Enhancement of hypermutation frequency in the chicken B cell line DT40 for efficient diversification of the antibody repertoire

A Novel DT40 Antibody Library for the Generation of Monoclonal Antibodies

Early etiological diagnosis is very important for the control of sudden viral infections, and requires antibodies with both high sensitivity and high specificity. Traditional antibody preparation methods have limitations, such as a long and arduous cycle, complicated operation, and high expenses. A chicken lymphoma cell line, DT40, is known to produce IgM-type antibodies and undergo gene conversion and somatic mutation in the variable region of the immunoglobulin gene during culture. Here, the DT40 cell line was developed to produce antibody libraries and prepare antibody rapidly in vitro. Since hypermutation in DT40 cells was regulated by the activation-induced cytidine deaminase (AID) gene, AID expression needs to be controlled to either fix the Ig sequence by stopping mutation or improve affinity by resuming mutation after the antibodies have been selected. In this study, we generated a novel AID-inducible DT40 cell line (DT40-H7), in which the endogenous AID gene was knocked out using the CRISPR/Cas9 genome editing system, and an inducible AID gene, based on the Tet-Off expression system, was stably transfected. AID expression was controlled in DT40-H7 cells in a simple and efficient manner; gene conversion and point mutations were observed only when AID was expressed. Using the antibody library generated from this cell line, we successfully obtained monoclonal antibodies against the NS1 protein of Zika virus. The DT40-H7 cell line represents a useful tool for the selection and evolution of antibodies and may also be a powerful tool for the rapid selection and generation of diagnostic antibodies for emerging infectious diseases.

Construction of a camelid VHH yeast two-hybrid library and the selection of VHH against haemagglutinin-neuraminidase protein of the Newcastle disease virus

BACKGROUND

Newcastle disease (ND), which is caused by the Newcastle disease virus (NDV), is one of the most important avian diseases in poultry. Since its discovery in 1926, ND has caused great economic losses to the world poultry industry and remains a threat to chickens and wild birds. Although a stringent vaccination policy is widely adopted to control ND, ND outbreaks still occur, and virulent NDV is sporadically isolated from chickens and wild birds. To study the pathogenesis of ND and provide tools to prevent its prevalence, novel antibody fragments should be developed. The variable domains of the heavy chain of the heavy-chain antibodies (VHH) are the smallest naturally occurring antibodies derived from camelid heavy-chain antibodies. The comparatively small size, high affinity, high solubility, low immunogenicity and ability to bind epitopes inaccessible to conventional antibodies of VHH make them ideal candidates for a considerable number of therapeutic and biotechnological applications. However, an anti-NDV VHH has not been reported to date.

RESULTS

In this study, a VHH yeast two-hybrid library was constructed from NDV vaccine immunized C. bactrianus, and seven VHH fragments to the haemagglutinin-neuraminidase (HN) protein of NDV were successfully screened and characterized for the first time. These selected VHH clones were all expressed as soluble protein in E. coli. ELISA, dot blot, immunocytochemistry and pull down results showed that the screened VHHs could interact with NDV virion, among which five had neutralizing activity. In addition, the seven VHHs could inhibit the haemagglutination activity of different NDV strains.

CONCLUSIONS

We constructed an NDV-immunized VHH yeast two-hybrid library and screened and characterized seven VHHs targeting NDV HN protein for the first time. The seven VHHs may have great potential for NDV diagnosis, pathogenesis and therapeutics.

Structural and genetic diversity in antibody repertoires from diverse species

The antibody repertoire is the fundamental unit that enables development of antigen specific adaptive immune responses against pathogens. Different species have developed diverse genetic and structural strategies to create their respective antibody repertoires. Here we review the shark, chicken, camel, and cow repertoires as unique examples of structural and genetic diversity. Given the enormous importance of antibodies in medicine and biological research, the novel properties of these antibody repertoires may enable discovery or engineering of antibodies from these non-human species against difficult or important epitopes.

Selection and characterization of single-chain recombinant antibodies against phosphoprotein of Newcastle disease virus

Phosphoprotein (P), involved in virus RNA replication and transcription, had become a new target for the research on treating Newcastle disease virus (NDV). Here we described the cloning and expression of phosphoprotein from NDV, and then screened the anti-P antibodies from the chicken single chain fragment variable (scFv) library, which were generated from chickens immunized with the ND vaccines. As a first step, the recombinant expression vector pET28a-P was successfully constructed. In a following step, two anti-P positive scFv clones from the scFv library were selected by indirect enzyme-linked immunosorbent assay (ELISA) method. The sequence analysis of two positive clones showed that there were more variation in complementary determine region (CDR) of VH and VL, and the CDR3 in VH exhibited a significant change in amino acid number and type. In another experiment, the purified scFv antibodies used in the assay was shown to be specific for NDV-P by western blot. The results indicated that the strategy we used in this experiment proved to be convenient way for screening scFv antibody, which paved a new way for the immunization diagnosis and the exploration of integrated control of NDV.

Activation-induced cytidine deaminase (AID)-dependent somatic hypermutation requires a splice isoform of the serine/arginine-rich (SR) protein SRSF1

Somatic hypermutation (SHM) of Ig variable region (IgV) genes requires both IgV transcription and the enzyme activation-induced cytidine deaminase (AID). Identification of a cofactor responsible for the fact that IgV genes are much more sensitive to AID-induced mutagenesis than other genes is a key question in immunology. Here, we describe an essential role for a splice isoform of the prototypical serine/arginine-rich (SR) protein SRSF1, termed SRSF1-3, in AID-induced SHM in a DT40 chicken B-cell line. Unexpectedly, we found that SHM does not occur in a DT40 line lacking SRSF1-3 (DT40-ASF), although it is readily detectable in parental DT40 cells. Strikingly, overexpression of AID in DT40-ASF cells led to a large increase in nonspecific (off-target) mutations. In contrast, introduction of SRSF1-3, but not SRSF1, into these cells specifically restored SHM without increasing off-target mutations. Furthermore, we found that SRSF1-3 binds preferentially to the IgV gene and inhibits processing of the Ig transcript, providing a mechanism by which SRSF1-3 makes the IgV gene available for AID-dependent SHM. SRSF1 not only acts as an essential splicing factor but also regulates diverse aspects of mRNA metabolism and maintains genome stability. Our findings, thus, define an unexpected and important role for SRSF1, particularly for its splice variant, in enabling AID to function specifically on its natural substrate during SHM.

Cooperation of dendritic cells with naïve lymphocyte populations to induce the generation of antigen-specific antibodies in vitro

The production of monoclonal antibodies by hybridoma technology is dependent on lymphocytes taken from vertebrates which have to be immunized against the corresponding antigen. We present here our first experiments which should allow the replacement of this in vivo immunization step by an in vitro immunization procedure. This work provides new possibilities for the specific activation of immune cells in order to use them for the generation of antibodies which are not of murine origin. Bone marrow-derived dendritic cells were loaded with antigen and co-cultured with naïve T and B lymphocytes of non-immunized mice. The interaction and activation of the different cell types were investigated by measuring the expression of specific cell surface markers, the release of activation-dependent interleukins and the secretion of antigen-specific antibodies. We could demonstrate that dendritic cells process and present antigen fragments and activate T cells, that T cells proliferate and release activation-induced interleukins, and that B cells maturate under the influence of activated T cells and secrete antigen-specific antibodies.