ReviewMultiple roles of LMP1 in Epstein-Barr virus induced immune escape
Introduction
Epstein–Barr virus (EBV) or human herpesvirus 4 (HHV4) is the first described human tumor virus, discovered in Burkitt lymphoma tumor cells, [1] and has since been associated with classic Hodgkin's Disease, various B-/T-/NK-cell non-Hodgkin lymphomas, immunoblastic B cell lymphomas in immunodeficient individuals [2] and epithelial derived malignancies such as nasopharyngeal carcinoma and EBV+ gastric carcinoma [3]. Despite the strong association with malignancies, EBV elicits remarkably little pathology as it persists unnoticed in most of the world's population. This indicates that the oncogenic properties of EBV are rarely exposed and that potentially dangerous viral gene expression is limited. In almost all EBV+ tumors except Burkitt lymphoma, which is driven by the cMyc translocation, LMP1 is considered the main viral oncogene driving cell growth, but it also promotes metastasis, apoptotic resistance and immune modulation. Indeed, LMP1 is a remarkably pleiotropic viral protein whose biological function relies heavily on the cellular context in which it is expressed. LMP1 (MW 63 kDa) has a N-terminal hydrophilic domain (AA1–23) followed by six hydrophobic clusters forming three membrane spanning domains (TM1-3; AA24–186) and a 200 AA C-terminal domain. LMP1 has clear oncogenic properties in multiple cell backgrounds and in transgenic mice. Interference with LMP1 function eliminates tumorigenic potential. To understand the role of LMP1 in modulating immune functions it is critical to discuss its natural expression, structure, trafficking and subcellular localization, and then consider implications for the pathogenic role of LMP1.
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Role of LMP1 in EBV persistence
EBV infects asymptomatically >90% of the world population and while EBV displays potent transforming activity in vitro, EBV rarely causes malignancies in vivo. Indeed, similar to other closely related herpesviruses, EBV has evolved to co-exist with its hosts [4]. Immune competent hosts elicit a vigorous cytotoxic T cell (CTL) response against EBV infected cells, predominantly aimed at controlling cells expressing the latency III program associated with active cell growth [5]. The current view
LMP1 expression patterns in vivo and in vitro
Unlike in healthy tissues containing EBV infected cells, in most EBV associated tumors, LMP1 protein is relatively highly expressed (Fig. 1A–C). Indeed, in immunoblastic lymphoma or post-transplant proliferative disease (PTLD) [28], Hodgkins disease [29] and in a subset of non-lymphoid poorly or undifferentiated nasopharyngeal carcinomas (NPC) LMP1 is readily detectable [117]. In contrast to the general belief that PTLD is associated with the EBV full growth program (latency III), in situ
Critical distinctions between CD40 and LMP1 function in vivo
LMP1 is arguably best recognized for its capacity to act as a classic oncogene in that it transforms rat fibroblasts [41] and is required for EBV-driven cell transformation [42], [43]. It is now understood that LMP1, besides its CD40-like growth and survival inducing activities, has evolved as a multifunctional molecule with active immune evasion functions exerted by distinct parts of the protein, mediating immune modulation both inside and outside the EBV infected cell. Although LMP1 and CD40
LMP1 structure, subcellular localization and signaling
Deciphering the precise role of LMP1 in EBV associated tumors has proven a formidable task, since LMP1 activates many intracellular signaling pathways. Following synthesis on ribosomes, the short positively charged hydrophilic N-terminus (AA1–24) serves to anchor the LMP1 protein to negatively charged membrane phospholipids essential for and ensuring proper transmembrane folding of the six hydrophobic domains (AA25–186), which are crucial for LMP1 self-assembly into oligomeric molecules and
LMP1 induced immunomodulation from within
It is generally assumed that the growth-promoting and oncogenic role of LMP1 in EBV+ tumors is due to its “normal” biological function in B cell development of healthy carriers gone awry. One possible strategy for EBV to limit its oncogenic potential associated with LMP1 expression is co-expression with highly immunogenic genes. LMP1 at high levels induces autophagy [38] which enhances Ag presentation thus making infected cells more visible to the immune system. However, it is more generally
Secreted LMP1 modulates the cellular microenvironment
Although the biological significance and mechanism of LMP1 secretion by EBV infected cells remains poorly understood, its possible presence in immunomodulatory exosomes [67], [89] suggests a function in immune modulation. Indeed, homogenously purified exosomes secreted by EBV infected lymphoblastoid cells (LCLs) are highly enriched in MHC-II molecules, have a distinct protein and lipid make-up [90], [91] and induce antigen-specific class II restricted T cell responses [92]. These vesicles
In vivo observations
In healthy carriers, dominant CD8+ T-cell responses controlling viral latency are not primarily directed against LMP1 but to the EBNA3 family of proteins, in addition to several (early) lytic gene products [5]. LMP1 is in fact a rather poor immunogen for both humoral and cellular immune responses [99], [100]. This may relate to an active role of LMP1 in preventing proper antigen presentation or the unusual proposed form of degradation of LMP1 [39]. Regardless of the underlying mechanism,
Conclusion
In summary, the life cycle of EBV closely follows normal B cell development. EBV encoded LMP1 is a carefully regulated potential oncogene with multiple functions, basically aiding to achieve long-life for the infected cell. When expressed, LMP1 functions as a constitutively active (ligand-independent) CD40 homologue, but has additional functions incorporated in its multi-domain sequence. Normally its expression is tightly controlled and non-pathogenic. However, when expression is deregulated
Conflict of interest statement
None declared.
Acknowledgements
Work by J.M. and M.P. related to this subject is funded by the Dutch Cancer Foundation (KWF-3775).
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