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Tumour antigen targeted antibodies (mAbs) can induce natural killer (NK) cells to kill tumours through antibody dependent cellular cytotoxicity (ADCC) upon engagement of NK cell expressed FcγRIIIa. FcγRIIIa polymorphisms partially dictate the potency of the ADCC response. The high affinity FcγRIIIa-158-valine (V) polymorphism is associated with more potent ADCC response than the low affinity FcγRIIIa-158-phenylalanine (F) polymorphism. Because approximately 45% of patients are homozygous for the FcγRIIIa-158-F polymorphism (FF genotype), their ability to mount ADCC is impaired. We investigated whether a novel mAb capable of binding multiple antigen specific targets and engaging multiple low affinity FcγRIIIa receptors could further enhance ADCC against colon cancer in vitro. Specifically, we generated a novel anti-epidermal growth factor receptor (EGFR) antibody (termed a stradobody) consisting of an unmodified Fab sequence and two Immunoglobulin G, subclass 1 (IgG1) Fc domains separated by an isoleucine zipper domain and the 12 amino-acid IgG2 hinge. The stradobody framework induced multimerisation and was associated with increased binding to the EGFR and FcγRIIIa. From a functional perspective, when compared to an unmodified anti-EGFR mAb with a sequence identical to cetuximab (a commercially available anti-EGFR mAb), stradobodies significantly enhanced ADCC. These effects were observed using both KRAS wild type HT29 and KRAS mutant SW480 colon cancer cells as targets, and by NK cells obtained from healthy donors and a cohort of patients with colon cancer. These data suggest that high avidity cross-linking of multiple tumour surface antigens and multiple NK cell associated FcγRIIIa molecules can enhance ADCC and partially overcome impaired ADCC by FF genotype individuals in vitro.

In vitro assessment of lymphocyte and natural killer (NK) cell cytotoxicity typically employs density gradient centrifugation and magnetic cell separation to isolate effector cells, and chromium release to assess cytotoxicity. In order to improve the rapidity and scalability of in vitro cytotoxicity assessment, we evaluated the efficacy of a protocol utilizing tetrameric antibody complexes and SepMate™ isolation tubes to negatively select NK cells (TACs/Sep), and calcein-AM release to measure cytotoxicity. We compared the efficiency and accuracy of this protocol to a conventional approach employing density gradient centrifugation and magnetically labeled antibodies (DG/MACS) to isolate NK cells and chromium release to measure cytotoxicity. The TACs/Sep method significantly decreased the time required for NK cell isolation (1h vs. 4h), but resulted in higher red blood cell contamination. NK cell activation marker expression (including CD94, NKG2D, NKp30, NKp46, DNAM-1, 2B4, KIR2DL1/S1, KIR2DL2/L3, intracellular granzyme B, and perforin) was similar when comparing NK cells isolated by the TACs/Sep or DG/MACS methods, but the TACs/Sep method induced higher expression of CD16. In vitro cytotoxicity against HT29 colon cancer and K562 leukemia cells was not affected by the isolation method. Lastly, by combining the TACs/Sep NK cell isolation method with calcein-acetoxymethyl diacetylester (calcein-AM) release, the time required to assess in vitro cytotoxicity was reduced by 33% (4h) compared to protocols employing DG/MACS and chromium release. Altogether, these results provide the foundation for the development of a rapid, high throughput functional assay, and make it practical for the multiplexing of downstream applications, such as flow cytometric analysis and enzyme-linked immunosorbent assays (ELISAs).