Characterization of two vaccinia CD36 recombinant-virus-generated monoclonal antibodies (10/5, 13/10): effects on malarial cytoadherence and platelet functions

Neonatal Intracranial Hemorrhage with a Dramatic Outcome Due to Maternal Anti CD36 Antibodies

Fetal/neonatal allo-immune thrombocytopenia (FNAIT) results from maternal immunization against fetal platelet-specific antigens (HPA) inherited from the father. Most cases involve HPA located on glycoproteins (GP) IbIX, IaIIa and IIbIIIa. Iso-immunizations can also occur in the absence of expression of membrane proteins, such as GPIIb or GPIIIa in Glanzmann patients. CD36 (also called glycoprotein GPIV) deficiency is observed in 3 to 5% of Asian and African populations. We report here the case of a 41-year-old Canadian woman originated from Africa, who delivered a male dead new-born at 39 weeks of gestation. A massive intracranial haemorrhage was identified as being the obvious cause of death. No platelet antibody against GPIbIX, IaIIa, and IIbIIIa was identified by the gold-standard Monoclonal Antibody-specific Immobilization of Platelet Antigens (MAIPA) assay. Surprisingly, anti CD36 iso-antibodies were identified in the maternal serum with a new bead-based multiplex assay. The CD36 gene was sequenced for both parents, and a mutation was identified on Exon 10 of the mother’s CD36 gene, which was absent for the father: NM_000072.3:c.975T>G inducing a STOP codon at position 325 of the mature protein. The absence of CD36 expression on the mother’s platelets was confirmed by flow cytometry.

OxLDL or TLR2-induced cytokine response is enhanced by oxLDL-independent novel domain on mouse CD36

OxLDL binding to CD36 is shown to result in macrophage activation and foam cell formation that have been implicated in atherosclerosis. However, CD36 has also been shown to induce inflammatory response to other ligands besides oxLDL. During the course of blocking CD36 oxLDL binding function using anti CD36 antibodies, we have identified a novel domain of CD36 that triggers inflammatory response-independent of oxLDL binding. OxLDL bound to the mouse reporter cell line RAW-Blue induced TNF-α and RANTES mRNA and protein expression. Pretreatment of RAW-Blue cells with an anti-mCD36 mAb, JC63.1, an activating mCD36 mAb, surprisingly did not inhibit oxLDL-induced response. Further, binding of this antibody to CD36 alone induced a pro-inflammatory cytokine response in RAW-Blue cells as well as primary mouse macrophages. The induction of cytokine response was specific only to this antibody and was CD36-dependent, since CD36(-/-) macrophages failed to induce a similar response. The interaction of the antibody to CD36 led to activation of NF-κB and MAP kinase. Notably, a CD36 peptide blocked oxLDL-induced foam cell formation and macrophage activation. However, the activating mCD36 mAb induced macrophage activation was not inhibited by CD36 peptide. Further, activating mCD36 mAb enhanced oxLDL- or TLR2- or TLR4-mediated inflammatory responses. Collectively, our data provide evidence that activating mCD36 mAb binds to a domain different from the oxLDL-binding domain on mouse CD36, and suggest that interaction at this domain may contribute to oxLDL-independent macrophage inflammatory responses that lead to chronic inflammatory diseases.

Adhesion of Plasmodium falciparum-infected erythrocytes to human cells: molecular mechanisms and therapeutic implications

Severe malaria has a high mortality rate (15–20%) despite treatment with effective antimalarial drugs. Adjunctive therapies for severe malaria that target the underlying disease process are therefore urgently required. Adhesion of erythrocytes infected with Plasmodium falciparum to human cells has a key role in the pathogenesis of life-threatening malaria and could be targeted with antiadhesion therapy. Parasite adhesion interactions include binding to endothelial cells (cytoadherence), rosetting with uninfected erythrocytes and platelet-mediated clumping of infected erythrocytes. Recent research has started to define the molecular mechanisms of parasite adhesion, and antiadhesion therapies are being explored. However, many fundamental questions regarding the role of parasite adhesion in severe malaria remain unanswered. There is strong evidence that rosetting contributes to severe malaria in sub-Saharan Africa; however, the identity of other parasite adhesion phenotypes that are implicated in disease pathogenesis remains unclear. In addition, the possibility of geographic variation in adhesion phenotypes causing severe malaria, linked to differences in malaria transmission levels and host immunity, has been neglected. Further research is needed to realise the untapped potential of antiadhesion adjunctive therapies, which could revolutionise the treatment of severe malaria and reduce the high mortality rate of the disease.

Heterogeneity of platelet responsiveness to anti-CD36 in plasma associated with adverse transfusion reactions

BACKGROUND AND OBJECTIVES

Antibodies to CD36 (anti-CD36) are clinically important. As some platelet immunoglobulins produced by transfusion or pregnancy have been shown to induce platelet activation and to play roles in non-haemolytic transfusion reactions (NHTRs), we investigated the in vitro response of platelets to plasma containing anti-CD36.

MATERIALS AND METHODS

Plasma containing anti-CD36, implicated in the development of NHTRs and subsequent thrombocytopenia, was incubated with CD36-positive platelets. Plasma-induced platelet activation was examined by evaluating platelet aggregation and RANTES (regulated on activation, normal, T-cell expressed, and presumably secreted) release.

RESULTS

Platelet activation was induced by plasma alone in four out of 20 CD36-positive subjects. In seven subjects, platelet activation was synergistically induced by the combination of epinephrine priming and the plasma. The platelets of the nine remaining subjects failed to respond to the plasma. Platelet activation induced by either the plasma alone or by synergy with epinephrine required the involvement of Fc gamma RIIa. The different responsiveness of the platelets was partially associated with the surface levels of CD36 and Fc gamma RIIa, but not with Fc gamma RIIa polymorphisms.

CONCLUSIONS

Plasma containing anti-CD36, implicated in the development of NHTRs, exhibited a platelet-activating capability. Additionally, platelets from healthy human subjects exhibited a considerable degree of heterogeneity in their responsiveness to this plasma. The heterogeneity of these responses may determine the occurrence of anti-CD36-related NHTRs.

The terminal six amino-acids of the carboxy cytoplasmic tail of CD36 contain a functional domain implicated in the binding and capture of oxidized low-density lipoprotein

CD36, a major adhesion molecule expressed by monocytes/macrophages, plays a key role in the binding and internalization of oxidized low-density lipoprotein (OxLDL). This adhesion molecule, a member of an important scavenger receptor family, contains a very short C-terminal cytoplasmic tail that is known to induce intracellular signalling events. However, the domains on the cytoplasmic tail involved in such signal transduction are unknown. In this study, we have investigated the functional components of the cytoplasmic tail by site-directed mutagenesis coupled with functional OxLDL and monoclonal antibody (mAb) binding studies. Seven truncated or punctual CD36 constructs, localized in the cytoplasmic tail, were produced by site-directed mutagenesis. Each construct was stably expressed in HEK293 cells. We used a quantitative and a qualitative method, labelling OxLDL with either iodine or rhodamine, to determine the functional importance of the cytoplasmic domains in OxLDL internalization. Results indicate that: (1) a deletion of the last amino-acid (construct K472STOP) significantly reduces, compared with wild-type, the binding, internalization and degradation of OxLDL; (2) truncation of the last six amino-acids (construct R467STOP) significantly reduces OxLDL binding; (3) the above two constructs (K472STOP and R467STOP) showed a reduced rate of OxLDL internalization compared with wild-type; (4) the binding and rate of internalization of an anti-CD36 monoclonal antibody (10/5) was not affected by the above mentioned mutants (K472STOP and R467STOP), compared with wild-type. This study shows, for the first time, a specific site on the CD36 cytoplasmic tail that is critical for the binding, endocytosis and targeting of OxLDL.

Sulfated glycoconjugates enhance CD36-dependent adhesion ofPlasmodium falciparum–infected erythrocytes to human microvascular endothelial cells

A novel adhesive pathway that enhances the adhesion ofPlasmodium falciparum-infected erythrocytes (IEs) to endothelial cells has been identified. The sulfated glycoconjugates heparin, fucoidan, dextran sulfate 5000, and dextran sulfate 500 000 caused a dramatic increase in adhesion of IEs to human dermal microvascular endothelial cells. The same sulfated glycoconjugates had little effect on IE adhesion to human umbilical vein endothelial cells, a CD36-negative cell line. The effect was abolished by a monoclonal antibody directed against CD36, suggesting that enhanced adhesion to endothelium is dependent on CD36. No effect was observed on adhesion to purified platelet CD36 cells immobilized on plastic. The same sulfated glycoconjugates enhanced adhesion of infected erythrocytes to COS cells transfected with CD36, and this was inhibited by the CD36 monoclonal antibody. These findings demonstrate a role for sulfated glycoconjugates in endothelial adherence that may be important in determining the location and magnitude of sequestration through endogenous carbohydrates. In addition, they highlight possible difficulties that may be encountered from the proposed use of sulfated glycoconjugates as antiadhesive agents in patients with severe malaria.

Sulfated glycoconjugates enhance CD36-dependent adhesion ofPlasmodium falciparum–infected erythrocytes to human microvascular endothelial cells

A novel adhesive pathway that enhances the adhesion ofPlasmodium falciparum-infected erythrocytes (IEs) to endothelial cells has been identified. The sulfated glycoconjugates heparin, fucoidan, dextran sulfate 5000, and dextran sulfate 500 000 caused a dramatic increase in adhesion of IEs to human dermal microvascular endothelial cells. The same sulfated glycoconjugates had little effect on IE adhesion to human umbilical vein endothelial cells, a CD36-negative cell line. The effect was abolished by a monoclonal antibody directed against CD36, suggesting that enhanced adhesion to endothelium is dependent on CD36. No effect was observed on adhesion to purified platelet CD36 cells immobilized on plastic. The same sulfated glycoconjugates enhanced adhesion of infected erythrocytes to COS cells transfected with CD36, and this was inhibited by the CD36 monoclonal antibody. These findings demonstrate a role for sulfated glycoconjugates in endothelial adherence that may be important in determining the location and magnitude of sequestration through endogenous carbohydrates. In addition, they highlight possible difficulties that may be encountered from the proposed use of sulfated glycoconjugates as antiadhesive agents in patients with severe malaria.

Alternatively spliced mRNA molecules of the thrombospondin receptor (CD36) in human PBMC

We employed reverse transcription polymerase chain reaction (RT-PCR) to detect alternatively spliced CD36 mRNA in human peripheral blood mononuclear cells (PBMC). Sequencing of cloned cDNA revealed alternatively spliced mRNA molecules in 13 out of 39 clones. We observed exon skipping of up to 10 out of 12 coding exons in eight alternative transcripts. Additionally, in five of the transcripts, alternative splice donor or acceptor sites were used during mRNA maturation. Considering the CD36 molecule serves many functions in coagulation, host defence, lipid metabolism, and scavenging, we speculate that the proteins encoded by the alternatively spliced mRNA molecules may be involved in regulation of both CD36 gene expression and function.

CD36 Peptides That Block Cytoadherence Define the CD36 Binding Region for Plasmodium falciparum-Infected Erythrocytes

Mature Plasmodium falciparum parasitized erythrocytes (PE) sequester from the circulation by adhering to microvascular endothelial cells. PE sequestration contributes directly to the virulence and severe pathology of falciparum malaria. The scavenger receptor, CD36, is a major host receptor for PE adherence. PE adhesion to CD36 is mediated by the malarial variant antigen, P. falciparumerythrocyte membrane protein 1 (PfEMP1), and particularly by its cysteine-rich interdomain region 1 (CIDR-1). Several peptides from the extended immunodominant domain of CD36 (residues 139-184), including CD36 139-155, CD36 145-171, CD36 146-164, and CD36 156-184 interfered with the CD36-PfEMP1 interaction. Each of these peptides affected binding at the low micromolar range in 2 independent assays. Two peptides, CD36 145-171 and CD36 156-184, specifically blocked PE adhesion to CD36 without affecting binding to the host receptor intercellular adhesion molecule-1 (ICAM-1). Moreover, an adhesion blocking peptide from the ICAM-1 sequence inhibits the PfEMP1–ICAM-1 interaction without affecting adhesion to CD36. These results confirm earlier observations that PfEMP1 is also a receptor for ICAM-1. Thus, the region 139-184 and particularly the 146-164 or the 145-171 regions of CD36 form the adhesion region for P. falciparum PE. Adherence blocking peptides from this region may be useful for modeling the PE/PfEMP1 interaction with CD36 and for development of potential anti-adhesion therapeutics.

CD36 Peptides That Block Cytoadherence Define the CD36 Binding Region for Plasmodium falciparum-Infected Erythrocytes

Mature Plasmodium falciparum parasitized erythrocytes (PE) sequester from the circulation by adhering to microvascular endothelial cells. PE sequestration contributes directly to the virulence and severe pathology of falciparum malaria. The scavenger receptor, CD36, is a major host receptor for PE adherence. PE adhesion to CD36 is mediated by the malarial variant antigen, P. falciparumerythrocyte membrane protein 1 (PfEMP1), and particularly by its cysteine-rich interdomain region 1 (CIDR-1). Several peptides from the extended immunodominant domain of CD36 (residues 139-184), including CD36 139-155, CD36 145-171, CD36 146-164, and CD36 156-184 interfered with the CD36-PfEMP1 interaction. Each of these peptides affected binding at the low micromolar range in 2 independent assays. Two peptides, CD36 145-171 and CD36 156-184, specifically blocked PE adhesion to CD36 without affecting binding to the host receptor intercellular adhesion molecule-1 (ICAM-1). Moreover, an adhesion blocking peptide from the ICAM-1 sequence inhibits the PfEMP1–ICAM-1 interaction without affecting adhesion to CD36. These results confirm earlier observations that PfEMP1 is also a receptor for ICAM-1. Thus, the region 139-184 and particularly the 146-164 or the 145-171 regions of CD36 form the adhesion region for P. falciparum PE. Adherence blocking peptides from this region may be useful for modeling the PE/PfEMP1 interaction with CD36 and for development of potential anti-adhesion therapeutics.

Recombinant glutathione S-transferase/CD36 fusion proteins define an oxidized low density lipoprotein-binding domain

CD36 is a multifunctional cell-surface receptor that binds adhesion molecules such as thrombospondin-1 and collagen and modified lipids and/or lipoproteins. It participates in cellular uptake of photoreceptor outer segments and scavenging of apoptotic cells and oxidized low density lipoprotein (Ox-LDL). Recognition and internalization of Ox-LDL by mononuclear phagocytes may play an important role in the development of atherosclerotic lesions. We have utilized a series of recombinant bacterial glutathione S-transferase/CD36 fusion proteins that span nearly all of the CD36 molecule to characterize the structural domain on CD36 that recognizes Ox-LDL. We found that the Ox-LDL-binding domain is different from the thrombospondin-1-binding domain located at amino acids 93-120. A fusion protein containing the region extending from amino acids 5 to 143 formed specific, saturable, and reversible complexes with Ox-LDL. As with intact CD36, binding was blocked by excess unlabeled Ox-LDL and antibodies to CD36. The stoichiometry and affinity of the fusion protein for Ox-LDL were similar to those of the intact protein. We also demonstrated that this fusion protein competitively inhibited binding of Ox-LDL to purified platelet CD36 and to CD36 expressed on peripheral blood monocytes and CD36 cDNA-transfected melanoma cells. The use of smaller peptides and fusion proteins including those spanning amino acids 28-93 and 5-93 has further narrowed the binding site to a region from amino acids 28 to 93, although participation of a sequence in the noncontiguous region 120-155 cannot be excluded. This study, for the first time, demonstrates unique regions of the scavenger receptor CD36 that bind the Ox-LDL ligand. Our structural analysis of the receptor provides information as to potential control of the trafficking of modified lipoproteins into the blood vessel wall.