The NK cell–activating receptor NKG2D interacts with three different cellular ligands, all of which are regulated by mouse cytomegalovirus (MCMV). We set out to define the viral gene product regulating murine UL16-binding protein-like transcript (MULT)-1, a newly described NKG2D ligand. We show that MCMV infection strongly induces MULT-1 gene expression, but surface expression of this glycoprotein is nevertheless completely abolished by the virus. Screening a panel of MCMV deletion mutants defined the gene m145 as the viral regulator of MULT-1. The MCMV m145-encoded glycoprotein turned out to be necessary and sufficient to regulate MULT-1 by preventing plasma membrane residence of MULT-1. The importance of MULT-1 in NK cell regulation in vivo was confirmed by the attenuating effect of the m145 deletion that was lifted after NK cell depletion. Our findings underline the significance of escaping MULT-1/NKG2D signaling for viral survival and maintenance.

Loewendorf A, Benedict CA (La Jolla Institute for Allergy and Immunology, La Jolla, CA, USA). Modulation of host innate and adaptive immune defenses by cytomegalovirus: timing is everything (Symposium). J Intern Med 2010; 267: 483–501. Human cytomegalovirus (HCMV) (HHV‐5, a β‐herpesvirus) causes the vast majority of infection‐related congenital birth defects, and can trigger severe disease in immune suppressed individuals. The high prevalence of societal infection, the establishment of lifelong persistence and the growing number of immune‐related diseases where HCMV is touted as a potential promoter is slowly heightening public awareness to this virus. The millions of years of co‐evolution between CMV and the immune system of its host provides for a unique opportunity to study immune defense strategies, and pathogen counterstrategies. Dissecting the timing of the cellular and molecular processes that regulate innate and adaptive immunity to this persistent virus has revealed a complex defense network that is shaped by CMV immune modulation, resulting in a finely tuned host–pathogen relationship.

Death receptors (DRs) of the TNFR superfamily contribute to antiviral immunity by promoting apoptosis and regulating immune homeostasis during infection, and viral inhibition of DR signaling can alter immune defenses. Here we identify the human cytomegalovirus (HCMV) UL141 glycoprotein as necessary and sufficient to restrict TRAIL DR function. Despite showing no primary sequence homology to TNF family cytokines, UL141 binds the ectodomains of both human TRAIL DRs with affinities comparable to the natural ligand TRAIL. UL141 binding promotes intracellular retention of the DRs, thus protecting virus infected cells from TRAIL and TRAIL-dependent NK cell-mediated killing. The identification of UL141 as a herpesvirus modulator of the TRAIL DRs strongly implicates this pathway as a regulator of host defense to HCMV and highlights UL141 as a pleiotropic inhibitor of NK cell effector function.

CD4 T lymphocytes regulate the adaptive immune response to most viruses, both by providing help to CD8 T cells and B cells as well as through direct antiviral activity. Currently, no mouse cytomegalovirus (MCMV)-specific CD4 T cell responses are known. In this study, we identify and characterize 15 I-Ab-restricted CD4 T cell responses specific for MCMV epitopes. CD4 T cells accumulate to high levels in the spleen and lungs during acute infection and produce multiple cytokines (IFN-γ, TNF, IL-2, IL-10, and IL-17). Interestingly, IL-17 and IFN-γ production within epitope-specific cells was found to be mutually exclusive. CD4 T cells recognizing a peptide derived from m09 were only detectable at later times of infection and displayed a unique cytokine production profile. In total, this study reveals that the MCMV-specific CD4 T cell response is complex and functionally diverse, highlighting its important role in controlling this persistent pathogen.

The Drosophila BTB domain containing gene mod(mdg4) produces a large number of protein isoforms combining a common N-terminal region of 402 aa with different C termini. We have deduced the genomic structure of this complex locus and found that at least seven of the mod(mdg4) isoforms are encoded on both of its antiparallel DNA strands, suggesting the generation of mature mRNAs by trans-splicing. In transgenic assays, we demonstrate the ability of Drosophila to produce mod(mdg4) mRNAs by trans-splicing of pre-mRNAs generated from transgenes inserted at distant chromosomal positions. Furthermore, evidence is presented for occurring of trans-splicing of mod(mdg4) -specific exons encoded by the parallel DNA strand. The mod(mdg4) locus represents a new type of comlpex gene structure in which genetic complexity is resolved by extensive trans-splicing, giving important implications for genome sequencing projects. Demonstration of naturally occurring trans-splicing in the model organism Drosophila opens new experimental approaches toward an analysis of the underlying mechanisms.