Human peripheral blood monocytes form a low permeability barrier in culture. (A) CD14⁺ human peripheral blood monocytes cultured on Transwells form a high-resistance barrier similar to mouse brain microvascular endothelial cells (MBECs), while CD14-depleted human peripheral blood mononuclear cells do not. HUVECs and HAECs do not develop a high transcellular resistance under the same culture conditions. (n = 3-4 for each cell type) P < .05 (CD14⁺, □; MBEC, ▴; HAEC, ○; HUVEC, ▪; and CD14-depleted, ×). (B-C) The permeability of ELMCs to FITC-labeled 4-kDa (B) and 20-kDa (C) dextran is lower than the permeability of HUVECs and similar to the permeability of MBECs. Cells were cultured on transwell inserts and the movement of FITC-labeled 4-kDa and 20-kDa dextrans through the cell layer was monitored and used to calculate permeability coefficients (HUVEC, ▵; ELMC, •; and MBEC, □) (n = 3). The permeability surface product (PS) was determined using the equation ΔCl/Δt where ΔCl is the incremental clearance volume and Δt is time. The permeability coefficient (P) was calculated using the equation PS/S=P where S is the filter surface area. To correct for the contribution of the filter, P was also determined for a fibronectin-coated filter and the permeability coefficient for the cell layer alone was calculated using the equation 1/Pc = 1/Pt - 1/Pf where Pc is the permeability of the cell layer, Pt is the permeability of the total system, and Pf is the permeability of the filter. The permeability of ELMCs for 4-kDa dextran (Pc = 3.8 ± 1.1 × 10^-6 cm/s) was significantly lower than the permeability of HUVECs for 4-kDa dextran (Pc = 5.5 ± 1.3 × 10^-6 cm/s; P < .05). This was also true for 20-kDa dextran (ELMC Pc = 2.23 ± 0.1 × 10^-6 cm/s vs HUVEC Pc = 3.8 × 0.4 × 10^-6 cm/s; P < .05). This experiment was performed twice with similar results. Data are expressed as mean ± SEM.