Borrelia burgdorferi is capable of persistently infecting a variety of hosts despite eliciting potent innate and adaptive immune responses. Preliminary studies indicated that IL-10-deficient (IL-10−/−) mice exhibit up to 10-fold greater clearance of B. burgdorferi from target tissues compared with wild-type mice, establishing IL-10 as the only cytokine currently known to have such a significant effect on spirochetal clearance. To further delineate these IL-10-mediated immune effects, kinetic studies indicated that spirochete dissemination to target tissues is similar in both wild-type and IL-10−/− mouse strains, and that enhanced clearance of B. burgdorferi in IL-10−/− mice is correlated with increased B. burgdorferi-specific Ab as early as 2 wk postinfection. Immunoblot analysis indicated that Abs produced by infected IL-10−/− and wild-type mice recognize similar ranges of spirochetal Ags. Immune sera from IL-10−/− and wild-type mice also exhibited similar bactericidal activity in vitro, and passive transfer of these immune sera into B. burgdorferi-infected SCID mice caused similar reductions of bacterial numbers in target tissues. Infectious dose studies indicated that 8-fold more B. burgdorferi were needed to efficiently infect naive IL-10−/− mice, suggesting these animals possess higher innate barriers to infection. Moreover, macrophages derived from IL-10−/− mice exhibit enhanced proinflammatory responses to B. burgdorferi stimulation compared with wild-type controls, and these responses are not significantly affected by the presence of immune serum. These findings confirm that B. burgdorferi clearance by innate immune responses is more efficient in the absence of IL-10, and these activities are not directly related to increased levels of B. burgdorferi-specific Ab.

Although IL-4 and IFN-γ often have opposite effects and suppress each other’s production by T cells, IL-4 can stimulate IFN-γ production. To characterize this, we injected mice with IL-4 and quantified IFN-γ production with the in vivo cytokine capture assay. IL-4 induced Stat6-dependent IFN-γ production by NK and, to a lesser extent, NKT cells, but not conventional T cells, in 2–4 h. Increased IFN-γ production persisted at a constant rate for >24 h, but eventually declined, even with continuing IL-4 stimulation. This eventual decline in IFN-γ production was accompanied by a decrease in NK and T cell numbers. Consistent with a dominant role for NK cells in IL-4-stimulated IFN-γ secretion, IL-4 induction of IFN-γ was B and T cell-independent; suppressed by an anti-IL-2Rβ mAb that eliminates most NK and NKT cells; reduced in Stat4-deficient mice, which have decreased numbers of NK cells; and absent in Rag2/γc-double-deficient mice, which lack T, B, and NK cells. IL-4-induced IFN-γ production was not affected by neutralizing IL-12p40 and was increased by neutralizing IL-2. IL-13, which signals through the type 2 IL-4R and mimics many IL-4 effects, failed to stimulate IFN-γ production and, in most experiments, suppressed basal IFN-γ production. Thus, IL-4, acting through the type 1 IL-4R, induces Stat6-dependent IFN-γ secretion by NK and NKT cells. This explains how IL-4 can contribute to Th1 cytokine-associated immune effector functions and suggests how IL-13 can have stronger proallergic effects than IL-4.