The molecular mechanisms that control the immune response to the spirochete B. burgdorferi
are not completely understood. In spite of extensive work showing that the cell-mediated Th1 response induced by B. burgdorferi
is responsible for the pathology associated with infection, little is known about the signal transduction pathways responsible for this response. p38 MAP kinase activity is involved in inflammation elicited during infection with B. burgdorferi
). Thus, mice that are deficient in the specific upstream activator of p38 MAP kinase, MKK3, developed a decreased Th1 response and reduced arthritis. MKK3-deficient effector CD4+
T cells produced lower levels of IFN-γ, the hallmark cytokine produced by Th1 cells, in the presence of MKK3+/+
). The role of p38 MAP kinase in the differentiation and effector function of CD4+
T cells remains unresolved. The previous studies were performed with in vitro differentiated CD4+
T cells from genetically modified mice or cell clones. Our results demonstrate for the first time that p38 MAP kinase activation in response to TCR- and IL-12-induced signals mediates the production of the Th1 cytokine in antigen-specific in vivo differentiated CD4+
T cells in a murine model of infection with B. burgdorferi
We show that the inhibition of p38 MAP kinase during infection with B. burgdorferi did not affect the ability of ex vivo restimulated cells to produce IFN-γ in conditions in which the kinase is not inhibited, suggesting that this pathway is not involved in the differentiation of CD4+ T cells into B. burgdorferi-specific Th1 effector cells. In contrast, the levels of IFN-γ in the sera of the infected, SB203580-treated mice were consistently reduced. The specific inhibition of p38 MAP kinase in T cells also resulted in lower serum levels of IFN-γ, indicating that this pathway regulates the production of the cytokine in T cells. These results strongly suggest that the inhibition of p38 MAP kinase activity in vivo during infection with the spirochete affected the effector function of antigen-specific CD4+ T cells rather than their differentiation, which resulted in lower levels of serum IFN-γ. Indeed, the restimulation in the presence of the inhibitor in response to specific antigen or anti-CD3 resulted in reduced levels of IFN-γ; an effect that was also mediated by the blockade of IL-12 induced signals that enhanced TCR-mediated induction of the Th1 cytokine.
It is likely that the effect of the inhibitor in vivo during the course of infection also affected other cell types, including cells that may produce IL-12 in response to B. burgdorferi
antigens. Since p38 MAP kinase is involved in IL-12 production by phagocytic cells (18
), it is conceivable that the presence of SB203580 during infection with the spirochete affected the production of the cytokine by antigen-presenting cells, as well as the response of antigen-specific T cells to the cytokine. However, the presence of lower IFN-γ levels in the sera of the dnp38 transgenic mice infected with B. burgdorferi
strongly suggests that Th1 p38 MAP kinase activation is responsible for the production of the cytokine. Moreover, in these mice the serum levels of IL-12 were also consistently reduced. The interaction of IFN-γ with phagocytic cells induces their activation and dramatically increases their phagocytic capacity and their ability to produce proinflammatory cytokines in response to specific ligands, such as B. burgdorferi
antigens (Olson and Anguita, unpublished observations), underscoring the importance of p38 MAP kinase activity on Th1 cells not only for the production of IFN-γ but also for the effector function of this cytokine in other cell types, including phagocytic cells.
The modulation of proinflammatory cytokine production in response to B. burgdorferi
has profound consequences for the ability of the spirochete to survive in the mammalian host and cause inflammation. The specific role of individual factors is still controversial. Conflicting findings regarding the development of murine inflammation are probably the result of several factors, including the route of infection and the identification of the pathological consequences of infection, such as arthritis. Indeed, three different reports on the pathology associated with the deficiency in the Toll-like receptor-mediated signaling molecule, MyD88, described contradictory results (7
). Moreover, a previously unrecognized role of this adaptor protein in IFN-γ-mediated responses in innate immune cells (25
) underscores the complexity of the signaling pathways that are involved in the response to the spirochete. These results therefore highlight the importance of a full understanding of the immune response associated with infection with B. burgdorferi
. The data presented here demonstrate the regulation of IFN-γ production by B. burgdorferi
-specific effector T cells mediated by p38 MAP kinase. Due to the effects of this cytokine on innate immune function, such as the activation of phagocytic cells, our findings shed light not only on the acquired immune response to the bacterium but also on the role of downstream targets of the T-cell cytokine, including phagocytic responses.