Patient-derived heavy chain antibody targets cell surface HSP90 on breast tumors

Background: Monoclonal antibodies have been used to effectively treat various tumors. We previously established a unique strategy to identify tumor specific antibodies by capturing B-cell response against breast tumor antigens from patient-derived sentinel lymph nodes. Initial application of this approach led to identification of a tumor specific single domain antibody. In this paper we optimized our previous strategy by generating heavy chain antibodies (HCAbs) to overcome the deficiencies of single domain antibodies. Here we identified and characterized a heavy chain antibody (HCAb2) that targets cell surface HSP90 antigen on breast tumor cells but not normal cells. Methods: Eight HCAbs derived from 4 breast cancer patients were generated using an in vitro expression system. HCAbs were screened against normal breast cells (MCF10A, HMEC) and tumor cell lines (MCF7, MDA-MB-231) to identify cell surface targeting and tumor specific antibodies using flow cytometry and immunofluorescence. Results observed with cell lines were validated by screening a cohort of primary human breast normal and tumor tissues using immunofluorescence. Respective antigens for two HCAbs (HCAb1 and HCAb2) were identified using immunoprecipitation followed by mass spectrometry. Finally, we generated MDA-MB-231 xenograft tumors in NOD scid gamma mice and performed in vivo tumor targeting analysis of HCAb1 and HCAb2. Results: Flow cytometry screen revealed that HCAb2 selectively bound to the surface of MDA-MB-231 cells in comparison to MCF10A and MCF7 cells. HCAb2 showed punctate membrane staining on MDA-MB-231 cells and preferential binding to human breast tumor tissues in comparison to normal breast tissues. In primary breast tumor tissues, HCAb2 showed positive binding to both E-cadherin positive and negative tumor cells. We identified and validated the target antigen of HCAb2 as Heat shock protein 90 (HSP90). HCAb2 also selectively targeted MDA-MB-231 xenograft tumor cells in vivo with little targeting to mouse normal tissues. Finally, HCAb2 specifically targeted calnexin negative xenograft tumor cells. Conclusions: From our screening methodology, we identified HCAb2 as a breast tumor specific heavy chain antibody targeting cell surface HSP90. HCAb2 also targeted MDA-MB-231 tumor cells in vivo suggesting that HCAb2 could be an ideal tumor targeting antibody.