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. 2014 Aug;12(8):563-74.
doi: 10.1038/nrmicro3309.

Adding New Dimensions: Towards an Integrative Understanding of HIV-1 Spread

Free PMC article

Adding New Dimensions: Towards an Integrative Understanding of HIV-1 Spread

Oliver T Fackler et al. Nat Rev Microbiol. .
Free PMC article


In vitro studies in primary or immortalized cells continue to be used to elucidate the essential principles that govern the interactions between HIV-1 and isolated target cells. However, until recently, substantial technical barriers prevented this information from being efficiently translated to the more complex scenario of HIV-1 spread in the host in vivo, which has limited our understanding of the impact of host physiological parameters on the spread of HIV-1. In this Review, we discuss the recent development of imaging approaches to visualize HIV-1 spread and the adaptation of these approaches to organotypic ex vivo models and animal models. We focus on new concepts, including the mechanisms and in vivo relevance of cell-cell transmission for HIV-1 spread and the function of the HIV-1 pathogenesis factor Nef, which have emerged from the application of these integrative approaches in complex cell systems.


Figure 1
Figure 1. Overview of crucial, but poorly understood steps in the biology of HIV-1 infection
(a) Under what conditions can HIV-1 cross the stratified epithelium of the vagina or the columnar epithelium of the uterine cervix during transmission? Can infectious virions penetrate mucosal tissues far enough to reach epithelial or subepithelial target cells? Which cells are initially infected? Is there a role of HIV trans-presentation by non-infected dendritic cells? (b) In what form does virus reach the draining lymph node? (c) How does the virus disseminate via the bloodstream to generate a systemic infection? (d) For all affected tissues, what is the role of (clockwise from top): free virus, trans-presentation by antigen-presenting cells, interactions with infected antigen-presenting cells, and direct T cell-T cell interactions for cell to cell spread ? (e) What is the role for different immunological anti-HIV effector functions, such as (clockwise from top) direct T cell-mediated cytotoxic lysis of infected cells, antibody-mediated neutralization of free virus, or antibody-mediated cellular effector functions, such as cytotoxicity or phagocytosis, at different time-points of infection and in different tissues?
Figure 2
Figure 2. Complex ex vivo cell systems for studies of HIV-1 spread
Schematic depiction and characteristics of complex ex vivo cells systems for studies of HIV-1 replication and spread. The top panel depicts the schematic organization of the model system, the lower panel present histology of a tissue section (a) (reproduced from with permission) or wide field images of cell organization (b, c). Scale bars = 40μm. (a) Organotypic histocultures of HIV-1 target organs. Tissue explants from cervix, vagina, tonsil or thymus are cultivated on support at the air-medium interface to maintain tissue organization and composition. (b) In synthetic 3D cultures systems, cells are embedded into matrices for cultivation, allowing for tightly controlled experimental variation of cell and matrix composition. (c) Classical cultures, even for suspension cells, result in unphysiologically dense cell packing at the bottom of the tissue culture plate and thus a 2D scenario. (d) Table summarizing key characteristics of the culture systems shown above.
Figure 3
Figure 3. Intravital microscopy analyses of retroviral spread in vivo
(a, b) MP-IVM of popliteal lymph nodes of HIV-1-infected BLT humanized mice has shown that HIV-1-infected T cells migrate half as fast as their uninfected counterparts (a) and that (b) they interact with CD4+ cells via gp120, which can lead to either tethering interactions or to formation of motile multinucleated syncytia. (c) Efficient viral dissemination from lymph nodes draining an HIV-1-inoculation site to distant tissues requires either HIV-1-infected or HIV-1-carrying cells as migratory vehicles (d) MLV-infection of C57BL/6 mice showed either unaltered migration or arrest of infected B cells in lymph nodes. (e) Visualizing Gag proteins revealed envelop-dependent polarization of virions in arrested B cells in vivo, suggesting formation of virological synapses during cellular interactions. Fluorescence micrograph in (e) is taken from the paper by Sewald et al. and shown with permission from Dr. Walther Mothes.
Figure 4
Figure 4. Model for the effects of HIV-1 Nef on T lymphocyte trafficking and induction of humoral immunity
(a) Schematic depiction of effects of Nef observed on in vivo T lymphocyte trafficking. The scheme summarizes observations made in lymph node sections of SIV-infected macaques , infected humanized mice and adoptive transfer of Nef-expression T lymphocytes into recipient mice . In the absence of Nef (left), T lymphocytes transmigrate from the HEV into the lymph node parenchyma where they remain fully motile. Infected cells spread virus but new infection may also occur by virus delivered via the afferent lymphatics. Exit of infected cells via the medulla may support dissemination of the virus. The presence of Nef (right) significantly reduces the ability of infected T lymphocytes to transmigrate to the lymph node lumen and may also reduce their ability to exit. Successfully transmigrated cells preferentially remain in the proximity of the HEV and display reduced intra lymph node motility. These properties may favor the formation of stable virological synapses or even polysynapses and thus optimize virus transmission. As indicated by the enrichment of viral antigen at the subcapsular sinus, infection with virus via the afferent lymphatics may be particularly efficient with nef-positive virus. (b) Hypothetical model of effects of Nef on humoral immunity. In the absence of Nef, T lymphocytes are readily engaged in productive immunological synapses with antigen presenting cells. In the case of B cells, CD4+ help induces B cell activation driving germinal centre reactions and ultimately mounting of a potent humoral immune response (left). Nef expressed in HIV-infected CD4+ T lymphocytes disrupts immunological synapse organization to reduce signal transmission and might thereby limit B cell activation. Moreover, infected, Nef expressing macrophages can trigger unspecific B cell activation by a paracrine mechanism involving ferritin and deposit the viral protein in B cells from via nanotubes to interfere with Ig class switching. Together these effects likely inhibit humoral immune responses to HIV infection and suggest Nef as a key determinant for B cell dysfunction in AIDS patients.

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