The blood-brain barrier (BBB) is compromised during progressive HIV-1 infection, but how this occurs is incompletely understood. We studied the integrity of tight junctions ( IntroductionHIV-1-associated dementia (HAD) is characterized by cognitive, behavioral, and motor abnormalities affecting up to 11% of infected individuals in the era of highly active antiretroviral therapy. 1 Clinical disease is often correlated with HIV-1 encephalitis (HIVE) and characterized by monocyte brain infiltration, productive infection of brain macrophages and microglia, giant cell formation, myelin pallor, astrogliosis, and neuronal injury. 2 The best histopathologic correlate of HAD is the number of inflammatory macrophages that accumulate in affected brain tissue. 3 This concept is further supported by more recent data demonstrating the importance of perivascular macrophages as viral reservoirs and perpetrators of disease. 4,5 It is now widely accepted that HAD neuronal dysfunction and death are caused by monocyte/ macrophage secretory products and viral proteins. [6][7][8][9][10][11][12][13] These observations strongly suggest that monocyte migration across the blood-brain barrier (BBB) is a pivotal event in disease.BBB compromise is associated with HAD. Examination of HIVE brain tissue reveals that expression of tight junctions ([TJs] providing structural integrity) decreases on brain microvascular endothelial cells (BMVECs). 14,15 HIV-1 patients exhibit signs of BBB compromise by neuroimaging studies. 16,17 Structurally, the BBB is composed of specialized nonfenestrated BMVECs connected by TJs in an impermeable monolayer devoid of transcellular pores. 18 TJs are composed of claudins and occludin (integral membrane proteins) and intracellular proteins, zonula occludens (ZO-1, ZO-2, ZO-3). 19 TJs formed by BMVECs maintain the structural integrity of the BBB, limiting paracellular passage of molecules and cells into the brain. Formation of TJs depends on the expression of high levels of occludin and claudin-5 and intracellular signaling processes that control phosphorylation of junctional proteins. 19,20 A recent study demonstrated that claudin-5 is a critical determinant of BBB permeability in mice. 21 The functional significance of occludin as compared with claudin-5 at TJs is not clear. Although claudin-5 is now considered to be the most important TJ protein, it is also expressed on endothelium of less tight barriers while occludin is detected principally in brain endothelial cells with TJs. 22 TJs are dynamic structures that readily adapt to a variety of physiologic or pathologic circumstances. 23 However, the precise mechanism(s) through which they operate is still unclear. It is widely accepted that F-actin filaments found at the TJ participate in TJ regulation, 24 and actin may be linked to occludin/claudins through ZO proteins. 25,26 While significant progress has been made in identification of the molecular mechanisms that attract leukocytes from the blood and promote their arrest on the vessel wall, less is known a...

Therapeutic strategies following spinal cord injury must address the multiple barriers that limit regeneration. Multiple channel bridges have been developed that stabilize the injury following implantation and provide physical guidance for regenerating axons. These bridges have now been employed as a vehicle for localized delivery of lentivirus. Implantation of lentivirus loaded multiple channel bridges produced transgene expression that persisted for at least 4 weeks. Expression was maximal at the implant at the earliest time point, and decreased with increasing time of implantation, as well as rostral and caudal to the bridge. Immunohistochemical staining indicated transduction of macrophages, Schwann cells, fibroblasts, and astrocytes within the bridge and adjacent tissue. Subsequently, the delivery of lentivirus encoding the neurotrophic factors NT3 or BDNF significantly increased the extent of axonal growth into the bridge relative to empty scaffolds. In addition to promoting axon growth, the induced expression of neurotrophic factors led to myelination of axons within the channels of the bridge, where the number of myelinated axons was significantly enhanced relative to control. Combining gene delivery with biomaterials to provide physical guidance and create a permissive environment can provide a platform to enhance axonal growth and promote regeneration.

The regeneration of tissues with complex architectures requires strategies that promote the appropriate cellular processes, and can direct their organization. Plasmid-loaded multiple channel bridges were engineered for spinal cord regeneration with the ability to support and direct cellular processes and promote gene transfer at the injury site. The bridges were manufactured with a gas foaming technique, and had multiple channels with controllable diameter and encapsulated plasmid. Initial studies investigating bridge implantation subcutaneously (SC) indicated transgene expression in vivo for 44 days, with gene expression dependent upon the pore size of the bridge. In the rat spinal cord, bridges implanted into a lateral hemisection supported substantial cell infiltration, aligned cells within the channels, axon growth across the channels, and high levels of transgene expression at the implant site with decreasing levels rostral and caudal. Immunohistochemistry revealed that the transfected cells at the implant site were present in both the pores and channels of the bridge and were mainly identified as Schwann cells, fibroblasts, and macrophages, in descending order of transfection. This synergy between gene delivery and the scaffold architecture may enable the engineering of tissues with complex architectures.

Bridges for treatment of the injured spinal cord must stabilize the injury site to prevent secondary damage and create a permissive environment that promotes regeneration. The host response to the bridge is central to creating a permissive environment, as the cell types that respond to the injury have the potential to secrete both stimulatory and inhibitory factors. We investigated multiple channel bridges for spinal cord regeneration and correlated the bridge structure to cell infiltration and axonal elongation. Poly(lactide-co-glycolide) bridges were fabricated by a gas foaming=particulate leaching process. Channels within the bridge had diameters of 150 or 250 mm, and the main body of the bridge was highly porous with a controllable pore size. Upon implantation in a rat spinal cord hemisection site, cells infiltrated into the bridge pores and channels, with the pore size influencing the rate of infiltration. The pores had significant cell infiltration, including fibroblasts, macrophages, S-100b-positive cells, and endothelial cells. The channels of the bridge were completely infiltrated with cells, which had aligned axially, and consisted primarily of fibroblasts, S-100b-positive cells, and endothelial cells. Reactive astrocytes were observed primarily outside of the bridge, and staining for chondroitin sulfate proteoglycans was decreased in the region surrounding the bridge relative to studies without bridges. Neurofilament staining revealed a preferential growth of the neural fibers within the bridge channels relative to the pores. Multiple channel bridges capable of supporting cellular infiltration, creating a permissive environment, and directing the growth of neural fibers have potential for promoting and directing spinal cord regeneration.