The American Journal Of Pathology
Proinflammatory circulating monocytes have important roles in the pathology of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). Yet there is limited information on their accumulation in blood during disease, the mechanisms that regulate their infiltration into the central nervous system (CNS), and whether medications affect their biology. We found a significant and prolonged elevation of CD11b(+)CCR2(+)Ly6C(high) proinflammatory monocytes in the blood of mice by the second day of immunization for EAE. At onset of clinical signs, levels of proinflammatory monocytes plummeted to those in naive mice. At day 16, when the majority of mice were at peak disease severity, clinical scores were inversely correlated to the proportion of proinflammatory monocytes in blood, and directly correlated with that in the spinal cord. Treatment with the MS medication laquinimod prevented EAE, correspondent with retention of proinflammatory monocytes in blood. The reduced entry of proinflammatory monocytes into the CNS by laquinimod was attributed to reduction of their levels of CD62L and matrix metalloproteinase-9. Moreover, the spinal cord of laquinimod-treated mice did not have elevated levels of CCR2 and CCL2, which provide chemotactic cues for monocytes. These results shed light on the important role of the trafficking of proinflammatory monocytes into the CNS to promote disease activity, and they identify a mechanism of action of laquinimod in MS.