Interleukin-15 in mycobacterial infection of antigen-presenting cells

Interleukin-15 (IL-15) shares many biological functions with IL-2 but also exhibits unique effects. Some of these represent the potent chemoattractant activity and expansion of distinct T-cell subsets, particularly memory T cells. IL-15 may therefore modulate the quality and quantity of cellular immune responses directed against intracellular pathogens. Immunohistochemical examination of skin lesions obtained from patients with the lepromatous or the tuberculoid form of Hansen's disease revealed intralesional IL-15 protein in both forms of the disease. In addition to Mycobacterium leprae, a number of different mycobacterial species are capable of effectively inducing IL-15 secretion in infected macrophages. In this work, increased IL-15 secretion was observed in IL-4/granulocyte-macrophage colony-stimulating factor (GM-CSF)-activated antigen-presenting cells (APC) compared with unstimulated macrophages. Immunocytological detection of intracellular IL-15 revealed that infection with different mycobacterial species resulted in different staining patterns of anti-IL-15 immunoreactive material in APC. In contrast to IL-2 or IL-7, IL-15 enhanced the cytolytic potential of immune effector cells in vitro and favoured the expansion of CD1b-restricted immune cells recognizing mycobacterial-associated antigens presented by autologous APC. IL-15 produced by infected cells in situ may represent one of the key cytokines involved in granuloma formation and may aid the augmentation of cellular immune responses directed against mycobacterial-infected cells.