Differing Effects of Interleukin-10 on Cutaneous and Pulmonary Francisella tularensis Live Vaccine Strain Infection

We investigated the role of interleukin-10 (IL-10) in cutaneous and pulmonary infection with Francisella tularensis. We found that after intradermal challenge of mice with the live vaccine strain (LVS) of F. tularensis, splenic IL-10 levels increased rapidly and reached a peak 5 days after infection. However, IL-10 expression after infection was detrimental, since IL-10^−/− mice showed increased bacterial clearance and were resistant to an infectious dose (>10⁶ CFU/mouse) that was uniformly lethal for IL-10^+/+ mice. Furthermore, IL-10^+/+ mice treated with neutralizing anti-IL-10R monoclonal antibody were able to survive lethal cutaneous LVS challenge. The presence of IL-10 appeared to restrain the expression of IL-17, since high levels of splenic IL-17 were observed after intradermal LVS infection only in IL-10^−/− mice. Furthermore, treatment with neutralizing anti-IL-17R antibody ablated the enhanced survival observed in IL-10^−/− mice. However, neutralization of IL-10 activity in IL-17R^−/− mice failed to provide protection. Thus, IL-10 suppresses a protective IL-17 response that is necessary for resistance to cutaneous LVS infection. Surprisingly, however, IL-10^−/− mice were significantly more susceptible to pulmonary infection with LVS. Finally, although IL-10 is a critical and novel regulator of immunity to F. tularensis LVS infection, its effects were masked during infection with the highly virulent SchuS4 strain. Taken together, these findings suggest that differentially regulating expression of the IL-10 pathway in various tissues could ultimately have prophylactic and therapeutic benefits for protection against tularemia.