Defective T cell control of EBV in MS

Pender MP, Csurhes PA, Burrows JM, Burrows SR. Defective T-cell control of Epstein-Barr virus infection in multiple sclerosis.
Clin Transl Immunology. 2017;6(1):e126.

Mounting evidence indicates that infection with Epstein-Barr virus (EBV) has a major role in the pathogenesis of multiple sclerosis (MS). Defective elimination of EBV-infected B cells by CD8+ T cells might cause MS by allowing EBV-infected autoreactive B cells to accumulate in the brain. Here we undertake a comprehensive analysis of the T-cell response to EBV in MS, using flow cytometry and intracellular IFN-γ staining to measure T-cell responses to EBV-infected autologous lymphoblastoid cell lines and pools of human leukocyte antigen (HLA)-class-I-restricted peptides from EBV lytic or latent proteins and cytomegalovirus (CMV), in 95 patients and 56 EBV-seropositive healthy subjects. In 20 HLA-A2+ healthy subjects and 20 HLA-A2+ patients we also analysed CD8+ T cells specific for individual peptides, measured by binding to HLA-peptide complexes and production of IFN-γ, TNF-α and IL-2. We found a decreased CD8+ T-cell response to EBV lytic, but not CMV lytic, antigens at the onset of MS and at all subsequent disease stages. CD8+ T cells directed against EBV latent antigens were increased but had reduced cytokine polyfunctionality indicating T-cell exhaustion. During attacks the EBV-specific CD4+ and CD8+ T-cell populations expanded, with increased functionality of latent-specific CD8+ T cells. With increasing disease duration, EBV-specific CD4+ and CD8+ T cells progressively declined, consistent with T-cell exhaustion. The anti-EBNA1 IgG titre correlated inversely with the EBV-specific CD8+ T-cell frequency. We postulate that defective CD8+ T-cell control of EBV reactivation leads to an expanded population of latently infected cells, including autoreactive B cells.

Mounting evidence indicates that infection with the Epstein–Barr virus (EBV) is a prerequisite for the development of multiple sclerosis (MS)

EBV, a ubiquitous double-stranded DNA γ-herpesvirus, is unique among human viruses in having the capability of infecting, activating, clonally expanding and persisting latently in B lymphocytes for the lifetime of the infected person. 

To accomplish this, EBV utilizes the normal pathways of B-cell differentiation. 

During primary infection EBV is transmitted through saliva to the tonsil where it infects naive  (mature) B cells and drives them out of the resting state into activated B blasts, which then progress through a germinal centre reaction to become circulating latently infected memory B cells. 

When latently infected memory B cells returning to the tonsil differentiate into plasma cells, the infection is reactivated by initiation of the lytic phase culminating in the generation of virions, which infect tonsil epithelial cells where the virus reproduces at a high rate and is released into saliva continuously for transmission to new hosts.

Newly formed virus also infects additional naive B cells in the same host, thereby completing the cycle necessary for its persistence as a lifelong infection.

To pass through the various stages of its life cycle, EBV makes use of a series of differing transcription programmes. 

After entering mature B cells, it first employs the latency III or ‘growth' programme expressing all viral latent proteins, namely the Epstein–Barr nuclear antigens (EBNA) 1, 2, 3A, 3B, 3C and LP, and the latent membrane proteins (LMP) 1, 2A and 2B, to activate the blast phase. 

After entering a germinal centre, the infected blast switches off expression of the EBNA proteins 2, 3A, 3B, 3C and LP and continues to express EBNA1, LMP1 and LMP2 (latency II or ‘default' programme) while it progresses through the germinal centre phase to differentiate into a memory B cell. 

Because latently infected memory B cells express no viral proteins they are unable to be detected by EBV-specific immune responses, except during cell division, when they express only EBNA1 (latency I), which is needed for duplication of the EBV genome and transmission to daughter cells. When latently infected memory B cells differentiate into plasma cells the virus is reactivated through the lytic transcription programme to generate infectious virions.

In healthy individuals, EBV infection is kept under rigorous control by EBV-specific immune responses, especially by cytotoxic CD8+ T cells, which kill proliferating and lytically infected B cells by targeting the various EBV-encoded latent and lytic proteins respectively.

It is suggested that defective elimination of EBV-infected B cells by cytotoxic CD8+ T cells might predispose to the development of MS by enabling the accumulation of EBV-infected autoreactive B cells in the central nervous system (CNS).

So what did this study find

These results are consistent with progressive T-cell exhaustion of EBV-specific CD4+ T cells and CD8+ T cells during the course of MS although they could also be due to other factors, for example an age-related decline in the tendency of EBV to reactivate.

T cell exhaustion is a state of T cell dysfunction that arises during many chronic infections and cancer. It is defined by poor effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T cells.

The CD8+ T-cell response to EBV lytic phase antigens is reduced at the onset of MS and throughout its course

CD8+ T cells recognizing EBV latent phase antigens in MS show T-cell exhaustion##

Proposed model of defective CD8+ T-cell control of EBV infection in MS. In healthy EBV carriers, (a) there is a dynamic equilibrium between the EBV-infected cell populations and the T-cell response. EBV-specific CD8+ T cells (T cell) exert a key role in controlling EBV infection by killing infected cells in the B blast, germinal centre (GC) B cell, plasma cell and tonsil epithelial cell, but not memory B cell, populations. The large arrows indicate the cycle of EBV infection: virion→B blast→GC B cell→memory B cell→plasma cell→virion→epithelial cell→virion→B blast. Smaller arrows indicate stimulation of T cells by EBV antigens from the infected populations. The relative sizes of the different EBV-infected cell populations are indicated by the circle sizes, based on the study by Hawkins et al.6 The relative sizes of the EBV-specific CD8+ T-cell populations are also indicated by the circle sizes; however, it is important to note that the EBV-specific CD8+ T-cell population is several orders of magnitude larger than the EBV-infected cell population, a distinction not depicted here. For simplicity, the EBV-specific CD4+ T-cell population and anti-EBV antibody response are not shown. At all stages of MS (b–d) the EBV-lytic-specific CD8+ T-cell population is decreased, allowing increased production of virions which infect naive B cells driving them into the blast phase. The resultant expansion of the infected blast population stimulates EBV-latent-specific CD8+ T cells which proliferate and restrict this expansion, but not without increased flow out of infected blast cells into a consequently enlarged EBV-infected GC cell population, which in turn is partially controlled by the augmented EBV-latent-specific CD8+ T-cell population. In the same way the EBV-infected memory B cell pool also grows, as does the population of plasma cells reactivating EBV infection. During clinical attacks of MS (c) there is increased differentiation of EBV-infected memory B cells into lytically infected plasma cells as a result of the various microbial infections that trigger attacks of MS. This EBV reactivation is inadequately regulated by the already deficient EBV-lytic-specific CD8+ T-cell response, resulting in increased virion production and increased infection of the blast pool, this in turn stimulating proliferation of the EBV-latent-specific CD8+ T-cell population which restricts further growth of the infected blast population. In progressive MS (d) the EBV-latent-specific CD8+ T-cell response becomes exhausted (indicated by fading), resulting in unchecked expansion of the infected GC population and the development of EBV-infected lymphoid tissue in the CNS.

we have shown that patients with MS have defective T-cell control of EBV infection which might underlie the accumulation of EBV-infected B cells in the CNS and subsequent development of the disease. We have proposed a model where decreased CD8+ T-cell control of EBV reactivation permits increased production of virus and consequent expansion of the latently infected B-cell population. 

To test this model they suggest that further studies are necessary to determine: 
(i) the cause of CD8+ EM/EMRA T-cell deficiency in MS, whether it genetically determined, and related to decreased type I IFN production; 
(ii) whether CD8+ T-cell deficiency precedes the onset of MS and is present in healthy first-degree relatives of people with MS,
(iii) whether sunlight deprivation and vitamin D deficiency aggravate the CD8+ T-cell deficiency
(iv) how and why the EBV-specific CD4+ T-cell response declines during the course of MS; 
(v) whether oral shedding of EBV is increased during clinical attacks; 
(vi) whether the frequency of EBV-infected memory B cells in the blood is increased in MS, as in rheumatoid arthritis and systemic lupus erythematosus;
(vii) whether EBV-infected B cells and plasma cells in the CNS in MS are autoreactive, and (viii) finally and most importantly, whether therapies aimed at controlling EBV infection, such as EBV-specific T-cell therapy,prevent and cure MS.

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