B-1a B cells that link the innate and adaptive immune responses are lacking in the absence of the spleen

Further insights into the anti-PF4/heparin IgM immune response

Anti-platelet factor 4 (PF4)/heparin antibodies are not only the cause of heparin-induced thrombocytopenia but might also play a role in the antibacterial host defence. Recently, marginal zone (MZ) B cells were identified to be crucial for anti-PF4/heparin IgG antibody production in mice. Combining human studies and a murine model of polymicrobial sepsis we further characterised the far less investigated anti-PF4/heparin IgM immune response. We detected anti-PF4/heparin IgM antibodies in the sera of paediatric patients < 6 months of age after cardiac surgery and in sera of splenectomised mice subjected to polymicrobial sepsis. In addition, PF4/heparin-specific IgM B cells were not only found in murine spleen, but also in peritoneum and bone marrow upon in vitro stimulation. Together, this indicates involvement of additional B cell populations, as MZ B cells are not fully developed in humans until the second year of life and are restricted to the spleen in mice. Moreover, PF4/heparin-specific B cells were detected in human cord blood upon in vitro stimulation and PF4-/- mice produced anti-PF4/heparin IgM antibodies after polymicrobial sepsis. In conclusion, the anti-PF4/heparin IgM response is a potential innate immune reaction driven by a B cell population distinct from MZ B cells.

Innate response activator B cells: origins and functions

Innate response activator (IRA) B cells are a subset of B-1a derived B cells that produce the growth factors granulocyte macrophage colony stimulating factor and IL-3. In mouse models of sepsis and pneumonia, B-1a B cells residing in serosal sites recognize bacteria, migrate to the spleen or lung, and differentiate to IRA B cells that then contribute to the host response by amplifying inflammation and producing polyreactive IgM. In atherosclerosis, IRA B cells accumulate in the spleen, where they promote extramedullary hematopoiesis and activate classical dendritic cells. In this review, we focus on the ontogeny and function of IRA B cells in acute and chronic inflammation.

A review of guidance on immunization in persons with defective or deficient splenic function

The spleen acts as a blood filter and lymphopoietic organ. Asplenic and hyposplenic individuals are more susceptible to serious infections caused by encapsulated bacteria but they can be protected by antibiotic prophylaxis and immunizations. Recent progress in vaccinology means prophylaxis is now successful in the vast majority of serious infections with pneumococci, meningococci and Haemophilus influenzae type b responsible for the majority of cases of overwhelming sepsis in asplenic patients. Current guidelines are coherent. Physicians treating patients with conditions associated with hyposplenism are ethically obliged to immunize their patients using the vaccines currently available to protect them from largely preventable, life-threatening infections.

Characteristics of natural antibody-secreting cells

Natural IgM plays a critical role in protection from pathogens and the prevention of autoimmunity. While its importance has been shown in many different settings, its origins are incompletely understood. This review focuses on the properties of the natural IgM antibody-secreting cells (ASCs), which arise mainly from the B-1 cell lineage. B-1 cells are generated in multiple waves during development, mostly in the fetal and early postfetal periods. The developmental time points can affect their repertoire: prenatal B-1 cells express a mainly germ line-encoded repertoire, while postnatally developing B-1 cells can express Ig with a greater degree of variation. Spleen and bone marrow, but not the body cavities, are primary sites of natural IgM secretion. Within these tissues heterogeneous populations of IgM ASCs can be found. While some ASCs express classical markers of B-1 lymphocytes, others express those of terminally differentiated plasma cells. A better understanding of the properties of these different natural IgM ASCs could aid their future therapeutic exploitation.

Natural and induced B-1 cell immunity to infections raises questions of nature versus nurture

Mouse B-1 cells are not only major producers of steady-state natural antibodies but also rapid responders to infections and inflammation. These discrete functions may be the outcomes of distinct environmental or developmental triggers that drive B-1 cells toward IgM production or an effector cell fate. Alternatively, distinct B-1 cell subsets may exist, which differ in their functional plasticity. In this paper, we summarize existing data suggesting that B-1 cells form a heterogeneous group of cells with distinct developmental requirements and nonoverlapping functions. Most spleen B-1 cells differ in development from that of bone marrow and peritoneal cavity B-1 cells, in that they develop in the absence of natural IgM. Functional heterogeneity is revealed by findings that B-1 cells in the bone marrow and spleen, but not the peritoneal cavity, generate natural serum IgM, while the latter are rapid responders to inflammatory and infectious insults, resulting in their relocation to secondary lymphoid tissues. A clearer understanding of the developmental and functional differences within the B-1 cell pool may reveal how they might be harnessed for prophylaxis or therapy.

N-3 polyunsaturated fatty acids modulate B cell activity in pre-clinical models: Implications for the immune response to infections

B cell antigen presentation, cytokine production, and antibody production are targets of pharmacological intervention in inflammatory and infectious diseases. Here we review recent pre-clinical evidence demonstrating that pharmacologically relevant levels of n-3 polyunsaturated fatty acids (PUFA) derived from marine fish oils influence key aspects of B cell function through multiple mechanisms. N-3 PUFAs modestly diminish B cell mediated stimulation of classically defined naïve CD4(+) Th1 cells through the major histocompatibility complex (MHC) class II pathway. This is consistent with existing data showing that n-3 PUFAs suppress the activation of Th1/Th17 cells through direct effects on helper T cells and indirect effects on antigen presenting cells. Mechanistically, n-3 PUFAs lower antigen presentation and T cell signaling by disrupting the formation of lipid microdomains within the immunological synapse. We then review data to show that n-3 PUFAs boost B cell activation and antibody production in the absence and presence of antigen stimulation. This has potential benefits for several clinical populations such as the aged and obese that have poor humoral immunity. The mode of action by which n-3 PUFA boost B cell activation and antibody production remains unclear, but may involve Th2 cytokines, enhanced production of specialized proresolving lipid mediators, and targeting of protein lateral organization in lipid microdomains. Finally, we highlight evidence to show that different n-3 PUFAs are not biologically equivalent, which has implications for the development of future interventions to target B cell activity.

Translating transitions - how to decipher peripheral human B cell development

During the last two decades our understanding of human B cell differentiation has developed considerably. Our understanding of the human B cell compartment has advanced from a point where essentially all assays were based on the presence or not of class-switched antibodies to a level where a substantial diversity is appreciated among the cells involved. Several consecutive transitional stages that newly formed IgM expressing B cells go through after they leave the bone marrow, but before they are fully mature, have been described, and a significant complexity is also acknowledged within the IgM expressing and class-switched memory B cell compartments. It is possible to isolate plasma blasts in blood to follow the formation of plasma cells during immune responses, and the importance and uniqueness of the mucosal IgA system is now much more appreciated. Current data suggest the presence of at least one lineage of human innate-like B cells akin to B1 and/or marginal zone B cells in mice. In addition, regulatory B cells with the ability to produce IL-10 have been identified. Clinically, B cell depletion therapy is used for a broad range of conditions. The ability to define different human B cell subtypes using flow cytometry has therefore started to come into clinical use, but as our understanding of human B cell development further progresses, B cell subtype analysis will be of increasing importance in diagnosis, to measure the effect of immune therapy and to understand the underlying causes for diseases. In this review the diversity of human B cells will be discussed, with special focus on current data regarding their phenotypes and functions.

Immune profile of asplenic patients following single or double vaccine administration: A longitudinal cross-sectional study

OBJECTIVE

Splenectomy poses a lifelong threat for the development of uncontrolled sepsis despite vaccination. As it is impractical to measure the levels of each antibody against 23 most frequent bacterial serotypes, different surrogate markers of immune response should be identified.

MATERIAL AND METHODS

Forty-eight patients with benign disorders were vaccinated with Pneumo-23 and Act-HIB before or at the day of surgery. The immunological response and opsonization capacity of the patients after splenectomy was analyzed through the quantitative measurement of IgG, IgM, C3, and C4 titers; flow-cytometric analysis of (CD3+) T-lymphocytes and (CD19+) B-lymphocytes; and isolation of CD27+ B cells by immunomagnetic positive selection. Blood samples were drawn at the sixth month and 5 and 7 years after surgery.

RESULTS

The mean follow-up period was 98.4 months. All the patients in this series had normal IgG, C3, C4 levels and a normal distribution of CD19+ B-cells and CD8+ T-cells in three follow-up periods. Moreover, C3 levels markedly improved to 133.5±37.3 mg/dL at 5 years and remained stable thereafter. CD19+ B-lymphocyte values have progressively improved to the normal range in 98% patients at 7 years. Further, low levels of CD27+ B-cell population (memory cells) was observed in only 12.5% patients at the last follow-up. Adequate seroconversion of IgG, IgM with normal C3, C4, and CD19+ B-cell levels were accomplished in almost all patients. Early postoperative death and late overwhelming infections did not occur.

CONCLUSION

Our results are indicative of the resumption of the immune function following Pneumo-23 and Act-HIB administrations, instigated by the probable activation of B cells and adequate production of C3, C4, IgG, and IgM antibodies in remote lymphoid tissues.

Sterile inflammation in the spleen during atherosclerosis provides oxidation-specific epitopes that induce a protective B-cell response

The B-cell response in atherosclerosis is directed toward oxidation-specific epitopes such as phosphorylcholine (PC) that arise during disease-driven oxidation of self-antigens. PC-bearing antigens have been used to induce atheroprotective antibodies against modified low-density lipoproteins (oxLDL), leading to plaque reduction. Previous studies have found that B-cell transfer from aged atherosclerotic mice confers protection to young mice, but the mechanism is unknown. Here, we dissected the atheroprotective response in the spleen and found an ongoing germinal center reaction, accumulation of antibody-forming cells, and inflammasome activation in apolipoprotein E-deficient mice (Apoe(-/-)). Specific B-cell clone expansion involved the heavy chain variable region (Vh) 5 and Vh7 B-cell receptor families that harbor anti-PC reactivity. oxLDL also accumulated in the spleen. To investigate whether protection could be induced by self-antigens alone, we injected apoptotic cells that carry the same oxidation-specific epitopes as oxLDL. This treatment reduced serum cholesterol and inhibited the development of atherosclerosis in a B-cell-dependent manner. Thus, we conclude that the spleen harbors a protective B-cell response that is initiated in atherosclerosis through sterile inflammation. These data highlight the importance of the spleen in atherosclerosis-associated immunity.

The omentum is a site of protective IgM production during intracellular bacterial infection

Infection of mice with the bacterium Ehrlichia muris elicits a protective T cell-independent (TI) IgM response mediated primarily by a population of CD11c-expressing plasmablasts in the spleen. Although splenic marginal zone (MZ) B cells are considered to be important for TI responses to blood-borne pathogens, MZ B cells were not responsible for generating plasmablasts in response to Ehrlichia muris. Moreover, antigen-specific serum IgM was decreased only modestly in splenectomized mice and in mice that lacked spleen, lymph nodes, and Peyer's patches (SLP mice). Both splenectomized and SLP mice were protected against lethal ehrlichial challenge infection. Moreover, we found a high frequency of Ehrlichia-specific plasmablasts in the omentum of both conventional and SLP mice. Omental plasmablasts elicited during Ehrlichia infection lacked expression of CD138 but expressed CD11c in a manner similar to that of their splenic counterparts. Selective ablation of CD11c-expressing B cells nearly eliminated the omental Ehrlichia-specific plasmablasts and reduced antigen-specific serum IgM, identifying the omental B cells as a source of IgM production in the SLP mice. Generation of the omental plasmablasts was route dependent, as they were detected following peritoneal infection but not following intravenous infection. Our data identify the omentum as an important auxiliary site of IgM production during intracellular bacterial infection.

Natural antibody repertoires: development and functional role in inhibiting allergic airway disease

In this review we discuss the effects of microbial exposure on the B cell repertoire. Neonatal exposure to conserved bacterial carbohydrates and phospholipids permanently reprograms the natural antibody repertoire directed toward these antigens by clonal expansion, alterations in clonal dominance, and increased serum antibody levels. These epitopes are present not only in bacterial cell walls, but also in common environmental allergens. Neonatal immunization with bacterial polysaccharide vaccines results in attenuated allergic airway responses to fungi-, house dust mite-, and cockroach-associated allergens in mouse models. The similarities between mouse and human natural antibody repertoires suggest that reduced microbial exposure in children may have the opposite effect, providing a potential mechanistic explanation for the hygiene hypothesis. We propose that understanding the effects of childhood infections on the natural antibody repertoire and the mechanisms of antibody-mediated immunoregulation observed in allergy models will lead to the development of prevention/interventional strategies for treatment of allergic asthma.

Natural IgM prevents autoimmunity by enforcing B cell central tolerance induction

It is unclear why selective deficiency in secreted (s)IgM causes Ab-mediated autoimmunity. We demonstrate that sIgM is required for normal B cell development and selection. The CD5(+) B cells that were previously shown to accumulate in body cavities of sIgM(-/-) mice are not B-1a cells, but CD19(int), CD43(-), short-lived, BCR signaling-unresponsive anergic B-2 cells. Body cavity B-1 cells were >10-fold reduced, including VH11(+) and phosphotidylcholine-specific B-1a cells, whereas splenic B-1 cells were unaffected and marginal zone B cells increased. Follicular B cells had higher turnover rates, survived poorly after adoptive transfer, and were unresponsiveness to BCR stimulation in vitro. sIgM bound to B cell precursors and provided a positive signal to overcome a block at the pro/pre-B stage and during IgVH repertoire selection. Polyclonal IgM rescued B cell development and returned autoantibody levels to near normal. Thus, natural IgM deficiency causes primary autoimmune disease by altering B cell development, selection, and central tolerance induction.

Impact of natural IgM concentration on gene therapy with adenovirus type 5 vectors

Natural IgM inhibits gene transfer by adenovirus type 5 (Ad5) vectors. We show that polyreactive natural IgM antibodies bind to Ad5 and that inhibition of liver transduction by IgM depends on Kupffer cells. By manipulating IgM concentration in vivo, we demonstrate that IgM inhibits liver transduction in a concentration-dependent manner. We further show that differences in natural IgM between BALB/c and C57BL/6 mice contribute to lower efficiency of Ad5 gene transfer in BALB/c mice.

Natural IgM is produced by CD5- plasma cells that occupy a distinct survival niche in bone marrow

Natural IgM is constitutively present in the serum, where it aids in the early control of viral and bacterial expansions. Natural IgM also plays a significant role in the prevention of autoimmune disease by promoting the clearance of cellular debris. Nevertheless, the origins of natural IgM have not been precisely defined. Previous studies focused on the role of CD5(+) B1 cells in the production of natural IgM, but we show in this article that a discrete population of CD5(-) IgM plasmablasts and plasma cells in the bone marrow (BM) produces the majority of serum IgM in resting mice. These Ab-secreting cells (ASC) originate from peritoneal cavity-resident cells, because transfer of peritoneal cells completely restores serum IgM and the specific compartment of BM ASC in Rag1-deficient mice. We show that BM natural IgM ASC arise from a fetal-lineage progenitor that is neither B1a nor B1b, and that this IgM ASC compartment contains a substantial fraction of long-lived plasma cells that do not occupy the IgG plasma cell survival niche in the BM; instead, they are supported by IL-5. In summary, we identified the primary source of natural IgM and showed that these ASC are maintained long-term in a unique survival niche within the BM.

Early and late B cell immune responses in lethal and self-cured rodent malaria

ICR mice have heterogeneous susceptibility to lethal Plasmodium yoelii yoelii 17XL from the first days of experimental infection as evidenced by the different parasitemia levels and clinical outcomes. This mouse model has revealed specific immune responses on peripheral blood correlating with the infection fate of the animals. To search for immune-markers linked to parasitemia we examined B lymphocytes in organs of the immune system as key effectors of rodent immunity against malaria. To determine changes in immune cellularity fostered by the different prognostic parasitemia we examined B cell subsets in low (<15%) and high (>50%) parasitized mice during the first days of the infection. In the case of surviving mice, we studied the preservation of memory immune response 500 days after the primary P. yoelii challenge. Correlating with the parasitemia level, it was observed an increase in total cellularity of spleen during the first week of infection which remained after 16 months of the infection in surviving animals. B cell subsets were also modified across the different infection fates. Subpopulation as follicular B cells and B-1 cells proportions behaved differently depending on the parasitemia kinetics. In addition, peritoneal cavity cells proliferated in response to high parasitemia. More significantly, P. yoelii -specific memory B cells remained in the spleen 500 days after the primo-infection. This study demonstrates that B cell kinetics is influenced by the different parasitemia courses which are naturally developed within a same strain of untreated mice. We show that high levels of parasitemia at the beginning of infection promote an extremely fast and exacerbate response of several cell populations in spleen and peritoneal cavity that, in addition, do not follow the kinetics observed in peripheral blood. Furthermore, our results describe the longest persistence of memory B cells long time upon a single malaria infection in mice.

Targeting B cells in atherosclerosis: closing the gap from bench to bedside

Atherosclerotic plaque formation is strongly influenced by different arms of the immune system, including B lymphocytes. B cells are divided into 2 main families: the B1 and the B2 cells. B1 cells are atheroprotective mainly via the production of natural IgM antibodies that bind oxidized low-density lipoprotein and apoptotic cells. B2 cells, which include follicular and marginal zone B cells, are suggested to be proatherogenic. Antibody-mediated depletion of B cells has become a valuable treatment option for certain autoimmune diseases, such as systemic lupus erythematosus and rheumatoid arthritis that are also characterized by the development of premature atherosclerosis. Thus, B cells represent a novel interesting target for therapeutic modulation of the atherosclerotic disease process. Here, we discuss the effect of different of B-cell subsets in experimental atherosclerosis, their mechanism of action as well as potential ways to exploit these findings for the treatment of human disease.

Spleen supports a pool of innate-like B cells in white adipose tissue that protects against obesity-associated insulin resistance

Lipid accumulation in obesity triggers a low-grade inflammation that results from an imbalance between pro- and anti-inflammatory components of the immune system and acts as the major underlying mechanism for the development of obesity-associated diseases, notably insulin resistance and type 2 diabetes. Innate-like B cells are a subgroup of B cells that respond to innate signals and modulate inflammatory responses through production of immunomodulatory mediators such as the anti-inflammatory cytokine IL-10. In this study, we examined innate-like B cells in visceral white adipose tissue (VAT) and the relationship of these cells with their counterparts in the peritoneal cavity and spleen during diet-induced obesity (DIO) in mice. We show that a considerable number of innate-like B cells bearing a surface phenotype distinct from the recently identified "adipose natural regulatory B cells" populate VAT of lean animals, and that spleen represents a source for the recruitment of these cells in VAT during DIO. However, demand for these cells in the expanding VAT outpaces their recruitment during DIO, and the obese environment in VAT further impairs their function. We further show that removal of splenic precursors of innate-like B cells through splenectomy exacerbates, whereas supplementation of these cells via adoptive transfer ameliorates, DIO-associated insulin resistance. Additional adoptive transfer experiments pointed toward a dominant role of IL-10 in mediating the protective effects of innate-like B cells against DIO-induced insulin resistance. These findings identify spleen-supplied innate-like B cells in VAT as previously unrecognized players and therapeutic targets for obesity-associated diseases.

B-1a transitional cells are phenotypically distinct and are lacking in mice deficient in IκBNS

B-1 cells mediate early protection against infection by responding to T cell-independent (TI) antigens found on the surface of various pathogens. Mice with impaired expression of the atypical IκB protein IκBNS have markedly reduced frequencies of B-1 cells. We used a mouse strain with dysfunctional IκBNS derived from an N-ethyl-N-nitrosourea (ENU) screen, named bumble, to investigate the point in the development of B-1 cells where IκBNS is required. The presence of wild-type (wt) peritoneal cells in mixed wt/bumble chimeras did not rescue the development of bumble B-1 cells, but wt peritoneal cells transferred to bumble mice restored natural IgM levels and response to TI antigens. The bumble and wt mice displayed similar levels of fetal liver B-1 progenitors and splenic neonatal transitional B (TrB) cells, both of which were previously shown to give rise to B-1 cells. Interestingly, we found that a subset of wt neonatal TrB cells expressed common B-1a markers (TrB-1a) and that this cell population was absent in the bumble neonatal spleen. Sorted TrB-1a (CD93(+)IgM(+)CD5(+)) cells exclusively generated B-1a cells when adoptively transferred, whereas sorted CD93(+)IgM(+)CD5(-) cells gave rise to B-2 cells and, to a lesser extent, B-1b and B-1a cells. This study identifies a phenotypically distinct splenic population of TrB-1a cells and establishes that the development of B-1a cells is blocked before this stage in the absence of IκBNS.

B cells and humoral immunity in atherosclerosis

Insights into the important contribution of inflammation and immune functions in the development and progression of atherosclerosis have greatly improved our understanding of this disease. Although the role of T cells has been extensively studied for decades, only recently has the role of B cells gained more attention. Recent studies have identified differential effects of different B-cell subsets and helped to clarify the still poorly understood mechanisms by which these act. B1 cells have been shown to prevent lesion formation, whereas B2 cells have been suggested to promote it. Natural IgM antibodies, mainly derived from B1 cells, have been shown to mediate atheroprotective effects, but the functional role of other immunoglobulin classes, particularly IgG, still remains elusive. In this review, we will focus on recent insights on the role of B cells and various immunoglobulin classes and how these may mediate their effects in atherosclerotic lesion formation. Moreover, we will highlight potential therapeutic approaches focusing on B-cell depletion that could be used to translate experimental evidence to human disease.

Nitric oxide regulates BAFF expression and T cell-independent antibody responses

Whereas NO is known to regulate T cell responses, its role in regulating B cell responses remains unclear. Previous studies suggested that inducible NO synthase 2 (NOS2/iNOS) is required for normal IgA Ab responses but inhibits antiviral IgG2a Ab responses. In this study we used NOS2(-/-) mice to determine the role of NO in T cell-dependent and T cell-independent (TI)-2 Ab responses. Whereas T cell-dependent Ab responses were only modestly increased in NOS2(-/-) mice, IgM and IgG3 Ab responses as well as marginal zone B cell plasma cell numbers and peritoneal B1b B cells were significantly elevated after immunization with the TI-2 Ag 4-hydroxy-3-nitrophenyl acetyl (NP)-Ficoll. The elevated TI-2 responses in NOS2(-/-) mice were accompanied by significant increases in serum levels of BAFF/BLyS and by increases in BAFF-producing Ly6C(hi) inflammatory monocytes and monocyte-derived dendritic cells (DCs), suggesting that NO normally inhibits BAFF expression. Indeed, we found that NOS2(-/-) DCs produced more BAFF than did wild-type DCs, and addition of a NO donor to NOS2(-/-) DCs reduced BAFF production. Bone marrow chimeric mice that lack NOS2 in either nonhematopoietic or hematopoietic cells had intermediate IgM and IgG3 Ab responses after NP-Ficoll immunization, suggesting that NOS2 from both hematopoietic and nonhematopoietic sources regulates TI-2 Ab responses. Similar to NOS2(-/-) mice, depletion of Ly6C(hi) inflammatory monocytes and monocyte-derived DCs enhanced NP-specific IgM and IgG3 responses to NP-Ficoll. Thus, NO produced by inflammatory monocytes and their derivative DC subsets plays an important role in regulating BAFF production and TI-2 Ab responses.

Splenic B-1a Cells Expressing CD138 Spontaneously Secrete Large Amounts of Immunoglobulin in Naïve Mice

B-1a cells constitutively secrete natural antibody that provides immediate protection against microbial pathogens and functions homeostatically to speed removal of apoptotic cell debris. Although B-1a cells are especially prominent in the peritoneal and pleural cavities, some B-1a cells reside in the spleen. A small subset of splenic B-1a cells in naïve, unimmunized mice express CD138, a recognized plasma cell antigen, whereas the bulk of splenic B-1a cells are CD138 negative. Splenic B-1a cells in toto have been shown to generate much more antibody per cell than peritoneal B-1a cells; however, specific functional information regarding CD138⁺ splenic B-1a cells has been lacking. Here, we find a higher proportion of CD138⁺ splenic B-1a cells spontaneously secrete more IgM as compared to CD138⁻ B-1a cells. Moreover, IgM secreted by CD138⁺ splenic B-1a cells is skewed with respect to N-region addition, and some aspects of V_H and J_H utilization, as compared to CD138⁻ splenic B-1a cells and peritoneal B-1a cells. The small population of CD138⁺ splenic B-1a cells is likely responsible for a substantial portion of natural IgM and differs from IgM produced by other B-1a cell subsets.

Differential immune response associated to malaria outcome is detectable in peripheral blood following Plasmodium yoelii infection in mice

Malaria infection in humans elicits a wide range of immune responses that can be detected in peripheral blood, but we lack detailed long-term follow-up data on the primary and subsequent infections that lead to naturally acquired immunity. Studies on antimalarial immune responses in mice have been based on models yielding homogenous infection profiles. Here, we present a mouse model in which a heterogeneous course of Plasmodium yoelii lethal malaria infection is produced in a non-congenic ICR strain to allow comparison among different immunological and clinical outcomes. Three different disease courses were observed ranging from a fatal outcome, either early or late, to a self-resolved infection that conferred long-term immunity against re-infection. Qualitative and quantitative changes produced in leukocyte subpopulations and cytokine profiles detected in peripheral blood during the first week of infection revealed that monocytes, dendritic cells and immature B cells were the main cell subsets present in highly-parasitized mice dying in the first week after infection. Besides, CD4(+)CD25(high) T cells expanded at an earlier time point in early deceased mice than in surviving mice and expressed higher levels of intracellular Foxp3 protein. In contrast, survivors showed a limited increase of cytokines release and stable circulating innate cells. From the second week of infection, mice that would die or survive showed similar immune profiles, although CD4(+)CD25(high) T cells number increased earlier in mice with the worst prognosis. In surviving mice the expansion of activated circulating T cell and switched-class B cells with a long-term protective humoral response from the second infection week is remarkable. Our results demonstrate that the follow-up studies of immunological blood parameters during a malaria infection can offer information about the course of the pathological process and the immune response.

Purification and immunophenotypic characterization of murine MZ and T2-MZP cells

B cells are generated every day in the bone marrow, but only a small fraction integrates the peripheral B-cell pool. In the murine spleen, we can find several B-cell subsets representing various maturation stages and/or cell functions. The spleen is a complex lymphoid organ organized in two main structures with different functions: the red and white pulp. The red pulp is flowed with blood while the white pulp is organized in primary follicles, with a B-cell area composed of follicular B cells and a T-cell area surrounding a periarterial lymphatic sheath. The frontier between the red and white pulp is defined as the marginal zone and contains the marginal zone B cells. Because B cells, localized in different areas, are characterized by distinct expression levels of B-cell receptor (BCR) and other surface markers, splenic B-cell subsets can be easily identified and purified by flow cytometry analyses and cell sorting (FACS).Here, we will focus on marginal zone B cells and their precursors giving some experimental hints to identify, generate, and isolate these cells. We will combine the use of FACS analysis and confocal microscopy to visualize marginal zone B cells in cell suspension and tissue sections, respectively.

Functional hyposplenism

Functional hyposplenism is a condition accompanying many diseases such as sickle cell disease, celiac disease, alcoholic liver disease, hepatic cirrhosis, lymphomas and autoimmune disorders. It is characterised mostly by defective immune responses against infectious agents, especially encapsulated organisms, since the spleen is thought to play an important role in the production and maturation of B-memory lymphocytes and other substances like opsonins, both of which are considered crucial elements of the immune system for fighting infections. It is also associated with thrombocytosis, which might lead to thromboembolic events. Functional hyposplenism is diagnosed by the presence of Howell-Jolly bodies and pitted erythrocytes in the peripheral blood smear, and by nuclear imaging modalities such as spleen scintigraphy with the use of Technetium-99m and/or spleen scintigraphy with the use of heat-damaged Technetium-99m labeled erythrocytes. Severe infections accompanying functional hyposplenism can lead to the overwhelming post infection syndrome, which can often be fatal. Identifying patients with functional hyposplenism is important because simple measures such as vaccination against common infective microorganisms (e.g. Streptococcus pneumonia, Neisseria meningitides and Haemophilous influenzae) and antibiotic therapy when needed are considered beneficial in diminishing the frequency and gravity of the infections accompanying the syndrome.

Specific antibody in IV immunoglobulin for postsplenectomy sepsis

OBJECTIVE

Overwhelming postsplenectomy infection progresses rapidly and shows unacceptably high mortality even when treated with optimal antibiotics. Recent reports have described beneficial effects of intravenous immunoglobulin in sepsis. However, the underlying mechanisms, especially the roles of the pathogen-specific antibody fraction in intravenous immunoglobulin, remain unclear. We investigated the effects of intravenous immunoglobulin on overwhelming postsplenectomy infection and the role of pathogen-specific antibody.

DESIGN

Prospective, controlled, and randomized animal study.

SETTING

University laboratory.

SUBJECTS

Male C57BL/6JJcl mice.

INTERVENTIONS

Mice underwent splenectomy or a sham operation at 8 weeks old. At 4 weeks after the operation, mice were injected intravenously with 106 colony-forming units pneumococcus. Intraperitoneal injection of 300 mg/kg IV immunoglobulins was conducted simultaneously with infection. Specific antibody-depleted IV immunoglobulin prepared using immunoprecipitation was also injected into the animals.

MEASUREMENTS AND MAIN RESULTS

IV immunoglobulin markedly improved splenectomized mice survival. Removal of pneumococcus-specific antibody canceled it completely. Fluorescence microscopy results indicated significantly increased phagocytosis of antibody-bound bacteria in the livers of splenectomized mice treated with intact IV immunoglobulins. Immunomodulation, including suppression of marginal zone B-cell activation, was induced by IV immunoglobulin.

CONCLUSIONS

IV immunoglobulin showed a significantly protective effect on overwhelming postsplenectomy infection via enhancement of specific antibody-mediated phagocytosis in the liver. Specific antibody, more than immunological modulation, is crucial for effects of IV immunoglobulin on overwhelming postsplenectomy infection.

Hyposplenism: A comprehensive review. Part I: Basic concepts and causes

Hyposplenism is not a rare condition and can complicate a remarkable number of illnesses. The two most time-honored diseases associated with the development of hyposplenism are sickle cell anemia and celiac disease. Hyposplenism is relatively easy to recognize by typical changes observed on the peripheral blood smear; including Howell-Jolly bodies, monocytosis, lymphocytosis, and increased platelet counts. Diagnosis can be confirmed by pitted RBC counts or 99Tc-labelled radiocolloid scan of the spleen; wherever available. Diagnosis needs to be made promptly to institute pneumococcal vaccination in a timely fashion and to recognize and treat bacterial infections promptly and aggressively because of the tendency of hyposplenic subject to develop fatal invasive disease. Overwhelming pneumococcal sepsis accounts for the major mortality cases in hyposplenic subjects; however severe infections with other encapsulated bacteria and protozoa have been reported. Hyposplenic individuals may also be at a higher risk for vascular, autoimmune and thrombotic diseases and they may have a higher risk of developing solid tumors. The commonly used pneumococcal polysaccharide vaccine is ineffective in asplenic subjects, because it requires the presence of IgM memory B cells, and should be given before splenectomy. In splenectomized, and functionally hyposplenic subjects, the pneumococcal conjugate vaccine is more effective, because it utilizes a T cell dependent mechanism, and should be the preferred vaccine in these circumstances.

Human B-1 cells take the stage

B-1 cells play critical roles in defending against microbial invasion and in housekeeping removal of cellular debris. B-1 cells secrete natural antibody and manifest functions that influence T cell expansion and differentiation and in these and other ways differ from conventional B-2 cells. B-1 cells were originally studied in mice where they are easily distinguished from B-2 cells, but their identity in the human system remained poorly defined for many years. Recently, functional criteria for human B-1 cells were established on the basis of murine findings, and reverse engineering resulted in identification of the phenotypic profile, CD20(+)CD27(+)CD43(+)CD70(-), for B-1 cells found in both umbilical cord blood and adult peripheral blood. Human B-1 cells may contribute to multiple disease states through production of autoantibody and stimulation/modulation of T cell activity. Human B-1 cells could be a rich source of antibodies useful in treating diseases present in elderly populations where natural antibody protection may have eroded. Manipulation of human B-1 cell numbers and/or activity may be a new avenue for altering T cell function and treating immune dyscrasias.

Liver-spleen axis: Intersection between immunity, infections and metabolism

Spleen has been considered a neglected organ so far, even though is strictly linked to liver. The spleen plays an important role in the modulation of the immune system and in the maintenance of peripheral tolerance via the clearance of circulating apoptotic cells, the differentiation and activation of T and B cells and production of antibodies in the white pulp. Moreover, splenic macrophages are able to remove bacteria from the blood and protect from sepsis during systemic infections. We review the spleen function and its assessment in humans starting from the description of spleen diseases, ranging from the congenital asplenia to secondary hyposplenism. From the literature data it is clear that obesity in humans affects different compartments of immune system, even thought there are still few data available on the implicated mechamisms. The intent is to enable clinicians to evaluate the newly recognized role of metabolic and endocrine functions of the spleen with special emphasis to obesity and nonalcoholic fatty liver disease in the context of the available literature. Moreover, understanding the spleen function could be important to develop appropriate prevention strategies in order to counteract the pandemia of obesity. In this direction, we suggest spleen longitudinal diameter at ultrasonography, as simple, cheap and largely available tool, be used as new marker for assessing splenic function, in the context of the so-called liver-spleen axis.

Multispecificity of immunoglobulin M antibodies raised against advanced glycation end products: involvement of electronegative potential of antigens

BACKGROUND

Advanced glycation end products (AGEs) can act as neoantigens to trigger immune responses.

RESULTS

Natural IgM antibodies against AGEs recognize multiple molecules, including DNA and chemically modified proteins.

CONCLUSION

There is a close relationship between the formation of AGEs and innate immune responses.

SIGNIFICANCE

Our findings highlight AGEs and related modified proteins as a source of multispecific natural antibodies Advanced glycation end products (AGEs) are a heterogeneous and complex group of compounds that are formed when reducing sugars, such as dehydroascorbic acid, react in a nonenzymatic way with amino acids in proteins and other macromolecules. AGEs are prevalent in the diabetic vasculature and contribute to the development of atherosclerosis. The presence and accumulation of AGEs in many different cell types affect the extracellular and intracellular structure and function. In the present study, we studied the immune response to the dehydroascorbic acid-derived AGEs and provide multiple lines of evidence suggesting that the AGEs could be an endogenous source of innate epitopes recognized by natural IgM antibodies. Prominent IgM titers to the AGEs were detected in the sera of normal mice and were significantly accelerated by the immunization with the AGEs. Patients with systemic lupus erythematosus (SLE), a potentially fatal systemic autoimmune disease characterized by the increased production of autoantibodies, showed significantly higher serum levels of the IgM titer against the AGEs than healthy individuals. A progressive increase in the IgM response against the AGEs was also observed in the SLE-prone mice. Strikingly, a subset of monoclonal antibodies, showing a specificity toward the AGEs, prepared from normal mice immunized with the AGEs and from the SLE mice cross-reacted with the double-stranded DNA. Moreover, they also cross-reacted with several other modified proteins, including the acetylated proteins, suggesting that the multiple specificity of the antibodies might be ascribed, at least in part, to the increased electronegative potential of the proteins. These findings suggest that the protein modification by the endogenous carbonyl compounds, generating electronegative proteins, could be a source of multispecific natural antibodies.

Why do we need IgM memory B cells?

Immunological memory is our reservoir of ready-to-use antibodies and memory B cells. Because of immunological memory a secondary infection will be very light or not occur at all. Antibodies and cells, generated in the germinal center in response to the first encounter with antigen, are highly specific, remain in the organism virtually forever and are mostly of IgG isotype. Long lived plasma cells homing to the bone marrow ensure the constant production of protective antibodies, whereas switched memory B cells proliferate and differentiate in response to secondary challenge. IgM memory B cells represent our first-line defense against infections. They are generated by a T-cell independent mechanism probably triggered by Toll-like receptor-9. They produce natural antibodies with anti-bacterial specificity and the spleen is indispensable for their maintenance. We will review the characteristics and functions of IgM memory B cells that explain their importance in the immediate protection from pathogens. IgM memory B cells, similar to mouse B-1a B cells, may be a remnant of a primitive immune system that developed in the spleen of cartilaginous fish and persisted throughout evolution notwithstanding the sophisticated tools of the adaptive immune system.

B cell subsets in atherosclerosis

Atherosclerosis, the underlying cause of heart attacks and strokes, is a chronic inflammatory disease of the artery wall. Immune cells, including lymphocytes modulate atherosclerotic lesion development through interconnected mechanisms. Elegant studies over the past decades have begun to unravel a role for B cells in atherosclerosis. Recent findings provide evidence that B cell effects on atherosclerosis may be subset-dependent. B-1a B cells have been reported to protect from atherosclerosis by secretion of natural IgM antibodies. Conventional B-2 B cells can promote atherosclerosis through less clearly defined mechanism that may involve CD4 T cells. Yet, there may be other populations of B cells within these subsets with different phenotypes altering their impact on atherosclerosis. Additionally, the role of B cell subsets in atherosclerosis may depend on their environmental niche and/or the stage of atherogenesis. This review will highlight key findings in the evolving field of B cells and atherosclerosis and touch on the potential and importance of translating these findings to human disease.

Quorum sensing contributes to activated IgM-secreting B cell homeostasis

Maintenance of plasma IgM levels is critical for immune system function and homeostasis in humans and mice. However, the mechanisms that control homeostasis of the activated IgM-secreting B cells are unknown. After adoptive transfer into immune-deficient hosts, B lymphocytes expand poorly, but fully reconstitute the pool of natural IgM-secreting cells and circulating IgM levels. By using sequential cell transfers and B cell populations from several mutant mice, we were able to identify novel mechanisms regulating the size of the IgM-secreting B cell pool. Contrary to previous mechanisms described regulating homeostasis, which involve competition for the same niche by cells having overlapping survival requirements, homeostasis of the innate IgM-secreting B cell pool is also achieved when B cell populations are able to monitor the number of activated B cells by detecting their secreted products. Notably, B cell populations are able to assess the density of activated B cells by sensing their secreted IgG. This process involves the FcγRIIB, a low-affinity IgG receptor that is expressed on B cells and acts as a negative regulator of B cell activation, and its intracellular effector the inositol phosphatase SHIP. As a result of the engagement of this inhibitory pathway, the number of activated IgM-secreting B cells is kept under control. We hypothesize that malfunction of this quorum-sensing mechanism may lead to uncontrolled B cell activation and autoimmunity.

Nuclear transferred embryonic stem cells for analysis of B1 B-lymphocyte development

The transfer of nuclei of fully differentiated cells into enucleated oocytes is a well-recognized method for the generation of embryonic stem (ES) cells. Here, we demonstrate that nuclear transferred ES (NT-ES) cells can be established with high efficiency using innate-like B lymphocytes as donor cells. We established two mouse lines carrying rearranged immunoglobulin heavy and light chains using NT-ES cells containing nuclei from peritoneal cavity B1 cells. Analysis of B1 clone lines revealed that the B1-cell generation critically depends on the interaction between antigen (possibly self-antigen) and surface immunoglobulin, while the B1-cell maintenance requires the peritoneal environment. The B1-cell expansion takes place in spleen, and is held in check by competitor B2 cells. The results indicate that the NT-ES method could replace the transgenic or knock-in mouse approaches currently used to study the biology of cells that undergo somatic rearrangements of their antigen receptor genes.

Splenic morphological changes are accompanied by altered baseline immunity in a mouse model of sickle-cell disease

Although functional asplenia from infarctions may be a major contributor to increased infectious mortality in sickle-cell disease (SCD), this relationship has not been fully defined. We used the transgenic Berkeley SCD mouse to define blood and splenic immunophenotypic differences in this model compared with C57BL/6 and hemizygous controls. In the serum of SCD mice, we found increased IgG2a and suppressed IgM, IgG2b, and IgA levels. Serum IL-6 levels in SCD mice were elevated, whereas IL-1α, CXCL10, and CCL5 levels were decreased. The blood of SCD mice had higher white blood cell counts, with an increased percentage of lymphocytes and decreases in other leukocytes. Immunophenotyping of lymphocytes revealed higher percentages of CD8(+) and T-regulatory cells and lower percentages of B cells. SCD mouse spleens exhibited histological disorganization, with reduction of defined lymphoid follicles and expansion of red pulp, a greater than fourfold increase in splenic mononuclear cells, marked expansion of the nucleated red blood cell fraction, and B-cell and CD8(+) T-cell lymphopenia. Within the splenic B-cell population, there was a significant decrease in B-1a B cells, with a corresponding decrease in IgA secreting plasma cells in the gut. Confocal microscopy of spleens demonstrated complete disruption of the normal lymphofollicular structure in the white pulp of SCD mice without distinct B, T, and marginal zones. Our findings suggest that altered SCD splenic morphological characteristics result in an impaired systemic immune response.

Protective role of natural IgM-producing B1a cells in atherosclerosis

Atherosclerosis initiated by hyperlipidemia is modulated by immune cells in its development, progression, and rupture that results in thrombotic arterial occlusion leading to strokes and myocardial infarction. B cells initially thought to be atheroprotective provide opposing roles by their different subsets. Unlike B2 cells that are atherogenic, serosal B1a cells are atheroprotective by producing natural IgM antibodies that clear modified low-density lipoprotein and apoptotic and necrotic debris. In addition to natural IgM antibodies, B1a cells may act as regulatory B cells by producing the anti-inflammatory cytokine interleukin-10, which inhibits proinflammatory cytokines secreted by activated macrophages and T cells in atherosclerotic lesions. These findings suggest in vivo expansion of atheroprotective B1a cells as a potential therapeutic strategy to augment the benefits of lipid-lowering statin therapy.

Innate stimulation of B1a cells enhances the autoreactive IgM repertoire in the NOD mouse: implications for type 1 diabetes

AIMS/HYPOTHESIS

We sought to determine whether the presence of natural autoreactive antibodies of B1a cell origin would play a role in the initiation of type 1 diabetes.

METHODS

We compared IgM repertoires and B1a cell compartments in NOD and C57BL/6 mice. Serum IgM autoreactivity profiles were determined by ELISA and the secretory properties and activation status of B1a cells were characterised by enzyme-linked immunosorbent spot (ELISPOT) assay and flow cytometry. B1a cell response to innate activation was analysed by gene expression assays, ELISA and [(3)H]thymidine incorporation. The effect of NOD IgM produced by B1a cells on NOD.severe combined immunodeficient (SCID) beta cells was examined in co-cultures: IgM binding was measured by flow cytometry and real-time PCR was used to study oxidative stress responses.

RESULTS

NOD mice displayed increased levels of serum anti-insulin IgM that were independent of the H2 locus, that were maintained up to prediabetic stages and that correlated with the NOD B1a cell secretion profile. NOD B1a cells had a naturally increased pattern of activation, expressed higher levels of toll-like-receptors (Tlrs) and responded to TLR stimulation in vitro with higher proliferation and increased capacity to secrete anti-type-1-diabetes-related IgM, but produced lower amounts of IL10. IgM of NOD B1a cell origin was able to bind to pancreatic beta cells in vitro and induce expression of inducible nitric oxide synthase (Nos2).

CONCLUSIONS/INTERPRETATION

NOD B1a cells had a lower innate activation threshold for secretion of autoreactive IgM capable of triggering oxidative stress responses on binding to pancreatic beta cells; this provides an early mechanism that contributes to diabetes in a mouse model of type 1 diabetes.

Lymphocytes and the adventitial immune response in atherosclerosis

Although much of the research on atherosclerosis has focused on the intimal accumulation of lipids and inflammatory cells, there is an increasing amount of interest in the role of the adventitia in coordinating the immune response in atherosclerosis. In this review of the contributions of the adventitia and adventitial lymphocytes to the development of atherosclerosis, we discuss recent research on the formation and structural nature of adventitial immune aggregates, potential mechanisms of crosstalk between the intima, media, and adventitia, specific contributions of B lymphocytes and T lymphocytes, and the role of the vasa vasorum and surrounding perivascular adipose tissue. Furthermore, we highlight techniques for the imaging of lymphocytes in the vasculature.

Immune Responses to pneumococcal vaccines in children and adults: Rationale for age-specific vaccination

Streptococcus pneumoniae is a significant human pathogen and currently available pneumococcal vaccines are designed to elicit anti-capsule antibodies. The 23-valent polysaccharide vaccine has been used in older adults for many years whereas 7-, 10-, and 13-valent pneumococcal conjugate vaccines have only been used commonly for young children in the last decade. In addition to their high protective efficacy among children, the use of conjugate vaccines in young children has had a number of additional effects, including production of a serotype shift and providing new herd immunity to adults. The immunogenicity of both of these types of vaccines can be determined by using an ELISA assay to measure antibody levels or an opsonophagocytosis assay to assess opsonic function. As these assays have improved over time, awareness of the analytical limitations of older studies has grown. While the 23-valent vaccine is effective among young adults, it is less effective among elderly adults. Aging-associated ineffectiveness may be due to aging-dependent changes in the antibody repertoire and/or a reduction in IgM antibody production associated with aging-dependent changes in B cell subpopulations. The immunologic basis of aging-associated immune defects thus remains an active area of research.

Phenotypic analysis of pneumococcal polysaccharide-specific B cells

The phenotype of B cells responsible for the production of anti-pneumococcal polysaccharide Ab has been unclear. Although individuals that respond poorly to the 23-valent pneumococcal polysaccharide (PPS) vaccine, Pneumovax, such as children <2 y, the asplenic, and a subset of common variable immunodeficiency patients, are profoundly deficient or lack IgM memory cells (CD27(+)IgM(+)), they are also deficient in the switched memory (CD27(+)IgM(-)) compartment. Direct characterization of PPS-specific B cells has not been performed. In this study, we labeled PPS14 and PPS23F with fluorescent markers. Fluorescently labeled PPS were used in FACSAria flow cytometry to characterize the phenotype of PPS-specific B cells obtained from 18 young adults pre- and postimmunization with Pneumovax. The labeled PPS were capable of inhibiting binding of Ab to the native PPS. Similarly, the native PPS were able to inhibit binding of PPS-specific B cells in a flow cytometric assay demonstrating specificity and functionality. Phenotypic analysis of unselected B cells, pre- and postimmunization, demonstrated a predominance of naive CD27(-)IgM(+) cells accounting for 61.5% of B cells. Likewise, the PPS-specific B cells obtained preimmunization consisted primarily of naive, CD27(-) B cells, 55.4-63.8%. In contrast, the PPS-specific B cells obtained postimmunization were predominantly IgM memory cells displaying the CD27(+)IgM(+), 54.2% for PPS14 and 66% for PPS23F, significantly higher than both unselected B cells and PPS-specific B cells. There was no significant difference in switched memory B cell populations (CD27(+)IgM(-)) between groups. These results suggest a dominant role of IgM memory cells in the immune response to pneumococcal polysaccharides.

LPS-induced migration of peritoneal B-1 cells is associated with upregulation of CXCR4 and increased migratory sensitivity to CXCL12

B-1 cells, which constitute a predominant lymphocyte subset in serosal cavities and produce most of natural antibodies, are subdivided into the CD5(+) B-1a and CD5(-) B-1b cell subpopulations, but the differential roles of B-1a and B-1b cells are not well understood. We report that B-1a cells preferentially migrate out of the peritoneal cavity and upregulate the expression of CXCR4 with heightened sensitivity to CXCL12 and CXCL13 upon LPS treatment compared to B-1b and B-2 cells. Whereas B-1a cells were slightly more abundant than B-1b and B-2 cells in the homeostatic condition, the number of B-1a cells preferentially decreased 48 hr after LPS treatment. The decrease in the peritoneal B-1a cell number was accompanied with increased migration of B-1a cells toward CXCL-12 and CXCL-13 in in vitro transmigration assay using peritoneal B cells from LPS treated mice. The expression level of CXCR4, but not of CXCR5, was also more prominently increased in B-1a cells upon LPS stimulation. LPS-stimulated B-1a cells did not accumulate in omental milky spots in contrast to B-2 cells. These results suggest that B-1a cells actively migrate out of the peritoneal cavity through the regulation of the migratory responsiveness to chemokines and actively participate in systemic immune responses.

B-1 cells in the bone marrow are a significant source of natural IgM

Natural IgM antibodies secreted in the absence of antigenic challenge are important contributors to antimicrobial immunity and tissue homeostasis. Early studies identified BM and, to a lesser extent the spleen, as main tissue sources of this spontaneously secreted IgM. However, the responsible B-cell subset has never been identified. Using multicolor flow cytometry, cell sorting and chimeric mice in which B-1 and B-2 cells and their secreted antibodies are distinguished by their Ig-allotype, we unequivocally identify the natural IgM-secreting cells in spleen and, for the first time, in the BM as IgM(+) IgD(lo/-) CD19(hi) CD43(+) CD5(+/-) B-1 cells. The newly identified population of BM B-1 cells shows many of the phenotypic characteristics of splenic B-1 cells but is distinct from B-1 cells in the peritoneal cavity, which generate at best very small amounts of IgM. Antibody-secreting spleen and BM B-1 cells are distinct also from terminally differentiated plasma cells generated from antigen-induced conventional B cells, as they express high levels of surface IgM and CD19 and lack expression of CD138. Overall, these data identify populations of non-terminally differentiated B-1 cells in spleen and BM as the most significant producers of natural IgM.

B1a B Lymphocytes Are Atheroprotective by Secreting Natural IgM That Increases IgM Deposits and Reduces Necrotic Cores in Atherosclerotic Lesions

Rationale:

Aggravated atherosclerosis in B lymphocyte-deficient chimeric mice and reduced atherosclerosis after transfer of unfractionated spleen B lymphocytes into splenectomized mice have led to the widely held notion that B lymphocytes are atheroprotective. However, B lymphocytes can be pathogenic, because their depletion by anti-CD20 antibody ameliorated atherosclerosis, and transfer of B2 lymphocytes aggravated atherosclerosis. These observations raise the question of the identity of the atheroprotective B-lymphocyte population.

Objective:

The purpose of the study was to identify an atheroprotective B-lymphocyte subset and mechanisms by which they confer atheroprotection.

Methods and Results:

Splenectomy of apolipoprotein E–deficient mice selectively reduced peritoneal B1a lymphocytes, plasma IgM, and oxidized low-density lipoprotein IgM levels and lesion IgM deposits. These reductions were accompanied by increased oil red O–stained atherosclerotic lesions and increased necrotic cores, oxidized low-density lipoproteins, and apoptotic cells in lesions. Plasma lipids, body weight, collagen, and smooth muscle content were unaffected. Transfer of B1a lymphocytes into splenectomized mice increased peritoneal B1a lymphocytes; restored plasma IgM, oxidized low-density lipoprotein IgM levels, and lesion IgM deposits; and potently attenuated atherosclerotic lesions, with reduced lesion necrotic cores, oxidized low-density lipoprotein, and apoptotic cells. In contrast, transfer of B1a lymphocytes that cannot secrete IgM failed to protect against atherosclerosis development in splenectomized mice despite reconstitution in the peritoneum.

Conclusions:

B1a lymphocytes are an atheroprotective B-lymphocyte population. Our data suggest that natural IgM secreted by these lymphocytes offers protection by depositing IgM in atherosclerotic lesions, which reduces the necrotic cores of lesions.

Partial splenectomy but not total splenectomy preserves immunoglobulin M memory B cells in mice

PURPOSE

The mechanism by which partial splenectomy preserves splenic immune function is unknown. Immunoglobulin (Ig) M memory B cells are critical for the immune response against encapsulated bacteria and are reduced in asplenic patients, although it is unknown whether partial splenectomy can preserve memory B cells. We hypothesized that IgM memory B cells (murine B-1a cells) would be preserved after partial splenectomy but not after total splenectomy in mice.

METHODS

We performed total splenectomy (n = 17), partial splenectomy (n = 10), or sham laparotomy (n = 16) on C57BL/6J mice. Mice were killed on postoperative day 10 or 30, and peritoneal washings were analyzed by multiparameter flow cytometry for expression of murine B-1a cells (IgM(pos)IgD(dull)CD5(pos)B220(dull)).

RESULTS

We found that B-1a cells were significantly reduced after both total and partial splenectomies compared with sham laparotomy in the early postoperative period, although normal levels of B-1a cells returned by postoperative day 30 in mice undergoing partial splenectomy but not total splenectomy.

CONCLUSION

Partial splenectomy but not total splenectomy preserves the B-1a B-cell population in mice within 30 days after surgery. Maintenance of these critical B cells may contribute to the preservation of a splenic-dependent immune response after partial splenectomy.