Mice lacking interleukin-2 (IL-2)/IL-15 receptor beta chain are susceptible to infection with avirulent Salmonella enterica subsp. enterica serovar choleraesuis but mice lacking IL-2 are resistant

Interleukin 15 in murine models of colitis

Inflammatory bowel diseases (IBDs) are characterized by abnormal, non-antigen specific chronic inflammation of unknown etiology. Genome-wide association studies show that many IBD genetic susceptibility loci map to immune function genes and compelling evidence indicate that environmental factors play a critical role in IBD pathogenesis. Clinical and experimental evidence implicate the pro-inflammatory cytokine IL-15 in the pathogenesis of IBD. IL-15 and IL-15α expression is increased in the inflamed mucosa of IBD patients. IL-15 contributes to the maintenance of different cell subsets in the intestinal mucosa. However, very few studies have addressed the role of IL-15 in pre-clinical models of colitis. In this study, we use three well-characterized models of experimental colitis to determine the contribution of IL-15 to pathological intestinal inflammation.

Interleukin-15 and NK1.1+ cells provide innate protection against acute Salmonella enterica serovar Typhimurium infection in the gut and in systemic tissues

Control of bacterial colonization at mucosal surfaces depends on rapid activation of the innate immune system. Interleukin-15 (IL-15) directs the development, maturation, and function of a population of cells positive for NK1.1, such as natural killer (NK) cells, which are critical components of the innate immune defense against several viral and bacterial pathogens. Using IL-15-deficient mice, in vivo depletion of NK1.1(+) cells from wild-type mice, and in vivo overexpression of IL-15 from a recombinant adenovirus, we tested the role of IL-15 and NK1.1(+) cells in innate protection of the murine gut and reticuloendothelial system from Salmonella enterica serovar Typhimurium infection. IL-15 and the NK1.1(+) cell population provided innate protection from serovar Typhimurium in mice at the enteric mucosae and in the reticuloendothelial system during murine typhoid. Interestingly, serovar Typhimurium extensively colonized the gut of IL-15(-/-) mice and wild-type C57BL/6 mice depleted of NK1.1(+) cells prior to infection, even though the animals were not pretreated with antibiotics to reduce colonization resistance and there was an absence of overt inflammation in the colon and cecum. Enhanced dissemination of Salmonella from the gut of mice depleted of NK1.1(+) cells correlated with a localized disruption of IL-17 in the colon. These data suggest a relationship between the gut ecosystem and the innate mucosal immune system, which may be linked via IL-15 and NK1.1(+) cells.

The biology of interleukin-2

Much data support an essential role for interleukin (IL)-2 in immune tolerance. This idea is much different from the early paradigm in which IL-2 is central for protective immune responses. This change in thinking occurred when a T regulatory cell defect was shown to be responsible for the lethal autoimmunity associated with IL-2/IL-2R deficiency. This realization allowed investigators to explore immune responses in IL-2-nonresponsive mice rendered autoimmune-free. Such studies established that IL-2 sometimes contributes to optimal primary immune responses, but it is not mandatory. Emerging findings, however, suggest an essential role for IL-2 in immune memory. Here, the current understanding of the dual role of IL-2 in maintaining tolerance and contributing to immunity in vivo is reviewed with some emphasis on T regulatory cell production and homeostasis. Also discussed are implications of this new appreciation concerning the immunobiology of IL-2 with respect to targeting IL-2 or its receptor in immunotherapy.

Effects of interleukin-15 on antifungal responses of human polymorphonuclear leukocytes against Fusarium spp. and Scedosporium spp

Fusarium spp. and Scedosporium spp. have emerged as important fungal pathogens that are frequently resistant to antifungal compounds. We investigated the effects of human interleukin-15 (IL-15) on human polymorphonuclear leukocyte (PMNL) activity against Fusarium solani and Fusarium oxysporum as well as Scedosporium prolificans and Scedosporium apiospermum. IL-15 (100 ng/ml) significantly enhanced PMNL-induced hyphal damage of both Fusarium spp. and S. prolificans after incubation for 22 h (P < 0.01) but not S. apiospermum. In addition, IL-15 enhanced PMNL oxidative respiratory burst evaluated as superoxide anion production in response to S. prolificans but not to the other fungi after 2 h incubation. IL-15 increased interleukin-8 (IL-8) release from PMNLs challenged with hyphae of F. solani and S. prolificans (P< or = 0.04). Release of tumor necrosis factor-alpha was not affected. The species-dependent enhancement of hyphal damage and induction of IL-8 release suggest that IL-15 plays an important role in the immunomodulation of host response to these emerging fungal pathogens.

Tolerance, not immunity, crucially depends on IL-2

Interleukin-2 (IL-2) was identified based on its potent T-cell growth-factor activity and is widely considered to be a key cytokine in T-cell-dependent immune responses. However, the main non-redundant activity of this cytokine centres on the regulation of T-cell tolerance, and recent studies indicate that a failure in the production of CD4(+)CD25(+) regulatory T cells is the underlying cause of autoimmunity in the absence of IL-2. In marked contrast to the importance of IL-2 in peripheral T-cell tolerance, T-cell immunity is readily elicited to various agents in the absence of IL-2 in vivo. Here, we discuss these findings and, in particular, the action of IL-2 on regulatory T cells and effector cells, and the targeting of IL-2 and/or the IL-2 receptor in clinical settings.

Efficient induction of primary and secondary T cell-dependent immune responses in vivo in the absence of functional IL-2 and IL-15 receptors

IL-2 and IL-15 are thought to be important cytokines for T cell-dependent immune responses. Mice deficient in IL-2, IL-2Ralpha, and IL-2Rbeta are each characterized by a rapid lethal autoimmune lymphoproliferative disorder that complicates their use in studies aimed at investigating the role of these cytokines and receptors for immune responses in vivo. We have previously characterized a novel transgenic (Tg) mouse on the IL-2Rbeta-/- genetic background (Tg-/- mice) that lacks autoimmune disease but still contains peripheral T cells that are nonresponsive to IL-2 and IL-15. In the present study, these mice were used to investigate the extent by which IL-2 and IL-15 are essential for T cell immunity in vivo. Tg-/- mice generated near normal primary and secondary Ab responses to OVA, readily mounted first and second set allogeneic skin graft rejection responses, and developed primary and recall CD8 T cell responses to vaccinia virus. However, Tg-/- mice generated a slightly lower level of IgG2a Abs to OVA, exhibited a somewhat delayed first set skin graft rejection response with lower allo-specific CTL, and developed a significantly lower number of IFN-gamma-producing vaccinia-specific CD8+ T cells. Thus, although T effector function is somewhat impaired, T cell immunity is largely functional in the absence of IL-2- and IL-15-induced signaling through IL-2Rbeta.