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Podocalyxin (Podxl) is a CD34-related cell surface sialomucin that is normally highly expressed by adult vascular endothelia and kidney podocytes where it plays a key role in blocking adhesion. Importantly, it is also frequently upregulated on a wide array of human tumors and its expression often correlates with poor prognosis. We previously showed that, in xenograft studies, Podxl plays a key role in metastatic disease by making tumor initiating cells more mobile and invasive. Recently, we developed a novel antibody, PODO447, which shows exquisite specificity for a tumor-restricted glycoform of Podxl but does not react with Podxl expressed by normal adult tissue. Here we utilized an array of glycosylation defective cell lines to further define the PODO447 reactive epitope and reveal it as an O-linked core 1 glycan presented in the context of the Podxl peptide backbone. Further, we show that when coupled to monomethyl auristatin E (MMAE) toxic payload, PODO447 functions as a highly specific and effective antibody drug conjugate (ADC) in killing ovarian, pancreatic, glioblastoma and leukemia cell lines in vitro . Finally, we demonstrate PODO447-ADCs are highly effective in targeting human pancreatic and ovarian tumors in xenografted NSG and Nude mouse models. These data reveal PODO447-ADCs as exquisitely tumor-specific and highly efficacious immunotherapeutic reagents for the targeting of human tumors. Thus, PODO447 exhibits the appropriate characteristics for further development as a targeted clinical immunotherapy., Competing Interests: The authors DC, MH, PD, IS, OB, KM, CR are inventors of a pending patent application on PODO447 and methods of using the same (US20180296673A1). MH, KM and CR are inventors of a pending patent application on PODO83 and methods of using the same (US20190367606A1). KM and CR possess an awarded patent on podocalyxin as a prognostic marker in cancer (US9309323B2). The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Canals Hernaez, Hughes, Li, Mainero Rocca, Dean, Brassard, Bell, Samudio, Mes-Masson, Narimatsu, Clausen, Blixt, Roskelley and McNagny.)

Background: The success of new targeted cancer therapies has been dependent on the identification of tumor-specific antigens. Podocalyxin (Podxl) is upregulated on tumors with high metastatic index and its presence is associated with poor outcome, thus emerging as an important prognostic and theragnostic marker in several human cancers. Moreover, in human tumor xenograft models, Podxl expression promotes tumor growth and metastasis. Although a promising target for immunotherapy, the expression of Podxl on normal vascular endothelia and kidney podocytes could hamper efforts to therapeutically target this molecule. Since pathways regulating post-translational modifications are frequently perturbed in cancer cells, we sought to produce novel anti-Podxl antibodies (Abs) that selectively recognize tumor-restricted glycoepitopes on the extracellular mucin domain of Podxl., Methods: Splenic B cells were isolated from rabbits immunized with a Podxl-expressing human tumor cell line. Abs from these B cells were screened for potent reactivity to Podxl + neoplastic cell lines but not Podxl + primary endothelial cells. Transcripts encoding heavy and light chain variable regions from promising B cells were cloned and expressed as recombinant proteins. Tumor specificity was assessed using primary normal tissue and an ovarian cancer tissue microarray (TMA). Mapping of the tumor-restricted epitope was performed using enzyme-treated human tumor cell lines and a glycan array., Results: One mAb (PODO447) showed strong reactivity with a variety of Podxl+ tumor cell lines but not with normal primary human tissue including Podxl+ kidney podocytes and most vascular endothelia. Screening of an ovarian carcinoma TMA (219 cases) revealed PODO447 reactivity with the majority of tumors, including 65% of the high-grade serous histotype. Subsequent biochemical analyses determined that PODO447 reacts with a highly unusual terminal N-acetylgalactosamine beta-1 (GalNAcβ1) motif predominantly found on the Podxl protein core. Finally, Ab-drug conjugates showed specific efficacy in killing tumor cells in vitro ., Conclusions: We have generated a novel and exquisitely tumor-restricted mAb, PODO447, that recognizes a glycoepitope on Podxl expressed at high levels by a variety of tumors including the majority of life-threatening high-grade serous ovarian tumors. Thus, tumor-restricted PODO447 exhibits the appropriate specificity for further development as a targeted immunotherapy., Competing Interests: Competing interests: The authors are inventors of a pending patent application on PODO447 and methods of using the same (US20180296673A1). MRH, PB, IS, CBG, KM and CR are inventors of a pending patent application on PODO83 and methods of using the same (US20190367606A1). KM and CR possess an awarded patent on podocalyxin as a prognostic marker in cancer (US9309323B2)., (© Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)

The CD34 cell surface antigen is widely expressed in tissues on cells with progenitor-like properties and on mature vascular endothelia. In adult human bone marrow, CD34 marks hematopoietic stem and progenitor cells (HSPCs) starting from the bulk of hematopoietic stem cells with long-term repopulating potential (LT-HSCs) throughout expansion and differentiation of oligopotent and unipotent progenitors. CD34 protein surface expression is typically lost as cells mature into terminal effectors. Because of this expression pattern of HSPCs, CD34 has had a central role in the evaluation or selection of donor graft tissue in HSC transplant (HSCT). Given its clinical importance, it is surprising that the biological functions of CD34 are still poorly understood. This enigma is due, in part, to CD34's context-specific role as both a pro-adhesive and anti-adhesive molecule and its potential functional redundancy with other sialomucins. Moreover, there are also critical differences in the regulation of CD34 expression on HSPCs in humans and experimental mice. In this review, we highlight some of the more well-defined functions of CD34 in HSPCs with a focus on proposed functions most relevant to HSCT biology., (Copyright © 2020 ISEH -- Society for Hematology and Stem Cells. Published by Elsevier Inc. All rights reserved.)

Podocalyxin (Podxl) is broadly expressed on the luminal face of most blood vessels in adult vertebrates, yet its function on these cells is poorly defined. In the present study, we identified specific functions for Podxl in maintaining endothelial barrier function. Using electrical cell substrate impedance sensing and live imaging, we found that, in the absence of Podxl, human umbilical vein endothelial cells fail to form an efficient barrier when plated on several extracellular matrix substrates. In addition, these monolayers lack adherens junctions and focal adhesions and display a disorganized cortical actin cytoskeleton. Thus, Podxl has a key role in promoting the appropriate endothelial morphogenesis required to form functional barriers. This conclusion is further supported by analyses of mutant mice in which we conditionally deleted a floxed allele of Podxl in vascular endothelial cells (vECs) using Tie2Cre mice ( Podxl ΔTie2Cre ). Although we did not detect substantially altered permeability in naïve mice, systemic priming with lipopolysaccharide (LPS) selectively disrupted the blood-brain barrier (BBB) in Podxl ΔTie2Cre mice. To study the potential consequence of this BBB breach, we used a selective agonist (TFLLR-NH 2 ) of the protease-activated receptor-1 (PAR-1), a thrombin receptor expressed by vECs, neuronal cells, and glial cells. In response to systemic administration of TFLLR-NH 2 , LPS-primed Podxl ΔTie2Cre mice become completely immobilized for a 5-min period, coinciding with severely dampened neuroelectric activity. We conclude that Podxl expression by CNS tissue vECs is essential for BBB maintenance under inflammatory conditions., Competing Interests: The authors declare no conflict of interest.