IgM-mediated B cell apoptosis

Optimization of a Plasmodium falciparum circumsporozoite protein repeat vaccine using the tobacco mosaic virus platform

RTS,S/AS01 is a circumsporozoite protein (CSP)-based malaria vaccine that confers partial protection against malaria in endemic areas. Recent reports have elucidated structures of monoclonal antibodies that bind to the central (NPNA) repeat region of CSP and that inhibit parasite invasion. Antigen configuration and copy number of CSP repeats displayed on a tobacco mosaic virus (TMV) particle platform were studied. A TMV vaccine containing CSP repeats displayed as a loop induced 10× better antibody titer than a nearly full-length CSP in mice. In rhesus model, this translated to a 5× improvement in titer. Rhesus antibodies potently inhibited parasite invasion up to 11 mo after vaccination. An optimized epitope-focused, repeat-only CSP vaccine may be sufficient or better than the existing CSP vaccines.

Plasmodium falciparum vaccine RTS,S/AS01 is based on the major NPNA repeat and the C-terminal region of the circumsporozoite protein (CSP). RTS,S-induced NPNA-specific antibody titer and avidity have been associated with high-level protection in naïve subjects, but efficacy and longevity in target populations is relatively low. In an effort to improve upon RTS,S, a minimal repeat-only, epitope-focused, protective, malaria vaccine was designed. Repeat antigen copy number and flexibility was optimized using the tobacco mosaic virus (TMV) display platform. Comparing antigenicity of TMV displaying 3 to 20 copies of NPNA revealed that low copy number can reduce the abundance of low-affinity monoclonal antibody (mAb) epitopes while retaining high-affinity mAb epitopes. TMV presentation improved titer and avidity of repeat-specific Abs compared to a nearly full-length protein vaccine (FL-CSP). NPNAx5 antigen displayed as a loop on the TMV particle was found to be most optimal and its efficacy could be further augmented by combination with a human-use adjuvant ALFQ that contains immune-stimulators. These data were confirmed in rhesus macaques where a low dose of TMV-NPNAx5 elicited Abs that persisted at functional levels for up to 11 mo. We show here a complex association between NPNA copy number, flexibility, antigenicity, immunogenicity, and efficacy of CSP-based vaccines. We hypothesize that designing minimal epitope CSP vaccines could confer better and more durable protection against malaria. Preclinical data presented here supports the evaluation of TMV-NPNAx5/ALFQ in human trials.

Valency and density matter: Deciphering impacts of immunogen structures on immune responses against a tumor associated carbohydrate antigen using synthetic glycopolymers

For successful carbohydrate based anti-cancer vaccines, it is critical that B cells are activated to secret antibodies targeting the tumor associated carbohydrate antigens (TACAs). Despite the availability of many TACA based constructs, systematic understanding of the effects of structural features on anti-glycan antibody responses is lacking. In this study, a series of defined synthetic glyco-polymers bearing a representative TACA, i.e., the Thomsen-nouveau (Tn) antigen, have been prepared to probe the induction of early B cell activation and antibody production via a T cell independent mechanism. Valency and density of the antigen in the polymers turned out to be critical. An average of greater than 6 Tn per chain was needed to induce antibody production. Glycopolymers with 40 antigens per chain and backbone molecular weight of 450 kDa gave the strongest stimulation to B cells in vitro, which correlated well with its in vivo activity. Deviations from the desired valency and density led to decreased antibody production or even antigen specific B cell non-responsiveness. These findings provide important insights on how to modulate anti-TACA immune responses facilitating the development of TACA based anti-cancer vaccines using glycopolymers.

Antibodies against CD20 or B-cell receptor induce similar transcription patterns in human lymphoma cell lines

Background

CD20 is a cell surface protein exclusively expressed on B cells. It is a clinically validated target for Non-Hodgkin's lymphomas (NHL) and autoimmune diseases. The B cell receptor (BCR) plays an important role for development and proliferation of pre-B and B cells. Physical interaction of CD20 with BCR and components of the BCR signaling cascade has been reported but the consequences are not fully understood.

Methodology

In this study we employed antibodies against CD20 and against the BCR to trigger the respective signaling. These antibodies induced very similar expression patterns of up- and down-regulated genes in NHL cell lines indicating that CD20 may play a role in BCR signaling and vice versa. Two of the genes that were rapidly and transiently induced by both stimuli are CCL3 and CCL4. 4 hours after stimulation the concentration of these chemokines in culture medium reaches a maximum. Spleen tyrosine kinase Syk is a cytoplasmic tyrosine kinase and a key component of BCR signaling. Both siRNA mediated silencing of Syk and inhibition by selective small molecule inhibitors impaired CCL3/CCL4 protein induction after treatment with either anti-CD20 or anti-BCR antibodies.

Conclusion

Our results suggest that treatment with anti-CD20 antibodies triggers at least partially a BCR activation-like response in NHL cell lines.

Comparative effects of human Ig alpha and Ig beta in inducing autoreactive antibodies against B cells in mice

Human and mouse Ig alpha molecules share only 58% amino acid sequence identity in their extracellular regions. However, mice immunized with a recombinant Fc fusion protein containing the extracellular portion of human Ig alpha produced significant amounts of IgG capable of binding to Ig alpha on mouse B cells. The induced auto/cross-reactive Abs could down-regulate B cell levels and the consequent humoral immune responses against an irrelevant Ag in treated mice. Analogous immunization with an Fc fusion protein containing the extracellular portion of human Ig beta gave a much weaker response to mouse Ig beta, although human and mouse Ig beta, like their Ig alpha counterparts, share 56% sequence identity in their extracellular regions. Protein sequence analyses indicated that a potential immunogenic segment, located at the C-terminal loop of the extracellular domain, has an amino acid sequence that is identical between human and mouse Ig alpha. A mAb A01, which could bind to both human and mouse Ig alpha, was found to be specific to a peptide encompassing this immunogenic segment. These findings suggest that specific auto/cross-reactivity against self Ig alpha can be induced by a molecular mimicry presented by a foreign Ig alpha.

Anti-immunoglobulin binding and activation of snapper (Pagrus auratus) leucocytes

In order to perform specific immunological assays we have produced and characterised three monoclonal antibodies (MAbs) that bind snapper (Pagrus auratus, Bloch and Schneider) immunoglobulin (Ig). Hybridomas were produced and screened for anti-Ig production using ELISA, Western blot and flow cytometry. All three MAbs (designated 2C5, 4A2 and 1C6) bound specifically to the heavy (H) chain of reduced Ig in Western blot. Furthermore, 1C6 was shown to bind to reduced skin mucus Ig H chain and all three MAbs cross-reacted with the H chain of Atlantic salmon and rainbow trout Ig. In flow cytometric analyses 2C5 and 4A2 bound to B cell populations in the peripheral blood and lymphoid organs. Furthermore, cross-linked 2C5 induced an increase in intracellular protein tyrosine phosphorylation in peripheral blood lymphocytes. Phosphorylated proteins exhibited similar molecular weights to those of mammalian Igalpha and Igbeta and may represent snapper mIg accessory molecule analogues. These data exhibit the potential use of 2C5, 4A2 and 1C6 in both cellular and biochemical analyses of populations of snapper leucocytes.

Control of B cell production by the adaptor protein lnk. Definition Of a conserved family of signal-modulating proteins

Lnk is an SH2 domain-containing adaptor protein expressed preferentially in lymphocytes. To illuminate the importance of Lnk, we generated lnk(-/-) mice. Whereas T cell development was unaffected, pre-B and immature B cells accumulated in the spleens. In the bone marrow, B-lineage cells were proportionately increased, reflecting enhanced production of pro-B cells that resulted in part from hypersensitivity of precursors to SCF, the ligand for c-kit. Hence, Lnk ordinarily acts to regulate B cell production. Further characterization of lnk(-/-) mice also revealed that full-length Lnk is a 68 kDa protein containing a conserved proline-rich region and a PH domain. Lnk is a representative of a multigene adaptor protein family whose members act, by analogy with Lnk, to modulate intracellular signaling.

CD27 signals through PKC in human B cell lymphomas

Tumour necrosis factor receptor (TNFR) superfamily members play critical roles in the regulation of cell proliferation and death. One member of the TNFR superfamily, CD27, is unique because it is the only covalently linked homodimer in the family. CD27 and its cellular ligand, CD70, have been implicated in the regulation of T cell and B cell interactions that lead to cellular activation and regulation of immunoglobulin expression. Due to the unique nature of CD27, we chose to screen a number of B cell lymphoma cell lines for CD27 and CD70 expression and evaluate CD27 activation by antibody cross-linking. Two cell lines, HT and SU-4, showed greater cellular proliferation when CD27 was cross-lined and this correlated with increased PKC activation. Additionally, in the HT cell line cell surface expression of IgG was increased by CD27 cross-linking. Thus we have identified cellular systems for the study of CD27 signal transduction that will allow definition of the CD27 signal cascade of some B cell lymphomas.

CD40- and HLA-DR-mediated cell death pathways share a lot of similarities but differ in their use of ADP-ribosyltransferase activities

CD40 and HLA-DR molecules are two major components of the immune system, and their engagement on several cell types leads to various cellular events that modulate cell function. In this study, we demonstrate that signaling via these molecules leads to a rapid B cell death. CD40-mediated cell death was mainly observed in Epstein-Barr virus (EBV)-transformed B cell lines, whereas, HLA-DR-induced response can be triggered in normal activated B cells as well as in EBV-transformed B cell lines. Cell death induced via both molecules does not require de novo protein synthesis, but involves the integrity of the cytoskeleton. The sensitivity of CD40- and HLA-DR-mediated cell death to various inhibitors is very similar to that previously reported for tumor necrosis factor receptor (TNFR)- and Fas-triggered apoptosis; however, caspases leading to poly(ADP-ribose) polymerase cleavage are not implicated in this response. Both B cell death forms do not involve Fas-Fas ligand and TNF-TNFR systems, but require LFA-1-independent cell-cell interactions mediated by still undefined molecules. Although CD40- and HLA-DR-mediated cell death appears to follow a common pathway, inhibitors of poly- and mono-ADP-ribosyltransferase activity differentially affect these responses. Defining the molecules involved in CD40- and HLA-DR-mediated death will provide a possible interrelation between the different B cell death programs that can lead to a better comprehension of regulation of B cell functions.

Cleavage of transcription factor SP1 by caspases during anti-IgM-induced B-cell apoptosis

Apoptosis is instrumental in the processes generating the diversity of the B-cell repertoire. Autoreactive B-cells are eliminated by anti-IgM crosslinking after encountering self-antigens, but precise mechanisms leading to B-cell apoptosis are still not well understood. We report here the cleavage of the transcription factor SP1 in the human Burkitt lymphoma cell line BL60 during anti-IgM-induced apoptosis. Western blot analysis revealed two cleavage products of approximately 68 kDa and 45 kDa after induction of apoptosis. Cleavage could be completely inhibited by zDEVD-fmk, an inhibitor specific for caspase 3-like proteases. In-vitro cleavage of recombinant SP1 by recombinant caspase 3 (CPP32) or caspase 7 (Mch 3) results in similar cleavage products as those observed in vivo. Recombinant caspase 6 (Mch 2) primarily generates a 68-kDa cleavage product, as observed after calcium ionophore (CaI) induced B-cell apoptosis. In contrast, caspase 1 (ICE) did not cleave SP1 in vitro. The time course of SP1 cleavage during anti-IgM-induced apoptosis is paralleled by an increase of caspase activity measured by DEVD-p-nitroanilide (DEVD-pNA) cleavage. DNA band-shift assays revealed a decrease in the intensity of the full length SP1/DNA complex and an increase in the intensity of a smaller complex due to the binding of one SP1 cleavage product. By Edman sequencing we could identify a caspase 3 cleavage site after Asp584 (D584AQPQAGR), generating a 22-kDa C-terminal SP1 protein fragment which still contains the DNA binding site. Our results show the cleavage of the human transcription factor SP1 in vivo and in vitro, underlining the central role of caspase 3-like proteases during the process of anti-IgM-induced apoptosis.