Bispecific antibody target pair discovery by high-throughput phenotypic screening using in vitro combinatorial Fab libraries

Bispecific antibodies can uniquely influence cellular responses, but selecting target combinations for optimal functional activity remains challenging. Here we describe a high-throughput, combinatorial, phenotypic screening approach using a new bispecific antibody target discovery format, allowing screening of hundreds of target combinations. Simple in vitro mixing of Fab-fusion proteins from a diverse library enables the generation of thousands of screen-ready bispecific antibodies for high-throughput, biologically relevant assays. We identified an obligate bispecific co-targeting CD79a/b and CD22 as a potent inhibitor of human B cell activation from a short-term flow cytometry signaling assay. A long-term, high-content imaging assay identified anti-integrin bispecific inhibitors of human cell matrix accumulation targeting integrins β1 and β6 or αV and β1. In all cases, functional activity was conserved from the bispecific screening format to a therapeutically relevant format. We also introduce a broader type of mechanistic screen whereby functional modulation of different cell subsets in peripheral blood mononuclear cells was evaluated simultaneously. We identified bispecific antibodies capable of activating different T cell subsets of potential interest for applications in oncology or infectious disease, as well as bispecifics abrogating T cell activity of potential interest to autoimmune or inflammatory disease. The bispecific target pair discovery technology described herein offers access to new target biology and unique bispecific therapeutic opportunities in diverse disease indications.

Creation of a gated antibody as a conditionally functional synthetic protein

The ability to conditionally direct antibodies is a potentially powerful application for Synthetic Biology in Medicine. Here we show that control of antibody binding through site-specific, chemical phosphorylation of a recognition domain creates a ‘gated’ antibody (Ab). This displays a crude Boolean logic where binding is induced in an enzyme-AND-antigen dependent manner. This ‘AND-Ab’ is therefore active only in the presence of two biomarker inputs: the simultaneous expression of a (cell surface) antigen and secreted enzyme to generate function in vitro, on cells and in mammalian tissue. Such gated Abs, either alone or in combination, could allow the application of logic strategies to enhance precision in biological interrogation, modulation and in therapy.

The ability to control antibody binding could have important medical implications. Here, the authors present a method to engineer phosphatase-controllable antibodies that bind to a specific recognition site in the presence of two biomarker inputs.

The loss of telomerase activity in highly differentiated CD8+CD28-CD27- T cells is associated with decreased Akt (Ser473) phosphorylation

The enzyme telomerase is essential for maintaining the replicative capacity of memory T cells. Although CD28 costimulatory signals can up-regulate telomerase activity, human CD8(+) T cells lose CD28 expression after repeated activation. Nevertheless, telomerase is still inducible in CD8(+)CD28(-) T cells. To identify alternative costimulatory pathways that may be involved, we introduced chimeric receptors containing the signaling domains of CD28, CD27, CD137, CD134, and ICOS in series with the CD3 zeta (zeta) chain into primary human CD8(+) T cells. Although CD3 zeta-chain signals alone were ineffective, triggering of all the other constructs induced proliferation and telomerase activity. However, not all CD8(+)CD28(-) T cells could up-regulate this enzyme. The further fractionation of CD8(+)CD28(-) T cells into CD8(+)CD28(-) CD27(+) and CD8(+)CD28(-)CD27(-) subsets showed that the latter had significantly shorter telomeres and extremely poor telomerase activity. The restoration of CD28 signaling in CD8(+)CD28(-)CD27(-) T cells could not reverse the low telomerase activity that was not due to decreased expression of human telomerase reverse transcriptase, the enzyme catalytic subunit. Instead, the defect was associated with decreased phosphorylation of the kinase Akt, that phosphorylates human telomerase reverse transcriptase to induce telomerase activity. Furthermore, the defective Akt phosphorylation in these cells was specific for the Ser(473) but not the Thr(308) phosphorylation site of this molecule. Telomerase down-regulation in highly differentiated CD8(+)CD28(-)CD27(-) T cells marks their inexorable progress toward a replicative end stage after activation. This limits the ability of memory CD8(+) T cells to be maintained by continuous proliferation in vivo.

Splice variants of human FOXP3 are functional inhibitors of human CD4+ T-cell activation

FOXP3 has been identified as a key regulator of immune homeostasis. Mutations within the FOXP3 gene result in dysregulated CD4+ T-cell function and elevated cytokine production, leading to lymphoproliferative disease. FOXP3 expression in CD4+ T cells is primarily detected with the CD4+ CD25+ regulatory T-cell population. In humans the protein is detected as a doublet following immunoblot analysis. The lower band of the doublet has been identified as a splice isoform lacking a region corresponding to exon 2. The aim of this study was to investigate whether the splice variant form lacking exon 2 and a new novel splice variant lacking both exons 2 and 7, were functional inhibitors of CD4+ T-cell activation. The data generated showed that full-length FOXP3 and both splice variant forms of the protein were functional repressors of CD4+ T-cell activation.

Activation of Resting Human Primary T Cells with Chimeric Receptors: Costimulation from CD28, Inducible Costimulator, CD134, and CD137 in Series with Signals from the TCRζ Chain

Chimeric receptors that include CD28 signaling in series with TCRzeta in the same receptor have been demonstrated to activate prestimulated human primary T cells more efficiently than a receptor providing TCRzeta signaling alone. We examined whether this type of receptor can also activate resting human primary T cells, and whether molecules other than CD28 could be included in a single chimeric receptor in series with TCRzeta to mediate the activation of resting human primary T cells. Human CD33-specific chimeric receptors were generated with CD28, inducible costimulator, CD134, or CD137 signaling regions in series with TCRzeta signaling region and transfected by electroporation into resting human primary T cells. Their ability to mediate Ag-specific activation was analyzed in comparison with a receptor providing TCRzeta signaling alone. Inclusion of any of the costimulatory signaling regions in series with TCRzeta enhanced the level of specific Ag-induced IL-2, IFN-gamma, TNF-alpha, and GM-CSF cytokine production and enabled resting primary T cells to survive and proliferate in response to Ag in the absence of any exogenous factors. Inclusion of CD28, inducible costimulator, or CD134 enhanced TCRzeta-mediated, Ag-specific target cell lysis. Chimeric receptors providing B7 and TNFR family costimulatory signals in series with TCRzeta in the same receptor can confer self-sufficient clonal expansion and enhanced effector function to resting human T cells. This type of chimeric receptor may now be used to discover the most potent combination of costimulatory signals that will improve current immunotherapeutic strategies.

Chimeric Receptors Providing Both Primary and Costimulatory Signaling in T Cells from a Single Gene Product

Single chain Fv chimeric receptors, or T-bodies, are described with intracellular sequences comprising the costimulatory signaling domain of CD28 in series with the ζ-chain from the TCR complex. Using an engineered human single chain Fv derived from P67, an mAb with specificity for human CD33, and a spacer comprising an Ab hinge region with either Fcγ or part of the CD28 extracellular region, fusion molecules were constructed to test the ability of single chain designs to mediate both primary signaling and costimulation from one extracellular binding event. Constructs with the CD28 signaling domain proximal and the ζ-chain distal to the membrane were found to express more efficiently in Jurkat than constructs with the opposite orientation and were capable of mediating up to 20 times more IL-2 production on stimulation with solid phase Ag when compared with transfectants expressing chimeric receptors with ζ-chain intracellular signaling domains only. IL-2 production was specific to Ag challenge and was completely inhibited by incubation with free Ab of the same specificity as the extracellular binding site of the construct, but not by an isotype-matched control Ab. The CD28 intracellular domain of these fusion proteins was shown to be capable of binding the p85 subunit of phosphatidylinositol 3′-kinase. These constructs represent the first of a new generation of single gene multidomain chimeric receptors capable of mediating both primary and costimulatory signaling specifically from a single extracellular recognition event.

Chimeric receptors providing both primary and costimulatory signaling in T cells from a single gene product

Single chain Fv chimeric receptors, or T-bodies, are described with intracellular sequences comprising the costimulatory signaling domain of CD28 in series with the zeta-chain from the TCR complex. Using an engineered human single chain Fv derived from P67, an mAb with specificity for human CD33, and a spacer comprising an Ab hinge region with either Fcgamma or part of the CD28 extracellular region, fusion molecules were constructed to test the ability of single chain designs to mediate both primary signaling and costimulation from one extracellular binding event. Constructs with the CD28 signaling domain proximal and the zeta-chain distal to the membrane were found to express more efficiently in Jurkat than constructs with the opposite orientation and were capable of mediating up to 20 times more IL-2 production on stimulation with solid phase Ag when compared with transfectants expressing chimeric receptors with zeta-chain intracellular signaling domains only. IL-2 production was specific to Ag challenge and was completely inhibited by incubation with free Ab of the same specificity as the extracellular binding site of the construct, but not by an isotype-matched control Ab. The CD28 intracellular domain of these fusion proteins was shown to be capable of binding the p85 subunit of phosphatidylinositol 3'-kinase. These constructs represent the first of a new generation of single gene multidomain chimeric receptors capable of mediating both primary and costimulatory signaling specifically from a single extracellular recognition event.