Paul A. Baars

Constitutive CD27/CD70 Interaction Induces Expansion of Effector-Type T Cells and Results in IFNγ-Mediated B Cell Depletion

The interaction between the TNF receptor family member CD27 and its ligand CD70 provides a costimulatory signal for T cell expansion. Normally, tightly regulated expression of CD70 ensures the transient availability of this costimulatory signal. Mice expressing constitutive CD70 on B cells had higher peripheral T cell numbers that showed increased differentiation toward effector-type T cells. B cell numbers in CD70 transgenic (TG) mice progressively decreased in primary and secondary lymphoid organs. This B cell depletion was caused by CD27-induced production of IFNgamma in T cells. We conclude that apart from its role in controlling the size of the activated T cell pool, CD27 ligation contributes to immunity by facilitating effector T cell differentiation.

Frequencies of Circulating Cytolytic, CD45RA+CD27−, CD8+ T Lymphocytes Depend on Infection with CMV

Viral infections may cause serious disease unless the adaptive immune system is able to clear the viral agents through its effector arms. Recent identification and functional characterization of subpopulations of human CD8+ T cells has set the stage to study the correlation between the appearance of particular subsets and common viral infections during childhood, i.e., EBV, CMV, varicella-zoster virus (VZV), and the attenuated measles-mumps-rubella (MMR) vaccine strains. In a cohort of 220 healthy children we analyzed lymphocytes and subpopulations of CD4+ and CD8+ T cells. The presence of the cytolytic CD45RA+CD27− subset of CD8+ T cells correlated with prior CMV infection as defined by seroconversion (p < 0.0001). The number of this CD8+ T cell subset remained stable during follow-up over 3 years in 40 children. The CD45RA+CD27− subset of CD8+ T cells first appeared during acute CMV infection and subsequently stabilized at an individual set-point defined by age and immunocompetence. The functional importance of these cells in CMV surveillance was reflected by their increased numbers in immunosuppressed pediatric kidney transplant patients. Preferential expansion of CD8+CD45RA+CD27− cytolytic T cells seems unique for 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.

CD27 Defines Phenotypically and Functionally Different Human NK Cell Subsets

The absence of the TNF-receptor family member CD27 marks the stable acquisition of cytolytic effector functions by both CD4+ and CD8+ T cells. We found that the majority of circulating human NK cells was CD27−. These cells were largely CD56dim, contained high levels of perforin and granzyme B, and were able to exert strong cytotoxic activity. In contrast, circulating CD27+ NK cells were mostly CD56dim/bright, had significant lower levels of perforin and granzyme B, and had a low cytolytic potential. Primary and secondary lymphoid organs were markedly enriched for CD27+ NK cells. When correlating the expression of CD27 to recently defined developmental stages of NK cells in tonsil, we observed that CD27 was exclusively found on mature CD94+, stage 4 NK cells. On these cells, regulation of CD27 expression appeared to be controlled by the common γ-chain cytokine IL-15, and down-regulation of CD27 was specifically induced by its ligand, CD70. Thus, the absence of CD27 expression allows the definition of cytotoxic effector cells within the known mature NK cell subsets in humans.

Cytolytic mechanisms and expression of activation-regulating receptors on effector-type CD8+CD45RA+CD27- human T cells.

Circulating CD8+ T cells with a CD45RA+CD27− phenotype resemble cytolytic effector cells because they express various cytolytic mediators and are able to execute cytotoxicity without prior stimulation in vitro. We here demonstrate that CD8+CD45RA+CD27− T cells can use both granule exocytosis and Fas/Fas ligand pathways to induce apoptosis in target cells. The availability of these cytolytic mechanisms in circulating T cells suggests that the activity of these cells must be carefully controlled to prevent unwanted tissue damage. For this reason, we analyzed the expression of surface receptors that either enhance or inhibit T cell function. Compared with memory-type cells, effector cells were found to express normal levels of CD3ε and TCRζ and relatively high levels of CD8. CTLA-4 was absent from freshly isolated effector cells, whereas a limited number of unstimulated memory cells expressed this molecule. In line with recent findings on CD8+CD28− T cells, CD45RA+CD27− T cells were unique in the abundant expression of NK cell-inhibitory receptors, both of Ig superfamily and C-type lectin classes. Binding of NK cell-inhibitory receptors to classical and nonclassical MHC class I molecules may inhibit the activation of the cytolytic machinery induced by either Ag receptor-specific or nonspecific signals in CD8+CD45RA+CD27− T cells.

Elevated levels of a soluble form of the T cell activation antigen CD27 in cerebrospinal fluid of multiple sclerosis patients

The expression of the T cell membrane molecule CD27--a molecule that has recently been shown to belong to the nerve growth factor receptor superfamily--is strongly increased after activation of T lymphocytes via the T cell receptor/CD3 complex. In addition, activated cells release a 28-32 kDa soluble form of CD27 in their supernatant which can also be detected in serum and urine of healthy individuals. In this study we show that levels of soluble (s) CD27 are significantly elevated in cerebrospinal fluid (CSF) of multiple sclerosis (MS) patients and of patients and of suffering from other inflammatory neurological diseases (OIND), whereas increased levels of sCD25 (soluble interleukin-2 receptor) were only found in CSF of patients with OIND. In MS patients, a significant correlation was found between CSF sCD27 titer and IgG index.

Properties of murine CD8+CD27– T cells

In humans, loss of CD27 expression is associated with the stable acquisition of effector functions by CD8+ T cells. We found that murine CD8+CD27– T cells were confined to the primed CD62Ldull/–CD44brightCCR7– T cell population. CD8+CD27– T cells were absent from lymph nodes but could be found in blood, spleen and in non‐lymphoid organs such as lung and liver. Late after primary influenza virus infection, low percentages of antigen‐specific CD27– cells emerged in the lung and spleen. After recovery from secondary influenza virus infection, high percentages of influenza‐specific CD27– T cells were found in the lung and the loss of CD27 on lung CD8+ T cells coincided with high granzyme B expression. After murine cytomegalovirus infection, loss of CD27 expression on virus‐specific CD8+ T cell populations was sustained and especially marked in liver and lung. We suggest that in mice, CD27 is lost from CD8+ T cells only after repetitive antigenic stimulation. Moreover, the high expression of both granzyme B and perforin in the CD27– T cells suggests that the lack of CD27 on murine CD8+ T cells can be used to identify memory T cells with expression of cytotoxic effector molecules.

Absence of Circulating Natural Killer and Primed CD8+ Cells in Life-Threatening Varicella

Five pediatric patients with no history of immunodeficiency had a life-threatening course of varicella. Strikingly, natural killer (NK) cells were absent from the circulation in all children, and, despite active viral infection, up to 98% of the CD8(+) cells were naive. Primary immunodeficiencies were excluded--NK cells and primed CD8(+) cells reappeared in the circulation, granzymes were detectable in plasma early during infection, and no abnormalities could be detected in interleukin-15 receptor function. Our data indicate that varicella-zoster virus (VZV) has a unique capability to seclude primed CD8(+) cells and NK cells from the circulating lymphocyte pool. This may be the consequence of an overwhelming immune response to VZV that is influenced by factors such as infectious dose, age, and the presence of maternal antibodies during infancy. Because both homozygous twin sisters in the study had a severe course of varicella, particular genetic factors may contribute to severe varicella.

Specific expression of GPR56 by human cytotoxic lymphocytes

We here report the existence of a new cluster of adhesion‐GPCRs in human immune cells. Analysis of a comprehensive immune cell transcriptome dataset indicated that expression of the closely related receptors, GPR56, GPR97, and GPR114, is associated with single lymphocyte and granulocyte subsets. Applying flow cytometric analysis with newly generated mAb, we show that expression of GPR56 is restricted to cytotoxic NK and T lymphocytes, including CD8+, CD4+, and γδ T cells. Primary infection with human CMV, which generates a vast population of CD8+ T cells with an effector phenotype, induced a strong increase in GPR56 expression in virus‐specific CD8+ T cells that remained detectable during latency. In NK‐92 cells, ectopic expression of GPR56 inhibited spontaneous and SDF‐1‐stimulated cell migration. Our data suggest that GPR56 expression is a common trait of human cytotoxic lymphocytes and might affect the migratory properties of these cells.

Cutting edge: CD95 maintains effector T cell homeostasis in chronic immune activation

The elimination of activated T cells is important to maintain homeostasis and avoid immunopathology. CD95 (Fas/APO-1) has been identified as a death mediator for activated T cells in vitro but the function of CD95 in death of mature T cells in vivo is still controversial. Here we show that triggering of the costimulatory TNF receptor family member CD27 sensitized T cells for CD95-induced apoptosis. CD95-deficient (lpr/lpr) T cells massively expanded and differentiated into IFN-γ-secreting effector cells in transgenic mice that constitutively express the CD27 ligand, CD70. Concomitantly, CD95-deficient CD70 transgenic mice became moribund by 4 wk of age with severe liver pathology and bone marrow failure. These findings establish that CD95 is a critical regulator of effector T cell homeostasis in chronic immune activation.