CD50 (INTERCELLULAR-ADHESION MOLECULE-3) IS EXPRESSED AT HIGHER LEVELS ON MEMORY THAN ON NAIVE HUMAN T-CELLS BUT INDUCES A SIMILAR CALCIUM MOBILIZATION ON BOTH SUBSETS

CD50, the intercellular adhesion molecule-3 (ICAM-3), is expressed alm ost exclusively on hematopoietic cells. T lymphocytes display a bimoda l distribution on CD50 expression levels. It was observed that CD45RO( +) cells expressed higher levels of CD50 than CD45RA(+) T lymphocytes. A similar situation was observed when CD4 and CD8 subpopulations were analyzed, with CD8(+) cells expressing higher levels of CD50 than CD4 (+) cells. When adult T lymphocytes were analyzed by three-color flow cytometry in CD8(+)CD45RA(+) cells both CD50(low) and CD50(high) expre ssing cells were detected, in accordance with several memory markers o n T lymphocytes, whereas only cells with a low level of CD50 were obse rved in CD4(+)CD45RA(+). The different level of CD50 expression was co nfirmed by analyzing purified CD45RA(+) and CD45RO(+) T cells. Moreove r, after the comparison of CD50 expression level in thymocytes, cord b lood and adult T lymphocytes, a progressive increase was observed. Whe n T cells were sorted by their intensity of CD50 expression, only CD50 (high) cells proliferated in response to tetanus toroid. Therefore, th e phenotypic and functional analysis of adult and cord blood T lymphoc ytes as well as thymocytes indicates that CD50 expression increases du ring the maturation process of T lymphocytes: from the lowest CD50 lev els present on CD1(+) thymocytes, to the highest levels of CD50 on hum an memory T cells. In addition, we have observed that after CD50 cross linking on human T lymphocytes, a transient increase in intracellular calcium concentration ([Ca2+](i)) is produced. When CD45RA(+) and CD45 RO(+) T cells were analyzed, in spite of the level of CD50 expression, the stimulation through CD50 induced a similar level of Ca2+ mobiliza tion in both subpopulations, contrasting with the higher rise in [Ca2](i) induced by CD3 stimulation on CD45RA(+) versus CD45RO(+). These d ata suggest that the signal transduction pathways activated after CD50 cross-linking are, at least partially, independent of those involved after CD3 stimulation.