Role of B-1a cells in autoimmunity

Autoimmunity Reviews 5 (2006) 403 – 408 www.elsevier.com/locate/autrev

Role of B-1a cells in autoimmunity Byian Duan, Laurence Morel * Department of Pathology, Immunology, and Laboratory Medicine, University of Florida, Gainesville, FL 32610, United States Received 15 September 2005; accepted 6 October 2005 Available online 9 December 2005

Abstract B-1a cells are distinguished from conventional B cells (B2) by their developmental origin, their surface marker expression and their functions. They were originally identified as a B cell subset of fetal origin that expresses the pan-T cell surface glycoprotein, CD5. B-1a cells also differ from B2 by the expression levels of several surface markers, including IgM, IgD, CD43 and B220 [R. Berland, H.H. Wortis, Origins and functions of B-1 cells with notes on the role of CD5. Ann Rev Immunol, 20 (2002) 253–300. [1]]. The majority of B-1a cells are located in peritoneal and pleural cavities. Compared to B2 cells, B-1a are long-lived, non-circulating, with reduced BCR diversity and affinity [A.B. Kantor, C.E. Merrill, L.A. Herzenberg, J.L. Hillson, An unbiased analysis of V–H–D–J(H) sequences from B-1a, B-1b, and conventional B cells. J Immunol, 158 (1997) 1175–1186. [2]]. B-1a cells are largely responsible for the production of circulating IgM referred to as natural antibodies. These low affinity antibodies are polyreactive and constitute as such a first line of defense against bacterial pathogens [M.C. Carroll, A.P. Prodeus, Linkages of innate and adaptive immunity. Curr Opin Immunol, 10 (1998) 36–40. [3]]. This polyreactivity also results into the recognition of autoantigens, which serves in the clearance of apoptosis products. The weak autoreactivity of the B-1a cells has been postulated to play a role in autoimmune pathogenesis. In addition, other characteristics, such as the production of high level of IL-10 [A. O’Garra, R. Chang, N. Go, R. Hastings, G. Haughton, M. Howard, et al. Ly-1 B (B-1) cells are the main source of B cell-derived interleukin 10. Eur J Immunol, 22 (1992) 711–717. [4]] and enhanced antigen presentation capacities [C. Mohan, L. Morel, P. Yang, E.K. Wakeland, Accumulation of splenic B1a cells with potent antigen-presenting capability in NZM2410 lupus-prone mice. Arthritis and Rheumatism, 41 (1998) 1652–1662. [5]], have implicated B-1a cells in autoimmunity. This review will discuss the current understandings of their role in autoimmune diseases with focus on lupus. D 2005 Elsevier B.V. All rights reserved. Keywords: B-1a cells; CD5; Antibodies; Autoimmunity; Lupus

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Characteristics of B-1a cells . . . . . . . . . . . . . . . . . . 1.1 Origins of B-1 cells . . . . . . . . . . . . . . . . . . . 1.2 B-1 cell functions . . . . . . . . . . . . . . . . . . . . B-1 cells in autoimmune diseases . . . . . . . . . . . . . . . 2.1 Association of B-1a cell functions and autoimmunity . . 2.2 Genetic dissection of B-1a cells in autoimmune models .

* Corresponding author. E-mail address: [email protected] (L. Morel). 1568-9972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.autrev.2005.10.007

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Take-home messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Systemic lupus erythematosus (SLE) is a chronic autoimmune disorder, characterized by the production of large amounts of autoantibodies that result in immune complex mediated end-organ damage. B cells are necessary players in the pathogenic process and at least two mechanisms, autoantibody production and antigen presentation has been evidenced in murine models [6]. As a long-lived, self-renewing B-cell subset, B-1 cells produce large amount of polyreactive, low affinity IgM, called natural antibody. High levels of B-1 cells have been reported in patients with SLE, Sjogren’s syndrome and rheumatoid arthritis, and numerous associations between expansion of this cell compartment and systemic autoimmunity have been found in murine models (reviewed in [1]). Extensive studies of B-1 cell phenotypes and functions have revealed that B1a cells possess a variety of characteristics which may contribute to the development of autoimmune diseases, which will be summarized here after a brief review of B-1 cell characteristics and functions. 1. Characteristics of B-1a cells The majority of B-1 cells reside in peritoneal and pleural cavities. B-1 cells are also found in the spleen in smaller numbers. Recent evidence suggests, however, that peritoneal and splenic B-1 cells are phenotypically and functionally different [7]. For practical reasons, most of the published work on B-1 cells refers to peritoneal B-1 cells, and this review will focus on this population. B-1 cells are larger than B-2 cells, and they are defined by a pattern of surface marker expression, B220(CD45)lo, IgMhi, IgDlo, CD9+, CD43+, CD23lo, as opposed to conventional circulating B-2 cells that are B220(CD45)hi, IgMhi/lo, IgD+, CD9 , CD43 , and CD23hi [1,8]. Interestingly peritoneal B-1 cells also express the myeloid marker CD11b. B-1a cells are a subset of B-1 cells defined by expressing pan-T cell marker CD5. The other B-1 sister population of B-1b cells share all the surface phenotypes with B-1a except CD5 expression, and contributes only to a small proportion of B-1 population. Very little is known of B-1b cells, and it is still not clear whether these two subsets are distinct cell types or two developmental stages of the same population. One argument in favor of the latter hypothesis is that CD5 is a negative regulator that is also expressed on anergic B cells, therefore its expression on

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B-1a cells would be a marker of autoantigen exposure [9]. B-1b cells have recently shown, however, to be critical in producing adaptive pneumococcal polysaccharide antibodies [10], providing evidence for the first time these cells with a specific function. Most of the studies, however, have targeted either the entire B-1 population or specifically B-1a cells, which will be the topic in the rest of the review. 1.1. Origins of B-1 cells Adoptive transfer experiments have shown that B-1 and B-2 cells belong to two distinct lineages. When transferred into irradiated mice, only fetal liver cells could reconstitute both the B-1 and B-2 compartments, while adult bone marrow only generated B-2 cells [11,12]. In addition, unlike conventional B-2 cells, B1 cells are long-lived and self-renewing [13]. Long term depletion of B-1 cells in peritoneal cavity can be achieved by osmotic lysis and transfer of B-1 cells to irradiated mice result in long-term reconstitutions [11]. As an alternative to this independent lineage model, an binduced-differentiationQ model has been proposed, in which the B-1 phenotype is a consequence of T independent type 2 (TI-2)-like activation following encounters with naturally occurring TI-2 antigens [14–16]. This model was based on the observation that BCR cross-linking on B-2 cells could induce some B-1 phenotypes, such as CD5 expression and proliferative response to phorbol ester. However, fully induction of B1 phenotypes on B-2 cells has not been obtained in vitro. B-1 cell production from BM precursors has been found sporadically [1], and we have reported that adult BM and splenocytes expressing the lupus susceptibility locus Sle2 produce large numbers of B-1a cells [17]. Transgenic models and targeted gene mutations have also shown that BCR signaling is critical for B-1 cell development. Mutations that disrupt positive regulators of BCR signaling, such as CD19, PI-3K p85, vav, BTK, or CR1/CR2, result in decreased B-1 cell numbers, while mutations in inhibitory regulators such as SHP1, CD22, CD72 or Lyn, can lead to increased B-1 cell numbers (see [1] for a review). In addition, in mice transgenic for B1-derived immunoglobulin genes, the predominant B cell population found in all lymphoid organs is CD5+ B-1a cells [18,19]. Conversely, the B2-

B. Duan, L. Morel / Autoimmunity Reviews 5 (2006) 403–408

derived immunoglobulin transgenes lead to B-2 cell phenotypes. These studies clearly show that adult lymphoid organs contain precursors that can develop into B-1 cells when the express the appropriate BCR specificity and/or signaling threshold. It is not known, however, if the fetal and adult-derived B-1 cells have equivalent characteristics and functions. Finally, B-1 cells are not maintained in the absence of spleen [20]. This intriguing finding demonstrates the obligate role, yet to be defined, of the adult lymphoid compartment for the development of this population of mainly fetal origin. 1.2. B-1 cell functions B-1 cells are the major source of natural antibodies, which are serum poly-reactive and weakly autoreactive. They recognize antigens from many common pathogens, and thus are very important for the early response to bacterial and viral infections. Mice lacking natural antibodies showed an increased susceptibility to influenza infections and an increased mortality [21]. B-1 cells have constitutively up-regulated expression of plasma cell specific genes, including Blimp-1 and XBP-1 [22], which may explain the spontaneous and continuous secretion of natural IgM. B-1 cells, along with marginal zone B cells, are also responsible for responses against TI-2 antigens to which they are frequently exposed due to their physical location. Finally, B-1 cells give rise to IgA-producing plasma cells in the gut lamina propria, which also contribute significantly against enteric pathogens [23]. In addition to their protective function against pathogens, antibodies produced by B-1 cells play an important role in the clearance of apoptotic cells and released autoantigens [24]. A critical role for B-1 cell produced IgM has also demonstrated in ischemia–reperfusion injury [25]. 2. B-1 cells in autoimmune diseases The secretion of autoantibodies has identified B-1 cells as potential contributors to the development of autoimmune diseases, such as lupus. In fact, elevated numbers of B-1a cell have been associated with autoimmunity both in human and mouse models. In patients with Sjogren’s syndrome, SLE and rheumatoid arthritis (RA), increased numbers of B-1 cells have been reported (reviewed in [1]). In the (NZB  NZW)F1 murine lupus model and its derivative NZM2410, large numbers of B1 cells accumulate in the peritoneal cavity and to a lesser extent, in the spleen [5].

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2.1. Association of B-1a cell functions and autoimmunity The most compelling evidence for the involvement of B-1 cells in lupus autoimmune pathogenesis in murine model was that the deletion of peritoneal B-1 cells by hypotonic shock reduced disease severity in (NZB  NZW)F1 mice [26]. The same group also showed that over-expression of osteopontin resulted in simultaneous increased B-1 cells and anti-dsDNA Ab production on a non-autoimmune genetic background [27]. A number of studies however do not support a role of B-1 cells in systemic autoimmunity. In the well-characterized in FAS-deficient models, B-1a cells do not contribute to autoantibody production [28]. Furthermore, over-expression of IL-5 in the (NZB  NZW)F1 model greatly increased the number of B-1a cells, but, surprisingly, significantly reduced anti-dsDNA antibody production and incidence of nephritis [29]. Several mechanisms have been suggested to explain the possible role of B-1 cells in autoimmune pathogenesis: production of pathogenic autoantibody; presentation of self-antigen to autoreactive T cells; and/or their ability to secret cytokines, such as IL-10. A direct role of the natural antibodies to autoimmune pathogenesis is difficult to show as these antibodies also exist in normal individuals. A diminished negative regulation has also been invoked since B-1 cells do not undergo the same tolerance checkpoints as B-2 cells do. Under normal conditions, B-1 cells are refractory to antigen stimulation and are always excluded from germinal center reactions. However, some studies have shown that human B-1a cells can be induced to differentiate to germinal center-like cells [30], and autoreactive B-1a cells carrying somatically mutated V genes have been found in some RA patients [31]. A number of BCR transgenic models have shown an association between elevated numbers of B-1 cells and autoimmune manifestations (reviewed in [1]). In particular the anti-red blood cell transgenic strain 4C8 developed by Murakami et al. has suggested that autoreactive B-1 cells are sequestered away from their cognate antigen in the peritoneum, and they can produce pathogenic autoantibodies when activated by pathogens or bystander effects [32]. One can imagine a genetic background that lowers the activation threshold for these cells, allowing their expansion and secretion of autoantibodies in response to normally subthreshold activation signals. B-1 cells are a major source of IL-10 [4], which is an anti-inflammatory cytokine involved in regulatory T cells functions. It is also a potent stimulator of B cell

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Table 1 Genetic linkage between murine chromosomal regions and increased numbers of B-1 cells Susceptible strain

Locus

Map (chromosome, cM)

References

NZW NZW NZW NZB C57BL/10 NZB NZM2410 (NZW)b NZM2410 (NZW) NZM2410 (NZB)

Bpal-1 Bpal-2 Bpal-3 Mott-1 H2-s Ltk a Sle2a Sle2b Sle2c

17, 20 cM 13, 13 cM 17, 3 cM 4, 66 cM 17, 20 cM 2, 67 cM 4, 29–38 cM 4, 38–44 cM 4, 55–56 cM

[41] [41] [41] [47] [43] [42] [46] [46] [46]

a

Gene. Indicates the NZW or NZB origin of the NZM2410 corresponding segment. b

differentiation, proliferation, and antibody production [33]. The role of IL-10 in systemic autoimmunity is controversial as it has been shown to both inhibit [34] and exacerbate [35] disease in animal models as well as in SLE patients [36,37]. This discrepancy implicates the cytokine functions differently during the stages of the disease development. Therefore, there is no definitive simple link linking B-1 cells to autoimmunity through the production of IL-10. B-1a cells express high levels of costimulatory molecules B7-1, B7-2 and display enhanced antigen presentation capabilities [5]. Accumulation of B1a cells in target organs have been observed in aged (NZB  NZW)F1 mice [38]. As the consequence, it has been suggested that B-1a cells may activate autoreactive T cells and produce autoantibodies in target organs, contributing significantly to immune-complex mediated pathology [39]. 2.2. Genetic dissection of B-1a cells in autoimmune models In murine models, genetic factors are also undeniably involved in determining the size of the B1 compartment [40], and both autoimmune-prone NZB and NZW are among the strains with the highest numbers of B-1 cells. A number of genetic loci linked to B-1 cells expansion have been mapped in NZB [41,42] and a gain-of-function polymorphism in the receptor-type protein tyrosine kinase LTK was identified as the causative allele for the chromosome 2 locus (Table 1). Several groups have also linked the MHC locus to the relative distribution of B-1 and B-2 cells [41,43]. We have shown by congenic analysis that chromosome 4 Sle2, a locus initially identified as linked to lupus nephritis in the NZM2410 strain [44], results into an

age dependent expansion of the B-1 compartment [45]. We have more recently reported that Sle2 corresponds to at least three independent additive loci, Sle2a, Sle2b, and Sle2c, all contributing to B-1 cell expansion [46]. Sle2a and Sle2b are from NZW origin, while Sle2c is of NZB origin [44]. The interferon a gene cluster is a candidate gene for Sle2b, as we have shown that Sle2 mice produce significantly less IFNa than B6 controls, and that IFNa deficiency is associated with an expansion of the B-1a cells in the peritoneal cavity [47]. On chromosome 4, but outside of the Sle2 interval, the NZB locus Mott-1 has been linked to an increased ability of B-1 cells to become Mott cells and secrete antibodies [48]. The contribution of these loci to lupus pathogenesis is important to link B-1a cells to the disease process. The NZB-derived Nba2 locus was shown to induce disease when crossed to NZW without affecting the size of the B-1 cell compartment [49] and the same observation was made for Sle1 (Morel et al., unpublished). The situation with Sle2 is more complex. Comparison of the various permutations of NZM2410 Sle loci clearly established that Sle2 contributes significantly to pathogenesis [50]. Sle2c, in particular, is the individual locus among the 3 Sle2 loci that is associated with the largest expansion of the B-1a cell pool. Sle2c, however, does not contribute significantly to pathogenesis in the NZM2410 model [46]. Although we have used the B-1a cell expansion to fine-map the Sle2 locus, Sle2 also results in a lower of activation threshold in conventional B-2 cells [45] and mediates a breach in tolerance of anti-nuclear specific B cells (Mohan et al., submitted). Genetics studies have provided a broader view on the role of B-1 cells in the development of autoimmunity. These studies together with other methods, such as gene-targeting, have shown that there is more than a direct functional link between increased B-1 cell numbers and autoimmunity. It is therefore possible that the accumulation of B-1 cells represents a bystander consequence of a dysregulated B-cell development driven by a greater BCR signaling, and under certain conditions, that these cells by themselves may or may not participate in the autoimmune pathogenesis. It will be of interest to compare the functions of B-1 cells in these different contexts. Take-home messages ! B-1a cells constitute a B cell subset that populates the peritoneal and pleural cavity and produce natural

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antibodies, which are polyreactive low affinity IgM antibodies. Natural antibodies provide a non-adaptive first line of defense against pathogens B-1a and are involved in clearance of apoptotic products. Expansion of the B-1a cells has been associated with autoimmunity. B-1a cells can contribute to autoimmunity by the autoreactivity of natural antibodies, the production of cytokines and enhanced antigen presentation capabilities. Many models have now shown that the participation of the B-1a population is neither necessary nor sufficient for autoimmune pathogenesis. The expansion of B-1a cell subset is under genetic control, which in autoimmune settings may represent a marker for a dysregulated B cell development driven by a greater B cell receptor signaling.

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Validity of screening tests for Sjogren’s syndrome in ambulatory patients with chronic disease To determine the validity of screening tests for Sjogren’s syndrome (SS) in ambulatory patients with chronic disease. In this study, Sanchez-Guerrero J. et. al. (J Rheumatol 2006; 33: 907-11), randomly selected 300 patients from the rheumatology and internal medicine clinics of a tertiary care center and assessed for SS. During the screening phase, an interview, the European questionnaire for sicca symptoms, Schirmer-I test, and the wafer test, were carried out in all patients. Patients with positive screening had confirmatory tests including fluorescein staining test, non-stimulated whole salivary flow, and autoantibody testing. Confirmatory tests were also done in 13 patients with negative screening. During the last phase, lip biopsy was proposed to patients who met preestablished criteria. Women made up 79% of the study population. Mean age of subjects was 42.8˘15.7 years. Two hundred twenty patients (73%) had positive screening. The distribution of positive test results was: xerophthalmia 118 (39%), xerostomia 103 (34%), Schirmer-I test 101 (34%), and wafer test 187 (62%) patients. Forty (13%) patients met criteria for SS. All screening tests were useful for identifying patients with SS; however, the model composed of at least one positive response to the European questionnaire (EQI), Schirmer-I test, and wafer test showed the best performance. Use of the European questionnaire, Schirmer-I test, and wafer test in parallel was useful for identifying patients with SS among ambulatory patients with chronic disease.