Low membrane transport activity for cystine in resting and mitogenically stimulated human lymphocyte preparations and human T cell clones

In order to determine whether the cysteine requirement of human T lineage cells is met primarily by extracellular cysteine or by cystine, amino‐acid‐transport activities were measured in resting and mitogenically stimulated human peripheral blood lymphocytes (PBL) and several human T cell clones and T cell tumors. The transport activity of the small neutral amino acids cysteine and alanine (ASC system) and the transport of the cationic amino acid arginine (y+ system) were found to be markedly increased after stimulation of PBL by the T cell mitogen phytohemagglutinin from Phaseolus vulgaris. The anionic transport activity for cystine and glutamate (xc− system), in contrast, was extremely weak in both resting and activated human PBL and also in all human T cell lines under test. The weak system xc− activity of human T lineage cells was further confirmed by an independent line of experiments showing that an increase of the extracellular concentration of glutamate, i.e. a competitive inhibitor of cystine transport, causes a decrease in the intracellular cystine levels in cells of the promonocytic line U937, but not in T lineage cells (Molt‐4). A third set of experiments showed that the rate of DNA synthesis in mitogenically stimulated human PBL is strongly influenced by variations of the extracellular cysteine level, even in cultures with relatively high and approximately physiological concentrations of cysteine. Cysteine cannot be replaced in this case by the addition of corresponding amounts of cystine or methionine. This demonstrates an important functional consequence of the weak cystine transport activity of human lymphocytes. The results may be relevant for the pathogenetic mechanism of the acquired immunodeficiency syndrome, since the mean plasma cysteine concentration of human‐immunodeficiency‐virus‐1‐seropositive persons was found to be strongly decreased in comparison with that of healthy blood donors, and since the cysteine level even of healthy persons is extremely low in comparison with all other protein‐forming amino acids.