Induction of inhibitory antibodies to the CCR5 chemokine receptor and their complementary role in preventing SIV infection in macaques

Development of a human leukocyte antigen-based HIV vaccine

Human immunodeficiency virus (HIV) carries abundant human cell proteins, particularly human leukocyte antigen (HLA) molecules when the virus leaves host cells. Immunization in macaques with HLAs protects the animals from simian immunodeficiency virus infection. This finding offers an alternative approach to the development of HLA molecule-based HIV vaccines. Decades of studies have enhanced a great deal of our understanding of the mechanisms of allo-immune response-mediated anti-HIV immunity. These include cell-mediated immunity, innate immunity, and antibody response. These studies provided a rationale for the future design of effective HIV vaccines.

Innate and Adaptive Mucosal Immunity in Protection against HIV Infection

The appalling toll on the populations of developing countries as a result of the HIV epidemic shows no signs of abatement. While costly drug therapies are effective in developed nations, the sheer scale of the epidemic elsewhere makes the need for a vaccine an ever more urgent goal. The prevalent DNA prime-viral boost strategy aims to elicit cytotoxic lymphocytes (CTL) against HIV, but this approach is undermined by the rapid mutation of HIV, which thereby escapes CTL control. Alloimmunity has been found to be protective in vertical transmission from infected mothers to their babies, in alloimmunization of women with their partners’ mononuclear cells, and in monkeys immunized with SIV grown in human T-cells. Vaginal mucosal immunization, as a result of unprotected sex with a regular partner, induced in vitro protection against HIV infection, and this was confirmed in macaques. The second type of natural protection is found in persons with the homozygous Δ32 CCR5 mutation, a 32-base-pair deletion of the CCR5 gene, which results in a lack of cell-surface expression of CCR5, which is associated with an increase in CC chemokines and the development of CCR5 antibodies. These two ‘experiments of nature’ have been used to develop vaccine strategies—first, in vaginal immunization of macaques with CCR5 peptides, in addition to HIV envelope (env) and SIV core (gag) antigens, all of which were linked to the 70-kD heat-shock protein (HSP70); and second, in mucosal allo-immunization of macaques, which also gave rise to in vitro protection from infection. Immunization with this vaccine elicited serum and vaginal IgG and IgA antibodies, IFNγ- and IL-12-producing cells, and increased concentrations of CCL-3 and CCL-4. Vaginal challenge with a simian immunodeficiency virus engineered to carry a human envelope protein (SHIV 89.6) showed significant clearance of SHIV in the immunized macaques. This platform strategy will now be developed to activate the co-stimulatory pathways with the aim of enhancing the primary allogeneic and CCR5-directed responses which are involved in natural protection against HIV infection. Abbreviations: IFN-γ, gamma interferon; IL-12, interleukin 12; MIP-1 α,β, Macrophage inflammatory protein-1; RANTES, Regulated on activation normal T-cell expressed and secreted; SDF-1, stromal-derived factor 1; SIV, simian immunodeficiency virus; and SHIV, engineered SIV carrying a human envelope protein.

ERK1-Based Pathway as a New Selective Mechanism To Modulate CCR5 with Natural Antibodies

Natural human Abs, recognizing an epitope within the first extramembrane loop of CCR5 (the main HIV coreceptor), induce a long-lasting internalization (48 h) of the protein, whereas all known CCR5 modulating molecules show a short-term kinetics (60-90 min). Despite extensive studies on the regulation of CCR5 signaling cascades, which are the effect of concomitant CCR5 internalization by exogenous stimuli such as Abs, downstream signaling continues to be poorly understood. In this article, we report a hitherto unrecognized mechanism of CCR5 modulation mediated by G protein-dependent ERK1 activity. We further demonstrate that ERK1 is localized mainly in the cytoplasmic compartment and that it interacts directly with the CCR5 protein, thus provoking possible CCR5 degradation with a subsequent de novo synthesis, and that re-expression of CCR5 on the cell membrane required several days. In contrast, the RANTES treatment induces a recovery of the receptor on the cell membrane in short-term kinetics without the involvement of de novo protein synthesis. The said new pathway could be relevant not only to better understand the molecular basis of all pathologic conditions in which CCR5 is involved but also to generate new tools to block viral infections, such as the use of recombinant Abs.

Alloantigen-based AIDS vaccine: revisiting a "rightfully" discarded promising strategy

This report revisits the accidental discovery that protection against simian immunodeficiency virus (SIV) infection in the early successful experimental AIDS vaccine studies in Rhesus macaques was due to antibodies directed against human leukocyte antigens (HLAs). The inactivated virus vaccine approach was discarded because protection was due to the host's immune reaction against the HLA acquired by SIV from the human cell lines in which it was grown, rather than against antigenic determinants of SIV itself. Subsequent studies have revealed that immune recognition of HLA on uninfected leukocytes also induces other factors that inhibit infection by both SIV and the human immunodeficiency virus. Pro and con aspects of immunization against HLA as a potential AIDS vaccine strategy are discussed.

Inhibition of human immunodeficiency virus type 1 infection of human CD4+ T cells by microbial HSP70 and the peptide epitope 407-426

Human immunodeficiency virus type 1 (HIV-1) virions contain heat shock proteins (HSP), but these proteins have received limited attention. The objectives of this study were to establish if the microbial 70-kDa HSP exerts an inhibitory effect on the HIV-1 infection of human CD4+ T cells, to identify an inhibitory peptide epitope within the sequence of HSP70, and to evaluate the kinetic features of any inhibitory activity. The results of these studies suggest that microbial HSP70 exerts dose-dependent inhibition on CCR5 (R5) strains of clades B, C, and D of HIV-1 infecting human CD4+ T cells. The site of the HIV-1-inhibitory function was identified within the C-terminal peptide binding domain of HSP70, and the function is expressed by the peptide epitope comprising amino acids 407 to 426. The mechanism of inhibition of HIV-1 infectivity by HSP70 is blocking of the CCR5 coreceptors directly and indirectly by inducing CC chemokines and APOBEC3G. The inhibitory effect of HSP70, its C-terminal fragment, or peptide 407-426 may make HSP70 useful as a microbicidal agent. A potentiating noncognate inhibition of HIV-1 infectivity by combined treatment with HSP70 and monoclonal or polyclonal antibody to CCR5 was demonstrated. This novel strategy may be utilized in therapeutic immunization against HIV-1 infection.

Suppression of Multiclade R5 and X4 Human Immunodeficiency Virus Type-1 Infections by a Coreceptor-Based Anti-HIV Strategy

A cyclic chimeric dodecapeptide (cCD) mimicking the conformation-specific domains of CCR5 and CXCR4 was prepared in which Gly-Asp links the amino and carboxyl termini of two combined pentapeptides (S169-G173 of CCR5; E179-R183 of CXCR4) derived from human immunodeficiency virus type-1 (HIV-1) coreceptors. The immunization of Balb/c mice with cCD conjugated with a multiple-antigen peptide (cCD-MAP) induced seven cCD-specific monoclonal antibodies (mAbs, CPMAb-I to -VII) that reacted with native CCR5 and CXCR4. Among the tested mAbs, CPMAb-I and -II potently inhibited the infection of both the R5 and X4 laboratory strains. CPMAb-III and -VI were effective against only R5 laboratory strains, and also against some X4 and R5 primary isolates. CPMAb-IV and -V had potent antiviral activities against the R5 and X4 primary isolates. In particular, CPMAb-VII was protective against not only R5 and X4 laboratory strains, but also most of the R5 and X4 primary isolates. Moreover, cCD-MAP immunization also induced antibodies that were effective against R5 and X4 multiclade HIV-1 isolates in vitro in two of three cynomolgus monkeys. Taken together, the results suggest that cCD-MAP is a candidate multiclade immunogen that can be used to block multiclade R5 and X4 HIV-1 infections.

Mucosal alloimmunization elicits T-cell proliferation, CC chemokines, CCR5 antibodies and inhibition of simian immunodeficiency virus infectivity

The hypothesis was tested that mucosal stimulation with unmatched mononuclear cells would induce systemic alloimmune responses. Rectal or vaginal mucosal administration of 10(4)-10(7) unmatched mononuclear cells induced significant dose-dependent T-cell proliferation stimulated by the allogeneic cells in rhesus macaques. This was associated with a significant upregulation of CD8(+) T-cell-derived suppressor factor, as well as the CC chemokines CCL3, CCL4 and CCL5. In addition, there was a dose-dependent increase in antibodies to CCR5. These responses were associated with decreased in vitro simian immunodeficiency virus (SIV) infectivity of CD4(+) T cells. A further investigation of SIV infectivity of CD4(+) T cells separated from multiparous macaques also showed significant inhibition compared with male macaques. It is suggested that vaginal or rectal exposure to allogeneic stimulation by a partner's HLA antigens in seminal fluid, as occurs during sexual intercourse, or immunization by semi-allogeneic fetuses in multiparous females may elicit protection against SIV or human immunodeficiency virus infection.

Natural antibodies to CCR5 from breast milk block infection of macrophages and dendritic cells with primary R5-tropic HIV-1

In the present study, we demonstrate that breast milk of 66% and 83% of HIV-seronegative and seropositive women, respectively, contains natural Abs of the secretory IgA and IgG isotypes directed against the CCR5 coreceptor for R5-tropic strains of HIV-1. Abs to CCR5 were affinity purified on a matrix to which a synthetic peptide corresponding to the second extracellular loop of CCR5 had been coupled. The purified Abs bound to the CCR5 peptide in a dose-dependent fashion and to both native CCR5 expressed by Chinese hamster ovary cells transfected with CCR5 gene, macrophages, and immature dendritic cells. Although the avidity differed, the amount of anti-CCR5 Abs did not significantly differ between breast milk of HIV-seropositive and -seronegative women. Purified anti-CCR5 Abs inhibited up to 75% infection of macrophages and dendritic cells with HIV(BaL) and HIV(JR-CSF). Our observations provide evidence for a role of natural Abs to CCR5 in breast milk in controlling transmissibility of HIV through breastfeeding.

Predictive value of anti-cell and anti-human immunodeficiency virus (HIV) humoral responses in HIV-1-exposed seronegative cohorts of European and Asian origin

Unconventional immune responses have been demonstrated in individuals who, despite repeated exposure to human immunodeficiency virus (HIV) infection, remain seronegative. As environmental exposure to pathogens and genetic background may modulate immune responses differentially, one Italian and two Asian populations of HIV-1-exposed seronegative individuals were studied. In serum samples from each group, IgG to CCR5, IgG to CD4 and IgA to gp41 were measured, which were previously described as markers of unconventional immunity in HIV-exposed seronegative Caucasians. Given the importance of conformational epitopes in virus-cell interactions, IgG to CD4-gp120 complex was also measured. It was found that markers of HIV exposure were present in all populations studied. HIV-specific humoral responses (IgA to gp41 and IgG to CD4-gp120 complex) were extremely significant predictors of HIV exposure (P<0.0001 in both cases), whereas the predictive values of anti-cell antibodies (anti-CCR5 and anti-CD4) varied between populations. Evidence is provided for the correlation of these differences with route of exposure to HIV and level of natural antibodies to cross-reactive microbial antigens. In conclusion, exposed seronegative individuals of ethnically different origins display similar signs of HIV-dependent unconventional immunity. A specific relevance must be attributed to different innate and acquired factors.

Innate immunity in experimental SIV infection and vaccination

Innate immunity represents the first line of defence to pathogens besides the physical barrier and seems to play a role in protection against HIV/SIV infection and disease progression. High production of beta-chemokines and CD8+ T cell anti-viral factors in naive as well as in vaccinated macaques has been associated with complete or partial protection against SIV infection indicating that genetic or environmental factors may influence their production. This innate immunity may help in generating HIV/SIV-specific responses upon the first exposure to HIV/SIV. SIV subunit vaccines given by the targeted iliac lymph node route have been shown to induce an increased production of CD8+ T cell suppressor factors and beta-chemokines. Only a few vaccine studies have focused on enhancing the innate immune response against HIV/SIV. The use of unmethylated CpG motifs, HSP and GM-CSF as adjuvants in SIV vaccines has been shown to induce production of HIV/SIV-inhibiting cytokines and beta-chemokines, which seem to be important in modulating and steering the adaptive immune responses. HSP has also been shown to induce gammadelta+ T cells, which contribute to the innate immunity. More knowledge about the interplay between the innate and adaptive immune responses is important to develop new HIV/SIV vaccine strategies.

CCR5 targeted SIV vaccination strategy preventing or inhibiting SIV infection

Cell-surface CCR5 is a major coreceptor with CD4 glycoprotein, mediating cellular entry of CCR5 strains of HIV-1 or SIV. We targeted the SIV CCR5 coreceptor in a combined CCR5-SIV antigen immunization strategy. Rhesus macaques were immunized i.m. with the 70 kDa heat shock protein (HSP70) covalently linked to the CCR5 peptides, SIV gpl20 and p27. Intravenous challenge with SIV mac 8980 prevented SIV infection or decreased the viral load with the CCR5-SIV combined vaccine. CC chemokines and antibodies which block and downmodulateCCR5 were induced, as well as immune responses to the subunit SIV antigens. This novel vaccination strategy complements cognate immunity to SIV with innate immunity to the CCR5 coreceptor of SIV.

Selection of Active ScFv to G-Protein-Coupled Receptor CCR5 Using Surface Antigen-Mimicking Peptides

This study describes the use of cyclic peptides for use in the selection of single-chain (ScFv) antibodies specific for the HIV-1 coreceptor CCR5, a representative G-protein-coupled receptor (GPCR). A tandem ligation strategy was developed for preparing biotinylated cyclic peptides, first through an orthogonal end-to-end ligation and then a chemoselective ligation with functionalized biotin. Cyclic peptides mimicking the extracellular loops of CCR5 and their unconstrained counterparts were then used for solution-phase selection of ScFv antibodies from a phage display antibody library. Antibodies reactive with CCR5 on cells were detected using a homogeneous high throughput assay. Of 19 isolated ScFv antibodies that bound to CCR5+ cells, three inhibited CCR5-mediated but not CXCR4-mediated HIV infection. Only ScFvs selected by binding to cyclic constrained peptides exhibited inhibitory activity. Our results demonstrate that surface-antigen mimetics of a GPCR are effective tools for selecting active, site-specific ScFv antibodies that hold promise as immunological reagents and therapeutics.

A novel HIV-CCR5 receptor vaccine strategy in the control of mucosal SIV/HIV infection

OBJECTIVE

To develop a novel SIV-CCR5 receptor vaccine strategy that will protect macaques from SHIV infection by the vaginal mucosal route.

DESIGN

The rationale for this strategy is that humans who express the homozygous delta32 CCR5 mutation and the associated upregulation of CC chemokines, the down-modulation of cell-surface expression of CCR5 and antibodies to CCR5 are protected against HIV infection.

METHODS

A vaccine was prepared consisting of three extracellular peptides of CCR5, an N-terminal HIV gp120 fragment generated in transgenic plants and recombinant SIV p27. These were linked to the 70 000 Mr microbial heat shock protein (HSP70) carrier. The vaccine was administered (x3) either by the vaginal mucosal route or by targeting the proximity of the draining iliac lymph nodes.

RESULTS

Serum and vaginal fluid IgG and IgA antibodies, IL-2 and IFN-gamma-producing cells, and macrophage-inflammatory protein (MIP) 1beta and MIP-1alpha (CCL4 and CCL3) were significantly raised in immunized macaques (P = 0.01-0.05). Vaginal challenge with SHIV(89.6P) infected all macaques, but sequential analysis over 24 weeks showed a significant variation in viral loads between the animals (P = 0.05). Whereas SHIV(89.6P) persisted in the four unimmunized macaques, in five of the eight immunized macaques the virus was cleared or became undetectable by reverse transcriptase-polymerase chain reaction. The CD4 cell counts in the immunized macaques were significantly higher than those in unimmunized animals (P < 0.05).

CONCLUSION

An immunization strategy that targets both the virus and its CCR5 receptor has significantly inhibited SHIV(89.6P) infection and may serve as a novel strategy in the prevention of HIV transmission.

The role of immunity in protection from mucosal SIV infection in macaques

The need for an effective vaccine against HIV has prompted a refocusing of attention on mucosal immunity. More than 75% of all infections are acquired across a mucosal surface. It is therefore a prerequisite for a vaccine to target directly the mucosal tissues or indirectly the regional lymph nodes in order to prevent or control viral replication. Although mucosal immunization has induced responses at the genital or rectal surfaces, immune mechanisms alone have not been shown to be sufficient to contain infections in macaques. A growing body of evidence suggests that a dual mechanism may be required for effective mucosal protection, mediated by specific CD4 and CD8 T cell and antibody responses to the immunizing agents, plus innate antiviral factors and beta chemokines that down-regulate CCR5 coreceptors. Targeted iliac lymph node immunization with SIV gp 120 and p27 in alum prevents SIV infection or significantly decreases the viral load when immunized macaques were challenged with SIV by the rectal route. Indeed, in addition to specific immunity, including significant SIgA antibody secreting cells in the iliac lymph nodes, CD8-suppressor factor and the 3beta chemokines (RANTES, MIP-1alpha and MIP-1beta) are significantly associated with protection against rectal mucosal SIV infection.

Serum IgA of HIV-exposed uninfected individuals inhibit HIV through recognition of a region within the alpha-helix of gp41

BACKGROUND

HIV-specific IgA is present in HIV-exposed uninfected individuals (EU) and neutralizes primary strains of HIV-1 in vitro.

OBJECTIVES

To analyse the antigenic correlates of HIV-1 neutralization using HIV epitopes and IgA from EU and HIV-seropositive individuals.

METHODS

Sera from six heterosexual couples discordant for HIV serostatus, six age-matched HIV-infected subjects and six healthy controls (HC; as negative controls) were analysed. IgA binding on HIV Env recombinant proteins was assayed. Serum IgA was affinity purified on specific Env peptides and tested in HIV neutralization using resting and activated peripheral blood mononuclear cells as target. Monoclonal antibody 2F5 was used as neutralizing positive control. BALB/c mice were immunized with specific gp41 peptide and anti-sera were tested in syncytia formation and in HIV viral replication.

RESULTS

IgA of EU exclusively bound an epitope within gp41; this epitope was restricted to residues 582-588 (QARILAV) and corresponded to the leucine zip motif in the alpha-helical region. IgA of HIV-positive patients recognized epitopes expressed both in gp120 and gp41; these epitopes were in the N-terminal portion of the extramembrane region. Additionally, IgA of EU and antisera of QARILAV-immunized Balb/C mice blocked syncytia formation and viral replication. The dose-dependent neutralization behaviour of specific QARILAV-purified IgA was very similar to that obtained with monoclonal antibody 2F5.

CONCLUSION

These results have important implications for the development of vaccines and therapeutical strategies against HIV infection.

Allo-immunization elicits CCR5 antibodies, SDF-1 chemokines, and CD8-suppressor factors that inhibit transmission of R5 and X4 HIV-1 in women

Studies in humans suggest that allo-immunization induces CC-chemokines, CD8-suppressor factors (SF) and anti-HIV immunity. Here we report that allo-immunization with unmatched leucocytes from partners of women with recurrent spontaneous abortion elicits specific antibodies to the CCR5 receptor. Such antibodies inhibit replication of M-tropic HIV-1 (R5) and MIP-1beta-mediated chemotaxis. These CCR5 antibodies were also found in the sera of multiparous women that were naturally immunized by semi-allogeneic fetal antigens. The specificity of these antibodies was demonstrated by adsorption with CCR5 transfected HEK-293 cells, a baculovirus CCR5 preparation and a peptide of the 2nd extra-cellular loop of CCR5. Allo-immunization also stimulated increased concentrations of the CXC chemokine, SDF-1alpha and CD8-SF that inhibit T-tropic HIV-1 (X4) replication. We suggest that allo- immunization may elicit (a) CC chemokines, CCR5 antibodies and CD8-SF that inhibit M-tropic HIV-1 infection and (b) the CXC chemokine SDF-1alpha and CD8-SF that inhibit T-tropic HIV-1 infection.

The basis for HIV immunotherapeutic vaccines

The drug treatments introduced in recent years for HIV infection have enabled a marked reduction in morbidity and prolongation of life. These treatments, however, are often associated with acute and chronic toxicities, the development of resistant virus can limit their effectiveness, and they are too expensive and difficult to administer in most third world settings. A successful HIV immunotherapeutic vaccine has the potential to overcome these problems, and would be a valuable advance. The most promising approaches have induced the type of immune response found to correlate with reduced activity of HIV in man, especially cytotoxic T-cell responses, or have led to reduced HIV or SIV viral load and increased CD4 counts in non-human primates or man. The agents that have led to one or both of these effects have been selected for review, and include inactivated envelope depleted virus, recombinant envelope glycoprotein, DNA vaccines utilising HIV peptides or gene products, viral vectors, such as canarypox or attenuated vaccinia, with HIV core proteins. There are other approaches, such as alloimmunity, for which no candidate products yet exist, but which conceptually appear promising. Currently, however, only a few phase III studies of HIV therapeutic vaccines have been completed in man, and there has been a modest therapeutic effect. Further development of both existing and new candidates remains one of the key priorities in our fight against HIV.

Immunogenicity of the extracellular domains of C-C chemokine receptor 5 and the in vitro effects on simian immunodeficiency virus or HIV infectivity

The C-C chemokine receptor CCR5 serves an important function in chemotaxis of lymphocytes, monocytes, and dendritic cells. CCR5 is also the major coreceptor in most macrophage-tropic HIV-1 infections. Immunization of rhesus macaques with a baculovirus-generated CCR5 construct or peptides derived from the sequences of the four extracellular domains of CCR5 elicited IgG and IgA Abs, inhibition of SIV replication, and CD4+ T cell proliferative responses to three of the extracellular domains of CCR5. The immune sera reacted with cell surface CCR5 expressed on HEK 293 cells. T and B cell epitope mapping revealed major and minor T and B cell epitopes in the N-terminal, first, and second loops of CCR5. The three C-C chemokines, RANTES, macrophage-inflammatory protein-1alpha, and macrophage-inflammatory protein-1beta, were up-regulated by immunization with the CCR5-derived peptides, and the cell surface expression of CCR5 was decreased. The CCR5 Abs were complementary to the C-C chemokines in inhibiting HIV replication in vitro. Immunization with the four extracellular domains of CCR5 suggests that three of them are immunogenic, with maximal T cell responses being elicited by the second loop peptide. However, maximal Abs to the cell surface CCR5 or viral inhibitory Abs in vitro were induced by the N-terminal peptide. Up-regulation of the three C-C chemokines and down-modulation of cell surface CCR5 were elicited by the second loop, N-terminal, and first loop peptides. The data suggest that a dual mechanism of C-C chemokines and specific Abs may engage and down-modulate the CCR5 coreceptors and prevent in vitro HIV or SIV replication.

Anti-CCR5 antibodies in sera of HIV-positive individuals

One of the proposed mechanisms for resistance to human immunodeficiency virus-1 (HIV-1) infection is the presence of antibodies against receptor for CC-chemokines (CCR5). These antibodies, detected in sera of uninfected individuals exposed to HIV, have been shown to downmodulate surface CCR5 in vivo and are able to neutralize the infectivity of CCR5 strains in vitro. To address the potential role of anti-CCR5 antibodies in HIV infection, we analyzed anti-CCR5 antibody levels in plasma from HIV-infected patients who present a wide range of CD4(+) T-cell counts and viral load. Increased levels of anti-CCR5 antibodies were found in plasma from 13/46 HIV-positive donors compared with healthy controls (0/36). However, antibody levels were not associated with disease stage evaluated by CD4(+) T-cell counts and viral load.

Induction of immune responses and break of tolerance by DNA against the HIV-1 coreceptor CCR5 but no protection from SIVsm challenge

An inactivating mutation in the human CCR5 gene reduces the risk of HIV-1 infection in individuals with homozygous alleles. We explored whether genetic immunization would induce an immune response directed to CCR5 structures and if immunological tolerance toward endogenous CCR5 could be broken. We also studied whether this immunization approach could protect cynomolgus monkeys from an infection, with SIVsm, which primarily uses CCR5 as a coreceptor. Epidermal but not intramuscular delivery of the CCR5 gene to mice elicited strong IgG antibody binding responses to CCR5. Intramucosal immunization of cynomolgus macaques with CCR5 DNA followed by boosts with CCR5 peptides induced prominent IgG and IgA antibody responses in serum and vaginal washings. The CCR5-specific antibodies neutralized the infectivity of primary human R5 HIV-1 strains, and the macaque SIVsm but not that of a tissue culture-adapted X4 HIV-1 strain. The consecutive CCR5 gene and CCR5 peptide immunizations induced B- and T-cell responses to peptides representing both human and macaque amino acid sequences of the respective CCR5 proteins. This indicates that tolerance was broken against endogenous macaque CCR5, which has a 98% homology to the human CCR5 gene. After the final boost, the vaccinated monkeys together with two control monkeys were challenged with SIVsm. Neither protection against nor enhancement of SIVsm infection was achieved.

Mechanisms for adaptation of simian immunodeficiency virus to replication in alveolar macrophages

In contrast to the simian immunodeficiency virus SIVmac239, which replicates poorly in rhesus monkey alveolar macrophages, a variant with nine amino acid changes in envelope (SIVmac239/316E) replicates efficiently and to high titer in these same cells. We examined levels of viral DNA, RNA, antigen, and infectious virus to identify the nature of the block to SIVmac239 replication in these cells. Low levels of viral antigen (0.1 to 1.0 ng of p27 per ml) and infectious virus (100 to 1,000 infectious units per ml) were produced in the supernatant 1 to 4 days after SIVmac239 infection, but these levels did not increase subsequently. SIVmac239 DNA was synthesized in these macrophage cultures during the initial 24 h after infection, but the levels did not increase subsequently. Quantitation of the numbers of infectious cells in cultures over time and the results of experiments in which cells were reexposed to SIVmac239 after the initial exposure indicated that only a small proportion of cells were susceptible to SIVmac239 infection in these alveolar macrophage cultures and that the vast majority (>95%) of cells were refractory to SIVmac239 infection. In contrast to the results with SIVmac239, the levels of viral antigen, infectious virus, and viral DNA increased exponentially 2 to 7 days after infection by SIVmac239/316E, reaching levels greater than 100 ng of p27 per ml and 100,000 infectious units per ml. Since SIVmac239/316E has previously been described as a virus capable of infecting cells in a relatively CD4-independent fashion, we examined the levels of CD4 expression on the surface of fresh and cultured alveolar macrophages from rhesus monkeys. The levels of CD4 expression were extremely low, below the limit of detection by flow cytometry, on greater than 99% of the macrophages. CCR5(+) cells were profoundly depleted only from alveolar macrophage cultures infected with SIVmac239/316E. High concentrations of an antibody to CD4 delayed but did not block replication of SIVmac239/316E. The results suggest that the adaptation of SIVmac316 to efficient replication in alveolar macrophages results from its ability to infect these cells in a CD4-independent fashion or in a CD4-dependent fashion even at extremely low levels of surface CD4 expression. Since resident macrophages in brains and lungs of humans also express little or no CD4, our findings predict the presence of human immunodeficiency virus type 1 that is relatively CD4 independent in the lung and brain compartments of infected people.