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Immature dendritic cells (DCs), unlike mature DCs, require the viral determinant nef to drive immunodeficiency virus (SIV and HIV) replication in coculture with CD4+ T cells. Since immature DCs may capture and get infected by virus during mucosal transmission, we hypothesized that Nef associated with the virus or produced during early replication might modulate DCs to augment virus dissemination. Adenovirus vectors expressing nef were used to introduce nef into DCs in the absence of other immunodeficiency virus determinants to examine Nef-induced changes that might activate immature DCs to acquire properties of mature DCs and drive virus replication. Nef expression by immature human and macaque DCs triggered IL-6, IL-12, TNF-α, CXCL8, CCL3, and CCL4 release, but without up-regulating costimulatory and other molecules characteristic of mature DCs. Coincident with this, nef-expressing immature DCs stimulated stronger autologous CD4+ T cell responses. Both SIV and HIV nef-expressing DCs complemented defective SIVmac239 delta nef, driving replication in autologous immature DC-T cell cultures. In contrast, if DCs were activated after capturing delta nef, virus growth was not exacerbated. This highlights one way in which nef-defective virus-bearing immature DCs that mature while migrating to draining lymph nodes could induce stronger immune responses in the absence of overwhelming productive infection (unlike nef-containing wild-type virus). Therefore, Nef expressed in immature DCs signals a distinct activation program that promotes virus replication and T cell recruitment but without complete DC maturation, thereby lessening the likelihood that wild-type virus-infected immature DCs would activate virus-specific immunity, but facilitating virus dissemination.

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IntroductionHIV-1 entry and fusion are thought to involve an initial interaction between the envelope glycoprotein, gp120, and cell-surface CD4. This interaction exposes a site within gp120 that then interacts with a coreceptor (ie, CCR5 or CXCR4), inducing a conformational change within the gp41 portion of the viral envelope; this set of events results in insertion of the fusion domain of gp41 into the cell membrane. Cell-surface expression of specific chemokine receptors and CD4 is necessary, but not sufficient, to confer HIV-1 permissivity. 1,2 Cellular resistance to HIV-1 infectivity can be due to fusion/entry failure, suggesting this differentiation-associated restriction is due to a positive factor or negative factor.Comparison of HIV-1 nonpermissive and permissive U937 subclones revealed relatively equivalent cell-surface CD4 and CXCR4 and a lack of CCR5. 1 A notable difference was that nonpermissive, but not permissive, subclones expressed detectable granule-associated proteinases, cathepsin G (CatG) and human leukocyte elastase (HLE). 3 These same proteinases are known to be cell-surface-associated in certain situations and to bind HIV-1 envelope proteins, 4,5 but in association with the lipid bilayer, enzymatic activity and antigen detection are absent or compromised. 6 This suggests that plasma membrane-associated proteinases may exhibit nonenzymatic receptor functions and that location, rather than gene expression, might impact HIV-1 permissivity.Traditionally, the proteinase activity of HLE has been characterized in aqueous environments, and cell-surface lipids are known to negatively influence its catalytic activity 6 further supporting a nonenzymatic function for plasma membrane HLE. 7 In fact, the primary actions of cell-surface HLE and CatG involve adhesion, chemotaxis, and stem-cell mobilization. [8][9][10] Granule-associated HLE rapidly translocates to the cell surface in response to many agonists including the bacterial endotoxin lipopolysaccharide (LPS), 11 suggesting these activation signals rapidly mobilize HLE to the cell surface in the absence of protein synthesis. The precise domains in HLE that allow its association with the plasma membrane are not completely known; however, the ␣ 1 antitrypsin (␣ 1 PI) domain that initiates chemotaxis has been identified as a hydrophobic pentapeptide concealed within its C-terminal region. 9 The corresponding hydrophobic pentapeptides found in various other serine proteinase inhibitors contain a pair of phenylalanine residues that share the motif FXFXX or FXXFX, where X represents the hydrophobic amino acids V, L, I, or M. 9 The identification of a pentapeptide having a similar motif within the fusion domain of HIV-1 gp41 (FLGFL) and other human viruses led to the discovery of a ligand-receptor interaction between gp41 and the ␣ 1 PI receptor HLE. 5 In HIV-1-seropositive patients, viral load is correlated with circulating levels of ␣ 1 PI. 12 In this study, 100% of patients in the asymptomatic category of disease manifested deficient levels of ␣ 1 PI....