IL-15 is a relatively novel cytokine that shares many biological activities with IL-2 [1,2]. The two cytokines have similar tertiary structure, and use β and γ chains of the IL-2 receptor complex for binding and signal transduction. However, unlike IL-2, which is produced by activated T cells, IL-15 is produced from a wide variety of cells and tissues in the body. Furthermore, it has a unique receptor component (α-chain), which also has much wider cell and tissue distribution compared with its corresponding α-chain for IL-2. In blood, monocyte-macrophages represent the main cell type that produces IL-15. This cytokine markedly increases the cytolytic potential of natural killer (NK) and cytotoxic T cells, and induces proliferation and antibody secretion in antigen-activated B cells. In vivo, it has been shown to be essential for the development and maturation of NK cells [1,2]. IL-15 is induced in the host as a defence to viral and non-viral intracellular pathogens. Human herpesviruses have been demonstrated to enhance NK activity of human peripheral blood mononuclear cells (PBMC) via IL-15 induction [3,4]. The enhanced NK activity is well known to play a role in the control of these infections .
AIDS, which results from infection with HIV-1, is accompanied by decreased resistance of the infected individuals to opportunistic infections including herpesviruses. Decreased NK activities and defective antimicrobicidal potential of the macrophages have been well documented in HIV-infected individuals [6,7]. The decreased NK activities occur even in the early stages of the infection . The reasons for this decrease are not fully understood. Reduced production of the NK activity-enhancing cytokines, e.g. IL-2 and IL-12 has been reported in HIV-infected individuals [8,9]. However, few studies have been conducted to investigate the role of IL-15 in these infections.
In order to investigate the capacity of HIV-infected individuals to induce IL-15 in response to a herpesviral infection, we obtained PBMC from 12 of these individuals as well as from age-matched HIV-seronegative control subjects after their informed consent. The HIV-infected individuals represented all stages of the infection, were not on highly active antiretroviral therapy and had one or more AIDS-defining conditions. Four million PBMC from each donor were infected in vitro with 50 μl of a herpes simplex virus type 1 (HSV-1) viral preparation that was produced in VERO cells and contained 1 × 108 plaque-forming units per millilitre. After infection at 37°C for 1 h, the cells were washed and incubated in one millilitre of the culture medium (RPMI-1640 with 10% heat-inactivated fetal bovine serum and antibodies). Twenty four hours later, the culture supernatants were collected and passed through 0.2 μ filters. These supernatants were concentrated 10-fold for proteins by using filters (Microconcentrator 10; Amicon, Beverly, MA, USA), and their IL-15 content was determined using a commercial enzyme-linked immunosorbent assay kit (Immunocorp, Montreal, Canada). The results obtained from these determinations are shown in Fig. 1. On average, the PBMC from HIV-infected/AIDS patients secreted less IL-15 than their HIV-seronegative counterparts in response to HSV-1 infection. IL-15 was not detectable in similarly prepared supernatants from mock-infected PBMC (except for three HIV-infected donors); the average optical density values of enzyme-linked immunosorbent assay between the two groups of donors did not differ significantly (P > 0.05; data not shown).
These results suggest that there is compromised induction of the IL-15 gene in the PBMC of HIV-infected individuals in response to HSV-1 infection. This virus is known to enhance the NK activity of human PBMC in vitro; however, this enhancement is much less in HIV-infected individuals compared with the HIV-seronegative controls . Our results suggest that impaired IL-15 production in the PBMC of the infected individuals may be one of the reasons for their less than normal increase in NK activity in response to HSV-1 infection. Our results are corroborated by those of Chehimi et al. , who also reported decreased production of IL-15 in response to a bacterial infection in the PBMC of HIV-infected individuals compared with HIV-seronegative control subjects. In addition to IL-2 and IL-12, HIV-infected individuals thus also seem to produce less IL-15 upon stimulation. Although IL-15 enhances immune function and the expansion of virus-specific cytotoxic T lymphocytes in HIV infections [11,12], it has also been incriminated in the polyclonal B cell activation and T cell-induced alveolitis in AIDS patients [13,14]. Clearly, further studies are needed to investigate how this important cytokine is modulated in the course of HIV infections.
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