Effect of secretory immunoglobulin A on bacterial translocation in an enterocyte-lymphocyte co-culture model

Intestinal secretory immunoglobulin A (sIgA) plays an important role in gut mucosal immunity in vivo; however, in-vitro enterocyte models for studying the mechanisms of these effects are lacking. This study utilizes a cell-culture model to investigate the effect of sIgA on bacterial translocation (BT) across human enterocytes co-cultured with human lymphoid cells (Raji cells). This model is intended to mimic in-vivo enterocyte/lymphocyte interactions found in intestinal follicle-associated epithelia. Human Caco-2 enterocytes were grown to confluence on porous filters in the apical chamber of a two-chamber cell-culture system. After differentiation, human B lymphoid cells (Raji cells) were added to the basolateral surface of Caco-2 monolayers for 3 days' co-culture, followed by washing away of unincorporated Raji cells. Transepithelial electrical resistance (TEER) was used to measure tight-junction permeability. Monolayers were treated with or without sIgA, IgG (negative control), or mannose (positive control). BT across the cell monolayer was determined 1.5 h after addition of Escherichia coli. Statistical analysis was by the Kruskal-Wallis test, P below 0.05 considered significant. In co-culture monolayers treated with sIgA, IgG, or mannose, there was no significant effect on TEER; however, the magnitude of BT across cells treated with sIgA (1.3 ± 0.4 log₁₀CFU/ml) and mannose (1.6 ± 1.1 log₁₀CFU/ml) was significantly decreased compared to PBS (3.9 ± 0.4 log₁₀CFU/ml) and IgG (2.9 ± 0.6 log₁₀CFU/ml) controls (P < 0.05). sIgA BT inhibition was dose-dependent. BT inhibition by sIgA and mannose was additive (0.5 ± 1 log₁₀CFU/ml). Inhibition of BT was negated when sIgA and mannose were removed by washing prior to E. Coli addition (3.6 ± 0.5 log₁₀CFU/ml), suggesting that both inhibitors act through bacterial binding.