Selective expression of CD45 isoforms and of maturation antigens during human thymocyte differentiation: observations and hypothesis.

Selective expression of high and low molecular weight isoforms of CD45 (T200) occurs coincident with activation of mature T lymphocytes. Expression of CD45 p180 and CDw29 defines antigen-experienced memory T cells in the periphery, and expression of CD45R (CD45 p205/220) defines naive T cells. Upon activation, CD45R+ T cells lose CD45R and acquire CD45 p180 and high density CDw29. In this review we discuss the expression of CD45 isoforms, CDw29, pgp-1 and CD1 on human thymocytes and develop the hypothesis that expression of CD45R marks the generative thymic lineage while expression of CD45 p180 marks those cells destined for intrathymic death. This hypothesis is supported by evidence from phenotypic, molecular and functional analysis of thymocyte subsets. We propose that expression of high molecular weight isoforms of CD45 is essential for growth and differentiation of immature T cells and that inappropriate rearrangement or specificity of TCR activates a transition from expression of CD45R to that of CD45 p180 and intrathymic death.