Illuminating intranodal natural killer cell behaviour using two-photon microscopy

NK cell development in the bone marrow proceeds from the initial commitment of a hematopoietic stem cell to the lymphoid cell lineage.Two-photon imaging to investigate fundamental immune processes as they occur in real time has revealed the choreography of cell movement and interactions that take place hidden from view inside lymphoid organs. This technique is now providing immunologists with the ideal tool for characterizing, at single-cell resolution in real time, leukocyte motility, migration patterns, and cell interaction dynamics within the native lymphoid-tissue environment. In this chapter we review advances in the field of natural killer (NK) cell imaging, with particular attention to a comparison of steady-state and activating conditions. The basic characterization of NK cell behavior using two-photon microscopy has contributed fundamental information about their migration, distribution pattern, motility, cellular interaction dynamics, and cytolytic effector function. These insights now provide a framework for understanding the role of intranodal NK cells in eliminating MHC-mismatched targets and establish the feasibility of functional immunoimaging approaches to investigate the molecular mechanism(s) that underlie NK cell-mediated immunosurvellience. Further NK cell-specific differentiation is defined by the acquisition of specific markers that qualitatively distinguish each stage of NK cell development. CD122 expression marks the transition from the bipotent T/NK progenitor to the NK cell progenitor stage. CD122 (IL-2R/15Rβ) trimerizes with IL-15Rα and common gamma chain (γc or CD132) to form the functional IL-15 receptor. A critical role of IL-15 signalling in promoting NK cell differentiation and survival is underscored by the dramatic defect in mature NK cells in IL-15-deficient animals. Although NK cell progenitors lack cytolytic activity at this stage in development, ∼10% of the cells express the activating receptor NKG2D. NK cell progenitor progression to the mature NK cell stage is characterized by further phenotypic and functional changes.