Immunosuppressive and autoimmune effects of thimerosal in mice.

The possible health effects of the organic mercury compound thimerosal (ethylmercurithiosalicylate), which is rapidly metabolized to ethylmercury (EtHg), have recently been much debated and the effect of this compound on the immune system is largely unknown. We therefore studied the effect of thimerosal by treating A.SW (H-2s) mice, susceptible to induction of autoimmunity by heavy metals, with 10 mg thimerosal/L drinking water (internal dose ca 590 microg Hg/kg body weight/day) for up to 30 days. The lymph node expression of IL-2 and IL-15 mRNA was increased after 2 days, and of IL-4 and IFN-gamma mRNA after 6 and 14 days. During the first 14 days treatment, the number of splenocytes, including T and B cells as well as Ig-secreting cells decreased. A strong immunostimulation superseded after 30 days treatment with increase in splenic weight, number of splenocytes including T and B cells and Ig-secreting cells, and Th2- as well as Th-1-dependent serum immunoglobulins. Antinucleolar antibodies (ANoA) targeting the 34-kDa nucleolar protein fibrillarin, and systemic immune-complex deposits developed. The H-2s strains SJL and B10.S also responded to thimerosal treatment with ANoA. The A.TL and B10.TL strain, sharing background genes with the A.SW and B10.S strain, respectively, but with a different H-2 haplotype (t1), did not develop ANoA, linking the susceptibility to H-2. Thimerosal-treated H-2s mice homozygous for the nu mutation (SJL-nu/nu), or lacking the T-cell co-stimulatory molecule CD28 (B10.S-CD28-/-), did not develop ANoA, which showed that the autoimmune response is T-cell dependent. Using H-2s strains with targeted mutations, we found that IFN-gamma and IL-6, but not IL-4, is important for induction of ANoA by thimerosal. The maximum added renal concentration of thimerosal (EtHg) and inorganic mercury occurred after 14 days treatment and was 81 microg Hg/g. EtHg made up 59% and inorganic mercury 41% of the renal mercury. In conclusion, the organic mercury compound thimerosal (EtHg) has initial immunosuppressive effects similar to those of MeHg. However, in contrast to MeHg, thimerosal treatment leads in genetically susceptible mice to a second phase with strong immunostimulation and autoimmunity, which is T-cell dependent, H-2 linked and may at least partly be due to the inorganic mercury derived from the metabolism of ethyl mercury.

Transposon mutagenesis of Mb0100 at the ppe1-nrp locus in Mycobacterium bovis disrupts phthiocerol dimycocerosate (PDIM) and glycosylphenol-PDIM biosynthesis, producing an avirulent strain with vaccine properties at least equal to those of M. bovis BCG.

The unusual and complex cell wall of pathogenic mycobacteria plays a major role in pathogenesis, with specific complex lipids acting as defensive, offensive, or adaptive effectors of virulence. The phthiocerol and phthiodiolone dimycocerosate esters (PDIMs) comprise one such category of virulence-enhancing lipids. Recent work in several laboratories has established that the Mycobacterium tuberculosis fadD26-mmpL7 (Rv2930-Rv2942) locus plays a major role in PDIM biosynthesis and secretion and that PDIM is required for virulence. Here we describe two independent transposon mutants (WAg533 and WAg537) of Mycobacterium bovis, both of which carry an insertion in Mb0100 (= M. tuberculosis Rv0097) to reveal a new locus involved in PDIM biosynthesis. The mutations have a polar effect on expression of the downstream genes Mb0101, Mb0102 (fadD10), Mb0103, and Mb0104 (nrp), and Mb0100 is shown to be in an operon comprising these genes and Mb0099. Reverse transcription-PCR analysis shows elevated transcription of genes in the operon upstream from the transposon insertion sites in both mutants. Both mutants have altered colony morphology and do not synthesize PDIMs or glycosylphenol-PDIM. Both mutants are avirulent in a guinea pig model of tuberculosis, and when tested as a vaccine, WAg533 conferred protective immunity against M. bovis infection at least equal to that afforded by M. bovis bacillus Calmette-Guerin.

Identification of the epitope of a monoclonal antibody to DJ-1.

Mutations in DJ-1 can cause early onset parkinsonism. Various antibodies have been generated to detect this protein, one of which is a commonly used monoclonal antibody (clone 3E8). Since results of in situ examinations of DJ-1 expression with this antibody have differed from analyses with species-specific antibodies (e.g. rat), it would be useful to know the epitope for this antibody. Using GFP-tagged deletion constructs of human DJ-1, we have localized the epitope region for this antibody to within residues 56-78 of human DJ-1. Mapping this region to the published three-dimensional structure of DJ-1 indicates that this is a solvent-accessible surface epitope. Immunonegativity of E64D mutant DJ-1 with the monoclonal antibody suggests that glutamate 64 of human DJ-1 contributes to the epitope recognized by this antibody. Moreover, the loss of immunoreactivity due to such a small substitution demonstrates the remarkable sensitivity of the monoclonal antibody 3E8 to DJ-1.

A species-specific determinant on beta2-microglobulin required for Ly49A recognition of its MHC class I ligand.

The mouse inhibitory NK cell receptor Ly49A recognizes the mouse MHC class I molecule H-2D(k). The present study focuses on the species specificity of beta(2)-microglobulin (beta(2)m), an invariant component of MHC class I, in the interaction between Ly49A and H-2D(k). Transfection of the beta(2)m-defective mouse cell line R1E/TL8x.1 with human (h) beta(2)m induced cell-surface expression of H-2D(k), but failed to protect the cells from killing by Ly49A(+) NK cells. In contrast, the cells transfected with mouse (m) beta(2)m were protected from killing by Ly49A(+) NK cells. These data indicate that Ly49A distinguishes mbeta(2)m from hbeta(2)m when it recognizes the H-2D(k) complexes. To identify the species-specific determinant of beta(2)m required for Ly49A recognition of H-2D(k), we prepared a panel of mbeta(2)m mutants and tested the H-2D(k) that included each of the beta(2)m mutants for its capacity to engage Ly49A on NK cells. Ly49A failed to functionally recognize the H-2D(k) that included the mbeta(2)m with K3R and Q29G mutations. Moreover, Ly49A was able to recognize the H-2D(k) that included the hbeta(2)m with R3K and G29Q mutations. These data indicate that Lys3 and Gln29 consist of the central part of the species-specific determinant of beta(2)m required for Ly49A recognition of H-2D(k). The two residues are conserved in the mouse and the rat, in which NK cells use Ly49 family molecules as the receptors specific for MHC class I. These results suggest functional importance of beta(2)m in NK cell recognition of target cells.

Epitope mapping of neutralizing TSST-1 specific antibodies induced by immunization with toxin or toxoids.

Toxic shock syndrome toxin-1 (TSST-1), a superantigen produced by Staphylococcus aureus, is a potent stimulator of the immune system. T-cells are activated by crosslinking of MHC class II molecules on antigen presenting cells with T-cell receptors (TCR). TSST-1 is associated with the majority of the cases of menstrual staphylococcal toxic shock, a severe and life-threatening multisystem disorder. Even though antibody mediated protection has been studied, information on antibody specificity directed to individual antigenic determinants of the protein is incomplete. To obtain immunogens with low toxicity, we generated a double-site mutant (dmTSST-1), modified at solvent-exposed residues predicted to be important for both MHC class II and TCR binding, and detoxified recombinantly expressed TSST-1 (rTSST-1) as well as native TSST-1 (nTSST-1) isolated from Staphylococcus aureus by treatment with formaldehyde. Rabbits were immunized with rTSST-1, nTSST-1, dmTSST-1, and formaldehyde inactivated toxoids. The sera obtained were used to map the antigen-reactive regions of the molecule and to identify specificities of antibodies induced by immunization with the different antigens. To detect linear antigenic epitopes of TSST-1 the reactivity of the sera with 11-meric peptides having an overhang of four residues, covering the entire molecule of TSST-1, have been studied. We found that sera of TSST-1 immunized rabbits predominantly reacted with N-terminal residues 1-15, while sera generated with formaldehyde inactivated toxoid recognized a total of 7 regions located at the N- and C-terminus and internal sites of TSST-1. Despite different specificities all sera were able to inhibit TSST-1 induced proliferation of human mononuclear cells.

The analgesic domain of interferon-alpha2b contains an essential proline(39) residue.

OBJECTIVE: It has been demonstrated that there are structural and functional similarities between interferon-alpha (INF-alpha) and endorphins. We have reported that there are distinct domains in the IFN-alpha molecule that mediate immune and analgesic effects, respectively, and that the opioid-like analgesic effect of IFN-alpha is mediated by the mu opioid receptor, thus inferring that the analgesic domain of IFN-alpha consists of Tyr(122) and the residues around Tyr(122) in the tertiary structure. The aim of this work was to further explore the molecular basis for the analgesic domain of IFN-alpha. METHODS: By using site-directed mutagenesis, the structure of IFN-alpha was changed and a mutant of IFN-alpha was obtained. Then, the antiviral activity and opioid-like analgesic activity of IFN-alpha were measured. RESULTS: When the Pro(39) residue of IFN-alpha, which is located close to the Tyr(122) residue in the tertiary structure, was mutated to Gly, the analgesic activity of this mutant was lost completely, but the antiviral activity of IFN-alpha was maintained compared with wild-type IFN-alpha. CONCLUSIONS: Combining the sequence of the endomorphin, it is suggested that the Pro(39) residue is one of the constituents of the analgesic domain of IFN-alpha and contributes to IFN-alpha binding to the mu opioid receptor.

Mapping of a hapten-binding site: molecular modeling and site-directed mutagenesis study of an anti-atrazine antibody.

A three-dimensional model of the variable domain of the atrazine-specific Fab fragment K411B was constructed by molecular modeling using known structures of highly homologous immunoglobulins as templates. Molecular dynamic simulations and cross-reactivity data were used to predict residues responsible for the binding of the hapten 4-chloro-6-(isopropylamino)-1,3,5-triazine-2-(6-aminohexanecarboxylic acid) (iPr/Cl/C6) instead of atrazine. Specific binding pockets could be defined for the chlorine, the isopropylamino group and the C6-spacer of the hapten. The influence of various amino acids on hapten binding was investigated by site-directed mutagenesis, and the effect of these mutations was analyzed by capture ELISA using the hapten iPr/Cl/C6 and 4-amino-6-chloro-1,3,5-triazine-2-(6-aminohexanecarboxylic acid) (H/Cl/C6). GlyH100a seems to be important in determining the conformation of the heavy-chain complementarity determining region H3; replacing it with any other residue prevented the binding of the hapten. Altering residues responsible for the binding of the chlorine atom (TrpH33, GluH50 and TyrL96) decreased the affinity significantly. Hapten-spacer recognition can be attributed to the interaction with PheL32; replacing PheL32 by leucine reduced the affinity towards iPr/Cl/C6. A triple mutant Fab fragment (GlnL89Glu, ValH37Ile and GluL3Val) showed an affinity 5-fold greater towards iPr/Cl/C6 compared to the wild-type K411B, as a result of better recognition of the isopropylamino group of iPr/Cl/C6.

Mutation of a single conserved residue in VH complementarity-determining region 2 results in a severe Ig secretion defect.

During an immune response, somatic mutations are introduced into the VH and VL regions of Ig chains. The consequences of somatic mutation in highly conserved residues are poorly understood. Ile51 is present in 91% of murine VH complementarity-determining region 2 sequences, and we demonstrate that single Ile51-->Arg or Lys substitutions in the PCG1-1 Ab are sufficient to severely reduce Ig secretion (1-3% of wild-type (WT) levels). Mutant H chains, expressed in the presence of excess L chain, associate with Ig binding protein (BiP) and GRP94 and fail to form HL and H2L assembly intermediates efficiently. The mutations do not irreversibly alter the VH domain as the small amount of mutant H chain, which assembles with L chain as H2L2, is secreted. The secreted mutant Ab binds phosphocholine-protein with avidity identical with that of WT Ab, suggesting that the combining site adopts a WT conformation. A computer-generated model of the PCG1-1 variable region fragment of Ig (Fv) indicates that Ile51 is buried between complementarity-determining region 2 and framework 3 and does not directly contact the L chain. Thus, the Ile51-->Arg or Ile51-->Lys mutations impair association with the PCG1-1 L chain via indirect interactions. These interactions are in part dependent on the nature of the L chain as the PCG1-1 VH single Ile51-->Arg or Ile51-->Lys mutants were partially rescued when expressed with the J558L lambda1 L chain. These results represent the first demonstration that single somatic mutations in V(H) residues can impair Ig secretion and suggest one reason for the conservation of Ile51 in so many Ig VH.

Defining the requirements for peptide recognition in gene therapy-induced T cell tolerance.

Expression of a retrovirally transduced MHC class I Ag, H-2K(b) (K(b)), in bone marrow-derived cells leads to specific prolongation of K(b) disparate skin grafts. To examine the extent to which peptides derived from K(b) contribute to the induction of tolerance, retroviruses carrying mutant K(b) genes designed to enter separate pathways of Ag presentation were constructed. Thymectomized and CD8 T cell-depleted mice that had been irradiated and reconstituted with bone marrow cells expressing a secreted form of K(b) showed prolongation of K(b) disparate skin graft survival. Skin graft prolongation was not observed when similar experiments were performed using mice that were not CD8 T cell depleted. This suggests that hyporesponsiveness can be induced in CD4 T cells, but not CD8 T cells by Ags presented via the exogenous pathway of Ag processing. Modest prolongation of skin allografts was observed in mice reconstituted with bone marrow cells transduced with retroviruses carrying a gene encoding a mutant K(b) molecule expressed only in the cytoplasm. Prolongation was also observed in similar experiments in mice that were thymectomized and CD4 T cell depleted following complete reconstitution, but not in mice that were reconstituted and then thymectomized and CD8 T cell depleted. Thus, hyporesponsiveness can be induced in a subset of CD8 T cells by recognition of peptides derived from K(b) through both the direct and indirect pathways of Ag recognition, while CD4 T cell hyporesponsiveness to MHC class I disparate grafts occurs only through the indirect pathway of Ag recognition.

Interleukin-2: structural and biological relatedness to opioid peptides.

Interleukin (IL)-2 is not only an immunoregulatory factor, but also an analgesic molecule. There are distinct domains of immune and analgesic functions in the IL-2 molecule. The analgesic domain is located around the 45th Tyr residue of human IL-2 in tertiary structure. Antiopioid (beta-endorphin, Leu-enkephalin, Met-enkephalin and dynorphin A1-13) sera partially neutralized the analgesic activity of IL-2. Monoclonal antibody against the IL-2 receptor alpha subunit (Tac) could not block the analgesic activity of IL-2. There existed cross-reactivity between IL-2 and antiopioid sera by indirect ELISA. These studies show strong structural and biological similarities between IL-2 and opioid peptides. The tertiary structure around the 45th residue of IL-2 composes the analgesic domain that is similar to that of endogenous opioids. These results are consistent with the hypothesis that multiple domains of cytokines serve as the structural bases for the immunoregulatory and neuroregulatory effects of cytokines.

Structure-function studies of an anti-asialo GM1 antibody obtained from a phage display library.

Although gangliosides elicit human autoantibodies, they are extremely weak immunogens in mice. We obtained a monoclonal antibody Fab fragment (clone 10) that is specific for asialo GM1 (GA1), from a phage display library. The Vkappa domain of clone 10 could be replaced by two different Vkappa domains without changing the specificity of the antibody. Mutagenesis of the third hypervariable regions of the heavy and light chains of clone 10 yielded three mutants that exhibited a 3 to 4 times increase in avidity for GA1. A molecular model of clone 10 indicated that the putative antigen-binding site contained a shallow surface pocket. These data illustrate the use of recombinant DNA techniques to obtain anti-ganglioside antibodies, and to explore the molecular basis of their antigen-binding activity.

DNA immunization with HIV-1 tat mutated in the trans activation domain induces humoral and cellular immune responses against wild-type Tat.

Intramuscular immunization of mice with plasmids encoding two transdominant negative mutants of the HIV-1 Tat protein (Tat22 and Tat22/37) elicited a humoral response to wild-type Tat that is comparable to that induced by inoculation of wild-type tat DNA or Tat protein. The percentage of the responders and the Ab titers continued to increase after three additional DNA boosts and pretreatment with bupivacaine at the site of inoculation, without a significant difference (p > 0.05) among the three groups of mice immunized with mutant and wild-type tat genes. By utilizing synthetic peptides representing the amino acid sequence of Tat, one major B cell epitope was defined within the cysteine-rich domain of Tat. Anti-Tat IgG Abs directed against this epitope were found in mice immunized with all tat DNA constructs, whereas different Tat epitopes were detected in mice immunized with the Tat protein. Similarly, IgG2a was the predominant isotype in DNA-immunized mice, with both mutants and wild-type tat genes, as compared with protein immunization, which induced mostly IgG1 and IgG3. Sera from most immunized mice neutralized the effect of extracellular Tat in activating HIV-1 replication. A cellular response was also elicited as indicated by the proliferation of splenocytes when stimulated with wild-type Tat. These results indicate that the wild-type Tat Ag is recognized by Abs and T cells induced by DNA immunization with mutated tat genes, suggesting the possible use of these Tat transdominant mutants, lacking viral trans activation activity and capable of blocking wild-type Tat activity, in the development of an anti-HIV-1 vaccine.

Cutting edge: effects of an allergy-associated mutation in the human IL-4R alpha (Q576R) on human IL-4-induced signal transduction.

A mutation in the human (hu) IL-4R alpha, Q576R, has been linked with allergy in humans. Increased sensitivity of patients cells with this mutation to IL-4 suggest that a Q576R change enhances IL-4 signaling. To directly test this hypothesis, we analyzed the ability of huIL-4R alpha cDNA bearing the Q576R and Y575F mutations to signal tyrosine phosphorylation, DNA-binding activity, proliferation, protection from apoptosis, and CD23 induction in response to huIL-4 in murine cells. Responses generated by the Q576R and Y575F mutants were similar to those of the wild-type receptor, using various concentrations of huIL-4 and times of stimulation. These results indicate that neither the Q576R nor the Y575F mutations have a significant direct effect on IL-4 signal transduction, and that hypersensitive induction of CD23 in cells derived from human allergy patients may be due to different and/or additional alterations in the IL-4 signaling pathway.

Functional characterization of the somatic hypermutation process leading to antibody D1.3, a high affinity antibody directed against lysozyme.

The impact of somatic hypermutation on the affinity of Abs directed against protein Ags remains poorly understood. We chose as a model the secondary response Ab D1.3 directed against hen egg lysozyme. During the maturation process leading to this Ab, five replacement somatic mutations occurred. After reconstituting the germline Ab from which D1.3 originated, we assessed the energetic and kinetic importance of each of the somatic mutations, individually or combined, using the BIAcore apparatus. We found that the mutations induced an overall 60-fold improvement of affinity, principally due to a decrease in the kinetic rate of dissociation. We showed that their effects were additive and context independent; therefore, in the case of D1.3, the order in which somatic mutations were introduced and selected is unimportant. Interestingly, most of the affinity improvement was due to a single somatic mutation (Asn50-->Tyr in VL), involving a residue that belongs to the functional interface between Ab D1.3 and lysozyme. This replacement could either establish new Van der Waals contacts between the Ab and the Ag or help stabilize the conformation of a closely situated crucial residue of the Ab paratope. The four other mutations played only a marginal part in affinity maturation; potential reasons for which these mutations were nevertheless selected are discussed.

Probing the interaction between a high-affinity single-chain Fv and a pyrimidine (6-4) pyrimidone photodimer by site-directed mutagenesis.

We have used site-directed mutagenesis to uncover the origin of the binding affinity differences exhibited by a series of monoclonal antibodies that recognize pyrimidine (6-4) pyrimidone photoproducts in the context of single- or double-stranded DNA. In this study, we have focused on two antibodies-64M3 and 64M5-that share the same binding specificity but differ in their affinities. We used single-chain Fv (scFv) derivatives for these studies since they can be easily expressed in Escherichia coli. To facilitate this, we also developed a simple, on-column refolding procedure for scFvs that is rapid and does not require high dilution. We took several precautions to ensure that the scFvs faithfully reflected the behavior of the parent monoclonal antibodies. Results obtained from chimeric scFvs constructed from 64M3 and 64M5 suggested that the higher affinity of the 64M5 antibody was mainly due to its VL region. Loop-grafting studies in which VH CDR loops of 64M3 were individually transplanted into 64M5 were consistent with this hypothesis. Since the VL sequences of 64M3 and 64M5 differ at only three positions (L30, L50, and L90), alanine-scanning mutagenesis was used to assess the importance of these three residues in DNA binding by 64M5. These studies highlighted the importance of all three VL CDR loops; furthermore, they suggested that photoproduct binding involved conformational changes within the VL region.

Improving the affinity and the fine specificity of an anti-cortisol antibody by parsimonious mutagenesis and phage display.

Immunoassays are widely used to determine steroid concentrations. However, they are limited by the specificity of antisteroid mAbs. We used the phage display system combined with molecular modeling and site-specific randomization to improve the affinity and the fine specificity of an anti-cortisol mAb. Using parsimonious mutagenesis, we have generated a library of mutant Ab fragments (scFv) derived from this Ab by randomizing five amino acids chosen by molecular modeling and Ab-hapten contact structural analysis. Anti-cortisol Ab fragments were selected from the library in the presence of steroid analogues to block cross-reacting binders. Specific elution with free cortisol allowed the recovery of clones with up to eightfold better affinity and fivefold less cross-reactivity than the wild-type scFv. This approach can be applied to any anti-hapten Ab and represents a useful approach for obtaining highly specific Abs for use in steroid immunoassays.

A site for CD4 binding in the beta 1 domain of the MHC class II protein HLA-DR1.

Using a lymphocyte binding assay, we have previously demonstrated that the CD4 protein can mediate cell adhesion by direct interaction with MHC class II molecules. In this report, we have used this assay to test whether synthetic peptides, corresponding to DR beta sequences, could inhibit CD4-class II adhesion. A peptide derived from sequences within the beta1 domain (DR beta 41-55), as well as two peptides derived from sequences within the beta 2 domain (DR beta 121-135 and DR beta 141-155), were shown to inhibit CD4-class II adhesion. Inasmuch as a site for CD4 binding in the beta 2 domain had been previously documented, these studies were designed to investigate the role of the beta 1 domain as an additional site of interaction with CD4. Sixteen site-specific mutations were engineered within the beta1 domain of DR beta 1*0101. Several mutations were shown to disrupt CD4-dependent T cell activation. Based on these results, we propose a model for the molecular interaction of CD4 with MHC class II proteins in which both the beta 1 and beta 2 domains of class II interact with the two amino-terminal Ig-like domains of CD4.

The leukocyte function-associated antigen-1 (LFA-1)-binding site on ICAM-3 comprises residues on both faces of the first immunoglobulin domain.

ICAM-3 (CD50), a member of the Ig superfamily, is a major ligand for the leukocyte integrin LFA-1 (CD11a/CD18). This interaction represents one of several Ig superfamily/integrin ligand-receptor pairs that have been described to date. ICAM-3 is highly expressed on resting leukocytes and on APCs. In addition to an adhesive function, ICAM-3 can act as a signal-transducing molecule on T cells, providing a costimulatory signal for cell proliferation. Eighteen point mutations in ICAM-3 were generated, and residues important for binding of functional blocking Abs were identified. Mutation of seven of the residues reduced or abrogated adhesion to LFA-1, including three residues that are located on strand A of the ABED face of domain 1. In contrast, extensive mutagenesis analysis of ICAM-1 has shown that only residues on the GFC face interact with LFA-1. Our results provide evidence for a more extensive binding interface between ICAM-3 and LFA-1 than has previously been described. ICAM-3 appears to be unique among the ICAMs in utilizing residues on both faces of domain 1 for interaction with its ligand LFA-1.

Conservation of the amino-terminal epitope of elongation factor Tu in eubacteria and Archaea.

An epitope of elongation factor Tu (EF-Tu), which is found in organisms in both the bacterial and archaeal domains, was recently defined by mAb 900. To localize the conserved epitope within the EF-Tu molecule and to determine its sequence, SPOTScan analysis of synthetic peptides, Western blot analysis of purified EF-Tu domains and site-directed mutagenesis studies were used. Analysis of mAb 900 binding to overlapping 15-mer peptides encompassing the complete sequence of EF-Tu of Escherichia coli was inconclusive, suggesting three distinct regions may be epitopes. Western blot analysis of EF-Tu domains 1-3 of Thermus thermophilus suggested that the epitope was located at the N terminus. This was confirmed by site-directed mutagenesis of EF-Tu domain 1 of Mycoplasma hominis. By C-terminal truncation of the N-terminal 15-mer peptide the epitope was mapped to EF-Tu residues 1-6. Replacement of each of the residues in the epitope peptide demonstrated that only positions 5 and 6 were indispensable for antibody binding. These data provide evidence that the highly conserved epitope recognized by mAb 900 in the bacterial and archaeal domains is located at the very end of the N terminus of the EF-Tu molecule.

Both PU.1 and nuclear factor-kappa B mediate lipopolysaccharide- induced HIV-1 long terminal repeat transcription in macrophages.

We recently reported that LPS stimulation of monocytic cells leads to the activation of PU.1, a member of the Ets family of transcription factors. Phosphorylation of PU.1 by protein kinase CK2 was found to up-regulate its trans-activation function, but not its DNA binding activity. Previous studies suggested that Ets proteins could bind to NF-kappa B motifs at the tetrameric core sequence TTCC. In macrophages, LPS-inducible HIV-1 gene expression is mediated in part by binding of NF-kappa B to identical tandem binding sites located within the long terminal repeat (LTR). Thus, we performed additional studies to determine whether PU.1 also played a role in regulating HIV-1 gene expression in macrophages. Our functional studies revealed that activation of the HIV-1 LTR in LPS-stimulated cells requires both NF-kappa B and PU.1. Extensive mutagenesis of the HIV-1 LTR revealed that PU.1-dependent activation requires the Ets motif within the upstream NF-kappa B site, whereas NF-kappa B itself binds to the downstream site. We also found that insertion of five additional nucleotides between the NF-kappa B sites abolished LPS inducibility, suggesting a direct interaction between factors that bind these sites. Lastly, we found that mutation of PU.1 at serine 148, which prevents its phosphorylation by CK2, blocked its ability to activate the HIV-1 LTR in response to LPS. These effects were promoter specific because PU.1 did not affect LPS-inducible activation of a distinct NF-kappa B-dependent promoter. While these data do not demonstrate direct binding of PU.1 to the HIV-1 LTR, they illustrate a novel role for PU.1 in activation of the HIV-1 LTR by LPS.