Gadofluorine M enhanced MRI reveals circumventricular organ involvement in CNS inflammation and facilitates occult lesion detection

Background: The central nervous system (CNS) may no longer be considered immune privileged but rather a site of selective immune activity. Although the bloodbrain barrier (BBB) covers most parts of the CNS, certain brain regions are devoid of it and are, therefore, in permanent contact to blood-born molecules and cells. These “exposed” areas include the choroid plexus and other small structures, which line the cavity of the third and of the fourth ventricle, and are known as circumventricular organs (CVO). CVO are characterized by a very dense capillary network with wide perivascular areas and provide an access route for immune cells into the brain parenchyma. Thus CVO might guide CNS immune surveillance. However, until recently no reliable method had been available to survey CVO in vivo. Assuming a crucial function as CNS “gate” for immune cells, the visualization of alterations in CVO might become of additional diagnostic and therapeutic value for the assessment of neuroinflammatory conditions. Figure legend: A Gf enhancement of the choroid plexus (arrows), the subfornicular organ (square), the organum vasculosum of the lamina terminalis (ovoid), the median eminance (arrowhead) as well as the area postrema (circle) are clearly marked in the EAE mice (column 1) and, much less pronounced, in naïve control mice (column 2). B Gf enhancement was quantitatively assessed computing mean intensity ratio of the CVO in T1-weighted images 24 hours after Gf application. The comparison between EAE and naïve control animals was performed using unpaired, two-tailed t-tests. Gf enhancement in EAE mice was significantly higher in the choroid plexus (*: p = 0,021), the subfornicular organ (*: p = 0,018) and the area postrema (++: p = 0,002 with Welch ́s correction for significantly different variances). C The signal intensity (mean intensity ratio) of the subfornicular organ (SFO) (1) and the area postrema (AP) (2) on Gf enhanced MRI 24 hours post injection correlated to the corresponding EAE scores at the time of scanning. Additionally, the time needed from T cell transfer to disease onset correlated inversely with the mean intensity ratio of the area postrema (3). Each dot depicts the data point of one mouse. D Visualization of optic neuritis (square) was markedly improved by Gf, since neighboring intravascular signal prohibited the unambiguous determination on Gd enhanced images (arrow). E Pericallosal inflammatory focus (B1) and corresponding H&E stained (A2,) and fluorescence microscopy (A3; red: Gf, blue: Hoechst 33258) slices depict typical inflammatory plaques, including cellular infiltrations (arrows), diffusely and halo-like surrounded by Gf. After 72 hours, Gf uptake into macrophages/microglia was evident (B2). In the choroid plexus (B1), internalized Gf became visible in numerous IBA-1 positive cells (B2; red: Gf; green: IBA-1; blue: Hoechst 33258).