Date Published: March 21, 2013
Publisher: Public Library of Science
Author(s): Ivana Nemčovičová, Chris A. Benedict, Dirk M. Zajonc, Daved H. Fremont.
The TRAIL (TNF-related apoptosis inducing ligand) death receptors (DRs) of the tumor necrosis factor receptor superfamily (TNFRSF) can promote apoptosis and regulate antiviral immunity by maintaining immune homeostasis during infection. In turn, human cytomegalovirus (HCMV) expresses immunomodulatory proteins that down-regulate cell surface expression of TNFRSF members as well as poliovirus receptor-related proteins in an effort to inhibit host immune effector pathways that would lead to viral clearance. The UL141 glycoprotein of human cytomegalovirus inhibits host defenses by blocking cell surface expression of TRAIL DRs (by retention in ER) and poliovirus receptor CD155, a nectin-like Ig-fold molecule. Here we show that the immunomodulatory function of HCMV UL141 is associated with its ability to bind diverse proteins, while utilizing at least two distinct binding sites to selectively engage TRAIL DRs or CD155. Binding studies revealed high affinity interaction of UL141 with both TRAIL-R2 and CD155 and low affinity binding to TRAIL-R1. We determined the crystal structure of UL141 bound to TRAIL-R2 at 2.1 Å resolution, which revealed that UL141 forms a homodimer that engages two TRAIL-R2 monomers 90° apart to form a heterotetrameric complex. Our structural and biochemical data reveal that UL141 utilizes its Ig-domain to facilitate non-canonical death receptor interactions while UL141 partially mimics the binding site of TRAIL on TRAIL-R2, which we found to be distinct from that of CD155. Moreover, UL141 also binds to an additional surface patch on TRAIL-R2 that is distinct from the TRAIL binding site. Therefore, the breadth of UL141-mediated effects indicates that HCMV has evolved sophisticated strategies to evade the immune system by modulating multiple effector pathways.
The immune system has evolved to protect against the many pathogens that are encountered throughout the lifetime of an individual. In turn, the selective pressure that is exerted by the immune system has shaped pathogen evolution. This co-evolutionary relationship between host and pathogen is particularly clear for viruses that establish persistent infections, such as human herpesviruses (HHV) , . Human cytomegalovirus (HCMV, a β-herpesvirus, HHV-5), is a large double-stranded DNA virus that causes a lifelong, persistent/latent infection in ∼50–80% of the US population, varying with age, geography and socioeconomic status. While HCMV infection is largely asymptomatic in healthy persons, it can induce serious disease in those with naïve or compromised immunity, and the high incidence of congenital infection has spurred a strong initiative for vaccine development . Primary clinical isolates carry at least 19 additional genes within the UL/b′ genomic region (UL133–151 locus) that have been lost in several commonly used HCMV strains that have been passaged extensively in tissue culture , , with several of them targeting signaling by the TNFR superfamily (e.g. UL144 and UL138) .
Human cytomegalovirus encodes several genes tightly linked to UL141 in the UL/b′ region that modulate host immune responses mediated by TNF-family proteins. These include UL138, which has recently been shown to promote the expression of TNFR-1, and UL144, a partial-mimic of HVEM (herpesvirus entry mediator) that exclusively binds the inhibitory receptor BTLA (B- and T-lymphocyte attenuator) . Although it is common for herpesvirus immune modulatory proteins to have evolved to target a specific protein, or a family of host proteins, targeting diverse proteins that contain unique folds is rare. We now add the modulation of the TRAIL DRs to the arsenal of UL141 immune modulatory activity, in addition to its previously known role in restricting CD155 and CD112 expression. TRAIL DRs and CD155 belong to two structurally distinct families, the classical TNF receptor superfamily and the nectin-like Ig superfamily, respectively. While the only currently known natural ligand, TRAIL, belongs to the TNF superfamily, our structural analysis shows that UL141 assumes an Ig-like fold and shows no structural homology to TRAIL. However, UL141 does mimic key TRAIL binding motifs of TRAIL to TRAIL-R2, even though the Ig-fold of UL141 is entirely different from the homo-trimeric fold of TRAIL and other TNF ligands. Since UL141 and the Ig-like poliovirus receptors share no primary sequence homology, we favor the view that UL141 evolved independently, mimicking the central binding motif of TRAIL in addition to an as-yet unidentified binding motif to engage CD155.