Spread of malignant lymphoid cells into rat central nervous system with intact and disrupted blood-brain barrier

The pathways of spread of malignant lymphoid cells into the central nervous system (CNS) were studied using a T lymphoblastic leukaemia/lymphoma model of inbred PVG rats. The effects of intraperitoneal, intracarotid, intravenous, intrathecal and intracerebral routes of transplantation were analysed, and the significance of the blood-brain barrier (BBB) in preventing neoplastic cell invasion was studied by disrupting the BBB with focal cold injury. Extraneurally transplanted cells appeared first in the dura and subarachnoid space. From the latter they spread further into the perivascular space of penetrating cortical vessels. Parenchymal tumour cell foci were only seen in terminally ill rats, usually associated with damage to the vessel wall. Intrathecal transplantation did not accelerate the progression of the disease. Intracerebrally transplanted cells readily produced parenchymal infiltrates with diffuse invasion into the white matter, perivascular spreading into the cortex, and contralateral extension along the corpus callosum. Parenchymal invasion did not occur immediately after disruption of the BBB, but in the chronic phase neoplastic cells infiltrated the injured area. In conclusion, the model closely resembles human CNS leukaemia. Malignant cells appeared to enter the CNS through the deficient BBB of the subarachnoid vessels, whereas the BBB of the intracerebral vessels and perivascular glia limitans were very resistant to leukaemic cell invasion. This underlines the difference between the subarachnoid and perivascular v. intraparenchymal compartments. Preceding BBB damage may predispose to brain metastases. The parenchymal dissemination of malignant cells was similar to that in primary CNS lymphoma and it followed the same spreading pathways as the extracellular fluid.