Surviving B cells complete antigen-independent maturation in the spleen, producing immunocompetent naïve mature B cells that subsequently develop into either follicular or marginal zone B cells. This is followed by positive and negative selection processes, to eliminate non-functional and self-reactive immature B cells. Ig gene rearrangement during these early stages of B-cell development results in the expression of a mature BCR that is capable of binding to antigen. B cells are routinely classified based on their maturation status, antibody isotype, and effector function. B cells originate from hematopoietic stem cells in the bone marrow, where they undergo several phases of antigen-independent development leading to the generation of immature B cells. However, the baseline phenotypic diversity in B cells and BCRs in the Kymouse has not been fully described.ī cells are an integral part of the humoral immune response due to their ability to produce antibodies against diverse antigens, providing protection against infection. Kymice exhibit normal B-cell production and maturation and the resulting B-cell receptors (BCRs) are diverse, with human-like CDRH3 lengths and evidence of somatic hypermutation ( Lee et al., 2014). The mouse constant regions were retained, preserving downstream interactions with endogenous intracellular signalling components and cell membrane Fc receptors, resulting in functional, fully active chimeric antibodies. In Kymab’s Intelliselect Transgenic mouse (Kymouse), a complete set of human variable (V), diversity (D), and junction (J) genes of the IGH locus as well as the V and J genes of the Igλ and Igκ loci were inserted at the sites of the endogenous mouse loci. As humanised animal models become the source of a growing number of therapeutics and play an increasingly important role in the evaluation of novel vaccine candidates, it is crucial to understand the degree to which their B-cell repertoires can be considered representative of humans.Ĭontemporary Ig transgenic animal models vary according to the number of genes and localisation of the inserted human Ig loci ( Green, 2014 Brüggemann et al., 2015). Transgenic platforms have also found a new application in vaccine response modelling ( Sok et al., 2016 Pantophlet et al., 2017 Walls et al., 2020). Humanised Ig loci-transgenic animal models have proven extremely useful in therapeutic antibody discovery 20 of the 127 therapeutic antibodies licensed in the US or EU as of April 2022 were derived from transgenic mouse platforms (data from Thera-SAbDab Raybould et al., 2020). Twenty-five years of progress in genetic engineering from the first immunoglobulin (Ig) transgenic mouse ( Brüggemann et al., 1989) culminated in 2014 in the integration of a complete human Ig haplotype in mice for the first time ( Lee et al., 2014). Our combined sequence and structural analysis indicates that the naïve Kymouse BCR repertoire is diverse with key similarities to human repertoires, while immunophenotyping confirms that selected naïve B cells are able to go through complete development. To compare the structural space explored by CDRH3s in each species’ repertoire, we used computational structure prediction to show that Kymouse naïve BCR repertoires are more human-like than mouse-like in their predicted distribution of CDRH3 shape. These differences result in Kymice having CDRH3 length and diversity intermediate between mice and humans. Comparison of the naïve B-cell receptor (BCR) repertoires of Kymice BCRs, naïve human, and murine BCR repertoires revealed key differences in germline gene usage and junctional diversification. In this study, we phenotypically characterised B-cell populations from the Intelliselect Transgenic mouse (Kymouse) demonstrating full B-cell development competence. Immunoglobulin loci-transgenic animals are widely used in antibody discovery and increasingly in vaccine response modelling.
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