Molecular characteristics of recombinant human CD4 as deduced from polymorphic crystals

Characterization of a variant CD4 molecule in Japanese Black cattle

Monoclonal antibodies (mAbs) that recognize cluster of differentiation (CD) molecules on lymphocytes are useful tools for the study of different lymphocyte subsets in flow cytometry (FCM) analysis. CD4 is a glycoprotein found on the surfaces of helper T cells, monocytes, macrophages, and dendritic cells. In this study, we describe Japanese Black (JB) calves in a farm whose peripheral blood mononuclear cells (PBMCs) did not react with a CD4-specific mAb. To identify calves with PBMCs with low mAb reactivity, PBMCs from 21 JB calves (1-12 months of age) bred at the same farm were examined using two different bovine CD4 mAbs (clones #CC8 and #CACT138A). FCM analysis indicated that the calves fell into two groups based on reactivity against the two mAbs, i.e., double-positive (DP) calves, whose PBMCs were recognized by both mAbs clones, and single-positive (SP) calves, whose PBMCs were only recognized by #CACT138A. PBMCs from seven calves were not recognized by #CC8, although they had normal reactivity with another mAb, #CACT138A. Sequencing analysis of the CD4 gene in these calves revealed four nucleotide substitutions (G918 T, A930C, G970A, and G1074A) in the coding region in the SP group when compared to the DP group. Three of the four mutations were associated with amino acid substitution (Q306H, K310 N, and A324 T). The substitution at A324 T was located in the D4 domain of CD4 gene. Homology modeling based on the amino acid sequences revealed that the surface structure of this part of the molecule was significantly different between the SP and the DP groups. Therefore, the epitope recognized by the #CC8 CD4 mAb was altered in calves with this genetic mutation, and this led to the low reactivity of the PBMCs from calves in the SP group aginst the #CC8 mAb. In conclusion, this is the first study to identify CD4 variants in JB cattle. We confirmed that the variants did not affect lymphocyte functions, such as mitogen stimulation or lipopolysaccharide-induced cytokine gene expression.

Preparation and characterization of monoclonal antibodies recognizing two CD4 isotypes of Microminipigs

Cluster of differentiation 4 (CD4) molecule expressed on the leukocytes is known to function as a co-receptor for class II major histocompatibility complex (MHC) binding to T cell receptor (TCR) on helper T cells. We previously identified two CD4 alleles (CD4.A and CD4.B) in a Microminipig population based on nucleotide sequencing and PCR detection of their gene sequences. However, CD4.B protein expression was not examined because of the unavailability of a reactive antibody to a CD4.B epitope. In this study, we have produced two swine-specific monoclonal antibodies (mAbs) against CD4.B molecules, one that recognizes only CD4.B (b1D7) and the other that recognizes both the CD4.A and CD4.B alleles (x1E10) and that can be used to distinguish CD4 T cell subsets by flow cytometry and immunohistochemistry. Using these two mAbs, we identified CD4.A and CD4.B allele-specific proteins on the surface of CD4.A (+/+) and CD4.B (+/+) T cells at a similar level of expression. Moreover, stimulation of peripheral blood mononuclear cells (PBMCs) derived from CD4.A (+/+) and CD4.B (+/+) swine with toxic shock syndrome toxin-1 (TSST-1) in vitro similarly activated both groups of cells that exhibited a slight increase in the CD4/CD8 double positive (DP) cell ratio. A large portion of the DP cells from the allelic CD4.A (+/+) and CD4.B (+/+) groups enhanced the total CD4 and class I swine leukocyte antigen (SLA) expression. The x1E10 mAb delayed and reduced the TSST-1-induced activation of CD4 T cells. Thus, CD4.B appears to be a functional protein whose expression on activated T cells is analogous to CD4.A.

A polymorphic CD4 epitope related to increased susceptibility to Babesia bovis in Canchim calves

Different allelic forms of bovine CD4 were previously described in cattle and were also observed in Canchim calves examined in the present experiment. However, the functional relevance of these different CD4 phenotypes has not yet been investigated. CD4 + T helper cells are known to play a central role in immune control against Babesia bovis infection. Thus, our study aimed to compare the profiles of immune cells, specific antibody titers and blood infection levels measured by qPCR (quantitative polymerase chain reaction) in calves naturally infected with B. bovis, phenotyped as CD4- (absence of anti-CD4 staining), CD4 + (intermediate staining) or CD4 ++ (high staining). The CD4 mRNA precursor was also measured in these animals. Calves with the CD4- phenotype showed higher amounts of B. bovis DNA in blood samples, compared to the other CD4 phenotypes. It was also observed that these calves with higher levels of infection had lower amounts of natural killer cells and higher expression of the CD4 gene, which can be interpreted as a compensation for the failure of the altered CD4 receptor to recognize relevant B. bovis epitopes.

MHC Molecules, T cell Receptors, Natural Killer Cell Receptors, and Viral Immunoevasins-Key Elements of Adaptive and Innate Immunity

Molecules encoded by the Major Histocompatibility Complex (MHC) bind self or foreign peptides and display these at the cell surface for recognition by receptors on T lymphocytes (designated T cell receptors-TCR) or on natural killer (NK) cells. These ligand/receptor interactions govern T cell and NK cell development as well as activation of T memory and effector cells. Such cells participate in immunological processes that regulate immunity to various pathogens, resistance and susceptibility to cancer, and autoimmunity. The past few decades have witnessed the accumulation of a huge knowledge base of the molecular structures of MHC molecules bound to numerous peptides, of TCRs with specificity for many different peptide/MHC (pMHC) complexes, of NK cell receptors (NKR), of MHC-like viral immunoevasins, and of pMHC/TCR and pMHC/NKR complexes. This chapter reviews the structural principles that govern peptide/MHC (pMHC), pMHC/TCR, and pMHC/NKR interactions, for both MHC class I (MHC-I) and MHC class II (MHC-II) molecules. In addition, we discuss the structures of several representative MHC-like molecules. These include host molecules that have distinct biological functions, as well as virus-encoded molecules that contribute to the evasion of the immune response.

Identification and characterization of two CD4 alleles in Microminipigs

Background

We previously identified two phenotypes of CD4+ cells with and without reactions to anti-pig CD4 monoclonal antibodies by flow cytometry in a herd of Microminipigs. In this study, we analyzed the coding sequences of CD4 and certified the expression of CD4 molecules in order to identify the genetic sequence variants responsible for the positive and negative PBMCs reactivity to anti-pig CD4 monoclonal antibodies.

Results

We identified two CD4 alleles, CD4.A and CD4.B, corresponding to antibody positive and negative, respectively, by nucleotide sequencing of PCR products using CD4 specific primer pairs. In comparison with the swine CD4 amino-acid sequence [GenBank: NP_001001908], CD4.A had seven amino-acid substitutions and CD4.B had 15 amino-acid substitutions. The amino-acid sequences within domain 1 of CD4.B were identical to the swine CD4.2 [GenBank: CAA46584] sequence that had been reported previously to be a modified CD4 molecule that had lost reactivity with an anti-pig CD4 antibody in NIH miniature pigs. Homozygous and heterozygous CD4.A and CD4.B alleles in the Microminipigs herd were characterised by using the RFLP technique with the restriction endonuclease, BseRI. The anti-pig CD4 antibody recognized pig PBMCs with CD4.AA and CD4.AB, but did not recognized those with CD4.BB. We transfected HeLa cells with the FLAG-tagged CD4.A or CD4.B vectors, and certified that transfected HeLa cells expressed FLAG in both vectors. The failure of cells to react with anti-CD4 antibodies in CD4.B pigs was associated to ten amino-acid substitutions in domain 1 and/or one amino-acid substitution in joining region 3 of CD4.B. We also found exon 8 was defective in some CD4.A and CD4.B resulting in the loss of the transmembrane domain, which implies that these CD4 proteins are secreted from helper T cells into the circulation.

Conclusions

We identified that amino-acids substitutions of domain 1 in CD4.B gave rise to the failure of some CD4 expressing cells to react with particular anti-pig CD4 monoclonal antibodies. In addition, we developed a PCR-RFLP method that enabled us to simply identify the CD4 sequence variant and the positive and negative PBMCs reactivity to our anti-pig CD4 monoclonal antibodies without the need to use flow cytometric analysis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-016-0856-8) contains supplementary material, which is available to authorized users.

A specific CD4 epitope bound by tregalizumab mediates activation of regulatory T cells by a unique signaling pathway

CD4⁺CD25⁺ regulatory T cells (Tregs) represent a specialized subpopulation of T cells, which are essential for maintaining peripheral tolerance and preventing autoimmunity. The immunomodulatory effects of Tregs depend on their activation status. Here we show that, in contrast to conventional anti-CD4 monoclonal antibodies (mAbs), the humanized CD4-specific monoclonal antibody tregalizumab (BT-061) is able to selectively activate the suppressive properties of Tregs in vitro. BT-061 activates Tregs by binding to CD4 and activation of signaling downstream pathways. The specific functionality of BT-061 may be explained by the recognition of a unique, conformational epitope on domain 2 of the CD4 molecule that is not recognized by other anti-CD4 mAbs. We found that, due to this special epitope binding, BT-061 induces a unique phosphorylation of T-cell receptor complex-associated signaling molecules. This is sufficient to activate the function of Tregs without activating effector T cells. Furthermore, BT-061 does not induce the release of pro-inflammatory cytokines. These results demonstrate that BT-061 stimulation via the CD4 receptor is able to induce T-cell receptor-independent activation of Tregs. Selective activation of Tregs via CD4 is a promising approach for the treatment of autoimmune diseases where insufficient Treg activity has been described. Clinical investigation of this new approach is currently ongoing.

Optimization of CD4/gp120 inhibitors by thermodynamic-guided alanine-scanning mutagenesis

As protein/protein interactions usually trigger signalling processes, inhibitors of those interactions must preclude protein binding without eliciting the signalling process themselves. To accomplish those goals, small molecules need to target those protein residues that contribute the most to binding (binding hotspots) without disturbing those residues that initiate signalling processes (allosteric hotspots). The availability of a blueprint identifying binding and allosteric hotspots will significantly aid inhibitor design and optimization. In this study, we show that in some situations the blueprint can be constructed by combining the standard technique of alanine-scanning mutagenesis with isothermal titration calorimetry (ITC). We demonstrate the approach by developing the combined binding and allosteric hotspots blueprint for CD4/gp120, the initial interaction leading to HIV-1 cell infection. A major finding of these studies is that not all binding hotspots are allosteric hotspots opening the possibility for the rational design of inhibitors and antagonist or agonist modulators.

Role of human CD4 D1D2 domain in HIV-1 infection

Broadly neutralizing antibodies and appropriate immunogens are critical for preexposure prophylaxis and therapeutic HIV vaccines. In this study, we aimed to explore effective antibodies against the genetically diverse HIV-1 strains by investigating the roles of human CD4 D1D2 domain and nonvariable immugens. The human CD4 D1D2 domain and the chimeric protein of mouse D1 domain/human D2 domain were expressed in Sf9 insect cells and purified by gel-filtration chromatography. The human CD4 D1D2 domain potently inhibited the infection of 77.8% HIV-1 pseudoviruses, including the clades AE, B' and BC, with less than 20 μg/mL of IC(50). pcDNA3.1-mhD1D2m and pcDNA3.1-mhD2m plasmids were used for the production of mouse anti-human CD4 polyclonal antibodies. The neutralizing activities of the polyclonal antibodies were determined by using pseudotyped HIV-1 viruses. The antibodies induced by plasmids containing human CD4 D1D2 domain were able to potently inhibit all pseudotyped HIV-1 strains. The antibodies from mhD1D2m-immunized mice also showed strong binding capacity to CD4 expressed on the surface of TZM-bl cells. The potent and broad inhibitory activity of antibodies against the human CD4 D1D2 domain may be used to develop effective passive immunization agent to control the spread of HIV infection.

Epitope mapping of ibalizumab, a humanized anti-CD4 monoclonal antibody with anti-HIV-1 activity in infected patients

Ibalizumab is a humanized monoclonal antibody that binds human CD4, the primary receptor for human immunodeficiency virus type 1 (HIV-1). With its unique specificity for domain 2 of CD4, this antibody potently and broadly blocks HIV-1 infection in vitro by inhibiting a postbinding step required for viral entry but without interfering with major histocompatibility complex class II (MHC-II)-mediated immune function. In clinical trials, ibalizumab has demonstrated anti-HIV-1 activity in patients without causing immunosuppression. Thus, a characterization of the ibalizumab epitope was conducted in an attempt to gain insight into the underlying mechanism of its antiviral activity as well as its safety profile. By studying mouse/human chimeric CD4 molecules and site-directed point mutants of CD4, amino acids L96, P121, P122, and Q163 in domain 2 were found to be important for ibalizumab binding, with E77 and S79 in domain 1 also contributing. All these residues appear to cluster on the interface between domains 1 and 2 of human CD4 on a surface opposite the site where gp120 and the MHC-II molecule bind on domain 1. Separately, the epitope of M-T441, a weakly neutralizing mouse monoclonal antibody that competes with ibalizumab, was localized entirely within domain 2 on residues 123 to 125 and 138 to 140. The results reported herein not only provide an appreciation for why ibalizumab has not had significant adverse immunological consequences in infected patients to date but also raise possible steric hindrance mechanisms by which this antibody blocks HIV-1 entry into a CD4-positive cell.

Structure and function of the HIV envelope glycoprotein as entry mediator, vaccine immunogen, and target for inhibitors

This chapter discusses the advances of the envelope glycoprotein (Env) structure as related to the interactions of conserved Env structures with receptor molecules and antibodies with implications for the design of vaccine immunogens and inhibitors. The human immunodeficiency virus (HIV) Env binds to cell surface–associated receptor (CD4) and coreceptor (CCR5 or CXCR4) by one of its two non-covalently associated subunits, gp120. The induced conformational changes activate the other subunit (gp41), which causes the fusion of the viral with the plasma cell membranes resulting in the delivery of the viral genome into the cell and the initiation of the infection cycle. As the only HIV protein exposed to the environment, the Env is also a major immunogen to which neutralizing antibodies are directed and a target that is relatively easy to access by inhibitors. A fundamental problem in the development of effective vaccines and inhibitors against HIV is the rapid generation of alterations at high levels of expression during long chronic infection and the resulting significant heterogeneity of the Env. The preservation of the Env function as an entry mediator and limitations on size and expression impose restrictions on its variability and lead to the existence of conserved structures.

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.

Marked differences in the structures and protein associations of lymphocyte and monocyte CD4: resolution of a novel CD4 isoform

The structures, molecular interactions and functions of CD4 in a subset of T lymphocytes have been well characterized. The CD4 receptors of other cell types have, however, been poorly documented. We have previously shown that lymphocytes and monocytes/macrophages differ in their expression of CD4 monomers and dimers. In the present study, we have shown further significant differences. Variability in the blocking of CD4 mAb binding by sulfated polyanions indicated differences in exofacial CD4 structures. In contrast to the well-documented 55 kDa monomers in lymphocytic cells, monocytic cells were found to coexpress two monomer isoforms: the 55 kDa form and a novel 59 kDa species. Experimental uncoupling of CD4 disulfides indicated that the oxidized 55 kDa monomer could be converted to the 59 kDa form. This was achieved by chemical reduction of purified native or recombinant CD4, or in cell transfection experiments by mutation of cysteine to alanine in domain 1 (D1) (Cys16 or Cys84) and in domain 4 (D4) (Cys303 or Cys345). All of these modifications promote CD4 distension on SDS-PAGE analysis and indicate that, when CD4 inter-beta-sheet disulfides in the D1 and D4 Ig folds are disrupted, there is an unravelling of the oxidized form to an extended 59 kDa unfolded state. We hypothesize that this may be a transition-state, structural-intermediate in the formation of disulfide-linked homodimers. Also identified were CD4-tyrosine kinase dissimilarities in which lymphocyte CD4 associated with Lck, but monocyte CD4 associated with HcK. These findings show that there is complex heterogeneity in structures and interactions in the CD4 of T lymphocytes and monocytes.

Real-time analysis of human immunodeficiency virus type 1 Env-mediated membrane fusion by fluorescence resonance energy transfer

Human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env)-mediated membrane fusion occurs as a sequence of events that is triggered by CD4 binding to the Env gp120 subunit. In this study, we analyzed the dynamics of Env-mediated membrane fusion at the single-cell level using fluorescent fusion proteins and confocal laser fluorescent microscopy. Either enhanced cyan or yellow fluorescent protein (CFP and YFP, respectively) was fused to the end of the cytoplasmic regions of the HIV-1 receptors (CD4 and CCR5) and Env proteins. Real-time imaging of membrane fusion mediated by these recombinant proteins revealed that the kinetics of fusion in our system was faster than that previously reported. Analysis of the receptor interaction by fluorescence resonance energy transfer (FRET) at the single-cell level demonstrated a tendency for oligomerization of CD4-CD4, but not of CD4-CCR5, in the absence of Env-expressing cells. However, when Env-expressing cells attached to the receptor cells, FRET produced by CD4-CCR5 interaction was increased; the FRET intensity began to decline before the formation of the fusion pore. These changes in FRET may represent the temporal association of these receptors, triggered by gp120 binding, and their dissociation during the formation of the fusion pore. In addition, the FRET analysis of receptor interactions in the presence of fusion inhibitors showed that not only inhibitors acting on CCR5 but also the gp41-derived peptide T-20 interfered with CD4-CCR5 interaction during fusion. These data suggest that T-20 could affect the formation of Env-receptors complexes during the membrane fusion.

CD4 is expressed by epidermal Langerhans' cells predominantly as covalent dimers

Langerhans' cells (LC) of skin are CD4 expressing, dendritic, antigen-presenting cells, that are essential for activation of primary immune responses and are productively infected by HIV. We have shown previously that lymphocytes and monocytes express CD4 both as monomers and covalently linked homodimers. In those cells the 55-kDa monomer structure predominates. LC in un-fractionated human epidermal cell (EC) suspension also expresses both forms of CD4, but in EC the dimer form is predominant. Because isolation of LC into single cell suspension by trypsin, as is routinely used for LC isolation, degrades CD4, a systematic study for an alternate procedure for LC isolation was performed. Thus it was found that collagenase blend F treatment can efficiently release LC into suspension, under conditions of only minimal degradation of control soluble recombinant CD4 or CEM-T4 or THP-1 cell CD4, or importantly of LC surface CD4. SDS-PAGE immunoblotting of purified LC extracted from EC by collagenase confirmed CD4 structure as predominantly 110-kDa dimers, with only minimal 55-kDa monomers. The suitability of LC prepared thus for functional studies was demonstrated with binding of functional ligand HIV gp120. It remains to be determined, however, why tissue embedded LC express mainly CD4 dimers, but single-celled blood lymphocytes and monocytes mainly monomers.

Enhancing protein crystallization through precipitant synergy

Suitable conditions for protein crystallization are commonly identified by screening combinations of independent factors that affect crystal formation. Because precipitating agents are prime determinants of crystallization, we investigated whether a systematic exploration of combinations of mechanistically distinct precipitants would enhance crystallization. A crystallization screen containing 64 precipitant mixtures was devised. Tests with ten HIV envelope-related proteins demonstrated that use of precipitant mixtures significantly enhanced both the probability of crystallization as well as the quality of optimized crystals. Tests with hen egg white lysozyme generated a novel C2 crystal from a salt/organic solvent mixture; structure solution at 2 A resolution revealed a lattice held together by both hydrophobic and electrostatic dyad interactions. The results indicate that mechanistically distinct precipitants can synergize, with precipitant combinations adding unique dimensions to protein crystallization.

Modulation of CD4 T cell function by soluble MHC II-peptide chimeras

Peptides antigens of 8 to 24 amino acid residues in length that are derived from processing of foreign proteins by antigen presenting cells (APC), and then presented to T cells in the context of major histocompatibility complex molecules (MHC) expressed by APC, are the only physiological ligands for T cell receptor (TCR). Co-ligation of TCR and CD4 co-receptor on T cells by MHC II-peptide complexes (signal 1) leads to various T cell functions depending on the nature of TCR and CD4 co-ligation, and whether costimulatory receptors (signal 2) such as CD28, CTLA-4, CD40L are involved in this interaction. Recently, the advance of genetic engineering led to the generation of a new class of antigen-specific ligands for TCR, i.e., soluble MHC class I-, and MHC class II-peptide chimeras. In principle, these chimeric molecules consist of an antigenic peptide which is covalently linked to the amino terminus of alpha-chain in the case of MHC I, or beta-chains in the case of MHC II molecules. Conceptually, such TCR/CD4 ligands shall provide the signal 1 to T cells. Since soluble MHC-peptide chimeras showed remarkable regulatory effects on peptide-specific T cells in vitro and in vivo, they may represent a new generation of immunospecific T cell modulators with potential therapeutic applicability in autoimmune and infectious diseases. This review is focused on the immunomodulatory effects of soluble, MHC class II-peptide chimeras, and discuss these effects in the context of the most accepted theories on T cell regulation.

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.

Inhibitory effect of interleukin-16 on interleukin-2 production by CD4+ T cells

Signalling through CD4 by human immunodeficiency virus (HIV)-1 envelope glycoprotein (gpl20) and/or anti-CD4 antibodies can promote T-cell activation and anergy. Interleukin (IL)-16 is a competence growth factor for CD4+ T cells that can induce a G0 to G1 cell cycle transition but cannot induce cell division. The receptor of this cytokine is thought to be the CD4 molecule, although the binding epitope of IL-16 differs from that of HIV. We have demonstrated that both HIV-1/gp120 and IL-16 induced CD4+ T-cell dysfunction, as indicated by suppression of mitogen-induced IL-2 production. Two anti-CD4 antibodies with different binding sites on CD4 also showed an inhibitory effect on IL-2 production. These results indicate that promotion of CD4+ T-cell anergy via the CD4 molecule does not depend on the binding sites of the CD4 ligands.

Staining of cell surface human CD4 with 2'-F-pyrimidine-containing RNA aptamers for flow cytometry

We have used recombinant human CD4 presented on beads as an affinity matrix to screen a 2'-F-pyrimidine-containing RNA library with a complexity of approximately 10(14) molecules. Affinity-selected aptamers bind recombinant CD4 with low nanomolar equilibrium dissociation constants. These high-affinity aptamers conjugated to different fluorophores such as fluorescein and phycoerythrin were used to stain cells, expressing human CD4 on cell surface, for analysis by flow cytometry. Aptamers, conjugated to fluorophores, stained mouse T cells that express human CD4 on the surface, but not the control mouse T cells lacking human CD4. The control cells, however, do express mouse CD4 whose extracellular domain has 55% sequence identity to the human form. These human CD4-specific aptamers selectively stained CD4(+) T cells in a preparation of human peripheral blood mononuclear cells. These results and others suggest that aptamers are emerging as a versatile class of molecules that can be used for various diagnostic applications performed under different formats or platforms.

A CXCR4/CD4 pseudotype rhabdovirus that selectively infects HIV-1 envelope protein-expressing cells

We show that a cellular virus receptor functions in the envelope of a virus, allowing selective infection of cells displaying the receptor ligand. A G-deficient rabies virus (RV) pseudotyped with CD4- and CXCR4-derived proteins selectively infected cells expressing HIV-1 envelope protein. Envelope protein or CD4 antibodies blocked virus entry. Pseudotype virus formation was most efficient with chimeric receptor proteins possessing the cytoplasmic tail of the RV G spike protein (CXCR4-RV and CD4-RV). While CXCR4-RV was incorporated when expressed alone, CD4-RV incorporation required CXCR4-RV as a carrier protein, indicating a mechanism by which oligomeric surface proteins are sorted into the RV envelope. Viral vectors bearing virus receptors in their envelope may be useful reagents for targeting virus-infected cells in vivo.

Dimeric association and segmental variability in the structure of human CD4

CD4 is a co-receptor in the cellular immune response. It increases the avidity of association between a T cell and an antigen-presenting cell by interacting with non-polymorphic portions of the complex between class II major histocompatibility complex (MHC) and T-cell receptor (TCR) molecules, and it contributes directly to signal transduction through its cytoplasmic association with the lymphocyte kinase Lck. CD4 also serves as the high-affinity receptor for cellular attachment and entry of the human immunodeficiency virus (HIV). The extracellular portion of CD4 comprises four immunoglobulin-like domains (D1-D4). This part of human CD4 (residues 1-369) has been characterized as a recombinant soluble protein (sCD4), and crystal structures have been described for the human D1D2 fragment and for the rat D3D4 fragment. We have now determined the structures of intact sCD4 in three crystal lattices. These structures have a hinge-like variability at the D1D2 to D3D4 junction that might be important in immune recognition and HIV fusion, and a common dimeric association through D4 domains. Dynamic light scattering measurements and chemical crosslinking of sCD4 corroborate dimerization at high protein concentration. We suggest that such dimers mayhave relevance as mediators of signal transduction in T cells.

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.

Production of crystallizable fragments of membrane proteins

Many membrane proteins feature autonomously folded extramembranous domains which, when isolated from the intact protein, perform biochemical functions relevant to biological activity. Whereas intact membrane proteins usually require detergent solubilization for purification, most extramembranous fragments are soluble in aqueous solution. If appropriately constructed, such fragments are often crystallizable and the resulting atomic structures can lead to important biological insight. In most instances, these fragments are produced in recombinant expression systems. To be crystallizable, molecular fragments should be uniform in composition and conformation and be available in abundance. Considerations for the production of crystallizable fragments of membrane proteins include the definition of fragment boundaries, the control of nonuniformities introduced by glycosylation of phosphorylation, and optimization of expression systems. These aspects are addressed here in general terms and in the case studies of applications to CD4, CD8, the insulin receptor kinase, and N-cadherin.

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.

Molecular recognition of antigen involves lattice formation between CD4, MHC class II and TCR molecules

Recent evidence indicates that CD4 stably binds to major histocompatibility complex (MHC) class II only after assuming an oligomeric state: the membrane-distal CD4 D1-D2 module interacts directly with MHC class II, whereas the membrane-proximal CD4 D3-D4 module mediates oligomerization. This results in the formation of aggregates critical for T-cell activation. The T-cell receptor (TCR) regulates specific crosslinking and is itself dependent on lattice formation to trigger physiological T-cell responses. Here, Toshiko Sakihama, Alex Smolyar and Ellis Reinherz discuss the molecular nature of CD4-MHC class II clustering and how, despite each of the component interactions being of low affinity, the molecular matrix renders T-cell recognition extremely specific and sensitive.

A human recombinant Fab identifies a human immunodeficiency virus type 1-induced conformational change in cell surface-expressed CD4

To explore the role of the CD4 molecule in human immunodeficiency virus (HIV) infection following initial virus-CD4 binding, we have characterized CD4-specific antibodies raised by immunizing an HIV-1-infected human with human recombinant soluble CD4 (rsCD4). Fabs were selected from a human recombinant Fab library constructed from the bone marrow of this immunized individual. Here, we describe a human rsCD4-specific recombinant Fab clone selected by panning the library over complexes of human rsCD4 and recombinant HIV-1 envelope protein. While this Fab does not bind to CD4-positive T-cell lines or to human T lymphocytes, it recognizes cell surface-expressed CD4 following the incubation of these cells with a recombinant form of HIV-1 gp120 or with HIV-1 virions. The Fab is not HIV-1 envelope specific, since it does not bind to recombinant gp120 or to native cell surface-expressed HIV-1 envelope proteins. As confirmation of its CD4 specificity, we show that this Fab immunoprecipitates a 55-kDa protein, corresponding to the molecular mass of cellular CD4, from an H9 cell lysate. The specificity of this human Fab provides evidence for a virus-induced conformational change in cell surface-expressed on CD4. The characterization of this altered CD4 conformation and its effects on the host cell will be important in defining postbinding events in HIV infection.

Oligomerization of CD4 is required for stable binding to class II major histocompatibility complex proteins but not for interaction with human immunodeficiency virus gp120

Previous studies have failed to detect an interaction between monomeric soluble CD4 (sCD4) and class II major histocompatibility complex (MHC) proteins, suggesting that oligomerization of CD4 on the cell surface may be required to form a stable class II MHC binding site. To test this possibility, we transfected the F43I CD4 mutant, which is incapable of binding to class II MHC or human immunodeficiency virus (HIV) gp120, into COS-7 cells together with wild-type CD4 (wtCD4). Expression of F43I results in a dominant negative effect: no class II MHC binding is observed even though wtCD4 expression is preserved. Apparently, F43I associates with wtCD4 oligomers and interferes with the formation of functional class II MHC binding structures. In contrast, F43I does not affect the binding of gp120 to wtCD4, implying that gp120 binds to a CD4 monomer. By production and characterization of chimeric CD4 molecules, we show that domains 3 and/or 4 appear to be involved in oligomerization. Several models of the CD4-class II MHC interaction are offered, including the possibility that one or two CD4 molecules initially interact with class II MHC dimers and further associate to create larger complexes important for facilitating T-cell receptor crosslinking.

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.

Crystal structure of an extracellular fragment of the rat CD4 receptor containing domains 3 and 4

BACKGROUND

CD4 is a transmembrane protein on the surface of T lymphocytes that interacts with MHC class II proteins at the surface of accessory cells, and is involved in the triggering of the lymphocytes by foreign antigens. It is also the major receptor for the human immunodeficiency virus. The extracellular portion of CD4 was predicted to contain four immunoglobulin superfamily domains and this has been confirmed by X-ray crystallography, but no detailed structure of domains 3 and 4 has been available.

RESULTS

We now report the expression of a form of rat CD4 containing only domains 3 and 4, its crystallization, and the refinement and analysis of its structure by X-ray crystallography with 2.6 A spacing data. Both domains show variations in core residues when compared with immunoglobulin domains. Features of the structure are discussed with respect to the structure of the complete extracellular part of CD4 and its function.

CONCLUSIONS

Domains 3 and 4 of CD4 show considerable similarity to domains 1 and 2, although there is a 25 degrees rotation in the relative positions of the domains with respect to one another. The absence of the disulphide bond in domain 3 is associated with an alteration in the packing of the beta-sheets, which may be important for interactions with domain 2 in the overall receptor structure. The location of N-linked glycosylation on one face of domain 3 appears to preclude the dimerization that is observed in antibodies.

Structures of an HIV and MHC binding fragment from human CD4 as refined in two crystal lattices

BACKGROUND

The T-cell surface glycoprotein CD4 interacts with class II molecules of the major histocompatibility complex (MHC) enhancing the signal for T-cell activation. Human CD4 also interacts, at high affinity, with the HIV envelope glycoprotein, gp120, to mediate T-cell infection by HIV. Crystal structures of amino-terminal two-domain (D1D2) fragments of human CD4, which contain the residues implicated in HIV and MHC interactions, have been reported earlier.

RESULTS

We have determined the crystal structure of a new D1D2 construct by molecular replacement from a previously described crystal structure of D1D2. This structure has more uniform lattice contacts than are in the first. This gives an improved image of domain D2, which in turn has permitted further refinement of the initial structure at 2.3 A resolution against a more complete data set. The structure of the second crystal form was also refined at 2.9 A resolution. In both models, all residues from 1 to 178 are now well defined, including the loop regions in D2.

CONCLUSIONS

Similarities of the molecular structure in the two lattices suggest that the D1D2 fragment works as a unit, with segmental flexibility largely restricted to the junction between domains D2 and D3. Variability of conformation in loops, including those implicated in MHC and HIV binding, requires an 'induced fit' in these interactions. Well defined density for the exposed side chain of Phe43 in both crystals confirms a prominent role for this residue in gp120 binding.

Human immunodeficiency virus type 1-associated CD4 downmodulation

This chapter discusses human immunodeficiency virus type 1 (HIV-1) associated with CD4 downmodulation. It also discusses the structure and function of CD4 and p56^lck and factors involved in hiv-1-associated cd4 downmodulation. There are, at present, at least three HIV-1 gene products known to be involved in cell surface CD4 downmodulation. These are Nef, Vpu, and gp160. Whereas Nef is expressed during the early phase of HIV-1 gene expression, both Vpu and gp160, which appear to act coordinately, are expressed during the late phase. This functional convergence of HIV-1 proteins on cell surface CD4 downmodulation, whether specific or nonspecific in activity, suggests that this event is of critical importance in the life cycle of HIV-1. Further elucidation of the mechanisms that underlie CD4 cell surface downmodulation may lead to the development of novel strategies aimed at preventing such events, and potentially to the development of new therapeutic approaches.

CD4 and the immunoglobulin superfamily

The CD4 membrane glycoprotein was one of the first cell surface antigens to be identified using monoclonal antibodies. It was shown to have a central role in the control of the recognition of foreign proteins by T lymphocytes and later as a receptor for the human immunodeficiency virus (HIV). The analysis of the amino acid sequence of CD4 showed that the extracellular region comprised four regions with sequence similarities to immunoglobulin domains. The structure of domains 3 and 4 of CD4 has been determined by X-ray crystallography and, like domains 1 and 2 previously determined, these have typical immunoglobulin-like folds. The results are discussed with respect to the identification of other domains with immunoglobulin-like folds from amino acid sequence data, and the evolution of CD4.

Structural properties of the myelin-associated glycoprotein ectodomain

Myelin-associated glycoprotein (MAG) has been proposed to mediate adhesive interactions during myelin development. We have used the baculovirus expression system to produce a truncated form of this molecule [soluble extracellular domain of MAG (sMAG)] consisting of the complete extracellular ectodomain. Spectroscopic studies indicate a high beta-sheet content, consistent with the prediction of Ig-like structure. Hydrodynamic studies indicate an asymmetric monomer, with a Stokes radius of 4.1-4.6 nm, a sedimentation coefficient of 3.6S, and a frictional ratio of approximately 1.6. We postulate that the outer two Ig-like domains form a unit that folds back over the rest of the molecule. Fluorescence quenching studies indicate that sMAG interacts with divalent cations and may have a functional lectin domain.

Three-dimensional structure of the human class II histocompatibility antigen HLA-DR1

The three-dimensional structure of the class II histocompatibility glycoprotein HLA-DR1 from human B-cell membranes has been determined by X-ray crystallography and is similar to that of class I HLA. Peptides are bound in an extended conformation that projects from both ends of an 'open-ended' antigen-binding groove. A prominent non-polar pocket into which an 'anchoring' peptide side chain fits is near one end of the binding groove. A dimer of the class II alpha beta heterodimers is seen in the crystal forms of HLA-DR1, suggesting class II HLA dimerization as a mechanism for initiating the cytoplasmic signalling events in T-cell activation.

Crystal structure of domains 3 and 4 of rat CD4: relation to the NH2-terminal domains

The CD4 antigen is a membrane glycoprotein of T lymphocytes that interacts with major histocompatibility complex class II antigens and is also a receptor for the human immunodeficiency virus. the extracellular portion of CD4 is predicted to fold into four immunoglobulin-like domains. The crystal structure of the third and fourth domains of rat CD4 was solved at 2.8 angstrom resolution and shows that both domains have immunoglobulin folds. Domain 3, however, lacks the disulfide between the beta sheets; this results in an expansion of the domain. There is a difference of 30 degrees in the orientation between domains 3 and 4 when compared with domains 1 and 2. The two CD4 fragment structures provide a basis from which models of the overall receptor can be proposed. These models suggest an extended structure comprising two rigid portions joined by a short and possibly flexible linker region.

Crystal structure at 2.8 A resolution of a soluble form of the cell adhesion molecule CD2

The crystal structure of a soluble form of the T lymphocyte antigen CD2 provides the first complete view of the extracellular region of a cell adhesion molecule. The topology of the molecule, which comprises two immunoglobulin-like domains, is the same as that of the first two domains of CD4 but the relative domain orientation is altered by a fairly flexible linker region. The putative ligand-binding beta-sheet forms a flat surface towards the top of the molecule. Crystal contacts between these surfaces suggest a plausible model for the adhesive interaction.

Unusual amino acid sequence of the second Ig-like domain of the feline CD4 protein

We have cloned and sequenced the cDNA for cat CD4. The overall amino-acid sequence of cat CD4 is similar to that from the primate and rodent CD4 molecules, with a 58% identity between the cat and human sequences. Comparison to the crystal structure of human CD4 does, however, reveal unusual features in the second Ig-like domain, D2, of cat CD4. First, a reciprocal substitution between a tryptophan and a cysteine, this latter involved in an intrasheet disulfide bond of human D2, is predicted to generate an intrastrand disulfide bond, a feature rarely observed in an Ig-fold. Second, a large serine-threonine-rich insertion is found between the A and B beta strands of D2. This sequence is a potential O-linked glycosylation site, and should protrude in a region that appears flexible in human CD4. This unusual insertion could affect the interaction of cat CD4 with class II molecules, or with FIV, a feline homolog of HIV. The expression of cat CD4 in different environment, or of a mutated human CD4 carrying the cat insertion, should help in understanding the role of cat CD4 as a putative receptor for FIV, and the CD4/MHC class II interaction.

Atomic structure of a fragment of human CD4 containing two immunoglobulin-like domains

The structure of an N-terminal fragment of CD4 has been determined to 2.4 A resolution. It has two tightly abutting domains connected by a continuous beta strand. Both have the immunoglobulin fold, but domain 2 has a truncated beta barrel and a non-standard disulphide bond. The binding sites for monoclonal antibodies, class II major histocompatibility complex molecules, and human immunodeficiency virus gp120 can be mapped on the molecular surface.