Further studies will be required to understand why UL18, alone among viral class I MHC homologs, associates with endogenous peptides and what effects peptide binding has on the HCMV lifecycle and the host anti-viral immune response. 

Structure of UL18, a peptide-binding viral MHC mimic, bound to a host inhibitory receptor.
Zhiru Yang and Pamela J. Bjorkman. 
PNAS July 22, 2008 vol. 105 no. 29 10095-10100. doi:10.1073/pnas.0804551105

Structure of UL18 (see below; click for larger versions) is highly similar to vertebrate MHC class I molecules. For more, see A Field Guide to the MHC Family.

Human cytomegalovirus UL18 HLA-A2
Side view HCMV UL18, side view HLA-A2, side view
Top view
(Peptide in green) 
HCMV UL18, top view HLA-A2, top view