Role of latent membrane protein 2 isoforms in Epstein–Barr virus latency Markus P. Rechsteiner * , Michele Bernasconi * , Christoph Berger and David Nadal Experimental Infectious Diseases and Cancer Research, Division of Infectious Diseases and Hospital Epidemiology, University Children’s Hospital of Zurich, CH-8032 Zurich, Switzerland The oncogenic Epstein–Barr virus (EBV) infects the majority of the human population without doing harm and establishes a latent infection in the memory B-cell compartment. To accomplish this, EBV hijacks B-cell differentiation pathways and uses its own viral genes to interfere with B-cell signalling to achieve life-long persistence. EBV latent membrane protein 2A (LMP2A) provides a surrogate B-cell receptor signal essential for cell survival and is believed to have a crucial role in the maintenance of latency by blocking B-cell activation which would otherwise lead to lytic EBV infection. These two functions demand tight control of LMP2A activity and expression levels. Based on recent insights in the function of LMP2B, an isoform of LMP2A, we propose a model for how LMP2B modulates the activity of LMP2A contributing to maintenance of EBV latency. Latent and lytic EBV infection Epstein–Barr virus (EBV) is a ubiquitous human gamma herpes virus that is associated with many malignancies, including Burkitt’s lymphoma (BL) and Hodgkin’s lym- phoma (HL) [1,2]. After primary infection, EBV enters the memory B-cell pool and establishes a life-long latent infection [3,4]. Once in its latent form, EBV persists as an episome in the nucleus and is maintained by segregation to the daughter cells after mitotic cell division [5]. EBV exhibits the unique ability to transform B cells in vitro into continuously proliferating lymphoblastoid cell lines (LCLs), indicating the oncogenic potential of the virus. LCLs express a restricted pattern of EBV genes (i.e. the latency genes including those essential for cell transform- ation). This gene expression pattern and variations thereof (EBV latency patterns) are found in distinct conditions (see Box 1 and Ref. [6]). Activation of a lytic infection leads to production of infectious particles and host cell death [7]. There is a large body of literature on the functions of EBV latency genes [3]. Here, we focus on the so far less extensively characterized EBV gene latent membrane protein 2 (LMP2) expression products, giving rise to the LMP2 isoforms LMP2A and LMP2B. Based on the known expression of LMP2 mRNA in B-cells in vivo and on results from recent studies on LMP2A and LMP2B in vitro, we propose a model for the interaction and regulation of LMP2A and LMP2B and outline their possible functions in the regulation of the balance between latent and lytic EBV infections and, thus, maintenance of EBV latency. LMP2 and its isoforms LMP2A and LMP2B LMP2 is transcribed into two mRNAs from two spatially distinct promoter elements [3]. Both transcripts are multi- ply spliced, and exons 2–9 are shared by both mRNAs. However, exon 1 of LMP2A encodes a hydrophilic, N- terminal cytoplasmic domain, whereas exon 1 of LMP2B is non-coding. The molecular characteristics of LMP2A were recently reviewed by Brinkmann and Schulz [8]. The main recognized function of LMP2A is the modulation of intracellular signalling pathways (e.g. Syk, Lyn, Btk, BLNK, AKT and PI3K) essential for B-cell survival and for switching to EBV lytic infection, providing either surrogate B-cell receptor (BCR) signalling [9] or by blocking B-cell activation after BCR cross-linking [10]. These functions are mediated by the cytoplasmic N-terminal domain of LMP2A. It remains unclear if LMP2A contributes to B- cell transformation in vivo [11,12]. LMP2B lacks the N-terminal cytoplasmic domain. Elu- cidation of its functions that might differ from those of LMP2A has been hampered by technical hurdles. Because of the 12 highly hydrophobic transmembrane domains, antibodies targeting LMP2 were tailored against the cyto- solic part of LMP2A. Additionally, the eight coding exons are shared between LMP2A and LMP2B, which do not seem to be further post-translationally modified in an isoform-specific manner. Thus, antibodies detecting LMP2B only cannot be produced. Accordingly, the studies on the localization of the LMP2 isoforms are done using either tagged LMP2B protein in vitro or subtractional antibody staining of LMP2 against LMP2A ex vivo [13]. Furthermore, there is virtually no information about the protein expression levels of LMP2B present in vivo. Sim- ilarly, given the lack of appropriate molecular tools there are no reports on the detection of LMP2B mRNA in vivo. Thus, much of the information on the function of LMP2B is derived from in vitro studies. These studies indicate that LMP2B negatively regulates LMP2A [14–16]. EBV gene expression during B-cell differentiation B-cell development takes place in the bone marrow and the periphery. B-cells within the bone marrow can be subdi- vided into pro- and pre-B-cells. They undergo rearrange- ments of the BCR, which is composed of heavy and light chains, and further subdivided into constant and variable regions. After functional rearrangement and expression of BCR on the cell surface, the B cell is termed as mature and naı¨ve and is released into the blood stream. In the so-called germinal centre (GC) reaction in secondary lymphoid Opinion Corresponding author: Nadal, D. (david.nadal@kispi.uzh.ch) * These authors contributed equally. 520 0966-842X/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tim.2008.08.007 Available online 3 October 2008