Chimeric antigen receptor T cells in the fast lane among autoimmune disease therapies

In this commentary, we highlight recent studies demonstrating the feasibility and promise of chimeric antigen receptor (CAR) T-cell therapy in treating a number of autoimmune disorders including systemic lupus erythematosus and compare CAR T cells to other therapies aimed at depleting B-lineage cells in treating such diseases.

Prevalence, risk factors, and prognosis of drug-resistant epilepsy in autoimmune encephalitis

OBJECTIVE

To evaluate the prevalence and biomarkers of drug-resistant epilepsy (DRE) in patients with autoimmune encephalitis (AIE).

METHODS

Sixty-nine patients with AIE were recruited retrospectively and electroencephalographies (EEGs) were reviewed using a standard reporting proforma. Associations between EEG biomarkers and DRE development at 12 months were examined using logistic regression modeling and were utilized to create a DRE risk score.

RESULTS

Sixteen percent of patients with AIE developed DRE at 12-month follow-up. The presence of status epilepticus (SE) (OR 11.50, 95% CI [2.81, 51.86], p-value <0.001), temporal lobe focality (OR 9.90, 95% CI [2.60, 50.71], p-value 0.001) and periodic discharges (OR 19.12, 95% CI [3.79, 191.10], p-value 0.001) on the admission EEG were associated with the development of DRE at 12 months. These variables were utilized to create a clinically applicable risk score for the prediction of DRE development.

CONCLUSIONS

Drug-resistant epilepsy is an infrequent complication of AIE. Electroencephalography changes during the acute illness can predict the risk of DRE at 12 months post-acute AIE.

SIGNIFICANCE

The identified EEG biomarkers provide the basis to generate a clinically applicable prediction tool which could be used to inform treatment, prognosis, and select patients for acute treatment trials.

Electroclinical biomarkers of autoimmune encephalitis

OBJECTIVE

To evaluate the utility of electroencephalography (EEG) changes as diagnostic and prognostic biomarkers in acute autoimmune encephalitis (AIE).

METHODS

One hundred and thirty-one patients with AIE were recruited retrospectively across 7 hospitals. Clinical data were collected during admission and at 12 months. EEGs were reviewed using a standard reporting proforma. Associations between EEG biomarkers, AIE subtypes, and clinical outcomes were assessed using logistic regression modeling.

RESULTS

Presence of superimposed fast activity (OR 34.33; 95% CI 3.90, 4527.27; p < 0.001), fluctuating EEG abnormality (OR 6.60; 95% CI 1.60, 37.59; p = 0.008), and hemispheric focality (OR 28.48; 95% CI 3.14, 3773.14; p < 0.001) were significantly more common in N-methyl-d-aspartate receptor (NMDAR) antibody-associated patients with AIE compared to other AIE subtypes. Abnormal background rhythm was associated with a poor mRS (modified Rankin score) at discharge (OR 0.29; 95% CI 0.10, 0.75; p = 0.01) and improvement in mRS at 12 months compared with admission mRS (3.72; 95% CI 1.14, 15.23; p = 0.04).

SIGNIFICANCE

We have identified EEG biomarkers that differentiate NMDAR AIE from other subtypes. We have also demonstrated EEG biomarkers that are associated with poor functional outcomes.

B cell memory: understanding COVID-19

Immunological memory is a mechanism to protect us against reinfection. Antibodies produced by B cells are integral to this defense strategy and underlie virtually all vaccine success. Here, we explain how B cell memory is generated by infection and vaccination, what influences its efficacy and its persistence, and how characterizing these parameters in the immune response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) will help achieve protective immunity through vaccination.

Lymph node stromal CCL2 limits antibody responses

Nonhematopoietic stromal cells in lymph nodes such as fibroblastic reticular cells (FRCs) can support the survival of plasmablasts and plasma cells [together, antibody-forming cells (AFCs)]. However, a regulatory function for the stromal compartment in AFC accumulation has not been appreciated. Here, we show that chemokine ligand 2 (CCL2)-expressing stromal cells limit AFC survival. FRCs express high levels of CCL2 in vessel-rich areas of the T cell zone and the medulla, where AFCs are located. FRC CCL2 is up-regulated during AFC accumulation, and we use lymph node transplantation to show that CCL2 deficiency in BP3⁺ FRCs and lymphatic endothelial cells increases AFC survival without affecting B or germinal center cell numbers. Monocytes are key expressers of the CCL2 receptor CCR2, as monocyte depletion and transfer late in AFC responses increases and decreases AFC accumulation, respectively. Monocytes express reactive oxygen species (ROS) in an NADPH oxidase 2 (NOX2)-dependent manner, and NOX2-deficient monocytes fail to reduce AFC numbers. Stromal CCL2 modulates both monocyte accumulation and ROS production, and is regulated, in part, by manipulations that modulate vascular permeability. Together, our results reveal that the lymph node stromal compartment, by influencing monocyte accumulation and functional phenotype, has a regulatory role in AFC survival. Our results further suggest a role for inflammation-induced vascular activity in tuning the lymph node microenvironment. The understanding of stromal-mediated AFC regulation in vessel-rich environments could potentially be harnessed to control antibody-mediated autoimmunity.

IRF4 Activity Is Required in Established Plasma Cells to Regulate Gene Transcription and Mitochondrial Homeostasis

The transcription factor interferon regulatory factor 4 (IRF4) is critical for the development, maintenance, and function of plasma cells. The mechanism by which IRF4 exerts its action in mature plasma cells has been elusive due to the death of all such cells upon IRF4 loss. While we identify apoptosis as a critical pathway for the death of plasma cells caused by IRF4 loss, we also determine that IRF4 did not regulate the intrinsic apoptotic pathway directly. By using an inducible IRF4 deletion system in the presence of the overexpression of anti-apoptotic BCL2, we identify genes whose expression is coordinated by IRF4 and that in turn specify plasma cell identity and mitochondrial homeostasis.

Do plasma cells contribute to the determination of their lifespan?

Longevity of plasma cells is dependent on their ability to access and reside in so-called survival niches that are predominantly located in the bone marrow. It is proposed that by some process a small fraction of the plasma cells generated in response to new antigen challenges can enter into the long-lived repertoire by displacing existing plasma cells. Several lines of research show that this process is not stochastic as not all resident, long-lived plasma cells appear equally likely to be displaced. The basis of these differences might reside in the niches, the plasma cells or a combination of both factors that intersect to create a distribution of susceptibility to replacement and lifespans. In this review, I consider factors that might vary in plasma cells and thus influence their access to niches and the ability of newly generated plasma cells to survive over the long term.

Innate Immunity in the Central Nervous System: A Missing Piece of the Autoimmune Encephalitis Puzzle?

The autoimmune encephalitides are a group of autoimmune conditions targeting the central nervous system and causing severe clinical symptoms including drug-resistant seizures, cognitive dysfunction and psychiatric disturbance. Although these disorders appear to be antibody mediated, the role of innate immune responses needs further clarification. Infiltrating monocytes and microglial proliferation at the site of pathology could contribute to the pathogenesis of the disease with resultant blood brain barrier dysfunction, and subsequent activation of adaptive immune response. Both innate and adaptive immune cells can produce pro-inflammatory molecules which can perpetuate ongoing neuroinflammation and drive ongoing seizure activity. Ultimately neurodegenerative changes can ensue with resultant long-term neurological sequelae that can impact on ongoing patient morbidity and quality of life, providing a potential target for future translational research.

Arginine methylation catalyzed by PRMT1 is required for B cell activation and differentiation

Arginine methylation catalyzed by protein arginine methyltransferases (PRMT) is a common post-translational modification in mammalian cells, regulating many important functions including cell signalling, proliferation and differentiation. Here we show the role of PRMT1 in B-cell activation and differentiation. PRMT1 expression and activity in human and mouse peripheral B cells increases in response to in vitro or in vivo activation. Deletion of the Prmt1 gene in mature B cells establishes that although the frequency and phenotype of peripheral B cell subsets seem unaffected, immune responses to T-cell-dependent and -independent antigens are substantially reduced. In vitro activation of Prmt1-deficient B cells with a variety of mitogens results in diminished proliferation, differentiation and survival, effects that are correlated with altered signal transduction from the B cell receptor. Thus PRMT1 activity in B cells is required for correct execution of multiple processes that in turn are necessary for humoral immunity.

PRMT1 is an arginine methyltransferase involved in a variety of cell functions. Here the authors delete PRMT1 specifically in mature B cells to show the importance of arginine methylation for B cell proliferation, differentiation and survival, and thereby for humoral immunity.

Regulation of plasma cell differentiation, migration and class-switch by c-Myb and T-bet

Humoral responses are tailored to respond most effectively to the invading pathogen. To achieve this, long lasting, protective antibodies of an affinity and isotype appropriate for the immunising antigen or infecting pathogen are produced mainly within germinal centres (GC). Here, we detail the critical, interdependent roles of the transcription factors c-Myb and T-bet in regulating GC egress and plasma cell differentiation. Deletion of c-Myb in mature B cells led to aberrant up-regulation of plasma cell transcription factors within the GC. Indeed, a subset of cycling cells within the GC secreted antibody and were CD138+ Blimp-1-GFP+. T-bet-regulated genes were also modulated, resulting in significantly elevated serum IgG2c and CXCR3 expression on plasma cells despite a Th2-biased immunization with NP-KLH in alum. These aberrant plasma cells within the GC were no longer evident upon deletion of T-bet in c-Myb-deficient mice. Surprisingly, this interplay between transcription factor networks also occurred during a response to influenza infection, in which T-bet expression is a normal component of B cell responses in wild-type mice. Chromatin accessibility studies of the T-bet promoter revealed that wild-type GC B cells from flu-infected mice had an increase in accessibility in comparison to naïve B cells, as expected. In line with the cellular data, however, there was an even further increase in chromatin accessibility in c-Myb-deficient GC B cells. Therefore, c-Myb establishes a transcription factor network during an immune response that can modulate the transcriptional network of T-bet and subsequent coordination of GC egress with plasma cell differentiation.

The BTB-ZF transcription factor Zbtb20 is driven by Irf4 to promote plasma cell differentiation and longevity

Zbtb20 facilitates terminal differentiation of B cells into antibody-secreting cells, and its expression is dependent on Irf4 and independent of Blimp1.

The transcriptional network regulating antibody-secreting cell (ASC) differentiation has been extensively studied, but our current understanding is limited. The mechanisms of action of known “master” regulators are still unclear, while the participation of new factors is being revealed. Here, we identify Zbtb20, a Bcl6 homologue, as a novel regulator of late B cell development. Within the B cell lineage, Zbtb20 is specifically expressed in B1 and germinal center B cells and peaks in long-lived bone marrow (BM) ASCs. Unlike Bcl6, an inhibitor of ASC differentiation, ectopic Zbtb20 expression in primary B cells facilitates terminal B cell differentiation to ASCs. In plasma cell lines, Zbtb20 induces cell survival and blocks cell cycle progression. Immunized Zbtb20-deficient mice exhibit curtailed humoral responses and accelerated loss of antigen-specific plasma cells, specifically from the BM pool. Strikingly, Zbtb20 induction does not require Blimp1 but depends directly on Irf4, acting at a newly identified Zbtb20 promoter in ASCs. These results identify Zbtb20 as an important player in late B cell differentiation and provide new insights into this complex process.

Diversity among memory B cells: origin, consequences, and utility

Immunological memory is the residuum of a successful immune response that in the B cell lineage comprises long-lived plasma cells and long-lived memory B cells. It is apparent that distinct classes of memory B cells exist, distinguishable by, among other things, immunoglobulin isotype, location, and passage through the germinal center. Some of this variation is due to the nature of the antigen, and some appears to be inherent to the process of forming memory. Here, we consider the heterogeneity in development and phenotype of memory B cells and whether particular functions are partitioned into distinct subsets. We consider also how understanding the details of generating memory may provide opportunities to develop better, functionally targeted vaccines.

B-cell differentiation: instructive one day, stochastic the next

Three recent papers provide striking insight into the mechanisms used to regulate B-cell differentiation. They demonstrate that B-cell fate choice can be stochastic, directed, inherited, or some combination of these, depending on the circumstances. The trick is going to be working out which is important when.

Sheepish B cells: evidence for antigen-independent antibody diversification in humans and mice

Antibody diversity is first generated by rearrangement of immunoglobulin (Ig) genes during B cell development in the bone marrow, and later by antigen-driven diversification in germinal centers (GCs). New data in humans and mice now identify specific B cell populations that may have undergone antigen-independent hypermutation outside GCs.

Plasma cell differentiation and survival

Humoral immunity depends on the regulated production and maintenance of antibody secreting cells during the course of an immune response. Recent insights into the transcriptional regulation of the initiation of plasma cell differentiation have clarified aspects of this process, particularly with respect to the choice between the memory B cell and plasma cell differentiation pathways. It is now possible to specify the conditions favouring these outcomes and to predict where they might occur within the germinal center. Once formed, plasma cell survival is critically dependent on accessing niches that are formed by stomal elements in both normal and inflamed tissues. The apparent homeostasis of plasma cell numbers means that new specificities can persist only at the expense of existing ones, raising questions on how immunological memory is maintained in the face of new immune responses. The answer appears to be through the reduction of the process to a single cell level, thereby introducing an element of stochasticity.

Antigen to the node: B cells go native

The participation of B and T cells in an immune response requires their separate activation by different forms of the target antigen. In this issue of Immunity, Pape et al. (2007) describe antigen entry into the B cell areas in the lymph node without degradation or cellular assistance.

B-cell memory: are subsets necessary?

B-cell memory is provided by populations of quiescent memory B cells and long-lived plasma cells. Whereas it is clear that both of these cell populations arise from germinal centres, the signals and circumstances that trigger germinal-centre B cells to enter and then persist in memory compartments are poorly defined. Here, I propose that germinal centres produce memory B cells and plasma cells throughout the immune response and that memory B cells arise by the emigration of B cells that are chosen at random from the pool available in the germinal centre. The ability of such emigrants to survive as memory B cells depends on their germinal-centre 'history', with the persistence of high-affinity B-cell variants being favoured.

SOCS5 is expressed in primary B and T lymphoid cells but is dispensable for lymphocyte production and function

Suppressors of cytokine signaling (SOCSs) are key regulators of cytokine-induced responses in hematopoietic as well as nonhematopoietic cells. SOCS1 and SOCS3 have been shown to modulate T-cell responses, whereas the roles of other SOCS family members in the regulation of lymphocyte function are less clear. Here, we report the generation of mice with a targeted disruption of the Socs5 gene. Socs5(-/-) mice were born in a normal Mendelian ratio and were healthy and fertile. We found that SOCS5 is expressed in primary B and T cells in wild-type mice. However, no abnormalities in the lymphocyte compartment were seen in SOCS5-deficient mice. We examined antigen- and cytokine-induced proliferative responses in B and T cells in the absence of SOCS5 and found no deviations from the responses seen in wild-type cells. Because SOCS5 has been implicated in Th1 differentiation, we also investigated the importance of SOCS5 in T helper cell responses. Unexpectedly, SOCS5-deficient CD4 T cells showed no abnormalities in Th1/Th2 differentiation and Socs5(-/-) mice showed normal resistance to infection with Leishmania major. Therefore, although SOCS5 is expressed in primary B and T cells, it appears to be dispensable for the regulation of lymphocyte function.

Architectural defects in the spleens of Nkx2-3-deficient mice are intrinsic and associated with defects in both B cell maturation and T cell-dependent immune responses

Mice lacking the homeodomain transcription factor Nkx2-3 are either asplenic or develop a spleen of significantly reduced size with poorly organized white pulp. In this report, we analyze the effect of this mutation on B lymphocyte development and differentiation. Follicular dendritic cells in spleen, but not lymph node, of Nkx2-3(-/-) mice fail to express a developmental Ag (follicular dendritic cell-M2) and show an abnormal association with B cells, despite essentially normal expression of several chemokine genes. Bone marrow reconstitution studies show the splenic disorganization and absence of marginal zone B cells to be of stromal rather than hemopoietic origin. Furthermore, Nkx2-3(-/-) mice show an excess of conventional B cells in mesenteric lymph node and peritoneal cavity, whereas transitional B cells are rare in spleen but overrepresented in bone marrow. Finally, immunization of Nkx2-3(-/-) mice with a T cell-dependent Ag elicits clusters of germinal center B cells, although these fail to develop to the same extent as in controls and there is no evidence of affinity maturation in serum Ab. Similarly, Ab-forming cells fail to aggregate into foci early in the response. Collectively, these data indicate a substantial role for Nkx2-3 in the correct association of lymphocytes and splenic stromal elements that is independent of chemokine expression.

The absence of Tssc6, a member of the tetraspanin superfamily, does not affect lymphoid development but enhances in vitro T-cell proliferative responses

The tetraspanins are a family of integral membrane proteins with four transmembrane domains. These molecules form multimolecular networks on the surfaces of many different cell types. Gene-targeting studies have revealed a role for tetraspanins in B- and T-lymphocyte function. We have isolated and deleted a novel tetraspanin, Tssc6, which is expressed exclusively in hematopoietic and lymphoid organs. Using a gene-trapping strategy, we generated an embryonic stem (ES) cell line with an insertion in the Tssc6 locus. Mice were derived from these ES cells and, using RNase protection and reverse transcription-PCR, we demonstrated that the insertion resulted in a null mutation of the Tssc6 allele. Mice homozygous for the gene trap insertion (Tssc6(gt/gt) mice) were viable and fertile, with normal steady-state hematopoiesis. Furthermore, responses to hemolysis and granulocyte colony-stimulating factor-induced granulopoiesis were equivalent to those of wild-type mice. Lymphoid development was normal in Tssc6(gt/gt) mice. Whereas Tssc6(gt/gt) B cells responded normally to lipopolysaccharide, anti-CD40, and anti-immunoglobulin M stimulation, Tssc6(gt/gt) T cells showed enhanced responses to concanavalin A, anti-CD3, and anti-CD28. This increased proliferation by Tssc6-deleted T lymphocytes was due to increased interleukin 2 production following T-cell receptor stimulation. These results demonstrate that Tssc6 is not required for normal development of the hematopoietic system but may play a role in the negative regulation of peripheral T-lymphocyte proliferation.

The combined absence of NF-kappa B1 and c-Rel reveals that overlapping roles for these transcription factors in the B cell lineage are restricted to the activation and function of mature cells

Transcription factors NF-kappaB1 and c-Rel, individually dispensable during embryogenesis, serve similar, yet distinct, roles in the function of mature hemopoietic cells. Redundancy among Rel/NF-kappaB family members prompted an examination of the combined roles of c-Rel and NF-kappaB1 by using mice that lack both proteins. Embryonic development and the maturation of hemopoietic progenitors were unaffected in nfkb1(-/-)c-rel(-/-) mice. Peripheral T cell populations developed normally, but follicular, marginal zone, and CD5(+) peritoneal B cell populations all were reduced. In culture, a failure of mitogen-stimulated nfkb1(-/-)c-rel(-/-) B cells to proliferate was caused by a cell cycle defect in early G(1) that prevented growth. In vivo, defects in humoral immunity and splenic architecture seen in nfkb1(-/-) and c-rel(-/-) mice were exacerbated in the double mutant mice. These findings demonstrate that in the B lineage overlapping roles for NF-kappaB1 and c-Rel appear to be restricted to regulating the activation and function of mature cells.

Homeodomain factor Nkx2-3 controls regional expression of leukocyte homing coreceptor MAdCAM-1 in specialized endothelial cells of the viscera

Regulated emigration of blood-borne leukocytes plays a defining role in lymphoid organ development, immune surveillance, and inflammatory responses. We report here that mice deficient in the homeobox gene Nkx2-3, expressed in developing visceral mesoderm, show a complex intestinal malabsorption phenotype and striking abnormalities of gut-associated lymphoid tissue and spleen suggestive of deranged leukocyte homing. Mutant Peyer's patches were reduced in number and size, intestinal villi contained few IgA(+) plasma cells, and mutant spleens were small and often atrophic, showing fused periarterial lymphoid sheaths, partially merged T and B cell zones, an absent marginal zone, and a dearth of macrophages in red pulp. Semiquantitative RT-PCR analysis and immunohistochemistry revealed down-regulation of mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in endothelial cells in which Nkx2-3 is normally expressed. MAdCAM-1 is a member of the immunoglobulin superfamily, acting as an endothelial cell ligand for leukocyte homing receptors L-selectin and alpha4beta7 integrin. Our data suggest a role for a homeodomain factor in establishing the developmental and positional cues in endothelia that regulate leukocyte homing through local control of cellular adhesion and identify MAdCAM-1 as a candidate target gene of Nkx2-3.

bcl-2 transgene expression inhibits apoptosis in the germinal center and reveals differences in the selection of memory B cells and bone marrow antibody-forming cells

Immunization with T cell-dependent antigens generates long-lived memory B cells and antibody-forming cells (AFCs). Both populations originate in germinal centers and, predominantly, produce antibodies with high affinity for antigen. The means by which germinal center B cells are recruited into these populations remains unclear. We have examined affinity maturation of antigen-specific B cells in mice expressing the cell death inhibitor bcl-2 as a transgene. Such mice had reduced apoptosis in germinal centers and an excessive number of memory B cells with a low frequency of V gene somatic mutation, including those mutations encoding amino acid exchanges known to enhance affinity. Despite the frequency of AFCs being increased in bcl-2-transgenic mice, the fraction secreting high-affinity antibody in the bone marrow at day 42 remained unchanged compared with controls. The inability of BCL-2 to alter selection of bone marrow AFCs is consistent with these cells being selected within the germinal center on the basis of their affinity being above some threshold rather than their survival being due to a selective competition for an antigen-based signal. Continuous competition for antigen does, however, explain formation of the memory compartment.

Genetic Evidence for Lyn as a Negative Regulator of IL-4 Signaling

IL-4 has multiple effects on B lymphocytes, many of which are concentration dependent. This is particularly so for Ig isotype switching, where different thresholds of IL-4 stimulation are needed to induce switching from IgM to either IgG1 or IgE. In this report we describe a critical role for the tyrosine kinase Lyn in setting IL-4 signaling thresholds in mouse B lymphocytes. Upon CD40 ligand stimulation of lyn−/− B cells, 10-fold less IL-4 was required to induce switching from IgM to IgG1 and IgE and an increased proportion of B cells isotype switched at each IL-4 concentration. These in vitro results correlate with the in vivo findings that in lyn−/− mice, IgG1 Ab-forming cells develop prematurely in ontogeny and that adult lyn−/− mice have an abnormally high proportion of IgG1-expressing B cells in their spleens. Adult lyn−/− mice also have significantly higher levels of IgE in their serum. These results identify Lyn as a molecule involved in modulating the IL-4 signal in B cells and provide insights into its regulation and how a B cell signaling imbalance may contribute to atopy.

Genetic evidence for Lyn as a negative regulator of IL-4 signaling

IL-4 has multiple effects on B lymphocytes, many of which are concentration dependent. This is particularly so for Ig isotype switching, where different thresholds of IL-4 stimulation are needed to induce switching from IgM to either IgG1 or IgE. In this report we describe a critical role for the tyrosine kinase Lyn in setting IL-4 signaling thresholds in mouse B lymphocytes. Upon CD40 ligand stimulation of lyn-/- B cells, 10-fold less IL-4 was required to induce switching from IgM to IgG1 and IgE and an increased proportion of B cells isotype switched at each IL-4 concentration. These in vitro results correlate with the in vivo findings that in lyn-/- mice, IgG1 Ab-forming cells develop prematurely in ontogeny and that adult lyn-/- mice have an abnormally high proportion of IgG1-expressing B cells in their spleens. Adult lyn-/- mice also have significantly higher levels of IgE in their serum. These results identify Lyn as a molecule involved in modulating the IL-4 signal in B cells and provide insights into its regulation and how a B cell signaling imbalance may contribute to atopy.

Germinal centers: getting there is half the fun

Several recent studies have provided new insights into how the infrequent B and T cells that are specific for a particular immunizing antigen come together to form the germinal centers that are crucial to the immune response.

Germinal centers: form and function

Our understanding of the formation of germinal centers has advanced considerably during the past year. Highlights include the elucidation of the role of cytokines and chemokines in splenic organization and lymphocyte migration and their roles in germinal center development. The functional consequences of recombination-activating gene re-expression in the germinal center have also been reported, as have in vitro models of somatic mutation. Finally the resolution of the germinal center reaction is being addressed by analysis of the individual cell types produced.

Adult mice with targeted mutation of the interleukin-11 receptor (IL11Ra) display normal hematopoiesis

Interleukin-11 (IL-11) is a pleiotropic growth factor with a prominent effect on megakaryopoiesis and thrombopoiesis. The receptor for IL-11 is a heterodimer of the signal transduction unit gp130 and a specific receptor component, the alpha-chain (IL-11R alpha). Two genes potentially encode the IL-11R alpha: the IL11Ra and IL11Ra2 genes. The IL11Ra gene is widely expressed in hematopoietic and other organs, whereas the IL11Ra2 gene is restricted to only some strains of mice and its expression is confined to testis, lymph node, and thymus. To investigate the essential actions mediated by the IL-11R alpha, we have generated mice with a null mutation of IL11Ra (IL11Ra-/-) by gene targeting. Analysis of IL11Ra expression by Northern blot and reverse transcriptase-polymerase chain reaction, as well as the absence of response of IL11Ra-/- bone marrow cells to IL-11 in hematopoietic assays, further confirmed the null mutation. Compensatory expression of the IL11Ra2 in bone marrow cells was not detected. IL11Ra-/- mice were healthy with normal numbers of peripheral blood white blood cells, hematocrit, and platelets. Bone marrow and spleen contained normal numbers of cells of all hematopoietic lineages, including megakaryocytes. Clonal cultures did not identify any perturbation of granulocyte-macrophage (GM), erythroid, or megakaryocyte progenitors. The number of day-12 colony-forming unit-spleen progenitors were similar in wild-type and IL11Ra-/- mice. The kinetics of recovery of peripheral blood white blood cells, platelets, and bone marrow GM progenitors after treatment with 5-flurouracil were the same in IL11Ra-/- and wild-type mice. Acute hemolytic stress was induced by phenylhydrazine and resulted in a 50% decrease in hematocrit. The recovery of hematocrit was comparable in IL11Ra-/ - and wild-type mice. These observations indicate that IL-11 receptor signalling is dispensable for adult hematopoiesis.

Germinal centers: a second childhood for lymphocytes

Recent observations show that, in the peripheral lymphoid organs known as germinal centers, lymphocytes appear to regain the phenotypic and molecular traits of immature cells; this cellular regression may play an important role in the affinity maturation of immune responses.

Mice lacking the c-rel proto-oncogene exhibit defects in lymphocyte proliferation, humoral immunity, and interleukin-2 expression

The c-rel proto-oncogene, which is expressed predominantly in hemopoietic cells encodes a subunit of the NF-kappa B-like family of transcription factors. In mice with an inactivated c-rel gene, whereas development of cells from all hemopoietic lineages appeared normal, humoral immunity was impaired and mature B and T cells were found to be unresponsive to most mitogenic stimuli. Phorbol ester and calcium ionophore costimulation, in contrast to certain membrane receptor-mediated signals, overcame the T cell-proliferative defect, demonstrating that T cell proliferation occurs by Rel-dependent and -independent mechanisms. The ability of exogenous interleukin-2 to restore T Cell, but not B cell, proliferation indicates that Rel regulates the expression of different genes in B and T cells that are crucial for cell division and immune function.

DH element reading frame selection is influenced by an Ig heavy chain transgene, but not by bcl-2

Mouse B cell precursors containing Ig DHJH junctions in one particular reading frame are selectively lost during B cell development. In this register, arbitrarily referred to as reading frame 2, DHJH junctions give rise to an open reading frame starting upstream of the DH element and including the DHJH-peptide fused to the constant region of IgM. Expression of this protein, called D mu, has been strongly implicated in the loss of B cell precursors containing reading frame 2 DHJH junctions. In an attempt to elucidate the means of D mu counterselection, we have examined the reading frame distribution of DHJH junctions in peripheral B cells from mice transgenic for either the human bcl-2 oncogene or for a functionally rearranged Ig mu heavy chain. In bcl-2 transgenic mice, reading frame 2 accounted for < 5% of the DHJH junctions in peripheral B cells, a value not significantly different from controls. Reading frames 1 and 3 were equally represented among the remaining junctions. By contrast, the reading frame distribution of endogenous DHJH junctions in splenic B cells from Ig mu heavy chain transgenic mice showed no evidence of bias against D mu encoding DHJH junctions. Reading frames 2 and 3 accounted for 27% and 30% of the sequenced DHJH junctions, respectively, and the remaining 43% were reading frame 1. Thus although the presence of BCL-2 cannot prevent the selective loss of reading frame 2 DHJH B cells, a functional mu heavy chain can. These results suggest that D mu-expressing B cell precursors may be selectively lost because of the premature and inappropriate cessation of heavy chain gene rearrangement rather than because of the induction of an apoptotic process which can be blocked by BCL-2.

DH element reading frame selection is influenced by an Ig heavy chain transgene, but not by bcl-2

Mouse B cell precursors containing Ig DHJH junctions in one particular reading frame are selectively lost during B cell development. In this register, arbitrarily referred to as reading frame 2, DHJH junctions give rise to an open reading frame starting upstream of the DH element and including the DHJH-peptide fused to the constant region of IgM. Expression of this protein, called D mu, has been strongly implicated in the loss of B cell precursors containing reading frame 2 DHJH junctions. In an attempt to elucidate the means of D mu counterselection, we have examined the reading frame distribution of DHJH junctions in peripheral B cells from mice transgenic for either the human bcl-2 oncogene or for a functionally rearranged Ig mu heavy chain. In bcl-2 transgenic mice, reading frame 2 accounted for &lt; 5% of the DHJH junctions in peripheral B cells, a value not significantly different from controls. Reading frames 1 and 3 were equally represented among the remaining junctions. By contrast, the reading frame distribution of endogenous DHJH junctions in splenic B cells from Ig mu heavy chain transgenic mice showed no evidence of bias against D mu encoding DHJH junctions. Reading frames 2 and 3 accounted for 27% and 30% of the sequenced DHJH junctions, respectively, and the remaining 43% were reading frame 1. Thus although the presence of BCL-2 cannot prevent the selective loss of reading frame 2 DHJH B cells, a functional mu heavy chain can. These results suggest that D mu-expressing B cell precursors may be selectively lost because of the premature and inappropriate cessation of heavy chain gene rearrangement rather than because of the induction of an apoptotic process which can be blocked by BCL-2.

Absence of MHC class II expression distinguishes fetal from adult B lymphopoiesis in mice

Early B-lineage progenitor cells (Pro-B) isolated from murine fetal liver and adult bone marrow can differentiate to the immature B cell stage in a stromal cell-dependent culture system and to mature B cells upon transfer into immunodeficient SCID mice. By using immunofluorescence analysis, we found that progenitor cells from day 16 fetus differentiating in culture lacked MHC class II expression during the Pre-B and immature B cell stages, whereas such expression was readily apparent on the surface of corresponding adult-derived populations. RT-PCR analysis of RNA message levels for the four class II genes (A alpha, A beta, E alpha, E beta) yielded completely concordant results. B cells of fetal progeny did eventually express class II upon further maturation in vivo. Thus, the onset of class II expression is uniquely delayed during fetal B cell differentiation. This result explains an apparent paradox, i.e., that class II expression is absent from B cells in neonatal spleen but present as early as the Pre-B cell stage in adult bone marrow. Furthermore, we suggest that such distinct programs of class II expression during fetal and adult lymphopoiesis could result in differences in susceptibility to tolerance.

Absence of MHC class II expression distinguishes fetal from adult B lymphopoiesis in mice

Early B-lineage progenitor cells (Pro-B) isolated from murine fetal liver and adult bone marrow can differentiate to the immature B cell stage in a stromal cell-dependent culture system and to mature B cells upon transfer into immunodeficient SCID mice. By using immunofluorescence analysis, we found that progenitor cells from day 16 fetus differentiating in culture lacked MHC class II expression during the Pre-B and immature B cell stages, whereas such expression was readily apparent on the surface of corresponding adult-derived populations. RT-PCR analysis of RNA message levels for the four class II genes (A alpha, A beta, E alpha, E beta) yielded completely concordant results. B cells of fetal progeny did eventually express class II upon further maturation in vivo. Thus, the onset of class II expression is uniquely delayed during fetal B cell differentiation. This result explains an apparent paradox, i.e., that class II expression is absent from B cells in neonatal spleen but present as early as the Pre-B cell stage in adult bone marrow. Furthermore, we suggest that such distinct programs of class II expression during fetal and adult lymphopoiesis could result in differences in susceptibility to tolerance.

Direct demonstration of MHC class II surface expression on murine pre-B cells

Using multi-parameter flow cytometry, MHC class II (Ia) antigens were found to be expressed at readily detectable levels on the majority of surface IgM- B lineage cells in the bone marrow of adult mice. Approximately 90% of the pre-B cells (B220+IgM-) in BALB/c bone marrow (BM) were MCH class II positive, while in C57BL/6 this figure was somewhat less, approximately 60%. Analysis of adult BM for the correlated expression of Ia both with heat stable antigen (HSA) and BP-1 on B lineage cells revealed in both cases a small fraction of cells expressing either the HSA or BP-1 but lacking Ia. This result suggests that the onset of expression of both HSA and BP-1 precede that of Ia in B cell development. When the B220loCD43+ pro-B cell compartment from BALB/c BM was analysed for the expression of Ia very few, if any, cells were found to be positive. Of the remaining B220+ cells, > 90% express detectable Ia. Thus the onset of Ia expression would appear to be soon after the pro-B cell compartment. The results presented here resolve the previous discrepancy between human and murine pre-B cell differentiation of when Ia is first expressed, and additionally, by identifying a new pre-B cell antigen, allow for the refinement of existing schemes of murine B cell development.

T cell-dependent antibody production by Ly-1 B cells

Through the use of a SCID transfer system, we have demonstrated that under certain conditions, the production of Ig by Ly-1 B cells can be modulated by T cells. This modulation can take the form of enhanced isotype production or isotype-switch induction and to some extent appears to be dependent on the activation state of the T cells. Furthermore we have shown that Ly-1 B cells can mount an idiotypically restricted T cell-dependent immune response to the antigen PC-KLH. This result suggests that the previous failure to observe T cell-dependent responses by Ly-1 B cells has been due to these B cells being "blind" to the antigens used and is not due to some inherent property of these B cells. When one considers the previous reports of the substantial contribution of Ly-1 B cells to the natural serum immunoglobulin levels and the ability of T cells to affect Ig production by Ly-1 B cells documented in this report, it is clear that the interaction of T cells with the Ly-1 B-cell population is important in determining the "natural" serum Ig repertoire of the mouse.

An explanation for the defect in secretion of IgM Mott cells and their predominant occurrence in the Ly-1 B cell compartment

Mott cells are a variant form of plasma cell in which the immunoglobulin (Ig), rather than being secreted, accumulates in rough endoplasmic reticulum-derived vesicles called Russell bodies. We have examined the molecular cause of this defect and the in vivo origin of IgM Mott cells. Our examination of the Ig variable region gene sequences of two IgM Mott hybridomas derived from C.B-20 Ly-1 B cells showed all to be germ line. In a series of mix and match transfection experiments, the Mott phenotype was only reconstituted when the original Mott specificity was expressed as an IgM, suggesting that both the specificity and the isotype were critical to the formation of Russell bodies. Based on our finding that Russell body formation was dependent on the Ig isotype being IgM, we suggest that the Mott phenotype is apparent only after differentiation of B cells into plasma cells and that probably the major cause of the IgM Mott phenotype is low-affinity interaction of the Mott Ig with some as yet unknown intracellular component(s) being stabilized by the intrinsic high avidity of the pentameric secreted form of IgM. Consistent with this proposal was the finding that after in vitro lipopolysaccharide (LPS) stimulation of sorted Ly-1 B cells derived from C.B-20 mice, Mott cells represented up to 5% of the IgM plasma cells in the culture. LPS stimulation of conventional B cells also induced the appearance of IgM Mott cells, but at the much reduced level of 0.1%, suggesting that the major, if not the only, source of Mott cells in vivo is the Ly-1 B cell population. A possible causal relationship between the elevated frequency of Mott cells in the Ly-1 B cell-derived LPS blasts and the repertoire selection inherent in the development of these B cells is discussed.