In vivo mechanical properties of leukocytes during adhesion to venular endothelium

Transient deformations of leukocytes (WBCs) were studied during their saltation along post-capillary venous endothelium (EC) in mesentery of the rat. During intermittent adhesion of WBCs to EC, prevailing fluid shear stresses, τ(wall), resulted in a stepwise loading of the WBC upon attachment with a transient increase in length, L(t), and reduction of height, H(t). Measurements of L(t) and H(t) from frame-by-frame analysis of video recordings were modelled as the simple shear of a standard linear viscoelastic solid to facilitate calculation of the elastic (k₁,k₂) and viscous (μ) elements with k₁ in parallel with serial elements k₂ and μ. The magnitude of τ(wall) was determined from measurements of red cell velocity within the venule. During the spontaneous adhesion of WBCs, a value of cell viscosity (μ) of 45 Poise was determined. Stimulating adhesion by topical application of the chemoattractant FMLP resulted in a 15-fold increase of μ to 668 Poise. Transient deformations during topical application of cytochalesin B to disrupt actin fibers within the WBC, yielded a 40% reduction in k₁, compared to an 80% reduction with colchicine which disrupts the microtubule structure. Thus, colchicine treated cells appear to be twice as deformable as cells treated with cytochalesin. During adhesion stimulated by the cytokine Interleukin-1, μ increased 50% without changes in k₁ and k₂, possibly dur to slight activation of the WBC.