Abstract 2893: Engineered release and presentation of antibody-bound viral antigens: A highly specific and novel immunotherapeutic approach to target cancer in vivo
Authors: Millar David G. , Morton Laura , Gaspar Manuela Carvalho , Mistry Punam , Peña Hugo De La , Joseph Ricky , Penny Sarah , Goodyear Oliver C. , Goodall Margaret , Pratt Guy E. , Cobbold Mark 
Published: May 2015

Abstract
Harnessing the power of adaptive immunity to combat cancer has been a long-term goal of translational immunotherapy. Tumor-specific immunity, where present, is typically at low frequency and affinity with compromised effector function. By contrast, immunity against persistent herpesviruses in man is characterised by high affinity cytotoxic T-lymphocytes (CTL) at high frequency with potent effector function. Furthermore, the immunosubversive mechanisms employed by herpesviruses show striking parallels to tumors, yet the associated anti-viral immunity limits these to life-long asymptomatic infections.
We reasoned that the delivery of immunodominant viral peptide epitopes to the tumor surface might facilitate passive-loading of peptides into empty MHC class-I molecules, effectively mimicking viral infection, rendering tumors susceptible to lysis by anti-viral immunity.
To address this we developed a new class of targeting antibodies: APEC (Antibody Peptide Epitope Complexes) that are able to deliver an antigenic payload at the cell surface through proteolytic release of covalently-coupled peptide antigens. As a proof-of-concept we used clinically-validated antibodies cetuximab (anti-EGFR) and rituximab (anti-CD20) to develop APECs that are able to target human tumors.
We screened 15 HLA-A*0201+ EGFR-expressing NCI-60 cell lines, CD20+ lymphoma cell lines, 20 primary CD20+ CLL tumor samples and four healthy B-cells against a library of 190 cetuximab-APECs (cAPEC) or rituximab (rAPEC) incorporating the immunodominant cytomegalovirus (CMV) pp65495-503 epitope and candidate protease cleavage sites following co-incubation with CMV-specific CTL (CMV-CTL). The most effective cAPEC and rAPEC were those incorporating MMP2, MMP9, Cathepsin B and Cathepsin D protease recognition domains. Very few (2/190) rAPEC were able to redirect CMV-CTL against healthy cells. Heterogeneity was observed for primary CLL tumors but a limited number of rAPEC were effective in all cases (5/190).
Mechanistic studies demonstrated that: (i) peptide loading occurred at the cell surface, (ii) required the expression of target antigens at the cell surface and (iii) T-cell recognition could be inhibited by unconjugated antibody (92%) or by incubation with protease inhibitors (83%).
T-cell specificity was examined using rAPEC treated tumor targets co-incubated with various HLA-matched effector T-cell populations. No activation of CD4+ was observed including CD4+CD25hi regulatory T-cell populations. Incubation with CD8+ T-cells revealed that only pp65495-503-specific CD8+ T-cells engaged with APEC-treated tumor cells. Lastly, xenograft studies using EGFR+ and CD20+ tumor cell lines demonstrated efficacy of both cAPEC and rAPEC to eliminate tumors in vivo by redirecting anti-viral CTL.
These data indicate that APECs represent a powerful new approach to combat cancer.
Citation Format: David G. Millar, Laura Morton, Manuela Carvalho Gaspar, Punam Mistry, Hugo De La Peña, Ricky Joseph, Sarah Penny, Oliver C. Goodyear, Margaret Goodall, Guy E. Pratt, Mark Cobbold. Engineered release and presentation of antibody-bound viral antigens: A highly specific and novel immunotherapeutic approach to target cancer in vivo. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2893. doi:10.1158/1538-7445.AM2014-2893