Table 1.

Methods to define T_RM.

Method	Description	Advantage(s)	Limitation(s)
Parabiosis [53]	Congenically distinct mice are joined via skin flaps that allows their vasculature to anastomose so that in ~2 weeks, blood contains equal cells of each partner	shows whether the cells are maintained independent of vascular input can be tested over many weeks can assess all organs/tissues at once	intensive procedure not appropriate for chronic infection can be difficult to interpret if the T cell population is mixed in the tissue(s) cannot assess cell egress from tissue of interest
Tissue transplant	Tissue from a congenically distinct subject is transplanted onto another	can answer whether host cells enter the transplant tissue and whether transplant cells exit	not appropriate for chronic infection inflammation induced by graft surgery could influence results only appropriate for organs that are easily transplanted (e.g., skin)
Photo-conversion [54]	Photo-convert transgenic cells (e.g., Kaede-expressing cells irreversibly change from green to red when exposed to violet light)	can assess infiltration and egress of cells from a tissue can work for chronic infection	limited by access of tissue to the violet light source protein turnover means converted fluorescent signal is lost in ~1 week
Dye-label	Inject a fluorescent dye into the tissues (e.g., CSFE) to label local cells	assesses infiltration and egress of cells from a tissue can work for chronic infection	dye diffusion away from local site can result in unintended cells getting labeled can have incomplete labeling of target cells loss of dye-labeling if cells proliferate may result in the cells of interest no longer being labelled
Intravascular antibody labeling [55]	Inject anti-CD45 (or anti-CD8, etc) i.v. 3 min prior to euthanization	reveals location of cells (vasculature vs. parenchyma) can work for chronic infection works well for highly vascularized tissue (e.g., lung)	does not reveal migration history can only determine cell location at time of euthanization
Peripheral antibody-mediated T cell depletion [44]	Inject anti-CD8 (or anti-Thy1.1, etc.) i.p. starting 8-10 dpi	can show cells are maintained independent of vasculature works well for solid organs with minimal antibody infiltration (e.g., brain, skin) can work for chronic infection can assess multiple organs/tissues at once	cannot assess cell egress from tissue(s) of interest limited use for tissues that are highly vascularized and/or mucosal if antibody can easily penetrate and deplete these cells