HIV Infection and Transmission close to Reality

Persistent infections such as HIV, hepatitis B and C virus are major causes of mortality world-wide. Dendritic cells (DC), which are essential for the generation of a protective antiviral immune response, are exploited by the viruses to evade innate and adaptive immunity. When HIV enters the body, virus becomes opsonized with complement fragments and antibodies within a short time which should help to eradicate the virions immediately by the innate immune system. The adaptive immune system needs longer for activation, and B cells and T cell responses further assist in the immune response against HIV. DC play a major role in this anti-viral responses but they can also get infected and transmit HIV. So far understanding of HIV infection and transmission was studied with non-opsonized virus and various subtypes of DC in the skin/mucosa showed different abilities to either eradicate or transmit HIV to T cells. Studies showed that Langerhans cells, the DC of the epidermis, degraded non-opsonized HIV particles after attachment via Langerin (C-type lectin) in Birbeck granules, while dermal DC enhanced HIV-infection via C- type lectins (DC-SIGN). In preliminary experiments we observed that Langerhans cells are unable to eliminate HIV when virions are opsonized with complement fragments. Since HIV only exists as complement-opsonized virus in the infected body, we believe that the real infectious situation differs from what has been reported so far. Thus, we want to investigate following topics in this project proposal: how are DC in the skin/mucosa infected when HIV is opsonized can DC transport and transmit opsonized HIV and is the antigen presentation capacity of DC affected by opsonization of HIV. During our investigations we will additionally characterize the exact cellular mechanisms and the various signaling pathways in LC and dermal/interstitial DC, which are occuring upon attachment of differentially opsonized HIV-1. The results will help to design new DC-based vaccination tools by manipulating DC in a way to elicit strong and protective immune responses. Thus, these findings are important in regard to efforts which are under way to develop immunotherapeutical interventions to prevent or treat HIV infection.

Within the project HIV infection and transmission close to reality that was performed from June 2012 to May 2017 at the Division of Hygiene and Medical Microbiology under the guidance of Associate-Professor Doris Wilflingseder the cat-and-mouse game between HIV-1 and the immune system was studied after initial contact with the virus. In detail interactions of various dendritic cell (DC) subsets with HIV-1 particles either naked virus or coated with fragments of our innate immune system (complement) were investigated. The complement system destroys intruders shortly after crossing the barriers of the host, but HIV-1 developed very efficient mechanisms to avoid this direct destruction by complement. Nevertheless, complement fragments highlight the presence of the virus due to covalent linking to the viral surface referred to as opsonization. This red coat makes HIV-1 visible for the next part of our innate immune system, dendritic cell (DC) subsets. DCs are also calledsentinels of our immune system, since these are the only cells capable of activating nave T cells in the lymph node. In turn T cells as part of the adaptive immune response are able to specifically destroy HIV-1.Earlier works of our group illustrated the improved recognition of complement-opsonized HIV-1 (HIV-C) by monocyte-derived as well as blood DCs compared to naked virus (HIV) or Antibody-opsonized HIV (HIV-Ig). During the project HIV infection and transmission close to reality the group now continued their characterization of DCs exposed to differentially opsonized virus preparations and extended their work to various DC subsets, Langerhans Cells (LCs) in the skin and dermal DCs (dDCs) in the dermis. Within the first project year, in vitro cultures of LCs and dDCs from umbilical cord, monocytes or isolated from skin explants were established. Infection experiments using differentially opsonized HIV-1 preparations illustrated a strong infection of those DC subsets using HIV-C independent of the cell origin. Even the described antiviral barrier of LCs was overcome by complement opsonization of HIV-1. Further steps focused on more detailed characterization of molecular mechanisms within DCs following exposure to HIV-C or HIV. We here gave insight into a substantial novel way of dendritic cell modulation at least during acute HIV-1 infection by triggering integrin receptor signaling. We found, that complement-opsonization of the virus was able to relieve SAMHD1 restriction in DCs, thereby initiating strong phosphorylation signals within DCs and increased DC maturation compared to DCs loaded with non-opsonized HIV-1. In addition, the co-stimulatory capacity of HIV-C-exposed DCs was significantly higher as well as their capacity to stimulating efficient cellular and humoral antiviral immune responses. This newly described way of DC modulation by complement might be exploited to find novel therapeutic targets promoting DC immune functions against HIV.