EL MATRIMONIO MILAGROSO
O VEREMOS LO QUE ES QUE LA MISERIA, O COMÓ SE COMENZO, Y CUANDO ACABARÁ EN EL MUNDO
latency and prevention of reactivation
The complexity of the intimate relationship between HSV-1 and its host is exemplified by the adaptive immune response to the latent virus. This is most obviously manifested as the
high frequency of reactivation of HSV-1 that results from immunosuppression as a result of cancer treatment or following transplantation (Djuric et al., 2009; Schubert et al., 1990). The interaction between the virus and the immune system is dominated by the CD8+ T cell response.
As described in Section 1.1.4, CD8+ T cells infiltrate the ganglia of mice from five days p.i. where they are retained following the establishment of latency, although some have reported that this infiltrate is cleared over time (Gebhardt and Hill, 1988; Khanna et al., 2003; Liu et al., 1996; Shimeld et al., 1995; Van Lint et al., 2005). Within HSV-1 infected C57Bl/6 mice, a large proportion of these cells are specific for the immunodominant epitope gB498, and non-HSV specific CD8+ T cells are selectively lost during latency (Khanna et al., 2003; Sheridan et al., 2009; St. Leger et al., 2011). Despite the presence of CD8+ T cells, very little neuronal loss or obvious pathology is observed (Tscharke and Simmons, 1999; Verjans et al., 2007).
A newly defined memory CD8+ T cell subset, named resident memory T (TRM) cells, are important for controlling HSV-1 latency. Gebhardt and colleagues (2009) found this population of CD8+ TRM cells are resident in the skin and sensory ganglia during latent HSV-1 infection of mice and are in disequilibrium with the circulating lymphocyte pool (Mackay et al., 2012). Mackay and colleagues (2012) showed that inflammation is sufficient to draw these cells into a highly localised area of skin in the absence of antigen recognition, where they become lodged and provide protection against local challenge with virus. Further, CD8+ TRM cells can mount a proliferative response entirely within DRG following challenge by reactivation (Wakim et al., 2008c).
Nearly all CD8+ T cells within latently infected ganglia express the early activation marker, CD69 and CD103, which functions in the survival and retention of these cells (Mackay et al., 2013). Later, the CD8+ T cells have a CD44hi phenotype, suggesting persistent activation. They also have a slow homeostatic turnover (Gebhardt et al., 2009). These cells have the capacity to produce IFN-γ and are gzmB+, indicating recent activation, with some cells show T cell receptor (TLR) polarization towards infected cells (Jiang et al., 2011; Khanna et al., 2003; Van Lint et al., 2005). By using bone marrow chimeras, it has been shown that the production of gzmB is completely dependent upon antigen presentation by parenchymal cells (Van Lint et al., 2005). IFN-α may also play a role in suppressing reactivation (De Regge et al., 2010). Inflammatory cytokines, including IFN-γ and TNF-α, and transcripts for molecules involved in chemoattraction, such asChemokine (C-C motif) ligand 5, have all been detected during latency in the ganglia of HSV-1 infected mice and
humans (Cantin et al., 1995; Chen et al., 2000; Halford et al., 1996a; Halford et al., 1997; Liu et al., 1996; Shimeld et al., 1997; Stock et al., 2011; Theil et al., 2003a).
It is highly likely that these CD8+ T cells are able to suppress reactivation. In ex vivo cultures of latently infected TG, both exogenous HSV-1 specific CD8+ T cells, or alternatively a gB498 specific CD8+ T cell clone, were able to suppress viral replication and cytopathic effect (CPE) in an MHC restricted manner (Khanna et al., 2003; Liu et al., 2000). However, viral genomes and some immediate early and early gene transcripts were still detectable in these cultures (Liu et al., 2000). The suppression of viral activity may be mediated in part by the IFN-γ that is produced by these cultures and augments the CD8+ T cell response (Liu et al., 2001). The addition of IFN-γ to ex vivo cultures of latently infected TG neurons reduced the frequency of reactivation by at least 50% and reduced the amount of CPE (Carr et al., 2009; Decman et al., 2005b; Liu et al., 2000). The addition of IFN-γ to these cultures was also associated with a reduction in the expression of ICP0 and gC (Decman et al., 2005b). Finally, using IFN-γ or IFN-γ receptor knockout mice in which normal levels of latency are established in the TG, reactivation following hyperthermic stress is enhanced, further implicating IFN-γ in the suppression of reactivation (Cantin et al., 1999).
There is some evidence that gzmB is an important mediator of HSV-1 latency. Firstly, gzmB has been shown to mediate cleavage of ICP4 (Knickelbein et al., 2008). Using mouse neuroblastoma lines infected with HSV, treatment with gzmB decreased cleavage of caspase3 and increased neuronal survival rates. This did not hold in cell lines in which LAT was deleted, linking LAT with protection from gzmB -induced apoptosis (Jiang et al., 2011). Secondly, latency is unstable in HSV-1 infected-perforin and gzmB deficient mice (Knickelbein et al., 2008).
The role of CD8 T cells in regulating HSV-1 latency has been partially verified in humans, with the detection of CD8+ T cells in the trigeminal, geniculate and vestibular ganglia of HSV-1 latently-infected humans (Arbusow et al., 2010; Derfuss et al., 2009; Derfuss et al., 2007; Theil et al., 2003a; Verjans et al., 2007). Some of these cells are found in the vicinity of or surrounding latently infected neurons (Derfuss et al., 2009; Theil et al., 2003a; Verjans et al., 2007). These cells have an activated phenotype, upregulating markers like CD69, gzmB and granzyme A (gzmA), and perforin, but lacked expression of the homing molecules C-C chemokine receptor type 7 and CD62L (Derfuss et al., 2007; Verjans et al., 2007). Further, CD8αα+ T cells, which closely resemble the TRM CD8+ T cells identified in mice, have been found in the skin of humans infected with HSV-2. The presence of these CD8αα+ T cells was found to be correlated with increased virus control (Schiffer et al.,
2010; Zhu et al., 2007; Zhu et al., 2013). These cells have direct cytotoxic action against newly infected cells in the skin, and produce antiviral cytokines (Zhu et al., 2007; Zhu et al., 2013). Therefore, this robust CD8+ T cell response at the skin is likely important for preventing virus reactivation and possible recrudescence.
While there is no direct role for CD4+ T cells in the maintenance of latency, they do infiltrate the DRG and are detectable at low levels throughout latency in both mice and humans (Liu et al., 1996; Shimeld et al., 1995; Theil et al., 2003a). CD4+ TRM cells are also found in the skin, but unlike the CD8+ TRM cells their role requires interplay between macrophages, antigen recognition and the production of IFN-γ and other downstream chemokines (Iijima and Iwasaki, 2014). CD4+ TRM cells also show a different pattern of localisation in the dermis as a part of a wider recirculation, while CD8+ TRM cells are found lodged in the epidermis (Gebhardt et al., 2011; Zhu et al., 2007). Ablation of CD4+ T cells revealed a failure to maintain the latent state, as evidenced by increased viral genome load. There was also a lower frequency of IFN-γ and TNF-α producing CD8+ T cells, suggesting that CD4+ T cell help is required to avert functional compromise of CD8+ T cells (Frank et al., 2010).
It is unlikely that antibody responses play a significant role in maintaining latency. There is a measureable antibody response during latency, with serum antibody levels in ocularly infected mice increasing until 30 days p.i. However, they then plateau throughout latency until at least 125 days p.i. (Halford et al., 1996a). Further, recurring mucocutaneous reactivations are associated with rising serum titers in humans (Zweerink and Stanton, 1981).
Finally, the innate immune response does have some role in regulating HSV reactivation. For example, it has been shown that plasmacytoid dendritic cells infiltrate the dermis of recurrent HSV-2 lesions in humans and are able to stimulate T cell proliferation (Donaghy et al., 2009). However, interaction of the innate immune system with HSV during latency and beyond is largely unexplored.