Constitutive expression and abnormal glycosylation of transferrin receptor in acute T-cell leukemia.

The expression of transferrin receptors (TrfRs) was investigated in acute T-cell leukemia (T-ALL) blasts at the molecular, biochemical, immunological, and functional level. TrfRs, although not detected on quiescent T-cells from normal adults, are constitutively expressed at high level on the blasts from all T-ALL patients and bind normally to transferrin. Their number is modulated by the intracellular iron level, but is independent of exogenous interleukin 2. They also exhibit immunological and biochemical abnormalities, in that: (a) they react preferentially with monoclonal antibodies (MAb) that recognize ligand-binding domains of TrfR (42/6 and 43/31), as compared to MAbs (B3/25, OKT9) that interact with the nonligand binding domains; (b) they have a reduced molecular weight, as compared to TrfR on normal thymocytes and activated T-lymphocytes: this phenomenon is apparently related to a defective glycosylation. It is noteworthy that expression of TrfR was not observed in a large series of other types of acute leukemias, i.e., pre-B, B, and myeloid leukemias, excluding erythroleukemias. The constitutive, high level expression of TrfRs on T-ALL blasts may play a key role in the stepwise progression of this malignancy and particularly provide a proliferative advantage to T-ALL blasts as compared to normal T-lymphocytes. Furthermore, indirect evidence suggests that the glycosylation defect of TrfR on T-ALL blasts contributes to their tumorigenic capacity.

Iron up-modulates the expression of transferrin receptors during monocyte-macrophage maturation.

We have investigated the effect of iron on the expression of transferrin receptors (TrfRs) and ferritin chains in cultures of human peripheral blood monocytes maturing to macrophages. Monocyte-macrophage maturation is associated with a gradual rise of Trf-binding capacity in the absence of cell proliferation. At all culture times, treatment with ferric ammonium citrate induces a dose-dependent rise of the Trf-binding level as compared with nontreated cells. Scatchard analysis revealed that this phenomenon is due to an increase in receptor number rather than an alteration in ligand-receptor affinity. Biosynthesis experiments indicated that the rise in number of TrfRs is due to an increase of receptor synthesis, which is associated with a sustained elevation of the TrfR RNA level. The up-regulation of TrfR synthesis is specific in that expression of other macrophage membrane proteins is not affected by iron addition. Conversely, addition of an iron chelator induced a slight decrease of TrfR synthesis. The expression of heavy and light ferritin chains at RNA and protein levels was markedly more elevated in cultured macrophages than in fresh monocytes, thus suggesting modulation of ferritin genes at transcriptional or post-transcriptional levels. Addition of iron salts to monocyte-macrophage cultures sharply stimulated ferritin synthesis but only slightly enhanced the level of ferritin RNA, thus indicating a modulation at the translational level. These results suggests that in cultured human monocytes-macrophages, iron up-regulates TrfR expression, thus in sharp contrast to the negative feedback reported in a variety of other cell types. These observations may shed light on the mechanism(s) of iron storage in tissue macrophages under normal conditions and possibly on the pathogenesis of diseases characterized by abnormal iron storage.

Fluctuations of plasma beta 2-microglobulin, soluble interleukin 2 receptor and interferon-gamma concentrations after adoptive immunotherapy with high-dose interleukin 2 and lymphokine-activated killer cells.

We report the serum levels of soluble interleukin 2 receptor (sIL2R), beta 2-microglobulin (beta 2-M) and interferon-gamma (IFN-gamma) in patients undergoing adoptive immunotherapy with rIL2 and lymphocyte-activated killer (LAK) cells. Our results indicate that rIL2 induced a marked increase of the serum concentration of these markers, although this increase varied considerably for different individuals. Parallel studies with the same patients also showed a marked rise in the number of IL2R+ lymphocytes: the IL2Rs expressed on these cells were mainly of the "low affinity" type. We suggest that evaluation of these markers may allow the monitoring of immune system activation induced by rIL2 in patients undergoing adoptive rIL2 and LAK cell immunotherapy.

Mechanisms underlying T-lymphocyte activation: mitogen initiates and IL-2 amplifies the expression of transferrin receptors via intracellular iron level.

Peripheral blood mononuclear cells (PBM) pulsed with lectin (PHA or Con A for 0.25-3 hr) show a low expression of interleukin-2 and transferrin receptors (IL-2Rs, TfRs) and a mild decline of intracellular ferritin level, compared to control cultures grown in continuous presence of mitogen. Interestingly, lectin-pulsed PBM do not release detectable amounts of IL-2 in the medium. Furthermore, expression of TfRs in these lymphocytes is not inhibited by addition of excess anti-IL-2 neutralizing monoclonal antibody, but is significantly inhibited by treatment with iron salts. These observations suggest that mitogen triggers an IL-2-independent expression of TfRs, at least in part via a decrease of intracellular iron level. Addition of either recombinant IL-2 (rIL-2) or an iron chelator (picolinic acid) to lectin-pulsed PBM induces both a marked enhancement of TfR synthesis and a sharp decline of intracellular ferritin level, which are comparable to the corresponding pattern observed in control cultures. Conversely, addition of iron salts fully inhibits the increase of TfR expression induced by rIL-2. These observations strongly suggest that the enhanced TfR synthesis elicited by rIL-2 is mediated by depletion of a regulatory intracellular iron pool. In line with these studies, greater than 99% purified T lymphocytes stimulated by lectin show a low expression of TfRs, which is markedly enhanced by addition of exogenous rIL-2. Altogether, we postulate that: (i) in resting T lymphocytes the gene encoding TfR is apparently in a 'closed' configuration; (ii) even in the absence of IL-2 activity, a mitogen pulse is sufficient to initiate the expression of TfRs, at least in part via a decline of intracellular iron level; and (iii) TfR synthesis is then largely amplified by IL-2, again via a decrease of the size of a regulatory intracellular iron pool.