Two-photon microscopy for imaging germinal centers and T follicular helper cells

One of the principle features of immune cells is their dynamic nature. Lymphocytes circulate in the blood between secondary lymphoid organs and tissues in an effort to maximize the likelihood of a rapid and appropriate immune response to invading pathogens and tissue damage. Conventional experimental techniques such as histology and flow cytometry have greatly increased our understanding of immune cells, but in the last decade, two-photon microscopy has revolutionized our ability to interrogate the dynamic behavior of immune cells, a facet so critical to their function. Two-photon microscopy relies on the excitation of fluorophores by simultaneous application of two photons of longer wavelength light. This allows a greater depth of imaging with minimal photodamage. Thus, living tissues can be imaged, including immune cells in lymph nodes. This technique has been used to interrogate the events occurring in a germinal center response and the interactions between cells in the germinal center, including T follicular helper cells (Tfh), germinal center B cells, and follicular dendritic cells (FDC). Herein, a method is described by which the interactions between Tfh and B cells within a germinal center in a popliteal lymph node can be imaged in a live mouse.