HIV-associated neuropathies: role of HIV-1, CMV, and other viruses

The role of the human immunodeficiency virus (HIV) and other viruses in the development of neuropathies associated with HIV infection is controversial. Distal symmetric polyneuropathy (DSP), the most common subtype of HIV-associated neuropathy, is characterized by an abundance of reactive macrophages within the peripheral nerve, but HIV replication is limited to a small percentage of the macrophages. Thus, the pathological destruction may be mediated by pro-inflammatory signals amplified by activated glial elements within the nerve, similar to the proposed mechanism of damage caused by HIV within the central nervous system. In contrast, in mononeuropathy multiplex (MM) and progressive polyneuropathy (PP), cytomegalovirus (CMV) replication in the peripheral nerve is consistently demonstrable, and this replication likely results in direct damage to the infected cells (neurons and glia). The rarest form of HIV-associated neuropathy, the diffuse infiltrative lymphocytosis syndrome (DILS), is characterized by an intense CD8+ T lymphocyte infiltration into the nerve and abundant HIV infection of macrophages. Finally, while other viruses (varicella zoster, herpes simplex) are associated with myelitis in HIV-infected individuals, there is little support for a role for these viruses in HIV-associated neuropathy.

Proton MR spectroscopy and magnetization transfer ratio in multiple sclerosis: correlative findings of active versus irreversible plaque disease

PURPOSE

To characterize plaques of multiple sclerosis (MS) using both proton MR spectroscopy and magnetization transfer (MT) imaging.

METHODS

The magnetization transfer ratio (MTR) was calculated from two series of three-dimensional gradient-recalled acquisition in the steady state (GRASS) images obtained with and without an MT saturation pulse. Proton spectra were acquired using the point-resolved spectroscopy (PRESS) sequence with a voxel size of 1.5 x 1.5 x 1.5 cm3. A total of 28 spectra were obtained in 13 patients who had clinically definitive MS. The spectra were analyzed together with the MTR.

RESULTS

A positive relationship was found between the N-acetylaspartate (NAA)/creatine (Cr) ratio and the MTR in MS plaques, whereas no significant correlation was found between the metabolite ratios and the signal intensity on fast spin-echo T2-weighted MR images.

CONCLUSION

Small changes in the MTR of MS plaques relative to the MTR of normal white matter may reflect inflammatory changes and edema, whereas larger changes in MTR correlate with decreased NAA/Cr ratio and therefore suggest demyelination and irreversible damage from chronic MS plaques.

Induction of apoptosis by La Crosse virus infection and role of neuronal differentiation and human bcl-2 expression in its prevention

La Crosse virus causes a highly cytopathic infection in cultured cells and in the murine central nervous system (CNS), with widespread neuronal destruction. In some viral infections of the CNS, apoptosis, or programmed cell death, has been proposed as a mechanism for cytopathology (Y. Shen and T. E. Shenk, Curr. Opin. Genet. Dev. 5:105-111, 1995). To determine whether apoptosis plays a role in La Crosse virus-induced cell death, we performed experiments with newborn mice and two neural tissue culture models. Newborn mice infected with La Crosse virus showed evidence of apoptosis with the terminal deoxynucleotidyl transferase-mediated nicked-end labeling (TUNEL) assay and, concomitantly, histopathological suggestion of neuronal dropout. Infection of tissue culture cells also resulted in DNA fragmentation, TUNEL reactivity, and morphological changes in the nuclei characteristic of apoptotic cells. As in one other system (S. Ubol, P. C. Tucker, D. E. Griffin, and J. M. Hardwick, Proc. Natl. Acad. Sci. USA 91:5202-5206, 1994), expression of the human proto-oncogene bcl-2 was able to protect one neuronal cell line, N18-RE-105, from undergoing apoptosis after La Crosse virus infection and prolonged the survival of infected cells. Nevertheless, expression of bcl-2 did not prevent eventual cytopathicity. However, a human neuronal cell line, NT2N, was resistant to both apoptosis and other types of cytopathicity after infection with La Crosse virus, reaffirming the complexity of cell death. Our results show that apoptosis is an important consequence of La Crosse virus infection in vivo and in vitro.

Tropism of bunyaviruses: evidence for a G1 glycoprotein-mediated entry pathway common to the California serogroup

The California serogroup is composed of antigenically and biologically related viruses within the Bunyavirus genus of the Bunyaviridae. We used a large panel of murine cells to study their tissue tropisms and found virtually identical patterns of viral replication among all of the members of this serogroup, in contrast to other members of the family (Bunyamwera, Cache Valley, and Punta Toro viruses). By analyzing the nonpermissive infections with both an RNA dot-blot and a virus binding assay, we determined that tropism for cultured cells was determined at the level of entry. A truncated soluble form of the La Crosse G1 glycoprotein (sG1) was expressed in a baculovirus system and, despite slight differences in glycosylation, was shown to resemble native G1 by immunoprecipitation with six monoclonal antibodies. sG1 bound to permissive but not to nonpermissive cell lines, as demonstrated by flow cytometry. The sG1 effectively blocked infection of permissive cell lines with all of the California serogroup viruses, but did not block infection of two other bunyaviruses. These results indicate that the California serogroup bunyaviruses share a common receptor on vertebrate cells which may differ from the receptor used by other Bunyaviridae and demonstrate that the G1 glycoprotein is the virus attachment protein. sG1 will be a useful reagent in the search for a putative receptor molecule.

Sodium butyrate stimulation of HIV-1 gene expression: a novel mechanism of induction independent of NF-kappa B

Nuclear factor-kappa B (NF-kappa B) has been shown to play a central role in stimulating human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR)-directed viral gene expression. We have previously described a cell line (TE671/RD) that fails to respond to phorbol myristate acetate (PMA) or tumor necrosis factor-alpha (TNF-alpha) in terms of amplifying HIV-1 LTR-driven gene expression unless it is concurrently treated with sodium butyrate. It was not determined whether this lack of response stemmed from an inability of these cells to produce free NF-kappa B or from ineffectual interaction of this sequence-specific transcriptional factor with its target. We now show that these cells are in fact capable of inducing a free nuclear NF-kappa B-binding activity when stimulated with PMA but not when treated with sodium butyrate alone. Furthermore, we show that sodium butyrate alone is equally potent in stimulating HIV-1 LTR-directed gene expression in latently infected U1 and ACH-2 cells in the absence of induction of nuclear NF-kappa B, as compared with PMA, which induces NF-kappa B activation in these cells. We also show that stimulation of HIV-1 expression in U1 cells with sodium butyrate is not blocked by N-acetylcysteine, whereas that of PMA stimulation is blocked. These observations are discussed in the context of a model where chromatin structure participates in the maintenance of restricted HIV-1 viral gene expression in these cells.

Protection from La Crosse virus encephalitis with recombinant glycoproteins: role of neutralizing anti-G1 antibodies

La Crosse virus, a member of the California serogroup of bunyaviruses, is an important cause of pediatric encephalitis in the midwestern United States. Like all bunyaviruses, La Crosse virus contains two glycoproteins, G1 and G2, the larger of which, G1, is the target of neutralizing antibodies. To develop an understanding of the role of each of the glycoproteins in the generation of a protective immune response, we immunized 1-week-old mice with three different preparations: a vaccinia virus recombinant (VV.ORF) that expresses both G1 and G2, a vaccinia virus recombinant (VV.G1) that expresses G1 only, and a truncated soluble G1 (sG1) protein prepared in a baculovirus system. Whereas VV.ORF generated a protective response that was mostly directed against G1, VV.G1 was only partially effective at inducing a neutralizing response and at protecting mice from a potentially lethal challenge with La Crosse virus. Nevertheless, a single immunization with the sG1 preparation resulted in a robust immune response and protection against La Crosse virus. These results indicate that (i) the G1 protein by itself can induce an immune response sufficient for protection from a lethal challenge with La Crosse virus, (ii) a neutralizing humoral response correlates with protection, and (iii) the context in which G1 is presented affects its immunogenicity. The key step in the defense against central nervous system infection appeared to be interruption of a transient viremia that occurred just after La Crosse virus inoculation.

Completion of the La Crosse virus genome sequence and genetic comparisons of the L proteins of the Bunyaviridae

La Crosse virus is a member of the Bunyavirus genus in the family Bunyaviridae, viruses with trisegmented RNA genomes of mostly negative polarity composed of large (L), medium (M), and small (S) segments. The sequences of the La Crosse/original M and S RNA segments have been previously characterized. Using reverse transcriptase in conjunction with PCR amplification, we have obtained the nucleotide sequence of the L RNA segment, which encodes the viral polymerase in a single large open reading frame. Comparison of the amino acid sequence of the LAC L protein with the sequence of other polymerases from members of the Bunyaviridae, demonstrated the presence of several conserved motifs, some of which are characteristic of many polymerase proteins. A genetic tree comparing the available polymerase proteins of the Bunyaviridae provides insights into the phylogenetic relationships within this large family. Members of the genus Bunyavirus, which are mosquito-borne and infect mammals, have a closer relationship to the plant viruses represented by tomato spotted wilt virus (Tospovirus genus) than to viruses of other genera in the family Bunyaviridae.

Synthetic multimeric peptides derived from the principal neutralization domain (V3 loop) of human immunodeficiency virus type 1 (HIV-1) gp120 bind to galactosylceramide and block HIV-1 infection in a human CD4-negative mucosal epithelial cell line

The glycosphingolipid galactosylceramide (GalCer), which binds gp120 with high affinity and specificity, is a potential alternative receptor for human immunodeficiency virus type 1 (HIV-1) in some CD4-negative neural and epithelial human cells, including the human colonic epithelial cell line HT-29. In the present study, we demonstrate that synthetic multibranched peptides derived from the consensus sequence of the HIV-1 V3 loop block HIV-1 infection in HT-29 cells. The most active peptide was an eight-branched multimer of the motif Gly-Pro-Gly-Arg-Ala-Phe which at a concentration of 1.8 microM induced a 50% inhibition of HIV-1 infection in competition experiments. This peptide was not toxic to HT-29 cells, and preincubation with HIV-1 did not affect viral infectivity, indicating that the antiviral activity was not due to a nonspecific virucidal effect. Using a high-performance thin-layer chromatography binding assay, we found that multibranched V3 peptides recognized GalCer and inhibited binding of recombinant gp120 to the glycosphingolipid. In addition, these peptides abolished the binding of an anti-GalCer monoclonal antibody to GalCer on the surface of live HT-29 cells. These data provide additional evidence that the V3 loop is involved in the binding of gp120 to the GalCer receptor and show that multibranched V3 peptides are potent inhibitors of the GalCer-dependent pathway of HIV-1 infection in CD4-negative mucosal epithelial cells.

Magnetization transfer effects in MR-detected multiple sclerosis lesions: comparison with gadolinium-enhanced spin-echo images and nonenhanced T1-weighted images

PURPOSE

To define the relationship between magnetization transfer and blood-brain-barrier breakdown in multiple sclerosis lesions using gadolinium enhancement as an index of the latter.

METHODS

Two hundred twenty lesions (high-signal abnormalities on T2-weighted images) in 35 multiple sclerosis patients were studied with gadolinium-enhanced spin-echo imaging and magnetization transfer. Lesions were divided into groups having nodular or uniform enhancement, ring enhancement, or no enhancement after gadolinium administration. For 133 lesions, T1-weighted images without contrast enhancement were also analyzed. These lesions were categorized as isointense or hypointense based on their appearance on the unenhanced T1-weighted images.

RESULTS

There was no difference between the magnetization transfer ratio (MTR) of lesions as a function of enhancement. MTR of hypointense lesions on unenhanced T1-weighted images was, however, lower than the MTR of isointense lesions.

CONCLUSION

We speculate that diminished MTR may reflect diminished myelin content and that hypointensity on T1-weighted images corresponds to demyelination. Central regions of ring-enhancing lesions had a lower MTR than the periphery, suggesting that demyelination in multiple sclerosis lesions occurs centrifugally. In addition, the short-repetition-time pulse sequence seems useful in the evaluation of myelin loss in patients with multiple sclerosis.

Interferon-gamma decreases cell surface expression of galactosyl ceramide, the receptor for HIV-1 GP120 on human colonic epithelial cells

HT-29-A7, a CD4-negative clonal derivative of the human colonic adenocarcinoma cell line HT-29, is particularly sensitive to infection by several isolates of HIV-1 and, correspondingly, expresses high amounts of galactosylceramide (galactocerebroside, GalCer). GalCer is a neutral glycolipid which binds to the HIV-1 envelope glycoprotein gp120 and is present at abundant levels in normal human epithelial cells of the small and large intestine. Treatment of the HT-29-A7 cells with recombinant gamma-interferon (rlFN gamma) induced a dose-dependent inhibition of GalCer expression on the cell surface, as demonstrated by indirect immunofluorescence and by enzymatic labeling of cell surface glycoconjugates with oxidase-tritiated sodium borohydride. The rIFN gamma effect was not associated with any toxicity and was specific for GalCer, since expression of carcinoembryonic antigen did not decrease following treatment. The decrease in GalCer expression was associated with resistance of the cells to HIV-1 infection. In contrast, rIFN gamma did not alter cell surface expression of CD4, the classical HIV receptor, in HT-29-A7 cells that had been transduced with a retroviral vector expressing full-length CD4, and there was no effect on their infection. These results strongly suggest that rIFN gamma blocks HIV-1 infection of HT-29-A7 cells by decreasing GalCer synthesis and expression. This effect on expression of a viral receptor is a novel antiviral property of rIFN gamma which should be exploited for antiviral therapeutics.

Suramin inhibits binding of the V3 region of HIV-1 envelope glycoprotein gp120 to galactosylceramide, the receptor for HIV-1 gp120 on human colon epithelial cells

The infection of human colonic epithelial cells HT-29 by human immunodeficiency virus type 1 (HIV-1) occurs independently of CD4, the main HIV-1 receptor expressed on lymphocytes and macrophages. Recent studies from our group have shown that HT-29 cells express the glycosphingolipid galactosylceramide (GalCer), a potential alternative receptor for the HIV-1 envelope glycoprotein gp120. The binding of recombinant gp120 to GalCer was blocked by monoclonal antibodies directed against the third variable region (V3) of gp120, suggesting that the V3 domain was implicated in GalCer recognition. In the present report, we show that suramin, a polysulfonyl naphtylurea known to inhibit retroviral reverse transcriptases in vitro, blocks HIV-1 infection in HT-29 cells. The effect is dose dependent, with a half-maximal inhibition (IC50) achieved for a suramin concentration of 54 micrograms/ml. Since [3H]suramin was not significantly internalized into HT-29 cells during our infection assay (i.e. 2 h), we have considered the possibility that the drug could act at an extracellular step of the HIV-1 cycle. Using a high performance thin layer chromatography binding assay, we show that suramin inhibits binding of HIV-1 gp120 to purified GalCer with an IC50 of 25 micrograms/ml. Suramin does not bind to GalCer, since preincubation of GalCer with suramin did not prevent the subsequent attachment of gp120. Using a solid-phase assay, we show that [3H]suramin specifically binds to recombinant gp120 and that this binding could be blocked by a monoclonal antibody specific for the conserved GPGRAF motif of the V3 domain of gp120. We also demonstrate that [3H]suramin binds to multibranched synthetic GPGRAF peptides that block HIV-1 infection in HT-29 cells. Binding of [3H]suramin to V3 peptides is specific and inhibited by unlabeled suramin (IC50 of 28 micrograms/ml). In contrast, the suramin derivative NF036, that is unable to block HIV-1 infection in HT-29 cells, does not inhibit the binding of [3H]suramin to V3 peptides. Taken together, these results suggest that suramin blocks HIV-1 infection in HT-29 cells because it binds to the V3 domain of gp120 and hence prevents the interaction between gp120 and the GalCer receptor.

Correlation of spectroscopy and magnetization transfer imaging in the evaluation of demyelinating lesions and normal appearing white matter in multiple sclerosis

Magnetization transfer imaging (MT) and localized proton spectroscopy (1H-MRS) were utilized in the evaluation of lesions (high signal abnormalities on T2-weighted images) and normal-appearing white matter (NAWM) in multiple sclerosis (MS). Eleven patients with a clinical diagnosis of MS were independently evaluated with both 1H-MRS and MT. The magnetization transfer ratio (MTR) of lesions was compared with the relative concentration of N-acetyl-aspartate (NAA) and a composite peak at 2.1 to 2.6 ppm termed "marker peaks." The MTR of white matter lesions in the MS patients was markedly decreased (6-34%; normal approximately 42%), and correlated well with increase in the marker peaks region (0.94-3.89). There was no correlation between the relative concentration of NAA and MTR. Increased resonance peaks in the 2.1 to 2.6 ppm range and marked decreases in MTR may be a relatively specific indicators of demyelination.

Human immunodeficiency virus type 1 Tat activity in human neuronal cells: uptake and trans-activation

Neurological dysfunction in AIDS occurs in the absence of productive infection of neurons, and may involve modulation of neuronal cell function by viral or cellular products released from surrounding infected cells. The human immunodeficiency virus type 1 (HIV-1) trans-activator protein Tat may be one such factor, as it can act as a neurotoxin, induces marked morphological changes in neurons and astrocytes in primary embryonic rodent brain cultures, and is released by certain HIV-1-infected cells. In addition, Tat can alter expression of cellular genes in several non-neuronal cell types. To explore the possibility that Tat may also mediate neuronal dysfunction in AIDS through non-lethal effects on neurons, we determined the trans-activating ability of Tat in human neuronal cells. We generated human neuronal cell lines stably expressing several HIV-1 tat genes, and also tested human neuronal cells exposed to extracellular recombinant Tat protein. Both endogenously expressed Tat as well as exogenous recombinant Tat protein up-regulated HIV-1 long terminal region (LTR)-driven gene expression by several hundred-fold. Only brief exposure to recombinant Tat was necessary and no toxic effects were seen at levels sufficient for trans-activation. Furthermore, Tat significantly enhanced virus expression in neuronal cells transfected with molecular clones of HIV-1. These results show that Tat is trans-activationally active in human neuronal cells, and can be taken up from the extracellular compartment by these cells in a biologically active form. Neurons represent an important potential target for Tat-mediated cellular dysfunction.

Replication in cultured C2C12 muscle cells correlates with the neuroinvasiveness of California serogroup bunyaviruses

The neuroinvasiveness of California serogroup bunyaviruses is determined by the ability of the virus to replicate in striated muscle after peripheral inoculation of mice. Neuroinvasiveness was mapped to the medium (M) RNA segment of the virus, which encodes the viral glycoproteins, when reassortants were made between La Crosse/original virus, a neuroinvasive isolate, and Tahyna-181/57 virus, a nonneuroinvasive clone. We have tested the murine muscle cell line C2C12 as a surrogate for myotropism and have found that there is a slight, but reproducible difference in the replication of virus clones bearing the M RNA segment of La Crosse/original virus compared to clones bearing the M RNA segment of Tahyna-181/57 virus, as determined by viral titer, antigen expression, and plaque formation.

Binding of human immunodeficiency virus type I (HIV-1) gp120 to galactosylceramide (GalCer): relationship to the V3 loop

The primary receptor for the human immunodeficiency virus (HIV) is the CD4 molecule. However, a large body of evidence has demonstrated that some cells that do not express the CD4 receptor can be infected by HIV-1 and HIV-2, indicating that an alternative mechanism of infection must exist for some cell types. Recently it was reported that antibodies against the glycosphingolipid, galactosylceramide (Gal beta 1-1'Cer;GalCer), blocked infection of several CD4 negative cell lines derived from the brain and colon. The hypothesis that GalCer might be involved in the process of HIV entry into these cells was further supported by the finding that recombinant gp120 bound GalCer with high affinity in a high performance thin layer chromatography (HPTLC) binding assay. We have examined the interactions between GalCer and gp120, and found that the oligosaccharides that constitute a large proportion of the molecular mass of this glycoprotein are not involved in binding to this glycolipid. Furthermore, using a panel of monoclonal and monospecific antibodies we have determined that gp120 binds GalCer, and the related molecule 3' sulfo galactosylceramide (sulfatide), at a site that is conformationally close to the its principal neutralizing domain (V3 loop) or at the V3 loop itself.

Human immunodeficiency virus type 1 causes productive infection of macrophages in primary placental cell cultures

To characterize the role of the placenta in vertical transmission of human immunodeficiency virus type 1 (HIV-1), the susceptibility of primary human placental cultures and of transformed trophoblast cell lines to infection by several HIV-1 isolates was examined. Placental cultures supported the replication of all strains tested, including lymphocyte-, macrophage-, and amphotropic isolates. All viruses replicated to modest levels, with production of both viral antigen and infectious virus in the culture supernatants. Placental cells demonstrated a pattern of permissiveness for HIV-1 isolates distinct from that seen with lymphocytes, blood-derived macrophages, or T cell lines. Immunofluorescent staining showed that 5%-10% of the cultured placental cells expressed viral antigens, and double labeling revealed that the HIV-positive cells were macrophages not trophoblasts. None of the trophoblast cell line (JEG-3, Jar, BeWo, HP-W1) could be infected by HIV. These results support the hypothesis that infection of the placenta could play a role in maternofetal transmission of HIV-1 and suggest that the placental macrophage is likely to be the primary cell type responsible.

Monoclonal anti-idiotypic antibodies that mimic the epitope on gp120 defined by anti-HIV-1 monoclonal antibody 0.5 beta

OBJECTIVE

To develop effective, specific and safe anti-idiotypic antibody (Ab2) vaccines against HIV-1.

DESIGN

Murine monoclonal Ab2 were generated against anti-HIV-1 antibody 0.5 beta (Ab1), which binds to gp120, neutralizes HIV-1 and inhibits virus-induced syncytia formation.

METHODS

Mice were immunized with Ab1, and Ab2 were produced from immunized mice by the hybridoma technique. The Ab2 were characterized in vitro, injected into rabbits, and the anti-anti-idiotypes (Ab3) induced in the rabbits were analyzed for binding and antiviral reactivities by enzyme-linked immunosorbent assay, p24gag release and syncytia formation assays.

RESULTS

Seven Ab2 bound to the antigen-combining site of Ab1, one of which (UD7) induced Ab3 in rabbits that were Ab1-like in their binding reactivities to PB1 (recombinant gp120 fragment) or peptides of gp120, and shared idiotypes with the Ab1. Crude Ab3-containing sera specifically and effectively neutralized the virus.

CONCLUSION

Monoclonal Ab2 UD7 has potential as a vaccine against HIV-1.

Replication of a macrophage-tropic strain of human immunodeficiency virus type 1 (HIV-1) in a hybrid cell line, CEMx174, suggests that cellular accessory molecules are required for HIV-1 entry

To investigate the mechanism underlying one aspect of the cellular tropism of human immunodeficiency virus type 1 (HIV-1), we used a macrophage-tropic isolate, 89.6, and screened its ability to infect a number of continuous cell lines. HIV-1 (89.6) was able to replicate robustly in a T-cell/B-cell hybrid line, CEMx174, while it replicated modestly or not at all in either of its parents, one of which is the CD4-positive line CEM.3. Analysis by transfection of a molecular clone, a virus uptake assay, and polymerase chain reaction all provided strong evidence that the block to HIV-1(89.6) replication in the CEM.3 line lies at the level of cellular entry. These results were complemented by preparing a CD4-expressing derivative of the B-cell parent, 721.174, and demonstrating that it is permissive for productive HIV-1(89.6) replication. Given these experimental findings, we speculate that there exist cellular accessory factors which facilitate virus entry and infection in CD4-positive cells. Furthermore, these cellular accessory factors may be quite virus strain specific, since not all macrophage-tropic strains of HIV-1 were able to replicate in the CEMx174 hybrid cell line. This experimental model provides a system for the identification of one or more of these putative cellular accessory factors.

HIV-1 pseudotype virus containing a Cocal virus genome and an HIV envelope: construction, assay and use

A method is described for the production and assay of pseudotype viruses between human immunodeficiency virus type 1 (HIV-1) and Cocal virus (COV), containing an HIV-1 envelope and a COV genome (COV(HIV)). COV(HIV) pseudotype virus is a useful tool for the investigation of a variety of questions regarding HIV entry into susceptible cells, including steps in virus binding, fusion, and internalization, and the role of molecules which inhibit entry. COV, a rhabdovirus closely related to vesicular stomatitis virus (VSV), replicated and caused cytopathic effect in primary cultures of human peripheral blood lymphocytes (PBLs) and monocyte-derived macrophages (MDM), and in human cell lines of lymphocytoid or monocytoid origin, making it an ideal candidate for pseudotype production. 174XCEM cells, which were permissive for selected macrophage-tropic strains as well as most lymphocyte-tropic strains of HIV-1, were used to produce stocks of putative pseudotype virus. To neutralize parental COV in these stocks, a rabbit antiserum was produced which had a neutralization index of > 10(7) at a dilution of 1:100. Using these methods, pseudotype viruses were produced with a titer of about 10(4) PFU per ml; these same stocks contained HIV-1 at a titer of about 10(5) TCD50 per ml and COV at a titer of about 10(8) PFU per ml. CD4-expressing HeLa cells were used to assay pseudotype stocks made with lymphocyte-tropic strains of HIV-1. The authenticity of the pseudotype stocks was validated by several controls, including their failure to register on congenic CD4-negative HeLa cells and their inhibition by monoclonal anti-CD4 antibodies such as Leu 3a.

Molecular determinants of the virulence and infectivity of California serogroup bunyaviruses

California bunyaviruses cause encephalitis in mammalian hosts after peripheral infection. The virulence of these viruses is determined by their ability to replicate sequentially in striated muscle, cause viremia, and invade and replicate in the central nervous system. These viruses are also able to infect vector mosquitoes following ingestion of a blood meal containing virus. Bunyaviruses are negative stranded RNA viruses with a trisegmented genome, and the large, medium, and small RNA segments encode the polymerase, the glycoproteins, and the nucleoprotein, respectively. Reassortants between virulent and avirulent virus clones have been used to map virulence determinants in mice as well as determinants of infectivity in mosquitoes. Attenuation in mice and infectivity in mosquitoes of some virus clones maps to the medium RNA segment, implying that the virus glycoproteins, which are involved in virus entry, play a role in virulence. Attenuation in mice and mosquito infectivity of other clones maps to the large RNA segment, suggesting that cell-specific differences in the function of the viral polymerase can also determine virulence and host range.

Sodium butyrate treatment of cells latently infected with HIV-1 results in the expression of unspliced viral RNA

To investigate potential mechanisms for HIV-1 proviral latency, we generated a set of chronically HIV-1 infected and stably long terminal repeat-chloramphenicol acetyl transferase (LTR-CAT)-transfected TE671/RD cells, and studied both their virus production and LTR-driven reporter gene expression. Established tissue culture models of retroviral latency in lymphoid and monocytoid cell lines have demonstrated that the induction of virus production is associated with a shift in HIV-1-specific mRNA from a predominance of singly and multiply spliced mRNA's to the production of full-length HIV-1 RNA. We found a similar pattern in TE671/RD cells, but in contrast to U1 and ACH2 cells, could not induce viral replication by exposure to phorbol myristate acetate (PMA) alone. We demonstrated instead that production of full-length viral RNA, viral replication, and LTR-driven CAT expression could be induced by exposure to sodium butyrate. The most proximate effect of sodium butyrate is inhibition of cellular histone deacetylase(s) which results in disruption of nucleosomes relieving one level of restriction to gene expression. Consistent with this mechanism of action, we further found that sodium butyrate's effects: (i) act synergistically with PMA and TNF-alpha; (ii) are independent of protein synthesis; (iii) do not affect the constitutively expressed creatine phosphokinase gene; (iv) do not map to a discrete sequence motif in the viral LTR; and (v) are not blocked by N-acetyl cysteine but (vi) are blocked by novobiocin, an inhibitor of cellular topoisomerase II. These data show that a similar pattern of restricted viral RNA expression exists in this nonlymphoid cellular model of HIV-1 latency. In contrast however, these results suggest that in these cells there is an additional block to viral gene expression, which is overcome with sodium butyrate. These results are discussed in the context of histone-mediated repression of HIV-1 gene expression.

Polygenic control of neuroinvasiveness in California serogroup bunyaviruses

The pathogenesis of the California serogroup bunyaviruses includes both extraneural and intraneural replicative phases that can be separated experimentally. The present study dissects the viral genetic determinants of extraneural replication. We have previously described two attenuated reassortant clones of California serogroup bunyaviruses which exhibit reduced neuroinvasiveness after subcutaneous inoculation into suckling mice. Clone B1-1a bears an attenuated middle RNA segment (neuroinvasiveness phenotype v alpha v), and clone B.5 bears an attenuated large RNA segment (neuroinvasiveness phenotype alpha vv). We prepared reassortant viruses between these two strains and found that the two attenuated gene segments acted independently and additively, since reassortants bearing two attenuated RNA segments were more attenuated than the parental clones. Reassortants bearing no attenuated RNA segments were much more neuroinvasive than either parental clone, indicating that a neuroinvasive strain can be derived from two attenuated clones. Pathogenesis studies demonstrated that after injection of 10(3) PFU, the attenuated reassortant clones did not replicate in peripheral tissue, failed to reach the brain, and did not cause disease. At a dose of 10(6) PFU, attenuated clones failed to replicate to a significant level in peripheral tissue and produced only a minimal passive plasma viremia during the first 24 h but nevertheless reached high titers in the brain and killed mice. Because of this result, we investigated the possibility that neuroinvasion occurs via retrograde axonal transport, by determining whether sciatic nerve sectioning could protect against virus infection after hind leg footpad inoculation. We found that nerve sectioning had no effect on lethality, ruling out this mode of entry and suggesting that passive viremia is likely to be sufficient for invasion of the central nervous system.

HIV-1 Tat alters normal organization of neurons and astrocytes in primary rodent brain cell cultures: RGD sequence dependence

The HIV-1 trans-activator protein Tat has been implicated as a mediator of neuronal dysfunction in several model systems. To explore the possibility that Tat can affect primary brain cells, we examined the effect of recombinant Tat protein on rat cortical brain cell cultures. Tat induced marked aggregation of neurons and astrocytes in developing cultures and caused the neuritic processes to coalesce into fascicles. Cell death was not seen and brain macrophages were not affected. These effects mapped to a different region from the trans-activation domain of Tat, as mutating the RGD (arginine-glycine-aspartic acid) sequence within the second exon abrogated aggregation and fascicle formation without affecting trans-activation capacity. Such effects on primary neurons and astrocytes may reflect specific interactions of Tat with uninfected cells within the CNS in vivo.

Infection of colonic epithelial cell lines by type 1 human immunodeficiency virus is associated with cell surface expression of galactosylceramide, a potential alternative gp120 receptor

The gastrointestinal tract plays a major role in the pathogenesis and pathophysiology of infection by the type 1 human immunodeficiency virus (HIV-1). It is a potential route for viral entry and it is the site of a number of complications, including both opportunistic infections and a primary HIV-induced enteropathy. Correspondingly, both in vivo and in vitro studies have demonstrated HIV infection of gastrointestinal cells of lymphoid and epithelial origin. HT-29, a human colonic epithelial cell line that is infectable with many HIV-1 strains, does not express CD4 protein or mRNA. Recent studies showed that antibodies recognizing a neutral glycolipid related to galactosylceramide (GalCer) in HT-29 cells inhibited HIV-1 infection of this cell line, extending previous findings in neural cells. In the current studies, we further analyzed the neutral glycolipids of HT-29 cells and showed that they contained authentic GalCer and that recombinant gp120 bound to this glycolipid. Moreover, by analyzing GalCer expression in clones derived from HT-29 and Caco-2 (another human colonic cell line), we observed that the level of expression of this glycolipid was associated with the sensitivity to HIV-1 infection. Subclones of Caco-2 did not express GalCer and were not infectable with any of three HIV-1 strains. These results strengthen the possibility that GalCer is an alternative receptor in CD4- cell lines. Furthermore, since GalCer is a major glycolipid in epithelial cells of the small intestine and colon, these results provide a structural basis for the binding of HIV-1 by gastrointestinal epithelial cells and the entry of the virus into those cells.

Expression of the La Crosse M segment proteins in a recombinant vaccinia expression system mediates pH-dependent cellular fusion

To study the expression of La Crosse virus (LAC) glycoproteins, G1 and G2, we constructed a cDNA copy of the open reading frame (ORF) of the middle RNA segment and expressed it in a recombinant vaccinia virus (VV.ORF). Cells infected with VV.ORF expressed G1 and G2 at the cell surface and formed syncytia with a pH profile similar to that of LAC. These experiments provide a system of studying the biological functions of the LAC glycoproteins, including processing, targeting, fusion, receptor binding, and antigenicity.

Replication of type 1 human immunodeficiency viruses containing linker substitution mutations in the -201 to -130 region of the long terminal repeat

In previous transfection analyses using the chloramphenicol acetyltransferase reporter gene system, we determined that linker substitution (LS) mutations between -201 and -130 (relative to the transcription start site) of the human immunodeficiency virus type 1 long terminal repeat (LTR) caused moderate decreases in LTR transcriptional activity in a T-cell line (S. L. Zeichner, J. Y. H. Kim, and J. C. Alwine, J. Virol. 65:2436-2444, 1991). In order to confirm the significance of this region in the context of viral replication, we constructed several of these LS mutations (-201 to -184, -183 to -166, -165 to -148, and -148 to -130) in proviruses and prepared viral stocks by cocultivation of transfected RD cells with CEMx174 cells. In addition, two mutations between -93 and -76 and between -75 and -58 were utilized, since they affect the nuclear factor kappa B (NF-kappa B)- and Sp1-binding sites and were expected to diminish viral replication. Our results suggest that while transfection analyses offer an adequate approximation of the effects of the LS mutations, the analysis of viral replication using a mutant viral stock presents a more accurate picture, which is sometimes at variance with the transfection results. Three mutants (-201/-184 NXS, -165/-148 NXS, and -147/-130 NXS) had effects on viral replication that were much more severe than the effects predicted from their performance in transfection analyses, and the effects of two LS mutations (-201/-184 NXS and -183/-166 NXS) were not predicted by their effects in transfection. In addition, we observed cell type-specific permissiveness to replication of some mutant viruses. In the cell types tested, the LS mutations indicated an apparent requirement not only for the intact NF-kappa B and SP1-binding sites but also for several regions between -201 and -130 not previously associated with viral infectivity.

Infection of cultured human adrenal cells by different strains of HIV

OBJECTIVE

To determine whether human adrenal cells can be infected by HIV.

METHODS

Cultured human fetal adrenal cells and the SW13 human adrenocortical carcinoma cell line were inoculated with several HIV-1 and HIV-2 strains. Virus replication was detected by viral core antigen enzyme-linked immunosorbent and reverse transcriptase assays. CD4 expression was measured by Northern blot and polymerase chain reaction procedures.

RESULTS

HIV infection of these adrenal cells was detected and was most evident after cocultivation of the inoculated cells with peripheral blood mononuclear cells. Infection does not involve the CD4 molecule, which is not expressed by these adrenal cells. The relative level of HIV replication depended on the viral strain used. Virus production occurred best in cells that maintained evidence of adrenal cell function. Infection did not appear to disturb steroidogenesis measured in the cells.

CONCLUSIONS

These observations indicate that human adrenal cells are susceptible to HIV infection, and provide further evidence of the polytropic nature of the virus.

Experimental allergic encephalomyelitis and multiple sclerosis: lesion characterization with magnetization transfer imaging

Magnetization transfer imaging (MTI) was initially performed in normal guinea pigs and human volunteers. A magnetization transfer ratio (MTR) was calculated in the normal white matter and was found to be 42%-44%, with less than 2.5% variation, which indicates the high reproducibility of the measurement. MTI was then applied to an animal model of white matter disease, acute experimental allergic encephalomyelitis (EAE). In this model of EAE, pathologically proved lesions were edematous with essentially no demyelination. MTRs decreased slightly but significantly (5%-8%) compared with the MTRs of the same tissue region measured before the onset of the lesion [corrected]. Fifteen patients with multiple sclerosis (MS) also underwent MTI. In the 15 patients with MS, all lesions (209 plaques) had a significantly decreased MTR (average, 26%). The authors believe that demyelination produced the lower MTR, and, thus, lesions varied in transfer ratio on the basis of the extent of myelin loss. In patients with MS, particularly those with chronic and/or progressive MS, the MTR of the normal-appearing white matter was significantly decreased. The data suggest that calculated MTR obtained with in vivo MTI may enable differentiation of edema from demyelination, and that MTI can demonstrate white matter abnormalities that cannot be seen with standard spin-echo or gradient-echo magnetic resonance imaging.

MR proton spectroscopy in multiple sclerosis

PURPOSE

To elucidate the natural history of visualized MR abnormalities in patients with multiple sclerosis using proton spectroscopy.

METHODS

MR imaging and proton spectroscopy (1H spectroscopy) were performed on 16 patients with clinically definite multiple sclerosis. All patients received gadopentetate dimeglumine (Gd-DTPA).

RESULTS

Decreased levels of N-acetylaspartate (NAA) were demonstrated in 17 out of 21 lesions. No correlation was found between decreased NAA and Gd-DTPA enhancement. In five out of seven enhancing lesions, abnormal 1H spectra with extra peaks (termed marker peaks) at 2.1-2.6 ppm (ranging in absolute concentration from 10-50 mM protons) were observed. In nine out of 14 unenhancing lesions, no elevated marker peaks were observed. In the five other unenhancing lesions, the levels of these marker peaks were generally lower than the enhancing group. No correlation was found between the NAA levels and the levels of the marker peaks. We suggest two distinct biochemical processes: 1) decreased NAA reflecting neuronal cell loss, and 2) elevated marker peaks reflecting ongoing demyelination.

CONCLUSIONS

Based upon these observations we infer that 1) the majority of enhancing lesions are demyelinating with extra peaks at 2.1-2.6 ppm representing a marker of this process, 2) enhancing lesions without this marker most likely represent edematous regions without significant demyelination, and 3) demyelination may be long in duration compared with transient blood-brain barrier disruption manifested by Gd-DTPA enhancement. Our results suggest that 1H spectroscopy has the ability to further categorize MR-demonstrated enhancing and unenhancing lesions in patients with multiple sclerosis and that it may be more sensitive than contrast enhancement in revealing the true time course of demyelination.

Neuroattenuation of an avirulent bunyavirus variant maps to the L RNA segment

The derivation and characterization of a neuroattenuated reassortant clone (RFC 25/B.5) of California serogroup bunyavirus was described previously (M. J. Endres, A. Valsamakis, F. Gonzalez-Scarano, and N. Nathanson, J. Virol. 64:1927-1933, 1990). To map the RNA segment responsible for this attenuation, a panel of reassortants was constructed between the attenuated clone B.5 (genotype TLL) and a virulent clone (B1-1a) of reciprocal genotype (LTT). Parent viruses and clones representing all of the six possible reassortants were examined for neurovirulence by intracerebral injection in adult mice. Reassortants bearing the large RNA segment from the virulent parent were almost as virulent as the virulent parent virus, while reassortants bearing the large RNA segment from the avirulent parent virus exhibited low or intermediate virulence. These results indicate that the large RNA segment is the major determinant of neuroattenuation of clone B.5. In addition to its neuroattenuation, clone B.5 was temperature sensitive and exhibited an altered plaque morphology. These phenotypes also segregated with the large RNA segment. The importance of the large RNA segment (which encodes the viral polymerase) in neurovirulence contrasts with prior studies which indicate that the ability to cause lethal encephalitis after peripheral injection of suckling mice (neuroinvasiveness) is primarily determined by the middle-sized RNA segment, which encodes the viral glycoproteins.

Galactosyl ceramide or a derivative is an essential component of the neural receptor for human immunodeficiency virus type 1 envelope glycoprotein gp120

This report demonstrates that galactosyl ceramide (GalCer) or a molecule derived from it may serve as an alternative receptor for human immunodeficiency virus in the nervous system. Recombinant gp120, an envelope glycoprotein of human immunodeficiency virus type 1, specifically binds to GalCer and its derivatives. This specificity was studied by inhibiting binding of radioiodinated gp120 to GalCer with antibodies to GalCer, antibodies to gp120, and an excess of unlabeled gp120. Binding activity was also removed by absorbing gp120 with liposomes containing GalCer. In addition, studies using natural and semisynthetic lipids indicate that the linkage between galactose and ceramide is essential for binding. The significance of an alternative receptor for human immunodeficiency virus in the nervous system is discussed.

Inhibition of entry of HIV-1 in neural cell lines by antibodies against galactosyl ceramide

Although the CD4 molecule is the principal cellular receptor for the human immunodeficiency virus (HIV), several CD4-negative cell lines are susceptible to infection with one or more HIV strains. These findings indicate that there are alternate modes of viral entry, perhaps involving one or more receptor molecules. Antibodies against galactosyl ceramide (galactocerebroside, or GalC) inhibited viral internalization and infection in two CD4-negative cell lines derived from the nervous system: U373-MG and SK-N-MC. Furthermore, recombinant HIV surface glycoprotein gp120 bound to GalC but not to other glycolipids. These results suggest a role for GalC or a highly related molecule in HIV entry into neural cells.

Entry of human immunodeficiency virus-1 into glial cells proceeds via an alternate, efficient pathway

Although the CD4 molecule is the cellular receptor for human immunodeficiency virus-1 (HIV-1) in cells of the lymphocyte/monocyte lineage, a number of investigators have also been able to infect cells, including several of central nervous system (CNS) origin, that do not express CD4 protein or mRNA. These infections are generally nonpermissive. To ascertain whether the nonpermissive nature of infection in glial cells is due to an inefficient entry pathway, we prepared a permanently transfected U373-MG cell line expressing the CD4 molecule and demonstrated that HIV-1 still replicates at a low level. Furthermore, a virus uptake assay indicated that HIV-1 enters glial cells effectively, even in the absence of CD4. These results demonstrate that HIV-1 entry is efficient and that the restrictive nature of the infection in glial cells is due to postentry mechanisms. In addition, these findings support the existence of an alternate, efficient, entry pathway in some glial cells.

Macrophage-tropic strains of human immunodeficiency virus type 1 utilize the CD4 receptor

To characterize the role of CD4 in human immunodeficiency virus type 1 (HIV-1) infection of macrophages, we examined the expression of CD4 by primary human monocyte-derived macrophages and studied the effect of recombinant soluble CD4 and anti-CD4 monoclonal antibodies on HIV-1 infection of these cells. Immunofluorescence and Western blot (immunoblot) studies demonstrated that both monocytes and macrophages display low levels of surface CD4, which is identical in mobility to CD4 in lymphocytes. Recombinant soluble CD4 and the anti-CD4 monoclonal antibody Leu3a blocked infection of macrophages by three different macrophage-tropic HIV isolates, and the cytopathic effects of HIV-1 infection were similarly prevented. Dose-response experiments using a prototype isolate which replicates in both macrophages and T lymphocytes showed that recombinant soluble CD4 inhibited infection of macrophages more efficiently than in lymphocytes. These results indicate that CD4 is the dominant entry pathway for HIV-1 infection of macrophages. In addition, recombinant soluble CD4 effectively blocks HIV-1 infection by a variety of macrophage-tropic strains and thus has the potential for therapeutic use in macrophage-dependent pathogenesis in HIV disease.

Neuroattenuated bunyavirus variant: derivation, characterization, and revertant clones

A neuroattenuated variant bunyavirus, designated RFC/25B.5 (B.5), was selected by serial passage of a reassortant clone (RFC virus) of a California serogroup virus in BHK-21 cells, followed by plaque purification of that passaged stock. Based on its virulence index (ratio of PFU/50% lethal dose), clone B5 was over 40,000-fold less virulent than its unpassaged RFC parent after intracerebral (i.c.) inoculation into adult mice. Clone B.5 also exhibited markedly reduced neuroinvasiveness after subcutaneous injection into neonatal mice, although it retained its ability to replicate and kill suckling mice after i.c. injection. A murine neuroblastoma line (NA cells) can be used as an in vitro surrogate for the adult mouse brain, since clone B.5 replicated to at least 1,000-fold-lower titers in NA cells than did several neurovirulent California serogroup viruses. Clone B.5 replicated in BHK-21 cells at 37 degrees C to titers similar to those achieved by other California serogroup viruses but was temperature sensitive (ts) since its replication was markedly restricted at 38.9 degrees C. Ten ts revertant clones of B.5 virus were selected at 38.9 degrees C, and all of them lost their ts phenotype and regained the ability to replicate to high titer in NA cells and to kill adult mice after i.c. injection. Clone B.5 is the first described California serogroup virus which is truly attenuated after i.c. inoculation, and its availability will permit genetic analysis of bunyavirus neurovirulence.

CD4-independent infection of human neural cells by human immunodeficiency virus type 1

A number of studies have indicated that central nervous system-derived cells can be infected with human immunodeficiency virus type 1 (HIV-1). To determine whether CD4, the receptor for HIV-1 in lymphoid cells, was responsible for infection of neural cells, we characterized infectable human central nervous system tumor lines and primary fetal neural cells and did not detect either CD4 protein or mRNA. We then attempted to block infection with anti-CD4 antibodies known to block infection of lymphoid cells; we noted no effect on any of these cultured cells. The results indicate that CD4 is not the receptor for HIV-1 infection of the glioblastoma line U373-MG, medulloblastoma line MED 217, or primary human fetal neural cells.

Human immunodeficiency virus can infect CD4-negative human fibroblastoid cells

Certain human immunodeficiency virus (HIV) strains were shown to infect human fibroblastoid cells. Replication was demonstrated only by coculturing normal peripheral blood mononuclear cells or human T-lymphotropic virus type 1-transformed T-cell lines with the infected human cells. This infection of human fibroblastoid cells did not involve the CD4 molecule and did not have the properties of endocytosis. Human sera could be distinguished by their ability to neutralize HIV infection of the fibroblastoid versus human T-cell lines. These observations demonstrate further that other mechanisms for viral entry, besides CD4 binding, must be considered for HIV. They also indicate the wide cellular host range and heterogeneity of HIV strains. The possibility that fibroblastoid cells serve as a reservoir for the AIDS virus and are involved in connective tissue disorders of infected individuals merits attention.

Human choroid plexus cells can be latently infected with human immunodeficiency virus

The human immunodeficiency virus (HIV) penetrates the central nervous system, particularly the cerebrospinal fluid, early in the course of HIV infection, and may cause a progressive encephalopathy in patients prior to the development of the acquired immunodeficiency syndrome. Neither the specific mechanism for penetration of the virus into the central nervous system nor the pathophysiological basis for these abnormalities is well understood. We cultured cells from the choroid plexus of 3 individuals who died of causes unrelated to the acquired immunodeficiency syndrome and demonstrated that these cells can be infected with type 1 HIV. Infection of cells of the choroid plexus may provide an initial route of entry of HIV into the cerebrospinal fluid and, together with the macrophage, a route of entry into the brain.

The large viral RNA segment of California serogroup bunyaviruses encodes the large viral protein

Reassortant bunyaviruses derived from two members of the California serogroup (La Crosse/original and Tahyna/181-57) viruses were used to demonstrate that the large Mr viral protein (L) is encoded by the L RNA segment. Radiolabelled viral proteins were analysed by discontinuous SDS-PAGE. The L protein of La Crosse virus was observed to migrate ahead of its Tahyna virus counterpart when electrophoresed through a 5% acrylamide resolving gel. Among the reassortant viruses, the L protein phenotype segregated with the viral L RNA segment. After confirming the genotype of the viruses used in this study, it was concluded that the L RNA species of California serogroup viruses codes for the L protein, the presumed viral polymerase.

Organization of the middle RNA segment of snowshoe hare Bunyavirus

The genetic organization of the M RNA segment of snowshoe hare (SSH) virus, a member of the Bunyavirus genus of the family Bunyaviridae, has been determined. The middle (M) RNA segment has a single open reading frame (ORF) of 1441 amino acids. We have used amino- and carboxy-terminus sequencing and synthetic peptides to map proteins within the ORF. The order of the proteins translated from the single large open reading frame is G2, NSm, G1. The G2 protein extends from amino acids 14 to 299. The molecule is 286 residues long, with a computed nonglycosylated molecular weight of 31,973 Da. It is preceded by a cleaved 13 amino acid signal sequence. G2 includes a long highly hydrophobic sequence and contains three potential N-linked glycosylation sites. The G1 protein occupies the C-terminal end of the open reading frame from amino acids 474 to 1441 (968 amino acid residues) and has a computed nonglycosylated, molecular weight of 108,981 kDa. It has two potential N-linked glycosylation sites, and a potential transmembrane region followed by a potential cytoplasmic domain at the C-terminal end. If membrane associated it has an orientation of N-terminus outer, C-terminus inner. Limited trypsin digestion removes a 33-kDa fragment from the N-terminal end, leaving a virion-associated truncated G1 molecule (amino acids 762 to 1441) with a single N-linked glycosylation site. Between the G2 and G1 molecules there are 174 amino acids, sufficient to code for 19 kDa of protein. Some antibodies raised against peptides within this region react with proteins of 11 kDa (NSm) and 10 kDa present in infected cell lysates, but the exact relationship of these proteins to the open reading frame remains to be determined.

Multiple sclerosis: serial study of gadolinium-enhanced MR imaging

Thirteen patients with definite multiple sclerosis (MS), studied 16-24 months previously with magnetic resonance (MR) imaging with and without enhancement by intravenously administered gadolinium diethylenetriaminepentaacetic acid (DTPA) dimeglumine, were reexamined with a similar protocol. Assessment of enhancement and clinical activity in both studies revealed that enhancement was observed in 13 of 14 cases in which clinical activity had changed within 4 weeks of the study and thus appeared more sensitive than clinical examination in determining active disease. The 3-minute postinjection, short repetition time image (TR) was the most efficient for depicting enhancement. Enhancing lesions (active plaques) arose from previously hyper- or isointense regions on long TR images. Previously active lesions reverted to areas of iso- or hyperintensity on long TR images. Serial comparison of long TR images in this population reveals a decrease in high-intensity lesions on long TR images in some cases and an increase in others. The findings of high-intensity regions on long TR images and previously enhancing lesions both becoming isointense suggests that transient inflammatory changes with concomitant edema without demyelination and/or with significant remyelination may occur in some MS lesions. MS lesions are dynamic; both active and inactive lesions may show dramatic change on longitudinal MR imaging studies.

Molecular pathogenesis of neurotropic viral infections

Classical virologists defined a number of viruses that affect the nervous system and identified tissue tropism, extraneural replication, and viremia as important parameters that determine whether viral infections will affect the central nervous system. Molecular techniques are expanding this knowledge by permitting us to relate specific genes and gene products to two defined phenotypes: neuroinvasion and neurovirulence. Two converging situations make this knowledge particularly useful: (1) the development of antiviral drugs and subunit vaccines, which mandate that pathogenesis be related to specific regions of the viral genome; and (2) the expanding problem of central nervous system infections in immunodeficient states.