Identification of high-affinity anti-CD16A allotype-independent human antibody domains

摘要

CD16A (Fc gamma RIIIA) is an activating receptor mostly expressed on natural killer (NK) cells and monocytes/macrophages. It can mediate antibody-dependent cell-mediated cytotoxicity (ADCC) through low-affinity interaction with human immunoglobulin G (IgG) Fc. It can also mediate cell lysis if NK cells are guided by bispecific killer cells engagers (BiKEs). BiKEs showed some success in clinical trials of cancer and are promising candidate therapeutics. However, currently reported BiKEs are based on antibody fragments (scFvs) of relatively large size. The CDI 6A-specific antibodies are also typically from animal origin. Decreasing the BiKE size could result in enhanced penetration into solid tumor and normal tissues, and using fully human antibodies could decrease the likelihood of immunogenicity. Here we report the identification and characterization of two antibody domains, D6 and E11, isolated from a very large human VH antibody domain library displayed on phage. D6 and E11 bound CD16A with EC50 of 4 nM and 8 nM, respectively, but not other Fc gamma receptors (Fc gamma Rs) such as CD64 (Fc gamma RI), CD32 (Fc gamma RII) and CD16B (Fc gamma RIIIB). They bound to both CD16A allotypes (158F,V) with equal affinity and competed with each other as well as with human IgG1 and the mouse anti-CD 16A antibody 3G8. These and other results were used to build a molecular docking model predicting that D6 and Ell may bind to the CD16A membrane proximal D2 domain by interacting with its BC, C'E and EF loops. Importantly, cross-linked (bivalent) D6 and Ell induced secretion of IL-2 after binding to CD16A-expressing Jurkat T cells. The small size of these antibody domains combined with their high-affinity, specific, allotype-independent, activating interactions with CDI 6A could allow generation of novel highly effective BiKEs and other candidate protein therapeutics. Published by Elsevier Inc.