DC-SIGN from African green monkeys is expressed in lymph nodes and mediates infection in trans of simian immunodeficiency virus SIVagm

Visualizing the Immune System: Providing Key Insights into HIV/SIV Infections

Immunological inductive tissues, such as secondary lymphoid organs, are composed of distinct anatomical microenvironments for the generation of immune responses to pathogens and immunogens. These microenvironments are characterized by the compartmentalization of highly specialized immune and stromal cell populations, as well as the presence of a complex network of soluble factors and chemokines that direct the intra-tissue trafficking of naïve and effector cell populations. Imaging platforms have provided critical contextual information regarding the molecular and cellular interactions that orchestrate the spatial microanatomy of relevant cells and the development of immune responses against pathogens. Particularly in HIV/SIV disease, imaging technologies are of great importance in the investigation of the local interplay between the virus and host cells, with respect to understanding viral dynamics and persistence, immune responses (i.e., adaptive and innate inflammatory responses), tissue structure and pathologies, and changes to the surrounding milieu and function of immune cells. Merging imaging platforms with other cutting-edge technologies could lead to novel findings regarding the phenotype, function, and molecular signatures of particular immune cell targets, further promoting the development of new antiviral treatments and vaccination strategies.

Non-human primates in HIV research: Achievements, limits and alternatives

An ideal model for HIV-1 research is still unavailable. However, infection of non-human primates (NHP), such as macaques, with Simian Immunodeficiency Virus (SIV) recapitulates most virological, immunological and clinical hallmarks of HIV infection in humans. It has become the most suitable model to study the mechanisms of transmission and physiopathology of HIV/AIDS. On the other hand, natural hosts of SIV, such as African green monkeys and sooty mangabeys that when infected do not progress to AIDS, represent an excellent model to elucidate the mechanisms involved in the capacity of controlling inflammation and disease progression. The use of NHP-SIV models has indeed enriched our knowledge in the fields of: i) viral transmission and viral reservoirs, ii) early immune responses, iii) host cell-virus interactions in tissues, iv) AIDS pathogenesis, v) virulence factors, vi) prevention and vii) drug development. The possibility to control many variables during experimental SIV infection, together with the resemblance between SIV and HIV infections, make the NHP model the most appropriate, so far, for HIV/AIDS research. Nonetheless, some limitations in using these models have to be considered. Alternative models for HIV/AIDS research, such as humanized mice and recombinant forms of HIV-SIV viruses (SHIV) for NHP infection, have been developed. The improvement of SHIV viruses that mimic even better the natural history of HIV infection and of humanized mice that develop a greater variety of human immune cell lineages, is ongoing. None of these models is perfect, but they allow contributing to the progress in managing or preventing HIV infection.

Innate Immunity in Simian Immunodeficiency Virus Infection

The past decade has seen the emergence of innate immunity as a mature field. The study of innate immunity has had a significant impact on the concepts of HIV immunity, pathogenesis, and vaccines. In this chapter, basic concepts of innate immunity at the anatomical, cellular, and molecular levels will be introduced from the perspective of their interplay with HIV and simian immunodeficiency virus (SIV). An emphasis will be placed on studies using SIV/non-human primate (NHP) models that shape current models of HIV pathogenesis. Finally, studies modulating the innate system in vivo in NHPs will be discussed.

HIV-associated chronic immune activation

Systemic chronic immune activation is considered today as the driving force of CD4(+) T-cell depletion and acquired immunodeficiency syndrome (AIDS). A residual chronic immune activation persists even in HIV-infected patients in which viral replication is successfully inhibited by anti-retroviral therapy, with the extent of this residual immune activation being associated with CD4(+) T-cell loss. Unfortunately, the causal link between chronic immune activation and CD4(+) T-cell loss has not been formally established. This article provides first a brief historical overview on how the perception of the causative role of immune activation has changed over the years and lists the different kinds of immune activation characteristic of human immunodeficiency virus (HIV) infection. The mechanisms proposed to explain the chronic immune activation are multiple and are enumerated here, as well as the mechanisms proposed on how chronic immune activation could lead to AIDS. In addition, we summarize the lessons learned from natural hosts that know how to 'show AIDS the door', and discuss how these studies informed the design of novel immune modulatory interventions that are currently being tested. Finally, we review the current approaches aimed at targeting chronic immune activation and evoke future perspectives.

Advances in research of fish immune-relevant genes: a comparative overview of innate and adaptive immunity in teleosts

Fish is considered to be an important model in comparative immunology studies because it is a representative population of lower vertebrates serving as an essential link to early vertebrate evolution. Fish immune-relevant genes have received considerable attention due to its role in improving understanding of both fish immunology and the evolution of immune systems. In this review, we discuss the current understanding of teleost immune-relevant genes for both innate and adaptive immunity, including pattern recognition receptors, antimicrobial peptides, complement molecules, lectins, interferons and signaling factors, inflammatory cytokines, chemokines, adaptive immunity relevant cytokines and negative regulators, major histocompatibility complexes, immunoglobulins, and costimulatory molecules. The implications of these factors on the evolutionary history of immune systems were discussed and a perspective outline of innate and adaptive immunity of teleost fish was described. This review may provide clues on the evolution of the essential defense system in vertebrates.

Understanding factors that modulate HIV infection at the female genital tract mucosae for the rationale design of microbicides

Women are now becoming the pivot of the epidemiological spread of HIV infection worldwide, especially in developing countries. Therefore, research to develop an efficient microbicide is now a priority for the prevention of HIV-1 acquisition in exposed women. However, recent disappointing failures in microbicide clinical trials revealed major gaps in basic and applied knowledge that hinder the development of effective microbicide formulations. Indeed, the inhibitory power of microbicide molecules may be affected by several physiological and immunological factors present in male and female genital tracts. Furthermore, mucosal crossing of HIV-1 to increase the ability to reach the submucosal target cells (macrophages, lymphocytes, and dendritic cells) may be modulated by supraepithelial factors such as seminal complement components (opsonized HIV-1), by epithelial factors released in the submucosal microenvironment such as antimicrobial soluble factors, cytokines, and chemokines, and by potent intraepithelial and submucosal innate immunity. The design of vaginal microbicide formulations should take into account an understanding of the intimate mechanisms involved in the crossing of HIV through the female genital mucosae, in the context of a mixture of both male and female genital fluids.

Non-human primate dendritic cells

Non-human primates (NHP) are essential translational models for biomedical research. Dendritic cells (DC) are a group of antigen presenting cells (APC) that play pivotal roles in the immunobiology of health and disease and are attractive cells for adoptive immunotherapy to stimulate and suppress immunity. DC have been studied extensively in humans and mice but until recently, have not been well characterized in NHP. This review considers the available data about DC across a range of NHP species and summarizes the understanding of in vitro-propagated DC and in vivo-isolated DC, which is now established. It is clear that although NHP DC exist within the paradigm of human DC, there are important functional and phenotypic differences when compared with human DC subsets. These differences need to be taken into account when designing preclinical, translational studies of DC therapy using NHP models.

Functional contributions of carbohydrate on AIDS virus glycoprotein

Envelope glycoprotein spikes on the surface of the human immunodeficiency virus (HIV) are used by the virus to bind to cellular receptors to gain entry into target cells. As such, the envelope spikes are the targets of antibodies that can neutralize viral infectivity. Fifty percent or more of the mass of the viral-encoded surface glycoprotein of HIV, and of its close monkey relative simian immunodeficiency virus (SIV), is actually carbohydrate; it is one of the most heavily glycosylated proteins that can be found in mammals. It has been clearly demonstrated that one of the functions of this carbohydrate is to shield viral epitopes that would otherwise be the direct target of antibodies that could neutralize viral infection. In addition, it is now generally accepted that the carbohydrate on the viral envelope glycoprotein is recognized by multiple cellular lectins of the host lymphoreticular system, and these interactions play a role in the dissemination of virus within the host as well as the release of modulatory cytokines. Our work recently demonstrated fundamental differences in the composition of the carbohydrate on HIV type 1, the cause of the AIDS pandemic, versus the SIV in the sooty mangabey monkey, a natural host that does not develop disease from its infection. We now speculate that this fundamental difference in carbohydrate composition reflects evolutionary pressures on both virus and host. Furthermore, carbohydrate composition on the virus and genetic differences in carbohydrate-sensing proteins of the host could be critically important for the generalized lymphoid activation that characterizes the acquired immunodeficiency syndrome (AIDS).

Tissue-specific reduction in DC-SIGN expression correlates with progression of pathogenic simian immunodeficiency virus infection

Studies were undertaken to determine whether previously described reductions in splenic DC-SIGN expression in simian acquired immune deficiency syndrome (AIDS) are limited to pathogenic simian immunodeficiency virus (SIV) infection. DC-SIGN expression was evaluated by immunohistochemistry in lymphoid tissues from AIDS-susceptible Asian macaque monkeys as compared with AIDS-resistant sooty mangabey monkeys in the presence and absence of SIV infection. The phenotype of DC-SIGN+ cells in susceptible and resistant species was identical and most consistent with macrophage identity. Significantly lower levels of DC-SIGN expression were identified in spleen, mesenteric lymph node, and bone marrow of macaques with AIDS (P<0.05). Reduced levels of splenic DC-SIGN correlated significantly with CD4T cell depletion in long-term pathogenic infection of macaques (P<0.01), whereas SIV-infected mangabeys retained high levels of DC-SIGN expression in spleen despite persistent infection. Reduced expression of DC-SIGN in spleen specifically characterizes pathogenic forms of SIV infection, correlates with disease progression, and may contribute to SIV pathogenesis.

Plasmacytoid dendritic cell dynamics and alpha interferon production during Simian immunodeficiency virus infection with a nonpathogenic outcome

We addressed the role of plasmacytoid dendritic cells (PDC) in protection against AIDS in nonpathogenic simian immunodeficiency virus (SIVagm) infection in African green monkeys (AGMs). PDC were monitored in blood and lymph nodes (LNs) starting from day 1 postinfection. We observed significant declines in blood during acute infection. However, PDC then returned to normal levels, and chronically infected AGMs showed no decrease of PDC in blood. There was a significant increase of PDC in LNs during acute infection. Blood PDC displayed only weak alpha interferon (IFN-alpha) responses to TLR9 agonist stimulation before infection. However, during acute infection, both blood and LN PDC showed a transiently increased propensity for IFN-alpha production. Bioactive IFN-alpha was detected in plasma concomitant with the peak of viremia, though levels were only low to moderate in some animals. Plasma interleukin 6 (IL-6) and IL-12 were not increased. In conclusion, PDC were recruited to the LNs and displayed increased IFN-alpha production during acute infection. However, increases in IFN-alpha were transient. Together with the lack of inflammatory cytokine responses, these events might play an important role in the low level of T-cell activation which is associated with protection against AIDS in nonpathogenic SIVagm infection.

Modulation of HIV and SIV neutralization sensitivity by DC-SIGN and mannose-binding lectin

The C-type lectin DC-SIGN binds to oligosaccharides on the human and simian immunodeficiency virus (HIV, SIV) envelope glycoproteins and promotes infection of susceptible cells. Here, we show that DC-SIGN recognizes glycans involved in SIV sensitivity to neutralizing antibodies and that binding to DC-SIGN confers neutralization resistance to an otherwise sensitive SIV variant. Moreover, we provide evidence that mannose-binding lectin (MBL) can interfere with HIV-1 neutralization by the carbohydrate-specific antibody 2G12.

Long oligonucleotide microarrays for African green monkey gene expression profile analysis

Nonhuman primates, including African green monkey (AGM), are important models for biomedical research. The information on monkey genomes is still limited and no versatile gene expression screening tool is available. We tested human whole genome microarrays for cross-species reactivity with AGM transcripts using both long oligonucleotide arrays (60-mer probes) and short oligonucleotide arrays (25-mer). Using the long oligonucleotide arrays, we detected 4-fold more AGM transcripts than with the short oligonucleotide technology. The number of detected transcripts was comparable to that detected using human RNA, with 87% of the detected genes being shared between both species. The specificity of the signals obtained with the long oligonucleotide arrays was determined by analyzing the transcriptome of concanavalin A-activated CD4+ T cells vs. nonactivated T cells of two monkey species AGM and macaque. For both species, the genes showing the most significant changes in expression, such as IL-2R, were those known to be regulated in human CD4+ T cell activation. Finally, tissue specificity of the signals was established by comparing the transcription profiles of AGM brain and tonsil cells. In conclusion, the ABI human microarray platform provides a highly valuable tool for the assessment of AGM gene expression profiles.

Dendritic cell-mediated viral transmission: a potential drug target?

Dendritic cells (DCs) are important in the sexual transmission of HIV-1, the most common route of acquiring HIV-1. HIV-1 subverts the biological function of DCs to facilitate its transport from site of entry at mucosal tissues to lymphoid tissues to infect T cells. Recent data have furthered our understanding of how DCs mediate viral transmission to T cells. DCs capture HIV-1 through specific attachment receptors, such as DC-SIGN, which not only facilitate HIV-1 transmission, but also infection of DCs. Therefore, these receptors are very promising targets for the design of inhibitors or vaccination strategies to prevent mucosal HIV-1 transmission. It is becoming evident that other viruses also use DCs for their transmission. This review will discuss the mechanism of HIV-1 transmission and potential intervention strategies.

Simian immunodeficiency virus SIVagm.sab infection of Caribbean African green monkeys: a new model for the study of SIV pathogenesis in natural hosts

Caribbean-born African green monkeys (AGMs) were classified as Chlorocebus sabaeus by cytochrome b sequencing. Guided by these phylogenetic analyses, we developed a new model for the study of simian immunodeficiency virus (SIV) infection in natural hosts by inoculating Caribbean AGMs with their species-specific SIVagm.sab. SIVagm.sab replicated efficiently in Caribbean AGM peripheral blood mononuclear cells in vitro. During SIVagm.sab primary infection of six Caribbean AGMs, the virus replicated at high levels, with peak viral loads (VLs) of 10(7) to 10(8) copies/ml occurring by day 8 to 10 postinfection (p.i.). Set-point values of up to 2 x 10(5) copies/ml were reached by day 42 p.i. and maintained throughout follow-up (through day 450 p.i.). CD4(+) T-cell counts in the blood showed a transient depletion at the peak of VL, and then returned to near preinfection values by day 28 p.i. and remained relatively stable during the chronic infection. Preservation of CD4 T cells was also found in lymph nodes (LNs) of chronic SIVagm.sab-infected Caribbean AGMs. No activation of CD4(+) T cells was detected in the periphery in SIV-infected Caribbean AGMs. These virological and immunological profiles from peripheral blood and LNs were identical to those previously reported in African-born AGMs infected with the same viral strain (SIVagm.sab92018). Due to these similarities, we conclude that Caribbean AGMs are a useful alternative to AGMs of African origin as a model for the study of SIV infection in natural African hosts.

Infection of dendritic cells (DCs), not DC-SIGN-mediated internalization of human immunodeficiency virus, is required for long-term transfer of virus to T cells

The C-type lectin DC-SIGN expressed on immature dendritic cells (DCs) captures human immunodeficiency virus (HIV) particles and enhances the infection of CD4+ T cells. This process, known as trans-enhancement of T-cell infection, has been related to HIV endocytosis. It has been proposed that DC-SIGN targets HIV to a nondegradative compartment within DCs and DC-SIGN-expressing cells, allowing incoming virus to persist for several days before infecting target cells. In this study, we provide several lines of evidence suggesting that intracellular storage of intact virions does not contribute to HIV transmission. We show that endocytosis-defective DC-SIGN molecules enhance T-cell infection as efficiently as their wild-type counterparts, indicating that DC-SIGN-mediated HIV internalization is dispensable for trans-enhancement. Furthermore, using immature DCs that are genetically resistant to infection, we demonstrate that several days after viral uptake, HIV transfer from DCs to T cells requires viral fusion and occurs exclusively through DC infection and transmission of newly synthesized viral particles. Importantly, our results suggest that DC-SIGN participates in this process by cooperating with the HIV entry receptors to facilitate cis-infection of immature DCs and subsequent viral transfer to T cells. We suggest that such a mechanism, rather than intracellular storage of incoming virus, accounts for the long-term transfer of HIV to CD4+ T cells and may contribute to the spread of infection by DCs.

Lentivirus degradation and DC-SIGN expression by human platelets and megakaryocytes

BACKGROUND AND AIM

As platelets are able to endocytose human immunodeficiency virus (HIV), we have investigated the fate of lentiviruses when endocytosed by human platelets and megakaryocytes (MK), and have characterized a specific receptor directly involved in this function.

METHODS

Genetically modified (non-replicative) lentiviruses with an HIV envelope (HIV-e) or with a vesicular stomatitis virus protein G envelope (VSV-e) were alternatively used and their interaction with platelets and MK analyzed by electron microscopy (EM) and immunoEM.

RESULTS

When incubated with platelets, HIV-e and VSV-e lentiviruses were internalized in specific endocytic vesicles and trafficked to the surface connected canalicular system (SCCS). Double immunolabeling for the viral P24 core protein and alpha-granule markers showed that lentiviruses were degraded in the SCCS after contact with alpha-granule proteins. In culture MK, lentiviruses were found in endocytic vesicles and accumulated in acid phosphatase-containing multivesicular bodies (MVB). The expression of the pathogen receptor dendritic cell-specific ICAM-grabbing non-integrin (DC-SIGN) was then demonstrated in platelets by flow cytometry, immunoEM and Western blot. Anti-DC-SIGN antibodies decreased HIV-e lentivirus internalization by platelets, showing that the receptor is functional. Specific signals for DC-SIGN protein and mRNA were also found in MK.

CONCLUSION

This study indicates that platelets and MK can internalize lentiviruses in a pathway, which either provide a shelter to lentiviral particles or alternatively disrupts viral integrity. The receptor DC-SIGN is involved in this function.

Phenotype and function of myeloid dendritic cells derived from African green monkey blood monocytes

Myeloid dendritic cells probably play an important role in the immune response against HIV and SIV, and in the enhancement of CD4+ T cell infection. Here, we have investigated phenotypic and functional features of myeloid monocyte-derived DC (MDDC) from African green monkeys (AGMs). AGMs are natural hosts of SIV and exhibit no signs of abnormal T cell activation despite high SIV plasma viremia. We identified mAbs that cross-react specifically with homologous molecules expressed on AGM DC. We adapted a protocol to derive AGM MDDC by culture in the presence of GM-CSF and IL-4. The differentiated cells possessed a typical dendritic morphology and the majority were CD11c+ DC-SIGN+. AGM MDDC displayed a high expression of typical maturation markers, such as CD83, CD86 and DC-LAMP, and moderate immunostimulatory capacity, suggesting that the cells were in a semi-mature state. Stimulation resulted in further maturation, as shown by up-regulation of CD80 and decrease of endocytosis ability. However, neither increase of HLA-DR or CD40 expression nor enhanced immunostimulatory capacity was observed. The latter was associated with a low pro-inflammatory cytokine production during mixed lymphocyte reactions and a cytokine balance in favour of IL-10 in contrast to human MDDC. This is the first characterization of AGM MDDC. The tools described here are a crucial step for future studies in vivo or in vitro on the function of myeloid DC using the AGM animal model.

Impact of viral factors on very early in vivo replication profiles in simian immunodeficiency virus SIVagm-infected African green monkeys

To better understand which factors govern the levels of viral loads in early lentiviral infections of primates, we developed a model that allows distinguishing between the influences of host and viral factors on viremia. Herein we report that two species of African green monkeys (Chlorocebus sabaeus and C. pygerythrus) infected with their respective wild-type simian immunodeficiency virus SIVagm viruses (SIVagm.sab92018 and SIVagm.ver644) consistently showed reproducible differences in viremia during primary infection but not at later stages of infection. Cross-infections of SIVagm.sab92018 and SIVagm.ver644 into, respectively, C. pygerythrus and C. sabaeus revealed that the dynamics of viral replication during primary infection were dependent on the viral strain used for the infection but not on the host. Hence, the kinetics of SIVagm.sab92018 and SIVagm.ver644 were similar in both sabaeus and vervet animals, indicating that the difference in viremia levels between the two groups during the early phase of infection was not associated with the host. Coreceptor usage for these two strains showed a larger coreceptor repertoire for SIVagm.sab92018, which is able to efficiently use CXCR4 in addition to CCR5, than for SIVagm.ver644, which showed a classical CCR5 coreceptor usage pattern. These differences could not be explained by different charges of the V3 loop for SIVagm.sab92018 and for SIVagm.ver644. In conclusion, our study showed that the extent of virus replication during the primary infection is primarily dependent on viral determinants.

Dendritic Cell-specific Intercellular Adhesion Molecule 3-grabbing Non-integrin (DC-SIGN)-mediated Enhancement of Dengue Virus Infection Is Independent of DC-SIGN Internalization Signals

Dengue virus (DV) is a mosquito-borne flavivirus that causes hemorrhagic fever in humans. In the natural infection, DV is introduced into human skin by an infected mosquito vector where it is believed to target immature dendritic cells (DCs) and Langerhans cells (LCs). We found that DV productively infects DCs but not LCs. We show here that the interactions between DV E protein, the sole mannosylated glycoprotein present on DV particles, and the C-type lectin dendritic cell-specific intercellular adhesion molecule 3-grabbing non-integrin (DC-SIGN) are essential for DV infection of DCs. Binding of mannosylated N-glycans on DV E protein to DC-SIGN triggers a rapid and efficient internalization of the viral glycoprotein. However, we observed that endocytosis-defective DC-SIGN molecules allow efficient DV replication, indicating that DC-SIGN endocytosis is dispensable for the internalization step in DV entry. Together, these results argue in favor of a mechanism by which DC-SIGN enhances DV entry and infection in cis. We propose that DC-SIGN concentrates mosquito-derived DV particles at the cell surface to allow efficient interaction with an as yet unidentified entry factor that is ultimately responsible for DV internalization and pH-dependent fusion into DCs.