Tissue resident memory T cells- A new benchmark for the induction of vaccine-induced mucosal immunity

Historically, the gold-standard benchmark for vaccine immunogenicity has been the induction of neutralizing antibodies detectable in the serum of peripheral blood. However, in recent years there has been a new appreciation for the mucosa as an important site for vaccine induced immunity. As a point of first contact, the mucosal tissue represents a major site of immune based detection and restriction of pathogen entry and dissemination. Tissue resident memory T cells (T_rm) are one of the critical cell types involved in this early detection and restriction of mucosal pathogens. Following tissue-specific infection or vaccination, T_rm lodge themselves within tissues and can perform rapid sensing and alarm functions to control local re-infections, in an effort that has been defined as important for restriction of a number of respiratory pathogens including influenza and respiratory syncytial virus. Despite this characterized importance, only minor attention has been paid to the importance of T_rm as a benchmark for vaccine immunogenicity. The purpose of this review is to highlight the functions of T_rm with particular emphasis on respiratory infections, and to suggest the inclusion of T_rm elicitation as a benchmark for vaccine immunogenicity in animal models, and where possible, human samples.