Neuronal CCL2 expression drives inflammatory monocyte infiltration into the brain during acute virus infection

Charles L Howe; Reghann G. LaFrance-Corey; Emma N Goddery; Kanish Mirchia

Given the hippocampal enrichment for CCL2 production at 6 hpi we sought to identify the relevant cellular locus ( Figure 2D -I). To our surprise, immunostaining for CCL2 in uninfected mice revealed weak fluorescence associated with the pyramidal neurons of CA1, CA3, and the dentate gyrus (DG) ( Figure 2D ). At higher magnification this signal, not present in the secondary-only controls (not shown), exhibited a diffuse pattern associated with the cell layers ( Figure 2G ). At 6 hpi we consistently observed a robust increase in expression of CCL2 in pyramidal neuron cell bodies in the CA1, CA3, and DG regions ( Figure 2E ). In addition, there was a clear increase in immunofluorescence in the terminal networks of the apical dendrites located in the stratum lacunosum moleculare ( Figure 2E , 2H). This observation is notable because we have previously reported that the hippocampal fissure, located just below the stratum lacunosum moleculare, is a primary site for infiltration of inflammatory monocytes [4]. Finally, the overall signal intensity faded by 24 hpi, though the pyramidal neuron layerassociated signal was more punctate and brighter than the earlier diffuse pattern ( Figure 2F , 2I). Whether some of this signal was associated with infiltrating monocytes was not resolved in this experiment. These observations suggest that the 6 hpi peak in hippocampal CCL2 measured by ELISA ( Figure 2C ) was the result of production by hippocampal neurons.