruited into the antiviral CD4 T cell pool during latency. In this study, we generate a mouse transgenic for a gp150-specific TCR and show epitope-specific activation of transgenic CD4 T cells during acute and latent infections. Furthermore, although only dendritic cells can stimulate virus-specific CD8 T cells during latency, we show that both dendritic cells and B cells stimulate transgenic CD4 T cells. These studies demonstrate that naive CD4 T cells specific for a viral glycoprotein can be stimulated throughout infection, even during quiescent latency, suggesting that CD4 T cell memory is maintained in part by the continual recruitment of naive cells.

Inflammatory chemokine receptors regulate CD8(+) T cell contraction and memory generation following infection

CD8⁺ T cells lacking CXCR3 and CCR5 expression have impaired contraction and generate an increased number of memory cells after virus infection.

The development of T cell memory from naive precursors is influenced by molecular cues received during T cell activation and differentiation. In this study, we describe a novel role for the chemokine receptors CCR5 and CXCR3 in regulating effector CD8⁺ T cell contraction and memory generation after influenza virus infection. We find that Ccr5^−/− Cxcr3^−/− cells show markedly decreased contraction after viral clearance, leading to the establishment of massive numbers of memory CD8⁺ T cells. Ccr5^−/− Cxcr3^−/− cells show reduced expression of CD69 in the lung during the peak of infection, which coincides with differential localization and the rapid appearance of memory precursor cells. Analysis of single chemokine receptor–deficient cells revealed that CXCR3 is primarily responsible for this phenotype, although there is also a role for CCR5 in the enhancement of T cell memory. The phenotype could be reversed by adding exogenous antigen, resulting in the activation and contraction of Ccr5^−/− Cxcr3^−/− cells. Similar results were observed during chronic Mycobacterium tuberculosis infection. Together, the data support a model of memory CD8⁺ T cell generation in which the chemokine-directed localization of T cells within infected tissues regulates antigen encounter and controls the extent of CD8⁺ T cell activation and differentiation, which ultimately regulates effector versus memory cell fate decisions.

Infection-induced myelopoiesis during intracellular bacterial infection is critically dependent upon IFN-γ signaling

Although microbial infections can alter steady-state hematopoiesis, the mechanisms that drive such changes are not well understood. We addressed a role for IFN-γ signaling in infection-induced bone marrow suppression and anemia in a murine model of human monocytic ehrlichiosis, an emerging tick-borne disease. Within the bone marrow of Ehrlichia muris-infected C57BL/6 mice, we observed a reduction in myeloid progenitor cells, as defined both phenotypically and functionally. Infected mice exhibited a concomitant increase in developing myeloid cells within the bone marrow, an increase in the frequency of circulating monocytes, and an increase in splenic myeloid cells. The infection-induced changes in progenitor cell phenotype were critically dependent on IFN-γ, but not IFN-α, signaling. In mice deficient in the IFN-γ signaling pathway, we observed an increase in myeloid progenitor cells and CDllb(lo)Gr1(lo) promyelocytic cells within the bone marrow, as well as reduced frequencies of mature granulocytes and monocytes. Furthermore, E. muris-infected IFN-γR-deficient mice did not exhibit anemia or an increase in circulating monocytes, and they succumbed to infection. Gene transcription studies revealed that IFN-γR-deficient CDllb(lo)Gr1(lo) promyelocytes from E. muris-infected mice exhibited significantly reduced expression of irf-1 and irf-8, both key transcription factors that regulate the differentiation of granulocytes and monocytes. Finally, using mixed bone marrow chimeric mice, we show that IFN-γ-dependent infection-induced myelopoiesis occurs via the direct effect of the cytokine on developing myeloid cells. We propose that, in addition to its many other known roles, IFN-γ acts to control infection by directly promoting the differentiation of myeloid cells that contribute to host defense.

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.

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.

Mycobacterium tuberculosis infection induces il12rb1 splicing to generate a novel IL-12Rbeta1 isoform that enhances DC migration

RNA splicing is an increasingly recognized regulator of immunity. Here, we demonstrate that after Mycobacterium tuberculosis infection (mRNA) il12rb1 is spliced by dendritic cells (DCs) to form an alternative (mRNA) il12rb1Δtm that encodes the protein IL-12Rβ1ΔTM. Compared with IL-12Rβ1, IL-12Rβ1ΔTM contains an altered C-terminal sequence and lacks a transmembrane domain. Expression of IL-12Rβ1ΔTM occurs in CD11c⁺ cells in the lungs during M. tuberculosis infection. Selective reconstitution of il12rb1^−/− DCs with (mRNA) il12rb1 and/or (mRNA) il12rb1Δtm demonstrates that IL-12Rβ1ΔTM augments IL-12Rβ1-dependent DC migration and activation of M. tuberculosis-specific T cells. It cannot mediate these activities independently of IL12Rβ1. We hypothesize that M. tuberculosis-exposed DCs express IL-12Rβ1ΔTM to enhance IL-12Rβ1-dependent migration and promote M. tuberculosis–specific T cell activation. IL-12Rβ1ΔTM thus represents a novel positive-regulator of IL12Rβ1-dependent DC function and of the immune response to M. tuberculosis.

In a murine tuberculosis model, the absence of homeostatic chemokines delays granuloma formation and protective immunity

Mycobacterium tuberculosis infection (Mtb) results in the generation of protective cellular immunity and formation of granulomatous structures in the lung. CXCL13, CCL21, and CCL19 are constitutively expressed in the secondary lymphoid organs and play a dominant role in the homing of lymphocytes and dendritic cells. Although it is known that dendritic cell transport of Mtb from the lung to the draining lymph node is dependent on CCL19/CCL21, we show in this study that CCL19/CCL21 is also important for the accumulation of Ag-specific IFN-gamma-producing T cells in the lung, development of the granuloma, and control of mycobacteria. Importantly, we also show that CXCL13 is not required for generation of IFN-gamma responses, but is essential for the spatial arrangement of lymphocytes within granulomas, optimal activation of phagocytes, and subsequent control of mycobacterial growth. Furthermore, we show that these chemokines are also induced in the lung during the early immune responses following pulmonary Mtb infection. These results demonstrate that homeostatic chemokines perform distinct functions that cooperate to mediate effective expression of immunity against Mtb infection.

Ixodes ovatus Ehrlichia exhibits unique ultrastructural characteristics in mammalian endothelial and tick-derived cells

Tick-borne pathogens in the genus Ehrlichia cause emerging zoonoses. Although laboratory mice are susceptible to Ehrlichia infections, many isolates do not cause clinical illness. In contrast, the Ixodes ovatus Ehrlichia-like agent (IOE) causes disease and immune responses in mice comparable to the human illness caused by Ehrlichia chaffeensis. No culture system had been developed for IOE, however, which limited studies of this pathogen. We reasoned that endothelial and tick cell lines could potentially serve as host cells, since the IOE is found in ticks and in endothelial cells in mice. Infected spleen cells from RAG-deficient mice were overlaid onto ISE6 and RF/6A cultures, and colonies typical of Ehrlichia were noted in RF/6A cells within 2 weeks. Infection of ISE6 cells was established after transfer of IOE from RF/6A cells. Electron microscopy revealed densely packed inclusions in infected RF/6A and ISE6 cells; these inclusions contained copious amounts of filamentous structures, apparently originating from Ehrlichial cells. In particular, within RF/6A cells the structures assumed an ordered morphology of finely combed hair. IOE from RF/6A cells, when inoculated into C57BL/6 and RAG-deficient mice, induced fatal disease. These data reveal unique structural features of IOE that may contribute to the pathogen's high virulence.

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.

Early T-cell responses in tuberculosis immunity

SUMMARY

Tuberculosis (TB) has plagued mankind for millennia yet is classified as an emerging infectious disease, because its prevalence in the human population continues to increase. Immunity to TB depends critically on the generation of effective CD4(+) T-cell responses. Sterile immunity has not been achieved through vaccination, although early T-cell responses are effective in controlling steady-state infection in the lungs. Although such early T-cell responses are clearly protective, the initiation of the Mycobacterium tuberculosis (Mtb) T-cell response occurs much later than is the case following other aerogenic infections. This fact suggests that there is a critical period, before the activation of the T-cell response, in which Mtb is able to establish infection. An understanding of the factors that regulate early T-cell activation should, therefore, lead to better control of the disease. This review discusses recent work that has investigated the early development of T-cell immunity following Mtb infection in the mouse.

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.

CD4 T-cell epitopes associated with protective immunity induced following vaccination of mice with an ehrlichial variable outer membrane protein

The ehrlichiae express variable outer membrane proteins (OMPs) that play important roles in both pathogenesis and host defense. Previous studies revealed that OMPs are immunodominant B-cell antigens and that passive transfer of anti-OMP antibodies can protect SCID mice from fatal ehrlichial infection. In this study, we used a model of fatal monocytotropic ehrlichiosis caused by Ehrlichia bacteria from Ixodes ovatus (IOE) to determine whether OMP immunization could generate protective immunity in immunocompetent mice. Immunization of C57BL/6 mice with a purified recombinant OMP expressed by IOE omp19 generated protection from fatal IOE infection and elicited robust humoral and CD4 T-cell responses. To identify CD4 T-cell epitopes within OMPs, we performed enzyme-linked immunospot analyses for gamma interferon (IFN-gamma) production using a panel of overlapping 16-mer peptides from IOE OMP-19. Five immunoreactive peptides comprising residues 30 to 45, 77 to 92, 107 to 122, 197 to 212, and 247 to 264 were identified; the strongest response was generated against OMP-19(107-122). Most of the peptides are conserved between E. muris and E. chaffeensis OMP-19, and they elicited IFN-gamma production in CD4 T cells from E. muris-infected mice, indicating that T-cell epitope cross-reactivity likely contributes to heterologous immunity. Accordingly, CD4 T-cell responses to both OMP-19 and OMP-19(107-122) were of greater magnitude following high-dose IOE challenge of mice that had been immunized by prior infection with E. muris. Our studies cumulatively identify B- and T-cell epitopes that are associated with protective homologous and heterologous immunity during ehrlichial infection.

Susceptibility and resistance to monocytic ehrlichiosis in the mouse

To address the role of cellular immunity during ehrlichia infection, we have utilized a model of monocytic ehrlichiosis that results from infection of mice by Ixodes ovatus ehrlichia (IOE). Although ehrlichiosis in humans is largely a disease of immunocompromised individuals, the use of the IOE model has allowed us to identify factors required for host defense in normal mice. Using a low-dose infection C57BL/6 mouse model, we have demonstrated that host defense requires immune mechanisms involving CD4 T cell-mediated, TNF-alpha-, IL-12-, and IFN-gamma-dependent, macrophage activation. We have also provided formal evidence that IFN-gamma produced by CD4 Th1 cells is sufficient for protective immunity. Our recent studies have demonstrated, in addition, an essential role for IL-10, which is probably important in inhibiting immunopathological responses, and for inducible nitric oxide synthase. The latter observation establishes an important role for reactive nitrogen intermediates in bacterial elimination in vivo. In contrast, evaluation of mice carrying wild-type and mutant alleles of Nramp1 revealed at most a modest role for this gene in resistance to fatal IOE infection. Other studies in low-dose infected mice have indicated that the generation of immunological memory may be impaired during low-dose IOE infection, possibly due to bacterial immune subversion. These studies highlight the utility of the IOE mouse model in identifying important parameters of the immune response during ehrlichiosis.

Immunity to the ehrlichiae: new tools and recent developments

PURPOSE OF REVIEW

Discusses recent developments in the study of immunity and host defense against the monocytic ehrlichiae in 2003 and 2004. The review does not address anaplasmoses, as the anaplasmae were recently re-classified into the genus Anaplasma, and are distinct in cell tropism from the ehrlichiae.

RECENT FINDINGS

The features of the immune responses against these emerging Gram-negative obligate intracellular pathogens are only beginning to be understood. Important advances in our ability to study host defense include the development of new experimental mouse models. Recent studies have defined possible mechanisms of innate immune subversion in human monocytes, as well as roles for lymphocyte subsets and type I cytokines during mouse infection. Other studies in the mouse suggest that cytokine production by CD8 T cells may contribute to immunopathology. New data also support a role for humoral immunity during host defense against these intracellular pathogens.

SUMMARY

The use of new animal models will facilitate research of the mechanisms of innate, adaptive, and pathological immune responses, and will enhance our understanding of human immunity to the ehrlichiae as well as to other pathogenic intracellular bacteria.

Expression of members of the 28-kilodalton major outer membrane protein family of Ehrlichia chaffeensis during persistent infection

The 28-kDa immunodominant outer membrane proteins (P28 OMPs) of Ehrlichia chaffeensis are encoded by a multigene family. As an indirect measure of the in vivo expression of the members of the p28 multigene family of E. chaffeensis, sera from two beagle dogs experimentally infected with E. chaffeensis were evaluated for the presence of specific antibodies to P28 OMPs by enzyme-linked immunosorbent assay. Antigenic peptides unique to each of the P28s were identified within the first hypervariable region of each P28 OMP. Serological responses to peptides derived from all P28 OMPs were detected from day 30 postinoculation to day 468 and from day 46 until day 159 in the two beagles. Although antibody titers to the peptides fluctuated, the peak response to all of the peptides appeared simultaneously in each dog. The antibody responses to another outer membrane protein of E. chaffeensis (GP120) showed similar temporal and quantitative changes. These data suggest that the P28 OMPs are expressed concurrently during persistent Ehrlichia infection.

Survival, replication, and antibody susceptibility of Ehrlichia chaffeensis outside of host cells

Ehrlichia chaffeensis, an obligate intracellular, tick-transmitted bacterium, is susceptible to antibody-mediated host defense, but the mechanism by which this occurs is not understood. One possible explanation is that antibodies directly access the bacteria in the extracellular environment of the host, perhaps during bacterial intercellular transfer. Accordingly, we investigated whether bacteria could be found outside of host cells during infection. Host cell-free plasma obtained from infected mice was found to contain ehrlichiae, and the host cell-free ehrlichiae readily transferred disease to susceptible SCID recipients. The host cell-free ehrlichiae were found during infection of both immunocompetent BALB/c and immunocompromised BALB/c-scid mice and reached levels as high as 10(8)/ml in plasma during persistent infection in SCID mice. Approximately 10% of the blood-borne bacteria were found outside of host cells. Although it is generally accepted that replication of ehrlichiae occurs only within host cells, the cell-free bacteria were shown to undergo DNA replication and cell division in vitro for 3 to 5 days when incubated at 37 degrees C in plasma. Paradoxically, both infectivity and virulence were lost after 24 h of ex vivo culture. The data indicate that E. chaffeensis is exposed to the extracellular milieu during infection, presumably during intercellular transfer, and reveal that these intracellular bacteria do not require the environment of the host cell for replication. Our findings reveal a possible mechanism by which antibodies can access the intracellular bacteria upon their release into the extracellular milieu and mediate host defense and also have implications for understanding the replication and transmission of this vector-borne pathogen.

Mechanisms of humoral immunity during Ehrlichia chaffeensis infection

Our laboratory has been investigating the mechanisms of host defense during Ehrlichia chaffeensis infection in the mouse. Although major roles are clearly played by T cells, we found that antibodies could also control infection in both normal and immunocompromised SCID mice, and could protect the latter from lethal infection. Antibodies are not generally effective during such intracellular infections, so we would like to understand exactly how antibodies can mediate immunity in this model. We have found that much of the humoral immune response is directed at the bacterial outer membrane proteins (OMPs), and that highly effective OMP antibodies (mostly IgG2a) exhibited picomolar affinities and very long binding half-lives. These antibodies, which could mediate bacterial clearance from tissues as early as 24 hours after administration, require host Fc receptors for their function(s). In contrast, we have failed to find any role for complement or reactive nitrogen intermediates, or for neutrophils, or for NK cells. One possible mechanism is that antibodies or immune complexes trigger microbiocidal activities in infected macrophages that lead to the elimination of bacteria residing inside host macrophages. Alternatively, it is proposed that antibodies opsonize bacteria exposed during intercellular transfer. This notion is supported by studies that have demonstrated the presence of bacteria in the extracellular milieu during infection, and suggests that our understanding of the behavior of the bacterium in the host may be key to our understanding of its susceptibility to antibody-mediated host defenses.

Persistence and turnover of antigen-specific CD4 T cells during chronic tuberculosis infection in the mouse

CD4 T cells are critical for resistance to Mycobacterium tuberculosis infection, but how effective T cell responses are maintained during chronic infection is not well understood. To address this question we examined the CD4 T cell response to a peptide from ESAT-6 during tuberculosis infection in the mouse. The ESAT-6(1-20)/IA(b)-specific CD4 T cell response in the lungs, mediastinal lymph nodes, and spleen reached maxima 3-4 wk postinfection, when the bacteria came under the control of the immune response. Once chronic infection was established, the relative frequencies of Ag-specific CD4 T cells were maintained at nearly constant levels for at least 160 days. ESAT-6(1-20)/IA(b)-specific CD4 T cells that responded in vitro expressed activation markers characteristic of chronically activated effector cells and used a limited Vbeta repertoire that was clonally stable in vivo for at least 12 wk. 5-Bromo-2-deoxyuridine incorporation studies indicated a relatively high rate of cell division among both total CD4 and ESAT-6(1-20)/IA(b)-specific CD4 T cells during acute infection, but the degree of 5-bromo-2-deoxyuridine incorporation by both the CD4 T cells and the Ag-specific cells declined at least 3-fold during chronic infection. The data indicate that the peripheral ESAT-6(1-20)/IA(b)-specific CD4 T cell response to M. tuberculosis is characterized during the acute phase of infection by a period of extensive proliferation, but once bacterial control is achieved, this is followed during chronic infection by an extended containment phase that is associated with a persistent response of activated, yet more slowly proliferating, T cells.

Antibodies highly effective in SCID mice during infection by the intracellular bacterium Ehrlichia chaffeensis are of picomolar affinity and exhibit preferential epitope and isotype utilization

Although often considered to be ineffective against intracellular bacteria, Abs, in the absence of lymphocytes, have been shown previously to protect SCID mice from lethal infection by the obligate intracellular bacterium Ehrlichia chaffeensis, even when administered well after infection has been established. To identify characteristics of Abs that are critical for host defense during this intracellular infection, a panel of Ehrlichia-specific mAbs was generated and analyzed. Among 100 Abs recovered, 39 recognized an amino-terminal hypervariable region of an outer membrane protein (OMP), demonstrating that the OMPs are both antigenically variable and immunodominant. A subset of 16 representative OMP-specific Abs was further examined to identify characteristics that were essential for in vivo efficacy. The highly effective Abs recognized a linear epitope within the first hypervariable region of OMP-1g. Only IgG were found to be effective, and among the effective IgG, the following hierarchy was observed: IgG2a > IgG3 = IgG2b. The most striking characteristics of the highly effective Abs were their picomolar binding affinities and long binding t(1/2). Thus, although epitope recognition and isotype use may contribute to efficacy, high affinity may be a critical characteristic of Abs that can act effectively during this intracellular bacterial infection.

Outer membrane protein-specific monoclonal antibodies protect SCID mice from fatal infection by the obligate intracellular bacterial pathogen Ehrlichia chaffeensis

Previous studies of Ehrlichia chaffeensis infection in the mouse have demonstrated that passive transfer of polyclonal Abs from resistant immunocompetent mice to susceptible SCID mice ameliorated infection and disease, even when Abs were administered during established infection. To identify particular Abs that could mediate bacterial clearance in vivo, E. chaffeensis-specific mAbs were generated and administered to infected SCID mice. Bacterial infection in the livers was significantly lowered after administration of either of two Abs of different isotypes (IgG2a and IgG3). Moreover, repeated administration of one Ab (Ec56.5; IgG2a) rescued mice from an otherwise lethal infection for at least 5 wk. Both protective Abs recognized the E. chaffeensis major outer membrane protein (OMP)-1g. Further studies revealed that both Abs recognized closely related epitopes within the amino terminus of the first hypervariable region of OMP-1g. Analyses of human sera showed that E. chaffeensis-infected patients also generated serological responses to OMP-1g hypervariable region 1, indicating that humans and mice recognize identical or closely related epitopes. These studies demonstrate that OMP-specific mAbs can mediate bacterial elimination in SCID mice, and indicate that Abs, in the absence of cell-mediated immunity, can play a significant role in host defense during infection by this obligate intracellular bacterium.

Detection of viral superantigen-class II MHC interactions at the cell surface

Mouse mammary tumor virus superantigens (vSAgs) must bind to class II MHC proteins to activate T cells. Although direct interaction of vSAgs with class II proteins has been demonstrated biochemically, the details of this interaction are largely unresolved. To facilitate the study of class II-vSAg interactions, a sensitive assay has been developed that can detect binding of vSAgs to class II proteins on the cell surface. The assay measures changes in vSAg surface expression upon enzymatic removal of a co-expressed glycan-phosphatidyl inositol-anchored form of the class II molecule IE(k). Because the vSAgs are synthesized as integral membrane proteins that undergo proteolytic processing, an event that is likely required to eliminate membrane tethering, the data provide further evidence that a proteolytic fragment of vSAg is bound to class II proteins on the cell surface. The assay was utilized to identify mutant vSAgs that either did not associate with IE(k) molecules, or did not undergo furin-dependent proteolytic processing. Class II protein binding was detected using vSAg7 mutants that lacked furin endoprotease recognition sites, and after expression of vSAg in furin-deficient cells. The data demonstrate that furin-mediated processing is not necessary for association of vSAg7 with class II proteins, supporting previous studies that have indicated a role for alternative endoproteases in vSAg activation. However, because class II interactions were also noted in the apparent absence of proteases that are known to activate vSAgs, the data suggest that yet other proteases may process vSAgs in a fashion that does not necessarily lead to activation of T cells.

Intercellular transfer of a soluble viral superantigen

Mouse mammary tumor virus (MMTV) superantigens (vSAgs) can undergo intercellular transfer in vivo and in vitro such that a vSAg can be presented to T cells by major histocompatibility complex (MHC) class II proteins on antigen-presenting cells (APCs) that do not express the superantigen. This process may allow T-cell activation to occur prior to viral infection. Consistent with these findings, vSAg produced by Chinese hamster ovary (CHO) cells was readily transferred to class II IE and IA (H-2(k) and H-2(d)) proteins on a B-cell lymphoma or mouse splenocytes. Fixed class II-expressing acceptor cells were used to demonstrate that the vSAg, but not the class II proteins, underwent intercellular transfer, indicating that vSAg binding to class II MHC could occur directly at the cell surface. Intercellular transfer also occurred efficiently to splenocytes from endogenous retrovirus-free mice, indicating that other proviral proteins were not involved. Presentation of vSAg7 produced by a class II-negative, furin protease-deficient CHO variant (FD11) was unsuccessful, indicating that proteolytic processing was a requisite event and that proteolytic activity could not be provided by an endoprotease on the acceptor APC. Furthermore, vSAg presentation was effected using cell-free supernatant from class II-negative, vSAg-positive cells, indicating that a soluble molecule, most likely produced by proteolytic processing, was sufficient to stimulate T cells. Because the membrane-proximal endoproteolytic cleavage site in the vSAg (residues 68 to 71) was not necessary for intercellular transfer, the data support the notion that the carboxy-terminal endoproteolytic cleavage product is an active vSAg moiety.

Antibody-mediated elimination of the obligate intracellular bacterial pathogen Ehrlichia chaffeensis during active infection

It is generally accepted that cellular, but not humoral immunity, plays an important role in host defense against intracellular bacteria. However, studies of some of these pathogens have provided evidence that antibodies can provide immunity if present during the initiation of infection. Here, we examined immunity against infection by Ehrlichia chaffeensis, an obligate intracellular bacterium that causes human monocytic ehrlichiosis. Studies with mice have demonstrated that immunocompetent strains are resistant to persistent infection but that SCID mice become persistently and fatally infected. Transfer of immune serum or antibodies obtained from immunocompetent C57BL/6 mice to C57BL/6 scid mice provided significant although transient protection from infection. Bacterial clearance was observed when administration occurred at the time of inoculation or well after infection was established. The effect was dose dependent, occurred within 2 days, and persisted for as long as 2 weeks. Weekly serum administration prolonged the survival of susceptible mice. Although cellular immunity is required for complete bacterial clearance, the data show that antibodies can play a significant role in the elimination of this obligate intracellular bacterium during active infection and thus challenge the paradigm that humoral responses are unimportant for immunity to such organisms.

Self-restricted dual receptor memory T cells

Enhanced immune responses during secondary exposure to Ag result from the development of memory cells. In the present report we show that stimulation through one receptor on dual receptor CD4 cells can promote the generation of T cells capable of giving a memory response through the second receptor, even though the cells had not been previously exposed to the Ag recognized by the second receptor. Cloned cells generated from dual receptor memory T cells proliferated and secreted the same lymphokines after stimulation with either Ag. Independent recognition of both Ags by distinct TCRs was shown by production of variants that had lost either Ag specificity along with the corresponding TCR. Recognition of both Ags is MHC restricted, since the cells recognize Ag presented by self, but not non-self, MHC class II molecules. These results raise the possibility that one potential mechanism of maintaining specific memory to a given Ag is through stimulation by an unrelated Ag via the second TCR.

Self-Restricted Dual Receptor Memory T Cells

Enhanced immune responses during secondary exposure to Ag result from the development of memory cells. In the present report we show that stimulation through one receptor on dual receptor CD4 cells can promote the generation of T cells capable of giving a memory response through the second receptor, even though the cells had not been previously exposed to the Ag recognized by the second receptor. Cloned cells generated from dual receptor memory T cells proliferated and secreted the same lymphokines after stimulation with either Ag. Independent recognition of both Ags by distinct TCRs was shown by production of variants that had lost either Ag specificity along with the corresponding TCR. Recognition of both Ags is MHC restricted, since the cells recognize Ag presented by self, but not non-self, MHC class II molecules. These results raise the possibility that one potential mechanism of maintaining specific memory to a given Ag is through stimulation by an unrelated Ag via the second TCR.

Redundant proteolytic activation of a viral superantigen

Proteolytic activation of viral superantigens (vSAgs)4 expressed in Chinese hamster ovary (CHO) cells is required for T cell stimulation, and is mediated primarily by the protein convertase (PC) furin. Three PC recognition sites are highly conserved in vSAgs, but it was not known which sites are required for PC dependent vSAg activation. Moreover, because the PC recognition sites are not conserved in all functional vSAgs it was possible that activation could occur by processing at any of several sites. To identify the location(s) where processing of vSAg7 generates an active superantigen, each of two PC recognition sites, and a third related site were altered by in vitro mutagenesis, and the mutant proteins were tested for their abilities to activate T cells. Mutation of the PC recognition site at position 68-71 in vSAg7 had no effect on its ability to activate T cells. Mutation of the processing site at position 169-172 completely abolished T cell activation, and indicated that cleavage at this position was obligatory for proteolytic activation of vSAg7. However, introduction of a PC recognition site at position 192-195, a position that in many other vSAgs encodes a PC recognition site, restored activity to a vSAg7 protein that lacked a recognition site at position 169-172. The data revealed that processing of vSAgs at either position 169-172 or 192-195 was sufficient for vSAg7 activation, and explain how vSAgs that lack some PC recognition sites can be activated by proteolytic processing.

Infection of the laboratory mouse with the intracellular pathogen Ehrlichia chaffeensis

To determine the basis of susceptibility and resistance to human monocytic ehrlichiosis (HME), immunocompetent and immunocompromised mice were infected with Ehrlichia chaffeensis and bacterial loads were measured by PCR and by immunohistochemistry. Immunocompetent (C. B-17 and C57BL/6) mice cleared the bacteria within 10 days, but immunocompromised SCID and SCID/BEIGE mice developed persistent infection in the spleen, liver, peritoneal cavity, brain, lung, and bone marrow and became moribund within 24 days. Both immunocompromised strains lack T and B lymphocytes, but the SCID/BEIGE strain is also deficient in natural killer (NK) cell function. During advanced stages of disease, the infections were associated with wasting, splenomegaly, lymphadenopathy, liver granulomas and necroses, intravascular coagulation, and granulomatous inflammation. Histochemical and immunohistochemical localization studies confirmed the presence of bacteria in tissues, and viable bacteria were cultured from infected animals. The data reveal that T and/or B cells play an essential role during resistance of immunocompetent mice to infection with E. chaffeensis and demonstrate the utility of immunocompromised mice as an experimental model for the study of HME.

Proteolytic processing activates a viral superantigen

Mouse mammary tumor virus (MMTV) superantigens (vSAg) undergo proteolytic processing at residues that have been demonstrated in vitro to be recognition sites for the endoprotease furin. To examine the role of furin in the presentation of vSAg7 to T cells, the vSAg7 and class II MHC IEk genes were introduced into Chinese Hamster Ovary (CHO) cells (furin-positive) and into a furin-negative CHO variant (FD11). Both transfected cell lines efficiently presented peptide antigen and bacterial superantigens to T cell hybridomas. However, while the furin-positive cells presented vSAg7 well, the furin-negative cells presented poorly. Transient transfection of the furin-negative cells with an expression plasmid containing the furin gene restored the ability to present vSAg7 efficiently. The marginal presentation of vSAg7 observed using the furin-negative transfectants was eliminated after culture with the protease inhibitor leupeptin, suggesting that one or more endoproteases other than furin have a detectable but limited capacity to proteolytically activate vSAg7. Biochemical analyses revealed that vSAg7 was largely unprocessed in the absence of furin. Thus, viral superantigens, unlike bacterial superantigens, require proteolytic processing to activate T cells.

Proteolytic processing is required for viral superantigen activity

The mouse mammary tumor virus-7 superantigen (vSAG7) is proteolytically processed in B cells at as many as three positions. Proteolytic processing appears to be important for superantigen activity because a processed form of vSAG7 was predominant among those forms that were found to bind to major histocompatibility complex class II molecules. To determine the functional significance of proteolytic processing, a mutation was introduced in vSAG7 at one of the sites where proteolytic cleavage is thought to take place in B cells. Elimination of the putative processing site at position 171 abrogated detectable vSAG7 surface expression in B cells, indicating that proteolytic processing is required for vSAG7 function. Coexpression in insect cells of vSAG7 and furin, a proprotein-processing enzyme, also demonstrated that furin could process vSAG7 at position 171.

Processing and major histocompatibility complex binding of the MTV7 superantigen

Mouse mammary tumor viruses produce superantigens (vSAGs) which interact with class II major histocompatibility complex (MHC) proteins and stimulate T cells. vSAGs are synthesized as Type II membrane proteins, but at least one of these proteins (vSAG7) is found on the cell surface in a proteolytically processed form. Monoclonal antibodies (MAbs) were used to characterize vSAG7 and its binding to class II molecules. vSAG7 is synthesized in the endoplasmic reticulum (ER) as a 45 kd glycoprotein containing N-asparagine-linked oligomannosyl carbohydrates. vSAG7 transits the golgi complex, where it is modified by the addition of complex-type glycans and proteolysed at three positions. After proteolysis, the amino and carboxyl termini remain noncovalently associated. The ER, golgi, and surface forms of vSAG7 are stably bound to class II, but one of the proteolysed forms comprises the majority of the class II-bound material.

Direct evidence for the role of COOH terminus of mouse mammary tumor virus superantigen in determining T cell receptor V beta specificity

It has recently been shown that open reading frames in the 3' long terminal repeats of mouse mammary tumor viruses encode superantigens. These viral superantigens (vSAGs) stimulate most T cells expressing appropriate V beta s almost regardless of the rest of the variable components of the T cell receptors (TCR) expressed by those cells. vSAGs produce a type II integral membrane protein with a nonessential short cytoplasmic domain and a large glycosylated extracellular COOH-terminal domain, which is predicted to interact with major histocompatibility complex class II molecules and the TCR. The transmembrane region of vSAG also has an internal positively charged lysine residue of unknown significance. A set of chimeric and mutant vSAG genes has been used in transfection experiments to show that only the extreme COOH-terminal portion of vSAGs determine their TCR V beta specificities, and to show that the lysine residue in the transmembrane domain is not essential for the function of vSAG.

The bacterial and mouse mammary tumor virus superantigens; two different families of proteins with the same functions

In conclusion, the bacterial toxins are completely unlike the MTV superantigens in primary sequence and structure. The former are soluble globular proteins which do not have to be proteolytically cleaved before they act. The latter are synthesized as type II membrane proteins and may be clipped before they reach the cell surface and act to stimulate T cells. Table III summarizes the similarities and differences between the two sets of superantigens. The most notable quality of these molecules is that both sets of families have developed strategies whereby they bind to Class II and engage V beta. As far as the microorganisms which produce them are concerned, these two properties appear to be essential since they are absolutely conserved over proteins of a number of different structures. Several questions can now be addressed as follows. a. Why do all known superantigens bind to Class II? For the microorganism which produces them, the function of superantigens appears to be T-cell and perhaps directly or indirectly B-cell and macrophage stimulation. Activation of virgin T cells requires engagement with antigen plus MHC on professional antigen-presenting cells. Unlike other cell surface proteins, for example Class I, most Class II in animals is expressed on such cells. Therefore it is likely that superantigens have evolved to engage Class II because presentation to T cells by Class II-bearing cells offers the superantigen the best chance of activating its target T cells. b. Why do superantigens engage TCR V beta and not V alpha or CD3? It is possible that superantigens bind to the V beta portion of the TCR rather than V alpha because the latter does not have a consistently well exposed face for engagement. The fact that it is perhaps relatively easier to produce anti-V beta rather than anti-V alpha antibodies supports this idea. We have shown that N-glycosylation of V beta can interfere with recognition by vSAGs (Pullen et al. 1991), perhaps glycosylation of V alpha tends to conceal otherwise available sites. As far as C beta, C alpha or CD3 engagement is concerned, this may be just too dangerous for MTVs. The role of MTVs SAgs in the life history of the virus seems to be to stimulate T cells in the suckling recipient and thereby create a pool of activated lymphocytes in which the virus may survive until the mouse gives birth and transmits the virus to her own progeny (Hainaut et al. 1990, Golovkina et al. 1992).(ABSTRACT TRUNCATED AT 400 WORDS)

Detection and biochemical characterization of the mouse mammary tumor virus 7 superantigen (Mls-1a)

Mouse mammary tumor viruses encode superantigens that bind to class II major histocompatibility complex proteins and engage T cells that bear particular V beta s. Among these superantigens is the long known, but previously uncharacterized, Mls-1a product, encoded by Mtv-7. Using a monoclonal antibody, we detect the Mtv-7 superantigen on the surface of activated B cells, but not on T cells or resting B cells. The superantigen is synthesized as a 45 kd transmembrane glycoprotein precursor, but is proteolytically processed to yield an 18.5 kd surface protein that we suggest is the functional form of the superantigen.