Switched and unswitched memory B cells detected during SARS-CoV-2 convalescence correlate with limited symptom duration

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the causative agent of the pandemic human respiratory illness COVID-19, is a global health emergency. While severe acute disease has been linked to an expansion of antibody-secreting plasmablasts, we sought to identify B cell responses that correlated with positive clinical outcomes in convalescent patients. We characterized the peripheral blood B cell immunophenotype and plasma antibody responses in 40 recovered non-hospitalized COVID-19 subjects that were enrolled as donors in a convalescent plasma treatment study. We observed a significant negative correlation between the frequency of peripheral blood memory B cells and the duration of symptoms for convalescent subjects. Memory B cell subsets in convalescent subjects were composed of classical CD24+ class-switched memory B cells, but also activated CD24-negative and natural unswitched CD27+ IgD+ IgM+ subsets. Memory B cell frequency was significantly correlated with both IgG1 and IgM responses to the SARS-CoV-2 spike protein receptor binding domain (RBD). IgM+ memory, but not switched memory, directly correlated with virus-specific antibody responses, and remained stable over time. Our findings suggest that the frequency of memory B cells is a critical indicator of disease resolution, and that IgM+ memory B cells play an important role in SARS-CoV-2 immunity.

Production of qualitatively different antibodies by T-bet+ IgM memory cells and plasmablasts

We are investigating the genesis of CD4 T cell-independent T-bet+ IgM+ plasmablasts and memory B cells during intracellular bacterial infection. Although the plasmablasts are generated early during ehrlichial infection, and the memory cells become established within 30 days, it is not known if the populations are related lineally. We showed previously that a high frequency of plasmablast-derived IgM was polyreactive, that is, these IgM bound structurally diverse antigens. Polyreactive IgM were particularly abundant among those Abs that bound the ehrlichial outer membrane protein, OMP-19. Here we addressed whether polyreactivity is also characteristic of memory cell-derived IgM. Antibodies were cloned from sorted CD11c+ T-bet+ IgM memory B cells by single cell Ig sequencing and expressed in HEK cells as chimeric human IgG1. Supernatants were tested by ELISA for reactivity to OMP-19, as well as for double-stranded DNA and insulin. Preliminary data showed that approximately 10% (29 out of 284 samples) of antibodies were OMP-19 specific, but unlike those generated by the plasmablasts, the memory cell-derived antibodies did not exhibit polyreactivity. These data reveal that T-bet+ IgM plasmablasts and memory cells produce distinctly different Igs, which may indicate that the two populations have different origins, or that the populations undergo different selection during their differentiation. Ongoing studies will help resolve these different interpretations.

IL-21-dependent Tbet+ B cells require Bcl6 and receive delayed help from non-Tfh T cells

Tbet+ B cells are a novel lymphocyte population that have been implicated in a multitude of diseases and infections, including hepatitis, AIDs, malaria, SLE, and age-related autoimmunity. Our laboratory has characterized a population of Tbet+ CD11c+ memory B cells that arise during Ehrlichia muris infection in mice, and we have used this infection model to study the ontogeny of these cells. We have previously demonstrated that the generation of Tbet+ memory B cells requires CD4 T cells and IL-21 signaling, implicating a role for Tfh cells in Tbet+ B cell development. However, Tfh cells were not required for the generation of Tbet+ memory B cells. Rather, these data suggest that Tbet+ B cells received help from a population of non-Tfh CD4 T cells capable of secreting IL-21, likely Th1 cells. As expected, the helper CD4 T cells provided CD40:CD40L signals to the Tbet+ B cells; however, these interactions occurred 16 days or later post-infection. While the helper CD4 T cells did not require Bcl6, the same was not true for the Tbet+ B cells, as Tbet+ memory B cells were not generated in the absence of B cell-intrinsic Bcl6. These data suggest that T-bet B cells require a novel transcriptional program whereby Tbet and Bcl6 function cooperatively, perhaps transiently. Collectively, these data support a model whereby a subset of non-Tfh T cells (likely Th1 cells) provide CD40 and IL-21 signals that drive the formation of Bcl6-dependent Tbet+ B cells.

Stem cell-like T-bet+ IgM memory cells generate multi-lineage effector B cells

Our laboratory has been studying the differentiation of CD11c+ T-bet+ IgM memory B cells during secondary ehrlichial infection.

These cells are closely related to B cells that have been described in a range of other contexts, including hepatitis, AIDs, malaria, SLE, and age-related autoimmunity.

Following secondary infection, IgM memory cells, as a population, undergo self-renewal, and differentiate into effector cells, including splenic and bone marrow antibody secreting cells (ASC).

Moreover, IgM memory cells enter germinal centers, where they undergo class-switch recombination and give rise to class-switched memory and effector cells.

Although these data suggest that a single memory cell has multi-lineage potential, we sought to formally address this question by searching for shared clones among IgM memory cell-derived effector cells.

Among the IgM memory cell-derived subsets, we identified several clones common to all effector cell populations. The number of common clones varied for each pairwise comparison of effector cells and suggested lineal relationships.

IgM memory cells accumulated mutations following rechallenge; although all memory cell-derived subsets displayed similar numbers of V region mutations.

Lineage analysis demonstrated that the effector cell subsets underwent varying degrees of clonal diversification, although this was clone-dependent.

These studies reveal that a single IgM memory cell clone can give rise to different memory and effector cell subsets, that is, they exhibit stem cell properties. This property distinguishes these T-bet+ IgM memory cells as a unique memory cell subset.

TNFα Inhibits the Expression of Germinal Center Phenotype on T-bet+ CD11c+ IgM Memory B Cells without Altering Clonal Diversity

Our studies have identified a population of T-bet+ CD11c+ IgM memory B cells that arise within 30 days following infection with the intracellular bacterium, E. muris. The memory cells require for their generation T cell help and IL-21; however, they are generated in a GC-independent fashion, because GCs are suppressed during infection. Previous studies have shown that TNFα is necessary for proper GC formation. Given that we detected an increase in serum TNFα following infection, we addressed a role for this cytokine in IgM memory cell development. Relative to wild-type, we observed a substantial increase in the frequency of developing IgM memory. These B cells were positive for GL7+ CD38lo CD95+ on day 16 post-infection. Imaging studies corroborated these findings by demonstrating the appearance of GC-like structures in infected TNFα-deficient, but not in wild-type, mice.

Increased GL7 expression on IgM memory cells in absence of TNFα was not correlated with changes in clonal diversity on day 30 post-infection. This suggests that TNFα does not affect B cell selection.

To address mechanisms whereby the absence of TNFα may facilitate formation of pseudo-GC structures, we quantitated CXCL13, a chemokine known for its role as a chemoattractant during GC morphogenesis. CXCL13 expression was 10-fold lower in TNFα-deficient mice, indicating its requirement, in part, for splenic disorganization in wild-type mice during infection.

Our findings suggest that although GCs are suppressed during ehrlichial and other bacterial infections, GC structures can, when present, magnify IgM memory cell generation. Ongoing studies will address whether changes in lymphoid architecture impact other properties and functions of T-bet+ CD11c+ IgM memory cells.

Novel functions CD11c+ IgM+ memory B cells suggested from transcriptome analysis

Our laboratory has identified a distinct subset of CD11c+ T-bet+ IgM memory B cells during ehrlichial infection in mice. These cells are likely closely related to phenotypically similar B cells that have been described in a range of other conditions, including hepatitis, AIDs, malaria, SLE, and age-related autoimmunity. In order to identify novel functions of CD11c+ T-bet+ IgM memory B cells, we performed RNAseq analysis. The cells were purified from infected mice on day 30 post-infection, and were compared to CD11c-negative CD19+ B cells isolated from the same mice. The CD11c+ B cells exhibited a distinct transcriptomic profile, compared to the CD11c-negative B cells, as over 900 genes were differentially regulated. Differentially-expressed genes included those encoding for integrins, Fc receptors, transcription factors, complement and receptors involved in innate immunity. Differentially-expressed complement-related genes included those encoding for the C1q complex, properdin, and complement factor B, which were all upregulated. Other genes that were upregulated included those encoding the adenosine receptor Adora2a, Bcl-6, which encodes a transcription factor required for GC B cells and the B cell inhibitory FcR, Fcgr2b. These data open new avenues of exploration of CD11c+ T-bet+ B cell function, and will lead to a better understanding of the role of these novel B cells in human health and disease.

CD11c+ T-bet+ IgM memory B cells provide multi-lineage reconstitution of effector and memory B cells following challenge infection

IgM memory B cells are now recognized as an important component of immunological memory. These cells have been proposed to act as a reservoir of broadly-reactive B cells that differentiate, in germinal centers, into high affinity class-switched effector B cells following antigen encounter. However, we propose IgM memory cells can differentiate into more than just germinal center and class-switched cells. To address this, we monitored the secondary response of CD11c+ T-bet+ IgM memory cells generated by Ehrlichia muris infection, by monitoring eYFP-labeled, flow cytometrically-purified, IgM memory cells following their transfer to naïve recipient mice and subsequent challenge infection. The IgM memory B cells differentiated into IgM-producing plasmablasts early following infection; this resulted in a 4-fold increase in IgM production, relative to control mice that did not receive memory cells. Other donor memory B cells entered germinal centers, down-regulated CD11c, and underwent class switching, generating switched memory B cells. Finally, some donor B cells were maintained as IgM memory cells. Upon secondary transfer and challenge, purified IgM memory cells underwent the same pattern of self-renewal and differentiation. Additionally, when IgM memory cell recipient mice were challenged with a virulent Ehrlichia strain, a reduction in bacterial burden was observed, indicating that IgM memory cells can provide immunity to challenge infection. Our findings also suggest that T-bet+ IgM memory cells act as a self-renewing population capable of repopulating the entire spectrum of effector and memory B cells during secondary infection, and demonstrate an important role for IgM memory cells in maintaining long-term immunity.

Helper T cells are required for the development of IgM memory cells, but not IgM bone marrow plasmablasts

The processes and factors that regulate B cell fate decisions during IgM memory cell development and differentiation are unknown, and may differ from those required by canonical memory cells. In our experimental model of Ehrlichia muris infection, we have characterized two long-lived B cell populations: IgM memory cells, and IgM bone marrow (BM) antibody secreting cells (ASCs). The IgM+ memory B cells are required for recall IgG responses, while the BM ASCs produce antigen-specific IgM responsible for maintaining long-term immunity. Both populations are derived from an early AID-expressing CD4 T cell-independent splenic CD11c+ plasmablast population. Because abundant T follicular helper cells are generated in the spleen during early infection, we investigated whether T cell help regulates B cell fate. Infection of MHC class II-deficient mice revealed that the IgM+ memory B cells required CD4+ T cell help, while the IgM+ BM ASCs did not. Moreover, IgM memory cell transfer studies indicated that T cell help was required for the generation of IgM+ memory cells, but not for the generation of BM ASCs. On the basis of these observations, we propose that T cells drive B cell fate: developing spleen plasmablasts that receive T cell signals mature to become IgM memory cells, while cells that do not receive these signals follow a default pathway to become long-lived IgM BM ASCs.

Regulation of IgM+ memory B cells by the inhibitory FγRIIb (IRM10P.610)

Our laboratory has described a long-term IgM+ memory population in the spleens of mice infected with Ehrlichia muris (J. Immunol. 191:1240). Among the many surface markers that distinguish the memory cells from conventional B cells is the inhibitory receptor FcγRIIb, which exhibited 4-fold higher expression. We hypothesized FcγRIIb negatively regulates IgM+ memory cells by binding immune complexes present during low-level chronic infection. To investigate this question, we monitored the population in infected FcγRIIb-deficient mice. Thirty days post-infection, the IgM+ memory B cells were increased by 3-fold in frequency in FcγRIIb-deficient mice, compared to control mice. This increase in the frequency of spleen IgM+ memory cells was due to an increase in the number of IgM+ memory cells, and not simply a decrease in the number of other B cells present. Preliminary studies revealed the IgM+ memory population generated in FcγRIIb-deficient mice was identical in phenotype to IgM+ memory cells generated in infected wild-type mice. The increase in the IgM+ memory population was associated with an increase in antigen-specific IgG. These data indicate that FcγRIIb plays an important role in regulating the expansion and/or persistence of IgM+ memory cells. We propose the inhibitory receptor acts to limit the contribution of IgM+ memory cells to recall responses under conditions where antigen-specific IgG is sufficient to control infection.

Long-term bone marrow IgM-producing plasmablasts are derived during acute infection and are maintained by active proliferation (LYM6P.767)

We have previously characterized a population of IgM-secreting B cells in the bone marrow (BM) of mice infected with the intracellular bacterium Ehrlichia muris (J. Immunol. 186:1011). The BM cells are responsible for IgM production, which is sufficient to maintain long-term immunity to fatal infection. In the present study, we have investigated the ontogeny and maintenance of the B cell population. We utilized AID-Cre-ERT2 X Rosa26loxP-EYFP mice (Nat. Immunol. 10:1292) to irreversibly mark activation-induced cytidine deaminase (AID)-expressing B cells. Following tamoxifen administration on day 10 post-infection, the time of peak spleen CD11c-positive plasmablast response, we detected EYFP+ CD138+ IgM+ BM cells for at least as long as 100 days post-infection; the EYFP+ cells were maintained at frequencies as high as 25% of the BM IgM+ B cells. These data indicated that the BM B cells were likely derived from T cell-independent spleen plasmablasts, and constituted a long-lived population. BrdU incorporation studies also demonstrated that the majority of the IgM+ cells were actively proliferating, indicating that B cells were plasmablasts. Finally, antibody-mediated blocking of CD40:CD40L signaling during acute infection did not impact the generation of the BM plasmablasts, indicating that the plasmablasts were likely generated in the absence of T cell help. These findings underscore the importance of BM IgM plasmablasts in long-term humoral immunity and host defense.

Pfit is a structurally novel Crohn's disease-associated superantigen

T cell responses to enteric bacteria are important in inflammatory bowel disease. I2, encoded by the pfiT gene of Pseudomonas fluorescens, is a T-cell superantigen associated with human Crohn's disease. Here we report the crystal structure of pfiT at 1.7Å resolution and provide a functional analysis of the interaction of pfiT and its homolog, PA2885, with human class II MHC. Both pfiT and PA2885 bound to mammalian cells and stimulated the proliferation of human lymphocytes. This binding was greatly inhibited by anti-class II MHC HLA-DR antibodies, and to a lesser extent, by anti HLA-DQ and DP antibodies, indicating that the binding was class II MHC-specific. GST-pfiT efficiently precipitated both endogenous and in vitro purified recombinant HLA-DR1 molecules, indicating that pfiT directly interacted with HLA-DR1. Competition studies revealed that pfiT and the superantigen Mycoplasma arthritidis mitogen (MAM) competed for binding to HLA-DR, indicating that their binding sites overlap. Structural analyses established that pfiT belongs to the TetR-family of DNA-binding transcription regulators. The distinct structure of pfiT indicates that it represents a new family of T cell superantigens.

Human inflammatory bowel disease (IBD) is a family of chronic inflammatory disorders of the gastrointestinal tract which affect genetically susceptible individuals. IBD is a lifelong disease involving mostly young people, often severely. Crohn's disease (CD) and ulcerative colitis are the two major forms of IBD. Although the exact cause of these diseases remains unknown, both genetic and environmental factors together play significant roles in the disease pathogenesis. Several lines of evidence implicate commensal bacteria as an important pathogenic element in clinical disease, particularly in CD. We recently identified a novel microbial gene, I2, encoded by Pseudomonas fluorescens, a gram-negative commensal, which may be involved in the pathogenesis of CD. Both molecular and immunological approaches were used to identify the human receptor for the microbial antigen encoded by I2, to characterize the ligand-receptor interactions, and to determine the three-dimensional structure of the microbial gene product. In particular, we show that the pfiT is a T cell superantigen, which may help to explain how microbial flora can trigger immune activation in IBD, and may provide the groundwork for novel therapies to treat CD.

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.

Inflammatory chemokine receptors regulate CD8 T cell contraction and memory generation following infection (150.3)

The development of T cell memory from naïve precursors is a complex process influenced by molecular cues received during T cell activation and differentiation. In the present study, we describe a novel role for the chemokine receptors CCR5 and CXCR3 in regulating effector CD8 T cell contraction and memory generation following influenza virus infection. Using mixed bone marrow chimeras, we find that Ccr5-/- Cxcr3-/- cells show markedly decreased contraction following viral clearance, leading to the establishment of massive numbers of memory CD8 T cells. Ccr5-/- Cxcr3-/- cells show preferential localization to the non-inflamed interstitial spaces of the infected lung, which coincides with reduced expression of the activation marker CD69 and the rapid appearance of CD127hi KLRG1lo memory precursor cells after viral clearance. Importantly, influenza-specific effector Ccr5-/- Cxcr3-/- CD8 T cells in the lung could be activated in vivo by the addition of exogenous antigen, which restored effector T cell contraction and significantly reduced memory T cell generation following viral clearance. Together, the data support a model of memory CD8 T cell generation where 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.

Hematopoietic stem cells undergo an IFNγ-dependent, MyD88-independent transition from dormancy to activity during acute bacterial infection (153.8)

How hematopoietic stem cells (HSCs) respond to and recover from immunologic stress is not well-understood. We demonstrate that infection with the intracellular bacteria, Ehrlichia muris, induces a transient expansion of bone marrow Lin-negative Sca-1+ cKit+ (LSK) progenitor cells. As inflammation can modulate the expression of surface markers used to phenotype stem and progenitor cells, we addressed whether the phenotypic changes correlated with functional changes in the progenitor cell population. The expansion of LSK cells was associated with a loss of dormant, long-term repopulating HSCs, reduced engraftment, and a bias towards myeloid lineage differentiation. The infection-induced changes required IFNγ signaling, and were accompanied by an expansion of more differentiated multipotent progenitor cells. IFNγ was required for the infection-induced loss of functionally-defined dormant HSCs in label retaining-cell assays. In competitive reconstitution experiments, LSK cells purified from infected IFNγR-deficient mice preferentially engrafted, relative to LSK cells purified from infected wild type mice. Furthermore, the infection-induced changes in progenitor cells were independent of intrinsic MyD88-signaling. We propose that IFNγ elicited during acute infection, provides a signal to dormant HSCs to undergo a rapid transition from dormancy to activity, ostensibly, to provide the host with additional or better-armed innate cells for host defense.

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.

Induction of a polyreactive IgM response by bacterial infection (99.26)

Infection of mice with the bacterium Ehrlichia muris induces a protective T cell-independent (TI) IgM response. A unique property of the TI IgM produced during early infection is that it is polyreactive; that is, it binds a number of unrelated foreign and self antigens, including influenza virus, Borrelia burgdorferi, ss- and dsDNA, insulin, and thyroglobulin. We hypothesized that polyreactivity results from cross-reactivity of antigen-specific IgM, or that antibodies of unrelated specificity were generated via polyclonal B cell activation. To address these hypotheses, we generated B cell hybridomas from infected spleen cells and determined whether they produced pathogen-specific IgM. Eighty percent of the IgM-producing hybridomas were ehrlichia-specific; 10% of the IgM bound an immunodominant ehrlichial antigen, OMP-19, and the remainder bound yet unidentified ehrlichial antigen(s). Polyreactivity was exclusively associated with OMP-19-specific IgM, however, indicating that the properties of the antigen (e.g., spatial distribution on the bacterium) influence the selection of polyreactive IgM. These data indicate that polyreactive IgM is generated by antigen-specific B cells. Our data support a model whereby polyreactivity among antigen-specific IgM allows for the heteroligation (Mouquet et al., 2010) of OMP-19 and additional ehrlichial antigens, and serves to increase the relative affinity of IgM.

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.

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.

CD27low CD4 T lymphocytes that accumulate in the mouse lungs during mycobacterial infection differentiate from CD27high precursors in situ, produce IFN-gamma, and protect the host against tuberculosis infection

The generation of effector, IFN-gamma producing T lymphocytes and their accumulation at sites of infection are critical for host protection against various infectious diseases. The activation and differentiation of naive T lymphocytes into effector memory cells starts in lymphoid tissues, but it is not clear whether the Ag-experienced cells that leave lymph nodes (LN) are mature or if they undergo further changes in the periphery. We have previously shown that CD44(high)CD62L(low) effector CD4 T lymphocytes generated during the course of mycobacterial infection can be segregated into two subsets on the basis of CD27 receptor expression. Only the CD27(low) subset exhibited a high capacity for IFN-gamma secretion, indicating that low CD27 expression is characteristic of fully differentiated effector CD4 T lymphocytes. We demonstrate now that CD27(low) IFN-gamma-producing CD4 T lymphocytes accumulate in the lungs but are rare in LNs. Several factors contribute to their preferential accumulation. First, CD27(low) CD4 T lymphocytes present in the LN are highly susceptible to apoptosis. Second, circulating CD27(low) CD4 T cells do not enter the LN but efficiently migrate to the lungs. Third, CD27(high) effector CD4 T cells that enter the lungs down-regulate CD27 expression in situ. In genetically heterogeneous mice that exhibit varying susceptibility to tuberculosis, the accumulation of mature CD27(low) CD4 T cells in the lungs correlates with the degree of protection against infection. Thus, we propose that terminal maturation of effector CD4 T lymphocytes in the periphery provides the host with efficient local defense and avoids potentially harmful actions of inflammatory cytokines in lymphoid organs.

Fatal recall responses mediated by CD8 T cells during intracellular bacterial challenge infection

The roles(s) of CD8 T cells during infections by intracellular bacteria that reside in host cell endocytic compartments are not well understood. Our previous studies in a mouse model of human monocytotropic ehrlichiosis indicated that CD8 T cells are not essential for immunity. However, we have observed an unexpected role for these cells during challenge infection. Although immunocompetent mice cleared a primary low-dose (nonfatal) Ixodes ovatus ehrlichia infection, a secondary low-dose challenge infection resulted in fatal disease and loss of control of infection. The outcome was CD8-dependent, because CD8-deficient mice survived secondary low-dose challenge infection. Moreover, effector and/or memory phenotype CD8 T cells were responsible, because adoptive transfer of purified CD44(high) CD8 T cells to naive mice induced fatal responses following a primary low-dose infection. The fatal responses were perforin- and Fas ligand-independent, and were associated with high serum concentrations of TNF-alpha and CCL2, and low levels of IL-10. Accordingly, blockade of either TNF-alpha or CCL2 ameliorated fatal recall responses, and in vitro coculture of memory CD8 T cells and Ixodes ovatus ehrlichia-infected peritoneal exudate cells resulted in substantial increases in TNF-alpha and CCL2. Thus, during monocytotropic ehrlichiosis, inflammatory cytokine production, by CD8 T cells and/or other host cells, can trigger chemokine-dependent disease. These findings highlight a novel role for CD8 T cells, and reveal that live vaccines for intracellular bacteria can, under some conditions, induce undesirable consequences.

Essential role for humoral immunity during Ehrlichia infection in immunocompetent mice

Although cellular immunity is essential for host defense during intracellular bacterial infections, humoral immunity can also play a significant role in host defense during infection by some intracellular bacteria, including the ehrlichiae. Antibodies can protect susceptible SCID mice from fatal Ehrlichia chaffeensis infection, an observation that has been hypothesized to involve the opsonization of bacteria released from host cells. To determine whether humoral immunity plays an essential role during ehrlichia infection in immunocompetent mice, we utilized a murine model of fatal monocytotropic ehrlichiosis caused by Ixodes ovatus ehrlichia. Mice lacking either B cells or FcgammaRI were unable to resolve a low-dose (sublethal) I. ovatus ehrlichia infection, which suggested that humoral immunity is essential for resistance. Polyclonal sera generated in I. ovatus ehrlichia-infected mice recognized a conserved ehrlichia outer membrane protein and, when administered to infected mice, caused a significant decrease in bacterial infection. Mice experimentally depleted of complement, or deficient for complement receptors 1 and 2, were also susceptible to sublethal I. ovatus ehrlichia infection, as were mice that lacked the phox91 subunit of NADPH oxidase. The data are consistent with a mechanism whereby bacteria released from infected cells are lysed directly by complement or undergo antibody-mediated FcgammaR-dependent phagocytosis and subsequent exposure to reactive oxygen intermediates. The findings suggest mechanisms whereby antibodies contribute to immunity against intracellular bacteria in immunocompetent mice.

Production of IFN-gamma by CD4 T cells is essential for resolving ehrlichia infection

To address the role of cellular immunity during ehrlichia infection, we have used a newly described model of monocytic ehrlichiosis that results from infection of mice by an ehrlichia that was isolated from an Ixodes ovatus tick (Ixodes ovatus ehrlichia, IOE). Immunocompetent C57BL/6 and BALB/c mice exhibited a dose-dependent susceptibility to IOE infection. Mice infected with a high dose inoculum ( approximately 1000 organisms) exhibited pronounced thrombocytopenia, lymphopenia, anemia, and morbidity within 12 days postinfection. Infection was associated with bacterial colonization of a number of tissues. In contrast, mice infected with a low dose inoculum ( approximately 100 organisms) exhibited only transient disease and were able to resolve the infection. SCID mice were highly susceptible to low-dose infection, indicating that adaptive immunity was required. Resistance to sublethal challenge in both C57BL/6 and BALB/c mice was CD4-, but not CD8-, dependent and required IL-12p40-dependent cytokines, IFN-gamma, and TNF-alpha, but not IL-4. CD4 T cells purified from infected mice proliferated in vitro in response to IOE Ags. T cell proliferation was associated with production of IFN-gamma, and the production of this cytokine by CD4 T cells rescued IFN-gamma-deficient mice from fatal infection. Exogenous IFN-gamma was capable of inducing microbiocidal activity in infected macrophages. The data suggest that classical immune mechanisms involving CD4 cells and type 1 cytokines are responsible for macrophage activation and for elimination of this intracellular bacterial pathogen.

Long-term ventilation in children with severe central nervous system impairment

Absent from the list of indications for long-term ventilation (LTV) is its use for children with severe central nervous system impairment, including those with severe mental retardation or in a permanent vegetative state. Over a two year period, we evaluated eight children with severe CNS dysfunction for whom long-term ventilation was being contemplated. Of these eight patients, three were in a permanent vegetative state and the remainder were severely neurologically impaired, with minimal cognition. The following recommendations were developed: (1) LTV for patients in a permanent vegetative state is inappropriate. (2) In a patient with severe neurologic disease, the process of informed consent must be viewed as dynamic; once the patient's condition is diagnosed, discussion should begin about the likely course of the disease (upper airway obstruction, respiratory failure, or both) and available treatment options. (3) Continued efforts must be made to resolve conflicts between healthcare professionals and surrogates concerning aggressive support of children with severe CNS dysfunction. Discussions should continue even after a decision to provide long-term ventilation is made. (4) Currently, requests by surrogates for LTV in patients with severe neurologic impairment are usually honored because of respect for family values. (5) Physicians and other healthcare professionals should develop an open and fair process for determining inappropriate care. (6) Once LTV is initiated, efforts to transfer the child to home or a long-term care facility should be made. Further life-saving support should be discouraged. (7) Irremediable patient suffering is reason to refuse a surrogate request for LTV. A patient's preservable existence might be so torturous, painful, or filled with suffering that continued medical intervention would be inhumane or abusive.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.

Cell surface expression of class II MHC proteins bound by a single peptide

On normal cells, the peptide-binding grooves of class II MHC proteins contain a wide spectrum of peptides. For some purposes, however, it would be helpful to have cells bearing class II proteins engaged by only one peptide species. In an attempt to make such cells we constructed a gene for a MHC class II beta-chain, IA beta b, covalently linked to a peptide, E alpha 52-68, which is known to bind to the peptide-binding groove of IAb. This gene, together with the gene for IA alpha b, was transfected into B lymphoma cells and fibroblasts. The IAb-E alpha complex was expressed on the surfaces of these cells where it could be recognized by a mAb and T cells specific for IAb plus E alpha 52-68. Most of the peptide on fibroblasts remained covalently attached to the IAb beta-chain, but the covalent linker and/or peptide were degraded to some extent on B lymphoma cells. Nearly all of the IAb expressed by transfected fibroblasts was occupied by the E alpha peptide. Of 16 IAb-reactive T cell hybridomas, only 3 could respond to the IAb-E alpha complex on fibroblasts, confirming the idea that recognition of MHC may often involve recognition of the peptides bound to the MHC as well.

Cell surface expression of class II MHC proteins bound by a single peptide

On normal cells, the peptide-binding grooves of class II MHC proteins contain a wide spectrum of peptides. For some purposes, however, it would be helpful to have cells bearing class II proteins engaged by only one peptide species. In an attempt to make such cells we constructed a gene for a MHC class II beta-chain, IA beta b, covalently linked to a peptide, E alpha 52-68, which is known to bind to the peptide-binding groove of IAb. This gene, together with the gene for IA alpha b, was transfected into B lymphoma cells and fibroblasts. The IAb-E alpha complex was expressed on the surfaces of these cells where it could be recognized by a mAb and T cells specific for IAb plus E alpha 52-68. Most of the peptide on fibroblasts remained covalently attached to the IAb beta-chain, but the covalent linker and/or peptide were degraded to some extent on B lymphoma cells. Nearly all of the IAb expressed by transfected fibroblasts was occupied by the E alpha peptide. Of 16 IAb-reactive T cell hybridomas, only 3 could respond to the IAb-E alpha complex on fibroblasts, confirming the idea that recognition of MHC may often involve recognition of the peptides bound to the MHC as well.

Co-stimulation via CD28 induces activation of a refractory subset of MRL-lpr/lpr T lymphocytes

Peripheral lymphoid tissues of lpr mice contain a large proportion of TCR alpha beta/CD3+CD4-CD8- T cells that lack surface CD2 and express the B cell isoform of CD45, B220. This subset of T cells does not proliferate or produce IL-2 in response to mitogenic signals or TCR-CD3 ligation. At the same time, these abnormal T cells display several characteristics of an activated phenotype. Collectively, these properties of lpr CD4-CD8- T cells have functional parallels with anergic T cells. A critical co-stimulatory molecule implicated in the prevention of or recovery from anergy is CD28, which binds the ligand BB1/B7 on certain accessory cells. lpr CD4-CD8- T cells express normal levels of CD28 which is capable of transducing a strong proliferative signal to these cells in co-stimulation with mitogens. However, proliferation of lpr CD4-CD8- T cells in response to CD28 co-stimulation does not reach the levels observed in normal T cells stimulated under similar conditions. Stimulation with anti-CD28 mAb in conjunction with phorbol myristate acetate and ionomycin promotes cell cycling in the CD2- subset of CD4-CD8- T cells, and results in a slight induction of CD2 levels during the course of the culture period. However, the majority of cells obtained at the end of the culture period remain TCR alpha beta+ CD4-CD8-, CD2low/- and B220high, similar to freshly isolated CD4-CD8- lpr T cells. In contrast, if IL-2 is included in the cultures, a strong shift toward a CD2+ phenotype is observed by a majority of the lpr T cells. Upon repeat stimulation, these lpr CD4-CD8- T cells can now proliferate in an IL-2-dependent manner when stimulated with only anti-CD3 mAb or mitogens, in the absence of exogenous IL-2 or anti-CD28 mAb. These data show that the hyporesponsiveness of lpr CD4-CD8- T cells does not result from a lack of CD28 expression, that it is not a fixed state, and that it can be reversed by the induction of cell cycling in the presence of IL-2. These observations extend the parallels between lpr CD4-CD8- T cells and anergic T cells.

Elevated production of interferon-gamma and interleukin 4 by mature T cells from autoimmune lpr mice correlates with Pgp-1 (CD44) expression

The cell surface glycoprotein, Pgp-1 (CD44), has been shown to be a marker of murine memory T lymphocytes. When activated, Pgp-1hi memory T cells produce strikingly higher amounts of interferon-gamma (IFN-gamma) than naive Pgp-1lo T cells, yet both subsets make similar levels of interleukin (IL)2. Whereas Pgp-1hi cells represent only 20%-25% of peripheral T cells from most strains, this marker is expressed by the vast majority (greater than 90%) of T cells from autoimmune MRL mice homozygous for the lymphoproliferation (lpr) gene. The massive lymphadenopathy that develops in lpr/lpr mice is composed of both non-mature (CD4-CD8-) T cells as well as a greatly expanded number (up to 300-fold) of mature (CD4+CD8-,CD4-CD8+) T cells. Paralleling the expression of high levels of Pgp-1, we find that compared to normal mouse T cells, the lpr mature T lymphocyte subsets are also very high producers on a per cell basis of IFN-gamma and, for the CD4+ subset, IL 4. Increased concentrations of IFN-gamma and IL 4 produced by large numbers of lpr Pgp-1hi mature T cells could contribute to the autoimmune syndrome in MRL lpr/lpr mice through the effects of these cytokines on augmenting MHC class II expression and production of certain classes of antibodies.

Intact antigen receptor-mediated generation of inositol phosphates and increased intracellular calcium in CD4 CD8 T lymphocytes from MRL lpr mice

The predominant T lymphocytes that accumulate in the peripheral lymphoid tissues of mice homozygous for the lpr gene bear the phenotype CD3+CD4-CD8-. By certain functional criteria these cells would appear to have impaired CD3-mediated signal transduction, in that they do not respond to alloantigen and produce little if any detectable IL-2 or other lymphokines. However, the signal pathway appears adequate for achieving other T cell functions, including induction of high affinity IL-2R, and thymic deletion. To clarify the basis of this seeming discrepancy, we examined transmembrane signal transduction in T cell subsets of lpr/lpr (lpr) and +/+ mice, as defined by increased [Ca2+]i and the generation of inositol phosphates (InsPs). Stimulation of lpr CD4-CD8- cells with anti-CD3 antibody produced prompt and sustained increases in the concentration of [C2+]i and in InsPs. Similar responses occurred in mature T cells from lpr and +/+ mice, except for the somewhat slower kinetics of their increased [Ca2+]i. In marked distinction to the anti-CD2-mediated response, Con A, even in high doses, could not stimulate any increase of [Ca2+]i in lpr CD4-CD8- cells, and only modest increases in InsPs. Mature T cells, whether of lpr or +/+ origin, yielded normal increased [Ca2+]i with Con A. The reason for the differences in signal transduction between anti-CD3 and Con A stimulation of lpr CD4-CD8- cells may relate to the absence of surface structures on these immature T cells that are required for activation by Con A but not by anti-CD3. The data demonstrate that the CD3 complex in lpr CD4-CD8- T cells can couple to phospholipase C to hydrolyze phosphoinositides. These activation properties of lpr CD4-CD8- T cells have interesting functional parallels to thymocytes at the time of thymic selection, as well as tolerance induction of mature T lymphocytes.

Intact antigen receptor-mediated generation of inositol phosphates and increased intracellular calcium in CD4 CD8 T lymphocytes from MRL lpr mice

The predominant T lymphocytes that accumulate in the peripheral lymphoid tissues of mice homozygous for the lpr gene bear the phenotype CD3+CD4-CD8-. By certain functional criteria these cells would appear to have impaired CD3-mediated signal transduction, in that they do not respond to alloantigen and produce little if any detectable IL-2 or other lymphokines. However, the signal pathway appears adequate for achieving other T cell functions, including induction of high affinity IL-2R, and thymic deletion. To clarify the basis of this seeming discrepancy, we examined transmembrane signal transduction in T cell subsets of lpr/lpr (lpr) and +/+ mice, as defined by increased [Ca2+]i and the generation of inositol phosphates (InsPs). Stimulation of lpr CD4-CD8- cells with anti-CD3 antibody produced prompt and sustained increases in the concentration of [C2+]i and in InsPs. Similar responses occurred in mature T cells from lpr and +/+ mice, except for the somewhat slower kinetics of their increased [Ca2+]i. In marked distinction to the anti-CD2-mediated response, Con A, even in high doses, could not stimulate any increase of [Ca2+]i in lpr CD4-CD8- cells, and only modest increases in InsPs. Mature T cells, whether of lpr or +/+ origin, yielded normal increased [Ca2+]i with Con A. The reason for the differences in signal transduction between anti-CD3 and Con A stimulation of lpr CD4-CD8- cells may relate to the absence of surface structures on these immature T cells that are required for activation by Con A but not by anti-CD3. The data demonstrate that the CD3 complex in lpr CD4-CD8- T cells can couple to phospholipase C to hydrolyze phosphoinositides. These activation properties of lpr CD4-CD8- T cells have interesting functional parallels to thymocytes at the time of thymic selection, as well as tolerance induction of mature T lymphocytes.

Mutations within the decoding site of Escherichia coli 16S rRNA: growth rate impairment, lethality and intragenic suppression

Several C----U transitions and small deletions were introduced into the conserved region centered on base C1400 in Escherichia coli 16S rRNA by in vitro mutagenesis. The mutations were placed within rrnB operons on multicopy plasmids under the transcriptional regulation of either the normal rrnB P1P2 promoters or the temperature-inducible PL promoter from bacteriophage lambda and introduced into E. coli hosts. When expressed from the P1P2 promoters, several of the mutant 16S rRNAs impaired cell growth while others, including one in which U replaced C at position 1400 within the ribosomal decoding site, had little or no effect on cell doubling time. However, C----U transitions at positions 1395 and 1407, as well as the deletion of C1400, appeared to render their hosts inviable. Cells in which these mutations were expressed from the lambdaPL promoter died within four generations after induction. Unexpectedly, the lethal phenotype was suppressed intragenically by replacement of G1505 with A, C or U. Suppression may alleviate a functional defect in 30S subunits containing the U1395, U1407 or deltaC1400 mutations.