In the present study, we hypothesized that IL-15 is a primary mediator involved in the induction of arthritis associated with Borrelia vaccination and infection of mice. In support of this hypothesis, we showed that treatment of Borrelia-vaccinated and -challenged mice with anti-IL-15 antibody prevented swelling of the hind paws. In addition, treatment of Borrelia-vaccinated and -infected mice with rIL-15 receptor alpha also prevented hind-paw swelling. More importantly, both anti-IL-15 antibody and rIL-15 receptor alpha-treated Borrelia-vaccinated and -challenged mice were free of major histopathologic indications of arthritis, including hyperplasia, hypertrophy, and vilus formation of the synovium. Similarly, the synovial space and perisynovium were free of inflammatory cells. By contrast, the synovium of non-anti-IL-15-treated Borrelia-vaccinated and -infected mice had overt hyperplasia, hypertrophy, and vilus formation. Moreover, the synovial space and perisynovium were infiltrated with neutrophils, macrophages, and lymphocytes. These results suggest that IL-15 plays a major role in the induction of arthritis associated with Borrelia-vaccinated and -infected mice.
Although IL-15 is involved in the induction of arthritis associated with Borrelia
vaccination and infection, we showed previously that IL-17 is also a major participant in the induction of this arthritis (8
). When Borrelia
-vaccinated and -infected mice were administered anti-IL-17 antibody, development of arthritis was prevented (8
). Likewise, treatment of Borrelia
-vaccinated and -infected mice with anti-IL-17 receptor antibody prevented arthritis, including cartilage and bone destruction (8
). Taken together, these results show that both IL-15 and IL-17 play a significant role in arthritis induced by Borrelia
vaccination and infection of mice.
What is the mechanism by which IL-15 and IL-17 drive the arthritis? Presumably, at the time of infection, some spirochetes are processed by neutrophils (14
) and macrophages (15
), leading to the release of IL-15 from these cells (19
). Since IL-15 is rarely detected in culture supernatants or tissues (4
), it probably binds rapidly to the IL-15 receptor alpha, the IL-2 receptor beta, and the common gamma-chain located especially on memory T cells (4
). The memory cells, or Borrelia
-vaccinated T lymphocytes, then release IL-17 that triggers the production of other proinflammatory cytokines, such as IL-1, IL-6, and IL-8 (3
), that contribute to the development of arthritis. In support of this theory, we showed that rIL-15 caused the release of IL-17 in ex vivo cultures of immune lymph node cells isolated from a site adjacent to the hind paws. Ziolkowska et al. (43
) and Kim et al. (24
) also showed that IL-15 triggers the release of IL-17. Our results, as well as those of Ziolkowska et al. (43
) and Kim et al. (24
), show a definite connection between IL-15 and IL-17. Additional studies are needed to determine if IL-15 directly causes IL-17 production. This linkage is important for the potential therapy of Borrelia
-induced arthritis as well as other inflammatory diseases (1
). Clearly, therapy with anti-IL-15 or anti-IL-17 (8
) antibody benefits vaccinated mice when it is started at the time of challenge with Borrelia
The arthritic mechanism, however, may be more complex than the simple linear stimulation of IL-17 production by IL-15. Histopathologic examination showed that treatment of Borrelia
-vaccinated and -infected mice with anti-IL-15 antibody or rIL-15 receptor alpha dramatically decreased the infiltration of inflammatory cells at the tibiotarsal joint, especially neutrophils. In contrast, neutrophils dominated the cell infiltrate in non-anti-IL-15-treated Borrelia
-vaccinated and -infected mice. It is known that resting neutrophils, along with infiltrating monocytes, express IL-2 receptor beta and IL-2 receptor gamma, which can readily bind high concentrations of IL-15 (43
). We speculate that initially resting neutrophils exposed to the infectious challenge release IL-15. Subsequently, IL-15 binds to these IL-2 receptors as well as the high-affinity IL-15 receptor alpha (23
). Interactions between IL-15 and its receptor complex lead to increased signaling and activation of neutrophils (19
). Further activation of these neutrophils by IL-15 also increases their phagocytic ability (19
). Therefore, IL-15 may have a dual role in the response to infection. It promotes the proinflammatory cytokine cascade via IL-17 production and may oversee the elimination of the initial spirochete challenge. In support of this theory, termination of anti-IL-15 treatment 6 days after challenge was followed immediately by an increase in hind-paw swelling in Borrelia
-vaccinated and -challenged mice. Histological samples from these animals taken 14 days after challenge (8 days after treatment cessation) revealed dramatic infiltration of immune cells as well as vilus formation and destruction of the synovial lining. In addition, others (18
) have shown that IL-15 can affect the migration and phagocytosis of neutrophils in patients with Lyme disease.
Although we showed that treatment of Borrelia
-vaccinated mice at the time of challenge with anti-IL-15 antibody prevented arthritis, treatment with anti-IL-15 antibody failed to ameliorate the arthritis once it was established. Specifically, no resolution of arthritis was detected when Borrelia
-vaccinated and -challenged mice were administered anti-IL-15 antibody 7 days after infection, as opposed to the case for controls. This was disappointing, and it suggests that anti-IL-15 therapy is not suitable for the treatment of established arthritis associated with Borrelia
vaccination and infection. In contrast, Baslund et al. (7
) have recently shown that treatment of humans with an antibody targeting IL-15 benefited patients with longstanding rheumatoid arthritis. This discrepancy may be related to differences in immune responses that maintain chronic arthritis in rheumatoid arthritis and arthritis induced by Borrelia burgdorferi
. Additional studies are under way to determine which mediators could be neutralized to hasten the resolution of Lyme arthritis.
The failure of anti-IL-15 antibody to ameliorate established arthritis does not lessen the cytokine's clinical importance. A major concern with development of a Lyme disease vaccine is the induction of adverse effects, such as arthritis. Once a Lyme disease vaccine has been shown to be protective, it could also be tested for its ability to cause the release of IL-15 or other proinflammatory cytokines from cells (peripheral lymphocytes) obtained from humans or animals exposed to B. burgdorferi. If proinflammatory cytokines are detected, the vaccine could be modified to remove epitopes that promote adverse effects but do not influence the ability of the vaccine to induce protection.
Finally, several investigators (2
) have shown that gamma interferon can up-regulate the production of IL-15. This suggests that gamma interferon may precede IL-15 in the cytokine cascade for the modulation of arthritis associated with Borrelia
vaccination and challenge. However, our studies were performed using gamma interferon-deficient mice. Additional studies are needed to define specific cytokines, Borrelia
antigens, or other immune factors that initiate the production of IL-15. Some of these approaches may provide a new perspective for the treatment of Borrelia
In conclusion, we show that IL-15 plays a major role in the induction of Lyme arthritis. Although rIL-15 treatment of immune cells led to increased production of IL-17 from these cells, additional studies are needed to determine if IL-15 solely promoted this production. Other studies are also needed to determine mediators that could be neutralized to enhance the resolution of established arthritis.