The cannabinoid system is known to be important in neuronal regulation, but is also capable of modulating immune function. Although the CNS resident microglial cells have been shown to express the CB₂ subtype of cannabinoid receptor during non‐immune‐mediated pathological conditions, little is known about the expression of the cannabinoid system during immune‐mediated CNS pathology. To examine this question, we measured CB₂ receptor mRNA expression in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) and, by real‐time PCR, found a 100‐fold increase in CB₂ receptor mRNA expression during EAE onset. We next determined whether microglial cells specifically express the CB₂ receptor during EAE, and found that activated microglial cells expressed 10‐fold more CB₂ receptor than microglia in the resting state. To determine the signals required for the up‐regulation of the CB₂ receptor, we cultured microglial cells with combinations of γ‐interferon (IFN‐γ) and granulocyte) macrophage‐colony stimulating factor (GM‐CSF), which both promote microglial cell activation and are expressed in the CNS during EAE, and found that they synergized, resulting in an eight to 10‐fold increase in the CB₂ receptor. We found no difference in the amount of the CB₂ receptor ligand, 2‐arachidonylglycerol (2‐AG), in the spinal cord during EAE. These data demonstrate that microglial cell activation is accompanied by CB₂ receptor up‐regulation, suggesting that this receptor plays an important role in microglial cell function in the CNS during autoimmune‐induced inflammation.

Multiple sclerosis (MS) is a CNS autoimmune disease believed to be triggered by T cells secreting Th1-specific proinflammatory cytokines, such as GM-CSF. In the animal model of MS, experimental autoimmune encephalomyelitis (EAE), Th1 but not Th2 cells have been shown to induce disease; however, to date, no single encephalitogenic T cell-derived cytokine has been shown to be required for EAE onset. Because GM-CSF-deficient mice have been shown to be resistant to EAE following immunization with myelin self-Ag, we investigated the cellular source of the required GM-CSF and found that GM-CSF production by encephalitogenic T cells, but not CNS resident or other peripheral cells, was required for EAE induction. Furthermore, we showed that microglial cell activation, but not peripheral macrophage activation, was a GM-CSF-dependent process. Activation of microglial cells by the injection of LPS abrogated the GM-CSF requirement for EAE induction, suggesting that microglial cell activation is required for EAE onset. These data also demonstrate that GM-CSF is a critical Th1 cell-derived cytokine required for the initiation of CNS inflammation associated with EAE, and likely MS.

Regulation of inflammation in the CNS is essential to prevent irreversible cellular damage that can occur in neurodegenerative diseases such as multiple sclerosis (MS). We investigated the role of interleukin-4 (IL-4) in regulating CNS inflammation using the animal model of MS, experimental autoimmune encephalomyelitis (EAE). We found that CNS-derived IL-4 was a critical regulator because mice with a deficiency in IL-4 production in the CNS, but not the periphery, had exacerbated EAE associated with a significant increase in the absolute number of infiltrating inflammatory cells. We also found that CNS-resident microglial cells in both the resting and activated state produced the protein Ym1, which is a marker of alternatively activated macrophages (aaMΦs), in an IL-4-dependent manner. This aaMΦ phenotype extended to the lack of nitric oxide (NO) production by activated microglial cells, which is a marker of classically activated macrophages. We also show that IL-4 induced the expression of Ym1 in peripheral infiltrating macrophages, which also produce NO. Thus, macrophages that migrate into the CNS exhibit a dual phenotype. These data indicate that IL-4 production in the CNS is essential for controlling autoimmune inflammation by inducing a microglial cell aaMΦ phenotype. Macrophages that have undergone alternative activation have been shown to be important in tissue repair; thus, our results suggest a new role for microglial cells in the regulation of inflammation in the CNS.