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Several strategies allow viruses to elude the surveillance of the immune system and to establish persistent infection in the host. One of such mechanisms is the immunosuppression caused by the direct infection and functional impairment of immune cells. Human Herpes virus type 6 (HHV-6) is a typical immunosuppressive agent, as suggested by its tropism for both CD4+ and CD8+ T cells, B cells, monocytes/macrophages, megakaryocytes and NK cells. In this study the production of IL-10 and IL-12 by peripheral blood mononuclear cells (PBMC) was evaluated during HHV-6 infection "in vitro". Our results demonstrate that HHV-6 up-regulates IL-10 production by PBMC. Furthermore, our data suggest that rhIFN gamma addition counteracts the effect of HHV-6 in promoting IL-10 release. To gain more insight into the role of IFN gamma, anti-IFN gamma monoclonal antibodies were added to PBMC stimulated with LPS. Neutralization of endogenous IFN gamma upregulated IL-10 release. Furthermore, HHV-6 infection inhibited IFN gamma release induced by LPS in PBMC. No basal production of IL-12 was found in PBMC. Moreover, HHV-6 infection did not induce IL-12 release by PBMC. On the contrary, IL-12 was detected in the supernatants of PBMC treated with LPS with or without rhIFN gamma. In these experimental conditions the further addition of HHV-6 markedly impaired IL-12 production. Moreover, the neutralization of IL-10 resulted in a significant up-regulation of IL-12. Finally our data suggest that the immunodysregulation induced by HHV-6 could be accounted for by a shift from a Th-1 to a Th-2 type cytokine profile.

In this work, the production of tumor necrosis factor alpha (TNF alpha) during interaction of human phagocytes with the intracellular parasite Leishmania major was further investigated. The human monocytic cell line U937, differentiated with a combination of 1 alpha, 25 dihydroxyvitamin D3 (VD) and retinoic acid (RA), or with granulocyte macrophage colony stimulating factor (GM-CSF) was used. Differentiated U937 cells were infected with Leishmania major promastigotes, and TNF alpha was assayed in cell culture supernatants. It was found that the cytokine was produced only by U937 cells differentiated with VD/RA and further incubated with GM-CSF and LPS or interferon gamma (IFN gamma). L. major induced TNF alpha production only in the presence of GM-CSF. No direct relationship was found, however, between production of TNF alpha and resistance of differentiated U937 cells to infection with L. major.

Several cytokines play a crucial role in the defense of the host against protozoa belonging to the genus Leishmania. However, the role of tumor necrosis factor alpha (TNF alpha) and interleukin 6 (IL-6) in human leishmaniasis is still controversial. The aim of this work was to study, in an "in vitro" model, the interactions of human phagocytes with L. major. The U937 human monocytic cell line, differentiated with phorbol myristate acetate (PMA) or a combination of 1 alpha,25 dihydroxyvitamin D3 (VD) and retinoic acid (RA), was used in all the experiments. The rate of infection, as well as the production of TNF alpha and IL-6 by cells upon infection with promastigotes, were studied. It was found that, depending on the agent used for differentiation, U937 cells produced different patterns of cytokines. PMA differentiated cells produced significantly more TNF alpha, but less IL-6 than cells differentiated with VD-RA. No direct relationship was found between the ability of differentiated U937 cells to release TNF alpha or IL-6 and their leishmanicidal activity.

This study characterizes the effect of differentiation on the resistance of the human monocytic cell line U937 to human herpes virus type 6 (HHV-6). The use of monocytic cell line has the advantage of avoiding genetic variations among different donors. The HHV-6 infection was compared in undifferentiated U937 cells and U937 cells differentiated with a combination of vitamin D3 and retinoic acid. Undifferentiated U937 cells were highly resistant to HHV-6 infection. Differentiation of U937 cells was accompanied by an increase in permissiveness for HHV-6 demonstrated in terms of extracellular virus production and viral antigen positive immunofluorescent cells. Tumor necrosis factor alpha (TNF alpha) appears to be an essential mediator during the first line defences of the host against viruses, even though its role during viral infection remains controversial. For this reason we examined the behaviour of TNF alpha in differentiated U937 upon HHV-6 infection. No basal production of TNF alpha was found in culture supernatants, while HHV-6 infection up-regulated TNF alpha release. The addition of human recombinant-TNF alpha to HHV-6 infected cells induced a marked cytotoxic effect accompanied by an increased release of extracellular virus, whereas it did not affect viral replication, as shown by the unmodified percentage of antigen positive cells. In conclusion, TNF alpha acts as a soluble mediator of cytotoxicity against HHV-6 infected U937 cells, but it fails to induce an antiviral state.

The release of monokines such as Tumor Necrosis Factor a (TNF alpha), Interleukin-1 beta (IL-1 beta) and Interleukin-6 (IL-6) by activated monocytes/macrophages is an important step in the immune as well as in the inflammatory response. In this study the production of TNF alpha, IL-1 beta and IL-6 by human monocytes (HM) and peripheral blood mononuclear cells (PBMC) was evaluated after HHV-6 infection. Our results demonstrate that HHV-6 can selectively regulate monokine synthesis, in a time-dependent manner. Moreover, we observed a different response closely related to the cellular population (HM or PBMC) examined. The hypothesis we evaluated was that IFN gamma is an important factor triggering the activation of HHV-6 infected human monocytes, to release monokines.