Research

Cell-to-cell Transmission of Retroviruses

The spread of retroviruses among cells is estimated to be 2-3 orders of magnitude more efficient when cells can physically interact with each other. The underlying mechanism is largely unknown, but transfer is believed to occur through large-surface interfaces, called virological or infectious synapses. To understand retroviral spreading, we co-culture infected cells generating fluorescent murine leukemia viruses (MLV) with receptor expressing target cells and monitor viral spreading by time-lapse microscopy. This resulted in the first documentation of cell-to-cell transmission of retroviruses in living cells. Viruses were observed to walk from cell to cell via thin filopodial bridges (see Movie below). While we are currently using this model system to understand the mechanism of cell-to-cell transmission of MLV, we have also turned our attention to HIV. The AIDS epidemic is driven by the ability of HIV to efficiently spread from cell to cell. Our focus in this area is directed towards the isolation of new host factors required for the spreading of HIV as well as the identification of small molecule inhibitors that can interfere with this step.

Murine leukemia virus (green/red) is seen walking along thin filopodial bridges from the infected cell, seen to the left, towards the target cells to the right. This mechanism of transmission involves the initial generation of a filopodial bridge between the infected cell and the target cell. The viral Env glycoprotein expressed in infected cells functions as an adhesion protein to stably anchor target cell filopodia in infected cells thereby generating a stable bridge. Viruses then move along the outer surface of the filopodial bridge toward the target cell.

TRIM proteins in innate immunity

A lot of excitement in the field of innate immunity to retroviruses such as HIV has come from the discovery of TRIM5 as a key player in cross species restriction. TRIM5 belongs to a family of E3 ligases with over 70 members, a number of which have exhibited antiviral activity. These findings have led to the hypothesis that several TRIM proteins may contribute to the innate immunity to retroviruses. Therefore, we systematically tested the antiviral activities of 55 human and mouse TRIM proteins. The results were astonishingly complex with activities affecting both, early and late stages of the retroviral life cycle. Importantly, a number of TRIM proteins that affected HIV or MLV replication upon overexpression, enhanced virus entry or release when downregulated by gene silencing. These experiments suggested that additional TRIM proteins contribute to the endogenous restriction of retroviruses. We are currently focusing on the mechanism by which some of these TRIM proteins interfere with retroviral replication and are testing a general role of these proteins in innate immunity.