Peripheral injections of Freund's adjuvant in mice provoke leakage of serum proteins through the blood-brain barrier without inducing reactive gliosis

Breakdown of the blood-brain barrier (BBB) and ensuing gliosis are common events following physical trauma to the central nervous system (CNS) or during autoimmune diseases such as experimental allergic encephalomyelitis (EAE). Some studies of EAE in rodents report that peripheral injections of complete Freund's adjuvant (CFA), which contains heat-inactivated Mycobacterium to provoke peripheral inflammation without adversely affecting the CNS, can itself lead to increased BBB permeability to small tracer molecules and certain serum proteins. To study the equivocal relationship between serum protein extravasation and reactive gliosis, we injected C57BL/6 mice with CFA and histologically assessed the permeability of various serum proteins and the reactivity of proximal microglia and astrocytes in the uninjured brainstem and spinal cord enlargements after 1-4 weeks. Our results confirm that CFA injections induce progressive increases in the perivascular extravasation of serum IgG, albumin, IgM, and exogenous horseradish peroxidase, all to varying degrees, most prominently in the brainstem and cervical spinal cord after 2-3 weeks. More importantly, neither microglial cells nor astrocytes in regions of focal serum protein leakage appeared morphologically reactive based on immunoreactivity for CR3 receptors (Mac-1) or glial fibrillary acidic protein (GFAP), respectively. Because we found no evidence of T cell infiltration accompanying the exudates, our results indicate that in the absence of physical trauma or inflammatory cells resident CNS neuroglia remain quiescent upon exposure to extravasated serum proteins.