Siglec-6 on chronic lymphocytic leukemia cells is a target for post-allogeneic hematopoietic stem cell transplantation antibodies

Jing Chang, Haiyong Peng, Brian C Shaffer, Sivasubramanian Baskar, Ina C Wecken, Matthew G Cyr, Gustavo J Martinez, Jo Soden, Jim Freeth, Adrian Wiestner, Christoph Rader

Although the five-year survival rate of chronic lymphocytic leukemia (CLL) patients has risen to >80%, the only potentially curative treatment is allogeneic hematopoietic stem cell transplantation (alloHSCT). To identify possible new monoclonal antibody (mAb) drugs and targets for CLL, we previously developed a phage display-based human mAb platform to mine the antibody repertoire of patients who responded to alloHSCT. We had selected a group of highly homologous post-alloHSCT mAbs that bound to an unknown CLL cell surface antigen. Here we show through next-generation sequencing of cDNAs encoding variable heavy-chain domains that these mAbs had a relative abundance of ~0.1% in the post-alloHSCT antibody repertoire and were enriched ~1,000-fold after three rounds of selection on primary CLL cells. Based on differential RNA-seq and a cell microarray screening technology for discovering human cell surface antigens, we now identify their antigen as Siglec-6. We verified this finding by flow cytometry, ELISA, siRNA knockdown, and surface plasmon resonance. Siglec-6 was broadly expressed in CLL and could be a potential target for antibody-based therapeutic interventions. Our study reaffirms the utility of post-alloHSCT antibody drug and target discovery.