The role of natural killer (NK) cells in hematopoietic stem cell transplantation and in the control of neonatal infections is not yet clear. Donor-versus-recipient NK cell alloreactivity was found to improve outcome in some settings of hematopoietic stem cell transplantation. We hypothesized that the role of NK cells in cord blood (CB) transplantation and neonatal infections may depend on CB NK cell maturation stage. We therefore analyzed the expression of NK cell differentiation/phenotypic markers in human CB, as well as functional properties of purified CB NK cells. CD8 and CD57 expression was lower in CB than in adult NK cells. However, the expression of other differentiation markers was similar, as was cell surface density of CD56, the percentage of late NK cell precursors, interferon-␥ production, and the proliferative response of purified NK cells to IL-2. Spontaneous cytotoxic activity of purified CB NK cells against NK-sensitive targets was low but reached adult levels after treatment with IL-15. Expression of perforin and granzyme B was higher in CB NK cells (90 versus 58% and 86 versus 69%, respectively Natural killer (NK) cells are innate immune lymphocytes defined by their cell surface expression of the CD56 antigen without expression of the CD3 antigen. They display a broad anti-infectious and antitumor cytolytic activity. They can also secrete various cytokines, such as interferon (IFN)-␥ and other cytokines that regulate the immune response and hematopoiesis (1). Although NK cells play an important role early in the infectious cycle, at a time when specific immunity has not yet fully developed, their role in the defense of the neonate against infection has not been studied (2). It was demonstrated recently that NK cells play an important role in the outcome of clinical HLA-haploidentical hematopoietic stem cell transplantation (1,3). A positive effect of NK cell alloreactivity has also been reported in a series of unrelated hematopoietic stem cell transplantations (4). We, as several other groups, use cord blood (CB) as a source of stem cells in partially HLA-mismatched transplantation with outcomes similar to those observed in HLA-identical bone marrow transplantation (5-9). CB contains a higher percentage of NK cells than adult blood-, bone marrow-, or cytokine-mobilized peripheral blood stem cell grafts (10). It thus is tempting to speculate that neonatal/CB NK cells may play a role in CB transplantation as well as in the control of neonatal infections.Several markers of NK cell immaturity have been described. Among these is the level of cell-surface expression of CD56.

Three exceptional cell lineA have been tested for the presence of the Epstein-Barr virus genome by nucleic acid hybridization (complementary RNA-DNA) and Epstein-Barr virus-determined nuclear antigen tests. Two lines were derived from Swedish lymphoma cases and one from an African Burkitt-like lymphoma biopsy that was negative for Epstein-Barr virus DNA and the virus-determined nuclear antigen. All three lines apparently lacked the viral genome. Two of the three lines clearly had characteristics of B-cells (bone-marrow-derived).

The marked tropism of human herpesvirus-6 (HHV-6) for natural killer (NK) cells and T lymphocytes has led us to investigate the effect of HHV-6 on cellular cytotoxicity. We describe here how HHV-6 infection of peripheral blood mononuclear cells (

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ADCC is an important immune effector mechanism against tumor and virus-infected cells that can be destroyed by the combined action of specific antibodies of IgG isotype against cell surface-associated antigens and effector cells, predominantly of the NK phenotype. ADCC has been shown to function in vivo in several systems. With regard to HIV, it can be readily demonstrated in vitro against the viral envelope proteins with serum and/or effector cells obtained from HIV-infected subjects. Several studies have demonstrated that the titers of the envy-specific ADCC-mediating antibodies decrease in the sera of HIV-infected individuals as the infection progresses toward AIDS. The cells that mediate ADCC also become functionally compromised in these individuals in early stages of the infection, thus depriving the host of the potential benefits of this process. Restoration of this process in the infected individuals by the administration of functionally competent effector cells (in vitro expanded and lymphokine-activated killer cells) and ADCC-mediating antibodies (hyperimmune serum or appropriate monoclonal antibodies), alone or in combination, may help slow the disease progress. Because of the multicomponent nature of the process, ADCC-mediating antibodies may prove a better correlate of protection and prognosis than the virus-specific neutralizing antibodies and cytotoxic T cells in assessing anti-HIV immunization and immunotherapy.

Human herpesvirus-6 (HHV-6), the etiologic agent of roseola, is ubiquitous, establishes latency in the host, and can infect a variety of immunocompetent cells, with CD4+ T lymphocytes being the targets in which it replicates most efficiently. The present study was undertaken to learn more about specific immunobiologic effects of HHV-6 infection on T-lymphocyte functions. Our data demonstrate that infection of peripheral blood mononuclear cells (PBMC) by HHV-6 results in suppression of T-lymphocyte functions, as evidenced by reduced interleukin-2 (IL-2) synthesis and cellular proliferation. In fact, HHV- 6-infected PBMC secreted 50% less IL-2 than mock-infected cells after mitogenic stimulation with OKT3 antibody or phytohemmaglutinin (PHA). The inhibition of IL-2 by HHV-6 was also observed in enriched T-cell cultures, suggesting a direct effect of this virus on this cell type. Messenger RNA (mRNA) analysis by reverse-transcriptase polymerase chain reaction (PCR) indicated that HHV-6 diminishes IL-2 mRNA levels in mitogen-stimulated peripheral blood T cells. These results were also confirmed by Northern blot using the leukemic T-cell line Jurkat. This inhibitory effect of HHV-6 did not require infectious virus, as the use of UV-irradiated HHV-6 produced similar results. Moreover, HHV-6- infected PBMC showed up to an 85% reduction in their mitogen-driven proliferative response, as compared with sham-infected cells. Proliferation of both CD4+ and CD8+ T cells was affected by HHV-6. Taken together, our data show that infection of T cells by HHV-6 results in immune suppression characterized by a downregulation of IL-2 mRNA and protein synthesis accompanied by diminished cellular proliferation.

One common attribute of herpesviruses is the ability to establish latent, lifelong infections. The role of virus-virus interaction in viral reactivation between or among herpesviruses has not been studied. Preliminary experiments in our laboratory had indicated that infection of Epstein-Barr virus (EBV) genome-positive human lymphoid cell lines with human herpesvirus 6 (HHV-6) results in EBV reactivation in these cells. To further our knowledge of this complex phenomenon, we investigated the effect of HHV-6 infection on expression of the viral lytic cycle proteins of EBV. Our results indicate that HHV-6 upregulates, by up to 10-fold, expression of the immediate-early Zebra antigen and the diffuse and restricted (85 kDa) early antigens (EA-D and EAR , respectively) in both EBV producer and nonproducer cell lines (i.e., P3HR1, Akata, and Raji). Maximal EA-D induction was observed at 72 h post-HHV-6 infection. Furthermore, expression of late EBV gene products, namely, the viral capsid antigen (125 kDa) and viral membrane glycoprotein gp350, was also increased in EBV producer cells (P3HR1 and Akata) following infection by HHV-6. By using dual-color membrane immunofluorescence, it was found that most of the cells expressing viral membrane glycoprotein gp350 were also positive for HHV-6 antigens, suggesting a direct effect of HHV-6 replication on induction of the EBV replicative cycle. No expression of late EBV antigens was observed in Raji cells following infection by HHV-6, implying a lack of functional complementation between the deleted form of EBV found in Raji cells and the superinfecting HHV-6. The susceptibility of the cell lines to infection by HHV-6 correlated with increased expression of various EBV proteins in that B95-8 cells, which are not susceptible to HHV-6 infection, did not show an increase in expression of EBV antigens following treatment with HHV-6. Moreover, UV lightirradiated or heat-inactivated HHV-6 had no upregulating effect on the Zebra antigen or EA-D in Raji cells, indicating that infectious virus is required for the observed effects of HHV-6 on these EBV products. These results show that HHV-6, another lymphotropic human herpesvirus, can activate EBV replication and may thus contribute to the pathogenesis of EBV-associated diseases.

The human herpesvirus 6 (HHV-6) is known to interact intimately with cells of the immune system. Here we report that HHV-6 is a potent inducer of interleukin-l (IL-113) and tumor necrosis factor alpha (TNF-a) in cultures of peripheral blood mononuclear cells. In contradistinction, HHV-6 has no effect on IL-6 synthesis. Maximal IL-1"3 and TNF-oa gene transcription, as detected by polymerase chain reaction amplification analysis, is observed at 12 and 6 h postinfection, respectively. Release of IL-1" and TNF-ot into the culture supernatants peaked at 24 h and gradually decreased with time. Heat-inactivated virus was unable to stimulate IL-1,8 and TNF-oa syntheses, whereas UV-irradiated virus retained the full monokine-inducing potential of the native particle. Preincubation of viral preparation with neutralizing anti-HHV-6 antibody resulted in the abrogation of this cytokine-inducing effect, whereas treatment of cells with phosphonoacetic acid (an inhibitor of viral DNA polymerase activity) had no effect on the ability of the virus to stimulate monokine release. These results indicate that HHV-6 can exert a strong immunomodulatory effect by stimulating the cells of myeloid lineage to produce these cytokines.