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 CD56(dim), contained high levels of perforin and granzyme B, and were able to exert strong cytotoxic activity. In contrast, circulating CD27(+) NK cells were mostly CD56(dim/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 gamma-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.

Persistent detection of varicella-zoster virus DNA in a previously healthy child after severe chickenpox.

In immunocompetent children with primary varicella-zoster virus (VZV) infection, peak viral loads are detected in peripheral blood near the onset of the vesicular rash. VZV DNA concentrations normally diminish and become undetectable within 3 weeks after the appearance of the exanthem. Here, we present a previously healthy, human immunodeficiency virus-negative, 4-year-old boy admitted with severe varicella. High viral loads (>340,000 copies/ml) were found in his blood, and the viral loads remained high for at least 1.5 years. Clinical recovery preceded complete clearance of the virus. General and VZV-specific immune reactivity were intact. NK cells and CD8(+) T cells were activated during acute infection, and VZV-specific CD4(+) T cells were detected at high frequencies. VZV DNA was initially detected in B cells, NK cells, and both CD4(+) and CD8(+) T cells. In contrast, during the persistent phase of VZV DNA detection, the viral DNA was primarily located in CD8(+) T cells. For the first time, we describe the persistent detection of VZV DNA in a previously healthy child.

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.

Development of virus-specific CD4+ T cells on reexposure to Varicella-Zoster virus.

Immunity to childhood diseases is maintained for decades by mechanisms that, at present, are still unclear. We longitudinally studied immune responses in 16 adults exposed to children experiencing varicella (chicken pox). None of the individuals showed clinical signs of infection, and varicella-zoster virus (VZV) DNA could not be detected in peripheral blood or cultured from nasopharyngeal swabs. Exposure to VZV, however, induced expansion of antigen-specific CD4(+) T cells in peripheral blood, with concomitant changes in cytotoxic CD8(+) T cells and natural killer cells. VZV-specific memory CD4(+) T cells were uniformly CD45RA(-) and enriched for CD27(-) cells. The virus-specific cells produced interferon- gamma, tumor necrosis factor- alpha, and interleukin-2. These memory responses to VZV were compared with the primary immune responses of children experiencing varicella. VZV-specific memory CD4(+) T cell responses largely resemble the primary immune response to VZV.

Spontaneous outgrowth of EBV-transformed B-cells reflects EBV-specific immunity in vivo; a useful tool in the follow-up of EBV-driven immunoproliferative disorders in allograft recipients.

During immunosuppressive medication, Epstein-Barr virus (EBV) infection is associated with a risk of developing posttransplant lymphoproliferative disease (PTLD). The appropriateness of a spontaneous EBV B-cell transformation (SET) assay as a monitor of EBV-specific immunity was evaluated to investigate if it safely allows reducing immunosuppressive medication, thereby decreasing the risk of developing PTLD. PBMC were isolated longitudinally from 20 pediatric renal allograft recipients treated with prednisone and cyclosporine combined with either azathioprine or mycophenolate mofetil. Most significantly, EBV-peptide-specific CD8+ T cells were detectable in the blood of patients with negative SET assays, coinciding with significantly lower EBV loads, whereas these cells were less frequent in the blood of patients with positive SET assays. Reducing the levels of immunosuppression resulted in normalization of the SET assays. Therefore, the SET assay is a reflection of the interaction between viral replication, transformation of B cells, and EBV-specific immunity in vivo and hence a valuable screening test for EBV-driven lymphoproliferative phenomena in allograft recipients.

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.