Macrophage-tropic HIV induces and exploits dendritic cell chemotaxis

New Approaches to Dendritic Cell-Based Therapeutic Vaccines Against HIV-1 Infection

Due to the success of combined antiretroviral therapy (cART) in recent years, the pathological outcome of Human Immunodeficiency Virus type 1 (HIV-1) infection has improved substantially, achieving undetectable viral loads in most cases. Nevertheless, the presence of a viral reservoir formed by latently infected cells results in patients having to maintain treatment for life. In the absence of effective eradication strategies against HIV-1, research efforts are focused on obtaining a cure. One of these approaches is the creation of therapeutic vaccines. In this sense, the most promising one up to now is based on the establishing of the immunological synapse between dendritic cells (DCs) and T lymphocytes (TL). DCs are one of the first cells of the immune system to encounter HIV-1 by acting as antigen presenting cells, bringing about the interaction between innate and adaptive immune responses mediated by TL. Furthermore, TL are the end effector, and their response capacity is essential in the adaptive elimination of cells infected by pathogens. In this review, we summarize the knowledge of the interaction between DCs with TL, as well as the characterization of the specific T-cell response against HIV-1 infection. The use of nanotechnology in the design and improvement of vaccines based on DCs has been researched and presented here with a special emphasis.

Schwann cell-derived CXCL1 contributes to human immunodeficiency virus type 1 gp120-induced neuropathic pain by modulating macrophage infiltration in mice

The neuroinflammatory responses to human immunodeficiency virus type 1 (HIV-1) coat proteins, such as glycoprotein 120 (gp120), are considered to be responsible for the HIV-associated distal sensory neuropathy. Accumulating evidences suggest that T-cell line tropic X4 gp120 increases macrophage infiltration into the peripheral nerves, and thereby induces neuroinflammation leading to pain. However, the mechanisms underlying X4 gp120-induced macrophage recruitment to the peripheral nervous systems remain unclear. Here, we demonstrated that perineural application of X4 gp120 from HIV-1 strains IIIB and MN elicited mechanical hypersensitivity and spontaneous pain-like behaviors in mice. Furthermore, flow cytometry and immunohistochemical studies revealed increased infiltration of bone marrow-derived macrophages into the parenchyma of sciatic nerves and dorsal root ganglia (DRG) 7 days after gp120 IIIB or MN application. Chemical deletion of circulating macrophages using clodronate liposomes markedly suppressed gp120 IIIB-induced pain-like behaviors. In in vitro cell infiltration analysis, RAW 264.7 cell (a murine macrophage cell line) was chemoattracted to conditioned medium from gp120 IIIB- or MN-treated cultured Schwann cells, but not to conditioned medium from these gp120-treated DRG neurons, suggesting possible involvement of Schwann cell-derived soluble factors in macrophage infiltration. We identified using a gene expression array that CXCL1, a chemoattractant of macrophages and neutrophils, was increased in gp120 IIIB-treated cultured Schwann cells. Similar to gp120 IIIB or MN, perineural application of recombinant CXCL1 elicited pain-like behaviors accompanied by macrophage infiltration to the peripheral nerves. Furthermore, the repeated injection of CXCR2 (receptor for CXCL1) antagonist or CXCL1 neutralizing antibody prevented both pain-like behaviors and macrophage infiltration in gp120 IIIB-treated mice. Thus, the present study newly defines that Schwann cell-derived CXCL1, secreted in response to X4 gp120 exposure, is responsible for macrophage infiltration into peripheral nerves, and is thereby associated with pain-like behaviors in mice. We propose herein that communication between Schwann cells and macrophages may play a prominent role in the induction of X4 HIV-1-associated pain.

CCR5 Revisited: How Mechanisms of HIV Entry Govern AIDS Pathogenesis

The chemokine receptor CCR5 has been the focus of intensive studies since its role as a coreceptor for HIV entry was discovered in 1996. These studies lead to the development of small molecular drugs targeting CCR5, with maraviroc becoming in 2007 the first clinically approved chemokine receptor inhibitor. More recently, the apparent HIV cure in a patient transplanted with hematopoietic stem cells devoid of functional CCR5 rekindled the interest for inactivating CCR5 through gene therapy and pharmacological approaches. Fundamental research on CCR5 has also been boosted by key advances in the field of G-protein coupled receptor research, with the realization that CCR5 adopts a variety of conformations, and that only a subset of these conformations may be targeted by chemokine ligands. In addition, recent genetic and pathogenesis studies have emphasized the central role of CCR5 expression levels in determining the risk of HIV and SIV acquisition and disease progression. In this article, we propose to review the key properties of CCR5 that account for its central role in HIV pathogenesis, with a focus on mechanisms that regulate CCR5 expression, conformation, and interaction with HIV envelope glycoproteins.

HIV-1 Trans Infection of CD4(+) T Cells by Professional Antigen Presenting Cells

Since the 1990s we have known of the fascinating ability of a complex set of professional antigen presenting cells (APCs; dendritic cells, monocytes/macrophages, and B lymphocytes) to mediate HIV-1 trans infection of CD4(+) T cells. This results in a burst of virus replication in the T cells that is much greater than that resulting from direct, cis infection of either APC or T cells, or trans infection between T cells. Such APC-to-T cell trans infection first involves a complex set of virus subtype, attachment, entry, and replication patterns that have many similarities among APC, as well as distinct differences related to virus receptors, intracellular trafficking, and productive and nonproductive replication pathways. The end result is that HIV-1 can sequester within the APC for several days and be transmitted via membrane extensions intracellularly and extracellularly to T cells across the virologic synapse. Virus replication requires activated T cells that can develop concurrently with the events of virus transmission. Further research is essential to fill the many gaps in our understanding of these trans infection processes and their role in natural HIV-1 infection.

R5 HIV-1 envelope attracts dendritic cells to cross the human intestinal epithelium and sample luminal virions via engagement of the CCR5

The gastrointestinal tract is a principal route of entry and site of persistence of human immunodeficiency virus type 1 (HIV-1). The intestinal mucosa, being rich of cells that are the main target of the virus, represents a primary site of viral replication and CD4⁺ T-cell depletion. Here, we show both in vitro and ex vivo that HIV-1 of R5 but not X4 phenotype is capable of selectively triggering dendritic cells (DCs) to migrate within 30 min between intestinal epithelial cells to sample virions and transfer infection to target cells. The engagement of the chemokine receptor 5 on DCs and the viral envelope, regardless of the genetic subtype, drive DC migration. Viruses penetrating through transient opening of the tight junctions likely create a paracellular gradient to attract DCs. The formation of junctions with epithelial cells may initiate a haptotactic process of DCs and at the same time favour cell-to-cell viral transmission. Our findings indicate that HIV-1 translocation across the intestinal mucosa occurs through the selective engagement of DCs by R5 viruses, and may guide the design of new prevention strategies.

Binding of HIV-1 gp120 to DC-SIGN promotes ASK-1-dependent activation-induced apoptosis of human dendritic cells

During disease progression to AIDS, HIV-1 infected individuals become increasingly immunosuppressed and susceptible to opportunistic infections. It has also been demonstrated that multiple subsets of dendritic cells (DC), including DC-SIGN(+) cells, become significantly depleted in the blood and lymphoid tissues of AIDS patients, which may contribute to the failure in initiating effective host immune responses. The mechanism for DC depletion, however, is unclear. It is also known that vast quantities of viral envelope protein gp120 are shed from maturing HIV-1 virions and form circulating immune complexes in the serum of HIV-1-infected individuals, but the pathological role of gp120 in HIV-1 pathogenesis remains elusive. Here we describe a previously unrecognized mechanism of DC death in chronic HIV-1 infection, in which ligation of DC-SIGN by gp120 sensitizes DC to undergo accelerated apoptosis in response to a variety of activation stimuli. The cultured monocyte-derived DC and also freshly-isolated DC-SIGN(+) blood DC that were exposed to either cross-linked recombinant gp120 or immune-complex gp120 in HIV(+) serum underwent considerable apoptosis after CD40 ligation or exposure to bacterial lipopolysaccharide (LPS) or pro-inflammatory cytokines such as TNFα and IL-1β. Furthermore, circulating DC-SIGN(+) DC that were isolated directly from HIV-1(+) individuals had actually been pre-sensitized by serum gp120 for activation-induced exorbitant apoptosis. In all cases the DC apoptosis was substantially inhibited by DC-SIGN blockade. Finally, we showed that accelerated DC apoptosis was a direct consequence of excessive activation of the pro-apoptotic molecule ASK-1 and transfection of siRNA against ASK-1 significantly prevented the activation-induced excessive DC death. Our study discloses a previously unknown mechanism of immune modulation by envelope protein gp120, provides new insights into HIV immunopathogenesis, and suggests potential therapeutic approaches to prevent DC depletion in chronic HIV infection.

HIV-1 infected individuals become increasingly immunocompromised and susceptible to opportunistic infection during disease progression, which is associated with significant reduction of the dendritic cell number in the peripheral blood or secondary lymphoid tissues. Because dendritic cells are the most powerful antigen-presenting cells, their survival is critical for host defence and inadequate dendritic cell number will fail to induce effective host immune responses. Here we describe a mechanism that may at least partly explain why dendritic cells become significantly depleted in chronic HIV-1 infection. We found that after binding of the HIV-1 envelope protein gp120 to the dendritic cell surface protein DC-SIGN, the subsequent activation by CD40 ligation, or by exposure to bacterial product lipopolysaccharide or pro-inflammatory cytokines such as TNF-α and IL-1β, will lead to overexpression of pro-apoptotic molecule ASK-1, resulting in excessive dendritic cell death. We also confirmed that DC-SIGN(+) dendritic cells in the blood of HIV-1 infected individuals have actually been pre-sensitized by viral gp120, which exists in vast amount in the blood, for activation-induced exorbitant death. Our study thus reveals a previously unknown pathway for dendritic cell depletion and provides clues for potential therapeutic approaches to prevent DC depletion in chronic HIV infection.

Slit2N/Robo1 inhibit HIV-gp120-induced migration and podosome formation in immature dendritic cells by sequestering LSP1 and WASp

Cell-mediated transmission and dissemination of sexually-acquired human immunodeficiency virus 1 (HIV-1) in the host involves the migration of immature dendritic cells (iDCs). iDCs migrate in response to the HIV-1 envelope protein, gp120, and inhibiting such migration may limit the mucosal transmission of HIV-1. In this study, we elucidated the mechanism of HIV-1-gp120-induced transendothelial migration of iDCs. We found that gp120 enhanced the binding of Wiskott-Aldrich Syndrome protein (WASp) and the Actin-Related Protein 2/3 (Arp2/3) complex with β-actin, an interaction essential for the proper formation of podosomes, specialized adhesion structures required for the migration of iDCs through different tissues. We further identified Leukocyte-Specific Protein 1 (LSP1) as a novel component of the WASp-Arp2/3-β-actin complex. Pretreating iDCs with an active fragment of the secretory glycoprotein Slit2 (Slit2N) inhibited HIV-1-gp120-mediated migration and podosome formation, by inducing the cognate receptor Roundabout 1 (Robo1) to bind to and sequester WASp and LSP1 from β-actin. Slit2N treatment also inhibited Src signaling and the activation of several downstream molecules, including Rac1, Pyk2, paxillin, and CDC42, a major regulator of podosome formation. Taken together, our results support a novel mechanism by which Slit2/Robo1 may inhibit the HIV-1-gp120-induced migration of iDCs, thereby restricting dissemination of HIV-1 from mucosal surfaces in the host.

Mycobacteria-infected bystander macrophages trigger maturation of dendritic cells and enhance their ability to mediate HIV transinfection

Synergistic interplay between Mycobacterium tuberculosis (Mtb) and HIV in coinfected individuals leads to the acceleration of both tuberculosis and HIV disease. Mtb, as well as HIV, may modulate the function of many immune cells, including DCs. To dissect the bystander impact of Mφs infected with Mtb on DC functionality, we here investigated changes in DC phenotype, cytokine profiles, and HIV-1 transinfecting ability. An in vitro system was used in which human monocyte-derived DCs were exposed to soluble factors released by Mφs infected with mycobacteria, including virulent clinical Mtb isolates and nonvirulent BCG. Soluble factors secreted from Mtb-infected Mφs, and to a lesser extent BCG-infected Mφs, resulted in the production of proinflammatory cytokines and partial upregulation of DC maturation markers. Interestingly, the HIV-1 transinfecting ability of DCs was enhanced upon exposure to soluble factors released by Mtb-infected Mφs. In summary, our study shows that DCs exposed to soluble factors released by mycobacteria-infected Mφs undergo maturation and display an augmented ability to transmit HIV-1 in trans. These findings highlight the important role of bystander effects during the course of Mtb-HIV coinfection and suggest that Mtb-infected Mφs may contribute to an environment that supports DC-mediated spread and amplification of HIV in coinfected individuals.

HIV impairment of immune responses in dendritic cells

Dendritic cells and their subsets are diverse populations of immune cells in the skin and mucous membranes that possess the ability to sense the presence of microbes and orchestrate an efficient and adapted immune response. Dendritic cells (DC) have the unique ability to act as a bridge between the innate and adaptive immune responses. These cells are composed of a number of subsets behaving with preferential and specific features depending on their location and surrounding environment. Langerhans cells (LC) or dermal DC (dDC) are readily present in mucosal areas. Other DC subsets such as plasmacytoid DC (pDC), myeloid DC (myDC), or monocyte-derived DC (MDDC) are thought to be recruited or differentiated in sites of pathogenic challenge. Upon HIV infection, DC and their subsets are likely among the very first immune cells to encounter incoming pathogens and initiate innate and adaptive immune responses. However, as evidenced during HIV infection, some pathogens have evolved subtle strategies to hijack key cellular machineries essential to generate efficient antiviral responses and subvert immune responses for spread and survival.In this chapter, we review recent research aimed at investigating the involvement of DC subtypes in HIV transmission at mucosal sites, concentrating on HIV impact on cellular signalling and trafficking pathways in DC leading to DC-mediated immune response alterations and viral immune evasion. We also address some aspects of DC functions during the chronic immune pathogenesis and conclude with an overview of the current and novel therapeutic and prophylactic strategies aimed at improving DC-mediated immune responses, thus to potentially tackle the early events of mucosal HIV infection and spread.

Innate immune responses and permissiveness to ranavirus infection of peritoneal leukocytes in the frog Xenopus laevis

Ranaviruses such as frog virus 3 ([FV3] family Iridoviridae) are increasingly prevalent pathogens that infect reptiles, amphibians, and fish worldwide. Whereas studies in the frog Xenopus laevis have revealed the critical involvement of CD8 T-cell and antibody responses in host resistance to FV3, little is known about the role played by innate immunity to infection with this virus. We have investigated the occurrence, composition, activation status, and permissiveness to infection of peritoneal leukocytes (PLs) in Xenopus adults during FV3 infection by microscopy, flow cytometry, and reverse transcription-PCR. The total number of PLs and the relative fraction of activated mononucleated macrophage-like cells significantly increase as early as 1 day postinfection (dpi), followed by NK cells at 3 dpi, before the peak of the T-cell response at 6 dpi. FV3 infection also induces a rapid upregulation of proinflammatory genes including arginase 1, interleukin-1beta, and tumor necrosis factor alpha. Although PLs are susceptible to FV3 infection, as evidenced by apoptotic cells, active FV3 transcription, and the detection of viral particles by electron microscopy, the infection is weaker (fewer infectious particles), more transitory, and involves a smaller fraction (less than 1%) of PLs than the kidney, the main site of infection. However, viral DNA remains detectable in PLs for at least 3 weeks postinfection, past the point of viral clearance observed in the kidneys. This suggests that although PLs are actively involved in anti-FV3 immune responses, some of these cells can be permissive and harbor quiescent, asymptomatic FV3.

HIV-1 gp120-induced migration of dendritic cells is regulated by a novel kinase cascade involving Pyk2, p38 MAP kinase, and LSP1

Targeting dendritic cell (DC) functions such as migration is a pivotal mechanism used by HIV-1 to disseminate within the host. The HIV-1 envelope protein is the most important of the virally encoded proteins that exploits the migratory capacity of DCs. In the present study, we elucidated the signaling machinery involved in migration of immature DCs (iDCs) in response to HIV-1 envelope protein. We observed that M-tropic HIV-1 glycoprotein 120 (gp120) induces phosphorylation of the nonreceptor tyrosine kinase, Pyk2. Inhibition of Pyk2 activity using a pharmacologic inhibitor, kinase-inactive Pyk2 mutant, and Pyk2-specific small interfering RNA blocked gp120-induced chemotaxis, confirming the role of Pyk2 in iDC migration. In addition, we also illustrated the importance of Pyk2 in iDC migration induced by virion-associated envelope protein, using aldithriol-2-inactivated M-tropic HIV-1 virus. Further analysis of the downstream signaling mechanisms involved in gp120-induced migration revealed that Pyk2 activates p38 mitogen-activated protein kinase, which in turn activates the F-actin-binding protein, leukocyte-specific protein 1, and enhances its association with actin. Taken together, our studies provide an insight into a novel gp120-mediated pathway that regulates DC chemotaxis and contributes to the dissemination of HIV-1 within an infected person.

In vivo CXCR4 expression, lymphoid cell phenotype, and feline immunodeficiency virus infection

Primary isolates of feline immunodeficiency virus (FIV) appear to require binding to CD134 in conjunction with CXCR4(X4) to infect IL-2-dependent T-cell-derived cells in culture. However, much less is known about the role of X4 for the infection of cells in vivo. To investigate the correlation between X4 expression and FIV infection in cats acutely infected with FIV-C-Pgmr we used high-speed fluorescence-activated cell sorting and realtime PCR to co-analyze cell phenotypes from lymph node, thymus, bone marrow and blood for FIV infection and X4 expression. X4 expression was greatest in lymph node, both in frequency and in mean fluorescence intensity. The thymus demonstrated a higher proviral burden in X4+ thymic T cells ( approximately 14% in X4+ thymic T cells and 7% in X4- cells) whereas, proviral loads were similar between X4+ and X4- cell populations in all other tissues examined. Assuming a minimum of one proviral copy per cell, a maximum of approximately 50% of FIV-positive cells were X4+. The highest fraction of FIV-infected X4- cells was present in bone marrow. Regardless of X4 status, proviral loads were higher in lymph node and blood T cells than in B cells. These studies provide both a positive association between X4 expression and FIV infection and introduce the probability that X4-independent infection occurs in other target cells in vivo.

Role of gp120 in dendritic cell dysfunction in HIV infection

Only a limited fraction of circulating virions are demonstrably infectious; therefore, exposure to inactivated viruses may mimic the most frequent type of CD4-HIV interactions that occur in vivo. Several studies have recently underscored the crucial role that those noninfectious viruses could play in defective immune function in HIV-infected individuals and in particular, in the dysregulation of dendritic cell (DC) function. In this review, we discuss how interactions between DC and HIV gp120 or inactivated virus, which harbor intact surface gp120, lead to impaired DC function through direct (direct contact) or indirect mechanisms (as a consequence of primary CD4+ T cell dysregulation, followed by defective CD4-DC interactions). It is important that these functionally impaired DCs fail to give optimal signal to T cells but appear to favor the emergence of regulatory T cells. gp120-mediated impairment of DC function could therefore play an important role in the pathogenesis of HIV disease.

An immunoglobulin fusion protein based on the gp120-CD4 receptor complex potently inhibits human immunodeficiency virus type 1 in vitro

Fusion proteins containing immunoglobulin Fc domains attached to bioactive moieties have been developed as therapeutic agents against several diseases. Here, we describe the development and characteristics of a novel fusion protein (FLSC R/T-IgG1) that targets CCR5, the major coreceptor for HIV-1 during primary infection. FLSC R/T-IgG1 was expressed from a synthetic gene that linked a single chain gp120-CD4 complex containing an R5 gp120 sequence with the hinge-CH2-CH3 portion of human immunoglobulin gamma subtype 1. Purified FLSC R/T-IgG1 exhibited a molecular mass of 189 kDa under reducing conditions, which matched the expected size of one polypeptide chain. Chemically crosslinked or untreated FLSC R/T-IgG1 exhibited a mass of a 360-kDa polypeptide under reducing and nonreducing conditions, which indicated that the molecule adopts a disulfide-linked bivalent structure. The chimeric molecule bound specifically to CCR5-expressing cells and to peptides derived from the CCR5 N-terminus. Such binding was more efficient than what was obtained with a monomeric single chain gp120-CD4 complex. FLSC R/T-IgG1 binding to CCR5 was blocked by preincubation of coreceptor-expressing cells with CCR5 ligands and by antibody to the coreceptor binding domain of gp120. Conversely, FLSC R/T-IgG1 blocked the binding of chemokine to CCR5. However, FLSC R/T-IgG1 did not trigger intracellular Ca2+ mobilization in peripheral blood mononuclear cells. FLSC R/T-IgG1 potently neutralized primary R5 HIV-1 in both a PBMC-based assay and cell line-based assays but did not affect the replication of X4 viruses. These findings suggest that FLSC R/T-IgG1 might be used as a possible therapeutic agent against HIV.

Chemokines: more than just road signs

Chemokines have long been known to orchestrate dendritic-cell migration in the body. However, recent evidence has shown that chemokines not only direct the trafficking of dendritic cells but also can regulate their maturation status. Here, we propose that this dual function of chemokines ensures that T cells and dendritic cells meet in T-cell regions of lymphoid organs and that antigen is presented in an immunologically optimal context for T-cell priming.

An aptamer that neutralizes R5 strains of human immunodeficiency virus type 1 blocks gp120-CCR5 interaction

We recently described the isolation and structural characterization of 2'-fluoropyrimidine-substituted RNA aptamers that bind to gp120 of R5 strains of human immunodeficiency virus type 1 and thereby potently neutralize the infectivity of phylogenetically diverse R5 strains. Here we investigate the physical basis of their antiviral action. We show that both N-linked oligosaccharides and the variable loops V1/V2 and V3 are not required for binding of one aptamer, B40, to gp120. Using surface plasmon resonance binding analyses, we show that the aptamer binds to the CCR5-binding site on gp120 in a relatively CD4-independent manner, providing a mechanistic explanation for its neutralizing potency.

HIV Type 1 Glycoprotein 120 Inhibits Human B Cell Chemotaxis to CXC Chemokine Ligand (CXCL) 12, CC Chemokine Ligand (CCL)20, and CCL21

We analyzed the modulation of human B cell chemotaxis by the gp120 proteins of various HIV-1 strains. X4 and X4/R5 gp120 inhibited B cell chemotaxis toward CXCL12, CCL20, and CCL21 by 40-50%, whereas R5 gp120 decreased inhibition by 20%. This gp120-induced inhibition was strictly dependent on CXCR4 or CCR5 and lipid rafts but not on CD4 or V(H)3-expressing BCR. Inhibition did not impair the expression or ligand-induced internalization of CCR6 and CCR7. Our data suggest that gp120/CXCR4 and gp120/CCR5 interactions lead to the cross-desensitization of CCR6 and CCR7 because gp120 does not bind CCR6 and CCR7. Unlike CXCL12, gp120 did not induce the activation of phospholipase Cbeta3 and PI3K downstream from CXCR4, whereas p38 MAPK activation was observed. Similar results were obtained if gp120-treated cells were triggered by CCL21 and CCL20. Our results are consistent with a blockade restricted to signaling pathways using phosphatidylinositol-4,5-bisphosphate as a substrate. X4 and X4/R5 gp120 induced the cleavage of CD62 ligand by a mechanism dependent on matrix metalloproteinase 1 and 3, CD4, CXCR4, Galpha(i), and p38 MAPK, whereas R5 gp120 did not. X4 and X4/R5 gp120 also induced the relocalization of cytoplasmic CD95 to the membrane and a 23% increase in CD95-mediated apoptosis. No such effects were observed with R5 gp120. The gp120-induced decrease in B cell chemotaxis and CD62 ligand expression, and increase in CD95-mediated B cell apoptosis probably have major deleterious effects on B cell responsiveness during HIV infection and in vaccination trials.

Covert human immunodeficiency virus replication in dendritic cells and in DC-SIGN-expressing cells promotes long-term transmission to lymphocytes

HIV-1 virions are efficiently captured by monocyte-derived immature dendritic cells (iDCs), as well as by cell lines expressing the lectin DC-SIGN. Viral infectivity can be retained for several days, and even enhanced, before transmission to CD4+ lymphocytes. The role of DC-SIGN in viral retention and enhancement of infection is not fully understood and varies according to the cell line expressing the lectin. We studied here the mechanisms underlying this process. We focused our study on X4-tropic human immunodeficiency virus (HIV) strains, since they were widely believed not to replicate in iDCs. However, we first show that X4 HIV replicates covertly and slowly in iDCs. This is also the case in Raji-DC-SIGN cells, which are classically used to study HIV transmission. We used either single-cycle or replicative HIV and measured viral RT and replication to further demonstrate that transfer of incoming virions from iDCs or DC-SIGN+ cells occurs only on the short-term (i.e., a few hours after viral exposure). There is no long-term storage of original HIV particles in these cells. A few days after viral exposure, replicative viruses, and not single-cycle virions, are transmitted to CD4+ cells. The cell-type-dependent activity of DC-SIGN reflects the ability of HIV to replicate covertly in some cells, and not in others.

Immunomodulatory effects of the HIV-1 gp120 protein on antigen presenting cells: implications for AIDS pathogenesis

Antigen presenting cell (APC) function is central to the development of an effective anti-viral immune response. Among APC, monocytes, macrophages and dendritic cells (DC) form the principal non-T cell compartment involved in in vivo HIV infection, and these cells play important and well-established roles in multiple aspects of viral pathogenesis. HIV infection may result in APC defects, which could ultimately contribute to the loss of CD4+ T cell responses observed early in HIV infection, when the CD4+ T cell number is still within the normal range. Extensive in vitro studies have demonstrated that the envelope glycoproteins of HIV-1 exert profound influences on various cell populations of the immune system, including hematopoietic progenitors, T and B lymphocytes, monocytes/ macrophages and DC, as well as on neuronal cells. The demonstration of the presence of envelope proteins both free in the circulation and bound to the surface of CD4+ cells suggests that gp120 interactions with non-infected cells can influence cellular functions in vivo, thus contributing to the immunopathogenesis of AIDS. This paper provides an overview of the present knowledge on gp120 binding, signal transduction triggering and interference with macrophage and DC functions and it highlights the importance of this interaction in the pathogenesis of AIDS.

Indoleamine 2, 3-dioxygenase (IDO) is essential for dendritic cell activation and chemotactic responsiveness to chemokines

Indoleamine 2, 3-dioxygenase (IDO) is a rate-limiting enzyme for the tryptophan catabolism. In human and murine cells, IDO inhibits antigen-specific T cell proliferation in vitro and suppresses T cell responses to fetal alloantigens during murine pregnancy. In mice, IDO expression is an inducible feature of specific subsets of dendritic cells (DCs), and is important for T cell regulatory properties. However, the effect of IDO and tryptophan deprivation on DC functions remains unknown. We report here that when tryptophan utilization was prevented by a pharmacological inhibitor of IDO, 1-methyl tryptophan (1MT), DC activation induced by pathogenic stimulus lipopolysaccharide (LPS) or inflammatory cytokine TNF-alpha was inhibited both phenotypically and functionally. Such an effect was less remarkable when DC was stimulated by a physiological stimulus, CD40 ligand. Tryptophan deprivation during DC activation also regulated the expression of CCR5 and CXCR4, as well as DC responsiveness to chemokines. These results suggest that tryptophan usage in the microenvironment is essential for DC maturation, and may also play a role in the regulation of DC migratory behaviors.

CXCR4-tropic HIV-1 envelope glycoprotein functions as a viral chemokine in unstimulated primary CD4+ T lymphocytes

Interaction of HIV-1 envelope glycoprotein gp120 with the chemokine receptor CXCR4 triggers not only viral entry but also an array of signal transduction cascades. Whether gp120 induces an incomplete or aberrant set of signals, or whether it can function as a full CXCR4 agonist, remains unclear. We report that, in unstimulated human primary CD4(+) T cells, the spectrum of signaling responses induced by gp120 through CXCR4 paralleled that induced by the natural ligand stromal cell-derived factor 1/CXCL12. gp120 activated heterotrimeric G proteins and the major G protein-dependent pathways, including calcium mobilization, phosphoinositide-3 kinase, and Erk-1/2 MAPK activation. Interestingly, gp120 caused rapid actin cytoskeleton rearrangements and profuse membrane ruffling, as evidenced by dynamic confocal imaging. This coordinated set of events resulted in a bona fide chemotactic response. Inactivated HIV-1 virions that harbored conformationally intact envelope glycoproteins also caused actin polymerization and chemotaxis, while similar virions devoid of envelope glycoproteins did not. Thus gp120, in monomeric as well as oligomeric, virion-associated form, elicited a complex cellular response that mimicked the effects of a chemokine. HIV-1 has therefore the capacity to dysregulate the vast CD4(+) T cell population that expresses CXCR4. In addition, HIV-1 may exploit its chemotactic properties to retain potential target cells and locally perturb their cytoskeleton, thereby facilitating viral transmission.

Role of cytokines and chemokines in the regulation of innate immunity and HIV infection

The earliest defense against microbial infection is represented by the responses of the innate (or natural) immune system, that also profoundly regulates the adaptive (or acquired) T- and B-cell immune responses. Activation of the innate immune system is primed by microbial invasion in response to conserved structures present in large groups of microorganisms (LPS, peptidoglycan, double-stranded RNA), and is finely tuned by different cell types (including dendritic cells, macrophages, natural killer cells, natural killer T cells, and gammadelta T cells). In addition, several soluble factors (complement components, defensins, mannose-binding lectins, interferons, cytokines and chemokines) can play a major role in the regulation of both the innate and adaptive immunity. In this review, we will briefly overview the regulation of some cellular subsets of the innate immune system particularly involved in human immunodeficiency virus (HIV) infection and then focus our attention on those cytokines and chemokines whose levels of expression are more profoundly affected by HIV infection and that, conversely, can modulate virus infection and replication.

Potency of HIV-1 envelope glycoprotein gp120 antibodies to inhibit the interaction of DC-SIGN with HIV-1 gp120

The interaction of DC-SIGN with gp120 provides an attractive target for intervention of HIV-1 transmission. Here, we have investigated the potency of gp120 antibodies to inhibit the DC-SIGN-gp120 interaction. We demonstrate that although the V3 loop is not essential for DC-SIGN binding, antibodies against the V3 loop partially inhibit DC-SIGN binding, suggesting that these antibodies sterically hinder DC-SIGN binding to gp120. Polyclonal antibodies raised against non-glycosylated gp120 inhibited both low and high avidity DC-SIGN-gp120 interactions in contrast to polyclonal antibodies raised against glycosylated gp120. Thus, glycans present on gp120 may prevent the generation of antibodies that block the DC-SIGN-gp120 interactions. Moreover, the polyclonal antibodies against non-glycosylated gp120 efficiently inhibited HIV-1 capture by both DC-SIGN transfectants and immature dendritic cells. Therefore, non-glycosylated gp120 may be an attractive immunogen to elicit gp120 antibodies that block the binding to DC-SIGN. Furthermore, we demonstrate that DC-SIGN binding to gp120 enhanced CD4 binding, suggesting that DC-SIGN induces conformational changes in gp120, which may provide new targets for neutralizing antibodies.

The human immune response to Mycobacterium tuberculosis in lung and lymph node

A key issue for the study of tuberculosis is to understand why individuals infected with Mycobacterium tuberculosis (Mtb) experience different clinical outcomes. To better understand the dynamics of Mtb infection and immunity, we have previously developed a temporal mathematical model that qualitatively and quantitatively characterizes the cellular and cytokine control network during infection. In this work we extend that model to a two compartmental model to capture the important processes of cellular activation and priming that occur between the lung and the nearest draining lymph node. We are able to reproduce typical disease progression scenarios including primary infection, latency or clearance. Then we use the model to predict key processes determining these different disease trajectories (i.e. identify bifurcation parameters), suggesting directions for further basic science study and potential new treatment strategies.

Early steps of retrovirus replicative cycle

During the last two decades, the profusion of HIV research due to the urge to identify new therapeutic targets has led to a wealth of information on the retroviral replication cycle. However, while the late stages of the retrovirus life cycle, consisting of virus replication and egress, have been partly unraveled, the early steps remain largely enigmatic. These early steps consist of a long and perilous journey from the cell surface to the nucleus where the proviral DNA integrates into the host genome. Retroviral particles must bind specifically to their target cells, cross the plasma membrane, reverse-transcribe their RNA genome, while uncoating the cores, find their way to the nuclear membrane and penetrate into the nucleus to finally dock and integrate into the cellular genome. Along this journey, retroviruses hijack the cellular machinery, while at the same time counteracting cellular defenses. Elucidating these mechanisms and identifying which cellular factors are exploited by the retroviruses and which hinder their life cycle, will certainly lead to the discovery of new ways to inhibit viral replication and to improve retroviral vectors for gene transfer. Finally, as proven by many examples in the past, progresses in retrovirology will undoubtedly also provide some priceless insights into cell biology.

Neutralization of infectivity of diverse R5 clinical isolates of human immunodeficiency virus type 1 by gp120-binding 2'F-RNA aptamers

Human immunodeficiency virus type 1 (HIV-1) has evolved a number of strategies to resist current antiretroviral drugs and the selection pressures of humoral and cellular adaptive immunity. For example, R5 strains, which use the CCR5 coreceptor for entry and are the dominant viral phenotype for HIV-1 transmission and AIDS pathogenesis, are relatively resistant to neutralization by antibodies, as are other clinical isolates. In order to overcome these adaptations, we raised nucleic acid aptamers to the SU glycoprotein (gp120) of the R5 strain, HIV-1(Ba-L). These not only bound gp120 with high affinity but also neutralized HIV-1 infectivity in human peripheral blood mononuclear cells (PBMCs) by more than 1,000-fold. Furthermore, these aptamers were able to neutralize the infectivity of R5 clinical isolates of HIV-1 derived from group M (subtypes A, C, D, E, and F) and group O. One aptamer defined a site on gp120 that overlaps partially with the conserved, chemokine receptor-binding, CD4-induced epitope recognized by monoclonal antibody 17b. In contrast to the antibody, the site is accessible to aptamer in the absence of CD4 binding. Neutralizing aptamers such as this could be exploited to provide leads in developing alternative, efficacious anti-HIV-1 drugs and lead to a deeper understanding of the molecular interactions between the virus and its host cell.

Cell surface receptors for gammaretroviruses

Evidence obtained during the last few years has greatly extended our understanding of the cell surface receptors that mediate infections of retroviruses and has provided many surprising insights. In contrast to other cell surface components such as lectins or proteoglycans that influence infections indirectly by enhancing virus adsorption onto specific cells, the true receptors induce conformational changes in the viral envelope glycoproteins that are essential for infection. One surprise is that all of the cell surface receptors for gamma-retroviruses are proteins that have multiple transmembrane (TM) sequences, compatible with their identification in known instances as transporters for important solutes. In striking contrast, almost all other animal viruses use receptors that exclusively have single TM sequences, with the sole proven exception we know of being the coreceptors used by lentiviruses. This evidence strongly suggests that virus genera have been prevented because of their previous evolutionary adaptations from switching their specificities between single-TM and multi-TM receptors. This evidence also implies that gamma-retroviruses formed by divergent evolution from a common origin millions of years ago and that individual viruses have occasionally jumped between species (zoonoses) while retaining their commitment to using the orthologous receptor of the new host. Another surprise is that many gamma-retroviruses use not just one receptor but pairs of closely related receptors as alternatives. This appears to have enhanced viral survival by severely limiting the likelihood of host escape mutations. All of the receptors used by gamma-retroviruses contain hypervariable regions that are often heavily glycosylated and that control the viral host range properties, consistent with the idea that these sequences are battlegrounds of virus-host coevolution. However, in contrast to previous assumptions, we propose that gamma-retroviruses have become adapted to recognize conserved sites that are important for the receptor's natural function and that the hypervariable sequences have been elaborated by the hosts as defense bulwarks that surround the conserved viral attachment sites. Previously, it was believed that binding to receptors directly triggers a series of conformational changes in the viral envelope glycoproteins that culminate in fusion of the viral and cellular membranes. However, new evidence suggests that gamma-retroviral association with receptors triggers an obligatory interaction or cross-talk between envelope glycoproteins on the viral surface. If this intermediate step is prevented, infection fails. Conversely, in several circumstances this cross-talk can be induced in the absence of a cell surface receptor for the virus, in which case infection can proceed efficiently. This new evidence strongly implies that the role of cell surface receptors in infections of gamma-retroviruses (and perhaps of other enveloped animal viruses) is more complex and interesting than was previously imagined. Recently, another gammaretroviral receptor with multiple transmembrane sequences was cloned. See Prassolov, Y., Zhang, D., Ivanov, D., Lohler, J., Ross, S.R., and Stocking, C. Sodium-dependent myo-inositol transporter 1 is a receptor for Mus cervicolor M813 murine leukemia virus.

DC-SIGN: escape mechanism for pathogens

Dendritic cells (DCs) are crucial in the defence against pathogens. Invading pathogens are recognized by Toll-like receptors (TLRs) and receptors such as C-type lectins expressed on the surface of DCs. However, it is becoming evident that some pathogens, including viruses, such as HIV-1, and non-viral pathogens, such as Mycobacterium tuberculosis, subvert DC functions to escape immune surveillance by targeting the C-type lectin DC-SIGN (DC-specific intercellular adhesion molecule-grabbing nonintegrin). Notably, these pathogens misuse DC-SIGN by distinct mechanisms that either circumvent antigen processing or alter TLR-mediated signalling, skewing T-cell responses. This implies that adaptation of pathogens to target DC-SIGN might support pathogen survival.

The HIV protease inhibitor indinavir reduces immature dendritic cell transendothelial migration

Indinavir (IDV) is a protease inhibitor that successfully suppresses HIV-1 replication as part of anti-retroviral therapy. There is evidence to suggest that IDV may also act non-specifically upon host proteases. In this study we investigated whether IDV could modulate protease-dependent molecules involved in dendritic cell (DC) migration - a pivotal process in immunoregulation. Human monocyte-derived DC were exposed to IDV (IDV-DC) and transendothelial migration (TEM) to inflammatory chemokines was determined. TEM of IDV-DC was significantly impaired compared to non-treated DC (p<0.01). Phenotypic analysis revealed that IDV-DC had reduced DC-SIGN expression, correlating with reduced adhesion to immobilized ICAM-2. Nevertheless, the reduction in migration following exposure to IDV could not be fully attributable to DC-SIGN interactions alone. Investigation of IDV-DC interactions with the underlying matrix protein, fibronectin, demonstrated that IDV significantly impaired DC binding to immobilized fibronectin (p<0.01). IDV appeared to act upon VLA-4 and VLA-5 since addition of antagonist monoclonal antibodies (mAb) similarly reduced adhesion of non-treated DC to fibronectin. Combined blockade of DC using anti-VLA-4, VLA-5 and anti-DC-SIGN mAb inhibited TEM to a similar extent as IDV. Our results strongly suggest that IDV inhibits host proteases necessary for DC migration and may, therefore, affect DC immunoregulation in HIV-1-infected patients.

Dendritic cells mediate NK cell help for Th1 and CTL responses: two-signal requirement for the induction of NK cell helper function

Early stages of viral infections are associated with local recruitment and activation of dendritic cells (DC) and NK cells. Although activated DC and NK cells are known to support each other's functions, it is less clear whether their local interaction in infected tissues can modulate the subsequent ability of migrating DC to induce T cell responses in draining lymph nodes. In this study, we report that NK cells are capable of inducing stable type 1-polarized "effector/memory" DC (DC1) that act as carriers of NK cell-derived helper signals for the development of type 1 immune responses. NK cell-induced DC1 show a strongly elevated ability to produce IL-12p70 after subsequent CD40 ligand stimulation. NK-induced DC1 prime naive CD4+ Th cells for high levels of IFN-gamma, but low IL-4 production, and demonstrate a strongly enhanced ability to induce Ag-specific CD8+ T cell responses. Resting NK cells display stringent activation requirements to perform this novel, DC-mediated, "helper" function. Although their interaction with K562 cells results in effective target cell killing, the induction of DC1 requires a second NK cell-activating signal. Such costimulatory signal can be provided by type I IFNs, common mediators of antiviral responses. Therefore, in addition to their cytolytic function, NK cells also have immunoregulatory activity, induced under more stringent conditions. The currently demonstrated helper activity of NK cells may support the development of Th1- and CTL-dominated type 1 immunity against intracellular pathogens and may have implications for cancer immunotherapy.

Changes in dendritic cell migration and activation during SIV infection suggest a role in initial viral spread and eventual immunosuppression

Dendritic cells (DC) serve an essential function in linking the innate and acquired immune responses to antigen. Peripheral DC acquire antigen and migrate to draining lymph nodes, where they localize to the T cell-rich paracortex and function as potent antigen presenting cells. We examined the effects of human immunodeficiency virus (HIV) infection on DC function in vivo using the rhesus macaque/simian immunodeficiency virus (SIV) model. Our data show that during acute SIV infection, Langerhans cell density is reduced in skin and activated DC are increased in proportion in lymph nodes, whereas during AIDS, DC migration from skin and activation within lymph nodes are suppressed. These findings suggest that changes in DC function at different times during the course of infection may serve to promote virus dissemination and persistence: early during infection, DC mobilization may facilitate virus spread to susceptible lymph node T cell populations, whereas depressed DC function during advanced infection could promote generalized immunosuppression.

Expression of human immunodeficiency virus (HIV)-binding lectin DC-SIGNR: Consequences for HIV infection and immunity

DC-SIGNR is a human immunodeficiency virus (HIV)-binding C-type lectin that is expressed on endothelium in the hepatic sinusoids, lymph node sinuses and placenta. Like closely related DC-SIGN, DC-SIGNR can bind both ICAM-3 and HIV and can potentiate HIV infection of T lymphocytes in trans. In the present study we have investigated reasons underlying the restricted distribution of DC-SIGNR and have examined DC-SIGNR expression in relation to HIV entry receptors. We show that DC-SIGNR expression does not depend on endothelial cell specialization or on activation state. DC-SIGNR-positive endothelium continues to express DC-SIGNR in conditions of hyperplasia, whereas the molecule is lost after neoplastic transformation, most likely as a result of changes in the microenvironment of the endothelial cells. We have further shown that CCR5, but not CD4, is coexpressed with DC-SIGNR on hepatic sinusoidal and placental capillary endothelial cells. However, CD4-positive CCR5-positive cells, such as hepatic Kupffer cells, placental Hofbauer cells, and CD4-positive T lymphocytes in lymph nodes, can be found adjacent to DC-SIGNR-positive endothelium. Therefore, DC-SIGNR may be able to mediate HIV infection of these cells in trans. Finally, we demonstrate that DC-SIGN and DC-SIGNR can be coexpressed on lymph node sinus endothelial cells, which may lead to modulation of the function of both molecules.

Disrupted homeostasis of Langerhans cells and interdigitating dendritic cells in monkeys with AIDS

Langerhans cells (LCs) are immature dendritic cells (DCs) that capture antigen in peripheral tissues and migrate to draining lymph nodes, where they reside in the paracortex as interdigitating dendritic cells (IDCs). We studied the effects of simian immunodeficiency virus (SIV) on LCs and IDCs during different stages of infection in monkeys. LCs isolated from monkeys with acute SIV infection or acquired immunodeficiency syndrome (AIDS) underwent normal maturation in vitro, including a switch in chemokine receptor expression from CCR5 to CXCR4 and CCR7. LCs migrated normally from skin in response to contact sensitization in monkeys with acute SIV infection. In contrast, LC migration from skin was markedly impaired during AIDS, associated with a reduction in antigen-bearing DCs in draining lymph nodes. Lymph node IDCs were increased in proportion during acute SIV infection and had an activated phenotype, whereas during AIDS IDCs had significantly lower expression of CD40 and the activation marker CD83. IDCs from monkeys with AIDS were refractory to stimulation with CD40L, demonstrating a functional consequence of decreased CD40 expression. SIV-infected DCs were not identified in lymph nodes or skin of monkeys with AIDS, suggesting an indirect effect of infection on DC populations in vivo. These data indicate that DCs are mobilized to lymph nodes during acute SIV infection, but that during AIDS this process is suppressed, with LC migration and IDC activation being impaired. We conclude that disruption of DC homeostasis may play a role in immunopathology induced by human immunodeficiency virus and suggest that therapeutic strategies targeting DCs may have limited efficacy during AIDS.

HIV-1 Nef-induced upregulation of DC-SIGN in dendritic cells promotes lymphocyte clustering and viral spread

DC-SIGN, a dendritic cell (DC)-specific lectin, mediates clustering of DCs with T lymphocytes, a crucial event in the initiation of immune responses. DC-SIGN also binds HIV envelope glycoproteins, allowing efficient virus capture by DCs. We show here that DC-SIGN surface levels are upregulated in HIV-1-infected DCs. This process is caused by the viral protein Nef, which acts by inhibiting DC-SIGN endocytosis. Upregulation of DC-SIGN at the cell surface dramatically increases clustering of DCs with T lymphocytes and HIV-1 transmission. These results provide new insights into how HIV-1 spreads from DCs to T lymphocytes and manipulates immune responses. They help explain how Nef may act as a virulence factor in vivo.

CD4+ T cell-independent vaccination against Pneumocystis carinii in mice

Host defenses are profoundly compromised in HIV-infected hosts due to progressive depletion of CD4+ T lymphocytes. Moreover, deficient CD4+ T lymphocytes impair vaccination approaches to prevent opportunistic infection. Therefore, we investigated a CD4+ T cell-independent vaccine approach to a prototypic AIDS-defining infection, Pneumocystis carinii (PC) pneumonia. Here, we demonstrate that bone marrow-derived dendritic cells (DCs) expressing the murine CD40 ligand, when pulsed ex vivo by PC antigen, elicited significant titers of anti-PC IgG in CD4-deficient mice. Vaccinated animals demonstrated significant protection from PC infection, and this protection was the result of an effective humoral response, since adoptive transfer of CD4-depleted splenocytes or serum conferred this protection to CD4-deficient mice. Western blot analysis of PC antigen revealed that DC-vaccinated, CD4-deficient mice predominantly reacted to a 55-kDa PC antigen. These studies show promise for advances in CD4-independent vaccination against HIV-related pathogens.

Dendritic cells and viral immunity: friends or foes?

Dendritic cells (DC) residing in epithelial tissues of various mucosae and the skin are characterized by the unique ability to capture antigens and migrate to draining lymph nodes, where they can activate naive and memory T cells. Although DC play a pivotal role in inducing protective immunity to viral infection, they can also be exploited by viruses to evade the host immune response, induce immune suppression, or serve as latent viral reservoirs. Thus, virus interactions with DC may lead to an immune response that can be protective, but does not necessarily lead to complete virus elimination, resulting in immunopathology.

Bitter-sweet symphony: defining the role of dendritic cell gp120 receptors in HIV infection

BACKGROUND

Dendritic cells (DC) are believed to be one of the first cell types infected during HIV transmission. Recently a single C-type lectin receptor (CLR), DC-SIGN, has been reported to be the predominant receptor on monocyte derived DC (MDDC) rather than CD4. The role of other CLRs in HIV binding and HIV binding by CLRs on other types of DC in vivo is largely unknown.

OBJECTIVES AND STUDY DESIGN

Review HIV binding to DC populations, both in vitro and in vivo, in light of the immense interest of a recently re-identified CLR called DC-SIGN.

RESULTS AND CONCLUSIONS

From recent work, it is clear that immature MDDC have a complex pattern of HIV gp120 binding. In contrast to other cell types gp120 has the potential to bind to several receptors on DC including CD4 and several types of C type lectin receptor, not just exclusively DC-SIGN. Given the diverse types of DC in vivo future work will need to focus on defining the receptors for HIV binding to these different cell types. Mucosal transmission of HIV in vivo targets immature sessile DCs, including Langerhans cells which lack DC-SIGN. The role of CLRs and DC-SIGN in such transmission remains to be defined.

Viral escape mechanisms--escapology taught by viruses

Viruses have 'studied' immunology over millions of years of coevolution with their hosts. During this ongoing education they have developed countless mechanisms to escape from the host's immune system. To illustrate the most common strategies of viral immune escape we have focused on two murine models of persistent infection, lymphocytic choriomeningitis virus (LCMV) and murine cytomegalovirus (MCMV). LCMV is a fast replicating small RNA virus with a genome prone to mutations. Therefore, LCMV escapes from the immune system mainly by two strategies: 'speed' and 'shape change'. At the opposite extreme, MCMV is a large, complex DNA virus with a more rigid genome and thus the strategies used by LCMV are no option. However, MCMV has the coding capacity for additional genes which interfere specifically with the immune response of the host. These escape strategies have been described as 'camouflage' and 'sabotage'. Using these simple concepts we describe the spectrum of viral escapology, giving credit not only to the researchers who uncovered this fascinating area of immunology but also to the viruses themselves, who still have a few lessons to teach.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

Directing the immune response to carbohydrate antigens

Peptide mimetics may substitute for carbohydrate antigens in vaccine design applications. At present, the structural and immunological aspects of antigenic mimicry, which translate into immunologic mimicry, as well as the functional correlates of each, are unknown. In contrast to screening peptide display libraries, we demonstrate the feasibility of a structure-assisted vaccine design approach to identify functional mimeotopes. By using concanavalin A (ConA), as a recognition template, peptide mimetics reactive with ConA were identified. Designed peptides were observed to compete with synthetic carbohydrate probes for ConA binding, as demonstrated by enzyme-linked immunosorbent assay and isothermal titration calorimetry (ITC) analysis. ITC measurements indicate that a multivalent form of one particular mimetic binds to ConA with similar affinity as does trimannoside. Splenocytes from mimeotope-immunized mice display a peptide-specific cellular response, confirming a T-cell-dependent nature for the mimetic. As ConA binds to the Envelope protein of the human immunodeficiency virus, type 1 (HIV-1), we observed that mimeotope-induced serum also binds to HIV-1-infected cells, as assessed by flow cytometry, and could neutralize T-cell line adapted HIV-1 isolates in vitro, albeit at low titers. These studies emphasize that mimicry is based more upon functional rather than structural determinants that regulate mimeotope-induced T-dependent antibody responses to polysaccharide and emphasize that rational approaches can be employed to develop further vaccine candidates.

Nef is required for efficient HIV-1 replication in cocultures of dendritic cells and lymphocytes

Dendritic cells (DCs) are thought to play a crucial role in the pathogenesis of HIV-1 infection. DCs are believed to transport virus particles to lymph nodes before transfer to CD4(+) lymphocytes. We have investigated the role of Nef in these processes. HIV-1 replication was examined in human immature DC-lymphocyte cocultures and in DCs or lymphocytes separately. Using various R5-tropic and X4-tropic HIV-1 strains and their nef-deleted (Deltanef) counterparts, we show that Nef is required for optimal viral replication in immature DC-T cells clusters and in T lymphocytes. Nef exerts only a marginal role on viral replication in immature DCs alone as well as on virion capture by DCs, long-term intracellular accumulation and transmission of X4 strains to lymphocytes. We also show that wild-type and Deltanef virions are similarly processed for MHC-I restricted exogenous presentation by DCs. Taken together, these results help explain how HIV-1 Nef may affect viral spread and immune responses in the infected host.

Human immunodeficiency virus type-1 and chemokines: beyond competition for common cellular receptors

The chemokines and their receptors have been receiving exceptional attention in recent years following the discoveries that some chemokines could specifically block human immunodeficiency virus type 1 (HIV-1) infection and that certain chemokine receptors were the long-sought coreceptors which, along with CD4, are required for the productive entry of HIV-1 and HIV-2 isolates. Several chemokine receptors or orphan chemokine receptor-like molecules can support the entry of various viral strains, but the clinical significance of the CXCR4 and CCR5 coreceptors appear to overshadow a critical role for any of the other coreceptors and all HIV-1 and HIV-2 strains best employ one or both of these coreceptors. Binding of the HIV-1 envelope glycoprotein gp120 subunit to CD4 and/or an appropriate chemokine receptor triggers conformational changes in the envelope glycoprotein oligomer that allow it to facilitate the fusion of the viral and host cell membranes. During these interactions, gp120 appears to be capable of inducing a variety of signaling events, all of which are still not defined in detail. In addition, the more recently observed dichotomous effects, of both inhibition and enhancement, that chemokines and their receptor signaling events elicit on the HIV-1 entry and replication processes has once again highlighted the intricate and complex balance of factors that govern the pathogenic process. Here, we will review and discuss these new observations summarizing the potential significance these processes may have in HIV-1 infection. Understanding the complexities and significance of the signaling processes that the chemokines and viral products induce may substantially enhance our understanding of HIV-1 pathogenesis, and perhaps facilitate the discovery of new ways for the prevention and treatment of HIV-1 disease.

Dendritic cells and transmission of HIV-1

Sexual transmission of HIV-1 requires that small amounts of virus at mucosal sites of inoculation gain access to replication-permissive cells. Recent observations have increased our understanding of the mechanisms by which this relatively inefficient virus can exploit the migratory nature of dendritic cells to establish infection within the host.

DC-SIGN, a dentritic cell-specific HIV-1 receptor present in placenta that infects T cells in trans-a review

Dendritic cells (DC) capture micro-organisms that enter peripheral mucosal tissues and then migrate to secondary lymphoid organs, where they present in antigenic form to resting T cells and thus initiate adaptive immune responses. Here we describe the properties of a DC-specific C-type lectin, DC-SIGN, that is highly expressed on DC present in mucosal tissues and binds to the HIV-1 envelope glycoprotein gp120. DC-SIGN does not function as a receptor for viral entry into DC, but instead promotes efficient infection in trans of cells that express CD4 and chemokine receptors. The interaction of DC-SIGN with HIV gp120 may be an important target for therapeutic intervention and vaccine development.

Chemokines: immunology's high impact factors

Chemokines facilitate leukocyte migration and positioning as well as other processes such as angiogenesis and leukocyte degranulation. The burgeoning knowledge on chemokines and their receptors has influenced many aspects of immunology, in part because cell migration is intimately related to leukocyte function. This overview assesses the impact that chemokines have had on our understanding of immunology and infectious diseases. These include the role of chemokines in leukocyte-endothelial cell interactions; dendritic cell function; T cell differentiation and function; inflammatory diseases; mucosal and subcutaneous immunity; and subversion of immune responses by viruses, including HIV-1. This knowledge heralds new opportunities for the manipulation of immune responses and the development of new anti-inflammatory therapies. It has also provided a new perspective on the functioning of the immune system.