The innate antiviral factor APOBEC3G targets replication of measles, mumps and respiratory syncytial viruses.

The cytidine deaminase APOBEC3G (apolipoprotein B mRNA-editing enzyme-catalytic polypeptide 3G; A3G) exerts antiviral activity against retroviruses, hepatitis B virus, adeno-associated virus and transposable elements. We assessed whether the negative-strand RNA viruses measles, mumps and respiratory syncytial might be affected by A3G, and found that their infectivity was reduced by 1-2 logs (90-99 %) in A3G overexpressing Vero cells, and in T-cell lines expressing A3G at physiological levels. Viral RNA was co-precipitated with HA-tagged A3G and could be amplified by RT-PCR. Interestingly, A3G reduced viral transcription and protein expression in infected cells by 50-70 %, and caused an increased mutation frequency of 0.95 mutations per 1000 nt in comparison to the background level of 0.22/1000. The observed mutations were not specific for A3G [cytidine to uridine (C→U) or guanine to adenine (G→A) hypermutations], nor specific for ADAR (adenosine deaminase acting on RNA, A→G and U→C transitions, with preference for next neighbour-nucleotides U = A>C>G). In addition, A3G mutants with inactivated catalytic deaminase (H257R and E259Q) were inhibitory, indicating that the deaminase activity is not required for the observed antiviral activity. In combination, impaired transcription and increased mutation frequencies are sufficient to cause the observed reduction in viral infectivity and eliminate virus replication within a few passages in A3G-expressing cells.

Target cell-mediated editing of HIV-1 cDNA by APOBEC3 proteins in human macrophages.

Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) proteins are encapsidated by assembling HIV-1 virions and edit viral cDNA in the next round of infection. Using alpha interferon (IFN-α)-treated monocyte-derived macrophages, we show that infrequent editing of HIV-1 reverse transcripts can also be mediated by APOBEC3 proteins supplied by the targets of infection. Based on the local sequence contexts of these mutations and the established characteristics of APOBEC3 protein expression in myeloid cells, we speculate that APOBEC3A may be responsible for a substantial proportion of this activity.

Human APOBEC3G-mediated editing can promote HIV-1 sequence diversification and accelerate adaptation to selective pressure.

Human apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3G (APOBEC3G, hereinafter referred to as A3G) is an innate virus restriction factor that inhibits human immunodeficiency virus type 1 (HIV-1) replication and induces excessive deamination of cytidine residues in nascent reverse transcripts. To test the hypothesis that this enzyme can also help generate viral sequence diversification and the evolution of beneficial viral variants, we have examined the impact of A3G on the acquisition of (-)2',3'-dideoxy-3'-thiacytidine (3TC) resistance in vitro. That characteristic resistance mutations are rapidly fixed in the presence of A3G and 3TC suggests that A3G-mediated editing can be an important source of genetic variation on which natural selection acts to shape the structure of HIV-1 populations.

Defining APOBEC3 expression patterns in human tissues and hematopoietic cell subsets.

Human APOBEC3 enzymes are cellular DNA cytidine deaminases that inhibit and/or mutate a variety of retroviruses, retrotransposons, and DNA viruses. Here, we report a detailed examination of human APOBEC3 gene expression, focusing on APOBEC3G (A3G) and APOBEC3F (A3F), which are potent inhibitors of human immunodeficiency virus type 1 (HIV-1) infection but are suppressed by HIV-1 Vif. A3G and A3F are expressed widely in hematopoietic cell populations, including T cells, B cells, and myeloid cells, as well as in tissues where mRNA levels broadly correlate with the lymphoid cell content (gonadal tissues are exceptions). By measuring mRNA copy numbers, we find that A3G mRNA is approximately 10-fold more abundant than A3F mRNA, implying that A3G is the more significant anti-HIV-1 factor in vivo. A3G and A3F levels also vary between donors, and these differences are sustained over 12 months. Responses to T-cell activation or cytokines reveal that A3G and A3F mRNA levels are induced approximately 10-fold in macrophages and dendritic cells (DCs) by alpha interferon (IFN-alpha) and approximately 4-fold in naïve CD4(+) T cells. However, immunoblotting revealed that A3G protein levels are induced by IFN-alpha in macrophages and DCs but not in T cells. In contrast, T-cell activation and IFN-gamma had a minimal impact on A3G or A3F expression. Finally, we noted that A3A mRNA expression and protein expression are exquisitely sensitive to IFN-alpha induction in CD4(+) T cells, macrophages, and DCs but not to T-cell activation or other cytokines. Given that A3A does not affect HIV-1 infection, these observations imply that this protein may participate in early antiviral innate immune responses.

Correlates of resistance to HIV-1 infection in homosexual men with high-risk sexual behaviour.

OBJECTIVE: To investigate possible correlates of HIV resistance in participants from the Amsterdam Cohort of Homosexual men who have remained HIV seronegative despite high-risk sexual behaviour. DESIGN/METHODS: We studied in vitro HIV-1 susceptibility and adaptive and innate immunity in 29 high-risk seronegative (HRSN) and 15 HIV-negative pre-seroconversion (pre-SC) homosexual men from the same Amsterdam Cohort Study (ACS) who seroconverted to HIV-1 positive during active follow-up. Host genetics were compared between HRSN and HIV-positive ACS participants. RESULTS: We found lower in vitro susceptibility for a CCR5-using (R5) HIV-1 variant, higher RANTES production levels, but no difference in coreceptor expression in HRSN as compared with pre-SC controls. Reduced R5 in vitro susceptibility of two HRSN tested was restored to normal levels by addition of antibodies against beta-chemokines. A higher proportion of HRSN carried the SDF-1 3'A variant and HLA-A*11, A*31 and Cw*15 alleles. ELIspot analysis with HIV-1 peptide stimulation revealed low frequencies of HIV-1-specific CD8 interferon-gamma producing cytotoxic T cells in both HRSN and pre-SC controls. CONCLUSIONS: Low in vitro R5 susceptibility of cells from the HRSN men was due to beta-chemokine mediated inhibition of virus replication. The presence of HIV-1 specific cytotoxic T cells in both HRSN and pre-SC participants may signify exposure to the virus rather than protection from infection. Host genetic characteristics and other factors affecting innate immunity may contribute to differential resistance to HIV-1 infection among exposed seronegative individuals.

Dynamics of HIV type 1 recombination following superinfection.

There are currently few detailed studies describing HIV-1 recombination events or the potential impact of recombination on drug resistance. We describe here the viral recombination dynamics in a drug-naive patient initially infected with a circulating recombinant form 19 (CRF19) virus containing transmitted drug resistance mutations followed by superinfection with "wild-type" subtype B virus. Single genome analysis showed replacement of the primary CRF19 virus by recombinants of the CRF19 virus and the superinfecting subtype B virus. The CRF19/B recombinant virus dominating after superinfection had lost drug resistance mutations and at no time was the superinfecting subtype B variant found to be dominant in blood plasma. Furthermore, the detection of recombinant viruses in seminal plasma indicates the potential for onward transmission of these strains.

Molecular evolution of human immunodeficiency virus type 1 upon transmission between human leukocyte antigen disparate donor-recipient pairs.

BACKGROUND: To address evolution of HIV-1 after transmission, we studied sequence dynamics in and outside predicted epitopes of cytotoxic T lymphocytes (CTL) in subtype B HIV-1 variants that were isolated from 5 therapy-naive horizontal HLA-disparate donor-recipient pairs from the Amsterdam Cohort Studies on HIV-1 infection and AIDS. METHODOLOGY/PRINCIPAL FINDINGS: In the first weeks after transmission, the majority of donor-derived mutations in and outside donor-HLA-restricted epitopes in Gag, Env, and Nef, were preserved in the recipient. Reversion to the HIV-1 subtype B consensus sequence of mutations in- and outside donor-HLA-restricted CTL epitopes, and new mutations away from the consensus B sequence mostly within recipient-HLA-restricted epitopes, contributed equally to the early sequence changes. In the subsequent period (1-2 years) after transmission, still only a low number of both reverting and forward mutations had occurred. During subsequent long-term follow-up, sequence dynamics were dominated by forward mutations, mostly (50-85%) in recipient-HLA-restricted CTL epitopes. At the end of long-term follow-up, on average 43% of the transmitted CTL escape mutations in donor-HLA-restricted epitopes had reverted to the subtype B consensus sequence. CONCLUSIONS/SIGNIFICANCE: The relatively high proportion of long-term preserved mutations after transmission points to a lack of back selection even in the absence of CTL pressure, which may lead to an accumulating loss of critical CTL epitopes. Our data are supportive for a continuous adaptation of HIV-1 to host immune pressures which may have implications for vaccine design.

APOBEC3F can inhibit the accumulation of HIV-1 reverse transcription products in the absence of hypermutation. Comparisons with APOBEC3G.

APOBEC3F (apolipoprotein B mRNA-editing enzyme catalytic polypeptide 1-like protein 3F) is a cytidine deaminase that, like APOBEC3G, is able to restrict the replication of HIV-1/delta vif. Initial studies revealed high numbers of mutations in the cDNA of viruses produced in the presence of these proteins, suggesting that cytidine deamination underpinned the inhibition of infection. However, we have recently shown that catalytically inactive APOBEC3G proteins, derived through mutation of the C-terminal cytidine deaminase motif, still exert a substantial antiviral effect. Here, we have generated a panel of APOBEC3F mutant proteins and show that the C-terminal cytidine deaminase motif is essential for catalytic activity and that catalytic activity is not necessary for the antiviral effect of APOBEC3F. Furthermore, we demonstrate that the antiviral activities of wild-type and catalytically inactive APOBEC3F and APOBEC3G proteins correspond well with reductions in the accumulation of viral reverse transcription products. Additional comparisons between APOBEC3F and APOBEC3G suggest that the loss of deaminase activity is more detrimental to APOBEC3G function than to APOBEC3F function, as reflected by perturbations to the suppression of reverse transcript accumulation as well as antiviral activity. Taken together, these data suggest that both APOBEC3F and APOBEC3G are able to function as antiviral factors in the absence of cytidine deamination, that this editing-independent activity is an important aspect of APOBEC protein-mediated antiviral phenotypes, but that APOBEC3F may be a better model in which to study it.

Low levels of human immunodeficiency virus type 1 DNA in high-risk seronegative men.

We detected human immunodeficiency virus type 1 (HIV-1) DNA at very low levels in sequential peripheral blood mononuclear cell samples of five out of six high-risk, seronegative, homosexual men and five out of five individuals 7.8 to 1.6 years prior to seroconversion. These data indicate a high prevalence of low-level HIV-1 DNA in exposed seronegative individuals.

Low-level CD4+ T cell activation is associated with low susceptibility to HIV-1 infection.

Different features have been associated with low susceptibility to HIV type 1 (HIV-1) infection in exposed seronegative individuals. These include genetic make-up such as homozygosity for the CCR5-Delta32 allele and the presence of HIV-specific CTLs. We studied immune activation and immune responsiveness in relation to HIV-1 susceptibility in 42 high-risk seronegative (HRSN) participants of the Amsterdam Cohort Studies and 54 men from the same cohort who were seronegative at the moment of analysis but later became HIV seropositive. HRSN had higher naive (CD45RO CD27) CD4 and CD8 T cell numbers and lower percentages of activated (HLADR CD38, CD70) CD4 and proliferating (Ki67) CD4 and CD8 T cells, irrespective of previous episodes of sexually transmittable infections. Furthermore, whole blood cultures from HRSN showed lower lymphoproliferative responses than healthy laboratory controls. These data suggest that low levels of immune activation and low T cell responsiveness may contribute to low HIV susceptibility.

Decreasing sensitivity to RANTES (regulated on activation, normally T cell-expressed and -secreted) neutralization of CC chemokine receptor 5-using, non-syncytium-inducing virus variants in the course of human immunodeficiency virus type 1 infection.

In approximately half of human immunodeficiency virus (HIV) type 1-infected individuals, the development of CXC chemokine receptor 4-using, syncytium-inducing (SI) virus variants precedes a rapid progression to acquired immunodeficiency syndrome (AIDS). In other individuals, only CC chemokine receptor 5-using (R5), non-SI (NSI) virus variants are present throughout infection. These individuals may be either long-term survivors (LTSs) or rapid progressors. The basis for this variable disease progression in individuals with only R5 virus variants is not yet fully understood. In this study, the beta-chemokine sensitivity of biological HIV-1 clones isolated from 13 individuals who harbored only R5, NSI virus variants (7 LTSs and 6 progressors) was investigated. We found a statistically significant decrease in sensitivity of virus variants to RANTES (regulated on activation, normally T cell-expressed and -secreted) neutralization during the course of progressive infection, but not during follow-up of LTSs. Our data suggest that a role exists for RANTES neutralization sensitivity of HIV-1 in AIDS pathogenesis.