The effect of human serum and some of its components on the process of transepithelial migration of human neutrophils was investigated in an in vitro system. 10% autologous serum caused an increase in neutrophil adherence to and migration across canine kidney epithelial cells. This increase in neutrophil binding also occurred if the epithelium but not the neutrophils had been preincubated with serum. The binding was lost if the serum was either preabsorbed over the kidney epithelium before use or heat inactivated. Indirect immunofluorescence studies indicated that IgG, IgM, and a component of C3 bound to the epithelial surface, whereas IgA, IgE, or C5a were not detectable. The majority of epithelial cells were immunofluorescent, however epithelial cells with varying degrees of reactivity were also apparent and approximately 5% of the epithelial cells did not bind IgG, IgM, and C3. When epithelia were simultaneously tested for the presence of either IgG, IgM, or C3, and bound neutrophils the few epithelial cells which did not bind IgG or IgM also did not bind C3 or neutrophils. Studies with monoclonal antibodies against Fc and C3 receptors indicate that neutrophil adherence to the epithelial surface was mediated predominately by the receptors for C3b and C3bi. In response to a chemotactic gradient, bound neutrophils were able to detach and migrate across the epithelium. A separate heat-stable factor(s) in serum was able to increase neutrophil migration across the epithelial monolayer. This factor acted independently of the factors which caused the increase in neutrophil binding as the increase in neutrophil migration also occurred under conditions (preabsorption over the kidney epithelium or heat inactivation) that prevented the increase in neutrophil binding. The increase in neutrophil migration may be caused by the permeability-increasing properties of this factor as both serum and heat-inactivated serum lowered the transepithelial electrical resistance an average of 38 and 35%, respectively, in 40 min. Upon removal of serum or heat-inactivated serum, the resistance returned 100 and 81%, respectively, in 5 h.

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