B-1 cells are deficient in Lck: defective B cell receptor signal transduction in B-1 cells occurs in the absence of elevated Lck expression

Potential role of viral infection and B cells as a linker between innate and adaptive immune response in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is an autoimmune disease that involves several organ systems. Although B cells play a key role in SLE pathogenesis, the mechanisms behind B cell dysregulation in SLE development remained controversial. Finding the modules containing highly co-expressed genes in B cells could explain biological pathways involved in the pathogenesis of SLE, which may further support the reasons for the altered function of B cells in SLE disease. A total of three microarray gene expression datasets were downloaded from Gene Expression Omnibus. SLE samples were prepared from the purified B lymphocyte cells of the patients who have not received immunosuppressive drugs as well as high dose immunocytotoxic therapies or steroids. A weighted gene co-expression network was then constructed to find the relevant modules implicated in the SLE progression. Among 17 identified modules, 3 modules were selected through mapping to STRING and finding the ones that had highly connection at the protein level. These modules clearly indicate the involvement of several pathways in the pathogenesis of SLE including viral infection, adaptive immune response, and innate immune response in B lymphocytes. The WGCN analysis further revealed the co-expressed genes involved in both innate and adaptive immune systems. Mix infections and primary immunodeficiency might also dysregulate B lymphocytes, which may facilitate SLE development. As such, identifying novel biomarkers and pathways in lupus would be of importance.

Higher expression of KCNK10 (TREK-2) K+ channels and their functional upregulation by lipopolysaccharide treatment in mouse peritoneal B1a cells

Innate-like CD5⁺ B1a cells localized in serous cavities are activated by innate stimuli, such as lipopolysaccharide (LPS), leading to T cell-independent antibody responses. Although ion channels play crucial roles in the homeostasis and activation of immune cells, the electrophysiological properties of B1a cells have not been investigated to date. Previously, in the mouse B cell lymphoma cells, we found that the voltage-independent two-pore-domain potassium (K2P) channels generate a negative membrane potential and drive Ca²⁺ influx. Here, we newly compared the expression and activities of K2P channels in mouse splenic follicular B (FoB), marginal zone B (MZB), and peritoneal B1a cells. Next-generation sequencing analysis showed higher levels of transcripts for TREK-2 and TWIK-2 in B1a cells than those in FoB or MZB cells. Electrophysiological analysis, using patch clamp technique, revealed higher activity of TREK-2 with the characteristic large unitary conductance (~ 250 pS) in B1a than that in FoB or MZB cells. TREK-2 activity was further increased by LPS treatment (>2 h), which was more prominent in B1a than that in MZB or FoB cells. The cytosolic Ca²⁺ concentration of B cells was decreased by high-K⁺-induced depolarization (ΔR_KCl (%)), suggesting the basal Ca²⁺ influx to be driven by negative membrane potential. The LPS treatment significantly increased the ΔR_KCl (%) in B1a, though not in FoB and MZB cells. Our study was the first to compare the K2P channels in mouse primary B cell subsets, elucidating the functional upregulation of TREK-2 and augmentation of Ca²⁺ influx by the stimulation of Toll-like receptor 4 in B1a cells.

Stem cell-derived CAR T cells traffic to HIV reservoirs in macaques

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) with CCR5^– donor cells is the only treatment known to cure HIV-1 in patients with underlying malignancy. This is likely due to a donor cell–mediated graft-versus-host effect targeting HIV reservoirs. Allo-HSCT would not be an acceptable therapy for most people living with HIV due to the transplant-related side effects. Chimeric antigen receptor (CAR) immunotherapies specifically traffic to malignant lymphoid tissues (lymphomas) and, in some settings, are able to replace allo-HSCT. Here, we quantified the engraftment of HSC-derived, virus-directed CAR T cells within HIV reservoirs in a macaque model of HIV infection, using potentially novel IHC assays. HSC-derived CAR cells trafficked to and displayed multilineage engraftment within tissue-associated viral reservoirs, persisting for nearly 2 years in lymphoid germinal centers, the brain, and the gastrointestinal tract. Our findings demonstrate that HSC-derived CAR⁺ cells reside long-term and proliferate in numerous tissues relevant for HIV infection and cancer.

Atypical Response of B-1 Cells to BCR Ligation: A Speculative Model

Peritoneal B-1a cells manifest unusual signaling characteristics that distinguish them from splenic B-2 cells. These include the failure of BCR engagement to trigger NF-κB activation and DNA replication. Despite extensive study, a clear explanation for these characteristics has not emerged. Here we aim to develop a unified paradigm based on previous reports and recent results, which proposes a central role for phosphatase activity. We hypothesize B-1a cells are unable to induce NF-κB or proliferate after BCR cross-linking due to increased phosphatase abundance or activity. This phosphatase abundance and/or activity may be the result of unique B-1a cell characteristics such as increased levels of HSP70 and/or constitutive secretion of IL-10. We speculate phosphatase activity cannot be overcome by BCR ligation alone due to insufficient Vav protein expression, which does not allow for proper production of reactive oxygen species, which inhibit phosphatases. Furthermore, constitutively active Lyn also plays a negative regulatory role in B-1a. We expect that a new focus on phosphatase activity and its suppression will be revealing for BCR signal transduction in B-1 cells.

Potentially autoreactive naturally occurring transitional T3 B lymphocytes exhibit a unique signaling profile

B lymphocytes exhibit phenotypic differences that correlate with their developmental or functional stages and affect humoral immune responses. One recently described subset of naturally occurring immature transitional type 3 (T3) B lymphocytes is believed to consist of potentially autoimmune cells whose signaling properties have not been studied in detail. This study characterizes intracellular signaling in T3 B cells in wildtype C57BL/6 mice. Protein phosphorylation and Ca(2+) responses upon B-cell antigen receptor (BCR) engagement were measured by multicolor flow cytometry. We observed high baseline signaling activity and reduced BCR-mediated responses in T3 cells, which confirmed their anergy - a functional state in which lymphocytes recognize chronically present self-antigens but cannot produce immune response due to intrinsic signaling inhibition. Our results also revealed a previously unknown T3-specific phosphorylation pattern of 24 key signaling molecules involved in BCR signal transduction. These characteristics reflect the balance between stimulatory and inhibitory BCR signaling pathways in anergy. Results obtained in the collagen-induced arthritis model demonstrate the loss of anergy in T3 B cells during the onset of the disease. Our findings provide rationale for further investigating alterations in B-cell signaling patterns as earliest functional biomarkers of changes in the immune tolerance of autoreactive B cells.

The double life of a B-1 cell: self-reactivity selects for protective effector functions

During their development, B and T cells with self-reactive antigen receptors are generally deleted from the repertoire to avoid autoimmune diseases. Paradoxically, innate-like B-1 cells in mice are positively selected for self-reactivity and form a pool of long-lived, self-renewing B cells that produce most of the circulating natural IgM antibodies. This Review provides an overview of the developmental processes that shape the B-1 cell pool in mice, outlines the functions of B-1 cells in both the steady state and during host defence, and discusses possible functional B-1 cell homologues that exist in humans.

Octamer binding protein 2 (Oct2) regulates PD-L2 gene expression in B-1 cells through lineage-specific activity of a unique, intronic promoter

Programmed death-1 ligand 2 (PD-L2) expression extends beyond macrophages/dendritic cells to B-1 B cells, a distinct B-cell lineage that is responsible for natural immunoglobulin and which is repertoire skewed toward autoreactive specificities. PD-L2 expression is constitutive in B-1 cells, whereas it is inducible in other cell types, suggesting that PD-L2 is regulated differently in the former versus the latter, and this proved to be the case, both in transcription and promotion. B-1 cells express a PD-L2 transcript that lacks exon 1, in contrast to macrophages/dendritic cells for which exon1 is included, reflecting a unique start site upstream of exon 2. PD-L2 transcription in B-1 cells is regulated by a novel intronic promoter located between exons 1 and 2. This intronic promoter binds Octamer binding protein 1 (Oct1) and Oct2, and although these transcription factors are present in all B cells, Oct2 binding is found in vivo only in B-1 cells and not PD-L2-negative B-2 cells. Moreover, the proximal promoter upstream of exon 1 that is active in macrophages is inactive in B-1 cells. Thus, PD-L2 expression is regulated by two different promoters that function in a lineage-specific manner, with the B-1-specific promoter being constitutively active as a result of Oct1 and Oct2 binding.

Fas apoptosis inhibitory molecule expression in B cells is regulated through IRF4 in a feed-forward mechanism

Fas apoptosis inhibitory molecule (FAIM) was originally cloned as an inhibitor of Fas-mediated apoptosis in B cells that has been reported to affect multiple cell types. Recently, we found that FAIM enhances CD40L-mediated signal transduction, including induction of IFN regulatory factor (IRF)4, in vitro and augments plasma cell production in vivo. These results have keyed interest in the regulation of FAIM expression, about which little is known. Here, we show that Faim is regulated by IRF4. The Faim promoter contains three IRF binding sites, any two of which promote Faim expression. Faim promoter activity is lost following mutation of all three IRF binding sites, whereas activity of the full promoter is enhanced by concurrent expression of IRF4. In stimulated primary B cells, IRF4 expression precedes FAIM expression, IRF4 binds directly to the Faim promoter, and loss of IRF4 results in the failure of stimulated Faim up-regulation. Finally, FAIM is preferentially expressed in germinal center B cells. Taken together, these results indicate that FAIM expression is regulated through IRF4 and that this most likely occurs as part of germinal center formation. Because FAIM enhances CD40-induced IRF4 expression in B cells, these results suggest that induction of FAIM initiates a positive reinforcing (i.e., feed-forward) system in which IRF4 expression is both enhanced by FAIM and promotes FAIM expression.

Adult BM generates CD5+ B1 cells containing abundant N-region additions

The self-renewing capacity of B1 cells infers homeostatic regulation; however, previous work suggests the low level of N-region addition characterizing B1 cells early in life increases with age, which implies that the B1-cell population is not a closed system. To explore this, we evaluated N-region addition in CD5(+) B1 cells generated from adult BM. Adult BM cells were marked with GFP introduced by mouse stem cell virus transduction, and were then adoptively transferred into lethally irradiated recipients. Within 2-3 months, we found GFP-marked CD5(+) B cells in the peritoneal cavities of recipients, which we demonstrate here meet a variety of criteria for B1-cell traits including Mac-1 surface expression; annexin, elfin, and Pax-5 gene expression; mitogenic responsiveness to phorbol ester; and spontaneous immunoglobulin secretion. Notably, we found by single-cell PCR that this population of BM-derived CD5(+) B1 cells expressed immunoglobulin with abundant N-region addition (and little V(H)11/V(H)12 skewing), unlike CD5(+) B1 cells obtained from unmanipulated animals but reminiscent of B2 cells. Further, we confirmed that native CD5(+) B1 cells from older mice contain more N-region additions than native CD5(+) B1 cells from younger mice. These results suggest that adult BM progenitors contribute to the peritoneal CD5(+) B1-cell pool over time.

Fas apoptosis inhibitory molecule enhances CD40 signaling in B cells and augments the plasma cell compartment

Fas apoptosis inhibitory molecule (FAIM) was cloned as a mediator of Fas resistance that is highly evolutionarily conserved but contains no known effector motifs. In this study, we report entirely new functions of FAIM that regulate B cell signaling and differentiation. FAIM acts to specifically enhance CD40 signaling for NF-kappaB activation, IRF-4 expression, and BCL-6 down-regulation in vitro, but has no effect on its own or in conjunction with LPS or anti-Ig stimulation. In keeping with its effects on IRF-4 and BCL-6, FAIM overexpression augments the plasma cell compartment in vivo. These results indicate that FAIM is a new player on the field of B cell differentiation and acts as a force multiplier for a series of events that begins with CD40 engagement and ends with plasma cell differentiation.

Continual signaling is responsible for constitutive ERK phosphorylation in B-1a cells

B-1a cells constitutively express phosphorylated, activated ERK, but the origin of pERK in B-1 cells has not been determined. To address this issue, we examined specific mediators of intracellular signaling in unmanipulated B-1a cells. We found that constitutive pERK was rapidly lost from B-1a cells following addition of metabolic inhibitors that block src kinase, Syk, PI-3K, and PLC function. We examined Syk and PLC in more detail and found rapid accumulation of phosphorylated forms of these molecules in B-1a cells, but not B-2 cells, when phosphatase activity was inhibited, and this change occurred in the majority of B-1a cells. Further, we showed that inhibition of src kinase activity eliminated "downstream" pSyk and pPLC accumulation in phosphatase-inhibited B-1a cells, indicating a pathway connection. CD86 expression is greater on B-1 than B-2 cells and plays a role in antigen presentation by B-1 cells to T cells. We found that when Syk or PI-3K was inhibited, CD86 expression was diminished in a reversible fashion. All together, these results indicate that continual activation of intracellular signaling leads to constitutive activation of ERK in B-1 cells, with attendant consequences for co-stimulatory molecule expression.

CD19 signaling is impaired in murine peritoneal and splenic B-1 B lymphocytes

B-1 cells reside predominantly within the coelomic cavities, tonsils, Peyer's patches, spleen (a minor fraction - approximately 5%) and are absent in the lymph nodes. They are the primary sources of natural IgM in the body. B-1 cells express polyreactive B cell receptors (BCRs) that cross react with self-antigens and are thus implicated in auto-immune disorders. Previously, we reported that peritoneal B-1 cells are deficient in CD19-mediated intracellular signals leading to Ca(2+) mobilization. Here, we find that splenic B-1 cells, like peritoneal B-1 cells, are defective in Ca(2+) release upon B cell activation by co-cross-linking BCR and CD19. In the absence of extracellular sources of Ca(2+), intracellular Ca(2+) flux is similar between B-1 and B-2 cells. Moreover, the intracellular component of Ca(2+) release in both subsets of B cells is mostly PI3K dependent. BCR and CD19 co-cross-linking activates Akt, a key mediator of survival and proliferation signals downstream of PI3K in splenic B-2 cells. Splenic B-1 cells, on the other hand, do not phosphorylate Akt (S473) upon similar treatment. Furthermore, BCR+CD19 cross-linking induced phosphorylation of JNK is much reduced in splenic B-1 cells. In contrast, B-1 cells exhibited increased levels of constitutively active pLyn which appears to have an inhibitory role. The CD19 induced Ca(2+) response and BCR induced proliferation response were restored by a partial inhibition of pLyn with Src kinase specific inhibitors. These findings suggest a defect in CD19-mediated signals in both peritoneal and splenic B-1 B lymphocytes, which is in part, due to higher levels of constitutively active Lyn.

Interleukin 5 in the link between the innate and acquired immune response

Interleukin-5 (IL-5) is an interdigitating homodimeric glycoprotein that is initially identified by its ability to support the in vitro growth and differentiation of mouse B cells and eosinophils. IL-5 transgenic mouse shows two predominant features, remarkable increase in B-1 cells resulting in enhanced serum antibody levels, predominantly IgM, IgA, and IgE classes and in expansion of eosinophil numbers in the blood and eosinophil infiltration into various tissues. Conversely, mice lacking a functional gene for IL-5 or IL-5 receptor alpha chain (IL-5Ralpha) display a number of developmental and functional impairments in B cells and eosinophils. IL-5 receptor (IL-5R) comprises alpha and betac chains. IL-5 specifically binds to IL-5Ralpha and induces the recruitment of betac to IL-5R. Although precise mechanisms on cell-lineage-specific IL-5Ralpha expression remain elusive, several transcription factors including Sp1, E12/E47, Oct-2, and c/EBPbeta have been shown to regulate its expression in B cells and eosinophils. JAK2 and JAK1 tyrosine kinase are constitutively associated with IL-5Ralpha and betac, respectively, and are activated by IL-5 stimulation. IL-5 activates at least three different signaling pathways including JAK2/STAT5 pathway, Btk pathway, and Ras/ERK pathway. IL-5 is one of key cytokines for mouse B cell differentiation in general, particularly for fate-determination of terminal B cell differentiation to antibody-secreting plasma cells. IL-5 critically regulates homeostatic proliferation and survival of and natural antibody production by B-1 cells, and enhances the AID and Blimp-1 expression in activated B-2 cells leading to induce mu to gamma1 class switch recombination and terminal differentiation to IgM- and IgG1-secreting plasma cells, respectively. In humans, major target cells of IL-5 are eosinophils. IL-5 appears to play important roles in pathogenesis of asthma, hypereosinophilic syndromes, and eosinophil-dependent inflammatory diseases. Clinical studies will provide a strong impetus for investigating the means of modulating IL-5 effects. We will discuss the role of IL-5 in the link between innate and acquired immune response, particularly emphasis of the molecular basis of IL-5-dependent B cell activation, allergen-induced chronic inflammation and hypereosinophilic syndromes on a novel target for therapy.

Hybrid cell vaccination resolves Leishmania donovani infection by eliciting a strong CD8+ cytotoxic T-lymphocyte response with concomitant suppression of interleukin-10 (IL-10) but not IL-4 or IL-13

There is an acute dearth of therapeutic interventions against visceral leishmaniasis that is required to restore an established defective cell-mediated immune response. Hence, formulation of effective immunotherapy requires the use of dominant antigen(s) targeted to elicit a specific antiparasitic cellular immune response. We implemented hybrid cell vaccination therapy in Leishmania donovani-infected BALB/c mice by electrofusing dominant Leishmania antigen kinetoplastid membrane protein 11 (KMP-11)-transfected bone marrow-derived macrophages from BALB/c mice with allogeneic bone marrow-derived dendritic cells from C57BL/6 mice. Hybrid cell vaccine (HCV) cleared the splenic and hepatic parasite burden, eliciting KMP-11-specific major histocompatibility complex class I-restricted CD8+ cytotoxic T-lymphocyte (CTL) responses. Moreover, splenic lymphocytes of HCV-treated mice not only showed the enhancement of gamma interferon but also marked an elevated expression of the Th2 cytokines interleukin-4 (IL-4) and IL-13 at both transcriptional and translational levels. On the other hand, IL-10 production from splenic T cells was markedly suppressed as a result of HCV therapy. CD8+ T-cell depletion completely abrogated HCV-mediated immunity and the anti-KMP-11 CTL response. Interestingly, CD8+ T-cell depletion completely abrogated HCV-induced immunity, resulting in a marked increase of IL-10 but not of IL-4 and IL-13. The present study reports the first implementation of HCV immunotherapy in an infectious disease model, establishing strong antigen-specific CTL generation as a correlate of HCV-mediated antileishmanial immunity that is reversed by in vivo CD8+ T-cell depletion of HCV-treated mice. Our findings might be extended to drug-nonresponsive visceral leishmaniasis patients, as well as against multiple infectious diseases with pathogen-specific immunodominant antigens.

Molecular and biochemical analysis of rainbow trout LCK suggests a conserved mechanism for T-cell signaling in gnathostomes

Two genes were identified in rainbow trout that display high sequence identity to vertebrate Lck. Both of the trout Lck transcripts are associated with lymphoid tissues and were found to be highly expressed in IgM-negative lymphocytes. In vitro analysis of trout lymphocytes indicates that trout Lck mRNA is up-regulated by T-cell mitogens, supporting an evolutionarily conserved function for Lck in the signaling pathways of T-lymphocytes. Here, we describe the generation and characterization of a specific monoclonal antibody raised against the N-terminal domains of recombinant trout Lck that can recognize Lck protein(s) from trout thymocyte lysates that are similar in size ( approximately 57kDa) to mammalian Lck. This antibody also reacted with permeabilized lymphocytes during FACS analysis, indicating its potential usage for cellular analyses of trout lymphocytes, thus representing an important tool for investigations of salmonid T-cell function.

Invariant NKT cells rapidly activated via immunization with diverse contact antigens collaborate in vitro with B-1 cells to initiate contact sensitivity

In cutaneous contact sensitivity there is an early elicited innate cascade of complement, mast cells, and platelets activated via IgM Abs. This response is required to initiate the elicitation of acquired classical contact sensitivity by leading to local recruitment of effector T cells. We recently performed in vivo experiments showing that collaboration is required between innate-like invariant Valpha14+ NKT cells (iNKT) and the innate-like B-1 B cell subset to induce this initiation process. Contact sensitization triggers iNKT cells to produce IL-4 to coactivate the B-1 cells along with specific Ag for production of the initiating IgM Abs. We now describe in vitro collaboration of iNKT and B-1 cells. Normal peritoneal B-1 cells, incubated in vitro with soluble Ag, and with 1-h in vivo immune iNKT cells producing IL-4, are activated to mediate the contact sensitivity-initiation cascade. The three components of this process can be activated by different Ag. Thus, 1-h iNKT cell activation, B-1 cell stimulation, and generation of immune effector T cells can be induced by sensitization with three different Ag to respectively generate IL-4 and Ag-specific IgM Abs, to recruit the Ag-specific effector T cells. These findings have relevance to allergic and autoimmune diseases in which infections can trigger exacerbation of T cell responses to allergens or to autoantigens.

B-1 B cells: development, selection, natural autoantibody and leukemia

B-1 (CD5+) B cells constitute a phenotypic and functionally distinct population of B cells in mouse that show enriched expression of autoreactive B-cell antigen receptors and that produce several types of natural autoantibodies. Recently, there has been much progress in this field of research. Evidence has appeared for the existence of distinctive B-cell precursors that preferentially generate B-1 B cells, and the crucial requirement for strong B-cell antigen receptor signaling in the maturation of B-1 B cells has been established. Other work focuses on a phenotypically similar population that lacks CD5, termed 'B-1b', which shows similarities and differences from most CD5+ B cells in both development and function. The relationship of normal B-1 cells with B-cell lymphomas and leukemias continues to be a subject of interest and debate.

Increased negative selection impairs neonatal B cell repertoire but does not directly lead to generation of disease-associated IgM auto-antibodies

To determine if increased negative B cell selection, due to lowered signaling threshold of responsiveness to a ligand as a result of SHP-1 deficiency, during ontogeny leads to the origin of disease-associated IgM auto-antibodies (AAbs), 47 V(H)J558+ VDJCmu rearrangements from SHP-1-deficient viable motheaten (me(v)/me(v)) and 24 J558+ VDJCmu rearrangements from normal me(v)/+ neonatal (<24 h post-birth) B cells were examined for their structural properties. None of the J558+ VDJCmu rearrangements from autoimmune-prone me(v)/me(v) had the characteristic CDR3H size restriction or arginine residues noted in disease-associated IgM AAbs. However, the MVAR2/10 genes are expressed at a high frequency in me(v)/me(v) (31.9%) as compared with me(v)/+ (16.7%), and pM11 gene expression is exclusively (14.9%) noted in me(v)/me(v) B cells. Clearly, there is a trend toward higher expression of pM11 genes (P-value < or = 0.09) in autoimmune-prone me(v)/me(v) strain. The CDR2H region of J558+ VDJCmu recombinations from me(v)/me(v) has increased hotspot triplets predisposing to mutations as compared with me(v)/+ (P-value < or = 0.01) mice. A higher DFL D-gene expression is noted in J558+ VDJCmu rearrangements from me(v)/me(v) (P-value < or = 0.1) in contrast to me(v)/+. The sophisticated logistic regression and odds ratio analysis of V-, D- and J-gene expressions in neonatal B cells from me(v)/me(v) and me(v)/+ mice demonstrates differential composition of the germ line IgM repertoire as a result of SHP-1 deficiency. These observations suggest that increased negative B cell selection during ontogeny impairs the developing IgM antibody repertoire but does not directly lead to generation of disease-associated IgM AAbs.

B-1 cells express transgelin 2: unexpected lymphocyte expression of a smooth muscle protein identified by proteomic analysis of peritoneal B-1 cells

B-1 cells constitute a unique B cell subset that differs phenotypically, biochemically, and functionally from the predominant population of conventional B-2 cells. Functional differences include constitutive secretion of natural immunoglobulin and failure of BCR signaling to initiate proliferation. The origin of these differences remains uncertain. We hypothesized that unbiased analysis of differences in protein expression between highly pure populations of B-1 and B-2 cells might provide information not readily available through other means. To pursue this, we undertook 2D gel analysis of B-1 and B-2 cells combined with mass spectrometry. We identified the smooth muscle protein, transgelin 2, in peritoneal (but not splenic) B-1 cells and did not find it in splenic B-2 cells; these results were confirmed by Western blot analysis, which showed a more than 60-fold difference in transgelin 2 expression between peritoneal B-1 and splenic B-2 cells. In contrast, levels of transgelin 2 RNA differed to a much lesser extent (3-fold) in the two B cell populations, so transgelin 2 is an example of a molecule whose subset-specific expression is more readily detected by proteomic than transcriptomic analyses. Finally, transgelin 2 protein expression was induced in splenic B-2 cells; thus, transgelin 2 joins a number of other inducible molecules that are constitutively expressed by peritoneal B-1 but not splenic B-2 cells. Although the role of transgelin 2 in B-1 cell function remains uncertain, identification of this molecule demonstrates the value of examining protein expression in this B cell subset.