Sivasubramanian Baskar

The B-cell tumor–associated antigen ROR1 can be targeted with T cells modified to express a ROR1-specific chimeric antigen receptor

Monoclonal antibodies and T cells modified to express chimeric antigen receptors specific for B-cell lineage surface molecules such as CD20 exert antitumor activity in B-cell malignancies, but deplete normal B cells. The receptor tyrosine kinase-like orphan receptor 1 (ROR1) was identified as a highly expressed gene in B-cell chronic lymphocytic leukemia (B-CLL), but not normal B cells, suggesting it may serve as a tumor-specific target for therapy. We analyzed ROR1-expression in normal nonhematopoietic and hematopoietic cells including B-cell precursors, and in hematopoietic malignancies. ROR1 has characteristics of an oncofetal gene and is expressed in undifferentiated embryonic stem cells, B-CLL and mantle cell lymphoma, but not in major adult tissues apart from low levels in adipose tissue and at an early stage of B-cell development. We constructed a ROR1-specific chimeric antigen receptor that when expressed in T cells from healthy donors or CLL patients conferred specific recognition of primary B-CLL and mantle cell lymphoma, including rare drug effluxing chemotherapy resistant tumor cells that have been implicated in maintaining the malignancy, but not mature normal B cells. T-cell therapies targeting ROR1 may be effective in B-CLL and other ROR1-positive tumors. However, the expression of ROR1 on some normal tissues suggests the potential for toxi-city to subsets of normal cells.

Unique Cell Surface Expression of Receptor Tyrosine Kinase ROR1 in Human B-Cell Chronic Lymphocytic Leukemia

Purpose: Gene expression profiling identified receptor tyrosine kinase ROR1, an embryonic protein involved in organogenesis, as a signature gene in B-cell chronic lymphocytic leukemia (B-CLL). To assess the suitability of ROR1 as a cell surface antigen for targeted therapy of B-CLL, we carried out a comprehensive analysis of ROR1 protein expression. Experimental Design: Peripheral blood mononuclear cells, sera, and other adult tissues from B-CLL patients and healthy donors were analyzed qualitatively and quantitatively for ROR1 protein expression by flow cytometry, cell surface biotinylation, Western blotting, and ELISA. Results: ROR1 protein is selectively expressed on the surface of B-CLL cells, whereas normal B cells, other normal blood cells, and normal adult tissues do not express cell surface ROR1. Moreover, cell surface expression of ROR1 is uniform and constitutive, i.e., independent of anatomic niches, independent of biological and clinical heterogeneity of B-CLL, independent of B-cell activation, and found at similar levels in all B-CLL samples tested. The antibody binding capacity of B-CLL cell surface ROR1 was determined to be in the range of 103 to 104 molecules per cell. A portion of B-CLL cell surface ROR1 was actively internalized upon antibody binding. Soluble ROR1 protein was detectable in sera of <25% of B-CLL patients and a similar fraction of healthy donors at concentrations below 200 ng/mL. Conclusions: The restricted, uniform, and constitutive cell surface expression of ROR1 protein in B-CLL provides a strong incentive for the development of targeted therapeutics such as monoclonal antibodies.

A novel adoptive transfer model of chronic lymphocytic leukemia suggests a key role for T lymphocytes in the disease.

Chronic lymphocytic leukemia (CLL) is an incurable adult disease of unknown etiology. Understanding the biology of CLL cells, particularly cell maturation and growth in vivo, has been impeded by lack of a reproducible adoptive transfer model. We report a simple, reproducible system in which primary CLL cells proliferate in nonobese diabetes/severe combined immunodeficiency/γc(null) mice under the influence of activated CLL-derived T lymphocytes. By co-transferring autologous T lymphocytes, activated in vivo by alloantigens, the survival and growth of primary CFSE-labeled CLL cells in vivo is achieved and quantified. Using this approach, we have identified key roles for CD4(+) T cells in CLL expansion, a direct link between CD38 expression by leukemic B cells and their activation, and support for CLL cells preferentially proliferating in secondary lymphoid tissues. The model should simplify analyzing kinetics of CLL cells in vivo, deciphering involvement of nonleukemic elements and nongenetic factors promoting CLL cell growth, identifying and characterizing potential leukemic stem cells, and permitting preclinical studies of novel therapeutics. Because autologous activated T lymphocytes are 2-edged swords, generating unwanted graph-versus-host and possibly autologous antitumor reactions, the model may also facilitate analyses of T-cell populations involved in immune surveillance relevant to hematopoietic transplantation and tumor cytoxicity.

Autologous lymphoma vaccines induce human T cell responses against multiple, unique epitopes

The clonotypic surface Ig receptor expressed by malignant B cells, idiotype, is a tumor-specific antigen and an attractive target for active immunotherapy. While Abs specific for tumor idiotype have been well described in patients with B cell malignancies, the precise antigenic epitopes in human idiotype recognized by autologous T cells remain largely unknown. We report here that T cell lines generated from lymphoma patients actively immunized with idiotype protein specifically recognized multiple, unique immunodominant epitopes in autologous tumor idiotype. Synthetic peptides corresponding to hypervariable, but not framework, regions of Ig heavy chain specifically stimulated CD4(+) and CD8(+) T cells to proliferate and secrete proinflammatory cytokines in an MHC-associated manner. Detailed analysis revealed a minimal determinant of an immunodominant epitope, comprising critical residues at the amino terminus that may be a product of somatic hypermutation. Association of idiotype-specific T cell responses with previously documented molecular remissions in idiotype-vaccinated patients suggests that the newly identified T cell epitopes may be clinically relevant. Such antigenic epitopes may serve as candidates for novel peptide-vaccine strategies, and as tools to selectively expand tumor antigen-specific T cells for adoptive immunotherapy and for monitoring T cell immunity in vaccinated patients.

Therapeutic Potential and Challenges of Targeting Receptor Tyrosine Kinase ROR1 with Monoclonal Antibodies in B-Cell Malignancies

Background Based on its selective cell surface expression in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL), receptor tyrosine kinase ROR1 has recently emerged as a promising target for therapeutic monoclonal antibodies (mAbs). To further assess the suitability of ROR1 for targeted therapy of CLL and MCL, a panel of mAbs was generated and its therapeutic utility was investigated. Methodology and Principal Findings A chimeric rabbit/human Fab library was generated from immunized rabbits and selected by phage display. Chimeric rabbit/human Fab and IgG1 were investigated for their capability to bind to human and mouse ROR1, to mediate antibody-dependent cellular cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and internalization, and to agonize or antagonize apoptosis using primary CLL cells from untreated patients as well as MCL cell lines. A panel of mAbs demonstrated high affinity and specificity for a diverse set of epitopes that involve all three extracellular domains of ROR1, are accessible on the cell surface, and mediate internalization. The mAb with the highest affinity and slowest rate of internalization was found to be the only mAb that mediated significant, albeit weak, ADCC. None of the mAbs mediated CDC. Alone, they did not enhance or inhibit apoptosis. Conclusions and Significance Owing to its relatively low cell surface density, ROR1 may be a preferred target for armed rather than naked mAbs. Provided is a panel of fully sequenced and thoroughly characterized anti-ROR1 mAbs suitable for conversion to antibody-drug conjugates, immunotoxins, chimeric antigen receptors, and other armed mAb entities for preclinical and clinical studies.

Targeting malignant B cells with an immunotoxin against ROR1

The selective cell surface expression of receptor tyrosine kinase-like orphan receptor 1 (ROR1) in chronic lymphocytic leukemia (CLL) and mantle cell lymphoma (MCL) has made ROR1 a novel and promising target for therapeutic monoclonal antibodies (mAbs). Four mouse mAbs generated by hybridoma technology exhibited specific binding to human ROR1. Epitope mapping studies showed that two mAbs (2A2 and 2D11) recognized N-terminal epitopes in the extracellular region of ROR1 and the other two (1A1 and 1A7) recognized C-terminal epitopes. A ROR1- immunotoxin (BT-1) consisting of truncated Pseudomonas exotoxin A (PE38) and the VH and VL fragments of 2A2-IgG was made recombinantly. Both 2A2-IgG and BT-1 showed dose-dependent and selective binding to primary CLL and MCL cells and MCL cell lines. Kinetic analyses revealed 0.12-nM (2A2-IgG) to 65-nM (BT-1) avidity/affinity to hROR1, depicting bivalent and monovalent interactions, respectively. After binding to cell surface ROR1, 2A2-IgG and BT-1 were partially internalized by primary CLL cells and MCL cell lines, and BT-1 induced profound apoptosis of ROR1-expressing MCL cell lines in vitro (EC50 = 16 pM–16 nM), but did not affect ROR1-negative cell lines. Our data suggest that ROR1-immunotoxins such as BT-1 could serve as targeted therapeutic agents for ROR1-expressing B cell malignancies and other cancers.

Generation, Affinity Maturation, and Characterization of a Human Anti-Human NKG2D Monoclonal Antibody with Dual Antagonistic and Agonistic Activity

In humans, NKG2D is an activating receptor on natural killer (NK) cells and a costimulatory receptor on certain T cells and plays a central role in mediating immune responses in autoimmune diseases, infectious diseases, and cancer. Monoclonal antibodies that antagonize or agonize immune responses mediated by human NKG2D are considered to be of broad and potent therapeutic utility. Nonetheless, monoclonal antibodies to NKG2D that are suitable for clinical investigations have not been published yet. Here, we describe the generation, affinity maturation, and characterization of a fully human monoclonal antibody to human NKG2D. Using phage display technology based on a newly generated naïve human Fab library in phage display vector pC3C followed by a tandem chain shuffling process designed for minimal deviation from natural human antibody sequences, we selected a human Fab, designated KYK-2.0, with high specificity and affinity to human NKG2D. KYK-2.0 Fab blocked the binding of the natural human NKG2D ligands MICA, MICB, and ULBP2 as potently as a commercially available mouse anti-human NKG2D monoclonal antibody in immunoglobulin G (IgG) format. Conversion of KYK-2.0 Fab to IgG1 resulted in subnanomolar avidity for human NKG2D. KYK-2.0 IgG1 was found to selectively recognize defined subpopulations of human lymphocytes known to express NKG2D, that is, the majority of human CD8+, CD16+, and CD56+ cells as well as a small fraction of human CD4+ cells. In solution, KYK-2.0 IgG1 interfered with the cytolytic activity of ex vivo expanded human NK cells. By contrast, immobilized KYK-2.0 IgG1 was found to strongly induce human NK cell activation. The dual antagonistic and agonistic activity promises a wide range of therapeutic applications for KYK-2.0 IgG1 and its derivatives.

Tumor cells expressing major histocompatibility complex class II and B7 activation molecules stimulate potent tumor-specific immunity.

In previous studies we have shown that the highly malignant mouse SaI sarcoma can be converted into an immunogenic tumor that is immunologically rejected by the autologous host if the tumor cells are transfected with and express syngeneic major histocompatibility complex (MHC) class II genes. Tumor cells expressing class II heterodimers truncated for the cytoplasmic regions of the alpha and beta chains, however, are as malignant as wild-type class II- tumors. These studies have contributed to the hypothesis that T-cell activation requires two signals: the engagement of the MHC class II/peptide complex of the antigen-presenting cell (APC) by the T cell receptor for antigen of the responding T cell and the transmittal of a second, or costimulatory, signal by the APC to the responding T cell. In this report we show that induction of tumor-specific immunity is facilitated by delivery of a costimulatory signal provided by the B7 activation molecule. Mice challenged with SaI cells bearing truncated class II molecules and transfected with B7 cDNA are immune to the transfectants and are protected against a challenge of wild-type class II-B7- ascites or solid SaI tumor. The induced immunity requires CD4+ T cells and is specific for the immunizing sarcoma cells. These results highlight the critical role of the B7 costimulatory pathway in stimulating long-term, tumor-specific immunity that is effective against high doses of challenging wild-type tumor and suggest a strategy for enhancing tumor rejection.

Restricted Cell Surface Expression of Receptor Tyrosine Kinase ROR1 in Pediatric B-Lineage Acute Lymphoblastic Leukemia Suggests Targetability with Therapeutic Monoclonal Antibodies

Background Despite high cure rates for pediatric B-lineage acute lymphoblastic leukemia (B-ALL), short-term and long-term toxicities and chemoresistance are shortcomings of standard chemotherapy. Immunotherapy and chemoimmunotherapy based on monoclonal antibodies (mAbs) that target cell surface antigens with restricted expression in pediatric B-ALL may offer the potential to reduce toxicities and prevent or overcome chemoresistance. The receptor tyrosine kinase ROR1 has emerged as a candidate for mAb targeting in select B-cell malignancies. Methodology and Principal Findings Using flow cytometry, Western blotting, immunohistochemistry, and confocal immunofluorescence microscopy, we analyzed the cell surface expression of ROR1 across major pediatric ALL subtypes represented by 14 cell lines and 56 primary blasts at diagnosis or relapse as well as in normal adult and pediatric tissues. Cell surface ROR1 expression was found in 45% of pediatric ALL patients, all of which were B-ALL, and was not limited to any particular genotype. All cell lines and primary blasts with E2A-PBX1 translocation and a portion of patients with other high risk genotypes, such as MLL rearrangement, expressed cell surface ROR1. Importantly, cell surface ROR1 expression was found in many of the pediatric B-ALL patients with multiply relapsed and refractory disease and normal karyotype or low risk cytogenetics, such as hyperdiploidy. Notably, cell surface ROR1 was virtually absent in normal adult and pediatric tissues. Conclusions and Significance Collectively, this study suggests that ROR1 merits preclinical and clinical investigations as a novel target for mAb-based therapies in pediatric B-ALL. We propose cell surface expression of ROR1 detected by flow cytometry as primary inclusion criterion for pediatric B-ALL patients in future clinical trials of ROR1-targeted therapies.

Tumor antigen ROR1 targeted drug delivery mediated selective leukemic but not normal B-cell cytotoxicity in chronic lymphocytic leukemia.

Selective cytotoxicity to cancer cells without compromising their normal counterparts pose a huge challenge for traditional drug design. Here we developed a tumor antigen-targeted delivery of immunonanoparticle carrying a novel non-immunosuppressive FTY720 derivative OSU-2S with potent cytotoxicity against leukemic B cells. OSU-2S induces activation of protein phosphatase 2A (PP2A), phosphorylation and nuclear translocation of SHP1S591 and deregulation of multiple cellular processes in chronic lymphocytic leukemia (CLL) resulting in potent cytotoxicity. To preclude OSU-2S-mediated effects on these ubiquitous phosphatases in unintended cells and avoid potential adverse effects, we developed an OSU-2S-targeted delivery of immunonanoparticles (2A2-OSU-2S-ILP), that mediated selective cytotoxicity of CLL but not normal B cells through targeting receptor tyrosine kinase ROR1 expressed in leukemic but not normal B cells. Developing a novel spontaneous CLL mouse model expressing human ROR1 (hROR1) in all leukemic B cells, we demonstrate the therapeutic benefit of enhanced survival with 2A2-OSU-2S-ILP in vivo. The newly developed non-immunosuppressive OSU-2S, its delivery using human CLL directed immunonanoparticles and the novel transgenic (Tg) mouse model of CLL that expresses hROR1 exclusively in leukemic B cell surface are highly innovative and can be applied to CLL and other ROR1+ malignancies including mantle cell lymphoma and acute lymphoblastic leukemia.