A case of tuberculous meningitis and the role of perivascular spaces in lymph cell migration in the brain.

Meningitis and encephalitis are inflammatory diseases in which acute and chronic inflammatory cells infiltrate leptomeninges, especially the arachnoid, and migrate through the subarachnoid space and by diapedesis, in order to extend around blood vessels and into the brain parenchyma. To what extent migrated/resident inflammatory cells participate to these interactions, or what are exactly the initial steps by which these cells reach the brain interstitium, it is not yet completely known. Recent years have brought new insights into the description of water flow circuits in the brain, suggesting that the cerebrospinal fluid enters the brain within the perivascular spaces of arteries, while interstitial fluid drains along perivascular venous sector. Moreover, it has been showed that vascular basement membranes have a complex multi-layered architecture that originates with epithelial, endothelial, smooth muscle cells and glial cells, and that the virtual space between these layers might be in fact an essential component of these perivascular spaces. Starting from a patient that presented with active pulmonary tuberculosis and with consecutive purulent-hemorrhagic meningitis and encephalitis, we have characterized here the compartments in which immune cells can be found in the brain tissue. Besides the classical histopathological description, what was of interest here, was that we identified for the first time mononucleated inflammatory cells that seemed to be present in pockets of the vascular basement membranes, small spaces devoid of red blood cells. Although this is mere a morphological observation, future high-resolution studies should clarify it this is a possible route for the immune cells entering the brain.