No abstract

Reverse transcription of the human immunodeficiency virus type 1 (HIV-1) RNA genome appears to be strictly regulated at the level of initiation. The primer binding site (PBS), at which the tRNA 3 Lys molecule anneals and reverse transcription is initiated, is present in a highly structured region of the untranslated leader RNA. Detailed mutational analysis of the U5 leader stem identified a sequence motif in the U5 region that is critical for activation of the PBS-bound tRNA 3 Lys primer. This U5 motif, termed the primer activation signal (PAS), may interact with the T⌿C arm of the tRNA 3 Lys primer, similar to the additional interaction proposed for the genome of Rous sarcoma virus and its tRNA Trp primer. This suggests that reverse transcription is regulated by a common mechanism in all retroviruses. In HIV-1, the PAS is masked through base pairing in the U5 leader stem. This provides a mechanism for positive and negative regulation of reverse transcription. Based on structure probing of the mutant and wild-type RNAs, an RNA secondary structure model is proposed that juxtaposes the critical PAS and PBS motifs.Infection of the host cell by a retroviral particle results in reverse transcription of the viral RNA genome into doublestranded DNA, which subsequently becomes integrated into the host cell genome (1). Reverse transcription is mediated by the virion-associated enzyme reverse transcriptase (RT), 1 and a cellular tRNA molecule is used as a primer (2). The tRNA primer binds with its 3Ј-terminal 18 nts to a complementary sequence, the primer binding site (PBS), that is located in the 5Ј-untranslated leader region of the viral RNA genome. Retroviral particles are competent to initiate reverse transcription shortly after budding from the producer cell, but there is also evidence that reverse transcription in virions is limited (3-7). This suggests that initiation of reverse transcription is restricted until a new host cell is infected. The mechanism that regulates reverse transcription is not known, but sequence motifs and RNA secondary structures in the region flanking the PBS have been implicated (8 -13). Alternatively, reverse transcription may be restricted in extracellular virions by the low concentration of dNTP molecules in virus particles.In the genome of human immunodeficiency virus type I (HIV-1), the PBS is predicted to be part of an extended RNA structure. Several RNA secondary structure models have been proposed for this region of the 5Ј-untranslated leader (14 -17), and there is recent evidence that this region can adopt alternate conformations (18,19). The model depicted in Fig. 1A shows the U5-PBS hairpin that occludes part of the PBS and the extended U5 leader stem, which is formed by base pairing of sequences in the upstream U5 and the downstream leader region. Similar RNA secondary structures have been predicted for other retroviruses (16, 20 -23). For the avian Rous sarcoma virus (RSV), these structures have been reported to regulate initiation of reverse transcription (8 -10, 24). In RSV, ...

SummaryMacrophages contribute to HIV-1 pathogenesis by forming a viral reservoir and mediating neurological disorders. Cell-free HIV-1 infection of macrophages is inefficient, in part due to low plasma membrane expression of viral entry receptors. We find that macrophages selectively capture and engulf HIV-1-infected CD4+ T cells leading to efficient macrophage infection. Infected T cells, both healthy and dead or dying, were taken up through viral envelope glycoprotein-receptor-independent interactions, implying a mechanism distinct from conventional virological synapse formation. Macrophages infected by this cell-to-cell route were highly permissive for both CCR5-using macrophage-tropic and otherwise weakly macrophage-tropic transmitted/founder viruses but restrictive for nonmacrophage-tropic CXCR4-using virus. These results have implications for establishment of the macrophage reservoir and HIV-1 dissemination in vivo.

DC-specific ICAM3-grabbing non-integrin (DC-SIGN), which is expressed on DCs, can interact with a variety of pathogens such as HIV-1, hepatitis C, Ebola, cytomegalovirus, Dengue virus, Mycobacterium, Leishmania, and Candida albicans. We demonstrate that human milk can inhibit the DC-SIGN-mediated transfer of HIV-1 to CD4 + T lymphocytes as well as viral transfer by both immature and mature DCs. The inhibitory factor directly interacted with DC-SIGN and prevented the HIV-1 gp120 envelope protein from binding to the receptor. The human milk proteins lactoferrin, α-lactalbumin, lysozyme, β-casein, and secretory leukocyte protease inhibitor did not bind DC-SIGN or demonstrate inhibition of viral transfer. The inhibitory effect could be fully alleviated with an Ab recognizing the Lewis X (Le X ) sugar epitope, commonly found in human milk. Le X in polymeric form or conjugated to protein could mimic the inhibitory activity, whereas free Le X sugar epitopes could not. We reveal that a Le X motif present in human milk can bind to DC-SIGN and thereby prevent the capture and subsequent transfer of HIV-1 to CD4 + T lymphocytes. The presence of such a DC-SIGN-binding molecule in human milk may both influence antigenic presentation and interfere with pathogen transfer in breastfed infants.