Tetravalent Bispecific Tandem Antibodies Improve Brain Exposure and Efficacy in an Amyloid Transgenic Mouse Model

Most antibodies display very low brain exposure due to the blood-brain barrier (BBB) preventing their entry into brain parenchyma. Transferrin receptor (TfR) has been used previously to ferry antibodies to the brain by using different formats of bispecific constructs. Tetravalent bispecific tandem immunoglobulin Gs (IgGs) (TBTIs) containing two paratopes for both TfR and protofibrillar forms of amyloid-beta (Aβ) peptide were constructed and shown to display higher brain penetration than the parent anti-Aβ antibody. Additional structure-based mutations on the TfR paratopes further increased brain exposure, with maximal enhancement up to 13-fold in wild-type mice and an additional 4–5-fold in transgenic (Tg) mice harboring amyloid plaques, the main target of our amyloid antibody. Parenchymal target engagement of extracellular amyloid plaques was demonstrated using in vivo and ex vivo fluorescence imaging as well as histological methods. The best candidates were selected for a chronic study in an amyloid precursor protein (APP) Tg mouse model showing efficacy at reducing brain amyloid load at a lower dose than the corresponding monospecific antibody. TBTIs represent a promising format for enhancing IgG brain penetration using a symmetrical construct and keeping bivalency of the payload antibody.

The development of early and mature B?cells is impaired in mice deficient for the Ets-1 transcription factor

The Ets-1 transcription factor is essential for normal development of the natural killer and T cell lineages; however, its role in B cell development remains poorly understood. To address this issue, we used gene targeting to inactivate Ets-1 in mice (Ets-1(-/-)). We show here that the development of B cell precursors, particularly steps requiring pre-B cell receptor function, is defective in Ets-1(-/-) mice. Peripheral B cell subsets were analyzed in RAG2-deficient mice reconstituted with Ets-1(-/-) fetal liver cells. In such Ets-1(-/-) chimeric mice, B cell precursors develop into IgM/IgD-bearing cells, but B-1a cells as well as transitional-2 and marginal zone B cell subsets of the spleen are absent. In response to B cell receptor stimulation, Ets-1(-/-) splenic B cells fail to express the CD69 and CD25 activation markers. Furthermore, despite activation of ERK and JNK signaling pathways, Ets-1-deficient B cells do not proliferate and die following BCR engagement. These findings demonstrate that the effect of Ets-1 inactivation is not restricted to the terminal B cell differentiation stage, but also affects the development and function of earlier B cell subsets.

The Ets-1 transcription factor is required for complete pre-T cell receptor function and allelic exclusion at the T cell receptor beta locus

The pre-T cell receptor (TCR) functions as a critical checkpoint during alphabeta T cell development. Signaling through the pre-TCR controls the differentiation of immature CD4(-)CD8(-)CD25(+)CD44(-) [double-negative (DN)3] thymocytes into CD4(+)CD8(+) double-positive (DP) cells through the CD4(-)CD8(-)CD25(-)CD44(-)(DN4) stage. In addition, pre-TCR activity triggers expansion and survival of thymocytes and inhibits TCRbeta gene rearrangement through a process referred to as allelic exclusion. Whereas many proteins involved in the pre-TCR transduction cascade have been identified, little is known about the nuclear factors associated with receptor function. Here, we use gene targeting to inactivate the Ets-1 transcription factor in mice and analyze pre-TCR function in developing Ets-1-deficient (Ets-1(-/-)) thymocytes. We find that inactivation of Ets-1 impairs the development of DN3 into DP thymocytes and induces an elevated rate of cell death in the DN4 subset. This defect appears specific to the alphabeta lineage because gammadelta T cells maturate efficiently. Finally, the percentage of thymocytes coexpressing two different TCRbeta chains is increased in the Ets-1(-/-) background and, in contrast with wild type, forced activation of pre-TCR signaling does not block endogenous TCRbeta gene rearrangement. These data identify Ets-1 as a critical transcription factor for pre-TCR functioning and for allelic exclusion at the TCRbeta locus.

Disruption of the lineage restriction of TCR beta gene rearrangements

Despite a common lymphoid recombinase, assembly of Ig genes is restricted to B cells, whereas TCR loci rearrange in T cells. Transcriptional promoters and enhancers are critical for the regulation of the recombination process. However, the specific function of such elements in conferring the lineage-restriction of V(D)J recombination remains poorly understood. To gain further insights into the mechanism restricting TCR beta-chain rearrangements to T cells, we generated mice in which an 11 kb region--containing the beta-chain constant region 2 and the TCR beta enhancer (E beta)--was replaced with the B cell specific Ig heavy-chain enhancer (E mu). Unlike the simple E mu to E beta replacement, this mutation allowed significant levels of D beta to J beta as well as V beta to DJ beta rearrangements in both T and B cells. Although the lineage restriction was disrupted, TCR beta allelic exclusion was still efficient in mutated T cells. Together these results demonstrate that changes in the activity of regulatory elements located at the TCR beta constant regions are sufficient to redirect the recombination pattern of TCR beta variable gene segments.

Transgenic expression of the p16(INK4a) cyclin-dependent kinase inhibitor leads to enhanced apoptosis and differentiation arrest of CD4-CD8- immature thymocytes

In the thymus, T cell development proceeds by successive steps of differentiation, expansion, and selection. Control of thymocyte proliferation is critical to insure the full function of the immune system and to prevent T cells from transformation. Deletion of the cell cycle inhibitor p16(INK4a) is frequently observed in human T cell neoplasias and, in mice, gene targeted inactivation of the Ink4a locus enhances thymocyte expansion and predisposes mutant animal to tumorigenesis. Here, we investigate the mechanism by which p16(Ink4a) controls thymocyte development by analyzing transgenic mice expressing the human p16(INK4a) into the T cell lineage. We show that forced expression of p16(INK4a) in thymocytes blocked T cell differentiation at the early CD4-CD8-CD3-CD25+ stage without significantly affecting the development of gammadelta T cells. Pre-TCR function was mimicked by the induction of CD3 signaling in thymocytes of recombinase activating gene (RAG)-2-deficient mice (RAG-2(-/-)). Upon anti-CD3epsilon treatment in vivo, p16(INK4a)-expressing RAG-2(-/-) thymocytes were not rescued from apoptosis, nor could they differentiate. Our data demonstrate that expression of p16(INK4a) prevents the pre-TCR-mediated expansion and/or survival of differentiating thymocytes.