IL-21 is a relatively newly discovered immune-enhancing cytokine that plays an essential role in controlling chronic viral infections. It is produced mainly by CD4+ T cells, which are also the main targets of HIV-1 and are often depleted in HIV-infected individuals. Therefore, we sought to determine the dynamics of IL-21 production and its potential consequences for the survival of CD4+ T cells and frequencies of HIV-specific CTL. For this purpose, we conducted a series of cross-sectional and longitudinal studies on different groups of HIV-infected patients and show in this study that the cytokine production is compromised early in the course of the infection. The serum cytokine concentrations correlate with CD4+ T cell counts in the infected persons. Among different groups of HIV-infected individuals, only elite controllers maintain normal production of the cytokine. Highly active antiretroviral therapy only partially restores the production of this cytokine. Interestingly, HIV infection of human CD4+ T cells inhibits cytokine production by decreasing the expression of c-Maf in virus-infected cells, not in uninfected bystander cells. We also show that the frequencies of IL-21–producing HIV-specific, but not human CMV-specific, Ag-experienced CD4+ T cells are decreased in HIV-infected viremic patients. Furthermore, we demonstrate in this study that recombinant human IL-21 prevents enhanced spontaneous ex vivo death of CD4+ T cells from HIV-infected patients. Together, our results suggest that serum IL-21 concentrations may serve as a useful biomarker for monitoring HIV disease progression and the cytokine may be considered for immunotherapy in HIV-infected patients.

The differentiation of monocytes into macrophages and dendritic cells is accompanied by induction of cell-surface neuraminidase 1 (Neu1) and cathepsin A (CathA), the latter forming a complex with and activating Neu1. To clarify the biological importance of this phenomenon we have developed the gene-targeted mouse models of a CathA deficiency (CathA S190A ) and a double CathA/Neu1 deficiency (CathA S190A-Neo ). Macrophages of CathA S190A-Neo mice and their immature dendritic cells showed a significantly reduced capacity to engulf Grampositive and Gram-negative bacteria and positively and negatively charged polymer beads as well as IgG-opsonized beads and erythrocytes. Properties of the cells derived from CathA S190A mice were indistinguishable from those of wild-type controls, suggesting that the absence of Neu1, which results in the increased sialylation of the cell surface proteins, probably affects multiple receptors for phagocytosis. Previous data showed that mammalian neuraminidase 1 (Neu1), 5 in addition to its role in the intralysosomal catabolism, may be also involved in cellular signaling during the immune response. In particular, during the activation of mouse T cells, Neu1 is expressed on the plasma membrane and is required for the early production of interleukin-4 and for the interaction of T cells with the antigen-presenting cells (8 -12). In addition, Neu1 of T cells converts the group-specific component (Gc protein) into a factor necessary for the inflammation-primed activation of macrophages (13,14). T cells derived from SM/J or B10.SM strains of mice with the reduced Neu1 activity, due to a missense mutation in the Neu1 gene (15), fail to convert Gc and synthesize interleukin-4, whereas B cells of these mice cannot produce IgG 1 and IgE after immunization with pertussis toxin (8,14,16). Strikingly, surface desialylation of macrophages by viral sialidase from Arthrobacter ureafaciens significantly increases their capacity for phagocytosis of influenza virus-infected HeLa cells (17), providing the direct link between the surface sialylation of antigen-presenting cells and their biological activity.Previously we showed that Neu1 increased 14-fold during the differentiation of human monocytes into macrophages (18). * This work was supported in part by Canadian Institutes of Health ResearchOperating Grants MOP 15079 and GOP 38107 and by an equipment grant from Canadian Foundation for Innovation (to A. V. P.

The host invariably responds to infecting viruses by activating its innate immune system and mounting virus-specific humoral and cellular immune responses. These responses are aimed at controlling viral replication and eliminating the infecting virus from the host. However, viruses have evolved numerous strategies to counter and evade host's antiviral responses. Providing specific examples from the published literature, we discuss in this review article various strategies that viruses have developed to evade antiviral cellular responses of the host. Unraveling these viral strategies allows a better understanding of the host-pathogen interactions and their coevolution. This knowledge is important for identifying novel molecular targets for developing antiviral reagents. Finally, it may also help devise new knowledge-based strategies for developing antiviral vaccines.