Characteristics of fetal thymus-derived T cell receptor γδ intestinal intraepithelial lymphocytes

We have previously demonstrated that grafting of CBF1(H-2^b/d) fetal thymus (FTG) under the kidney capsule of congenitally athymic nude mice of BALB/c background (H-2^d) generates a substantial number of T cell receptor (TCR) γδ intestinal intraepithelial lymphocytes (IEL) that were of FTG origin (H-2^b+) (see accompanying report). Here we investigated the characteristics of these FTG-derived TCR γδ IEL and compared them to the extrathymically derived TCR γδ IEL found in nude mice. Phenotypically, FTG-derived TCR γδ IEL were similar to their extrathymically derived counterparts in that most were Thy-1 ^?, CD5^? and CD8αα (homodimer). Vγ and Vδ gene usage in thymus-derived and extrathymically derived TCR γδ IEL were found to be virtually the same. Functionally, FTG-derived TCR γδ IEL were similar to the TCR γδ IEL found in euthymic mice as both were relatively anergic to TCR cross-linking in vitro. However, FTG-derived TCR γδ IEL differed slightly from extrathymically derived TCR γδ IEL, which were completely nonresponsive to the same in vitro stimulation. Overall, these findings support the view that FTG-derived and extrathymically derived TCR γδ IEL are almost indistinguishable. Lastly, we demonstrate that despite their thymic origin, development of FTG-derived TCR γδ IEL partially takes place extrathymically; that is positive selection of FTG-derived Vδ4 IEL occurs extrathymically. In addition, we demonstrate that the CD8 molecule is not necessary for development and homing of FTG-derived TCR γδ IEL. This later finding suggests that the CD8αα molecule develops extrathymically for FTG-derived CD8αα TCR γδ IEL.