The outcome following HIV infection depends on the nature and durability of the HIV-specific T cell response induced initially. The activation of protective T cell responses depends upon dendritic cells (DC), antigen-presenting cells which have the capacity to process and present viral antigens. DC pulsed with aldrithiol-2-inactivated HIV and delivered in vivo were reported to induce immune responses and promote virologic control in chronically HIV-1-infected subjects. To gain an understanding of this phenomenon, we characterized the steps involved in the presentation of antigens derived from aldrithiol-2-treated vs. infectious HIV-1 by DC. Antigen presentation, on both MHC class I and II, was independent of DC-specific ICAM-3-grabbing integrin, DEC-205 and macrophage mannose receptor, C-type lectins expressed by the DC. Inhibitor studies showed that presentation on MHC class I was dependent on viral fusion in a CD4/coreceptor-dependent manner, both at the cell surface and within endosomes, and access to the classical endosomal processing pathway. MHC class II presentation of HIV-associated antigens was dependent on active endocytosis, probably receptor-mediated, and subsequent degradation of virions in acidified endosomes in the DC. Our study brings forth new facts regarding the binding, uptake, and processing of chemically inactivated virions leading to efficient antigen presentation and should aid in the design of more effective HIV vaccines. © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Interaction with the complement system is an underappreciated aspect of HIV-1 infection; even in primary infection, complement fragments are found on virions with potential to affect the interplay between the virus and dendritic cells (DC). Since opsonization may affect the efficiency of uptake and the type of receptors utilized, we compared the interactions of DC with free HIV-1 (F-HIV) and complement opsonized HIV-1 (C-HIV). We demonstrate that C-HIV significantly enhanced the uptake by immature DC (IDC) and mature DC (MDC) and that the internalization rate was dependent on both opsonization of the virus and DC maturation state. Increased DC uptake of C-HIV was not due to opsonization related increased binding of virus to the surface of DC but rather increased internalization of C-HIV despite utilizing a similar repertoire of receptors as F-HIV. Both F-HIV and C-HIV interacted with C-type lectins, integrins, and CD4 and blocking these receptor families prevented HIV-1 from binding to DC at 4 degrees C. Blocking integrins significantly reduced the binding and uptake of F-HIV and C-HIV implicating the involvement of several integrins such as beta 1-integrin, CR3, LFA-1, and alpha 4 beta 7. Distinctive for C-HIV was usage of beta 1-integrin and for F-HIV, usage of beta 7-integrin, whereas both F-HIV and C-HIV utilized both integrin chains of CR3. We have in this study identified the receptor types used by both F-HIV and C-HIV to bind to DC. Noteworthy, C-HIV was internalized more efficiently by DC than F-HIV, probably via receptor mediated endocytosis, which may entail different intracellular processing of the virus leading to both elevated infection and altered activation of HIV specific immune responses.

The antigen processing and presentation processes occurring in dendritic cells (DCs) required for induction of HIV-1 specific T cell responses, which are essential for controlling the viral infection in vivo. The initial interactions of DCs with free HIV-1 (FHIV), or complement opsonized HIV-1 (C-HIV) might influence the routing and nature of pathways used for MHC class I and II restricted presentation. We have examined FHIV, C-HIV, and complement and antibody opsonized HIV-1 (C-IgG-HIV) effects on immature DCs (IDCs) and mature DCs (MDCs) antigen proteolysis, MHC class I and II antigen presentation, and the role of endocytic receptors in presentation of antigens derived from HIV-1. We found that opsonized virions promoted MHC class I presentation by both IDCs and MDCs compared to F-HIV. Indicative of that complement opsonization routes more virions towards the MHC class I presentation pathway. We found that blocking macrophage mannose receptor (MMR) rerouted the HIV-1 to a path leading to higher levels of MHC class I and II presentation. Furthermore, the blocking of β7-integrin also gave an enhanced MHC class I and II presentation by both IDCs and MDCs, whereas the block of αMβ2 integrins, i.e. complement receptor 3 (CR3), decreased the MHC class I and II presentation. In addition, we found that IDCs and MDCs proteolytic activities were modulated by the HIV-1 exposure, for example C-HIV induced an increased proteasome activity in IDCs. Taken together, these findings indicated that endocytic receptors, such as MMR, CR3, and β7 integrin, can promote or disfavor antigen presentation by routing HIV-1 into different endosomal compartments with distinct properties and efficiencies for degradation of viral antigens and MHC class I and II presentation and that HIV-1 affects the antigen processing machineries.

Slightly more than half of the HIV-1 infected individuals in the world are women and almost all acquire the infection through sexual intercourse. The initial interaction between HIV-1 and the host occurs at the mucosa site and the most common mode to access the submucosa is through dendritic cells (DCs). In the cervical mucosa, HIV-1 exist both as free and opsonized virions and this might influence initial infection. We used a cervical tissue explant model and both free and opsonized virions to study HIV-1 transmission and how it can be prevented.

We found that complement opsonization significantly enhanced HIV-1 infection of DCs compared to free HIV-1, but this increased infection was not seen for CD4+ T cells. Blocking of α4, β7, and β1 integrins demonstrated significant inhibition of infection of both DCs and CD4+ T cells emigrating from mucosa, independent of the use of free or complement opsonized HIV-1. We found a higher impairment of HIV-1 infection in emigrating DCs for complement opsonized virions compared to free virions when the use of αM/β2 and α4 integrins was blocked.

This study showed that block of integrins decreased the HIV-1 infection of both DCs and CD4+ T cells emigrating from the cervical explant tissues and, remarkably, the establishment of founder populations in these tissues. This indicates that preventing or severely lowering initial infection of the cervical mucosa could avert systemic HIV-1 infection and should be considered for development of microbicides to prevent HIV infection and transmission.