Cell surface expression of the 300 kDa mannose-6-phosphate receptor by activated T lymphocytes

Phosphosugars, such as mannose-6-phosphate (M6P), have been shown previously to display anti-inflammatory properties, notably inhibition of experimental autoimmune encephalomyelitis (EAE) and adjuvant-induced arthritis in rats. It has been proposed that M6P exerts its anti-inflammatory effect by displacing lysosomal enzymes, which are involved in T-cell extravasation into inflammatory sites, from the 300 kDa mannose-6-phosphate receptor (MPR-300) on the surface of T cells. If this model is correct MPR-300 should be selectively expressed on the surface of activated T cells, as T cell entry into the central nervous system in EAE depends on the T cells being in an activated state. Thus, the present study examines whether cell surface expression of MPR-300 by T lymphocytes correlates with their state of activation and whether T cells in inflammatory sites express the receptor. Flow cytometric studies showed MPR-300 to be absent from the surface of unstimulated rat T cells isolated from peripheral blood and lymphoid tissues, and T cells resident within the peritoneal cavity. In contrast, MPR-300 was expressed on activated T cells derived from an inflammatory peritoneal exudate. In vitro studies demonstrated transient expression of MPR-300 on the surface of splenic T cells following stimulation with Con A. MPR-300 was also induced on T-cell lines by antigen stimulation. These data demonstrate that T cells in inflammatory sites express MPR-300 on their surface and activation of T lymphocytes induces cell surface expression of MPR-300. Such findings are consistent with the hypothesis that cell surface MPR-300 is required for the entry of T cells into inflammatory sites.