CD45R as a primary signal transducer stimulating IL-2 and IL-2R mRNA synthesis by CD3-4-8- thymocytes

CD45R is a high molecular weight (p205/220) form of a series of transmembrane glycoproteins, collectively known as CD45 and present in some form on all lymphoid cells. We have proposed that CD45R+ thymocytes, a minority (15 to 30%) of total thymocytes, represent the generative thymic lineage whereas CD45 p180+ thymocytes are destined for intrathymic death. To test this hypothesis, we prepared human thymus fractions enriched for the expression of CD45R by exhaustive depletion of CD45 p180+ cells, as well as progenitor CD3-4-8- "multinegative" thymocytes which are predominantly CD45R+. Northern analysis of RNA extracted from CD45 p180- and multinegative thymus fractions demonstrated that these populations are enriched for cells able to synthesize mRNA encoding IL-2 and IL-2R after mitogenic stimulation, as compared to unfractionated thymus, consistent with the properties expected for generative thymocytes. Postulating that the CD45R glycoprotein might represent an important signal delivery molecule, we analyzed the ability of mAb specific for CD45 epitopes to synergize with suboptimal amounts of PHA and PMA in the stimulation of IL-2 mRNA production by multinegative thymocytes. We found that CD45R-specific mAb synergizes strongly with PHA/PMA to stimulate IL-2 and IL-2R mRNA expression. In contrast, mAb to CD45 common determinants were unable to synergize. Multinegative thymocytes depleted of all CD45 p180+ cells were compared to total multinegative cells and found to synthesize fourfold greater levels of IL-2 mRNA after stimulation with anti-CD45R mAb. This CD45 p180- multinegative subset is enriched for cells expressing a high density of CD45R, and for CD45- thymus cells, suggesting a possible enrichment for nonlymphoid cells which may play a role in the stimulation process. Our results suggest that the extended amino acid insert of CD45R plays a fundamental role in transmembrane signalling, and that CD45R may be a primary signal transducer for developing thymic progenitor cells.