A mutation in X-linked inhibitor of apoptosis (G466X) leads to memory inflation of Epstein-Barr virus-specific T cells.

Mutations in the X-linked inhibitor of apoptosis (XIAP) gene have been associated with XLP-like disease, including recurrent Epstein-Barr virus (EBV)-related haemophagocytic lymphohystiocytosis (HLH), but the immunopathogenic bases of EBV-related disease in XIAP deficiency is unknown. We present the first analysis of EBV-specific T cell responses in functional XIAP deficiency. In a family of patients with a novel mutation in XIAP (G466X) leading to a late-truncated protein and varying clinical features, we identified gradual hypogammaglobulinaemia and large expansions of T cell subsets, including a prominent CD4(+) CD8(+) population. Extensive ex-vivo analyses showed that the expanded T cell subsets were dominated by EBV-specific cells with conserved cytotoxic, proliferative and interferon (IFN)-γ secretion capacity. The EBV load in blood fluctuated and was occasionally very high, indicating that the XIAP(G466X) mutation could impact upon EBV latency. XIAP deficiency may unravel a new immunopathogenic mechanism in EBV-associated disease.

IL-7R alpha versus CCR7 and CD45 as markers of virus-specific CD8+ T cell differentiation: contrasting pictures in blood and tonsillar lymphoid tissue.

In humans, circulating CD8(+) memory T cells to a nonpersistent virus (influenza) lie within CCR7(+)CD45RA(-) central memory, whereas memory to Epstein-Barr virus (EBV) latent, EBV lytic, and cytomegalovirus (CMV) antigens are progressively larger in size and are more biased toward CCR7(-)CD45RA(-) effector memory and CCR7(-)CD45RA(+) terminally differentiated compartments. We found that these populations are also distinguished by progressively lower expression of the interleukin-7 receptor (IL-7R alpha) and by lower IL-7 responsiveness; indeed, percentage IL-7R alpha -positive values showed a tight inverse correlation with population size. However, these relationships among size, differentiation phenotype, and IL-7R alpha status in blood did not hold in tonsillar tissue. In tonsil tissue, although EBV reactivities outnumbered their CMV and influenza counterparts, the distinct CCR7/CD45 isoform signatures of the different virus-specific populations were retained. Moreover, all detectable reactivities showed high levels of IL-7R alpha expression. As a discriminator between different virus-specific populations, IL-7R alpha therefore appears to be more susceptible to tissue location than the classical CCR7/CD45 markers.

The T cell response to persistent herpes virus infections in common variable immunodeficiency.

We show that at least half of patients with common variable immunodeficiency (CVID) have circulating CD8(+) T cells specific for epitopes derived from cytomegalovirus (CMV) and/or the Epstein-Barr virus (EBV). Compared to healthy age-matched subjects, more CD8(+) T cells in CVID patients were committed to CMV. Despite previous reports of defects in antigen presentation and cellular immunity in CVID, specific CD4(+) and CD8(+) T cells produced interferon (IFN)-gamma after stimulation with CMV peptides, and peripheral blood mononuclear cells secreted perforin in response to these antigens. In CVID patients we found an association between a high percentage of circulating CD8(+) CD57(+) T cells containing perforin, CMV infection and a low CD4/CD8 ratio, suggesting that CMV may have a major role in the T cell abnormalities described previously in this disease. We also show preliminary evidence that CMV contributes to the previously unexplained severe enteropathy that occurs in about 5% of patients.

HLA-A11-restricted epitope polymorphism among Epstein-Barr virus strains in the highly HLA-A11-positive Chinese population: incidence and immunogenicity of variant epitope sequences.

An individual's CD8(+)-cytotoxic-T-lymphocyte (CTL) response to Epstein-Barr virus (EBV) latent cycle antigens focuses on a small number of immunodominant epitopes often presented by just one of the available HLA class I alleles; for example, HLA-A11-positive Caucasians frequently respond to two immunodominant HLA A11 epitopes, IVTDFSVIK (IVT) and AVFDRKSDAK (AVF), within the nuclear antigen EBNA3B. Here, we reexamine the spectrum of EBV strains present in the highly HLA-A11-positive Chinese population for sequence changes in these epitopes relative to the Caucasian type 1 prototype strain B95.8. The IVT epitope was altered in 61 of 64 Chinese type 1 viruses, with four different sequence variants being observed, and the AVF epitope was altered in 46 cases with six different sequence variants; by contrast, all 10 Chinese type 2 viruses retained the prototype 2 epitope sequences. All but one of the type 1 epitope variants were poorly recognized by IVT- or AVF-specific CTLs in pulse-chase assays of peptide-mediated target cell lysis. More importantly, we screened HLA-A11-positive Chinese donors carrying viruses with known epitope mutations for evidence of epitope-specific CTL memory by enzyme-linked immunospot assays: none of the type 1 variants tested, nor the type 2 prototype, appeared to be immunogenic in vivo. The data remain consistent with the possibility that, during virus-host coevolution, pressure from the host CTL-mediated immune response has given A11 epitope-loss viruses a selective advantage.

EBV-specific CD8+ T cell memory: relationships between epitope specificity, cell phenotype, and immediate effector function.

EBV infection in humans induces CD8+ T cell memory to viral epitopes derived from both lytic and latent cycle Ags. We have analyzed the relationship between the phenotype and function of the memory pool of T cells specific for these Ags. Lytic epitope-specific populations were heterogeneous in terms of CD45RO/RA and CD28 expression, whereas latent epitope-specific populations were uniformly CD45RO+ and CD28+, consistent with the higher antigenic challenge from lytic epitopes driving some memory cells toward a CD45RA+, CD28- phenotype. However, both types of memory population showed immediate epitope-specific cytotoxicity and type 1 cytokine production in ex vivo assays. Cytotoxic function was not associated with preactivated T cells, as EBV-specific populations were negative for activation markers such as CD69 or CD38, nor could cytotoxic function be ascribed to CD27- or CD56+ subsets, as such cells were not detected in EBV-specific memory. Furthermore, cytotoxicity was not limited to CD45RA+ and/or CD28- fractions, but also was observed in CD45RO+, CD28+ populations in lytic and latent epitope-specific memory. Cytokine (IFN-gamma, TNF-alpha) responses, measured by intracytoplasmic staining after peptide stimulation, also were detectable in CD45RO+ and RA+ subsets as well as CD28+ and CD28- subsets. Of other markers that were heterogeneous in both lytic and latent epitope populations, CCR7 gave the best discrimination of functionality; thus, CCR7+ cells consistently failed to give an IFN-gamma or TNF-alpha response, whereas many CCR7- cells were responsive. Our data are consistent with effector functions having a broad distribution among phenotypically distinct subsets of "effector memory" cells that have lost the CCR7 marker.

Memory T cells constitute a subset of the human CD8+CD45RA+ pool with distinct phenotypic and migratory characteristics.

Using HLA class I-viral epitope tetramers to monitor herpes virus-specific CD8(+) T cell responses in humans, we have shown that a significant fraction of responding cells revert from a CD45RO(+) to a CD45RA(+) state after priming. All tetramer-binding CD45RA(+) cells, regardless of epitope specificity, expressed a phenotype LFA-1(high)CCR7(low) that was stable for at least 10 years in infectious mononucleosis patients and indefinitely in asymptomatic carriers. CD8(+)CD45RA(+)LFA-1(high) cells were not present in cord blood but in adults account for up to 50% of CD8(+)CD45RA(+) cells. These CD45RA(+)LFA-1(high) cells have significantly shorter telomeres than CD45RA(+)LFA-1(low) cells, suggesting that the latter represent a naive population, while the former are memory cells. CD45RA(+) memory cells are a stable population of noncycling cells, but on stimulation they are potent producers of IFN-gamma, while naive CD8(+) cells produce only IL-2. The chemokine receptor profile and migratory potential of CD45RA(+) memory cells is very similar to CD45RO(+) cells but different to naive CD8 cells. In accord with this, CD45RA(+) memory cells were significantly underrepresented in lymph nodes, but account for virtually all CD8(+)CD45RA(+) T cells in peripheral tissues of the same individuals.

Vaccine-induced cytotoxic T lymphocytes protect against retroviral challenge.

The development of prophylactic vaccines against retroviral diseases has been impeded by the lack of obvious immune correlates for protection. Cytotoxic T-lymphocyte (CTL), CD4-lymphocyteS, chemokine and/or antibody responses have all been associated with protection against HIV and AIDS; however, effective and safe vaccination strategies remain elusive. Here we show that vaccination with a minimal ovine CTL peptide epitope identified within gp51 of the retrovirus bovine leukemia virus (BLV), consistently induced peptide-specific CTLs. Only sheep whose CTLs were also capable of recognizing retrovirus-infected cells were fully protected when challenged with BLV. This retrovirus displays limited sequence variation; thus, in the relative absence of confounding CTL escape variants, virus-specific CTLs targeting a single epitope were able to prevent the establishment of a latent retroviral infection.