NOD-like receptors in host defence against viruses

Innate immunity represents the first line of defence against pathogens. The human genome contains 22 genes encoding NLR (NACHT and Leucine Rich Repeat) proteins, which are typically found in the cytosol, representing intracellular analogues to the TLRs (Toll-like receptors), central mediators of innate immunity. Accumulating data indicate an important role of NLR proteins in host defence to viral infection. For example, the presence of viral DNA in the cytosol triggers activation of certain NLR family members, defining a mechanism by which host cells recognize and initiate immune responses to DNA viruses. Gamma-herpesviruses, such as human herpesvirus 8 (HHV8), are responsible for extensive morbidity and mortality on a worldwide basis. These DNA viruses possess genes that counteract immune host defences, ensuring their livelong maintenance in infected hosts and promoting transmission to new hosts. At present, little is known about the mechanisms by which gamma-herpesviruses evade host immune defences. Interactions of herpesviruses with NLRs and cellular NLR-interacting proteins have not been defined, thus leaving a gap in our knowledge. In this Project, the hypothesis that NLR proteins represent an important class of immune receptors in host response to HHV8 infection will be tested. Interactions of NLRs with viral proteins will be investigated and functionally dissected. The proposed Specific Aims are to: (1) Determine the roles of NLR proteins in host responses to HHV8 infection. (2) Define the downstream effectors of NLR proteins involved in defence against HHV8. (3) Elucidate how HHV8 inhibits NLR-mediated host responses. Altogether, these studies will provide insights into the roles of NLR-family proteins in host defence against gamma-herpesviruses, while also laying a foundation for a more general understanding of innate immune defences against DNA viruses. Aims #1 and #2 will be conducted during two years of research abroad in the group of Prof. John Reed at Sanford- Burnham Institute for Medical Research (La Jolla, California). Aim #3 will be completed during the third year (Return Phase) in the group of Prof. Robert Schwarzenbacher at the University of Salzburg (Austria). Please refer to SCIENTIFIC EXPERTISE OF THE SUPERVISORS for detailed information on Prof. Reed and Prof. Schwarzenbacher as well as their host organizations.

The main goal of the funded project was to assess the contribution of NLR proteins, a major class of innate immune receptors, to the host antiviral response. To this end interactions of NLR proteins and their downstream effectors with viral factors, including factors encoded by human immunodeficiency virus (HIV), were investigated. HIV continues to be a global public health issue. The emergence of resistant viruses is a constant threat and development of a cure or an effective vaccine has been elusive so far. Like all viruses, HIV relies on the cellular machinery for replication. During its life cycle HIV also overcomes multiple measures of cellular restriction and efficiently evades host immune responses. Since the HIV genome encodes only 15 proteins, it is not surprising that many HIV proteins are optimized to perform multiple functions, and HIV-1 protease (PR) appears to be no exemption. Long been recognized for its essential role during virus maturation, PR was amongst the first targets for drug development against HIV. PR as a viral countermeasure to cellular restriction is a novel concept though. It has been realized that PR cannot only process viral polypeptide precursors but also host cell factors to benefit virus replication. Through a combination of genome-wide siRNA library screening and protein interaction studies, two candidate PR-interacting host factors were identified that have known connections to NLRs. We found that RIPK1 and RIPK2, two critical cellular kinases involved in NLR signaling, are novel substrates of PR. During infection of T cells, RIPK1 and RIPK2 associate with and are cleaved by PR. This cleavage disrupts their function and likely represents a novel viral mechanism to manipulate the host response to HIV infection.