Primary EBV Infection Induces an Acute Wave of Activated Antigen-Specific Cytotoxic CD4+ T Cells.

CD4+ T cells are essential for immune protection against viruses, yet their multiple roles remain ill-defined at the single-cell level in humans. Using HLA class II tetramers, we studied the functional properties and clonotypic architecture of EBV-specific CD4+ T cells in patients with infectious mononucleosis, a symptomatic manifestation of primary EBV infection, and in long-term healthy carriers of EBV. We found that primary infection elicited oligoclonal expansions of TH1-like EBV-specific CD4+ T cells armed with cytotoxic proteins that responded immediately ex vivo to challenge with EBV-infected B cells. Importantly, these acutely generated cytotoxic CD4+ T cells were highly activated and transcriptionally distinct from classically described cytotoxic CD4+ memory T cells that accumulate during other persistent viral infections, including CMV and HIV. In contrast, EBV-specific memory CD4+ T cells displayed increased cytokine polyfunctionality but lacked cytotoxic activity. These findings suggested an important effector role for acutely generated cytotoxic CD4+ T cells that could potentially be harnessed to improve the efficacy of vaccines against EBV.

Asymptomatic Primary Infection with Epstein-Barr Virus: Observations on Young Adult Cases.

Epstein-Barr virus (EBV) is typically acquired asymptomatically in childhood. In contrast, infection later in life often leads to infectious mononucleosis (IM), a febrile illness characterized by anti-EBV IgM antibody positivity, high loads of circulating latently infected B cells, and a marked lymphocytosis caused by hyperexpansion of EBV-specific CD8+ T cells plus a milder expansion of CD56dim NKG2A+ KIR- natural killer (NK) cells. How the two situations compare is unclear due to the paucity of studies on clinically silent infection. Here we describe five prospectively studied patients with asymptomatic infections identified in a seroepidemiologic survey of university entrants. In each case, the key blood sample had high cell-associated viral loads without a marked CD8 lymphocytosis or NK cell disturbance like those seen in patients during the acute phase of IM. Two of the cases with the highest viral loads showed a coincident expansion of activated EBV-specific CD8+ T cells, but overall CD8+ T cell numbers were either unaffected or only mildly increased. Two cases with slightly lower loads, in whom serology suggests the infection may have been caught earlier in the course of infection, also showed no T or NK cell expansion at the time. Interestingly, in another case with a higher viral load, in which T and NK cell responses were undetectable in the primary blood sample in which infection was detected, EBV-specific T cell responses did not appear until several months later, by which time the viral loads in the blood had already fallen. Thus, some patients with asymptomatic primary infections have very high circulating viral loads similar to those in patients during the acute phase of IM and a cell-mediated immune response that is qualitatively similar to that in IM patients but of a lower magnitude. However, other patients may have quite different immune responses that ultimately could reveal novel mechanisms of host control.IMPORTANCE Epstein-Barr virus (EBV) is transmitted orally, replicates in the throat, and then invades the B lymphocyte pool through a growth-transforming latent infection. While primary infection in childhood is usually asymptomatic, delayed infection is associated with infectious mononucleosis (IM), a febrile illness in which patients have high circulating viral loads and an exaggerated virus-induced immune response involving both CD8+ T cells and natural killer (NK) cells. Here we show that in five cases of asymptomatic infection, viral loads in the blood were as high as those in patients during the acute phase of IM, whereas the cell-mediated responses, even when they resembled those in patients during the acute phase of IM in timing and quality, were never as exaggerated. We infer that IM symptoms arise as a consequence not of the virus infection per se but of the hyperactivated immune response. Interestingly, there were idiosyncratic differences among asymptomatic cases in the relationship between the viral load and the response kinetics, emphasizing how much there is still to learn about primary EBV infection.

Early T Cell Recognition of B Cells following Epstein-Barr Virus Infection: Identifying Potential Targets for Prophylactic Vaccination.

Epstein-Barr virus, a B-lymphotropic herpesvirus, is the cause of infectious mononucleosis, has strong aetiologic links with several malignancies and has been implicated in certain autoimmune diseases. Efforts to develop a prophylactic vaccine to prevent or reduce EBV-associated disease have, to date, focused on the induction of neutralising antibody responses. However, such vaccines might be further improved by inducing T cell responses capable of recognising and killing recently-infected B cells. In that context, EBNA2, EBNA-LP and BHRF1 are the first viral antigens expressed during the initial stage of B cell growth transformation, yet have been poorly characterised as CD8+ T cell targets. Here we describe CD8+ T cell responses against each of these three "first wave" proteins, identifying target epitopes and HLA restricting alleles. While EBNA-LP and BHRF1 each contained one strong CD8 epitope, epitopes within EBNA2 induced immunodominant responses through several less common HLA class I alleles (e.g. B*3801 and B*5501), as well as subdominant responses through common class I alleles (e.g. B7 and C*0304). Importantly, such EBNA2-specific CD8+ T cells recognised B cells within the first day post-infection, prior to CD8+ T cells against well-characterised latent target antigens such as EBNA3B or LMP2, and effectively inhibited outgrowth of EBV-transformed B cell lines. We infer that "first wave" antigens of the growth-transforming infection, especially EBNA2, constitute potential CD8+ T cell immunogens for inclusion in prophylactic EBV vaccine design.

Early virological and immunological events in asymptomatic Epstein-Barr virus infection in African children.

Epstein-Barr virus (EBV) infection often occurs in early childhood and is asymptomatic. However, if delayed until adolescence, primary infection may manifest as acute infectious mononucleosis (AIM), a febrile illness characterised by global CD8+ T-cell lymphocytosis, much of it reflecting a huge expansion of activated EBV-specific CD8+ T-cells. While the events of AIM have been intensely studied, little is known about how these relate to asymptomatic primary infection. Here Gambian children (14-18 months old, an age at which many acquire the virus) were followed for the ensuing six months, monitoring circulating EBV loads, antibody status against virus capsid antigen (VCA) and both total and virus-specific CD8+ T-cell numbers. Many children were IgG anti-VCA-positive and, though no longer IgM-positive, still retained high virus loads comparable to AIM patients and had detectable EBV-specific T-cells, some still expressing activation markers. Virus loads and the frequency/activation status of specific T-cells decreased over time, consistent with resolution of a relatively recent primary infection. Six children with similarly high EBV loads were IgM anti-VCA-positive, indicating very recent infection. In three of these donors with HLA types allowing MHC-tetramer analysis, highly activated EBV-specific T-cells were detectable in the blood with one individual epitope response reaching 15% of all CD8+ T-cells. That response was culled and the cells lost activation markers over time, just as seen in AIM. However, unlike AIM, these events occurred without marked expansion of total CD8+ numbers. Thus asymptomatic EBV infection in children elicits a virus-specific CD8+ T-cell response that can control the infection without over-expansion; conversely, in AIM it appears the CD8 over-expansion, rather than virus load per se, is the cause of disease symptoms.

CD8+ T cell responses to lytic EBV infection: late antigen specificities as subdominant components of the total response.

EBV elicits primary CD8(+) T cell responses that, by T cell cloning from infectious mononucleosis (IM) patients, appear skewed toward immediate early (IE) and some early (E) lytic cycle proteins, with late (L) proteins rarely targeted. However, L Ag-specific responses have been detected regularly in polyclonal T cell cultures from long-term virus carriers. To resolve this apparent difference between responses to primary and persistent infection, 13 long-term carriers were screened in ex vivo IFN-γ ELISPOT assays using peptides spanning the two IE, six representative E, and seven representative L proteins. This revealed memory CD8 responses to 44 new lytic cycle epitopes that straddle all three protein classes but, in terms of both frequency and size, maintain the IE > E > L hierarchy of immunodominance. Having identified the HLA restriction of 10 (including 7 L) new epitopes using memory CD8(+) T cell clones, we looked in HLA-matched IM patients and found such reactivities but typically at low levels, explaining why they had gone undetected in the original IM clonal screens. Wherever tested, all CD8(+) T cell clones against these novel lytic cycle epitopes recognized lytically infected cells naturally expressing their target Ag. Surprisingly, however, clones against the most frequently recognized L Ag, the BNRF1 tegument protein, also recognized latently infected, growth-transformed cells. We infer that BNRF1 is also a latent Ag that could be targeted in T cell therapy of EBV-driven B-lymphoproliferative disease.

Cytotoxic CD4+ T cell responses to EBV contrast with CD8 responses in breadth of lytic cycle antigen choice and in lytic cycle recognition.

EBV, a B lymphotropic herpesvirus, encodes two immediate early (IE)-, >30 early (E)-, and >30 late (L)-phase proteins during its replication (lytic) cycle. Despite this, lytic Ag-induced CD8 responses are strongly skewed toward IE and a few E proteins only, all expressed before HLA I presentation is blocked in lytically infected cells. For comparison, we examined CD4(+) T cell responses to eight IE, E, or L proteins, screening 14 virus-immune donors to overlapping peptide pools in IFN-γ ELISPOT assays, and established CD4(+) T cell clones against 12 defined epitopes for target-recognition assays. We found that the lytic Ag-specific CD4(+) T cell response differs radically from its CD8 counterpart in that it is widely distributed across IE, E, and L Ag targets, often with multiple reactivities detectable per donor and with IE, E, or L epitope responses being numerically dominant, and that all CD4(+) T cell clones, whether IE, E, or L epitope-specific, show strong recognition of EBV-transformed B cell lines, despite the lines containing only a small fraction of lytically infected cells. Efficient recognition occurs because lytic Ags are released into the culture and are acquired and processed by neighboring latently infected cells. These findings suggested that lytic Ag-specific CD4 responses are driven by a different route of Ag display than drives CD8 responses and that such CD4 effectors could be therapeutically useful against EBV-driven lymphoproliferative disease lesions, which contain similarly small fractions of EBV-transformed cells entering the lytic cycle.

Impaired Epstein-Barr virus-specific CD8+ T-cell function in X-linked lymphoproliferative disease is restricted to SLAM family-positive B-cell targets.

X-linked lymphoproliferative disease (XLP) is a condition associated with mutations in the signaling lymphocytic activation molecule (SLAM)-associated protein (SAP; SH2D1A). SAP functions as an adaptor, binding to and recruiting signaling molecules to SLAM family receptors expressed on T and natural killer cells. XLP is associated with extreme sensitivity to primary Epstein-Barr virus (EBV) infection, often leading to a lethal infectious mononucleosis. To investigate EBV-specific immunity in XLP patients, we studied 5 individuals who had survived EBV infection and found CD8(+) T-cell responses numerically comparable with healthy donors. However, further investigation of in vitro-derived CD8(+) T-cell clones established from 2 of these donors showed they efficiently recognized SLAM ligand-negative target cells expressing EBV antigens, but showed impaired recognition of EBV-transformed, SLAM ligand-positive, lymphoblastoid cell lines (LCLs). Importantly, LCL recognition was restored when interactions between the SLAM receptors CD244 and natural killer-, T-, and B-cell antigen (NTBA) and their ligands on LCLs were blocked. We propose that XLP patients' particular sensitivity to EBV, and not to other viruses, reflects at least in part EBV's strict tropism for B lymphocytes and the often inability of the CD8(+) T-cell response to contain the primary infection of SLAM ligand-expressing target cells.

CD8+ immunodominance among Epstein-Barr virus lytic cycle antigens directly reflects the efficiency of antigen presentation in lytically infected cells.

Antigen immunodominance is an unexplained feature of CD8+ T cell responses to herpesviruses, which are agents whose lytic replication involves the sequential expression of immediate early (IE), early (E), and late (L) proteins. Here, we analyze the primary CD8 response to Epstein-Barr virus (EBV) infection for reactivity to 2 IE proteins, 11 representative E proteins, and 10 representative L proteins, across a range of HLA backgrounds. Responses were consistently skewed toward epitopes in IE and a subset of E proteins, with only occasional responses to novel epitopes in L proteins. CD8+ T cell clones to representative IE, E, and L epitopes were assayed against EBV-transformed lymphoblastoid cell lines (LCLs) containing lytically infected cells. This showed direct recognition of lytically infected cells by all three sets of effectors but at markedly different levels, in the order IE > E >> L, indicating that the efficiency of epitope presentation falls dramatically with progress of the lytic cycle. Thus, EBV lytic cycle antigens display a hierarchy of immunodominance that directly reflects the efficiency of their presentation in lytically infected cells; the CD8+ T cell response thereby focuses on targets whose recognition leads to maximal biologic effect.

CD4+ T-cell clones recognizing human lymphoma-associated antigens: generation by in vitro stimulation with autologous Epstein-Barr virus-transformed B cells.

Epstein-Barr virus (EBV)-specific T-cell preparations, generated by stimulating immune donor lymphocytes with the autologous virus-transformed B-lymphoblastoid cell line (LCL) in vitro, can be used to target EBV-positive malignancies. Although these preparations are enriched for EBV antigen-specific CD8(+) T cells, most also contain a CD4(+) T-cell population whose specificity is unknown. Here, we show that, although CD4(+) T-cell clones derived from such cultures recognize HLA class II-matched LCLs but not mitogen-activated B lymphoblasts, many (1) do not map to any known EBV antigen, (2) can be raised from EBV-naive as well as EBV-immune persons, and (3) can recognize a broad range of human B lymphoma-derived cell lines irrespective of EBV genome status, providing those lines to express the relevant HLA class II-restricting allele. Importantly, such CD4(+) clones not only produce IFNgamma but are also cytotoxic and can control the outgrowth of HLA-matched lymphoma cells in cocultivation assays. We infer that such CD4(+) T cells recognize cellular antigens that are preferentially up-regulated in EBV-transformed but not mitogen-activated B lymphoblasts and that are also expressed in a range of B-cell malignancies. Such antigens are therefore of potential value as targets for CD4(+) T cell-based immunotherapy.

Cyclical expression of EBV latent membrane protein 1 in EBV-transformed B cells underpins heterogeneity of epitope presentation and CD8+ T cell recognition.

CD8(+) T cells specific for EBV latent cycle epitopes can be reactivated in vitro by stimulating with the autologous EBV-transformed B lymphoblastoid cell line (LCL). The resultant CD8(+) clones kill epitope peptide-loaded targets, but frequently do not kill or show only low levels of lysis of the unmanipulated LCL in 5-h cytotoxicity assays. However, they reproducibly show clear LCL recognition in cytokine (IFN-gamma) release assays and inhibit LCL outgrowth in long-term coculture assays. We show that this growth inhibition is not mediated by cytokines, but by slow killing detectable in extended cytotoxicity assays. The paradoxical earlier findings reflect the fact that cytokine assays are more sensitive indicators of Ag-specific recognition in situations in which the target population is heterogeneous at the single-cell level in terms of epitope display. Such heterogeneity exists within LCLs with, at any one time, subpopulations showing large differences in sensitivity to T cell detection. These differences are not cell cycle related, but correlate with differing levels of EBV latent membrane protein (LMP)1 expression at the single-cell level. In this study, LMP1 is not itself a CD8(+) T cell target, but its expression enhances Ag-processing capacity and HLA class I expression. We propose that LMP1 levels fluctuate cyclically in individual cells and, over time, all cells within a LCL pass through a LMP1(high) T cell-detectable phase.

Epitope-specific evolution of human CD8(+) T cell responses from primary to persistent phases of Epstein-Barr virus infection.

Primary virus infection often elicits a large CD8(+) T cell response which subsequently contracts to a smaller memory T cell pool; the relationship between these two virus-specific populations is not well understood. Here we follow the human CD8(+) T cell response to Epstein-Barr virus (EBV) from its primary phase in infectious mononucleosis (IM) through to the persistent carrier state. Using HLA-A2.1 or B8 tetramers specific for four lytic cycle and three latent cycle epitopes, we find marked differences in the epitope-specific composition of the T cell populations between the two phases of infection. The primary response is dominated by lytic epitope specificities which are severely culled (and in one case extinguished) with resolution of the acute infection; in contrast latent epitope specificities are less abundant, if present at all, in acute IM but often then increase their percentage representation in the CD8 pool. Even comparing epitopes of the same type, the relative size of responses seen in primary infection does not necessarily correlate with that seen in the longer term. We also follow the evolution of phenotypic change in these populations and show that, from a uniform CD45RA(-)RO(+)CCR7(-) phenotype in IM, lytic epitope responses show greater reversion to a CD45RA(+)RO(-) phenotype whereas latent epitope responses remain CD45RA(-)RO(+) with a greater tendency to acquire CCR7. Interestingly these phenotypic distinctions reflect the source of the epitope as lytic or latent, and not the extent to which the response has been amplified in vivo.

Upregulation of interleukin 7 receptor alpha and programmed death 1 marks an epitope-specific CD8+ T-cell response that disappears following primary Epstein-Barr virus infection.

In immunocompetent individuals, the stability of the herpesvirus-host balance limits opportunities to study the disappearance of a virus-specific CD8(+) T-cell response. However, we noticed that in HLA-A 0201-positive infectious mononucleosis (IM) patients undergoing primary Epstein-Barr virus (EBV) infection, the initial CD8 response targets three EBV lytic antigen-derived epitopes, YVLDHLIVV (YVL), GLCTLVAML (GLC), and TLDYKPLSV (TLD), but only the YVL and GLC reactivities persist long-term; the TLD response disappears within 10 to 27 months. While present, TLD-specific cells remained largely indistinguishable from YVL and GLC reactivities in many phenotypic and functional respects but showed unique temporal changes in two markers of T-cell fate, interleukin 7 receptor alpha (IL-7Ralpha; CD127) and programmed death 1 (PD-1). Thus, following the antigen-driven downregulation of IL-7Ralpha seen on all populations in acute IM, in every case, the TLD-specific population recovered expression unusually quickly post-IM. As well, in four of six patients studied, TLD-specific cells showed very strong PD-1 upregulation in the last blood sample obtained before the cells' disappearance. Our data suggest that the disappearance of this individual epitope reactivity from an otherwise stable EBV-specific response (i) reflects a selective loss of cognate antigen restimulation (rather than of IL-7-dependent signals) and (ii) is immediately preceded, and perhaps mediated, by PD-1 upregulation to unprecedented levels.

Tonsillar homing of Epstein-Barr virus-specific CD8+ T cells and the virus-host balance.

Patients with infectious mononucleosis (IM) undergoing primary EBV infection show large expansions of EBV-specific CD8+ T cells in the blood. While latent infection of the B cell pool is quickly controlled, virus shedding from lytically infected cells in the oropharynx remains high for several months. We therefore studied how responses localize to the tonsil, a major target site for EBV, during primary infection and persistence. In acute IM, EBV-specific effectors were poorly represented among CD8+ T cells in tonsil compared with blood, coincident with absence of the CCR7 lymphoid homing marker on these highly activated cells. In patients who had recently recovered from IM, latent epitope reactivities were quicker than lytic reactivities both to acquire CCR7 and to accumulate in the tonsil, with some of these cells now expressing the CD103 integrin, which mediates retention at mucosal sites. By contrast, in long-term virus carriers in whom both lytic and latent infections had been controlled, there was 2- to 5-fold enrichment of lytic epitope reactivities and 10- to 20-fold enrichment of latent epitope reactivities in tonsil compared with blood; up to 20% of tonsillar CD8+ T cells were EBV specific, and many now expressed CD103. We suggest that efficient control of EBV infection requires appropriate CD8+ T cell homing to oropharyngeal sites.

MHC II tetramers visualize human CD4+ T cell responses to Epstein-Barr virus infection and demonstrate atypical kinetics of the nuclear antigen EBNA1 response.

Virus-specific CD4(+) T cells are key orchestrators of host responses to viral infection yet, compared with their CD8(+) T cell counterparts, remain poorly characterized at the single cell level. Here we use nine MHC II-epitope peptide tetramers to visualize human CD4(+) T cell responses to Epstein-Barr virus (EBV), the causative agent of infectious mononucleosis (IM), a disease associated with large virus-specific CD8(+) T cell responses. We find that, while not approaching virus-specific CD8(+) T cell expansions in magnitude, activated CD4(+) T cells specific for epitopes in the latent antigen EBNA2 and four lytic cycle antigens are detected at high frequencies in acute IM blood. They then fall rapidly to values typical of life-long virus carriage where most tetramer-positive cells display conventional memory markers but some, unexpectedly, revert to a naive-like phenotype. In contrast CD4(+) T cell responses to EBNA1 epitopes are greatly delayed in IM patients, in line with the well-known but hitherto unexplained delay in EBNA1 IgG antibody responses. We present evidence from an in vitro system that may explain these unusual kinetics. Unlike other EBNAs and lytic cycle proteins, EBNA1 is not naturally released from EBV-infected cells as a source of antigen for CD4(+) T cell priming.

EBV-associated mononucleosis leads to long-term global deficit in T-cell responsiveness to IL-15.

In mice, interleukin-7 (IL-7) and IL-15 are involved in T-cell homeostasis and the maintenance of immunologic memory. Here, we follow virus-induced responses in infectious mononucleosis (IM) patients from primary Epstein-Barr virus (EBV) infection into long-term virus carriage, monitoring IL-7 and IL-15 receptor (IL-R) expression by antibody staining and cytokine responsiveness by STAT5 phosphorylation and in vitro proliferation. Expression of IL-7Ralpha was lost from all CD8+ T cells, including EBV epitope-specific populations, during acute IM. Thereafter, expression recovered quickly on total CD8+ cells but slowly and incompletely on EBV-specific memory cells. Expression of IL-15Ralpha was also lost in acute IM and remained undetectable thereafter not just on EBV-specific CD8+ populations but on the whole peripheral T- and natural killer (NK)-cell pool. This deficit, correlating with defective IL-15 responsiveness in vitro, was consistently observed in patients up to 14 years after IM but not in patients after cytomegalovirus (CMV)-associated mononucleosis, or in healthy EBV carriers with no history of IM, or in EBV-naive individuals. By permanently scarring the immune system, symptomatic primary EBV infection provides a unique cohort of patients through which to study the effects of impaired IL-15 signaling on human lymphocyte functions in vitro and in vivo.