γ/δ T cells express a unique surface molecule appearing late during thymic development

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Abstract
The vast majority of T cells in man and mouse use the α/β form of T cell receptor (TcR), and express either CD4 or CD8, whereas the small subset of γ/δ T cells are usually CD4-CD8-. In contrast to man and mouse, the γ/δ subset in sheep, defined here using an anti-γ/δ monoclonal antibody (mAb), comprises 30%–60% of T cells. We show that γ/δ T cells in sheep express a unique surface molecule termed T19 which is 215 kDa in size and unrelated to either CD45 or the TcR. The T19 molecule was expressed at a distinct stage during γ/δ T cell ontogeny within the thymus, since γ/δ thymocytes which appeared early in fetal ontogeny were T19- and also major histocompatibility complex (MHC) class I- and localized almost exclusively to the outer cortex and cortex of the thymus. “Mature-type” γ/δ thymocytes which emerged late in thymic development were T19+ and MHC class I+ and localized predominantly to the thymic medulla. The sequence of events indicated that these cells were most likely derived from the early γ/δ thymocytes. These medullary γ/δ thymocytes showed a very distinctive association with Hassall's corpuscles, suggesting a role for these structures in γ/δ thymocyte maturation. In the periphery, T19 was expressed exclusively within the γ/δ T cell subset, however some γ/δ T cells were T19-. In particular, a large proportion of γ/δ T cells within intestinal epithelium lacked T19, indicating a correlation between T19 expression and either function or homing patterns of γ/δ T cells. Both T19+ and T19- γ/δ T cells were CD2-, and expressed low levels of LFA-1 and CD5. In addition, γ/δ T cells recirculated differently from other T cells, and appeared not to enter mesenteric lymph nodes at all from the blood. We propose that T19 is a maturation marker for γ/δ T cells. In addition, the exclusive expression of T19 by γ/δ T cells indicates that this molecule most likely serves a fundamental role in the interactions and function of γ/δ T cells.