CCL2 and Alzheimer disease: Many scientists and histologists have argued that elevated CCL2 levels lead to increased blood–brain barrier permeability by inducing macrophage recruitment, cytokine production, and directly altering endothelial cell tight junction protein expression, which are observed in a variety of pathological processes, such as neuroinflammation,133 stroke,134 MS,135 and Alzheimer's disease (AD).136