Pilarski L M, Gillitzer R, Zola H, Shortman K, Scollay R
Department of Immunology, University of Alberta, Edmonton, Canada.
Eur J Immunol. 1989 Apr;19(4):589-97. doi: 10.1002/eji.1830190403.
Selective expression of CD45 isoforms distinguishes naive and memory T cells in peripheral blood. Paradoxically, although the most recent thymic emigrants are CD45R+ CD45 p180-, the majority of thymocytes are CD45 p180+. Speculating that the small subset of thymocytes selectively expressing only the high molecular weight isoforms of CD45 constitute the thymic generative lineage giving rise to peripheral T cells, we characterized the phenotypic and functional properties of CD45 p180- thymocytes. All cells bearing CD45 p180 were removed by rigorous depletion or all CD45R+ thymocytes were removed in a parallel depletion. CD45R- thymocytes were essentially the same in phenotype and CD4/CD8 subset distribution as unfractionated thymus, and dissimilar to naive peripheral blood lymphocyte (PBL) T cells. In contrast, CD45 p180- thymocytes, mainly CD45R+, were CD1- CD38- pgp 1+, corresponding closely to the phenotype of naive CD45R+ PBL T cells. This subset is enriched in CD4+ or CD8+ single positives, includes a high proportion of CD4-8- thymocytes which are predominantly CD3-, and appears to have a medullary location. Approximately 40%-50% of CD45 p180- thymocytes expressed a high density of CDw29 (4B4), which in the periphery is expressed at high density only on CD45 p180+ memory T cells and at low density on CD45R+ naive T cells. However, the expression of high density CDw29 in the absence of CD45 p180 indicates a close resemblance to fetal lymphocytes and suggests an essential role for CDw29 in both the least and the most mature of T cells. If CD45 p180- thymocytes constitute the generative lineage and CD45 p180+ cells are commited to intrathymic death, then the CD45 p180- subset should have enhanced proliferative potential. By combining depletion methods with a limiting dilution assay for clonogenic potential, we found that 100% of the clonogenic precursors present in unfractionated thymus were CD45R+ CD45 p180- cells. This indicates that the CD45 p180+ majority of thymocytes has a very limited capability for proliferation consistent with a commitment to intrathymic death. The clonogenic potential of CD45 p180- thymocytes indicates a greater functional resemblance to PBL T cells than to CD45 p180+ thymocytes. In so far as clonogenic potential in vitro reflects generative potential in vivo, expression of high molecular weight CD45 isoforms appears to define the generative thymic lineage. Our working hypothesis proposes that expression of CD45 p180 implements the mechanism for eliminating thymocytes with self-reactive receptor specificities.
CD45异构体的选择性表达可区分外周血中的初始T细胞和记忆T细胞。矛盾的是，尽管最新迁出胸腺的细胞是CD45R⁺ CD45 p180⁻，但大多数胸腺细胞是CD45 p180⁺。推测选择性表达仅CD45高分子量异构体的一小部分胸腺细胞构成了产生外周T细胞的胸腺生成谱系，我们对CD45 p180⁻胸腺细胞的表型和功能特性进行了表征。通过严格去除所有携带CD45 p180的细胞或平行去除所有CD45R⁺胸腺细胞。CD45R⁻胸腺细胞在表型以及CD4/CD8亚群分布上与未分离的胸腺基本相同，与初始外周血淋巴细胞（PBL）T细胞不同。相反，主要为CD45R⁺的CD45 p180⁻胸腺细胞为CD1⁻ CD38⁻ pgp 1⁺，与初始CD45R⁺ PBL T细胞的表型密切对应。该亚群富含CD4⁺或CD8⁺单阳性细胞，包括高比例主要为CD3⁻的CD4⁻8⁻胸腺细胞，并且似乎位于髓质。大约40% - 50%的CD45 p180⁻胸腺细胞表达高密度的CDw29（4B4），在外周，CDw29仅在CD45 p180⁺记忆T细胞上高密度表达，而在CD45R⁺初始T细胞上低密度表达。然而，在没有CD45 p180的情况下高密度CDw29的表达表明其与胎儿淋巴细胞非常相似，并提示CDw29在最不成熟和最成熟的T细胞中都起着重要作用。如果CD45 p180⁻胸腺细胞构成生成谱系且CD45 p180⁺细胞注定在胸腺内死亡，那么CD45 p180⁻亚群应该具有增强的增殖潜力。通过将去除方法与用于克隆形成潜力的有限稀释分析相结合，我们发现未分离胸腺中存在的100%的克隆形成前体是CD45R⁺ CD45 p180⁻细胞。这表明大多数CD45 p180⁺胸腺细胞的增殖能力非常有限，这与注定在胸腺内死亡一致。CD45 p180⁻胸腺细胞的克隆形成潜力表明其与PBL T细胞的功能相似性大于与CD45 p180⁺胸腺细胞的相似性。就体外克隆形成潜力反映体内生成潜力而言，高分子量CD45异构体的表达似乎定义了胸腺生成谱系。我们的工作假设提出，CD45 p180的表达实现了消除具有自身反应性受体特异性的胸腺细胞的机制。
