Mechanisms of HIV cell-to-cell transmission

Contact between HIV-infected and uninfected T cells can lead to the formation of infection enhancing structures called virological synapses (VS). VS are initiated by the HIV surface Env glycoprotein on the infected cell and CD4 on the target cell. We design and employ infectious fluorescent viruses to visualize and quantify viral transfer across the VS. High resolution, live video rate confocal microscopy has allowed us to directly record the changes in viral protein distribution that occur during VS formation. We are characterizing the efficient transfer of viral particles into target T cells through an endocytic route. Our ongoing studies are directed at understanding the host factors that participate in and regulate VS assembly and transfer.  We are also studying the different paths that key components of the virus, Gag, Env and the genomic RNA, take to the VS. The work will help us understand how this mode of efficient viral dissemination may allow HIV to spread efficiently in vivo.

Visualizing HIV virological synapses in vivo
We have set out to characterize the sequence of events that occurs during transmission of HIV via the intravenous route.  To model this important mode of transmission, we have examining the role of cell-to-cell and cell-free virus in venous transmission using humanized mouse models.  Humanized mouse systems utilize immunodeficient mouse strains that can be transplanted with human hematopoietic stem cells.  They support the development of diverse lineages of human immune cells including those that can be infected by HIV.  The engrafted human immune systems are highly susceptible to HIV and support sustained HIV viral loads in animals that are challenged.  By challenging mice with different mixed genotypes from single cells, we have examined the efficiency of genetic coinheritance, a phenomenon that occurs efficiently through virological synapses.  We are using whole animal imaging approaches to examine the distribution of cells within animals over time.  We have developed imaging methods to visualize cell-cell interactions that play a role in the establishment of infection, and have examined the spatial organization of viral spread using fluorescent viral markers. The animal models we study will help us to understand how the dynamic trafficking of T cells, and cell-cell interactions contribute to viral spread amongst T cells.

Evasion of antibody responses by virological synapses
The VS-mediated viral infection can be resistant to patient antibodies that are capable of neutralizing homologous cell free virus. We are working to understand how the VS provides a mechanism for HIV to evade humoral immune responses.  We found that the cytoplasmic tail of the Env glycoprotein, which is plays an important role in regulating fusion activity of Env, plays a role in the resistance of cell-cell infection to neutralization.  We are testing a model whereby the conformational regulation of Env during VS formation is what makes cell-cell transmission more resistant to neutralization.  We are studying patient neutralizing responses, and cloning B cells from patients to characterize potent cell-cell neutralizing activities.

HIV induced inflammation and pathogenesis in the kidney
In addition to forming virological synapses with uninfected T cells, HIV infected cells can also mediate interactions with non-immune cells, such as the tubular cells within the kidney. During chronic HIV infection, infected cells can be found in the kidney where they can transfer virus to renal cells. In response to the interactions with HIV infected cells, renal cells can elaborate an inflammatory cascade of chemokines that can attract more T cells to the organ. We are investigating the mechanisms by which the virus is internalized by renal cells, and what pathogen associated patterns on HIV are recognized to initiate this inflammatory response. These studies may provide clues as to why HIV can disrupt the function of non-immune organs, and why infected patients have elevated markers of inflammation even when treated with anti-retroviral drugs.

Modeling HIV-1 reservoirs in humanized mice
Although antiretroviral treatment is very effective at limiting the spread of HIV, the persistence of virus in latent pools within memory T cell thwarts the ability of the immune system to eliminate the virus from the body. As a result, when antiretroviral treatment is stopped, virus replication immediately resumes at a high rate. A current challenge in the field is therefore to find a way to identify the cells that harbor latent virus, and to find ways to eliminate them either by activating the virus within them or by somehow targeting them for descruction. To address the problem of latency, we are building genetic reporter mouse models that can become irreversibly marked when HIV infects the cells. The cells that persist and harbor latent virus should remain marked even in the presence of antiretroviral therapy. The marking system is designed to help uncover specific genes or cell states that distinguish latent HIV reservoirs from non-infected cells.