#immunosupression

The Measles virus (MV) is a pleomorphic single-stranded negative-sense RNA virus that belongs to the genus Morbillivirus within the family Paramyxoviridae. The measles virus is the causal agent of Measles, a disease that is responsible for more than 40 million infections globally each year and more than 250,000 deaths, predominantly among children (Moss and Griffin, 2009).

Like many other viruses, the measles virus has evolved an elegant repertoire of strategies to evade host immunity and has become adept at exploiting and manipulating many aspects of its normal functioning to enhance infection and facilitate transmission (Hahm et al. 2004). A number of mechanisms interplay to induce immunosuppression in measles and infection is characterised by lymphopenia, initially impaired immunoglobulin synthesis, inhibition of T cell proliferation, lymphocyte apoptosis, and NK-cell functioning and aberrant Th2- biased cytokine production (Griffin, 2010).

Transmission and entry

Measles is transmitted by aerosol or direct contact with respiratory secretions; viral particles are inhaled and establish infection at the alveolar level in the lung (Lemon et al. 2011). The measles virus is highly lymphotropic, preferentially parasitizing alveolar macrophages and dendritic cells, utilising the CD150 receptor, a member of the immunoglobulin superfamily to which the MV haemagglutinin (MV-H) protein binds with high affinity (Lemon et al. 2011). Attachment and subsequent infection is enhanced by the binding of the glycoprotein designated ‘fusion’ (MV-F) and MV-H to the C-type lectin receptor DC-SIGN, which is expressed on immature dendritic cells (Lemon et al. 2011).

Evading innate immunity

Upon entering the cytosol, the measles virus eludes recognition by RIG-1-like receptors (Nunes-Alves, 2014). These receptors constitute the first line defence against viral pathogens and when viral RNA is detected, RIG-1 and MDA5 complex with the mitochondrial antiviral signalling protein (MAVS) by associating with their caspase activation and recruitment domains (Nunes-Alves, 2014). This sequence of events culminates in the production of antiviral type 1 interferons and is negatively modulated by phosphorylation of RIG-1 and MDA5 (Davis et al. 2014). Phosphorylation prevents MAVS, RIG-1 and MDA5 from forming a complex. Conversely, PP1 phosphatase dephosphorylates the same residues, allowing these proteins to complex and thus triggering type 1 interferon production (Davis et al. 2014; Mesman et al. 2014). MV replication is known to be sensitive to type 1 interferon and has evolved numerous cell-type specific strategies to suppress their expression and evade innate immunity (Davis et al. 2014).

The non-structural V protein (MV-V) blocks the phosphatase responsible for the regulation of type 1 interferon production (PPI) and is a well-studied virulence factor (Mesman et al. 2014). When MV-V inhibits PP1, RIG-1 and MDA5 remain phosphorylated and inactive and cannot complex with MAVS; as a result the measles virus remains undetected (Mesman et al. 2014). It has been also shown that MV prevents dephosphorylation of RIG-1 and MDA5 by activating RAF1, a regulatory kinase (Davis et al. 2014). RAF-1 phosphorylates a type-1 phosphatase inhibitor, designated inhibitor-1. Inhibitor-1 in turn reduces the function of PP1 and subsequently the production of antiviral IFN-β (Davis et al. 2014; Mesman et al.2014; Nunes-Alves, 2014).

Replication within immune cellsIn the absence of antiviral type 1 interferons, the measles virus can replicate within activated CD150+ immune cells (Fontana et al. 2008; Fugier-Vivier, 1997). Dendritic cells infected with MV do not attain the mature phenotype and exhibit a marked decrease in expression of major histocompatibility complex (MHC) class II and associated co-stimulatory cytokines, such as IL-12 (Servet-Delprat et al. 2000). This down-regulation of MHC II impedes the ability of infected cells to prime specific cytotoxic CD8+ T cell responses (Yilla et al. 2003). Infected DCs also maintain expression the chemokine receptor CCR5 and antigen uptake (Servet-Delprat et al. 2000). As a result, infected DCs remain functional and are continually recruited from the luminal alveolar surface to regional lymph nodes and bronchus associated lymphoid tissue (Moss et al. 2004).

Ordinarily, maturation signals are given in the secondary lymphoid tissues by naïve T cells expressing the ligand for CD40 receptors expressed on the dendritic cell surface (Servet-Delprat et al. 2000). This ligation is fundamental to the terminal differentiation of dendritic cells and for IL-12 production. In the absence of IL-12, natural killer cells are not recruited and cannot eliminate parasitized cells in the early stages of infection (Griffin, 2010). When infected DCs interact with T cells, CD40-CD40L ligation triggers a burst of vigorous viral replication (Servet-Delprat et al. 2000). This has been demonstrated in-vitro and the absence of this signal prevents synthesis of the MV nucleoprotein (NP) (Laine et al.2005).

DC escape and dissemination: crippling adaptive immunityDCs that are parasitized by MV are observed as being markedly more sensitive to the pro-apoptotic transmembrane protein Fas ligand (FasL) (Servet-Delprat et al. 2000). FasL is expressed on activated T cells, and expression of the receptor (Fas) is upregulated on MV infected DCs (Servet-Delprat et al. 2000). When Fas interacts with FasL, infected DCs die by apoptosis, releasing vast quantities of progeny virion within secondary lymphoid organs and the circulation (Servet-Delprat et al. 2000). 

It is also known that infected DCs are able to trigger apoptosis of infected and uninfected T cells (Vidalain et al. 2000). Studies have shown that this process is Fas-independent, occurring instead through the tumour necrosis factor (TNF)-related inducing apoptosis ligand (TRAIL) pathway (Vidalain et al. 2000). MV-infected dendritic cells highly express TRAIL and are cytotoxic by this mechanism (Vidalain et al. 2000). Strikingly, implementation of this death ligand to eliminate T cells has also been demonstrated in HIV-1 infection and is likely fundamental to the suppressive and evasive properties of the measles virus (Katsikis et al. 1997; Vidalain et al. 2000). Adaptive cellular immunity is critical in the elimination of MV infected cells and clearance of the virus from the body (Slifka et al 2003). In the absence of adaptive cellular responses due pronounced lymphopenia, MV replicates in huge quantities and disseminates throughout the body to multiple organs (de Vries et al. 2012). 

DC-SIGN+ DCs are able to directly infect T and B-lymphocytes in secondary lymphoid organs as well as in peripheral blood, although the former is the most common (de Vries et al. 2012). As infected DCs undergo Fas-mediated apoptosis, and lymphocytes are induced to die by the TRAIL pathway, copious amounts of progeny virion and infectious cellular debris are released into the lymphatic and circulatory systems resulting in viraemia and further infection of CD150+ DC-SIGN+ immune cells (Servet-Delprat et al. 2000; de Vries et al. 2012). It has also been demonstrated that MV-NP blocks immunoglobulin synthesis and secretion by activated B cells by binding to the Fc-Receptor II, dampening adaptive cellular immune responses further (Ravanel et al. 1997; Laine et al. 2003).

Targeting respiratory epithelia: exit, shedding and transmission

It was initially believed that respiratory epithelia constituted primary replicative sites for MV, however this has since been discredited (Lemon et al. 2011). Although establishing infection in these cells is critical to pathogenesis and transmission, in contrast to other respiratory viruses such as respiratory syncytial virus (RSV), the respiratory epithelium is the ultimate goal (de Vries et al. 2012). Hitherto, each aspect of infection has contributed to crippling host immunity to a sufficient extent so as to permit unregulated viral replication and dissemination (de Witte et al. 2006; de Vries et al. 2012). Having successfully accomplished this, the virus disseminates unrestricted, finally targeting the basolateral surface of the respiratory epithelium (Noyce, 2011). 

Until recently, it was not known precisely how MV reached the luminal surface (Noyce, 2011; Muhlebach et al. 2011). However in 2011, two separate groups found that this occurs by transmission of the virus from infected lymphocytes and dendritic cells in the respiratory submucosa by utilisation of a novel attachment receptor in the immunoglobulin superfamily, now known as nectin-4 (Noyce, 2011; Muhlebach et al. 2011). At this stage, the virus is detectable at high quantities in numerous tissues including the buccal mucosa, the trachea, the nose, conjunctiva and the skin (fig.1) (Lemon et al. 2011; de Vries et al. 2012). Having finally reached these sites, virion extrude by budding from the luminal surface, facilitating copious shedding into respiratory secretions de Vries et al. 2012). 

Considerable damage to the respiratory epithelium is a hallmark of measles infection, heralding the onset of symptoms that include pyrexia, coughing, rhinitis, conjunctivitis and sneezing as well as the characteristic ‘Koplik’s spots’ within the oral cavity (Schneider-Schaulies and Meulen, 2009). Around 3-7 days after the onset of these prodromal symptoms, the pathognomonic generalised maculopapular rash is observed, covering the whole body (Schneider-Schaulies and Meulen, 2009). 

Figure 1: The fundamental steps in the pathogenesis of measles are illustrated using rMV expressing enhanced green fluorescent protein (EGFP) in a cynomolgus macaque model (Macaca fascicularis). MV viraemia is demonstrated using fluorescence-activated cellular sorting data plots, showing EGFP+ CD150+ cells in the peripheral circulation. Image sourced from de Vries et al. 2012.

The appearance of the rash marks the point at which the immune system finally begins to respond to the infection, mounting an adaptive immune response and subsequently allowing for the elimination of infected cells and reservoirs by natural killer cells and CD8+ T cells (Ludlow et al. 2015). Transmission and infectivity steeply decline from this point, with antibodies specific to the measles virus increasing exponentially, correlated with the decrease in viral load (Ludlow et al. 2015; Lemon et al. 2011).

Evasion is critical to virus survival and success

MV has a basic reproductive number (R0) of 15-17, among the highest of all human pathogens (de Vries et al. 2012). Unlike many other respiratory viruses, recovery from measles confers lifelong immunity (Griffin et al. 2012). This is a hindrance for the measles virus, which must subsequently implement a number of strategies to maximise potential for transmission to new susceptible hosts (de Vries et al. 2012). 

As discussed, this is achieved by inducing pronounced, transient immunosuppression, disabling host immunity and buying the virus enough time to replicate to sufficient numbers, promoting infection and thus transmission (Lemon et al. 2011). Peak levels of viral replication occur prior to the onset of symptoms due to these strategies as MV paralyses the antiviral action of both innate and adaptive immunity (Ludlow et al. 2015; de Vries et al. 2012).

MV immune evasion is essential to the survival of the virus, which has circulated in human populations since antiquity (Schneider-Schaulies and Meulen, 2009). It is evident that not only is there still much to learn from measles, but it also seems pertinent to re-examine what we think we know about this unique Morbillivirus. Learning from practical experience (de Swart et al. 2012), the virus is seems very much a step ahead; evading not only human immunity, but also our comprehension and our control efforts.

REFERENCES

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