Leukocytes roll on blood vessel walls

Leukocytes in the circulatory system have the remarkable ability to migrate to particular areas of the body. They adhere to the endothelium, or inner wall, of post-capillary venules before entering the appropriate tissues. During the first stage of this process, leukocytes roll along the vascular endothelium. Rolling adhesion is mediated by selectin adhesion: weak, transitory interactions between selectins, proteins on the vascular endothelium, and oligosaccharides on the surface of the leukocytes. Selectins are only present in post-capillary blood vessels: primarily in venules and rarely in larger veins.

Physiological variables, such as shear stress, affect the ability of leukocytes to engage in rolling adhesion. Shear stress is applied along the endothelial surface by blood flow and is measured as force per unit area. It is applied in the same direction as blood flow. Generally, higher numbers of leukocytes are able to adhere at lower levels of shear stress. In vivo shear stress can be calculated by the following equation:

where $\tau$‍ is shear stress, $\mu$‍ is blood viscosity, $v$‍ is mean blood velocity, and $d$‍ is blood vessel diameter. Shear stress levels vary throughout the circulatory system.

Figure 1: Shear Stress Values in Various Vessels

A team of researchers explored the function of a CD-62, a selectin involved in rolling adhesion, at varying levels of shear stress. They utilized a flow chamber in which leukocytes rolled along a flat, artificial endothelium containing CD-62 at varying densities.

Figure 2: Leukocyte Rolling Velocity as a Function of CD-62 Density and Shear Stress. Rolling velocity values represent means of adherent cell rolling velocities. CD-62 were incorporated into the artificial endothelium at the indicated density (50, 200, or 400) in sites$\cdot$‍μm${}^{-2}$‍. Nonadherent cell velocities were too fast (>1700 μm$\cdot$‍s${}^{-1}$‍) to be accurately measured.

Which of the following physiological changes can be inferred to increase the number of leukocytes that are able to adhere to the vascular endothelium?