1 3 Med Microbiol Immunol (2015) 204:295–305 DOI 10.1007/s00430-015-0405-2 REVIEW Principles for studying in vivo attenuation of virus mutants: defining the role of the cytomegalovirus gH/gL/gO complex as a paradigm Jürgen Podlech 1 · Matthias J. Reddehase 1 · Barbara Adler 2 · Niels A. W. Lemmermann 1 Received: 30 January 2015 / Accepted: 4 March 2015 / Published online: 18 March 2015 © Springer-Verlag Berlin Heidelberg 2015 established primary organ infection or with recurrent infec- tion after virus reactivation from latency within tissue cells. The demonstration in the murine model of alternative gH/ gL complexes gH/gL/gO and gH/gL/MCK-2, substituting one another in a redundant fashion for securing viral spread in tissues, has the medically interesting bearing that target- ing the gH/gL core complex directly may be a promising approach to preventing primary, established, and recurrent CMV infections. Keywords Antiviral intervention · Envelope glycoproteins · Mathematical modeling of virus multiplication · Transcomplementation · Virus entry · Virus spread Introduction Glycoprotein complexes inserted in the virion envelope of enveloped viruses are supposed to be critically involved in the process of virus entry into host cells (reviewed in [1]), a process basic to virus multiplication and pathogenesis. In the case of cytopathogenic viruses, such as the cytomegalo- viruses (CMVs), virus spread from initially infected cells to neighboring cells in tissues directly accounts for tissue destruction that can end up in functional organ failure associ- ated with morbidity and often mortality. Based primarily on cell culture studies with human CMV (hCMV), four virion envelope glycoprotein complexes, largely conserved among CMV species, are thought to be more directly involved in host cell attachment and entry, and thus in viral host cell tro- pism: the gB homotrimer [2–7], the gM/gN heterodimer [8, 9], the trimeric complex gH/gL/gO [10–12], and an alterna- tive gH/gL complex identified for hCMV as a pentameric complex composed of the gH/gL core complex [13, 14] and Abstract Initial virus entry into cells of host organs and subsequent spread of viral progeny between tissue cells are events fundamental to viral pathogenesis. Glycopro- tein complexes inserted in the virion envelope are criti- cally involved in the cell entry process. Here we review and discuss recent work that has shed light on the in vivo role of the trimeric glycoprotein complex gH/gL/gO of murine cytomegalovirus (mCMV) as a model to propose the role of the corresponding complex of human CMV, for which experimental studies in vivo are not feasible due to the host species specificity of CMVs and evident ethical con- straints. A novel approach combining gO transcomplemen- tation of a genetically gO-deficient virus and a mathemati- cal log-linear regression analysis of the viral multiplication kinetics in host tissues revealed a critical role of mCMV gH/gL/gO only in first target cell entry of virions arriving with the circulation, whereas intra-tissue spread proceeded unaffected also in the absence of gH/gL/gO. These find- ings predict that targeting gO for an antiviral intervention may be of prophylactic value in preventing the seeding of virus to organs, but will likely fail to interfere with an B. Adler and N. A. W. Lemmermann are co-senior authors. This article is part of the Special Issue on Cytomegalovirus. * Niels A. W. Lemmermann lemmermann@uni-mainz.de 1 Institute for Virology, University Medical Center of the Johannes Gutenberg-University Mainz and Research Center for Immunotherapy (FZI), Obere Zahlbacher Strasse 67, Hochhaus am Augustusplatz, 55131 Mainz, Germany 2 Max von Pettenkofer-Institute for Virology, Ludwig- Maximilians-University Munich, Pettenkoferstraße 9A, 80336 Munich, Germany