New targeted therapies for treatment of thrombosis in antiphospholipid syndrome

Antiphospholipid (aPL) antibodies (Abs) are associated with thrombosis and pregnancy loss in antiphospholipid syndrome (APS), a disorder initially characterised in patients with systemic lupus erythematosus (SLE) but now known to occur in the absence of other autoimmune disease. There is strong evidence that aPL Abs are pathogenic in vivo, from studies of animal models of thrombosis, endothelial cell activation and pregnancy loss. In recent years, progress has been made in characterising the molecular basis of this pathogenicity, which includes direct effects on platelets, endothelial cells and monocytes as well as activation of complement. This review summarises the clinical manifestations of APS and current modalities of treatment, and explains recent advances in understanding the molecular events triggered by aPL Abs on target cells in coagulation pathways as well as effects of aPL Abs on complement activation. Based on this information and on additional scientific evidence using in vitro and in vivo models, new potential targeted therapies for treatment and/or prevention of thrombosis in APS are proposed and discussed.     

Signaling Dependent and Independent Mechanisms in Pemphigus Vulgaris Blister Formation

Pemphigus vulgaris (PV) is an autoimmune epidermal blistering disease caused by autoantibodies directed against the desmosomal cadherin desmoglein-3 (Dsg3). Significant advances in our understanding of pemphigus pathomechanisms have been derived from the generation of pathogenic monoclonal Dsg3 antibodies. However, conflicting models for pemphigus pathogenicity have arisen from studies using either polyclonal PV patient IgG or monoclonal Dsg3 antibodies. In the present study, the pathogenic mechanisms of polyclonal PV IgG and monoclonal Dsg3 antibodies were directly compared. Polyclonal PV IgG cause extensive clustering and endocytosis of keratinocyte cell surface Dsg3, whereas pathogenic mouse monoclonal antibodies compromise cell-cell adhesion strength without causing these alterations in Dsg3 trafficking. Furthermore, tyrosine kinase or p38 MAPK inhibition prevents loss of keratinocyte adhesion in response to polyclonal PV IgG. In contrast, disruption of adhesion by pathogenic monoclonal antibodies is not prevented by these inhibitors either in vitro or in human skin explants. Our results reveal that the pathogenic activity of polyclonal PV IgG can be attributed to p38 MAPK-dependent clustering and endocytosis of Dsg3, whereas pathogenic monoclonal Dsg3 antibodies can function independently of this pathway. These findings have important implications for understanding pemphigus pathophysiology, and for the design of pemphigus model systems and therapeutic interventions.     

Incidence,pathophysiology, and clinical manifestations of antiphospholipid syndrome

Antiphospholipid syndrome (APLS) is a complex systemic disease with a wide variety of clinical manifestations. In the obstetric population, recurrent early pregnancy loss, fetal loss, and thrombosis are hallmarks of the disease. Patients with APLS have developed one or more pathogenic auto‐antibodies directed against plasma and cell surface proteins. These antibodies are characterized by their affinity for anionic phospholipids. Interactions between APLS antibodies and their protein targets influence a wide variety of biological systems and signaling pathways, including monocytes, platelets, the complement system, and endothelial cells. While much research is currently directed at understanding the mechanisms involved in this autoimmune disease, the key clinical presentation is the hypercoagulable state resulting in thrombosis occurring in essentially any arterial or venous location, as well as numerous obstetrical complications. Treatment of APLS is generally directed at preventing thrombosis and poor pregnancy outcomes by ameliorating the hypercoagulable state. Birth Defects Research (Part C) 105:201–208, 2015. © 2015 Wiley Periodicals, Inc.     

Monoclonal anti-annexin V antibody inhibits trophoblast gonadotropin secretion and induces syncytiotrophoblast apoptosis.

The pathogenic role of anti-annexin V antibodies remains unclear. Anti-annexin V antibodies are frequently associated with higher incidences of intrauterine fetal loss, preeclampsia, and arterial and venous thrombosis. The present study investigated the in vitro ability of anti-annexin V antibody to bind human trophoblast cells, to affect trophoblast gonadotropin secretion and invasiveness, and to induce placental apoptosis. Cytotrophoblast cells were dispersed in Ringer bicarbonate buffer containing trypsin and DNase I, filtered, and layered over a Percoll gradient in Hanks balanced salt solution. In the case of monoclonal anti-annexin V antibody, the highest binding was found when the cells displayed the greatest amount of syncytium formation. Anti-annexin V antibody, but not its negative control, induced trophoblast apoptosis and significantly reduced trophoblast gonadotropin secretion. These findings suggest that recognition by anti-annexin V antibody of adhered annexin V on trophoblast cell structures might represent a potential pathogenic mechanism by which these antibodies can cause defective placentation.     

Immunochemical characteristics of synthetic peptides incorporating T- and B-cell epitopes nonspecific porins of pathogenic Yersinia

Multiple antigenic peptides (MAPs), a sequence which include common antigenic epitopes of outer membrane porins (OM) bacteria of the genus Yersinia (Y. pseudotuberculosis, Y. enterocolitica, Y. pestis), pathogenic for humans have been synthesized. After immunization of BALB/c mice the antiserum to the peptide have been obtained. With the help of ELISA we showed that these sera interact with porins isolated from OM pathogenic Yersinia, and MAP interact with antibodies in sera from rabbits immunized with individual porins, and with antibodies in sera of patients with intestinal yersiniosis and pseudotuberculosis.     

Differential prevalence of anti-heparin-PF4 immunoglobulin subtypes in patients treated with clivarin and heparin: Implications in the HIT pathogenesis

Heparin-induced thrombocytopenia (HIT) syndrome is a catastrophic complication of heparin therapy that may result in arterial/venous thromboembolic events. The pathophysiology of HIT is mediated by the generation of a functionally and molecularly heterogeneous group of anti-heparin-platelet factor 4 (AHPF4) antibodies that cause platelet/endothelial cell activation/destruction. These AHPF4 antibodies may be of various subtypes and cause differential pathogenic responses during HIT. This study evaluated the differential prevalence and functionality of AHPF4 Ig subtypes (IgA, IgG, and IgM) in plasma samples obtained from clinically suspected HIT patients (n = 111) and two clinical trials. In these trials, a low-molecular-weight heparin, clivarin and unfractionated heparin (UFH) were used to treat deep-vein thrombosis (CORTES) and for prophylaxis of the orthopedic surgery (ECHOS). In the CORTES study, three randomized groups of patients (n = 312-328) received prophylactic treatment with either UFH or clivarin (o.d. or b.i.d.). In the ECHOS study, there were approximately 600 patients per group. Citrated plasma samples were analyzed for cumulative IgA/IgG/IgM and individual Ig subtypes of AHPF4 utilizing ELISA. Functionality of the ELISA-positive samples was ascertained by 14C-serotonin release assay. In clinically confirmed HIT patients (and UFH-treated CORTES and ECHOS samples), the Ig subtyping revealed a predominance of IgG AHPF4 antibodies in contrast to the asymptomatic high AHPF4 antibody titers, which were found to be mostly IgM and/or IgA subtypes. In the clivarin-treated patients in both trials, the prevalence of AHPF4 antibodies was found to be lower (2-3 fold, p < 0.01) in comparison to UFH group. In addition, the clivarin-treated patients with positive AHPF4 antibodies were found to be predominantly of the non-functional type and were found in the order of IgM > IgA > IgG Together, these observations demonstrate that ELISA-detectable IgG subtype in UFH-treated patients may be more likely to cause functional/pathologic responses during HIT syndrome. Thus, determination of IgG subtype of AHPF4 antibodies during HIT syndrome may be crucial in the diagnosis; however, the relevance of the pathologically non-functional (IgA and/or IgM) antibodies and the overall mechanism(s) of these HIT-associatied antibodies need further investigation.     

Characterization of monoclonal antibodies to Treponema pallidum

Thirteen hybrid cell lines which produce mouse monoclonal antibodies to Treponema pallidum, the causative agent of syphilis, have been established. All of the monoclonal antibodies react with T. pallidum, Nichols strain, in ELISA and in immunofluorescence assays, but do not react with normal rabbit testicular tissue in the ELISA. Two of these antibodies were demonstrated to react with the nonpathogenic treponemes T. phagedenis, biotype Reiter, T. refringens (Noguchi strain), T. vincentii, and T. denticola (strains 11 and W), as well as with Borrelia recurrentis, Leptospira interrogans, serogroup Canicola, and the swine pathogen T. hyodysenteriae. The remaining 11 antibodies react with four recently isolated strains of T. pallidum, but with none of the related nonpathogens nor with Borrelia or Leptospira. Thus, our results to date indicate that these monoclonal antibodies may identify antigenic determinants that are specific either for T. pallidum alone or for those treponemes which are pathogenic for humans. The molecular specificities of six of the 13 antibodies were determined by Western blotting. We anticipate potential usefulness of these antibodies in the investigation of the antigenic structure of T. pallidum, the taxonomic study of the pathogenic and nonpathogenic treponemes, and in the diagnosis of syphilis.     

Anti-SSA/Ro antibodies and the heart: more than complete congenital heart block? A review of electrocardiographic and myocardial abnormalities and of treatment options

Apart from complete and incomplete congenital heart block (CHB), new cardiac manifestations related to anti-SSA/Ro antibodies have been reported in children born to mothers bearing these antibodies. These manifestations include transient fetal first-degree heart block, prolongation of corrected QT (QTc) interval, sinus bradycardia, late-onset cardiomyopathy, endocardial fibroelastosis and cardiac malformations. Anti-SSA/Ro antibodies are not considered pathogenic to the adult heart, but a prolongation of the QTc interval has recently been reported in adult patients and is still a matter of debate. Treatment of CHB is not well established and needs to be assessed carefully. The risks and benefits of prenatal fluorinated steroids are discussed.     

Human single-chain Fv antibodies against Burkholderia mallei and Burkholderia pseudomallei

Much effort has been devoted to the development of mouse monoclonal antibodies that react specifically with Burkholderia mallei and Burkholderia pseudomallei for diagnostic and/or therapeutic purposes. Our present study focused on the screening of a phage-displayed nonimmune human single-chain Fv (scFv) antibody library against heat-killed B. mallei and B. pseudomallei for the generation of human scFv antibodies specific to the two pathogenic species of bacteria. Using two different panning procedures, we obtained seven different scFv phage antibodies that interacted with the heat-killed whole bacterial cells of B. mallei and B. pseudomallei. Our results demonstrate that panning of a human scFv antibody library against heat-killed whole bacterial cells may provide a valuable strategy for developing human monoclonal antibodies against the highly pathogenic bacteria.     

Anti-beta 2 glycoprotein I antibodies in systemic lupus erythematosus: a marker of thrombosis associated with a circulating anticoagulant]

An ELISA technique for the detection of anti-beta 2 glycoprotein I (beta 2gp I) antibodies was developed. Among 47 systemic lupus erythematosus patients, 17 had anti-beta 2gp I antibodies. These antibodies were statistically associated with anticardiolipin antibodies, lupus anticoagulant and thrombosis. Out of 18 patients with anticardiolipin antibodies without anti-beta 2gp I antibodies or lupus anticoagulant, only one had thrombosis (due to nephrotic syndrome). Therefore the presence of anti-beta 2gp I antibodies is a new immunologic marker of lupus patients with thrombosis. In addition, we propose that anti-beta 2gp I antibodies may be directly responsible for lupus anticoagulant activity.     

Pathogenic antibody recognition of cartilage

Antibodies against cartilage proteins are highly prevalent in the sera and synovial fluids of rheumatoid arthritis (RA) patients and also precede disease induction in various spontaneous and induced animal models of arthritis. These antibodies play an important role in the induction and perpetuation of the clinical disease. Antibodies binding to cartilage protein(s), especially the major articular cartilage protein, collagen type II (CII) can induce, in naive mice, an acute form of arthritis that can substantially destroy the cartilage and bone architecture. More importantly, these anti-CII antibodies can also directly cause the destruction of the target tissue preceding and independently of disease development and in the absence of any other pathogenic inflammatory factors or the action of immune cells. Alternatively, antibodies to citrullinated protein antigens and rheumatoid factor are well-validated prognostic and diagnostic markers of severe erosive RA, although their arthritogenic potential is questioned. Recently, we have found that the monoclonal antibodies to citrulline-modified cartilage protein can bind cartilage and synovial tissue and mediate arthritis in mice. Similarly, one of the pathogenic anti-CII monoclonal antibodies has rheumatoid-factor-like activity, suggesting a disease-inducing role for these commonly prevalent antibodies in RA patients. Interestingly, recent findings have also shown that the enzymatic cleavage or modification of pathogenic IgG antibodies protects the cartilage surface, thereby opening up new therapeutic possibilities for protecting the cartilage from inflammatory damage.     

Lepidopteran cells,an alternative for the production of recombinant antibodies?

Monoclonal antibodies are used with great success in many different therapeutic domains. In order to satisfy the growing demand and to lower the production cost of these molecules, many alternative systems have been explored. Among them, the baculovirus/insect cells system is a good candidate. This system is very safe, given that the baculoviruses have a highly restricted host range and they are not pathogenic to vertebrates or plants. But the major asset is the speed with which it is possible to obtain very stable recombinant viruses capable of producing fully active proteins whose glycosylation pattern can be modulated to make it similar to the human one. These features could ultimately make the difference by enabling the production of antibodies with very low costs. However, efforts are still needed, in particular to increase production rates and thus make this system commercially viable for the production of these therapeutic agents.     

Lepidopteran cells,an alternative for the production of recombinant antibodies?

Monoclonal antibodies are used with great success in many different therapeutic domains. In order to satisfy the growing demand and to lower the production cost of these molecules, many alternative systems have been explored. Among them, the baculovirus/insect cells system is a good candidate. This system is very safe, given that the baculoviruses have a highly restricted host range and they are not pathogenic to vertebrates or plants. But the major asset is the speed with which it is possible to obtain very stable recombinant viruses capable of producing fully active proteins whose glycosylation pattern can be modulated to make it similar to the human one. These features could ultimately make the difference by enabling the production of antibodies with very low costs. However, efforts are still needed, in particular to increase production rates and thus make this system commercially viable for the production of these therapeutic agents.     

Tissue plasminogen activator inhibitor in patients with systemic lupus erythematosus and thrombosis.

OBJECTIVE--To examine the relations among tissue plasminogen activator antigen, plasminogen activator inhibitor, the lupus anticoagulant, and anticardiolipin antibodies in patients with systemic lupus erythematosus. DESIGN--Prospective study of blood samples (a) from selected patients with systemic lupus erythematosus whose disease was and was not complicated by a history of thrombosis or recurrent abortions, or both, and (b) from a series of healthy controls with a similar age and sex distribution. SETTING--University based medical clinic. SUBJECTS--23 Patients with definite systemic lupus erythematosus (American Rheumatism Association criteria), of whom 11 (eight women) aged 26-51 had a history of thrombosis or recurrent abortions, or both, and 12 (10 women) aged 23-53 had no such history. 15 Healthy subjects (10 women) aged 25-58 served as controls. MAIN OUTCOME MEASURES--Tissue plasminogen activator concentrations, plasminogen activator inhibitor activities, detection of the lupus anticoagulant, and values of anticardiolipin antibodies in the two groups of patients and in the patients with a history of thrombosis or abortions compared with controls. Other measurements included concentrations of proteins that are known to change during the acute phase of systemic lupus erythematosus--namely, fibrinogen, C3 and C4, and C reactive protein. RESULTS--Patients with a history of thrombosis or abortions, or both, had significantly higher values of tissue plasminogen activator and plasminogen activator inhibitor than patients with no such history. A significant correlation between tissue plasminogen activator and plasminogen activator inhibitor (r = 0.80) was found only in the patients with a history of complications of their disease. The lupus anticoagulant was detected in six of the 11 patients with a history of thrombosis or abortions when tested by measuring the activated partial thromboplastin time but was found in all 11 patients when tested by measuring the diluted activated partial thromboplastin time. Nine of these 11 patients had raised values of anticardiolipin antibodies. The findings showed no relation to the activity of the disease. CONCLUSIONS--A significant correlation between tissue plasminogen activator concentrations and plasminogen activator inhibitor activities was found only in patients whose systemic lupus erythematosus was complicated by a history of thrombosis or recurrent abortions. The findings show that these patients have raised plasminogen activator inhibitor activities, and the frequent association between these raised activities and the presence of the lupus anticoagulant suggests that the two may be linked.     

Development and validation of species-specific nested PCRs for diagnosis of acute sarcocystiosis in sheep

Sheep may be infected by four species of Sarcocystis. Two of these species, Sarcocystis tenella and Sarcocystis arieticanis, are pathogenic. They may cause abortion or acute disease during the early phase of infection, and chronic disease during the late phase of infection. Thus far, diagnosis of sarcocystiosis in sheep has been limited, because traditional diagnostic tests based on the detection of Sarcocystis-specific antibodies are only genus-specific and, thus, cannot differentiate between pathogenic and non-pathogenic species. In addition, most of these tests show a reasonable sensitivity only for the late phase of infection. Therefore, diagnosis of acute sarcocystiosis has been based mainly on post-mortem examination, i.e. after the animal had succumbed to the disease. Here we established species-specific nested PCR assays based on unique small subunit ribosomal RNA gene sequences of S. tenella and S. arieticanis. These PCR assays specifically detect DNA of the homologous species in blood samples of sheep. No cross-reactions were observed with the heterologous pathogenic species, the non-pathogenic species Sarcocystis gigantea, or the closely related coccidia Toxoplasma gondii and Neospora caninum. In sheep experimentally infected with S. tenella or S. arieticanis, positive PCR results were correlated with the early phases of multiplication (endopolygeny) of the parasites. By contrast, Sarcocystis-specific antibodies were detected by an enzyme-linked immunosorbent assay only during the terminal phase of endopolygeny or thereafter. Thus, the nested PCR assays developed here enable, for the first time, the diagnosis and differentiation of infections with S. tenella and S. arieticanis in living sheep during the acute phase of the disease and facilitate comprehensive studies on the epidemiology and importance of infections with pathogenic Sarcocystis species in sheep.     

Lupus anticoagulant and antiplatelet properties of human hybridoma autoantibodies

The clinical association of lupus anticoagulant antibodies with thrombocytopenia and thrombosis was the rationale for investigating the in vitro reactivity of these human hybridoma lupus anticoagulant antibodies with platelets. Fifty human hybridoma antibodies from 13 patients with systemic lupus erythematosus, 2 women with multiple spontaneous abortions, and 4 normal individuals were analyzed for lupus anticoagulant, antiplatelet, anti-DNA, and antiphospholipid reactivities. Of the hybridoma antibodies studied, 25 had lupus anticoagulant activity, 21 had antiplatelet reactivity, and 7 of these antibodies had both lupus anticoagulant and antiplatelet properties. No correlation was found between lupus anticoagulant antibody activity and antiplatelet, anti-denatured DNA, anticardiolipin, anti-egg phosphatidylethanolamine, antiphosphatidylserine, antiphosphatidylinositol, and antiphosphatidylcholine reactions. In contrast, antiplatelet activity was strongly correlated with antiphosphatidylethanolamine (rho = 0.761, p less than 0.001), anticardiolipin (rho = 0.748, p less than 0.001), and anti-dDNA (rho = 0.745, p less than 0.001) reactivities. Pretreatment of platelets with deoxyribonuclease, ribonuclease, trypsin, or phospholipases A2 and C resulted in different effects on the binding of individual hybridoma antibodies to platelets, suggesting that antiplatelet antibodies may recognize different epitopes on the platelet membrane. Our data demonstrate that most hybridoma lupus anticoagulant antibodies did not bind directly to platelets in vitro. This suggests that additional serum factors may be required in vivo to explain the association of these antibodies with thrombocytopenia and thrombosis.     

Ligation of Signal Inhibitory Receptor on Leukocytes-1 Suppresses the Release of Neutrophil Extracellular Traps in Systemic Lupus Erythematosus

Neutrophil extracellular traps (NETs) have been implicated in the pathogenesis of systemic Lupus erythematosus (SLE), since netting neutrophils release potentially immunogenic autoantigens including histones, LL37, human neutrophil peptide (HNP), and self-DNA. In turn, these NETs activate plasmacytoid dendritic cells resulting in aggravation of inflammation and disease. How suppression of NET formation can be targeted for treatment has not been reported yet. Signal Inhibitory Receptor on Leukocytes-1 (SIRL-1) is a surface molecule exclusively expressed on phagocytes. We recently identified SIRL-1 as a negative regulator of human neutrophil function. Here, we determine whether ligation of SIRL-1 prevents the pathogenic release of NETs in SLE. Peripheral blood neutrophils from SLE patients with mild to moderate disease activity and healthy donors were freshly isolated. NET release was assessed spontaneously or after exposure to anti-neutrophil antibodies or plasma obtained from SLE patients. The formation of NETs was determined by microscopic evaluation using DNA dyes and immunostaining of NET components, as well as by live cell imaging. We show that SLE neutrophils spontaneously release NETs. NET formation is enhanced by stimulation with antibodies against LL37. Inhibition of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and MEK-ERK signaling prevents NET release in response to these antibodies. Signaling via the inhibitory receptor SIRL-1 was induced by ligation with anti-SIRL-1 specific antibodies. Both spontaneous and anti-neutrophil antibody-induced NET formation is suppressed by engagement of SIRL-1. Furthermore, NET release by healthy neutrophils exposed to SLE plasma is inhibited by SIRL-1 ligation. Thus, SIRL-1 engagement can dampen spontaneous and anti-neutrophil antibody-induced NET formation in SLE, likely by suppressing NAPDH oxidase and MEK-ERK activity. Together, these findings reveal a regulatory role for SIRL-1 in NET formation, potentially providing a novel therapeutic target to break the pathogenic loop in SLE.     

V region sequences of an idiotypically connected family of pathogenic anti-DNA autoantibodies

The H and L chain V region sequences of nine anti-DNA mAb that are representative of a pathogenic population of autoantibodies produced by the nephritis prone (SWR x NZB)F1 (SNF1) mice, were determined. These nine anti-DNA autoantibodies were idiotypically connected members of a cross-reactive Id family called the Id564 cluster. Moreover, these autoantibodies were all cationic in charge, IgG2b in isotype, and their H chain C regions had the normal SWR parent's allotype. Although derived from two different SNF1 animals, these pathogenic autoantibodies possessed highly homologous Leader-VH sequences that could account for their idiotypic cross-reactivity. Furthermore, the VH region sequences of these anti-DNA antibodies contained numerous basic residues that could impart their cationic charge. The Leader-VH sequences of these autoantibodies were also highly homologous to that of an anti-NP antibody-related germ-line gene of C57BL/6 mice, called VH-23. Among these nine pathogenic autoantibodies, three sets of clonally related anti-DNA antibodies could be identified. Thus the Id564 cluster of cationic anti-DNA autoantibodies of SNF1 mice are encoded by highly related VH genes, and this idiotypically connected population of pathogenic autoantibodies are selected to undergo an oligoclonal expansion in the lupus-prone SNF1 mice.     

Thrombophilia Associated with Anti-DFS70 Autoantibodies

Context

Anti-DFS70 antibodies are the most frequent antinuclear antibodies (ANA) found in healthy individuals. We assessed the clinical significance of the presence of anti-DFS70 antibodies.

Methods

We defined a group of patients (n = 421) with anti-DFS70 antibodies and a group of patients (n = 63) with a history of idiopathic arterial and/or venous thrombotic disease and/or obstetric complication (i.e. ≥3 miscarriages, fetal death or premature birth with eclampsia). Anti-DFS70 antibodies prevalence was also assessed in a cohort of 300 healthy blood donors.

Results

The prevalence of thrombotic disease and/or obstetric complication in the 421 patients with anti-DFS70 antibodies was 13.1% (n = 55) and the prevalence of connective tissue disease was 19% (n = 80). Among the 63 patients with a history of thrombosis and/or obstetric complications, 7 (11.1%) had anti-DFS70 antibodies and among the latter, 5 had no common thrombophilic factor. In contrast, the prevalence of anti-DFS70 antibodies was of 3.0% (9 out of 300) in healthy donors. Finally, the Activated Partial Thromboplastin Time (aPTT) ratio of patients with a history of thrombosis and anti-DFS70 antibodies was lower than the aPTT ratio of other patients, suggesting that thrombotic patients with anti-DFS70 antibodies may have a hypercoagulable state.

Conclusion

We described here for the first time an immune procoagulant state involving anti-DFS70 antibodies.     

Protection against High-Dose Highly Pathogenic Mucosal SIV Challenge at Very Low Serum Neutralizing Titers of the Antibody-Like Molecule CD4-IgG2

Passive transfer studies using monoclonal or polyclonal antibodies in the macaque model have been valuable for determining conditions for antibody protection against immunodeficiency virus challenge. Most studies have employed hybrid simian/human immunodeficiency virus (SHIV) challenge in conjunction with neutralizing human monoclonal antibodies. Passive protection against SIV, particularly the pathogenic prototype virus SIVmac239, has been little studied because of the paucity of neutralizing antibodies to this virus. Here, we show that the antibody-like molecule CD4-IgG2 potently neutralizes SIVmac239 in vitro. When administered by an osmotic pump to maintain concentrations given the short half-life of CD4-IgG2 in macaques, the molecule provided sterilizing immunity/protection against high-dose mucosal viral challenge to a high proportion of animals (5/7 at a 200 mg dose CD4-IgG2 and 3/6 at a 20 mg dose) at serum concentrations below 1.5 μg/ml. The neutralizing titers of such sera were predicted to be very low and indeed sera at a 1∶4 dilution produced no neutralization in a pseudovirus assay. Macaque anti-human CD4 titers did develop weakly at later time points in some animals but were not associated with the level of protection against viral challenge. The results show that, although SIVmac239 is considered a highly pathogenic virus for which vaccine-induced T cell responses in particular have provided limited benefit against high dose challenge, the antibody-like CD4-IgG2 molecule at surprisingly low serum concentration affords sterilizing immunity/protection to a majority of animals.