Evasion of host immune response recognition plays a key role in the establishment of systemic viral infection. Many viruses have evolved complex strategies for this evasion (one example being the down-regulation of cell surface MHC-I display). Such strategies are readily observable among the herpesviruses, a family of large DNA viruses that efficiently produce persistent infections and disease in their host. Kaposi's sarcoma-associated herpesvirus (KSHV; also called human herpesvirus 8) is a lymphotropic herpesvirus that is the etiologic agent of Kaposi's sarcoma as well as of two AIDS-related lympho-proliferative syndromes.
We are focusing our energy on understanding the molecular mechanisms of immune evasion employed by KSHV. This will improve our understanding of KSHV pathogenesis and provide insights into basic cellular and immunological processes. Additionally, this could lead to new-antiviral strategies and the ability to exploit viral function as a tool with medical relevance.
MHC-I downregulation by the KSHV encoded MIR proteins. I have recently shown that KSHV encodes two transmembrane proteins (MIR1 and MIR2) that downregulate cell surface molecules (MHC-I, B7.2, and ICAM-1) which are involved in the immune recognition of infected cells. The pathway of this downregulation is novel: the target proteins are efficiently synthesized, matured and reach the cell surface normally. From that locale, however, they are rapidly and efficiently endocytosed and subsequently degraded in the endolysosomal compartment. I found that expression of MIR1 and MIR2 leads to ubiquitination of the cytosolic tail of their target proteins and that this ubiquitination is essential for their removal from the cell surface. MIR proteins thus define a novel class of membrane-bound E3 ubiquitin ligases that modulate the trafficking of host cell membrane proteins.
We are now pursuing the characterization of the molecular mechanism of MIR-mediated downregulation. Our recent experiments have suggested that host proteins are involved in this process. We are currently pursuing the identification of these MIR-interacting host components. These results should shed light on the molecular details of the connection between ubiquitination and endocytosis, and further inform our understanding of this unusual pathway of immune evasion.
Interestingly, structural homologues of the MIRs can be found in most eukaryotic organisms although no function has yet been attributed to their expression. Thus, dissection of the mechanism of MIR-mediated ubiquitination and endocytosis may lead to the discovery of novel cellular mechanisms involved in protein trafficking.
Identification of additional KSHV proteins implicated in immune evasion. Herpesviruses generally employ several different strategies to achieve immune evasion, including the sabotage of MHC antigen processing pathways, the molecular mimicry of host signaling molecules and the expression of decoys that inhibit natural killer (NK)-cell-mediated killing. It is thus unlikely that the MIR proteins alone are sufficient to allow for a complete immune evasion by the virus. This idea is supported by our observation that MIR expressing cells have an increased sensitivity to natural killer cell lysis. Using various approaches, we will identify additional KSHV proteins implicated in other forms of immune evasion (NK cell avoidance, resistance to complement lysis, etc.).