The inappropriate trafficking and retention of immune subsets in tissue is a hallmark of many diseases prevalent in the developed world. These issues include autoimmunity, chronic inflammation, and unbalanced tissue repair responses which represent a significant disease burden: diabetes, coronary artery disease, inflammatory bowel disease, hepatitis, rheumatoid arthritis, and asthma. Although knowledge about cell trafficking to inflamed sites has grown in the past decade, there is a current gap in knowledge in identifying when and how immune cells decide to stay or leave these tissues.

To determine phenotypic and functional differences between those Th1 cells that remain at the inflamed site and those that exit, we employed the photoconvertible Kaede protein system to enable the labelling of Th1 cells in time and space. Kaede+OT-II Th1 T cells were adoptively transferred into WT mice immunized with CFA/OVA. Th1 cells recruited to the inflamed ear skin were photoconverted from Kaede green to Kaede red. Kaede red Th1 cells retained at the inflamed site could be distinguished from newly recruited Kaede green cells, while Kaede red Th1 cells that subsequently exited the inflamed skin were readily detected in the draining lymph node. Using this system, we are able to analyze the phenotypic (multicolor flow cytometry) and transcriptional signatures (RNA-seq) of Th1 cells that are retained at the inflamed site and those Th1 cells that exit (tissue-migrants). Given the temporal component of our model, we are gaining new insight into when and where expression of conventional markers of trafficking and activation are regulated (CD62L, CD44, and CD69) and seek to identify new pathways that dictate T cell retention and egress at inflammatory sites.