Practical intravital two-photon microscopy for immunological research: faster, brighter, deeper.

With its emphasis on minimally invasive high-speed imaging of intact tissues at depth, video-rate two-photon microscopy has revolutionized cell biology. This is particularly true in immunology, where the orchestration of cell migration, cell-cell interactions and intracellular signalling events in multiple distinct anatomical compartments within secondary lymphoid organs is fundamental for achieving an effective immune response. Until recently, access to this powerful tool has been limited to a handful of laboratories with the necessary skills and resources to either custom-build or purchase a commercial two-photon microscope. However, with the entry of more commercial vendors into the market and availability of turnkey solutions, two-photon microscopy is now becoming more accessible. Here, we discuss the practical aspects of establishing a basic intravital two-photon microscopy facility specifically for immunological research and how recent advances in ultrafast lasers, non-linear optics and localized photochemistry can be used to build more sophisticated instruments to support applications such as photoactivation and photobleaching, spectral fingerprinting and automated single-cell tracking. In addition, we discuss the next generation of fluorescent dyes and reporter mice and some of the microsurgical principles required to expose the relevant biology to interrogation by two-photon excitation.