Ineke J. M. ten Berge

Human CD8 + T-cell differentiation in response to viruses

CD8+ T cells are essential in the defence against viruses. Recently, peptide–HLA class I tetramers have been used to study immune responses to viruses in humans. This approach has indicated consecutive stages of human CD8+ T-cell development in acute viral infection and has illustrated the heterogeneity of CD8+ T cells that are specific for latent viruses. Here, we summarize these findings and discuss their significance for our understanding of antigen-induced CD8+ T-cell maturation in humans.

Emergence of a CD4+CD28− Granzyme B+, Cytomegalovirus-Specific T Cell Subset after Recovery of Primary Cytomegalovirus Infection

Cytotoxic CD4+CD28− T cells form a rare subset in human peripheral blood. The presence of CD4+CD28− cells has been associated with chronic viral infections, but how these particular cells are generated is unknown. In this study, we show that in primary CMV infections, CD4+CD28− T cells emerge just after cessation of the viral load, indicating that infection with CMV triggers the formation of CD4+CD28− T cells. In line with this, we found these cells only in CMV-infected persons. CD4+CD28− cells had an Ag-primed phenotype and expressed the cytolytic molecules granzyme B and perforin. Importantly, CD4+CD28− cells were to a large extent CMV-specific because proliferation was only induced by CMV-Ag, but not by recall Ags such as purified protein derivative or tetanus toxoid. CD4+CD28− cells only produced IFN-γ after stimulation with CMV-Ag, whereas CD4+CD28+ cells also produced IFN-γ in response to varicella-zoster virus and purified protein derivative. Thus, CD4+CD28− T cells emerge as a consequence of CMV infection.

Development of virus-specific CD4+ T cells during primary cytomegalovirus infection

Although virus-specific CD4(+) T cells have been characterized extensively in latently infected individuals, it is unclear how these protective T-cell responses develop during primary virus infection in humans. Here, we analyzed the kinetics and characteristics of cytomegalovirus-specific (CMV-specific) CD4(+) T cells in the course of primary CMV infection in kidney transplant recipients. Our data reveal that, as the first sign of specific immunity, circulating CMV-specific CD4(+) T cells become detectable with a median of 7 days after first appearance of CMV-DNA in peripheral blood. These cells produce the T helper 1 type (Th1) cytokines IFNgamma and TNFalpha, but not the T helper 2 type (Th2) cytokine IL4. In primary CMV infection, the vast majority of these circulating virus-specific T cells have features of recently activated naive T cells in that they coexpress CD45RA and CD45R0 and appear to be in the cell cycle. In contrast, in people who have recovered from CMV infection earlier in life, virus-specific T cells do not cycle and express surface markers characteristic of memory T cells. After the initial rise, circulating virus-specific CD4(+) T cells decline rapidly. During this phase, a strong rise in IgM and IgG anti-CMV antibody titers occurs, concomitant with the reduction of CMV-DNA in the circulation.

Clinical and immunologic aspects of cytomegalovirus infection in solid organ transplant recipients.

Primary cytomegalovirus (CMV) infection is a major cause of morbidity and mortality in recipients after solid organ transplantation (SOT). Widespread and prolonged use of antiviral drugs has changed the natural course of CMV disease by delaying its onset and causing drug resistance. CMV induces a strong cellular immune response, even in immunosuppressed patients, and has developed strategies to evade this immune surveillance. This review summarizes challenges in managing CMV infection in transplant recipients and highlights current insights in the cellular immune response against CMV.

Proliferation Requirements of Cytomegalovirus-Specific, Effector-Type Human CD8+ T Cells

Two prototypic types of virus-specific CD8+ T cells can be found in latently infected individuals: CD45R0+CD27+CCR7− effector-memory, and CD45RA+CD27−CCR7− effector-type cells. It has recently been implied that CD45RA+CD27−CCR7− T cells are terminally differentiated effector cells and as such have lost all proliferative capacity. We show in this study, however, that stimulation of CMV-specific CD45RA+CD27−CCR7− T cells with their cognate peptide in concert with either CD4+ help or IL-2, IL-15, or IL-21 in fact induces massive clonal expansion. Concurrently, these stimulated effector T cells change cell surface phenotype from CD45RA to CD45R0 and regain CCR7, while effector functions are maintained. Our data imply that CD45RA+CD27−CCR7− effector-type T cells contribute to immunity not only by direct execution of effector functions, but also by yielding progeny in situations of viral reinfection or reactivation.

CD103 is a marker for alloantigen-induced regulatory CD8(+) T cells

The αEβ7 integrin CD103 may direct lymphocytes to its ligand E-cadherin. CD103 is expressed on T cells in lung and gut and on allograft-infiltrating T cells. Moreover, recent studies have documented expression of CD103 on CD4+ regulatory T cells. Approximately 4% of circulating CD8+ T cells bear the CD103 molecule. In this study, we show that the absence or presence of CD103 was a stable trait when purified CD103− and CD103+CD8+ T cell subsets were stimulated with a combination of CD3 and CD28 mAbs. In contrast, allostimulation induced CD103 expression on ∼25% of purified CD103−CD8+ T cells. Expression of CD103 on alloreactive cells was found to be augmented by IL-4, IL-10, or TGF-β and decreased by addition of IL-12 to MLCs. The alloantigen-induced CD103+CD8+ T cell population appeared to be polyclonal and retained CD103 expression after restimulation. Markedly, in vitro-expanded CD103+CD8+ T cells had low proliferative and cytotoxic capacity, yet produced considerable amounts of IL-10. Strikingly, they potently suppressed T cell proliferation in MLC via a cell-cell contact-dependent mechanism. Thus, human alloantigen-induced CD103+CD8+ T cells possess functional features of regulatory T cells.

Serine proteases of the human immune system in health and disease.

Serine proteases form a large family of protein-cleaving enzymes that play an essential role in processes like blood coagulation, apoptosis and inflammation. Immune cells express a wide variety of serine proteases such as granzymes in cytotoxic lymphocytes, neutrophil elastase, cathepsin G and proteinase 3 in neutrophils and chymase and tryptase in mast cells. Regulation of proteolysis induced by these serine proteases is essential to prevent self-induced damage. Hence, there are specialized serine protease inhibitors, serpins, which are broadly distributed. Here, we discuss the function of human serine proteases in inflammation, apoptosis and tissue remodeling. Furthermore, we address their impact on development and progression of immune mediated-diseases. Understanding the mode of action of serine proteases will help to unravel molecular processes involved in immunological disorders and will facilitate the identification of new therapeutic targets.

Molecular profiling of cytomegalovirus-induced human CD8 + T cell differentiation

CD8+ T cells play a critical role in the immune response to viral pathogens. Persistent human cytomegalovirus (HCMV) infection results in a strong increase in the number of virus-specific, quiescent effector-type CD8+ T cells with constitutive cytolytic activity, but the molecular pathways involved in the induction and maintenance of these cells are unknown. We show here that HCMV infection induced acute and lasting changes in the transcriptomes of virus-reactive T cells collected from HCMV-seropositive patients at distinct stages of infection. Enhanced cell cycle and metabolic activity was restricted to the acute phase of the response, but at all stages, HCMV-specific CD8+ T cells expressed the Th1-associated transcription factors T-bet (TBX21) and eomesodermin (EOMES), in parallel with continuous expression of IFNG mRNA and IFN-γ-regulated genes. The cytolytic proteins granzyme B and perforin as well as the fractalkine-binding chemokine receptor CX3CR1 were found in virus-reactive cells throughout the response. During HCMV latency, virus-specific CD8+ T cells lacked the typical features of exhausted cells found in other chronic infections. Persistent effector cell traits together with the permanent changes in chemokine receptor usage of virus-specific, nonexhausted, long-lived CD8+ T cells may be crucial to maintain lifelong protection from HCMV reactivation.

Cytomegalovirus Infection Reduces Telomere Length of the Circulating T Cell Pool

Short telomeres of circulating leukocytes are a risk factor for age-related diseases, such as atherosclerosis, but the exact mechanisms generating variations in telomere length are unknown. We hypothesized that induction of differentiated T cells during chronic CMV infection would affect T cell telomere length. To test this, we measured the amount of differentiated T cells and telomere length of lymphocytes during primary CMV infection as well as CMV-seropositive and -seronegative healthy individuals. After primary CMV infection, we observed an increase in highly differentiated cells that coincided with a steep drop in telomere length. Moreover, we found in a cohort of 159 healthy individuals that telomere shortening was more rapid in CMV-seropositive individuals and correlated with the amount of differentiated T cells in both CD4+ T cells and CD8+ T cells. Finally, we found that telomere length measured in blood leukocytes is correlated with lymphocyte telomere length. Thus, CMV infection induces a strong decrease in T cell telomere length, which can be explained by changes in the composition of the circulating lymphocyte pool.

The Size and Phenotype of Virus-Specific T Cell Populations Is Determined by Repetitive Antigenic Stimulation and Environmental Cytokines

Based on the expression of the TNFR SFP CD27, two Ag-primed CD8+ T cell subsets can be discerned in the circulation of healthy individuals: CD27+ T cells that produce a variety of cytokines but do not display immediate cytolytic activity; and cytotoxic CD27− T cells, which secrete only IFN-γ and TNF-α. The mechanism that controls the generation of these different phenotypes is unknown. We show that CMV reactivation not only increases the number of virus-specific T cells but also induces their transition from a CD27+ to a CD27− phenotype. In support of a relation between pool size and phenotype in a cohort of latently infected individuals, the number of Ag-specific CD27− CD8+ T cells was found to be linearly related to the total number of CMV-specific CD8+ T cells. In vitro studies revealed that the acquisition of the CD27− phenotype on CMV-specific T cells depended on the interaction of CD27 with its cellular ligand, CD70. Expression of CD70 was proportional to the amount of antigenic stimulation and blocked by the CD4+ T cell-derived cytokine IL-21. Thus, induction of CD70, which may vary in distinct viral infections, appears to be a key factor in determining the size and phenotype of the CMV-specific T cell population in latently infected individuals.