CD19 is a pan B-cell surface receptor expressed from pro-B-cell development until its down-regulation during terminal differentiation into plasma cells. CD19 represents an attractive immunotherapy target for cancers of lymphoid origin due to its high expression levels on the vast majority of non-Hodgkin's lymphomas and some leukemias. A humanized anti-CD19 antibody with an engineered Fc domain (XmAb5574) was generated to increase binding to Fc; receptors on immune cells and thus increase Fc-mediated effector functions. In vitro, XmAb5574 enhanced antibodydependent cell-mediated cytotoxicity 100-fold to 1,000-fold relative to an anti-CD19 IgG1 analogue against a broad range of B-lymphoma and leukemia cell lines. Furthermore, XmAb5574 conferred antibody-dependent cell-mediated cytotoxicity against patient-derived acute lymphoblastic leukemia and mantle cell lymphoma cells, whereas the IgG1 analogue was inactive. XmAb5574 also increased antibody-dependent cellular phagocytosis and apoptosis. In vivo, XmAb5574 significantly inhibited lymphoma growth in prophylactic and established mouse xenograft models, and showed more potent antitumor activity than its IgG1 analogue. Comparisons with a variant incapable of Fc; receptor binding showed that engagement of these receptors is critical for optimal antitumor efficacy. These results suggest that XmAb5574 exhibits potent tumor cytotoxicity via direct and indirect effector functions and thus warrants clinical evaluation as an immunotherapeutic for CD19 + hematologic malignancies.

The human kallikreins are a large multigene family of closely related serine-type proteases. In this regard, they are similar to the multigene kallikrein families characterized in mice and rats. There is a much more extensive body of knowledge regarding the function of mouse and rat kallikreins in comparison with the human kallikreins. Human kallikrein 6 has been proposed as the homologue to rat myelencephalon-specific protease, an arginine-specific degradative-type protease abundantly expressed in the central nervous system and implicated in demyelinating disease. We present the xray crystal structure of mature, active recombinant human kallikrein 6 at 1.75-Å resolution. This high resolution model provides the first three-dimensional view of one of the human kallikreins and one of only a few structures of serine proteases predominantly expressed in the central nervous system. Enzymatic data are presented that support the identification of human kallikrein 6 as the functional homologue of rat myelencephalon-specific protease and are corroborated by a molecular phylogenetic analysis. Furthermore, the xray data provide support for the characterization of human kallikrein 6 as a degradative protease with structural features more similar to trypsin than the regulatory kallikreins.Recent studies demonstrate that humans have a large multigene family of at least 15 different kallikreins (serine type proteases, abbreviated as KLK 1 in reference to the gene, or hK in reference to the protein) (1). Similarly, the mouse and rat kallikrein gene families are characterized by a large number of closely related members that presumably arose because of gene duplication events (2-6). The different members of the mouse and rat kallikreins are characterized by a high degree of amino acid identity, but typically exhibit different preferences toward peptide substrates (7-12). Several human kallikreins have been identified as potentially useful diagnostic markers for breast (KLK3 and KLK6), prostate (KLK2 and KLK3), and ovarian (KLK6, KLK9, KLK10, and KLK11) cancers as well as neurodegenerative diseases such as Alzheimer's (KLK6) (1, 13-17).Myelencephalon-specific protease (MSP) is a member of the rat kallikrein gene family that is abundantly expressed in the rodent central nervous system and shown to be up-regulated in response to glutamate receptor-mediated excitotoxic injury (18). Potential human homologues to rat MSP have also been identified (18) and have been alternatively named protease M (19), Zyme (20), and neurosin (21). Mouse homologues to MSP have been reported as brain and skin serine protease (BSSP) (22) and brain serine protease (BSP) (23). It has been postulated that MSP/protease M/neurosin may play a key role in the regulation of myelin turnover and in demyelinating disease (18, 24 -27), including the development of multiple sclerosis lesions (25). Furthermore, this kallikrein may also play a role in the degradation of ␤-amyloid or turnover of amyloid precursor protein (28,29). The kinetic properties of MSP have ident...

Kallikrein 6 (K6, MSP) is a newly identified member of the Kallikrein family of serine proteases that is preferentially expressed in the adult central nervous system (CNS). We have previously demonstrated that K6 is abundantly expressed by inflammatory cells at sites of CNS inflammation and demyelination in animal models of multiple sclerosis (MS) and in human MS lesions. To test the hypothesis that this novel enzyme is a mediator of pathogenesis in CNS inflammatory disease, we have evaluated whether autonomously generated K6 antibodies alter the clinicopathological course of disease in murine proteolipid protein139-151-induced experimental autoimmune encephalomyelitis (PLP139-151 EAE). We demonstrate that immunization of mice with recombinant K6 generates antibodies that block K6 enzymatic activity in vitro, including the breakdown of myelin basic protein (MBP), and that K6-immunized mice exhibit significantly delayed onset and severity of clinical deficits. Reduced clinical deficits were reflected in significantly less spinal cord pathology and meningeal inflammation and in reduced Th1 cellular responses in vivo and in vitro. These data demonstrate for the first time that K6 participates in enzymatic cascades mediating CNS inflammatory disease and that this unique enzyme may represent a novel therapeutic target for the treatment of progressive inflammatory disorders, including MS.

Myelencephalon-specific protease (MSP), first identified in the rat and now known to have a human homologue (human kallikrein 6), is preferentially expressed in the central nervous system (CNS), compared with nonneural tissues. MSP has been postulated to have trypsin-like activity, is upregulated in response to glutamate receptor-mediated excitotoxic injury in the CNS, and is downregulated in the brain of Alzheimer's patients. The preferential expression of this enzyme by oligodendrocytes in CNS white matter points to a role in myelin homeostasis. To further characterize the activity and substrate specificity of this newly identified enzyme, we have heterologously expressed MSP in a baculovirus/insect cell line system. We demonstrate that recombinant MSP exhibits a broad specificity for cleavage after arginine but not lysine residues, with kinetic characteristics intermediate between trypsin and pancreatic kallikrein. We show that the pro form of MSP does not self-activate but, rather, requires cleavage after lysine, indicating that mature active MSP is regulated by a distinct protease. MSP may be regulated in part by autolysis, since the active protein is readily inactivated through autolysis at specific internal arginine positions. Additionally, we show that MSP is abundantly expressed in inflammatory cells at sites of demyelination in the Theiler's murine encephalomyelitis virus (TMEV) model of multiple sclerosis (MS). In conjunction with data demonstrating the ability of MSP to degrade myelin-associated as well as several extracellular matrix proteins, these findings delineate MSP as a broad-specificity arginine-specific protease with the potential to play a key role in immune-mediated demyelination.

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