Inhibition of NETosis by a Nuclear-Penetrating Anti-DNA Autoantibody

Nuclear-penetrating anti-DNA autoantibodies have therapeutic potential as delivery agents and in targeting DNA and the DNA damage response (DDR). Derivatives of such Abs have advanced to human testing in genetic disease and are in preparation for oncology clinical trials. DNA release associated with neutrophil extracellular traps (NETs) contributes to immunity, inflammation, and the pathophysiology of multiple diseases. The DDR contributes to mechanisms of NETosis, and we hypothesize that anti-DNA autoantibodies that localize into live cell nuclei and inhibit DNA repair will suppress release of NETs by activated neutrophils. In the current study we evaluated the impact of a nuclear-penetrating anti-DNA autoantibody that interferes with the DDR on decondensation and release of DNA and NETs by activated human granulocyte-like differentiated PLB-985 cells and neutrophils isolated from C57BL/6 mice. The response of cells pretreated with control or autoantibody to subsequent stimulators of NETosis, including PMA and the calcium ionophore ionomycin, was evaluated by DAPI and SYTOX Green stains, measurement of DNA release, analysis of histone citrullination by Western blot, or visualization of NETs by immunostaining and confocal fluorescence microscopy. Autoantibody treatment of the cells yielded significant inhibition of NADPH oxidase-dependent and independent NETosis. These findings establish the concept of nuclear-penetrating anti-DNA autoantibodies as modulators of neutrophil biology with potential for use in strategies to suppress NETosis.

ENT2 facilitates brain endothelial cell penetration and blood-brain barrier transport by a tumor-targeting anti-DNA autoantibody

The blood-brain barrier (BBB) prevents antibodies from penetrating the CNS and limits conventional antibody-based approaches to brain tumors. We now show that ENT2, a transporter that regulates nucleoside flux at the BBB, may offer an unexpected path to circumventing this barrier to allow targeting of brain tumors with an anti-DNA autoantibody. Deoxymab-1 (DX1) is a DNA-damaging autoantibody that localizes to tumors and is synthetically lethal to cancer cells with defects in the DNA damage response. We found that DX1 penetrated brain endothelial cells and crossed the BBB, and mechanistic studies identify ENT2 as the key transporter. In efficacy studies, DX1 crosses the BBB to suppress orthotopic glioblastoma and breast cancer brain metastases. ENT2-linked transport of autoantibodies across the BBB has potential to be exploited in brain tumor immunotherapy, and its discovery raises hypotheses on actionable mechanisms of CNS penetration by neurotoxic autoantibodies in CNS lupus.

Abstract PD13-08: An ENT2-dependent, cell-penetrating, and DNA-damaging lupus autoantibody crosses the blood-brain barrier to target breast cancer brain metastases

Introduction: Antibody-based immunotherapy has potential to change paradigms in the management of breast cancer brain metastases, if the blood-brain barrier (BBB) can be overcome. The autoimmune disease systemic lupus erythematosus offers an unexpected new approach to this problem. 3E10 is a lupus anti-DNA autoantibody that localizes to DNA at tumors, penetrates cells via the ENT2 nucleoside transporter, inhibits DNA repair, and is synthetically lethal to BRCA1/2 or PTEN-deficient cancer cells with impaired homologous recombination (HR). Breast cancer brain metastases exhibit increased HR defects and PTEN loss compared to primary breast tumors, and ENT2 expressed in brain endothelial cells (BECs) regulates nucleoside flux at the BBB. Further, 3E10 has previously delivered cargo protein to ischemic brain. We hypothesized that ENT2 may facilitate transport of 3E10 across the BBB, and that 3E10 could be used to treat breast cancer brain metastases. Deoxymab-1 (also known as PAT-DX1 or DX1) is a re-engineered and optimized fragment of 3E10 in pre-clinical development for use against HR-deficient tumors. In the present study we examined the ability of DX1 to cross the BBB and suppress breast cancer brain metastases.

BBB study results: ENT2-dependent transport of DX1 across the BBB was evaluated in vitro and in vivo. DX1 penetrated hCMEC/D3 BECs and crossed from apical to basolateral chambers in an hCMEC/D3 transwell model of the BBB. Integrity of the model was confirmed by measuring transendothelial electrical resistance and demonstrating the barrier prevented movement of control protein into the basolateral chamber. Expression of ENT2 in hCMEC/D3 cells was confirmed by immunostaining, and the ENT2 inhibitor dipyridamole (DP) inhibited both penetration by DX1 into the cells and its transport across the BBB transwell model. Immunodeficient mice with orthotopic GBM tumors were treated with IV and IP control buffer (n=2), IV DX1 (20 mg/kg) and IP control buffer (n=4), or IV DX1 (20 mg/kg) and IP DP (ENT2 inhibitor, 70 mg/kg) (n=4). DX1 was labeled with Alexa Fluor 750 (AF750) to allow detection by IVIS. Twenty-four hours after treatment, mice treated with DX1 in the absence of DP exhibited strong AF750 signal in the brain correlating to tumor. Co-treatment with DP reduced uptake of DX1 into the brain tumors by ~78% (P<0.001). These findings are consistent with DX1 crossing the BBB and localizing into brain tumors in an ENT2-dependent manner.

Efficacy study results: The 231-BR brain-seeking subclone of the MDA-MB-231 triple negative breast cancer cell line exhibits PTEN loss relative to parental cells. DX1 penetrated and killed 231-BR cells in vitro. For in vivo testing, brain metastases were generated in immunodeficient mice by intracardiac injection of 231-BR cells engineered for expression of luciferase. Brain metastases were confirmed by IVIS one week later, and then mice were treated with IV control buffer (PBS, n=7) or DX1 (20 mg/kg, n=7). In separate studies DX1 was delivered as a single cycle or as four consecutive cycles, with one cycle defined as control or DX1 3X/week. DX1 significantly suppressed tumor growth, evidenced by weekly IVIS. At week 5 brain radiance efficiencies (x105) in the single cycle study in control and DX1-treated mice were 264.8±72.0 and 71.9±31.3 (P<0.04), and 320±66 and 20.2±8.5 (P≤0.01) in the four-cycle study. One cycle of DX1 yielded a non-significant increase in median survival from 30 to 35 days (P=0.42). Four cycles of DX1 had greater impact, with median survival increased by 14 days (from 31 to 45) (P<0.002). DX1 was not associated with nonspecific toxicity.

Conclusion: Our findings support an ENT2-mediated mechanism of BBB penetration by DX1 and establish proof of concept for use of a DNA-targeting autoantibody against breast cancer brain metastases.

Citation Format: James E Hansen, Shenqi Zhang, Anupama Shirali, Christopher May, Benedette Cuffari, Valentina Dubljevic, James A Campbell, Jiangbing Zhou. An ENT2-dependent, cell-penetrating, and DNA-damaging lupus autoantibody crosses the blood-brain barrier to target breast cancer brain metastases [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD13-08.

64. AN ENT2-DEPENDENT, CELL-PENETRATING, AND DNA-DAMAGING LUPUS AUTOANTIBODY CROSSES THE BLOOD-BRAIN BARRIER TO TARGET BRAIN TUMORS

The blood-brain barrier (BBB) limits conventional antibody-based approaches to brain tumors. ENT2, an equilibrative nucleoside transporter, facilitates penetration of autoantibodies into live cells and is expressed in the BBB. PAT-DX1 (also known as Deoxymab-1 or DX1) is an ENT2-dependent, cell-penetrating, and DNA-damaging lupus autoantibody that is synthetically lethal to cancer cells with defects in the DNA damage response. PTEN loss renders sensitivity to DX1 and is common in primary and metastatic brain tumors. We show that DX1 is toxic to spheroids derived from primary PTEN-deficient glioblastoma (GBM), and crosses the BBB to suppress the growth of orthotopic GBM and breast cancer brain metastases. Mechanistically, we find the ENT2 inhibitor dipyridamole blocks DX1 penetration into brain endothelial cells and transport across the BBB in vitro and in vivo, consistent with ENT2-mediated uptake of DX1 into brain tumors. Autoantibodies that hijack nucleoside transporters to cross cell membranes may open new frontiers in brain tumor therapy.

BSCI-04. TARGETING TRIPLE-NEGATIVE BREAST CANCER BRAIN METASTASES WITH A RE-ENGINEERED LUPUS AUTOANTIBODY

An unusual lupus anti-DNA autoantibody, 3E10, has potential to be used against triple-negative breast cancer (TNBC) brain metastases. 3E10 penetrates live cell nuclei, inhibits DNA repair, and is selectively toxic to cancer cells with the PTEN and/or DNA-damage response (DDR)-deficiencies that are associated with brain metastases in TNBC. The ENT2 nucleoside transporter that 3E10 uses to cross cell membranes is highly expressed in tumors and in brain endothelial cells (BECs) at the blood-brain barrier (BBB), and 3E10 has previously delivered cargo proteins to ischemic brain in a rat stroke model. We have re-engineered 3E10 into an optimized fragment, called Deoxymab-1 (PAT-DX1), that has increased effect on PTEN/DDR-deficient tumor cells. In the present study we tested the ability of PAT-DX1 to cross the BBB and improve outcomes in a mouse model of TNBC brain metastases. PAT-DX1 crossed from apical to basolateral chambers in an hCMEC/D3 Transwell filter model of the BBB, and penetrated the nuclei of and was toxic to the brain-seeking 231-BR subclone of MDA-MB-231 TNBC cells, which harbors a loss of PTEN compared to parental cells. Brain metastases were generated in nude mice by intracardiac injection of 1.75x10⁵ 231-BR cells engineered for expression of luciferase, as confirmed by IVIS one week after injection. Mice with brain metastases were treated by tail vein injection of control (PBS, n=7) or DX1 (20 mg/kg, n=7) 3x/week for 4 weeks. Mice were observed for behavior and weights, and brain radiance efficiency was monitored by weekly IVIS to track metastatic tumor growth. PAT-DX1 significantly suppressed growth of brain metastases based on absolute and relative radiance efficiencies in the brain, increased the median survival of the mice from 38 to 52 days (P< 0.02), and was well tolerated. These results provide proof of concept for use of a re-engineered autoantibody against brain metastases.

Abstract 393: Deoxymab: A targeted biologic that is synthetically lethal to TNBC brain metastases

Brain metastases in triple-negative breast cancer (TNBC) harbor defects in homology-directed DNA repair that may be exploited to develop new therapeutic strategies. However, these tumors are protected by the blood-brain barrier (BBB), and mechanisms of DNA repair are sequestered in the nucleus. Methods to cross the BBB and target intranuclear processes are needed.

3E10 is a nuclear-penetrating lupus anti-DNA autoantibody that inhibits the DNA damage response to selectively kill cancer cells with defects in DNA repair. 3E10 localizes to DNA in tumor environments, and penetrates cells via the ENT2 equilibrative nucleoside transporter. ENT2 is expressed in the BBB, and 3E10 has previously delivered cargo proteins to the brain. We believe 3E10 can be turned against TNBC brain metastases, and used MDA-MB-231-BR brain-seeking human TNBC cells and tumors as a model system to test this hypothesis.

We re-engineered 3E10 into an optimized fragment, called Deoxymab-1 (DX1), to maximize effect on cancer cells and minimize toxicity. DX1 expressed in CHO cells was purified over a HiTrap Capto S column by FPLC, and purity and quality was confirmed by SDS-PAGE and SEC-HPLC. Immunostaining and colony formation assays demonstrated that DX1 penetrated the nuclei of ~100% of MDA-MB-231-BR cells in culture and was synthetically lethal to the cells, with doses of 5 and 10 μM reducing surviving fractions to 0.47±0.15 (P&lt;0.03) and 0.29±0.07 (P&lt;0.01), respectively.

TNBC brain metastases were established in nude mice by intracardiac injection of 1.75x105 MDA-MB-231-BR cells transfected with luciferase. One week later the presence of brain metastases was confirmed by IVIS. Mice were treated with tail vein injection of vehicle control (n=7) or DX1 (20 mg/kg) (n=7) 3x/week for 3 weeks. Tumor burden was monitored by weekly IVIS. DX1 significantly suppressed growth of the brain metastases (Table 1). These results establish proof of the novel concept of use of a modified lupus autoantibody against TNBC brain metastases.

Table 1:Brain Radiance Efficiencies (x105)TreatmentWeek 0Week 1Week 2Week 3Control1.2±0.22.5±0.45.1±0.720±5DX11.3±0.21.4±0.32.0±0.23.8±0.8Pns&lt;0.01&lt;0.01&lt;0.02

Citation Format: Shenqi Zhang, Christopher May, Anupama Shirali, Valentina Dubljevic, James A. Campbell, Jiangbing Zhou, James E. Hansen. Deoxymab: A targeted biologic that is synthetically lethal to TNBC brain metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 393.

Abstract 2773: A DNA-damaging lupus autoantibody synergizes with PARP inhibitors against DNA repair-deficient tumor cells

The lupus anti-DNA autoantibody 3E10 is a compelling candidate for development as a targeted therapy for DNA repair-deficient malignancies. 3E10 has previously been shown to localize to tumors due to its attraction to DNA released by dying cancer cells, penetrate into cell nuclei, inhibit DNA repair, and kill cancer cells with defects in homology-directed repair (HDR) of DNA double-strand breaks. A more potent derivative of 3E10 with increased affinity for DNA has been developed (referred to here as 3E10EN), and identification of optimal combination therapies with 3E10EN is needed to facilitate planning for upcoming clinical trials. In the present study, we found that 3E10EN increases the activity of the DNA repair enzyme poly (ADP-ribose) polymerase (PARP) in HDR-deficient cells and hypothesized that combination treatment with 3E10EN and PARP inhibitors (PARPi) would yield synergistic effects on HDR-deficient cancer cell survival.

PARP content and activity in HDR-deficient and proficient cells prior to and following treatment with 3E10EN was evaluated. 3E10EN did not impact PARP protein content but yielded a significant increase in pADPr signal in HDR-deficient cells, which suggests a compensatory increase in PARP activity in response to DNA damage accumulation in HDR-deficient cells. Combinations of 3E10EN and the PARPi olaparib were tested on a panel of HDR-deficient cells, and a matched pair of BRCA2-deficient and proficient DLD1 cells. Olaparib inhibited the increase in pADPr caused by 3E10EN, and colony formation assays analyzed by the Chou-Talalay method confirmed that 3E10EN and olaparib synergized against HDR-deficient cancer cells. Conversely, HDR-proficient cells were resistant to 3E10EN and olaparib combination treatment.

The original 3E10 is a murine antibody isolated from a lupus mouse model, and in preparation for its further development as a new drug we have recently designed Deoxymab 1 (DX1), a humanized version of 3E10EN. DX1 exhibits improved activity relative to the 3E10EN prototype, and when tested on a panel of HDR-deficient and proficient cells, DX1 and olaparib exhibited synergistic effects similar to that observed with the 3E10EN prototype.

In conclusion, we have found that both the prototype 3E10EN and humanized DX1 synergize with PARPi against HDR-deficient tumor cells. These findings provide the rationale for further studies to determine the potential for this approach to be translated into a clinically relevant therapeutic strategy.

Citation Format: Zahra Rattray, Jaymin M. Patel, Philip W. Noble, Valentina Dubljevic, Deanne L. Greenwood, James A. Campbell, James E. Hansen. A DNA-damaging lupus autoantibody synergizes with PARP inhibitors against DNA repair-deficient tumor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2773.

EGFRvIII-mediated transactivation of receptor tyrosine kinases in glioma: mechanism and therapeutic implications

A truncation mutant of the epidermal growth factor receptor, EGFRvIII, is commonly expressed in glioma, an incurable brain cancer. EGFRvIII is tumorigenic, in part, through its transactivation of other receptor tyrosine kinases (RTKs). Preventing the effects of this transactivation could form part of an effective therapy for glioma; however, the mechanism by which the transactivation occurs is unknown. Focusing on the RTK MET, we show that MET transactivation in U87MG human glioma cells in vitro is proportional to EGFRvIII activity and involves MET heterodimerization associated with a focal adhesion kinase (FAK) scaffold. The transactivation of certain other RTKs was, however, independent of FAK. Simultaneously targeting EGFRvIII (with panitumumab) and the transactivated RTKs themselves (with motesanib) in an intracranial mouse model of glioma resulted in significantly greater survival than with either agent alone, indicating that cotargeting these RTKs has potent antitumor efficacy and providing a strategy for treating EGFRvIII-expressing gliomas, which are usually refractory to treatment.

GRP78-directed immunotherapy in relapsed or refractory multiple myeloma - results from a phase 1 trial with the monoclonal immunoglobulin M antibody PAT-SM6

The primary objective of this phase 1 study was to evaluate the safety and tolerability of the anti-glucose regulated protein 78 monoclonal immunoglobulin M antibody PAT-SM6 in subjects with relapsed or refractory multiple myeloma. Twelve heavily pretreated patients received four intravenous infusions of PAT-SM6 at doses of 0.3, 1, 3, and 6 mg/kg within 2 weeks. Efficacy, pharmacokinetics and immunogenicity were followed up until the end of the trial (day 36). In addition, immune cell patterns in peripheral blood were assessed by flow cytometry and glucose regulated protein 78 expression status was evaluated in bone marrow specimens by immunohistochemistry and flow cytometry at screening. All doses administered were found to be safe and well tolerated; the maximum tolerated dose was not reached. The most common treatment emergent adverse event was leukopenia (grades 1 and 2) in eight out of the 12 multiple myeloma patients. Pharmacokinetic analysis demonstrated dose-proportional increases in drug serum concentration. The terminal half-life ranged from 5.86 to 8.41 h, the apparent volume of distribution ranged from 101 to 150 mL/kg, and clearance ranged from 8.11 to 16.1 mL/h/kg. All patients showed glucose regulated protein 78 surface expression on multiple myeloma cells. Four out of the 12 patients (33.3 %) had stable disease, according to the International Myeloma Working Group criteria, after PAT-SM6 treatment across the doses 1, 3 and 6 mg/kg. In summary, single-agent PAT-SM6 was well tolerated with modest clinical activity in relapsed or refractory multiple myeloma. Further trials exploring the combination of PAT-SM6 with existing myeloma therapies are planned.

TRIAL REGISTRATION

clinicaltrials.gov identifier: NCT01727778.

Antibody-functionalized porous silicon nanoparticles for vectorization of hydrophobic drugs

We describe the preparation of biodegradable porous silicon nanoparticles (pSiNP) functionalized with cancer cell targeting antibodies and loaded with the hydrophobic anti-cancer drug camptothecin. Orientated immobilization of the antibody on the pSiNP is achieved using novel semicarbazide based bioconjugate chemistry. To demonstrate the generality of this targeting approach, the three antibodies MLR2, mAb528 and Rituximab are used, which target neuroblastoma, glioblastoma and B lymphoma cells, respectively. Successful targeting is demonstrated by means of flow cytometry and immunocytochemistry both with cell lines and primary cells. Cell viability assays after incubation with pSiNPs show selective killing of cells expressing the receptor corresponding to the antibody attached on the pSiNP.

The microtubule depolymerizing agent CYT997 causes extensive ablation of tumor vasculature in vivo

The orally active microtubule-disrupting agent (S)-1-ethyl-3-(2-methoxy-4-(5-methyl-4-((1-(pyridin-3-yl)butyl)amino)pyrimidin-2-yl)phenyl)urea (CYT997), reported previously by us (Bioorg Med Chem Lett 19:4639-4642, 2009; Mol Cancer Ther 8:3036-3045, 2009), is potently cytotoxic to a variety of cancer cell lines in vitro and shows antitumor activity in vivo. In addition to its cytotoxic activity, CYT997 possesses antivascular effects on tumor vasculature. To further characterize the vascular disrupting activity of CYT997 in terms of dose and temporal effects, we studied the activity of the compound on endothelial cells in vitro and on tumor blood flow in vivo by using a variety of techniques. In vitro, CYT997 is shown to potently inhibit the proliferation of vascular endothelial growth factor-stimulated human umbilical vein endothelial cells (IC(50) 3.7 ± 1.8 nM) and cause significant morphological changes at 100 nM, including membrane blebbing. Using the method of corrosion casting visualized with scanning electron microscopy, a single dose of CYT997 (7.5 mg/kg i.p.) in a metastatic cancer model was shown to cause destruction of tumor microvasculature in metastatic lesions. Furthermore, repeat dosing of CYT997 at 10 mg/kg and above (intraperitoneally, b.i.d.) was shown to effectively inhibit development of liver metastases. The time and dose dependence of the antivascular effects were studied in a DLD-1 colon adenocarcinoma xenograft model using the fluorescent dye Hoechst 33342. CYT997 demonstrated rapid and dose-dependent vascular shutdown, which persists for more than 24 h after a single oral dose. Together, the data demonstrate that CYT997 possesses potent antivascular activity and support continuing development of this promising compound.

Interaction of the exported malaria protein Pf332 with the red blood cell membrane skeleton

Intra-erythrocytic Plasmodium falciparum malaria parasites synthesize and export numerous proteins into the red blood cell (RBC) cytosol, where some bind to the RBC membrane skeleton. These interactions are responsible for the altered antigenic, morphological and functional properties of parasite-infected red blood cells (IRBCs). Plasmodium falciparum protein 332 (Pf332) is a large parasite protein that associates with the membrane skeleton and who's function has recently been elucidated. Using recombinant fragments of Pf332 in in vitro interaction assays, we have localised the specific domain within Pf332 that binds to the RBC membrane skeleton to an 86 residue sequence proximal to the C-terminus of Pf332. We have shown that this region partakes in a specific and saturable interaction with actin (K(d)=0.60 microM) but has no detectable affinity for spectrin. The only exported malaria protein previously known to bind to actin is PfEMP3 but here we demonstrate that there is no competition for actin-binding between PfEMP3 and Pf332, suggesting that they bind to different target sequences in actin.

Plasmodium falciparum: genetic and immunogenic characterisation of the rhoptry neck protein PfRON4

The Apicomplexan parasites Toxoplasma and Plasmodium, respectively, cause toxoplasmosis and malaria in humans and although they invade different host cells they share largely conserved invasion mechanisms. Plasmodium falciparum merozoite invasion of red blood cells results from a series of co-ordinated events that comprise attachment of the merozoite, its re-orientation, release of the contents of the invasion-related apical organelles (the rhoptries and micronemes) followed by active propulsion of the merozoite into the cell via an actin-myosin motor. During this process, a tight junction between the parasite and red blood cell plasma membranes is formed and recent studies have identified rhoptry neck proteins, including PfRON4, that are specifically associated with the tight junction during invasion. Here, we report the structure of the gene that encodes PfRON4 and its apparent limited diversity amongst geographically diverse P. falciparum isolates. We also report that PfRON4 protein sequences elicit immunogenic responses in natural human malaria infections.

Interactions of Plasmodium falciparum erythrocyte membrane protein 3 with the red blood cell membrane skeleton

Plasmodium falciparum parasites express and traffick numerous proteins into the red blood cell (RBC), where some associate specifically with the membrane skeleton. Importantly, these interactions underlie the major alterations to the modified structural and functional properties of the parasite-infected RBC. P. falciparum Erythrocyte Membrane Protein 3 (PfEMP3) is one such parasite protein that is found in association with the membrane skeleton. Using recombinant PfEMP3 proteins in vitro, we have identified the region of PfEMP3 that binds to the RBC membrane skeleton, specifically to spectrin and actin. Kinetic studies revealed that residues 38-97 of PfEMP3 bound to purified spectrin with moderately high affinity (K(D(kin))=8.5 x 10(-8) M). Subsequent deletion mapping analysis further defined the binding domain to a 14-residue sequence (IFEIRLKRSLAQVL; K(D(kin))=3.8 x 10(-7) M). Interestingly, this same domain also bound to F-actin in a specific and saturable manner. These interactions are of physiological relevance as evidenced by the binding of this region to the membrane skeleton of inside-out RBCs and when introduced into resealed RBCs. Identification of a 14-residue region of PfEMP3 that binds to both spectrin and actin provides insight into the potential function of PfEMP3 in P. falciparum-infected RBCs.

Major immunoreactive domains of human ribosomal P proteins lie N-terminal to a homologous C-22 sequence: application to a novel ELISA for systemic lupus erythematosus

The aim of this study was to identify immunoreactive domains on human ribosomal P0, P1 and P2 proteins, other than the C-22 peptide, to develop a novel ELISA using a combination of these proteins and to compare this ELISA with one using the C-22 peptide. Human recombinant P0, P1, P2 and mutant P0 lacking the homologous C-22 peptide (N-P0) were produced in bacteria and tested by ELISA and immunoblotting using sera from 48 patients with systemic lupus erythematosus (SLE), 48 with an unrelated inflammatory disorder (Crohn's disease) and 47 healthy controls. ELISA with P0, P1 and P2, premixed at equimolar concentrations, gave higher OD readings than each protein tested individually. Eighteen SLE sera tested positive by ELISA with premixed P0, P1, P2 but only 3 tested positive with the C-22 peptide. Twenty-two SLE sera reacted positively, as determined by immunoblotting, with 5 different P protein combinations: P1P2, P0P1P2, P1, P0P1, P0 and P1. Only sera reactive with all three P proteins reacted with the C-22 peptide, with absent or minimal reactivity with N-P0. Native antigens yielded sensitivity (6/48, 13%) similar to the C-22 peptide assay. An ELISA with premixed P1 and P2 gave higher OD values than the arithmetic means with P1 or P2. Fifteen SLE patients had antibodies to double stranded (ds)-DNA, of which 6 also had antibodies to P0P1P2 by ELISA but 12 reactive with P0P1P2 did not have discernable ds-DNA antibodies. Ribosomal P autoantibodies react mainly with epitopes N-terminal to a homologous C-22 peptide. An ELISA with premixed P0, P1 and P2 has 5-fold greater sensitivity (38%) for SLE than an assay with the conventional C-22 peptide (7%). The combined sensitivity for SLE for antibodies to P0P1P2 and ds-DNA is 56%, higher than C-22 and ds-DNA, 38%. Only one of the SLE patients had neuropsychiatric lupus.

Structural basis of allotypes of ecto-nucleotide pyrophosphatase/phosphodiesterase (plasma cell membrane glycoprotein PC-1) in the mouse and rat, and analysis of allele-specific xenogeneic antibodies

Ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPPs) have been implicated in bone calcification, type II diabetes, control of purinergic signalling and tumour invasion. The gene for the plasma cell membrane glycoprotein PC-1 in the mouse (Enpp1) has been known since 1970 to exist in two allelic forms, but their structural basis was heretofore unknown. We show that the Enpp1a and Enpp1b alleles differ by only two amino acids, at positions 650 and 679 in the C-terminal nuclease-like domain. Histidine 650 but not arginine 679 forms an essential part of the Enpp1a epitope recognized by monoclonal antibody IR-518. Sequences of LEW and LOU rats and the rat glioma cell line C6 differ from that of the mouse by about 60 amino acids. The LOU and C6 cell line sequences differ by only three amino acids, but differ from the LEW sequence by 10 amino acids. All three rat strains possess the mouse Enpp1b allele at positions 650 and 679. Despite numerous other differences from the mouse, rats immunized with Enpp1a mouse cells have generated monoclonal antibodies specific for the Enpp1a allele, suggesting that amino acids 650 and 679 may be particularly immunogenic. The cytoplasmic tails of the mouse and rat are highly conserved, but are significantly different from human cytoplasmic tails.

Characterization of a di-leucine-based signal in the cytoplasmic tail of the nucleotide-pyrophosphatase NPP1 that mediates basolateral targeting but not endocytosis

Enzymes of the nucleotide pyrophosphatase/phosphodiesterase (NPPase) family are expressed at opposite surfaces in polarized epithelial cells. We investigated the targeting signal of NPP1, which is exclusively expressed at the basolateral surface. Full-length NPP1 and different constructs and mutants were transfected into the polarized MDCK cell line. Expression of the proteins was analyzed by confocal microscopy and surface biotinylation. The basolateral signal of NPP1 was identified as a di-leucine motif located in the cytoplasmic tail. Mutation of either or both leucines largely redirected NPP1 to the apical surface. Furthermore, addition of the conserved sequence AAASLLAP redirected the apical nucleotide pyrophosphatase/phosphodiesterase NPP3 to the basolateral surface. Full-length NPP1 was not significantly internalized. However, when the cytoplasmic tail was deleted upstream the di-leucine motif or when the six upstream flanking amino acids were deleted, the protein was mainly found intracellularly. Endocytosis experiments indicated that these mutants were endocytosed from the basolateral surface. These results identify the basolateral signal of NPP1 as a short sequence including a di-leucine motif that is dominant over apical determinants and point to the importance of surrounding amino acids in determining whether the signal will function as a basolateral signal only or as an endocytotic signal as well.

A conserved RGD (Arg-Gly-Asp) motif in the transferrin receptor is required for binding to transferrin

The transferrin receptor contains a highly conserved Arg-Gly-Asp (RGD) sequence in the C-terminal region where transferrin is thought to bind. RGD sequences are commonly involved in cell adhesion. This sequence is crucial for transferrin binding, suggesting possible evolutionary links between molecules mediating iron uptake and cell adhesion.