Bidirectional transfer of Anelloviridae lineages between graft and host during lung transplantation

Solid organ transplantation disrupts virus-host relationships, potentially resulting in viral transfer from donor to recipient, reactivation of latent viruses, and new viral infections. Viral transfer, colonization, and reactivation are typically monitored using assays for specific viruses, leaving the behavior of full viral populations (the "virome") understudied. Here we sought to investigate the temporal behavior of viruses from donor lungs and transplant recipients comprehensively. We interrogated the bronchoalveolar lavage and blood viromes during the peritransplant period and 6-16 months posttransplant in 13 donor-recipient pairs using shotgun metagenomic sequencing. Anelloviridae, ubiquitous human commensal viruses, were the most abundant human viruses identified. Herpesviruses, parvoviruses, polyomaviruses, and bacteriophages were also detected. Anelloviridae populations were complex, with some donor organs and hosts harboring multiple contemporaneous lineages. We identified transfer of Anelloviridae lineages from donor organ to recipient serum in 4 of 7 cases that could be queried, and immigration of lineages from recipient serum into the allograft in 6 of 10 such cases. Thus, metagenomic analyses revealed that viral populations move between graft and host in both directions, showing that organ transplantation involves implantation of both the allograft and commensal viral communities.

Alcohol use and abuse among patients with multidrug-resistant tuberculosis in Botswana

BACKGROUND

Data on alcohol abuse as a risk factor for the development of multidrug-resistant tuberculosis (MDR-TB) are scarce.

OBJECTIVE

To describe the patterns of alcohol use in MDR-TB patients and to determine whether alcohol use is associated with the development of MDR-TB in Botswana.

METHODS

We compared the level of alcohol use among MDR-TB patients against three control groups: 1) non-MDR-TB patients, 2) human immunodeficiency virus (HIV) infected patients without a history of TB, and 3) the general population. Alcohol use and abuse was measured with the Alcohol Use Disorders Identification Test 10 (AUDIT) questionnaire.

RESULTS

Of a total national population of 164 MDR-TB cases, 114 (70%) were interviewed. MDR-TB cases had a lifetime prevalence of alcohol use of 35.1%, which was lower than that of all control groups (P < 0.001). MDR-TB cases had higher 1-month prevalence of alcohol dependence symptoms and a lower 1-year period prevalence of alcohol dependence symptoms (P < 0.01 and P = 0.01 respectively). Among patients with TB, alcohol abuse was found to be a risk factor for the development of MDR-TB.

CONCLUSION

MDR-TB patients in Botswana have high rates of alcohol use and abuse. Among TB patients, alcohol abuse is associated with the diagnosis of MDR-TB, and could be an important modifiable factor.

HIV-1 gp120 and chemokine activation of Pyk2 and mitogen-activated protein kinases in primary macrophages mediated by calcium-dependent, pertussis toxin-insensitive chemokine receptor signaling

Human immunodeficiency virus type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry. It was recently demonstrated that HIV-1 glycoprotein 120 (gp120) elevated calcium and activated several ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. This study shows that chemokine receptor engagement by both CCR5-dependent (R5) and CXCR4-dependent (X4) gp120 led to rapid phosphorylation of the focal adhesion-related tyrosine kinase Pyk2 in macrophages. Pyk2 phosphorylation was also induced by macrophage inflammatory protein-1beta (MIP-1beta) and stromal cell-derived factor-1alpha, chemokine ligands for CCR5 and CXCR4. Activation was blocked by EGTA and by a potent blocker of calcium release-activated Ca++ (CRAC) channels, but was insensitive to pertussis toxin (PTX), implicating CRAC-mediated extracellular Ca++ influx but not Galpha(i) protein-dependent mechanisms. Coreceptor engagement by gp120 and chemokines also activated 2 members of the mitogen-activated protein kinase (MAPK) superfamily, c-Jun amino-terminal kinase/stress-activated protein kinase and p38 MAPK. Furthermore, gp120-stimulated macrophages secreted the chemokines monocyte chemotactic protein-1 and MIP-1beta in a manner that was dependent on MAPK activation. Thus, the gp120 signaling cascade in macrophages includes coreceptor binding, PTX-insensitive signal transduction, ionic signaling including Ca++ influx, and activation of Pyk2 and MAPK pathways, and leads to secretion of inflammatory mediators. HIV-1 Env signaling through these pathways may contribute to dysregulation of uninfected macrophage functions, new target cell recruitment, or modulation of macrophage infection.

Concordant utilization of macrophage entry coreceptors by related variants within an HIV type 1 primary isolate viral swarm

There is considerable diversity among HIV-1 strains in terms of their ability to use entry coreceptors on macrophages, especially CXCR4, but it is not known whether virus-specific differences exist among related members of a viral swarm. Defining how entry coreceptors on primary target cells are utilized by the spectrum of HIV-1 variants that emerge in vivo is important for understanding the relationship between coreceptor selectivity and pathogenesis. HIV-1 89.6(PI) is a dual-tropic primary isolate, and the prototype 89.6-cloned R5X4 Env uses both CXCR4 and CCR5 on macrophages. We generated a panel of env clones from the 89.6(PI) quasispecies and found a mixture of R5, R5X4, and X4 variants on the basis of fusion and infection of coreceptor-transfected cell lines. Here we address the use of macrophage coreceptors by these related Envs by analyzing fusion and infection of primary monocyte-derived macrophages mediated specifically through each coreceptor. All R5X4 Envs utilized both CXCR4 and CCR5 on macrophages, while R5 variants used CCR5 only. One variant characterized in cell lines as X4 used both CXCR4 and CCR5 on macrophages. No Env variant fused with macrophages through alternative coreceptor pathways. Thus, there was heterogeneity in coreceptor use among the related Env variants, but use of each coreceptor specifically in macrophages was consistent among members of the viral swarm. Coreceptor use in transfected cells generally predicted use in primary macrophages, although for some Envs macrophages may be a more sensitive indicator of CCR5 use than transfected cell lines.

A CCR5/CXCR4-independent coreceptor pathway on human macrophages supports efficient SIV env-mediated fusion but not infection: implications for alternative pathways of viral entry

Several coreceptors in addition to CCR5 and CXCR4 support immunodeficiency virus entry in transfected cells, but whether they could play a role in HIV-1 pathogenesis is uncertain. To probe whether human macrophages express potentially functional alternative entry pathways, we analyzed cell-cell fusion and infection of primary macrophage by several SIVmac Envs. All Envs fused with normal macrophages. One, SIVmac316, also fused efficiently with macrophages lacking CCR5. CCR5-independent fusion was not mediated by CXCR4 and was CD4 dependent, while CCR5-mediated fusion was partly independent of CD4. However, pseudotype virions carrying the SIVmac316 Env and HIV-1 core could not infect macrophages through the CCR5-independent pathway, although they did infect wild-type macrophages. Thus, human macrophages possess an alternative coreceptor pathway that mediates SIV Env fusion but does not support infection. Macrophage entry pathways other than CCR5 and CXCR4 may have limited potential in pathogenesis given their restricted capacity for infection despite efficient fusion.

Substance P antagonist (CP-96,345) inhibits HIV-1 replication in human mononuclear phagocytes

Substance P (SP) is a potent modulator of neuroimmunoregulation. We recently reported that human immune cells express SP and its receptor. We have now investigated the possible role that SP and its receptor plays in HIV infection of human mononuclear phagocytes. SP enhanced HIV replication in human blood-isolated mononuclear phagocytes, whereas the nonpeptide SP antagonist (CP-96,345) potently inhibited HIV infectivity of these cells in a concentration-dependent fashion. CP-96,345 prevented the formation of typical giant syncytia induced by HIV Bal strain replication in these cells. This inhibitory effect of CP-96,345 was because of the antagonism of neurokinin-1 receptor, a primary SP receptor. Both CP-96,345 and anti-SP antibody inhibited SP-enhanced HIV replication in monocyte-derived macrophages (MDM). Among HIV strains tested (both prototype and primary isolates), only the R5 strains (Bal, ADA, BL-6, and CSF-6) that use the CCR5 coreceptor for entry into MDM were significantly inhibited by CP-96,345; in contrast, the X4 strain (UG024), which uses CXCR4 as its coreceptor, was not inhibited. In addition, the M-tropic ADA (CCR5-dependent)-pseudotyped HIV infection of MDM was markedly inhibited by CP-96,345, whereas murine leukemia virus-pseudotyped HIV was not affected, indicating that the major effect of CP-96,345 is regulated by Env-determined early events in HIV infection of MDM. CP-96,345 significantly down-regulated CCR5 expression in MDM at both protein and mRNA levels. Thus, SP-neurokinin-1 receptor interaction may play an important role in the regulation of CCR5 expression in MDM, affecting the R5 HIV strain infection of MDM.

Coreceptor choice and T cell depletion by R5, X4, and R5X4 HIV-1 variants in CCR5-deficient (CCR5delta32) and normal human lymphoid tissue

Coreceptor utilization by HIV-1 is an important determinant of pathogenesis. However, coreceptor selectivity is defined in vitro, while in vivo critical pathogenic events occur in lymphoid tissues. Using pharmacological inhibitors, we recently provided evidence that coreceptor selectivity by the R5X4 dual-tropic isolate 89.6 was more restricted in ex vivo infected lymphoid tissue than in vitro [S. Glushakova, Y. Yi, J. C. Grivel, A. Singh, D. Schols, E. De Clercq, R. G. Collman, and L. Margolis (1999). J. Clin. Invest. 104, R7-R11]. Here we extend those observations using CCR5-deficient (CCR5Delta32) lymphoid tissue as well as additional primary isolates. We definitively show that neither CCR5 nor secondary coreceptors used in vitro mediate 89.6 infection in lymphoid tissue. We also demonstrate that restricted coreceptor use in lymphoid tissue ex vivo compared with in vitro utilization occurs with other dual-tropic primary isolates and is not unique to 89.6. For all strains tested that are dual tropic in vitro, severe CD4 T cell depletion in lymphoid tissue correlated with preferential CXCR4 use in this ex vivo system.

Chemokine-receptor activation by env determines the mechanism of death in HIV-infected and uninfected T lymphocytes

There is considerable confusion concerning the mechanism of lymphocyte death during HIV infection. During the course of HIV infection, M-tropic viruses (R5) that use CCR5 chemokine coreceptors frequently evolve to T-tropic viruses (X4) that use CXCR4 receptors. In this study we show that activation of the CD4 or CCR5 receptor by R5 HIVenv causes a caspase 8-dependent death of both uninfected and infected CD4 T cells. In contrast, CXCR4 activation by X4 HIVenv induces a caspase-independent death of both uninfected CD4 and CD8 T cells and infected CD4 cells. These results suggest that activation of the chemokine receptor by HIVenv determines the mechanism of death for both infected and uninfected T lymphocytes.

Heterogeneous spectrum of coreceptor usage among variants within a dualtropic human immunodeficiency virus type 1 primary-isolate quasispecies

Human immunodeficiency virus type 1 (HIV-1) variants that use the coreceptor CCR5 for entry (R5; macrophage tropic) predominate in early infection, while variants that use CXCR4 emerge during disease progression. Some late-stage variants use CXCR4 alone (X4; T-cell tropic), while others use both CXCR4 and CCR5 (R5X4; dualtropic). It has been proposed that dualtropic R5X4 strains are intermediates in the evolution from R5 to X4, and we hypothesized that a dualtropic primary-isolate quasispecies might contain variants that represent the spectrum of coreceptor use in vivo. We generated a panel of 35 functional full-length env clones from the primary-isolate quasispecies of a dualtropic prototype strain, HIV-1 89.6(PI). Thirty of the functional env clones (86%) were R5X4, four (11%) were R5, and one (3%) was predominantly X4. V3 to V5 sequences did not reveal clustering by coreceptor usage, and no specific sequence motif or V3 charge pattern corresponded to coreceptor utilization. Complete sequencing of seven functionally divergent Env proteins revealed > or =98.7% homology and conservation of structurally important domains. Chimeras between the R5X4 89.6 prototype and an R5 variant indicated that multiple regions contributed to the use of CXCR4, while chimeras with the X4 variant implicated a single residue in V4 in CCR5 use. These results confirm, at the molecular level, both that dualtropic variants are a predominant component of late-stage syncytium-inducing isolates and that variants restricted to each coreceptor coexist with dualtropic species in vivo. Coreceptor-restricted minority variants may reflect residual R5 species from earlier in disease as well as emerging X4 variants.

Chemokine signaling and HIV-1 fusion mediated by macrophage CXCR4: implications for target cell tropism

To better understand CXCR4 function on macrophages and the relationship between coreceptor use and macrophage tropism among diverse HIV-1 isolates, we analyzed macrophage pathways involved in Env-mediated fusion, productive HIV-1 infection, and chemokine-elicited signaling. We found that both CXCR4 and CCR5 transduced intracellular signals in monocyte-derived macrophages, activating K+ and Cl- ion channels and elevating intracellular calcium in response to their chemokine ligands stromal cell-derived factor-1alpha and macrophage inflammatory protein-1beta, respectively. The prototype T-tropic X4 strain IIIB infected macrophages poorly, and this was associated with failure of the IIIB Env to fuse efficiently with target macrophages despite functional CXCR4. In contrast, several primary X4 isolates mediated efficient CXCR4-dependent fusion and productive macrophage infection. Several R5X4 strains could fuse with and infect macrophages through both CCR5 and CXCR4. Thus, macrophages express functional CXCR4 and CCR5 but primary and prototype X4 isolates differ in their ability to utilize macrophage CXCR4. Isolates classified as X4 based on coreceptor use may be phenotypically either T-tropic or dual-tropic and, conversely, phenotypically dual-tropic isolates may be either R5X4 or X4 based on coreceptor use.

Co-receptor usage was more predictive than NSI/SI phenotype for HIV replication in macrophages: is NSI/SI phenotyping sufficient?

A monocyte-derived macrophage (MDM) culture assay was used to define the replication kinetics of HIV isolates. Ten-day-old MDMs were infected with HIV. Supernatants were collected and assayed for HIV p24 on days 3, 7, 10, and 14 post-infection (PI). In this assay, SF162 (macrophage tropic, NSI) produced increasing amounts of HIV p24 antigen with increasing time in culture. BRU (nonmacrophage tropic, SI) infection resulted in low levels of HIV p24 antigen with no increase in production during the culture period. A panel of 12 clinical isolates was evaluated. All isolates produced detectable levels of HIV p24 antigen in MDMs. However, the NSI viruses had significantly higher log10 HIV p24 antigen values at all times PI (P < 0.01). Co-receptor usage was determined for all 12 isolates (8 NSI and 4 SI). All SI isolates used CXCR4 for entry; two used CXCR4 only, one used CXCR4, CCR5, and CCR3, and one was a mixture of two isolates using CXCR4 and CCR5. None of the NSI viruses used CXCR4 for entry. All used CCR5 as their predominant co-receptor. Of the eight NSI isolates, three used CCR5 only, two used CCR5 and CCR2b, one used CCR5 and CCR3, and one used CCR5, CCR3, and CCR2b. Log10 HIV p24 antigen production on day 14 PI for viruses that used CCR5+CCR3 (3.79 + 1.40) was greater than for viruses that used CCR5+CCR2b (3.22 + 1.55) or CCR5 (3.32 + 1.49), and all were greater than those that used CXCR4 only (1.69 + 0.28), regardless of SI phenotype (P < 0.05). Thus, in these primary isolates, macrophage tropism and replication kinetics were closely linked to CCR5 utilization, whereas SI capacity was closely linked to CXCR4 utilization. Furthermore, viruses, which could use CCR5 and CCR3 for entry, had a replication advantage in macrophages, regardless of SI phenotype.

HIV-1 gp120 and chemokines activate ion channels in primary macrophages through CCR5 and CXCR4 stimulation

HIV type 1 (HIV-1) uses the chemokine receptors CCR5 and CXCR4 as coreceptors for entry into target cells. Here we show that the HIV-1 envelope gp120 (Env) activates multiple ionic signaling responses in primary human macrophages, which are important targets for HIV-1 in vivo. Env from both CCR5-dependent JRFL (R5) and CXCR4-dependent IIIB (X4) HIV-1 opened calcium-activated potassium (K(Ca)), chloride, and calcium-permeant nonselective cation channels in macrophages. These signals were mediated by CCR5 and CXCR4 because macrophages lacking CCR5 failed to respond to JRFL and an inhibitor of CXCR4 blocked ion current activation by IIIB. MIP-1beta and SDF-1alpha, chemokine ligands for CCR5 and CXCR4, respectively, also activated K(Ca) and Cl(-) currents in macrophages, but nonselective cation channel activation was unique to gp120. Intracellular Ca(2+) levels were also elevated by gp120. The patterns of activation mediated by CCR5 and CXCR4 were qualitatively similar but quantitatively distinct, as R5 Env activated the K(Ca) current more frequently, elicited Cl(-) currents that were approximately 2-fold greater in amplitude, and elevated intracellular Ca(+2) to higher peak and steady-state levels. Env from R5 and X4 primary isolates evoked similar current responses as the corresponding prototype strains. Thus, the interaction of HIV-1 gp120 with CCR5 or CXCR4 evokes complex and distinct signaling responses in primary macrophages, and gp120-evoked signals differ from those activated by the coreceptors' chemokine ligands. Intracellular signaling responses of macrophages to HIV-1 may modulate postentry steps of infection and cell functions apart from infection.

Role of CXCR4 in cell-cell fusion and infection of monocyte-derived macrophages by primary human immunodeficiency virus type 1 (HIV-1) strains: two distinct mechanisms of HIV-1 dual tropism

Dual-tropic human immunodeficiency virus type 1 (HIV-1) strains infect both primary macrophages and transformed T-cell lines. Prototype T-cell line-tropic (T-tropic) strains use CXCR4 as their principal entry coreceptor (X4 strains), while macrophagetropic (M-tropic) strains use CCR5 (R5 strains). Prototype dual tropic strains use both coreceptors (R5X4 strains). Recently, CXCR4 expressed on macrophages was found to support infection by certain HIV-1 isolates, including the dual-tropic R5X4 strain 89.6, but not by T-tropic X4 prototypes like 3B. To better understand the cellular basis for dual tropism, we analyzed the macrophage coreceptors used for Env-mediated cell-cell fusion as well as infection by several dual-tropic HIV-1 isolates. Like 89.6, the R5X4 strain DH12 fused with and infected both wild-type and CCR5-negative macrophages. The CXCR4-specific inhibitor AMD3100 blocked DH12 fusion and infection in macrophages that lacked CCR5 but not in wild-type macrophages. This finding indicates two independent entry pathways in macrophages for DH12, CCR5 and CXCR4. Three primary isolates that use CXCR4 but not CCR5 (tybe, UG021, and UG024) replicated efficiently in macrophages regardless of whether CCR5 was present, and AMD3100 blocking of CXCR4 prevented infection in both CCR5 negative and wild-type macrophages. Fusion mediated by UG021 and UG024 Envs in both wild-type and CCR5-deficient macrophages was also blocked by AMD3100. Therefore, these isolates use CXCR4 exclusively for entry into macrophages. These results confirm that macrophage CXCR4 can be used for fusion and infection by primary HIV-1 isolates and indicate that CXCR4 may be the sole macrophage coreceptor for some strains. Thus, dual tropism can result from two distinct mechanisms: utilization of both CCR5 and CXCR4 on macrophages and T-cell lines, respectively (dual-tropic R5X4), or the ability to efficiently utilize CXCR4 on both macrophages and T-cell lines (dual-tropic X4).

Preferential coreceptor utilization and cytopathicity by dual-tropic HIV-1 in human lymphoid tissue ex vivo

Many HIV-1 isolates at the late stage of disease are capable of using both CXCR4 and CCR5 in transfected cell lines, and are thus termed dual-tropic. Here we asked whether these dual-tropic variants also use both coreceptors for productive infection in a natural human lymphoid tissue microenvironment, and whether use of a particular coreceptor is associated with viral cytopathicity. We used 3 cloned dual-tropic HIV-1 variants, 89.6 and its chimeras 89-v345.SF and 89-v345.FL, which use both CCR5 and CXCR4 in transfected cell lines. In human lymphoid tissue ex vivo, one variant preferentially used CCR5, another preferentially used CXCR4, and a third appeared to be a true dual-tropic variant. The 2 latter variants severely depleted CD4(+) T cells, whereas cytopathicity of the virus that used CCR5 only in lymphoid tissue was mild and confined to CCR5(+)/CD4(+) T cells. Thus, (a) HIV-1 coreceptor usage in vitro cannot be unconditionally extrapolated to natural microenvironment of human lymphoid tissue; (b) dual-tropic viruses are not homogeneous in their coreceptor usage in lymphoid tissue, but probably comprise a continuum between the 2 polar variants that use CXCR4 or CCR5 exclusively; and (c) cytopathicity toward the general CD4(+) T cell population in lymphoid tissue is associated with the use of CXCR4.

A macrophage fusion assay for rapid screening of cloned HIV-1 Env using dual recombinant vaccinia viruses expressing distinct RNA polymerases

HIV-1 cell tropism is determined initially at the level of fusion mediated by the viral envelope glycoprotein (Env). Cell-cell fusion assays are employed widely to study Env-mediated fusion, and generally require transfection of target cells with a reporter plasmid that is activated upon fusion with Env-expressing effector cells. Macrophages are an important target for HIV-1, but fusion studies using primary macrophages are limited by their resistance to transfection. An assay described previously used recombinant vaccinia virus to express T7 polymerase in macrophages, and effector cells transfected with a T7-driven reporter plasmid and infected with recombinant vaccinia virus expressing Env. However, this requires a recombinant vaccinia virus for each Env. We developed a method to study fusion using primary macrophages and HIV-1 env plasmid clones under control of the T7 promoter. Macrophages were infected with a recombinant vaccinia virus expressing the SP6 RNA polymerase. Effector 293T cells were infected with a recombinant vaccinia virus expressing T7 polymerase, and co-transfected with T7-driven env plasmids and an SP6-driven reporter gene plasmid. Cell-cell fusion mediated by T7-driven Env results in SP6-driven reporter gene transactivation. This approach is suitable for rapid analysis of multiple primary isolate, chimeric, or mutant env genes cloned into plasmid vectors.

Patterns of chemokine receptor fusion cofactor utilization by human immunodeficiency virus type 1 variants from the lungs and blood

Human immunodeficiency virus type 1 (HIV-1) infection is highly compartmentalized, with distinct viral genotypes being found in the lungs, brain, and other organs compared with blood. CCR5 and CXCR4 are the principal HIV-1 coreceptors, and a number of other molecules support entry in vitro but their roles in vivo are uncertain. To address the relationship between tissue compartmentalization and the selective use of entry coreceptors, we generated functional env clones from primary isolates derived from the lungs and blood of three infected individuals and analyzed their use of the principal, secondary, orphan, and virus-encoded coreceptors for fusion. All Env proteins from lung viruses used CCR5 but not CXCR4, while those from blood viruses used CCR5 or CXCR4 or both. The orphan receptor APJ was widely used for fusion by Env proteins from both blood and lung viruses, but none used the cytomegalovirus-encoded receptor US28. Fusion mediated by the secondary coreceptors CCR2b, CCR3, CCR8, and CX3CR1 and orphan receptors GPR1, GPR15, and STRL33 was variable and heterogeneous, with relatively broad utilization by env clones from isolates of one subject but limited use by env clones from the other two subjects. However, there was no clear distinction between blood and lung viruses in secondary or orphan coreceptor fusion patterns. In contrast to fusion, none of the secondary or orphan receptors enabled efficient productive infection. These results confirm, at the level of cofactor utilization, previous observations that HIV-1 populations in the lungs and blood are biologically distinct and demonstrate diversity within lung-derived as well as blood-derived quasispecies. However, the heterogeneity in coreceptor utilization among clones from each isolate and the lack of clear distinction between lung- and blood-derived Env proteins argue against selective coreceptor utilization as a major determinant of compartmentalization.

Cofactors for human immunodeficiency virus entry into primary macrophages

Macrophages are permissive for macrophage-tropic (M-tropic) human immunodeficiency virus type 1 (HIV-1) isolates that use CCR5 for entry but are resistant to CXCR-4-dependent T cell-tropic prototype strains. M-tropic variants are critical for HIV-1 transmission, and persons who are homozygous for an inactivating mutation of CCR5 are resistant to HIV-1 in vivo. In vitro, their macrophages and lymphocytes are resistant to M-tropic strains that depend on CCR5. It is shown that CCR5-deficient macrophages are permissive for a dual-tropic isolate, 89.6, that uses CCR5, CXCR-4, and other cofactors. Entry by 89.6 into CCR5-deficient macrophages was blocked by the CXCR-4 ligand SDF and by an anti-CXCR-4 antibody. Immunoflorescence staining and reverse transcription PCR confirmed macrophage CXCR-4 expression. Thus, CXCR-4 on macrophages mediates entry of certain dual-tropic but not T cell-tropic isolates. Therefore, HIV-1 strains differ in how they utilize chemokine receptors as cofactors for entry, and the ability of a chemokine receptor to facilitate entry depends on the cell in which it is expressed.

Case of the month: May 1998--a patient with HIV infection and multiple cranial neuritis

A 36-year old male with a three year history of HIV infection and more recently, CMV retinitis, had several episodes of polyradiculitis with severe bilateral leg pain and urinary retention which resolved slowly over several months. He then presented with high fevers and severe dysphagia with dehydration. Examination showed oral thrush, dyarthric speech and mild memory impairment. Fundoscopic exam showed CMV retinitis and HIV retinopathy. Further examination revealed other cranial nerve signs and leg weakness. MRI scans showed several contrast enhancing abnormalities of cranial nerve roots. The patient died from massive barium aspiration. At autopsy the brain showed multiple CMV cranial neuritis, CMV polyradiculitis and CMV ventriculo-ependymitis. While spinal nerve root involvement by CMV may occur in up to 1% of AIDS patients, involvement of cranial nerves is unusual and CMV infection of multiple cranial nerves is distinctly rare.

An orphan seven-transmembrane domain receptor expressed widely in the brain functions as a coreceptor for human immunodeficiency virus type 1 and simian immunodeficiency virus

Both CD4 and an appropriate coreceptor are necessary for infection of cells by human immunodeficiency virus type 1 (HIV-1) and most strains of HIV-2. The chemokine receptors CCR5 and CXCR4 are the major HIV-1 coreceptors, although some virus strains can also utilize alternative coreceptors such as CCR3 to infect cells. In contrast, most if not all simian immunodeficiency virus (SIV) strains use CCR5 as a coreceptor, and many SIV strains can use CCR5 independently of CD4. In addition, several orphan seven-transmembrane receptors which can serve as HIV-1 and SIV coreceptors have been identified. Here we report that APJ, an orphan seven-transmembrane domain receptor with homology to the angiotensin receptor family, functions as a coreceptor for a number of HIV-1 and SIV strains. APJ was expressed widely in the human brain and in NT2N neurons. APJ transcripts were also detected by reverse transcription-PCR in the CD4-positive T-cell line C8166, but not in peripheral blood leukocytes, microglia, phytohemagglutinin (PHA)- or PHA/interleukin-2-stimulated peripheral blood mononuclear cells, monocytes, or monocyte-derived macrophages. The widespread distribution of APJ in the central nervous system coupled with its use as a coreceptor by some HIV-1 strains indicates that it may play a role in neuropathogenesis.

Influence of the CCR2-V64I polymorphism on human immunodeficiency virus type 1 coreceptor activity and on chemokine receptor function of CCR2b, CCR3, CCR5, and CXCR4

The chemokine receptors CCR5 and CXCR4 are used by human immunodeficiency virus type 1 (HIV-1) in conjunction with CD4 to infect cells. In addition, some virus strains can use alternative chemokine receptors, including CCR2b and CCR3, for infection. A polymorphism in CCR2 (CCR2-V64I) is associated with a 2- to 4-year delay in the progression to AIDS. To investigate the mechanism of this protective effect, we studied the expression of CCR2b and CCR2b-V64I, their chemokine and HIV-1 coreceptor activities, and their effects on the expression and receptor activities of the major HIV-1 coreceptors. CCR2b and CCR2b-V64I were expressed at similar levels, and neither molecule affected the expression or coreceptor activity of CCR3, CCR5, or CXCR4 in cotransfected cell lines. Peripheral blood mononuclear cells (PBMCs) from CCR2-V64I heterozygotes had normal levels of CCR2b and CCR5 but slightly reduced levels of CXCR4. CCR2b and CCR2b-V64I functioned equally well as HIV-1 coreceptors, and CCR2-V64I PBMCs were permissive for HIV-1 infection regardless of viral tropism. The MCP-1-induced calcium mobilization mediated by CCR2b signaling was unaffected by the polymorphism, but MCP-1 signaling mediated by either CCR2b- or CCR2-V64I-encoded receptors resulted in heterologous desensitization (i.e., limiting the signal response of other receptors) of both CCR5 and CXCR4. The heterologous desensitization of CCR5 and CXCR4 signaling by both CCR2 allele receptor types provides a mechanistic link that might help explain the in vivo effects of CCR2 gene variants on progression to AIDS as well as the reported antiviral activity of natural CCR2 ligands.

Determinants of entry cofactor utilization and tropism in a dualtropic human immunodeficiency virus type 1 primary isolate

Human immunodeficiency virus type 1 strain 89.6 is a dualtropic isolate that replicates in macrophages and transformed T cells, and its envelope mediates CD4-dependent fusion and entry with CCR5, CXCR-4, and CCR3. To map determinants of cofactor utilization by 89.6 and determine the relationship between cofactor use and tropism, we analyzed recombinants generated between 89.6 and T-cell-tropic (HXB) or macrophage-tropic (JRFL) strains. These chimeras showed that regions of 89.6 env outside V3 through V5 determine CXCR-4 utilization and T-cell line tropism as well as CCR5 utilization and macrophage tropism. However, the 89.6 env V3 domain also conferred on HXB the ability to use CCR5 for fusion and entry but not the ability to establish productive macrophage infection. CCR3 use was conferred on HXB by 89.6 env V3 or V3 through V5 sequences. While replacement of the 89.6 V3 through V5 region with HXB sequences abrogated CCR3 utilization, replacement of V3 or V4 through V5 separately did not. Thus, CCR3 use is determined by sequences within V3 through V5 and most likely can be conferred by either the V3 or the V4 through V5 domains. These results indicate that cofactor utilization and tropism in this dualtropic isolate are determined by complex interactions among multiple env segments, that distinct regions of the Env glycoprotein may be important for utilization of different chemokine receptors, and that determinants in addition to cofactor usage participate in postentry stages in the virus replication cycle that contribute to target cell tropism.

Neutralizing antibodies in sera from macaques infected with chimeric simian-human immunodeficiency virus containing the envelope glycoproteins of either a laboratory-adapted variant or a primary isolate of human immunodeficiency virus type 1

The magnitude and breadth of neutralizing antibodies raised in response to infection with chimeric simian-human immunodeficiency virus (SHIV) in rhesus macaques were evaluated. Infection with either SHIV-HXB2, SHIV-89.6, or SHIV-89.6PD raised high-titer neutralizing antibodies to the homologous SHIV (SHIV-89.6P in the case of SHIV-89.6PD-infected animals) and significant titers of neutralizing antibodies to human immunodeficiency virus type 1 (HIV-1) strains MN and SF-2. With few exceptions, however, titers of neutralizing antibodies to heterologous SHIV were low or undetectable. The antibodies occasionally neutralized heterologous primary isolates of HIV-1; these antibodies required >40 weeks of infection to reach detectable levels. Notable was the potent neutralization of the HIV-1 89.6 primary isolate by serum samples from SHIV-89.6-infected macaques. These results demonstrate that SHIV-HXB2, SHIV-89.6, and SHIV-89.6P possess highly divergent, strain-specific neutralization epitopes. The results also provide insights into the requirements for raising neutralizing antibodies to primary isolates of HIV-1.

Evidence that antibody-mediated neutralization of human immunodeficiency virus type 1 by sera from infected individuals is independent of coreceptor usage

Human immunodeficiency virus type 1 (HIV-1) uses a variety of chemokine receptors as coreceptors for virus entry, and the ability of the virus to be neutralized by antibody may depend on which coreceptors are used. In particular, laboratory-adapted variants of the virus that use CXCR4 as a coreceptor are highly sensitive to neutralization by sera from HIV-1-infected individuals, whereas primary isolates that use CCR5 instead of, or in addition to, CXCR4 are neutralized poorly. To determine whether this dichotomy in neutralization sensitivity could be explained by differential coreceptor usage, virus neutralization by serum samples from HIV-1-infected individuals was assessed in MT-2 cells, which express CXCR4 but not CCR5, and in mitogen-stimulated human peripheral blood mononuclear cells (PBMC), where multiple coreceptors including CXCR4 and CCR5 are available for use. Our results showed that three of four primary isolates with a syncytium-inducing (SI) phenotype and that use CXCR4 and CCR5 were neutralized poorly in both MT-2 cells and PBMC. The fourth isolate, designated 89.6, was more sensitive to neutralization in MT-2 cells than in PBMC. We showed that the neutralization of 89.6 in PBMC was not improved when CCR5 was blocked by having RANTES, MIP-1alpha, and MIP-1beta in the culture medium, indicating that CCR5 usage was not responsible for the decreased sensitivity to neutralization in PBMC. Consistent with this finding, a laboratory-adapted strain of virus (IIIB) was significantly more sensitive to neutralization in CCR5-deficient PBMC (homozygous delta32-CCR5 allele) than were two of two SI primary isolates tested. The results indicate that the ability of HIV-1 to be neutralized by sera from infected individuals depends on factors other than coreceptor usage.

CXCR-4 is expressed by primary macrophages and supports CCR5-independent infection by dual-tropic but not T-tropic isolates of human immunodeficiency virus type 1

Primary macrophages are infected by macrophage (M)-tropic but not T-cell line (T)-tropic human immunodeficiency virus type 1 (HIV-1) strains, and CCR5 and CXCR-4 are the principal cofactors utilized for CD4-mediated entry by M-tropic and T-tropic isolates, respectively. Macrophages from individuals homozygous for an inactivating mutation of CCR5 are resistant to prototype M-tropic strains that depend on CCR5 but are permissive for a dual-tropic isolate, 89.6, that can use both CCR5 and CXCR-4, as well as CCR2b, CCR3, and CCR8. Here we show that 89.6 entry into CCR5-deficient macrophages is blocked by an anti-CXCR-4 antibody and by the CXCR-4-specific chemokine SDF but not by the ligands to CCR2b or CCR3. Reverse transcription-PCR demonstrated expression of CXCR-4 but not CCR3 or CCR8 in macrophages, while CCR2b was variable. Macrophage surface expression of CXCR-4 was confirmed by immunofluorescence staining and flow cytometry. Thus, CXCR-4 is expressed by primary macrophages and functions as a cofactor for entry by dual-tropic but not T-tropic HIV-1 isolates, and macrophage resistance to T-tropic strains does not result from a lack of the T-tropic entry cofactor CXCR-4. Since CXCR-4 on macrophages can be used by some but not other isolates, these results indicate that HIV-1 strains differ in how they utilize chemokine receptors as cofactors for entry and that the ability of a chemokine receptor to mediate HIV-1 entry differs, depending on the cell type in which it is expressed.

Utilization of chemokine receptors, orphan receptors, and herpesvirus-encoded receptors by diverse human and simian immunodeficiency viruses

Human immunodeficiency virus type 1 (HIV-1) requires both CD4 and a coreceptor to infect cells. Macrophage-tropic (M-tropic) HIV-1 strains utilize the chemokine receptor CCR5 in conjunction with CD4 to infect cells, while T-cell-tropic (T-tropic) strains generally utilize CXCR4 as a coreceptor. Some viruses can use both CCR5 and CXCR4 for virus entry (i.e., are dual-tropic), while other chemokine receptors can be used by a subset of virus strains. Due to the genetic diversity of HIV-1, HIV-2, and simian immunodeficiency virus (SIV) and the potential for chemokine receptors other than CCR5 or CXCR4 to influence viral pathogenesis, we tested a panel of 28 HIV-1, HIV-2, and SIV envelope (Env) proteins for the ability to utilize chemokine receptors, orphan receptors, and herpesvirus-encoded chemokine receptor homologs by membrane fusion and virus infection assays. While all Env proteins used either CCR5 or CXCR4 or both, several also used CCR3. Use of CCR3 was strongly dependent on its surface expression levels, with a larger number of viral Env proteins being able to utilize this coreceptor at the higher levels of surface expression. ChemR1, an orphan receptor recently shown to bind the CC chemokine I309 (and therefore renamed CCR8), was expressed in monocyte and lymphocyte cell populations and functioned as a coreceptor for diverse HIV-1, HIV-2, and SIV Env proteins. Use of ChemR1/CCR8 by SIV strains was dependent in part on V3 loop sequences. The orphan receptor V28 supported Env-mediated cell-cell fusion by four T- or dual-tropic HIV-1 and HIV-2 strains. Three additional orphan receptors failed to function for any of the 28 Env proteins tested. Likewise, five of six seven-transmembrane-domain receptors encoded by herpesviruses did not support Env-mediated membrane fusion. However, the chemokine receptor US28, encoded by cytomegalovirus, did support inefficient infection by two HIV-1 strains. These findings indicate that additional chemokine receptors can function as HIV and SIV coreceptors and that surface expression levels can strongly influence coreceptor use.

CCR5 genotypes in sexually active couples discordant for human immunodeficiency virus type 1 infection status

Persons who are homozygous for the delta32 polymorphism of the CCR5 chemokine receptor gene are highly protected against human immunodeficiency virus type 1 (HIV-1) infection. Previous studies described 54 HIV-1-discordant couples in whom no virus transmission occurred despite extensive sexual contact. The possible role of the delta32 polymorphism in the lack of HIV-1 transmission between these partners was studied. No participants were homozygous for the delta32 allele, but the proportion that was heterozygous was higher among HIV-1-seronegative than HIV-1-seropositive partners (28% vs. 11%, P = .05). This association was seen in heterosexual couples (P = .03) but not in homosexual couples (P = .74). Among white persons, who are most likely to carry the delta32 allele, 38.9% of HIV-1-uninfected and 5.6% of HIV-1-infected heterosexual partners were heterozygous (P = .04). These data are consistent with a possible association between the heterozygous delta32 genotype in heterosexual sex partners and partial protection against HIV-1 infection, and they emphasize the importance of analyzing different risk groups in studies of host factors that influence infection.

Chemokine receptors and HIV

The discovery that chemokine receptors are the human cofactors required along with CD4 for fusion and infection by HIV has opened new directions in AIDS research on mechanisms of viral entry, tropism, and pathogenesis. A possible mechanism of co-receptor function has been demonstrated that involves the formation of a complex on the cell surface between the HIV-1 envelope, CD4, and the coreceptor. Functional studies indicate that this interaction is structurally complex, that it probably involves multiple domains of the coreceptor, and that different virus isolates interact with coreceptors in distinct ways. Other immunodeficiency viruses including simian immunodeficiency virus and feline immunodeficiency virus also utilize chemokine receptors for entry. The identification of genetic polymorphisms helps explain why some people, with alterations in the CCR5 gene that prevent expression, are protected from HIV-1 infection. The discovery of specific HIV-1 fusion coreceptor molecules has not only provided new insights into the mechanisms of viral entry and tropism, but also led to new avenues of investigation on strategies to block HIV infection.

A monoclonal antibody (12G5) directed against CXCR-4 inhibits infection with the dual-tropic human immunodeficiency virus type 1 isolate HIV-1(89.6) but not the T-tropic isolate HIV-1(HxB)

We used a monoclonal antibody (12G5) directed against an extracellular domain of CXCR-4 to investigate the role of this receptor in infection of immortalized lymphoid cell lines, peripheral blood mononuclear cells (PBMCs), and primary brain microglia with a dual-tropic strain of human immunodeficiency virus (HIV-1(89.6)) and a T-tropic strain (HIV-1(IIIB)). Addition of antibody 12G5 to cells prior to and during infection with HIV-1(89.6) inhibited p24 production 100- to 10,000-fold in CEMx174 and 174-CD4 cells and about 10-fold in PBMC cultures but had no activity against infection of either monocyte-derived macrophages or brain microglia. In contrast, 12G5 had little or no effect on infection of CEMx174 cells with HIV-1(IIIB) or HIV-1(HxB). To identify the region of the HIV-1(89.6) envelope that confers sensitivity to 12G5, we used chimeric molecular clones. Chimeras containing the V3 loop region of HIV-1(89.6) were inhibited by 12G5 to the same degree as wild-type HIV-1(89.6) whereas replication of those viruses containing the V3 loop of HIV-1(HxB) was not inhibited by the antibody. A similar pattern was seen in infections of a U87 glioblastoma line that coexpresses CD4 and CXCR-4. Antibody 12G5 was also able to block fusion between HeLa-CD4 cells and CEMx174 cells chronically infected with HIV-1(89.6) but had no effect on fusion mediated by cells chronically infected with HIV-1(IIIB). Taken together, these results suggest that different strains of HIV-1 may interact with different sites on CXCR-4 or may have different binding affinities for the coreceptor.

Abortive infection in HeLaCD4 cells by a primary HIV type 1 isolate: implications for differential host cell tropism

The emergence of T cell-tropic, syncytium-inducing (T-tropic/SI) HIV-1 variants from the background of macrophage-tropic, non-syncytium-inducing (M-tropic/NSI) strains is associated with disease progression in infected individuals. HIV89.6 is a primary isolate with a transitional phenotype: like M-tropic strains it replicates in primary macrophages and lymphocytes but not in most transformed cells, yet it is also syncytium inducing. We have shown that HIV89.6 can utilize both the M-tropic and T-tropic cofactors CCR-5 and CXCR-4, respectively, in conjunction with CD4 for fusion and entry into otherwise nonpermissive nonhuman cells. To better understand the nature of restricted HIV89.6 infection of transformed cells, we analyzed its interaction with CD4-expressing transformed human HeLaCD4-LTR/beta-Gal cells, which contain the beta-galactosidase gene linked to the HIV-1 LTR. Here we show that HIV89.6 enters these cells and undergoes reverse transcription and integration. Furthermore, HIV89.6 induces LTR-driven beta-galactosidase expression, indicating Tat-dependent trans-activation, in a similar number of cells as the permissive T-tropic/SI isolate HIV(HXB). Acute infection with HIV89.6, however, produces markedly lower levels of p24 antigen and infectious virus per trans-activation-positive cell than HIV(HXB). In contrast, transfection results in high levels of expression for both viruses but HIV89.6 still fails to establish spreading infection. HIV89.6 is also blocked after entry in two other nonpermissive cell lines, SUP-T1 and U937. HIV89.6 arrest in HeLaCD4-LTR/beta-Gal cells at a stage subsequent to entry, reverse transcription, integration, and Tat expression is a novel level at which HIV-1 strain- and cell-specific restrictions define host cell tropism. These studies emphasize that complex patterns of tropism are determined by the interplay of permissive or restricted virus-cell interactions at multiple steps in the replication cycle.

Deletion of nef slows but does not prevent CD4-positive T-cell depletion in human immunodeficiency virus type 1-infected human-PBL-SCID mice

The pathogenicity of four human immunodeficiency virus type 1 (HIV-1) isolates with nef deleted for SCID mice repopulated with human peripheral blood leukocytes (hu-PBL-SCID mice) was studied. Deletion of nef led to a substantial reduction in CD4-positive T-cell depletion and delayed kinetics of plasma viremia in infected hu-PBL-SCID mice. Deletion of the nef gene impacts both the efficiency of primary infection and the cytopathicity of virus for infected CD4-positive T cells in this animal model of HIV-1 infection.

Role of CCR5 in infection of primary macrophages and lymphocytes by macrophage-tropic strains of human immunodeficiency virus: resistance to patient-derived and prototype isolates resulting from the delta ccr5 mutation

The alpha-chemokine receptor fusin (CXCR-4) and beta-chemokine receptor CCR5 serve as entry cofactors for T-cell (T)-tropic and macrophage (M)-tropic human immunodeficiency virus type 1 (HIV-1) strains, respectively, when expressed with CD4 in otherwise nonpermissive cells. Some M-tropic and dual-tropic strains can also utilize other beta-chemokine receptors, such as CCR2b and CCR3. A mutation of CCR5 (delta ccr5) was recently found to be common in certain populations and appears to confer protection against HIV-1 in vivo. Here, we show that this mutation results in a protein that is expressed intracellularly but not on the cell surface. Primary CD4 T cells from delta ccr5 homozygous individuals were highly resistant to infection with prototype M-tropic HIV-1 strains, including an isolate (YU-2) that uses CCR5 and CCR3, but were permissive for both a T-tropic strain (3B) and a dual-tropic variant (89.6) that uses CXCR-4, CCR5, CCR3, or CCR2b. These cells were also resistant to M-tropic patient isolates but were readily infected by T-tropic patient isolates. Primary macrophages from delta ccr5 homozygous individuals were also resistant to infection with M-tropic strains, including YU-2, but the dual-tropic strain 89.6 was able to replicate in them even though macrophages are highly resistant to CXCR-4-dependent T-tropic isolates. These data show that CCR5 is the essential cofactor for infection of both primary macrophages and T lymphocytes by most M-tropic strains of HIV-1. They also suggest that CCR3 does not function for HIV-1 entry in primary lymphocytes or macrophages, but that a molecule(s) other than CCR5 can support entry into macrophages by certain virus isolates. These studies further define the cellular basis for the resistance to HIV-1 infection of individuals lacking functional CCR5.

Regions in beta-chemokine receptors CCR5 and CCR2b that determine HIV-1 cofactor specificity

Macrophage-tropic (M-tropic) HIV-1 strains use the beta-chemokine receptor CCR5, but not CCR2b, as a cofactor for membrane fusion and infection, while the dual-tropic strain 89.6 uses both. CCR5/2b chimeras and mutants were used to map regions of CCR5 important for cofactor function and specificity. M-tropic strains required either the amino-terminal domain or the first extracellular loop of CCR5. A CCR2b chimera containing the first 20 N-terminal residues of CCR5 supported M-tropic envelope protein fusion. Amino-terminal truncations of CCR5/CCR2b chimeras indicated that residues 2-5 are important for M-tropic viruses, while 89.6 is dependent on residues 6-9. The identification of multiple functionally important regions in CCR5, coupled with differences in how CCR5 is used by M- and dual-tropic viruses, suggests that interactions between HIV-1 and entry cofactors are conformationally complex.

Resistance to HIV-1 infection in caucasian individuals bearing mutant alleles of the CCR-5 chemokine receptor gene

HIV-1 and related viruses require co-receptors, in addition to CD4, to infect target cells. The chemokine receptor CCR-5 (ref.1) was recently demonstrated to be a co-receptor for macrophage-tropic (M-tropic) HIV-1 strains, and the orphan receptor LESTR (also called fusin) allows infection by strains adapted for growth in transformed T-cell lines (T-tropic strains). Here we show that a mutant allele of CCR-5 is present at a high frequency in caucasian populations (allele frequency, 0.092), but is absent in black populations from Western and Central Africa and Japanese populations. A 32-base-pair deletion within the coding region results in a frame shift, and generates a non-functional receptor that does not support membrane fusion or infection by macrophage- and dual-tropic HIV-1 strains. In a cohort of HIV-1 infected caucasian subjects, no individual homozygous for the mutation was found, and the frequency of heterozygotes was 35% lower than in the general population. White blood cells from an individual homozygous for the null allele were found to be highly resistant to infection by M-tropic HIV-1 viruses, confirming that CCR-5 is the major co-receptor for primary HIV-1 strains. The lower frequency of heterozygotes in seropositive patients may indicate partial resistance.

A dual-tropic primary HIV-1 isolate that uses fusin and the beta-chemokine receptors CKR-5, CKR-3, and CKR-2b as fusion cofactors

Here, we show that the beta-chemokine receptor CKR-5 serves as a cofactor for M-tropic HIV viruses. Expression of CKR-5 with CD4 enables nonpermissive cells to form syncytia with cells expressing M-tropic, but not T-tropic, HIV-1 env proteins. Expression of CKR-5 and CD4 enables entry of a M-tropic, but not a T-tropic, virus strain. A dual-tropic primary HIV-1 isolate (89.6) utilizes both Fusin and CKR-5 as entry cofactors. Cells expressing the 89.6 env protein form syncytia with QT6 cells expressing CD4 and either Fusin or CKR-5. The beta-chemokine receptors CKR-3 and CKR-2b support HIV-1 89.6 env-mediated syncytia formation but do not support fusion by any of the T-tropic or M-tropic strains tested. Our results suggest that the T-tropic viruses characteristic of disease progression may evolve from purely M-tropic viruses prevalent early in virus infection through changes in the env protein that enable the virus to use multiple entry cofactors.

Vaginal transmission of chimeric simian/human immunodeficiency viruses in rhesus macaques

Chimeric simian/human immunodeficiency viruses (SHIVs) that express the env genes derived from distinct HIV type 1 (HIV-1) isolates were tested for the ability to infect rhesus macaques following intravaginal inoculation. SHIVs containing either the HIV-1 HXBc2 or the HIV-1 89.6 envelope glycoproteins were capable of replicating in intravenously inoculated rhesus macaques. However, intravaginal inoculation of animals with these two SHIVs resulted in infection only with the SHIV containing the HIV-1 89.6 glycoprotein. Thus, properties conferred by the envelope glycoproteins in the chimeric virus affect the ability of particular SHIVs to initiate a systemic infection following vaginal inoculation. These results provide indirect support for the hypothesis that the selection of specific viral variants occurs in the genital tracts of individuals exposed to HIV by sexual contact.

An env gene derived from a primary human immunodeficiency virus type 1 isolate confers high in vivo replicative capacity to a chimeric simian/human immunodeficiency virus in rhesus monkeys

To explore the roles played by specific human immunodeficiency virus type 1 (HIV-1) genes in determining the in vivo replicative capacity of AIDS viruses, we have examined the replication kinetics and virus-specific immune responses in rhesus monkeys following infection with two chimeric simian/human immunodeficiency viruses (SHIVs). These viruses were composed of simian immunodeficiency virus SIVmac239 expressing HIV-1 env and the associated auxiliary HIV-1 genes tat, vpu, and rep. Virus replication was assessed during primary infection of rhesus monkeys by measuring plasma SIVmac p27 levels and by quantifying virus replication in lymph nodes using in situ hybridization. SHIV-HXBc2, which expresses the HIV-1 env of a T-cell-tropic, laboratory-adapted strain of HIV-1 (HXBc2), replicated well in rhesus monkey peripheral blood leukocytes (PBL) in vitro but replicated only to low levels when inoculated in rhesus monkeys. In contrast, SHIV-89.6 was constructed with the HIV-1 env gene of a T-cell- and macrophage-tropic clone of a patient isolate of HIV-1 (89.6). This virus replicated to a lower level in monkey PBL in vitro but replicated to a higher degree in monkeys during primary infection. Moreover, monkeys infected with SHIV-89.6 developed an inversion in the PBL CD4/CD8 ratio coincident with the clearance of primary viremia. The differences in the in vivo consequences of infection by these two SHIVs could not be explained by differences in the immune responses elicited by these viruses, since infected animals had comparable type-specific neutralizing antibody titers, proliferative responses to recombinant HIV-1 gp120, and virus-specific cytolytic effector T-cell responses. With the demonstration that a chimeric SHIV can replicate to high levels during primary infection in rhesus monkeys, this model can now be used to define genetic determinants of HIV-1 pathogenicity.

The carboxy-terminal domain is essential for stability and not for virion incorporation of HIV-1 Vpr into virus particles

Vpr is one of the auxiliary gene products encoded by HIV-1 genome. Vpr is a 14-kDa protein and exhibits several interesting characteristics including incorporation into virus particles, oligomerization, localization in the nucleus, and positive regulation of virus replication in primary cells. In an effort to define the structure-function relationship of Vpr, the role of the C-terminus of Vpr was investigated. Site-specific mutagenesis involving deletion, insertion, and substitution of residues at the C-terminus was utilized to generate variants of Vpr. Mutations introduced at the C-terminus affected properties of Vpr in different ways: (i) Vpr containing amino acids 1-72 showed the virion incorporation phenotype, indicating that the C-terminus is not essential for this function, (ii) the C-terminus contributes to the stability of Vpr, and (iii) substitution mutagenesis involving the basic residues showed stability similar to that of wild type, indicating the lack of involvement of these residues in this biochemical property of Vpr. The data generated in this study and our early mutagenic analyses on Vpr suggest that domains noncontiguous in primary sequence contribute to the stability of Vpr through overall conformation of the protein.

Acetylcholinesterase expression in NTera 2 human neuronal cells: a model for developmental expression in the nervous system

Acetylcholinesterase (AChE; EC 3.1.1.7) is expressed in the central nervous system in multiple molecular forms that may subserve multiple functions and may be selectively lost in neurodegenerative illnesses such as Alzheimer's disease. AChE expression has been studied in primary cultures of developing vertebrate nervous system, but investigation has been limited by the lack of a suitable human CNS surrogate cell model system for in vitro studies and the inability of primary brain cultures to provide large numbers of pure neurons. To develop an in vitro model for studies of neuronal AChE expression and function, we utilized a neuronally committed human teratocarcinoma cell line, NTera 2, that can be induced to differentiate to a post-mitotic CNS neuronal phenotype. We found that NTera 2 cells express multiple molecular forms of AChE that are similar to CNS-derived AChE isoforms in velocity sedimentation profile, anion exchange elution profile, and sensitivity to inhibitors. At least two forms of AChE are expressed (G1 and G4), similar to human and rodent brain, and induction of NTera 2 cell differentiation results in an increased G4/G1 ratio, which is characteristic of mature neurons. As in primary CNS neurons, AChE is present in NTera 2 cells in both the cytosolic fraction and in the outer membrane, and is also released in a soluble form. These observations indicate that NTera 2 cells provide a useful human model system for studies of expression of cell-associated and soluble cell-free AChE in developing and mature human neurons and for elucidating the potential role(s) of acetylcholinesterase metabolism in both normal development and neurodegenerative disease states.

Human immunodeficiency virus type 1 infection of SK-N-MC cells: domains of gp120 involved in entry into a CD4-negative, galactosyl ceramide/3' sulfo-galactosyl ceramide-positive cell line

The primary receptor for human immunodeficiency virus (HIV) is the CD4 molecule; however, in vitro evidence suggests that a neutral glycolipid, galactosyl ceramide (GalCer) or a derivative molecule, 3' sulfogalactosyl ceramide (GalS), may serve as an alternative receptor for HIV type 1 (HIV-1) in cells of neural and colonic origin. Biochemical studies have demonstrated that recombinant gp120 envelope protein binds to GalCer/GalS in both solid-phase enzyme-linked immunosorbent assay and high-performance thin-layer chromatography overlays. We have used the SK-N-MC cell line, a CD4-negative, GalCer/GalS-positive cell line previously characterized as susceptible to HIV-1 infection, to identify virus isolates with either a positive infection phenotype, HIVHxB2, or a negative infection phenotype, HIV-1(89.6). Using a solid-phase virus binding assay, we determined the level of restriction in HIV-1(89.6) infection to be at the level of virus-glycolipid binding. Furthermore, using HIV-1HxB2-HIV-1(89.6) chimeras, we have identified a 193-amino-acid fragment from the envelope region of HIV-1HxB2 containing the V3, V4, and V5 regions which confers a positive infection phenotype on the HIV-1(89.6) background. Recombinant viruses which separate this 193-amino-acid fragment into two distinct chimeras are each able to confer a positive infection phenotype on the background of HIV89.6, suggesting that a stable GalCer/GalS-envelope interaction is dependent on the conformation of the envelope protein in the context of the viral membrane. Alternatively, the GalCer/GalS-gp120 bond may involve multiple sites on the oligomeric envelope protein.

Functional analysis of HIV-1 Vpr: identification of determinants essential for subcellular localization

Vpr is a conserved HIV-1 auxiliary protein that localizes to the nuclear region of cells. Vpr is also present in virions, and it is directed into the assembling virus when coexpressed with Gag. Each of these two localization activities may be important for Vpr function, and we recently identified regions of Vpr that are critical for virion incorporation. In this study we analyzed the Vpr domains involved in subcellular localization. Immunofluorescence staining of transfected cells showed that wild-type Vpr localized exclusively to the nuclear region. Mutations in the N-terminal domain that were designed to disrupt a predicted alpha-helical structure resulted in aberrant localization, while conservative substitutions showed a wild-type pattern. A region in the central portion of the protein also has the potential for helical structure, and mutagenesis of two conserved amino acids in this domain (A59, H71) impaired localization, while substitution of a third (Q65) did not. In contrast, neither the conserved Gly and Cys at positions 75-76 nor the C-terminal basic residues (R87, K95) were necessary for nuclear localization. In addition, two-residue insertions within and between the two putative helices disrupted localization but insertion in the C-terminal region did not. Thus, Vpr's subcellular localization function depends on the two putative helical domains but is independent of the conserved Gly-Cys motif and of specific C-terminal basic residues.

Role of the conserved dipeptide Gly75 and Cys76 on HIV-1 Vpr function

Vpr is one of the accessory proteins encoded by the HIV-1 genome. Several interesting features associated with Vpr include incorporation into virus particles, ability to oligomerize, localization in the nucleus, and positive effect on virus production and replication. In order to understand the structure-function relationship of Vpr, we have analyzed the role of the Gly75 and Cys76 (GC) residues which are highly conserved in HIV-1 Vpr and in Vpr and Vpx of HIV-2/SIV. We have generated several substitution mutants involving this dipeptide and have evaluated for expression, stability, nuclear localization, and virion incorporation of Vpr. Our data demonstrate that the GC residues are not essential for virion incorporation and nuclear localization of Vpr. Serine substitution for Cys, however, restricted the localization of Vpr in the cytoplasm without affecting the Gag-directed incorporation of Vpr into virus-like particles. Interestingly, the cysteine-substituted mutants showed altered stability in comparison to the wild type, and substitution mutants for glycine showed minimal effect on stability. These results indicate that the glycine and cysteine do not play a role in nuclear localization or virion incorporation properties of Vpr and further suggest that these two functions of Vpr may not be interdependent.

Mutagenesis of the putative alpha-helical domain of the Vpr protein of human immunodeficiency virus type 1: effect on stability and virion incorporation

vpr is one of the auxiliary genes of human immunodeficiency virus type 1 (HIV-1) and is conserved in the related HIV-2/simian immunodeficiency virus lentiviruses. The unique feature of Vpr is that it is the only nonstructural protein incorporated into the virus particle. Secondary structural analysis predicted an amphipathic alpha-helical domain in the amino terminus of Vpr (residues 17-34) which contains five acidic and four leucine residues. To evaluate the role of specific residues of the helical domain for virion incorporation, mutagenesis of this domain was carried out. Substitution of proline for any of the individual acidic residues (Asp-17 and Glu-21, -24, -25, and -29) eliminated the virion incorporation of Vpr and also altered the stability of Vpr in cells. Conservative replacement of glutamic residues of the helical domain with aspartic residues resulted in Vpr characteristic of wild type both in stability and virion incorporation, as did substitution of glutamine for the acidic residues. In contrast, replacement of leucine residues of the helical domain (residues 20, 22, 23, and 26) by alanine eliminated virion incorporation function of Vpr. These data indicate that acidic and hydrophobic residues and the helical structure in this region are critical for the stability of Vpr and its efficient incorporation into virus-like particles.

V3-independent determinants of macrophage tropism in a primary human immunodeficiency virus type 1 isolate

Human immunodeficiency virus type 1 isolates differ in their ability to productively infect macrophages, and several groups have mapped the genetic basis for macrophage tropism to regions of env that include the third hypervariable region (V3 loop). We recently described a primary isolate (89.6) which is highly macrophage tropic and yet differs from other macrophage-tropic strains studied in that it is cytopathic in T cells. Genetic mapping of macrophage tropism determinants in this virus was done by using chimeras generated with the prototypic non-macrophage-tropic strain HXB2. Replacement of a 2.7-kb env-containing region of HXB with corresponding sequences from 89.6 conferred the macrophage-tropic phenotype, but insertion of the 89.6 V3 loop along with V4/V5 sequences did not. Conversely, placement of HXB sequences that included V3 into 89.6 did not impair this strain's ability to replicate in macrophages. Sequence analysis of V3 shows that 89.6 differs markedly from previously described macrophage-tropic consensus sequences and that it is more similar to highly charged non-macrophage-tropic strains. This suggests either that macrophage tropism is defined by structural determinants resulting from complex interactions among multiple env regions rather than V3 sequence-specific requirements or that there are multiple mechanisms by which different strains may establish productive macrophage infection. In addition, because the HXB V3 loop supports productive macrophage infection in the background of 89.6, phenotypic characterization of V3 sequences should be considered specific to the viral context in which they are placed.

Identification of residues in the N-terminal acidic domain of HIV-1 Vpr essential for virion incorporation

Vpr is one of the auxiliary proteins encoded by the HIV-1 genome and is selectively incorporated into the virus particle. It has been shown that Vpr incorporation in the virus particle requires only the core protein Gag. In an effort to identify the domains of Vpr which are essential for incorporation into the HIV-1 virion, site-specific mutagenesis of vpr was carried out. Mutation of the highly conserved acidic residues in the N-terminal domain (amino acid positions 17-34) eliminated virion incorporation. These mutations disrupt a predicted amphipathic alpha-helical structure that is highly conserved among Vpr sequences. In contrast, alterations of the conserved cysteine (Cys76), basic domain (Arg87 and Lys95), and other residues (Gln65) did not impair the incorporation of Vpr into virus-like particles directed by HIV-1 Gag. The results presented here suggest that protein-protein interactions mediated through the putative helical domain of Vpr may participate in its incorporation into the virus particle.