Peptide and nucleotide sequences of rat CD4 (W3/25) antigen: evidence for derivation from a structure with four immunoglobulin-related domains

Proteoglycan 4 (PRG4) treatment improves skin wound healing in a porcine model

Proteoglycan 4 (PRG4) is a boundary lubricant originally identified in articular cartilage and has been since shown to have immunomodulation and antifibrotic properties. Previously, we have demonstrated that recombinant human (rh)PRG4 treatment accelerates auricular cartilage injury closure through an inhibition of the fibrotic response, and promotion of tissue regeneration in mice. The purpose of the current study was to examine the effects of rhPRG4 treatment (vs. a DMSO carried control) on full-thickness skin wound healing in a preclinical porcine model. Our findings suggest that while rhPRG4 did not significantly accelerate nor impede full-thickness skin wound closure, it did improve repair quality by decreasing molecular markers of fibrosis and increasing re-vascularization. We also demonstrated that rhPRG4 treatment increased dermal adipose tissue during the healing process specifically by retaining adipocytes in the wound area but did not inhibit lipolysis. Overall, the results of the current study have demonstrated that rhPRG4 acts as antifibrotic agent and regulates dermal adipose tissue during the healing processes resulting in a tissue with a trajectory that more resembles the native skin vs. a fibrotic patch. This study provides strong rationale to examine if rhPRG4 can improve regeneration in human wounds.

Impact of a CD4 gene haplotype on the immune response in minipigs

The cluster of differentiation 4 (CD4) molecule functions as a co-receptor for MHC class II binding to TCR in T helper cells. A CD4 epitope deficiency was identified in the swine MeLiM (melanoblastoma-bearing Libechov minipig) strain, a model for spontaneous cutaneous melanoma development and regression. Extensive sequencing revealed a high genetic variability of CD4 and the existence of several haplotypes segregating in MeLiM. Forty polymorphisms were identified in the coding sequence, out of which 20 correspond to non-synonymous variants and 10 are located in the 3'UTR of CD4 transcripts. One of the haplotypes segregating in the MeLiM explained the epitope deficiency observed. An association analysis between CD4 genotype and several phenotypes related to tumor regression was performed in 267 animals. An association was evidenced between a MeLiM alternative CD4 haplotype and skin and eye depigmentation, as well as the extent of hair depigmentation. Also, seric IgG concentration was shown to be higher in pigs carrying the alternative haplotype at the homozygous state. In conclusion, the genetic variability of the CD4 gene is associated with immune response-related phenotypes in MeLiM minipigs.

Development of an anti-ferret CD4 monoclonal antibody for the characterisation of ferret T lymphocytes

The ferret is an established animal model for a number of human respiratory viral infections, such as influenza virus and more recently, Ebola virus. However, a paucity of immunological reagents has hampered the study of cellular immune responses. Here we describe the development and characterisation of a novel monoclonal antibody (mAb) against the ferret CD4 antigen and the characterisation of ferret CD4 T lymphocytes. Recombinant production and purification of the ferret CD4 ectodomain soluble protein allowed hybridoma generation and the generation of a mAb (FeCD4) showing strong binding to ferret CD4 protein and lymphoid cells by flow cytometry. FeCD4 bound to its cognate antigen post-fixation with paraformaldehyde (PFA) which is routinely used to inactivate highly pathogenic viruses. We have also used FeCD4 in conjunction with other immune cell markers to characterise ferret T cells in both primary and secondary lymphoid organs. In summary, we have developed an important reagent for the study of cellular immunological responses in the ferret model of infectious disease.

Comparative and evolutionary insights into CD4 gene across mammalian and avian taxa

The present day genetic architecture of a species bears much significance to its closely related species which is due to species-specific differences, shaped by different evolutionary forces across time scale. With the availability of whole genome sequence of several closely related species, it is now possible to infer evolutionary patterns of genes and genomes in specific lineages. To this respect, CD4 gene, primarily responsible for defensive mechanism in human, is conserved across a few taxa, and thus, comparative genomic studies could be useful for better understanding of host-pathogen biology. Comparative and evolutionary analyses were performed in eleven taxa (10 mammalian and avian) with different statistical algorithms. Phylogenetic inferences revealed recent divergence of human and chimpanzee, and pig was found to be diverged from rest of the taxa significantly. Additionally, gene length, microsatellites, and secondary structures were observed across taxa. The genetic architecture of CD4 gene and its evolutionary history in different mammalian taxa provide crucial evidence in support of the fact that this gene might have been evolving at a similar rate to other human immune system genes. Future population-based study and structural modeling would unravel the differential ability to interact with HIV virus and influence immune system in humans.

CD4 and CD8 homologues in Japanese flounder, Paralichthys olivaceus: Differences in the expressions and localizations of CD4-1, CD4-2, CD8α and CD8β

CD4 and CD8 molecules are co-receptors of T cell receptors which interact specifically with MHC class II and I, respectively, during antigen presentation. Here we investigated CD4 and CD8 expression patterns in a fish, Japanese flounder, Paralichthys olivaceus in response to infection and tuberculin injection. The CD4-1 mRNA level was gradually and weakly increased in trunk kidney after infection with Streptococcus iniae, Edwardsiella tarda and viral hemorrhagic septicemia virus (VHSV), while the CD4-2 mRNA level was dramatically increased after E. tarda and VHSV infection, but not increased after S. iniae infection. CD4-2 mRNA but not CD4-1mRNA increased in the kidney during tuberculin response which is mediated by memory Th1 cells. The patterns for the change of mRNA level in CD8α and CD8β were similar to those of the CD4-2 during the infections and tuberculin response. Fluorescent in situ hybridization detected CD4-1 mRNAs on melano-macrophage centers and CD4-2 mRNAs at some cell clusters located near the melano-macrophage centers. CD8α and CD8β mRNAs were detected at the same cell clusters in the spleen and head kidney. These results suggest that CD4-1 and CD4-2 are expressed in different cells and that CD4-2-positive cells, rather than CD4-1-positive cells, have a main role in Th1-related immune responses collaborating with CD8α- and CD8β-positive cells in Japanese flounder.

Candidate antibody-based therapeutics against HIV-1

Antibody-based therapeutics have been successfully used for the treatment of various diseases and as research tools. Several well characterized, broadly neutralizing monoclonal antibodies (bnmAbs) targeting HIV-1 envelope glycoproteins or related host cell surface proteins show sterilizing protection of animals, but they are not effective when used for therapy of an established infection in humans. Recently, a number of novel bnmAbs, engineered antibody domains (eAds), and multifunctional fusion proteins have been reported which exhibit exceptionally potent and broad neutralizing activity against a wide range of HIV-1 isolates from diverse genetic subtypes. eAds could be more effective in vivo than conventional full-size antibodies generated by the human immune system. Because of their small size (12∼15 kD), they can better access sterically restricted epitopes and penetrate densely packed tissue where HIV-1 replicates than the larger full-size antibodies. HIV-1 possesses a number of mechanisms to escape neutralization by full-size antibodies but could be less likely to develop resistance to eAds. Here, we review the in vitro and in vivo antiviral efficacies of existing HIV-1 bnmAbs, summarize the development of eAds and multispecific fusion proteins as novel types of HIV-1 inhibitors, and discuss possible strategies to generate more potent antibody-based candidate therapeutics against HIV-1, including some that could be used to eradicate the virus.

Mapping the interaction between the cytoplasmic domains of HIV-1 viral protein U and human CD4 with NMR spectroscopy

Viral protein U (VpU) of HIV-1 plays an important role in downregulation of the main HIV-1 receptor CD4 from the surface of infected cells. Physical binding of VpU to newly synthesized CD4 in the endoplasmic reticulum is an early step in a pathway leading to proteasomal degradation of CD4. In this study, regions in the cytoplasmic domain of VpU involved in CD4 binding were identified by NMR spectroscopy. Amino acids in both helices found in the cytoplasmic region of VpU in membrane-mimicking detergent micelles experience chemical shift perturbations upon binding to CD4, whereas amino acids between the two helices and at the C-terminus of VpU show no or only small changes, respectively. The topology of the complex was further studied with paramagnetic relaxation enhancement. Paramagnetic spin labels were attached at three sequence positions of a CD4 peptide comprising the transmembrane and cytosolic domains of the receptor. VpU binds to a membrane-proximal region in the cytoplasmic domain of CD4.

Phylogenetic and developmental study of CD4, CD8 α and β T cell co-receptor homologs in two amphibian species, Xenopus tropicalis and Xenopus laevis

CD4 and CD8 co-receptors play critical roles in T cell development and activation by interacting both with T cell receptors and MHC molecules. Although homologs of these genes have been identified in many jawed vertebrates, there are still unresolved gaps concerning their evolution and specialization in MHC interaction and T cell function. Using experimental and computational procedures we identified CD4, CD8α and CD8β gene homologs both in Xenopus tropicalis, whose full genome has been sequenced, and its sister species Xenopus laevis. Multiple alignments of deduced amino acid sequences reveal a poor conservation of the residues involved in binding of CD4 to MHC class II, and CD8α to class I in non-mammalian species, presumably related to the co-evolutionary pressure of MHC I and II genes. Phylogenetic study suggests that Xenopodinae co-receptor genes are more closely related to their homologs in other tetrapods than those of bony fish. Furthermore, the developmental and cell-specific expression patterns of these genes in X. laevis are very similar to that of mammals. X. laevis CD4 is mainly expressed by peripheral non-CD8 T cells and detected in the thymus as early as four days post-fertilization (dpf) at the onset of thymic organogenesis. CD8β expression is specific to adult surface CD8(+) T cells and thymocytes, and is first detected in the thymus at 5 dpf in parallel with productive TCRγ transrcipts, whereas productive TCRβ and α rearrangements are not detected before 7-9 dpf.

NMR structure of the transmembrane and cytoplasmic domains of human CD4 in micelles

The human cluster determinant 4 (CD4) is a type I transmembrane glycoprotein involved in T-cell signalling. It is expressed primarily on the surface of T helper cells but also on subsets of memory and regulatory T lymphocytes (CD4(+) cells). It serves as a coreceptor in T-cell receptor recognition of MHC II antigen complexes. Besides its cellular functions, CD4 serves as the main receptor for human immunodeficiency virus type I (HIV-1). During T-cell infection, the CD4 extracellular domain is bound by HIV-1 gp120, the viral surface glycoprotein, which triggers a number of conformational changes ultimately resulting in virion entry of the cell. Subsequently, CD4 is downregulated in infected cells by multiple strategies that involve direct interactions of the HIV-1 proteins VpU and Nef with the cytoplasmic part of CD4. In the present work, we describe the NOE-based solution structure of the transmembrane and cytoplasmic domains of the cystein-free variant of CD4 (CD4mut) in dodecylphosphocholine (DPC) micelles. Furthermore, we have characterized micelle-inserted CD4mut by paramagentic relaxation enhancement (PRE) agents and (1)H-(15)N heteronuclear NOE data. CD4mut features a stable and well-defined transmembrane helix from M372 to V395 buried in the micellar core and a cytoplasmic helix ranging from A404 to L413. Experimental data suggest the amphipathic cytoplasmic helix to be in close contact with the micellar surface. The role of the amphipathic helix and its interaction with the micellar surface is discussed with respect to the biological function of the full-length CD4 protein.

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

Immune activation is required for NT-3-induced axonal plasticity in chronic spinal cord injury

After an unilateral lesion of the corticospinal tract (CST) at the level of the medulla over-expression of Neurotrophin-3 (NT-3) in lumbar spinal cord motoneurons induced axonal sprouting of the intact CST in the acutely injured but not uninjured or chronically injured spinal cord in rats. This suggested that processes associated with immune-mediated wound healing may act with NT-3 to induce neuroplasticity. To test whether immune processes were involved we measured NT-3-induced axonal sprouting in immunosuppressed compared to immunocompetent rats. Rats were immunosuppressed with anti-leukocyte antibodies 1 day before receiving a CST lesion and then 2 weeks later NT-3 was over-expressed in the lumbar spinal motoneurons with an adenoviral vector carrying the NT-3 gene targeted to the motoneurons by retrograde transport. At 35 days post-lesion no axonal sprouting was measured in immunosuppressed rats whereas axonal sprouting was measured in the immunocompetent rats. We then tested whether re-evoking an immune response in chronically lesioned rats would induce neuroplasticity. Rats received CST lesions and then 4 months later were treated with systemic injections of lipopolysaccharide (LPS) 7 days before NT-3 was over-expressed in the lumbar spinal motoneurons. Axonal sprouting was observed in the LPS treated rats but not in control animals that were not treated with LPS. Further studies showed that lesioning the CST activated and LPS reactivated microglia and CD4(+) T-cells in the acutely lesioned and chronically lesioned rats, respectively. However, immunosuppression only decreased the number of activated CD4(+) T-cells suggesting they were responsible for the support of axonal growth. These observations demonstrate that processes associated with immune-mediated wound healing play a role in NT-3-induced neuroplasticity after injury.

Molecular cloning and characterization of carp (Cyprinus carpio L.) CD8beta and CD4-like genes

Partial cDNA sequences of both CD8beta and CD4-like (CD4L) genes of common carp (Cyprinus carpio L.) were isolated from thymus cDNA library by the method of suppression subtractive hybridization (SSH). Subsequently the full length cDNAs of carp CD8beta and CD4L were obtained by means of 3' RACE and 5' RACE, respectively. The full length cDNA of carp CD8beta is 1164 bp and encodes 207 amino acids including a signal peptide region of 24 amino acids, a transmembrane region of 23 amino acids from aa 167 to aa189 and an immunoglobulin V-set from aa 19 to aa 141. Similar to other species CD8betas, carp CD8beta also lacks p56(lck) domain in the cytoplasmic region. The full length cDNA of carp CD4L is 2001 bp and encodes 458 amino acids including four immunoglobulin (Ig)-like domains in the extracellular region, a transmembrane region of 23 amino acids at the C-terminal region from aa 402 to aa 424 and a cytoplasmic tail. Similar to mammalian, avian CD4s and fugu CD4L, carp CD4L also has the conserved p56(lck) tyrosine kinase motif (C-X-C) in the cytoplasmic region. RT-PCR analysis demonstrated that carp CD8beta and CD4L genes were both expressed predominantly in thymus. The results from this study can be used to understand the evolution of both the CD8beta and CD4 molecules which can be used as markers for cytotoxic and helper T cells in carp.

Expression, purification, and membrane reconstitution of a CD4 fragment comprising the transmembrane and cytoplasmic domains of the receptor

The transmembrane glycoprotein CD4 plays a prominent role in the adaptive immune response. CD4 is displayed primarily on the surface of T helper cells, but also on subsets of memory and regulatory T lymphocytes, macrophages, and dendritic cells. Binding of the lymphocyte specific tyrosine kinase p56(lck) to the cytoplasmic domain of CD4 is crucial for antigen receptor-mediated signal transduction. The human immunodeficiency virus (HIV) utilizes CD4 as the main receptor for T cell invasion. The virus has developed multiple strategies for down-regulation of CD4 in infected cells. Physical interactions of viral proteins VpU and Nef with the cytoplasmic tail of CD4 initiate a cascade of events leading to degradation of CD4. Here we report heterologous expression and purification of a CD4 fragment comprising the transmembrane and cytoplasmic domains of human CD4. A synthetic gene encoding CD4 amino acid residues 372-433 and a protease cleavage site was cloned into the pTKK19xb/ub plasmid. The CD4 fragment was expressed in Escherichia coli C43(DE3) cells as a ubiquitin fusion with an N-terminal His-tag, isolated, released by PreScission proteolytic cleavage, and purified to homogeneity. Incorporation of the recombinant CD4 fragment in lipid membranes and physical interaction with the cytoplasmic domain of VpU was demonstrated by centrifugation assays followed by reversed phase chromatographic analysis of the composition of the proteoliposomes. A high resolution NMR spectrum of uniformly (15)N-labeled CD4 peptide in membrane simulating micelles proves the possibility of solution NMR studies of this CD4 fragment and of its molecular complexes.

Improvement of kidney failure with fetal kidney precursor cell transplantation

BACKGROUND

Current therapies for end-stage renal disease have severe limitations. Dialysis is only a temporary treatment and does not restore kidney function. Transplantation is limited by donor organ shortage and immune-related problems. Here, we show that the transplantation of fetal kidney precursor cells reconstitutes kidney tissues, reduces uremic symptoms, and provides life-saving metabolic support in kidney failure animal models.

METHODS

Kidney failure was surgically induced by resecting kidneys, leaving approximately 1/6 of the total kidney mass (5/6 nephrectomy). Fetal kidney precursor cells were isolated from metanephroi of E17.5 rat fetuses using collagenase/dispase digestion. Five weeks after the nephrectomy procedure, isolated fetal kidney precursor cells were transplanted under the kidney capsule of rats using fibrin gel matrix. Six and ten weeks after transplantation, animals were analyzed biochemically and the grafts were retrieved for histological analyses.

RESULTS

Five weeks after the nephrectomy, glomerular hypertrophy, and increased blood urea nitrogen and serum creatinine levels were observed. The cell transplantation into the kidneys of kidney failure-induced rats resulted in kidney tissue reconstitution and the transplanted cells were observed in the reconstitution region of the kidneys as evidenced by the presence of fluorescently labeled cells. In addition, biochemical parameters from serum and urine samples showed improved kidney functions compared with non-treated group without severe immune response after ten weeks.

CONCLUSION

Transplanting fetal kidney precursor cells showed the potential for the partial augmentation of kidney structure and function in the treatment of kidney failure.

Identification of two allelic forms of ovine CD4 exhibiting a Ser183/Pro183 polymorphism in the coding sequence of domain 3

The ovine CD4 cDNA sequence from four sheep sources (Australian Merino, Indonesian Thin Tail, Canadian cross bred, Prealpes du sud) predicts a protein of 455 residues with position 130 in the V2 domain exhibiting a W instead of C suggesting that, like the white whale, dog and cat sequences, sheep CD4 contains only two disulphide bonds. The sequence shows 73% amino acid identity and 83% nucleotide identity to a CD4 sequence from the white whale and significant identity to a partial sequence (314 residues) of bovine CD4 (87% amino acid identity, 93% nucleotide identity). Phylogenetic analysis showed that the ovine CD4 sequence forms a clade with the pig, white whale, dolphin, dog and cat CD4. Two forms of ovine CD4 were identified which differ by a single base pair (T/C) in their cDNA sequence at position 622. This polymorphism is also present in sheep genomic DNA in Hardy-Weinberg equilibrium, suggesting that at least two alleles of CD4 exist in the ovine genome with no selection for a particular allele. This polymorphism changes the first codon position of amino acid 183 and results in a Pro/Ser substitution in the N-terminal region of domain 3 of the CD4 protein.

CD4 expression decrease by antisense oligonucleotides: inhibition of rat T CD4+ cell reactivity

In previous studies, we have demonstrated the inhibition of CD4 expression in rat lymphocytes treated with phorbol myristate acetate (PMA) by antisense oligonucleotides (AS-ODNs) directed against the AUG start region of the cd4 gene. The aim of the present study was to inhibit CD4 expression in lymphocytes without promoting CD4 synthesis and to determine the effect of this inhibition on CD4+ T cell function. Four 21-mer ODNs against the rat cd4 gene (AS-CD4-1 to AS-CD4-4) were used. Surface CD4 expression was measured by immunofluorescence staining and flow cytometry, and mRNA CD4 expression was measured by RT-PCR. T CD4+ cell function was determined by specific and unspecific proliferative response of rat-primed lymphocytes. After 24 hours of incubation, AS-CD4-2 and AS-CD4-4 reduced lymphocyte surface CD4 expression by 40%. This effect remained for 72 hours and was not observed on other surface molecules, such as CD3, CD5, or CD8. CD4 mRNA expression was reduced up to 40% at 24 hours with AS-CD4-2 and AS-CD4-4. After 48 hours treatment, CD4 mRNA decreased up to 27% and 29% for AS-CD4-2 and AS-CD4-4, respectively. AS-CD4-2 and AS-CD4-4 inhibited T CD4+ cell proliferative response upon antigen-specific and unspecific stimuli. Therefore, AS-ODNs against CD4 molecules inhibited surface and mRNA CD4 expression, under physiologic turnover and, consequently, modulate T CD4+ cell reactivity.

CD4 dimers constitute the functional component required for T cell activation

The CD4 molecule plays a key role in the development and activation of helper T cells. Dimerization and oligomerization is often a necessary step in the function of several cell surface receptors. Herein, we provide direct biochemical evidence confirming the presence of CD4 as dimers in transfected cells from hemopoetic and fibroblastic origin as well as in primary T cells. Such dimers are also observed with murine CD4 confirming selective pressure during evolution to maintain such a structure. Using a series of point mutations, we have precisely mapped the dimerization site at residues K318 and Q344 within the fourth extracellular domain of CD4. These residues are highly conserved and their mutation results in interference with dimer formation. More importantly, we demonstrate that dimer formation is essential for the coligand and coreceptor functions of CD4 in T cell activation. These data strongly suggest that CD4 dimerization is necessary for helper T cell function.

Inhibition of CD4 expression by antisense oligonucleotides in PMA-treated lymphocytes

To decrease CD4 expression on T helper (Th) lymphocyte surface, antisense oligonucleotides (AS-ODNs), delivered by the cationic liposome DOTAP, were assayed in vitro on rat spleen lymphocytes. Four 21-mer ODNs (AS-CD4-1, AS-CD4-2, AS-CD4-3, and AS-CD4-4) directed against the translation start region of the cd4 gene were designed. AS-CD4-1 was phosphorothioate (PS)-modified in each base, and the other three were PS-modified at both ends and in the internal pyrimidine residues. Four ODN controls (fully PS-modified ODN-A and partially modified ODN-B, ODN-C, and ODN-D) were also assayed. CD4 resynthesis was stimulated by treatment with phorbol 12-myristate 13-acetate (PMA) at the same time as the incubations with the ODN. After 24 hours of treatment, CD4 expression was measured by immunofluorescence staining and flow cytometry. CD4 reexpression in rat PMA-treated lymphocytes was counteracted by 40% by means of AS-CD4-2 and AS-CD4-4 treatments. On the other hand, AS-CD4-3 produced only 20% inhibition, similar to that produced by ODN-B, and AS-CD4-1 did not have any significant effect compared with control ODNs. Both AS-CD4-2 and AS-CD4-4 decreased CD4 mRNA, as determined by RT-PCR, and in addition, they did not affect the expression of other surface lymphocyte molecules. Inhibition of surface CD4 expression remained at least 72 hours. The addition of both AS-ODNs did not further increase the effect obtained separately by each AS-ODN. Treatment of rat PMA-lymphocytes with two concentrations of AS-CD4-2 and AS-CD4-4 added 24 hours apart did not further decrease CD4 expression. In summary, AS-CD4-2 and AS-CD4-4 could constitute a good strategy to inhibit CD4 expression on Th lymphocytes and modulate their function.

An organic CD4 inhibitor reduces the clinical and pathological symptoms of acute experimental allergic encephalomyelitis

CD4(+) T cells have an important role in mediating the pathogenesis of many human and experimental autoimmune diseases including experimental allergic encephalomyelitis (EAE), a demyelinating animal model for multiple sclerosis (MS). We applied a computer screening approach to select a small organic molecule, TJU103, that would specifically inhibit autoreactive CD4(+) T cells by disrupting the function of the CD4 molecule during activation. Upon studying the therapeutic effect of TJU103 in acute EAE, it was found that administration shortly before or after the onset of clinical symptoms reduced the severity of disease in both SJL and SWXJ-14 mouse models. In addition, TJU103 treatment could affect both in vivo responses to EAE rechallenge and secondary in vitro proliferation and cytokine production of T cells responding to proteolipid protein epitope 139-151 (PLPe). These results demonstrate the potential of the TJU103 organic inhibitor for future clinical application in CD4(+) T cell-mediated diseases.

Molecular function of the CD4 D1 domain in coreceptor-mediated entry by HIV type 1

The surface molecule CD4 plays a key role in initiating cellular entry by the human immunodeficiency virus type 1 (HIV-1), and it is now recognized as acting synergistically with select chemokine receptors (coreceptors) in the infection process. The present study was undertaken to determine whether the extracellular region of CD4 is sufficient to induce fusion of HIV-1 virions with target cells in the absence of its anchoring function. Using pseudotype reporter viruses to quantitate infection, soluble CD4 (sCD4) was tested for its ability to induce fusion by viruses utilizing CCR5 as their coreceptor. We found that sCD4 was competent to replace membrane-bound CD4 to trigger infection mediated by several HIV-1 envelopes. Furthermore, in a comparison of the envelopes of HIV-1 NL4-3 and a chimera containing the gp120 V3 loop of Ba-L, the V3 region was found to be one factor affecting susceptibility to induction by sCD4. In addition, using truncated and mutant derivatives of sCD4, the amino-terminal D1 domain of CD4 was found to be necessary and sufficient for induction of fusion and to require an intact gp120-binding site for this activity. These results delineate determinants on CD4 and gp120 required for fusion induction in collaboration with a coreceptor, and suggest a mechanism whereby CD4 may contribute to viral infection in trans.

Signaling lymphocytic activation molecule (CDw150) is homophilic but self-associates with very low affinity

Signaling lymphocytic activating molecule ((SLAM) CDw150) is a glycoprotein that belongs to the CD2 subset of the immunoglobulin superfamily and is expressed on the surface of activated T- and B-cells. It has been proposed that SLAM is homophilic and required for bidirectional signaling during T- and B-cell activation. Previous work has suggested that the affinity of SLAM self-association might be unusually high, undermining the concept that protein interactions mediating transient cell-cell contacts, such as those involving leukocytes, have to be weak in order that such contacts are readily reversible. Using surface plasmon resonance-based methods and analytical ultracentrifugation (AUC), we confirm that SLAM is homophilic. However, we also establish a new theoretical treatment of surface plasmon resonance-derived homophilic binding data, which indicates that SLAM-SLAM interactions (solution K(d) approximately 200 micrometer) are in fact considerably weaker than most other well characterized protein-protein interactions at the cell surface (solution K(d) approximately 0.4-20 micrometer), a conclusion that is supported by the AUC analysis. Whereas further analysis of the AUC data imply that SLAM could form "head to head" dimers spanning adjacent cells, the very low affinity raises important questions regarding the physiological role and/or properties of such interactions.

Wnt/frizzled-2 signaling induces aggregation and adhesion among cardiac myocytes by increased cadherin-beta-catenin complex

Wingless is known to be required for induction of cardiac mesoderm in Drosophila, but the function of Wnt family proteins, vertebrate homologues of wingless, in cardiac myocytes remains unknown. When medium conditioned by HEK293 cells overexpressing Wnt-3a or -5a was applied to cultured neonatal cardiac myocytes, Wnt proteins induced myocyte aggregation in the presence of fibroblasts, concomitant with increases in beta-catenin and N-cadherin in the myocytes and with E- and M-cadherins in the fibroblasts. The aggregation was inhibited by anti-N-cadherin antibody and induced by constitutively active beta-catenin, but was unaffected by dominant negative and dominant positive T cell factor (TCF) mutants. Thus, increased stabilization of complexed cadherin-beta-catenin in both cell types appears crucial for the morphological effect of Wnt on cardiac myocytes. Furthermore, myocytes overexpressing a dominant negative frizzled-2, but not a dominant negative frizzled-4, failed to aggregate in response to Wnt, indicating frizzled-2 to be the predominant receptor mediating aggregation. By contrast, analysis of bromodeoxyuridine incorporation and transcription of various cardiogenetic markers showed Wnt to have little or no impact on cell proliferation or differentiation. These findings suggest that a Wnt-frizzled-2 signaling pathway is centrally involved in the morphological arrangement of cardiac myocytes in neonatal heart through stabilization of complexed cadherin- beta-catenin.

Down regulation of CD4 expression following isolation and culture of human monocytes

The down regulation of CD4 by cultured monocytes has been observed by our group and by other investigators. Flow cytometric experiments were done to examine which factors might influence this phenomenon. The addition of lipopolysaccharide, granulocyte-macrophage colony-stimulating factor, macrophage colony-stimulating factor, or interleukin-10 to monocyte cultures failed to inhibit the decrease in monocyte CD4 expression routinely observed following overnight culture. The down regulation was an adherence-independent phenomenon and was not influenced by the type of anticoagulant into which the peripheral blood was collected or by the presence or absence of lymphocytes within the cultures. The avoidance of the use of Ficoll-Paque to isolate peripheral blood mononuclear cells did not prevent monocyte CD4 down regulation. Finally, by tagging monocyte CD4 with an anti-CD4 phycoerythrin-conjugated monoclonal antibody prior to culture, we were able to determine that the down regulation observed was the result of the internalization of the molecule. At this time, we conclude that the observed down regulation of monocyte CD4 is probably due to the differentiation of blood monocytes into tissue culture-derived macrophages rather than to some artifact of the isolation procedure.

Identification of a CD4 domain required for interleukin-16 binding and lymphocyte activation

Interleukin-16 (IL-16) activates CD4(+) cells, possibly by direct interaction with CD4. IL-16 structure and function are highly conserved across species, suggesting similar conservation of a putative IL-16 binding site on CD4. Comparison of the human CD4 amino acid sequence with that of several different species revealed that immunoglobulin-like domain 4 is the most conserved extracellular region. Potential interaction of this domain with IL-16 was studied by testing murine D4 sequence-based oligopeptides for inhibition of IL-16 chemoattractant activity and inhibition of IL-16 binding to CD4 in vitro. Three contiguous 12-residue D4 region peptides (designated A, B, and C) blocked IL-16 chemoattractant activity, with peptide B the most potent. Peptides A and B were synergistic for inhibition, but peptide C was not. Peptides A and B also blocked IL-16 binding to CD4 in vitro, whereas peptide C did not. CD4, in addition to its known function as a receptor for major histocompatibility complex class II, contains a binding site for IL-16 in the D4 domain. The D4 residues required for IL-16 binding overlap those previously shown to participate in CD4-CD4 dimerization following class II major histocompatibility complex binding, providing a mechanistic explanation for the known function of IL-16 to inhibit the mixed lymphocyte reaction.

Direct evidence for native CD4 oligomers in lymphoid and monocytoid cells

CD4 is expressed by T lymphocytes and monocytes and is generally considered a monomer even though its structure was originally modelled on the REI Bence-Jones homodimer. However, native CD4 was demonstrated as both monomer and dimers of 55 and 110 kDa in lymphoid and monocytoid cells by immunoprecipitation and immunoblotting after solubilization with alkylating (iodoacetamide) or reducing (dithiothreitol, 2-mercaptoethanol) reagents. Full reduction yielded only the 55-kDa monomeric form. Purified CD4 oligomers from CEM-T4 cells were also resolved as homodimers by MALDI-Tof mass fingerprinting after tryptic digestion. Cell treatment with the membrane impermeable, free-thiol reactive, 5,5'-dithiobis-2-nitrobenzoic acid enhanced cell surface CD4 dimers and tetramers. The interaction sites producing dimerization were probably in the D4 domain as OKT4 inhibited self association of recombinant CD4 (rCD4). Oligomerization of rCD4 by glutathione and thioredoxin indicates that thiol exchange interactions were responsible. Enhanced CD4 dimer expression was also observed after PMA (20 ng/ml) activation of THP-1 cells. These findings demonstrate that different quaternary forms of CD4 such as monomers, homodimers and tetramers are expressed by T lymphocytes and monocytes/macrophages.

Cloning and Modeling of the First Nonmammalian CD4

We have cloned and sequenced the first nonmammalian CD4 cDNA from the chicken using the COS cell expression method. Chicken CD4 contains four extracellular Ig domains that, in analogy to mammalian CD4, are in the order V, C2, V, and C2. The molecule is 24% identical with both human and mouse sequences. The extracellular domains were modeled using human and rat CD4 crystal structures as templates. In the first domain there are two extra Cys residues that are at suitable distance to form an intra-β-sheet disulfide bridge in addition to the canonical one in the V domain. The region responsible for the interaction with MHC class II is relatively nonconserved in chicken. However, there are positively charged amino acids in the C″ region of the N-terminal domain that may mediate the association to the negatively charged residues of the MHC class II β-chain. Molecular modeling also implies that the membrane-proximal domain mediates dimerization of chicken CD4 in a similar way as it does for human CD4. Furthermore, the cytoplasmic tail is highly conserved, containing the protein tyrosine kinase p56lck recognition site that is preceded by an adjacent di-leucine motif for the internalization of the molecule. Interestingly, there are no Ser residues in the cytoplasmic part, which may explain the slow down-regulation of chicken CD4 after phorbol ester stimulation.

Differential expression of inflammatory mediators in rat microglia cultured from different brain regions

Microglial cells show a rather uniform distribution of cell numbers throughout the brain with only minor prevalences in some brain regions. Their in situ morphologies, however, may vary markedly from elongated forms observed in apposition with neuronal fibers to spherical cell bodies with sometimes extremely elaborated branching. This heterogeneity gave rise to the hypothesis that these cells are differentially conditioned by their microenvironment and, therefore, also display specific patterns of differential gene expression. In this study, microglia were isolated from 2-4 week-old mixed CNS cultures that had been prepared from neonatal rat diencephalon, tegmentum, hippocampus, cerebellum and cerebral cortex, and were investigated 24 h later. Messenger RNA levels of proteins involved in crucial immune functions of this cell type (TNF-alpha, CD4, Fcgamma receptor II, and IL-3 receptor beta-subunit) have been determined by semi-quantitative RT-PCR. The results clearly show, that three of these mRNAs (TNF-alpha, CD4, Fcgamma receptor II) are differentially expressed in microglia with hippocampal microglia displaying the highest levels of these mRNAs. The data strongly support the notion that the status of microglial gene expression depends on their localization in brain and on specific interactions with other neural cell types. Consequently, it is hypothesized that their responsiveness to signals arising in injury or disease may vary from one brain region to another.

MFR, a putative receptor mediating the fusion of macrophages

We had previously identified a macrophage surface protein whose expression is highly induced, transient, and specific, as it is restricted to actively fusing macrophages in vitro and in vivo. This protein is recognized by monoclonal antibodies that block macrophage fusion. We have now purified this protein and cloned its corresponding cDNA. This protein belongs to the superfamily of immunoglobulins and is similar to immune antigen receptors such as the T-cell receptor, B-cell receptor, and viral receptors such as CD4. We have therefore named this protein macrophage fusion receptor (MFR). We show that the extracellular domain of MFR prevents fusion of macrophages in vitro and therefore propose that MFR belongs to the fusion machinery of macrophages. MFR is identical to SHPS-1 and BIT and is a homologue of P84, SIRPalpha, and MyD-1, all of which have been recently cloned and implicated in cell signaling and cell-cell interaction events.

Focal ischaemia of the rat brain elicits an unusual inflammatory response: early appearance of CD8+ macrophages/microglia

Cerebral ischaemia leads to profound glial activation and leukocyte infiltration into the infarct area. In this study, we provide evidence for a dual macrophage response in focal ischaemic lesions of the rat brain. We show that a considerable proportion of macrophages in the ischaemic lesions express the CD8alphabeta heterodimer to date only described on CD8+ T cells. As known from other lesion paradigms, CD4+ macrophages were also present. Interestingly, CD8- and CD4-expressing macrophages formed two non-overlapping subpopulations. CD8+ macrophages reached their maximum during the first week with pronounced downregulation thereafter whereas CD4+ cells persisted at high levels into the second week. In contrast to cerebral ischaemia, macrophages in the spleen and in Wallerian degeneration after optic nerve axotomy expressed CD4, but not CD8. In experimental autoimmune encephalomyelitis, CD8 was mainly associated with T cells and very weakly detectable on some ramified cells resembling activated microglia. In conclusion, we show that cerebral ischaemia triggers an unusual inflammatory response characterized by the appearance of CD8+/CD4- macrophages that might exert specific functions in the pathogenesis of ischaemic brain damage.

Expression, purification, and characterization of a murine CD4 fragment containing the first two N-terminal domains

CD4 is a membrane glycoprotein on T lymphocytes that binds to the same peptide:major histocompatibility complex (MHC) class II molecules recognized by the antigen-specific T cell receptor (TcR). Recent evidence supports the importance of coaggregation of CD4 and TcR for effective T cell activation. Here, we report that a transfected Chinese hamster ovary (CHO) cell line expressing a murine CD4 fragment containing the first two N-terminal domains secretes both monomeric molecules and disulfide-linked multimers. Elimination of the predicted N-linked glycosylation site at residue 161 that is next to the fourth cysteine does not affect the formation of interchain disulfide bonds. N-Terminal amino acid sequencing of the purified CD4 fragment demonstrates that the leader signal sequence is properly cleaved off the expressed protein. Circular dichroism studies suggest that both monomeric and disulfide-linked proteins are folded as primarily beta-sheet.

The design of a specific ligand of HIV gp120

The crystal structure of CD4 suggested that the C/G38 and C/L44 replacements with the consequent cystine bridge formation are compatible with the native structure of that molecular moiety. As the NQGSF sequence, corresponding to the 39-43 fragment of human CD4 protein, was found to be involved in the HIV gp120 interaction, it has been synthesized in a cyclic form by adding two cysteine residues at the amino and carboxy termini. 1H-NMR studies show that the predominant solution conformation of cyclo-[CNQGSFC] is a type II beta-turn centred on the NQGS segment. Structural and dynamic properties of the peptide are also analysed in relation to the in vitro activity.

CD80 (B7-1) binds both CD28 and CTLA-4 with a low affinity and very fast kinetics

The structurally related T cell surface molecules CD28 and CTLA-4 interact with cell surface ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells (APC) and modulate T cell antigen recognition. Preliminary reports have suggested that CD80 binds CTLA-4 and CD28 with affinities (Kd values approximately 12 and approximately 200 nM, respectively) that are high when compared with other molecular interactions that contribute to T cell-APC recognition. In the present study, we use surface plasmon resonance to measure the affinity and kinetics of CD80 binding to CD28 and CTLA-4. At 37 degrees C, soluble recombinant CD80 bound to CTLA-4 and CD28 with Kd values of 0.42 and 4 microM, respectively. Kinetic analysis indicated that these low affinities were the result of very fast dissociation rate constants (k(off)); sCD80 dissociated from CD28 and CTLA-4 with k(off) values of > or = 1.6 and > or = 0.43 s-1, respectively. Such rapid binding kinetics have also been reported for the T cell adhesion molecule CD2 and may be necessary to accommodate-dynamic T cell-APC contacts and to facilitate scanning of APC for antigen.

Cellular inflammatory response after spinal cord injury in Sprague-Dawley and Lewis rats

The distribution of microglia, macrophages, T-lymphocytes, and astrocytes was characterized throughout a spinal contusion lesion in Sprague-Dawley and Lewis rats by using immunohistochemistry. The morphology, spatial localization, and activation state of these inflammatory cells were described both qualitatively and quantitatively at 12 hours, 3, 7, 14, and 28 days after injury. By use of OX42 and ED1 antibodies, peak microglial activation was observed within the lesion epicenter of both rat strains between three and seven days post-injury preceding the bulk of monocyte influx and macrophage activation (seven days). Rostral and caudal to the injury site, microglial activation plateaued between two and four weeks post-injury in the dorsal and lateral funiculi as indicated by morphological transformation and the de-novo expression of major histocompatibility class II (MHC II) molecules. Similar to the timing of microglial reactions, T-lymphocytes maximally infiltrated the lesion epicenter between three and seven days post-injury. Reactive astrocytes, while present in the acute lesion, were more prominent at later survival times (7-28 days). These cells were interspersed with activated microglia but appeared to surround and enclose tissue sites occupied by reactive microglia and phagocytic macrophages. Thus, trauma-induced central nervous system (CNS) inflammation, regardless of strain, occurs rapidly at the site of injury and involves the activation of resident and recruited immune cells. In regions rostral or caudal to the epicenter, prolonged activation of inflammatory cells occurs preferentially in white matter and primarily consists of activated microglia and astrocytes. Differences were observed in the magnitude and duration of macrophage activation between Sprague-Dawley (SD) and Lewis (LEW) rats throughout the lesion. Increased expression of complement type 3 receptors (OX42) and macrophage-activation antigens (ED1) persisted for longer times in LEW rats while expression of MHC class II molecules was attenuated in LEW compared to SD rats at all times examined. Variations in the onset and duration of T-lymphocyte infiltration also were observed between strains with twice as many T-cells present in the lesion epicenter of Lewis rats by 3 days post-injury. These strain-specific findings potentially represent differences in corticosteroid regulation of immunity and may help predict a range of functional neurologic consequences affected by neuroimmune interactions.

Immunocytochemical study of retinal diode laser photocoagulation in the rat

AIM

To determine the nature of the cellular infiltrate, alterations in cell adhesion molecules, and MHC II antigen expression in the rat retina following diode laser retinal photocoagulation.

METHOD

20 normal Lister rats underwent diode laser photocoagulation of the retina. Frozen sections from eyes enucleated at 0, 1, 5, 13, and 33 days post laser were examined for T cells (R7.3), CD4 T cells (W3/25), activated CD4 T cells (OX-40), CD8 T cells (OX-8), B cells (OX-33), and macrophages (OX-42), MHC II antigen (OX-6), and E-Selectin-1, VCAM-1, and ICAM-1.

RESULTS

Retinal diode laser photocoagulation stimulated a wound healing response in the outer retina and choroid. The cellular infiltrate included macrophages and activated CD4 T cells at 13 and 33 days post laser. Glial cells in the inner plexiform and inner nuclear layers expressed MHC II antigen at 24 hours only. ICAM-1 antigen was induced in RPE cells and in Muller cells in the inner retina at all time intervals post laser and intense staining for ICAM-1 was present around intraretinal migrated cells at 13 and 33 days post laser. VCAM-1 antigen expression was induced in the choroidal vascular endothelium and RPE at 13 and 33 days after laser as was E-Selectin-1 antigen expression which was also evident focally at the external limiting membrane in association with migrated cells adjacent to the burn.

CONCLUSIONS

These results suggest that alterations in cell adhesion molecules may regulate the migration and activation of retinal pigment epithelium, macrophages and CD4 T cells at the outer blood-retinal barrier and choroid following diode laser photocoagulation of the normal Lister rat retina.

Postbinding events mediated by human immunodeficiency virus type 1 are sensitive to modifications in the D4-transmembrane linker region of CD4

Evidence from both structural and functional studies of the CD4 molecule suggests that several domains, including the transmembrane (TM) domain and the adjoining extracellular region (D4-TM linker), contribute to the post-gp12O-binding events leading to human immunodeficiency virus-mediated membrane fusion. To investigate such a role in syncytium formation and cell-free infectivity, we generated several deletion and substitution mutations in the TM and D4-TM linker regions of the CD4 molecule. We found that while the TM domain of CD4 was dispensable for cell-cell and virus-cell interactions, modifications in the D4-TM linker led to perturbations in both processes. Deletion of the five amino acid residues linking D4 to the TM domain resulted in a delayed and reduced capacity to form syncytia, whereas replacement of the residues with the heterologous sequence from the CD8 molecule restored the kinetic profile to wild-type CD4 levels. On the other hand, both mutants of the CD4 D4-TM linker demonstrated delayed cell-free human immunodeficiency virus type 1 infectivity profiles. The defective fusion capacity may be linked to structural perturbations identified with anti-CD4 monoclonal antibodies in the D1-D2 interface and D3 domain of the deletion mutant yet absent in D1 and D4. While all cells were found to bind comparable levels of gp120, both D4-TM linker mutants appeared to induce a decrease in the V3 loop exposure of bound gp120. This underexposure may explain the delays in cell-free infectivities observed for both of these mutants. Together, these findings confirm a role for regions of the CD4 molecule located outside D1 in post-gp120-binding events and suggest that the D4-TM interface contributes to the conformational changes that direct the fusion process.

Envelope glycoproteins of human immunodeficiency virus type 1: profound influences on immune functions

Infection by human immunodeficiency virus type 1 (HIV-1) leads to progressive destruction of the CD4+ T-cell subset, resulting in immune deficiency and AIDS. The specific binding of the viral external envelope glycoprotein of HIV-1, gp120, to the CD4 molecules initiates viral entry. In the past few years, several studies have indicated that the interaction of HIV-1 envelope glycoprotein with cells and molecules of the immune system leads to pleiotropic biological effects on immune functions, which include effects on differentiation of CD34+ lymphoid progenitor cells and thymocytes, aberrant activation and cytokine secretion patterns of mature T cells, induction of apoptosis, B-cell hyperactivity, inhibition of T-cell dependent B-cell differentiation, modulation of macrophage functions, interactions with components of complement, and effects on neuronal cells. The amino acid sequence homologies of the envelope glycoproteins with several cellular proteins have suggested that molecular mimicry may play a role in the pathogenesis of the disease. This review summarizes work done by several investigators demonstrating the profound biological effects of envelope glycoproteins of HIV-1 on immune system cells. Extensive studies have also been done on interactions of the viral envelope proteins with components of the immune system which may be important for eliciting a "protective immune response." Understanding the influences of HIV-1 envelope glycoproteins on the immune system may provide valuable insights into HIV-1 disease pathogenesis and carries implications for the trials of HIV-1 envelope protein vaccines and immunotherapeutics.

Degenerating and regenerating skeletal muscles contain several subpopulations of macrophages with distinct spatial and temporal distributions

Macrophages of different phenotypes can be detected using a panel of antibodies. We have used such antibodies to demonstrate that the macrophages in freeze-lesioned skeletal muscles are heterogeneous, with each subtype having a distinct location within the lesion as well as distinct times of arrival and departure from the lesion. ED1+ monocytes and macrophages began invading the lesion within 3 h and were abundant until necrotic tissue had been removed. In some macrophages, the ED1 antigen aggregated into a single or a few clumps and such cells persisted in the regenerated area for at least 21 d. ED2+/Ox6-/ED1-/RM1- cells are one of the major subpopulations of resident macrophages within skeletal muscle. Cells of this phenotype accumulated in the epimysia and perimysia surrounding the lesions but did not penetrate into the lesion until extensive phagocytosis had occurred (usually 1 or 2 d). ED2+ cells were subsequently concentrated in the regenerating connective tissues and empty remnants of phagocytosed fibres. They only rarely invaded necrotic tissue, even when immediately adjacent to it, suggesting that this type of macrophage has a specialised function which is unrelated to removal of damaged tissue. The ED2+ macrophages were CD4+ and it is probably that macrophages of this type have been previously misclassified as CD4+ T cells. Skeletal muscles also contain numerous Ox6(Ia)+/ED2- resident macrophages. Unlike ED2+ macrophages, Ox6+ macrophages invaded the damaged muscles half a day after lesioning and were abundant in necrotic tissue. As regeneration occurred, the Ox6+ macrophages became restricted to the connective tissues of the muscle, which is their normal location.

Analysis of recombinant and native CD4 by one- and two-dimensional gel electrophoresis

Knowledge of CD4 conformation within the membranes of human lymphoid and monocytoid cells is essential for a clear understanding of its function as a ligand for major histocompatibility complex II (MHC) molecules in T cell activation and for gp120 in human immunodeficiency virus (HIV) infection. The charge and structure of native (nCD4) and soluble recombinant CD4 (rCD4) were examined by one- and two-dimensional (2-DE) electrophoresis antigen mapping and silver staining. Recombinant CD4 was partitioned by nonequilibrium pH gradient electrophoresis (NEPHGE) and revealed a number of differentially charged 44 kDa species (pI > 9.5). Biotinylation (4 h, room temperature) of rCD4 yielded a single labelled species on sodium dodedyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) with an increased apparent molecular mass to 50 kDa, consistent with a maximal incorporation of approximately 18 molecules of biotin per rCD4 molecule. The milder biotinylation (15 min, 4 degrees C) of cell-(CEM-T4, THP-1) expressed CD4 was not accompanied by any apparent alteration in molecular weight, nor abrogation of CD4 antigenicity. This was determined by isolation of nCD4 by immunoprecipitation and SDS-PAGE immunoblotting, using anti-CD4 mAbs (leu3a, OKT4A, Q4120, T4, OKT4, Q425) and by flow cytometry (leu4a, T4). The immunoprecipitation of full-length native CD4 from lymphoid MT2 and CEM-T4 cell extracts, however, revealed both monomeric and higher-order CD4 antigen complexes by immunoblotting. These studies describe the biotinylation, 1-DE and 2-DE of CD4 preparations, and indicate the capacity of CD4 of lymphocytes to form complexes which may influence CD4 conformation and epitope availability.

Mechanisms of acquired thymic tolerance in experimental autoimmune encephalomyelitis: thymic dendritic-enriched cells induce specific peripheral T cell unresponsiveness in vivo

Experimental autoimmune encephalomyelitis (EAE), an experimental model for the study of multiple sclerosis, is an autoimmune disease of the central nervous system that can be induced in a number of species by immunization with myelin basic protein (MBP). MBP-reactive CD4+ T cells, predominantly expressing the V beta 8.2 T cell receptor (TCR), migrate from the peripheral lymphoid organs and initiate the inflammatory response in the brain. We have previously shown that a single intrathymic injection of MBP or its major encephalitogenic peptide (p71-90), but not a nonencephalitogenic peptide (p21-40), induces antigen-specific systemic tolerance and inhibits the induction of EAE in Lewis rats. In this study, we investigated the mechanisms of induction and maintenance of acquired thymic tolerance in this model. First, we investigated which thymic cell is responsible for "induction" of systemic tolerance. Thymic dendritic-enriched cells, isolated by plastic adherence, when incubated in vitro with p71-90 and injected intravenously into Lewis rats, were capable of preventing the development of EAE, but his protection was lost in thymectomized recipients. In addition, intravenous injection of thymic dendritic cells isolated from animals that had been previously injected intrathymically with p71-90 but not p21-40 also prevented the development of EAE. Second, to determine the "effector" mechanisms involved in acquired thymic tolerance, we compared TCR expression in the brains of animals with actively induced EAE with TCR expression in animals that received intrathymic injection of p71-90 or p21-40. Using a semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) technique, we found increased expression of CD4 and V beta 8.2 message in brains of immunized animals compared with those of naive animals. In animals intrathymically injected with p71-90 but not p21-40, CD4 and V beta 8.2 transcript levels were significantly reduced compared with immunized controls. Immunohistologic studies of brain tissue and spleens with specific V beta 8.2 and control V beta 10 monoclonal antibodies confirmed these observations in vivo. These findings, taken together with recent data demonstrating that activated T cells circulate through the thymus, suggest that interaction of thymic dendritic cells with specific TCR of activated peripheral T cells can lead to inactivation of these antigen-specific cells and confirm the role of V beta 8.2-expressing T cells in EAE.

Capturing genes encoding membrane and secreted proteins important for mouse development

A strategy based on the gene trap was developed to prescreen mouse embryonic stem cells for insertional mutations in genes encoding secreted and membrane-spanning proteins. The "secretory trap" relies on capturing the N-terminal signal sequence of an endogenous gene to generate an active beta-galactosidase fusion protein. Insertions were found in a cadherin gene, an unc6-related laminin (netrin) gene, the sek receptor tyrosine kinase gene, and genes encoding two receptor-linked protein-tyrosine phosphatases, LAR and PTP kappa. Analysis of homozygous mice carrying insertions in LAR and PTP kappa showed that both genes were effectively disrupted, but neither was essential for normal embryonic development.

The human immunodeficiency virus type 1 (HIV-1) CD4 receptor and its central role in promotion of HIV-1 infection

Interactions between the viral envelope glycoprotein gp120 and the cell surface receptor CD4 are responsible for the entry of human immunodeficiency virus type 1 (HIV-1) into host cells in the vast majority of cases. HIV-1 replication is commonly followed by the disappearance or receptor downmodulation of cell surface CD4. This potentially renders cells nonsusceptible to subsequent infection by HIV-1, as well as by other viruses that use CD4 as a portal of entry. Disappearance of CD4 from the cell surface is mediated by several different viral proteins that act at various stages through the course of the viral life cycle, and it occurs in T-cell lines, peripheral blood CD4+ lymphocytes, and monocytes of both primary and cell line origin. At the cell surface, gp120 itself and in the form of antigen-antibody complexes can trigger cellular pathways leading to CD4 internalization. Intracellularly, the mechanisms leading to CD4 downmodulation by HIV-1 are multiple and complex; these include degradation of CD4 by Vpu, formation of intracellular complexes between CD4 and the envelope precursor gp160, and internalization by the Nef protein. Each of the above doubtless contributes to the ultimate depletion of cell surface CD4, although the relative contribution of each mechanism and the manner in which they interact remain to be definitively established.

Molecular dynamics studies of the human CD4 protein

A dynamical model for an N-terminal fragment of the human CD4 protein has been determined by computer simulation. The protein has been studied both in vacuo and in solution. Data from both simulations agree moderately well with each other and with the crystal structure. All elements of secondary structure were retained during simulation. Point mutation and sequence replacement studies have shown that a loop in CD4, residues 40-52 is involved in binding with gp120, the human immunodeficiency virus surface glycoprotein. Our results show that the gp120-binding loop and a few regions which bind to monoclonal antibodies and class II MHC molecules are the most highly motile areas of the protein. These results are consistent with the suggestion that CD4 binds to target molecules by using induced-fit contacts.

Human cell-adhesion molecule CD2 binds CD58 (LFA-3) with a very low affinity and an extremely fast dissociation rate but does not bind CD48 or CD59

CD2 is a T lymphocyte cell-adhesion molecule (CAM) belonging to the immunoglobulin superfamily (IgSF) which mediates transient adhesion of T cells to antigen-presenting cells and target cells. Reported ligands for human CD2 include the structurally-related IgSF CAMs CD58 (LFA-3) and CD48 as well as, more controversially, the unrelated cell-surface glycoprotein CD59. Using surface plasmon resonance technology, which avoids several pitfalls of conventional binding assays, we recently reported that rat CD2 binds rat CD48 with a very low affinity (Kd 60-90 microM) and dissociates rapidly (koff > or = 6 s-1) [van der Merwe, P. A., Brown, M. H., Davis, S. J., & Barclay, A. N. (1993) EMBO J. 12, 4945-4954]. In contrast, a study using conventional equilibrium binding methods reported a much higher affinity (Kd 0.4 microM) for human CD2 binding CD58 which suggested that the weak binding of rat CD2 to CD48 may not represent a typical CAM interaction. In the present study we have used surface plasmon resonance to obtain definitive affinity and kinetic data on the interactions of a soluble, recombinant form of human CD2 with soluble forms of CD58, CD48, and CD59. Binding of CD2 to CD58 was readily detected but we were unable to detect any direct interaction between CD2 and either CD59 or CD48 under conditions in which very low affinity interactions (Kd approximately 0.5 mM) would have been detected. In contrast to previous reports we found that human CD2 bound CD58 with a very low affinity (Kd 9-22 microM) and dissociated with an extremely fast dissociation rate constant (koff > or = 4 s-1). The association rate constant (kon) could not be measured directly but was calculated to be > or = 400,000 M-1s-1. Taken together, these results provide conclusive evidence that CAM interactions can have very low affinities and extremely fast dissociation rate constants.