Trafficking of natural killer cells

Natural killer (NK) cells comprise a set of lymphocytes that is capable of mediating innate immune responses to viral infections, malignancies, and allogeneic bone marrow grafts. This review summarizes what is known about the mechanisms NK cells use to arrive at their sites of action. NK cells express a wide array of adhesion molecules including α_Lβ₂, α_Mβ ₂, α_Xβ₂, and α ₄β₁ integrins, ICAM-1, PSGL-1, and L-selectin. Like other immune and inflammatory cells, NK cells use the blood circulation to enter tissues and organs, which requires that they interact with the vessel wall under flow conditions, arrest, and transmigrate. NK cells are able to chemotax to a variety of cytokines and chemokines, including IL-12, IFN-α/β, CCL2, 3, 4, 5, 7, 8, CXCL8, and CX₃CL1. In many cases, NK cells appear to migrate towards these soluble factors without any kind of priming. These cells also appear to distribute in secondary and tertiary lymphoid sites (i.e., spleen, bone marrow, liver, lung, and lymph nodes) both with and without stimulation. In addition to their ability to move throughout the body in an unprimed state, activated NK cells may have increased specificity in homing to sites of inflammation. NK cells not only react to, but also proauce IFN-γ, TNF-α, GM-CSF, CCL3, CCL4, and CCL5, enabling them to recruit various immune cells to sites of immune response.