Electron microscopic analysis of HIV-host cell interactions

Hypothetically, antibodies may neutralize enveloped viruses by diverse mechanisms, such as disruption of receptor binding, interference with conformational changes required for virus entry, steric hindrance, or virus aggregation. Here, we demonstrate that retroviral infection mediated by the avian sarcoma-leukosis virus (ASLV-A) envelope glycoproteins can be neutralized by an antibody directed against a functionally unimportant component of a chimeric receptor protein. Thus, the binding of an antibody in proximity to the retroviral envelope glycoprotein-receptor complex, without binding to the entry machinery itself, results in neutralization. This finding provides additional support for the hypothesis that steric hindrance is sufficient for antibody-mediated neutralization of retroviruses.The generation of virus-neutralizing antibodies contributes to the success of prophylactic vaccines against viral infections. In vitro, neutralization can be defined as the disruption of the viral life cycle prior to viral gene expression due to the direct binding of antibody molecules to viral surface antigens, without the necessary participation of other factors (2). Once bound to the envelope glycoprotein (Env) spike of a virus, an antibody can hypothetically effect neutralization by steric hindrance, direct receptor competition, prevention of necessary conformational changes or induction of deleterious changes in the viral Env, causing virion aggregation, or occupation of a large fraction of the virion surface (11,12).Studies of the stoichiometries of neutralization of different strains of human immunodeficiency virus type 1 (HIV-1) by nine different representative antibodies revealed that the binding of one antibody molecule is sufficient to neutralize the function of the whole Env trimer (23). As the nine antibodies tested bind to very different structural and functional elements on the HIV-1 gp120 and gp41 envelope glycoproteins, the shared stoichiometry implies that a generic mechanism underlies HIV-1 neutralization by antibodies. One such mechanism is steric hindrance, in which the bulk of the antibody molecule interferes with the virus entry process. This hypothesis is supported by experiments demonstrating that an unrelated antibody, the M2 anti-FLAG antibody, can effectively neutralize HIV-1 virions that carry an exogenous FLAG epitope in the functionally unimportant V4 variable region of gp120 (14). Importantly, M2 antibody binding to the FLAG-tagged gp120 does not compete for binding to the CD4/CCR5 receptors and does not inhibit CD4-induced conformational changes within gp120. As these results suggest the hypothesis that steric hindrance is sufficient for antibody-mediated neutralization of HIV-1, we sought to test this hypothesis using a novel experimental design. We investigated whether a model antibody can achieve neutralization when targeted to the vicinity of the viral Env spike and its cognate receptor without actually binding to the entry machinery per se.Avian sarcoma-leukosis virus (ASLV-A) Env was sele...

Section: Vol 81 2007 Notes 8811supporting

confidence: 48%

Antibody Binding in Proximity to the Receptor/Glycoprotein Complex Leads to a Basal Level of Virus Neutralization

Yang

Lipchina

Lifton

et al. 2007

“…However, more recent studies have reported that human immunodeficiency virus type 1 (HIV-1) entry specificity may be dependent on the specific target cell, as HIV-1 evidently can establish productive infection in certain cell types by receptor-specific, clathrinmediated endocytosis (15). In addition, other studies indicate that HIV-1 entry may or may not require internalization of CD4 receptor (36)(37)(38)46) and that HIV-1 can infect certain cultured CD4-negative human fibroblast cells, suggesting alternatives to CD4-mediated entry (8,48,56). These apparent variations reported from independent studies of HIV-1 entry pathways naturally raise the question of the type of entry mechanisms used by other animal lentiviruses, especially those with defined cell receptors.…”

mentioning

confidence: 99%

Receptor-Mediated Entry by Equine Infectious Anemia Virus Utilizes a pH-Dependent Endocytic Pathway

Jin

Zhang²,

Weisz

et al. 2005

Previous studies of human and nonhuman primate lentiviral entry mechanisms indicate a predominant use of pH-independent pathways, although more recent studies of human immunodeficiency virus type 1 entry appear to reveal the use of a low-pH-dependent entry pathway in certain target cells. To expand the characterization of the specificity of lentiviral entry mechanisms, we have in the current study examined the entry pathway of equine infectious anemia virus (EIAV) during infection of its natural target, equine macrophages, permissive equine fibroblastic cell lines, and an engineered mouse cell line expressing the recently defined equine lentivirus receptor-1. The specificity of EIAV entry into these various cells was determined by assaying the effects of specific drug treatments on the level of virus entry as measured by quantitative real-time PCR assay of early reverse transcripts or by measurements of virion production. The results of these studies demonstrated that EIAV entry into all cell types was substantially inhibited in a dose-dependent manner by treatment with the vacuolar H ؉ -ATPase inhibitors concanamycin A and bafilomycin A1 or the lysosomotropic weak base ammonium chloride. In contrast, treatments with sucrose to block clathrin-mediated endocytosis or with chloroquine to block organelle acidification failed to inhibit EIAV entry into the same target cells. The observed inhibition of EIAV entry was shown not to be related to cytotoxicity. Taken together, these experiments reveal for the first time that EIAV receptor-mediated entry into target cells is via a low-pH-dependent endocytic pathway.

“…The HIV-1 Nef protein accelerates viral pathogenesis and enhances viral infectivity, in part by increasing the cytoplasmic delivery of HIV cores through the fusion pathway (9,17,20,25,28,32). In contrast, the endocytic pathway culminates in virion inactivation in acidified endosomes or degradation in lysosomes and, in most cells, nonproductive infection (19,22,27,29,30,33).…”

mentioning

confidence: 99%

Compensatory Link between Fusion and Endocytosis of Human Immunodeficiency Virus Type 1 in Human CD4 T Lymphocytes

Schaeffer

Soros

Greene

2004

104

101

Although fusion remained dependent on CD4 and chemokine receptor binding, the endosome inhibitors did not alter surface expression of CD4 and CXCR4. These results suggest that fusion in the presence of the endosome inhibitors likely occurs within nonacidified endosomes. However, the ability of these inhibitors to impair vesicle trafficking from early to late endosomes in some cells could also increase the recycling of these virioncontaining endosomes to the cell surface, where fusion occurs. In summary, our results reveal an unexpected, CD4-mediated reciprocal relationship between the pathways governing HIV virion fusion and endocytosis.