HIV-C escapes Restriction not Sensing in DCs

The complement system is one of the first lines of defense against invading pathogens. Importantly, complement coats the surface of HIV-1 immediately due to a complement binding site within the HIV-1 envelope. Since the virus is very well protected against the attacks of this early recognition system in our body, it is able to bind to complement receptor (CR)-expressing cells, such as dendritic cells (DCs) or macrophages. Similar to HIV-2, complement-coated HIV-1 infects DCs to significantly higher levels as naked HIV-1 particles. This enhanced DC infection is associated with improved antiviral and adaptive immune responses against the virus. HIV-2, which is controlled by the immune system and less pathogenic than HIV-1, too, shows higher infection of DCs because of Viral protein X (Vpx). Vpx is a protein within HIV-2, that mediates degradation of a restriction factor in DCs, thereby allowing infection and trigger of an antiviral response similar to HIV-C, which does not contain Vpx. Today, HIV-1 infection is a chronic disease with a lifelong treatment associated with side effects. In this project we aim in finding novel therapeutic targets for HIV-1 treatment - for that, we will study pathways activated by complement- coated HIV-1 in DCs and those similarly activated in HIV-2 infection. Upon comparison of HIV-C with HIV-2, we will search for compounds, which could mimic the antiviral state and innate/adaptive activation. Within the project period we will characterize cellular compounds, which make the virus more susceptible for dendritic cell sensing. Specifically, we will: -Unveil, how complement-coated HIV-1 and HIV-2 activate innate immune pathways, but naked HIV-1 is able to hide from the innate immune sensors. -Untangle the role of viral accessory proteins in the infectivity and sensing of complement-coated HIV- 1. -Discover new targets for activating innate and adaptive immunity against HIV-1. Understanding the intracellular pathways leading to a better immune defense against HIV-1 will help developing novel strategies for HIV clearance and prophylaxis. Furthermore, understanding in detail the differences in infectivity and pathways activated, opens new avenues of treatment, shock-and-kill and prevention.

In the FWF-funded project P33510, led by Univ.-Prof. Doris Wilflingseder at the Institute of Hygiene and Medical Microbiology, the focus was on understanding how our innate immune system responds at mucosal surfaces immediately after first contact with viruses-particularly the interaction between the complement system and dendritic cells. Building on earlier work by the project leader, the team demonstrated for the first time how complement-opsonized HIV-1 activates a strong antiviral immune response in dendritic cells via the CCR5/RIG-I/MDA5/MAVS pathway. A groundbreaking finding was that the complement receptor CR4 (CD11c) plays a pivotal role in this process, representing a promising new target for therapeutic interventions against HIV. Another study revealed that the virus exploits tunneling nanotubes (TNTs) to spread between cells-a mechanism that can be effectively disrupted by blocking complement anaphylatoxin receptors C3aR and C5aR and anaphylatoxin signaling, offering a new strategy to prevent viral dissemination. With the global outbreak of COVID-19, the team swiftly shifted focus to SARS-CoV-2. Thanks to a rapid and productive collaboration with the local clinical infectious disease department, they obtained the first patient samples in April 2020. These were expanded in the BSL-3 high-security lab and used to infect an established, highly differentiated 3D model of the human airway epithelium to investigate the initial steps of infection with this then-unknown virus. The results were as striking as they were concerning: SARS-CoV-2 triggered massive mucus hypersecretion, overactivation of the complement system, and destruction of the epithelial barrier in the tissue models. The team showed for the first time that targeted blockade of the anaphylatoxin receptors C3aR and C5aR in non-immune respiratory cells significantly reduced inflammation and tissue damage-offering a highly promising therapeutic approach. Equally relevant to real-world applications was the investigation of a licensed medical mouth spray and additional plant-based mouth/nasal sprays and lozenges. A simple pre-treatment with these products prevented viral binding and uptake, blocked intracellular complement activation, and preserved epithelial integrity. Even lozenges dissolved in saliva demonstrated antiviral effects. These findings powerfully highlight how safe, scientifically tested products can serve as effective and rapid frontline defenses against viral infections. Overall, this project exemplifies how excellent basic research, interdisciplinary collaboration, and cutting-edge models can generate new insights into viral infections-while simultaneously delivering tangible solutions for urgent public health challenges.