Contrasting effects of filamin A and B proteins in modulating filovirus entry

Ebola (EBOV) and Marburg viruses (MARV) cause severe hemorrhagic fever associated with high mortality rates in humans. A better understanding of filovirus-host interactions that regulate the EBOV and MARV lifecycles can provide biological and mechanistic insight critical for therapeutic development. EBOV glycoprotein (eGP) and MARV glycoprotein (mGP) mediate entry into host cells primarily by actin-dependent macropinocytosis. Here, we identified actin-binding cytoskeletal crosslinking proteins filamin A (FLNa) and B (FLNb) as important regulators of both EBOV and MARV entry. We found that entry of pseudotype psVSV-RFP-eGP, infectious recombinant rVSV-eGP-mCherry, and live authentic EBOV and MARV was inhibited in filamin A knockdown (FLNaKD) cells, but was surprisingly enhanced in filamin B knockdown (FLNbKD) cells. Mechanistically, our findings suggest that differential regulation of macropinocytosis by FLNa and FLNb likely contributes to their specific effects on EBOV and MARV entry. This study is the first to identify the filamin family of proteins as regulators of EBOV and MARV entry. These findings may provide insight into the development of new countermeasures to prevent EBOV and MARV infections.

Standardized IMGT® Nomenclature of Salmonidae IGH Genes, the Paradigm of Atlantic Salmon and Rainbow Trout: From Genomics to Repertoires

In teleost fish as in mammals, humoral adaptive immunity is based on B lymphocytes expressing highly diverse immunoglobulins (IG). During B cell differentiation, IG loci are subjected to genomic rearrangements of V, D, and J genes, producing a unique antigen receptor expressed on the surface of each lymphocyte. During the course of an immune response to infections or immunizations, B cell clones specific of epitopes from the immunogen are expanded and activated, leading to production of specific antibodies. Among teleost fish, salmonids comprise key species for aquaculture. Rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) are especially important from a commercial point of view and have emerged as critical models for fish immunology. The growing interest to capture accurate and comprehensive antibody responses against common pathogens and vaccines has resulted in recent efforts to sequence the IG repertoire in these species. In this context, a unified and standardized nomenclature of salmonid IG heavy chain (IGH) genes is urgently required, to improve accuracy of annotation of adaptive immune receptor repertoire dataset generated by high-throughput sequencing (AIRRseq) and facilitate comparisons between studies and species. Interestingly, the assembly of salmonids IGH genomic sequences is challenging due to the presence of two large size duplicated IGH loci and high numbers of IG genes and pseudogenes. We used data available for Atlantic salmon to establish an IMGT standardized nomenclature of IGH genes in this species and then applied the IMGT rules to the rainbow trout IGH loci to set up a nomenclature, which takes into account the specificities of Salmonid loci. This unique, consistent nomenclature for Salmonid IGH genes was then used to construct IMGT sequence reference directories allowing accurate annotation of AIRRseq data. The complex issues raised by the genetic diversity of salmon and trout strains are discussed in the context of IG repertoire annotation.

Origin of Public Memory B Cell Clones in Fish After Antiviral Vaccination

Vaccination induces “public” antibody clonotypes common to all individuals of a species, that may mediate universal protection against pathogens. Only few studies tried to trace back the origin of these public B-cell clones. Here we used Illumina sequencing and computational modeling to unveil the mechanisms shaping the structure of the fish memory antibody response against an attenuated Viral Hemorrhagic Septicemia rhabdovirus. After vaccination, a persistent memory response with a public VH5JH5 IgM component was composed of dominant antibodies shared among all individuals. The rearrangement model showed that these public junctions occurred with high probability indicating that they were already favored before vaccination due to the recombination process, as shown in mammals. In addition, these clonotypes were in the naïve repertoire associated with larger similarity classes, composed of junctions differing only at one or two positions by amino acids with comparable properties. The model showed that this property was due to selective processes exerted between the recombination and the naive repertoire. Finally, our results showed that public clonotypes greatly expanded after vaccination displayed several VDJ junctions differing only by one or two amino acids with similar properties, highlighting a convergent response. The fish public memory antibody response to a virus is therefore shaped at three levels: by recombination biases, by selection acting on the formation of the pre-vaccination repertoire, and by convergent selection of functionally similar clonotypes during the response. We also show that naive repertoires of IgM and IgT have different structures and sharing between individuals, due to selection biases. In sum, our comparative approach identifies three conserved features of the antibody repertoire associated with public memory responses. These features were already present in the last common ancestors of fish and mammals, while other characteristics may represent species-specific solutions.

Teleost fish mount complex clonal IgM and IgT responses in spleen upon systemic viral infection

Upon infection, B-lymphocytes expressing antibodies specific for the intruding pathogen develop clonal responses triggered by pathogen recognition via the B-cell receptor. The constant region of antibodies produced by such responding clones dictates their functional properties. In teleost fish, the clonal structure of B-cell responses and the respective contribution of the three isotypes IgM, IgD and IgT remain unknown. The expression of IgM and IgT are mutually exclusive, leading to the existence of two B-cell subsets expressing either both IgM and IgD or only IgT. Here, we undertook a comprehensive analysis of the variable heavy chain (VH) domain repertoires of the IgM, IgD and IgT in spleen of homozygous isogenic rainbow trout (Onchorhynchus mykiss) before, and after challenge with a rhabdovirus, the Viral Hemorrhagic Septicemia Virus (VHSV), using CDR3-length spectratyping and pyrosequencing of immunoglobulin (Ig) transcripts. In healthy fish, we observed distinct repertoires for IgM, IgD and IgT, respectively, with a few amplified μ and τ junctions, suggesting the presence of IgM- and IgT-secreting cells in the spleen. In infected animals, we detected complex and highly diverse IgM responses involving all VH subgroups, and dominated by a few large public and private clones. A lower number of robust clonal responses involving only a few VH were detected for the mucosal IgT, indicating that both IgM(+) and IgT(+) spleen B cells responded to systemic infection but at different degrees. In contrast, the IgD response to the infection was faint. Although fish IgD and IgT present different structural features and evolutionary origin compared to mammalian IgD and IgA, respectively, their implication in the B-cell response evokes these mouse and human counterparts. Thus, it appears that the general properties of antibody responses were already in place in common ancestors of fish and mammals, and were globally conserved during evolution with possible functional convergences.

Is IgT the evolutionary equivalent of IgA? Insights into its structure and function (81.21)

From an evolutionary perspective, immunoglobulins emerged for the first time in fish. Teleost fish are known to contain at least three immunoglobulin (Ig) isotypes, IgM, IgD and the recently discovered IgZ/IgT. Thus far nothing is known about the protein structure, function and distribution of IgZ/IgT. We have raised monoclonal antibodies against IgT that have enabled us to purify it from trout plasma. Our analysis show that IgT is a 180 kDa monomeric Ig and represents the first monomeric immunoglobulin to be shown in teleost fish. Thus far, IgM is the only known teleost fish Ig involved in eliciting antigen-specific immune responses in teleost fish. However, here we present for the first time exciting data showing that rainbow trout produces IgT-specific responses upon antigenic stimulation. In addition we have identified a unique B cell subset expressing exclusively surface IgT. Importantly, the distribution of IgT+ B cells is predominant in the fish mucosal surfaces, suggesting a possible role of IgT in mucosal immunity.

Adjuvant properties of trout C5a in subunit protein and DNA vaccines (47.31)

The activation of the complement system results in the generation of C3a, C4a and C5a anaphylotoxins. C5a is the most potent and pro-inflammatory of all three anaphylatoxins. In mammals, C5a has been shown to influence the generation ofantigen-specific antibody responses, thereby acting as a molecular adjuvant. We have previously reported that rainbow trout C5a has potent chemotactic properties. In addition, we have also characterized a trout C5a receptor that is expressed at high levels both in granulocytes and B cells. In this study, we tested the possible role of trout C5a as a molecular adjuvant. To this end, we generated recombinant infectious hematopoietic necrosis virus glycoprotein (G), and a G-C5a fusion protein in E.coli. We showed that in contrast to G alone, G-C5a displayed a potent chemoattractant activity, indicating that the C5a portion of the fusion protein was functional. G-C5a was injected in incomplete freunds adjuvant (IFA) to one group of trout, while two other groups of trout were inoculated with the same dose of recombinant G in incomplete or complete freunds adjuvant (CFA) respectively. At six week postinjection, the serum antibody titers of the group injected with recombinant G-C5a in IFA were significantly higher than those of the group injected with G protein in IFA. Thus, these results suggest for the first time a role of C5a as molecular adjuvant in teleost fish. Experiments in progress will evaluate the effect of a boost injection on enhancing secondary and memory immune responses in the same fish. In addition, the presence of neutralizing antibodies against IHNV in the immunized fish will be reported.

Identification of a phagocytic complement C3d receptor in rainbow trout (53.15)

In higher vertebrates, activation of the complement system leads to cleavage of C3 into C3b which binds covalently to complement-activating particles. The C3d degradation fragment of C3b is thereafter produced upon cleavage by factor I and is known to bind to complement receptor type 2 (CR2). The presence of CR2 on B cells enables the uptake of C3d-antigen complexes and together with the B cell receptor, CR2 plays a synergistic role in the activation of these cells. While CR2 has been characterized in mammals, its presence in non-mammalian species remains to be studied. Here we present evidence for the existence of phagocytic C3d receptors in teleost fish B lymphocytes. To evaluate the presence of C3d receptors in trout B cells, we generated FITC-labeled monomers and tetramers of trout C3d and assessed their binding to trout leukocytes. Binding of C3d to trout B cells could be inhibited both by anti-trout C3 antibodies and by unlabeled C3d. Biotinylated C3d was then used to coat 2μm FITC-conjugated streptavidin beads. Trout B lymphocytes were able phagocytose C3d-beads at a much higher ratio than BSA-beads or beads alone. Internalization of beads was inhibited by soluble C3d. To address further the identity of this receptor, we have recently cloned a trout molecule that shows the highest degree of homology to CR1/2 molecules from mammals. Thus far, our results suggest the existence of a trout C3d receptor that has a role in the phagocytosis of C3d-coated particles. Whether this receptor plays a role in adaptive immunity is currently being investigated in our laboratory.

B-Lymphoma cells with epigenetic silencing of Pax5 trans-differentiate into macrophages, but not other hematopoietic lineages

In mice, zygotic or pro-B-cell-specific knock-out of the Pax5 gene allows differentiation of pro-B-cells into all hematopoietic lineages. We previously generated and characterized a murine B-cell lymphoma, dubbed Myc5, whose cells spontaneously lose Pax5 expression when cultured in vitro, but regain it when re-injected into syngeneic mice. In cultured Myc5 cells, the loss of Pax5 correlates with the acquisition of myeloid markers, such as CD11b and F4/80. Here, we sought to determine whether these cells are truly B-macrophage-restricted or, like Pax5-null progenitors, can give rise to additional hematopoietic lineages. In vitro differentiation assays with various cytokines showed that Myc5 cells do not differentiate into NK cells, dendritic cells, neutrophils, or osteoclasts. At the same time, in the presence of macrophage colony-stimulating factor (M-CSF), they readily phagocytose latex beads and provide T-cell help. Both phenomena are indicative of the bona fide macrophage phenotype. Conversely, enforced Pax5 re-expression in macrophage-like Myc5 cells led to down-regulation of the M-CSF receptor and re-acquisition of some B-cell surface markers (e.g., CD79a) and lineage-specific transcription factors (e.g., IRF4 and Blimp). Retrovirally encoded Pax5 also restored expression of several master B-cell differentiation proteins, such as the IL-7 receptor and transcription factor E2A. In contrast, levels of EBF were unaffected by Pax5 suggesting that EBF acts exclusively upstream of Pax5 and might contribute to Pax5 expression. Indeed, transduction with an EBF-encoding retrovirus partly reactivated endogenous Pax5. Our data reveal the complex relationship between B-cell-specific transcription factors and suggest the existence of numerous feedback mechanisms.