In vivo imaging of the T cell response to infection

The induction and execution of a successful anti-pathogen immune response requires a consecutive series of cellular interactions that begin in lymphoid environments and later extend into the periphery. Much of our current knowledge of these events has been gained using ex vivo approaches that yield important static information but do not convey the dynamic nature of cellular behavior in vivo. The application of multiphoton-laser based microscopic analysis to the ongoing immune response has provided new insight into cellular interactions leading to T cell activation and the behavior of primed immune effectors. Here we discuss recent insights on anti-pathogen immune responses revealed using live imaging of both lymphoid and non-lymphoid tissues.

Introduction

The priming of robust T cell responses begins in the lymph node draining the site of infection, or in the spleen if the pathogen gains entry to the blood. Pathogens that penetrate the epithelial barrier are rapidly transported to lymph nodes, either as a lymph-borne particulate or via cells that capture the pathogen and migrate to the lymph node. While the first form of transit can occur very rapidly (within seconds to minutes), the second occurs from hours to days post-infection [1•, 2, 3, 4•, 5]. T cells constantly enter lymph nodes through high endothelial venules (HEVs) and survey resident or migrant antigen presenting cells (APC) for the presence of foreign antigen [6, 7]. Once cognate, pathogen-derived antigen is detected in the form of a peptide–MHC molecule complex, T cells arrest on the expressing APC, forming stable interactions that result in their activation [8]. Activated T cells rapidly proliferate, exit the lymph node and enter the circulation before reaching infected tissue. In the periphery, effector T cells use a number of methods to control and eliminate pathogen-infected cells, including the release of interferon-γ and granzyme B.

This sequence of events has largely been elucidated via the dissociation of lymphoid organs or infected tissues and performing flow cytometric analysis, or via the use of immunohistochemistry on fixed tissues to discern the types, numbers, and functional phenotype of T cells at different time points post-infection. The static nature of these approaches necessitates the use of complementary techniques to understand the kinetic behavior of cells in their environment. How long do naïve T cells interact with infected APC? How do effector cells maneuver through a complex tissue? To answer questions such as these requires the direct visualization of the cells in action.