Interleukin-15 as an activator of natural killer cell-mediated antiviral response

Natural killer (NK) cells are large granular lymphocytes capable of efficient killing of virus-infected and tumor cells in a major histocompatibility complex-independent manner. The cytotoxic killing potential of NK cells can be modulated by a variety of factors, including cytokines such as interleukin-12 (IL-12), IL-15, and interferon (IFN). IL-15 also plays an important role in NK cell development and survival. Killing of virally infected cells by NK cells is likely to represent an important antiviral defense mechanism, especially during the early phase of infection when antigen-specific immunity has yet to be generated. In the present work, we studied the potential of IL-15 to act as a modulator of NK cell-mediated antiviral defense. Our results clearly indicate that IL-15 can curtail infections by 3 human herpesviruses: Herpes simplex virus type 1, Epstein-Barr virus, and human herpesvirus 6. The antiviral activity of IL-15 is dose-, time-, and NK cell-dependent. IL-15-treated NK cells showed an increased killing potential against a variety of cells, including virus-infected target cells. Lastly, using highly purified cell population, we report that IL-15 triggers the synthesis of IFN-gamma from both CD4(+) and NK cells, which can act in both autocrine and paracrine fashion to modulate NK cells cytotoxic potential. In conclusion, IL-15 is a cytokine that can contribute to the establishment of an antiviral state in 2 ways: first by increasing the killing ability of NK cells and second by stimulating the synthesis and secretion of IFN.