Transferrin receptor induction is required for human B-lymphocyte activation but not for immunoglobulin secretion

The critical roles of iron during the journey from fetus to adolescent: Developmental aspects of iron homeostasis

Iron is indispensable for human life. However, it is also potentially toxic, since it catalyzes the formation of harmful oxidative radicals in unbound form and may facilitate pathogen growth. Therefore, iron homeostasis needs to be tightly regulated. Rapid growth and development require large amounts of iron, while (especially young) children are vulnerable to infections with iron-dependent pathogens due to an immature immune system. Moreover, unbalanced iron status early in life may have effects on the nervous system, immune system and gut microbiota that persist into adulthood. In this narrative review, we assess the critical roles of iron for growth and development and elaborate how the body adapts to physiologically high iron demands during the journey from fetus to adolescent. As a first step towards the development of clinical guidelines for the management of iron disorders in children, we summarize the unmet needs regarding the developmental aspects of iron homeostasis.

Down-regulation of CXCR4 and CD62L in chronic lymphocytic leukemia cells is triggered by B-cell receptor ligation and associated with progressive disease

Progressive cases of B-cell chronic lymphocytic leukemia (CLL) are frequently associated with lymphadenopathy, highlighting a critical role for signals emanating from the tumor environment in the accumulation of malignant B cells. We investigated on CLL cells from 30 untreated patients the consequence of B-cell receptor (BCR) triggering on the membrane expression of CXCR4 and CD62L, two surface molecules involved in trafficking and exit of B-lymphocytes from lymph nodes. BCR stimulation promoted a strictly simultaneous down-regulation of CXCR4 and CD62L membrane expression to a variable extent. The variable BCR-dependent decrease of the two proteins was strikingly representative of the heterogeneous capacity of the CLL cells to respond to BCR engagement in a given patient. Functionally, cells down-regulating CXCR4 and CD62L in response to BCR engagement displayed a reduction in both migration toward CXCL12 and adhesion to lymphatic endothelial cells. Remarkably, the ability of CLL cells to respond to BCR ligation was correlated with unfavorable prognostic markers and short progression-free survival. In conclusion, BCR signaling promotes decrease of CXCR4 and CD62L membrane expression in progressive cases only. These results are consistent with the hypothesis that BCR-mediated signaling pathways favor accumulation of a proliferative pool within the lymph nodes of progressive CLL cases.

Transferrin receptor 1 is differentially required in lymphocyte development

Transferrin receptor (TfR) facilitates cellular iron uptake by mediating endocytosis of its ligand, iron-loaded transferrin. Although TfR is widely believed to be important for iron acquisition by all mammalian cells, direct experimental evidence is lacking. We have previously shown that mouse embryos homozygous for a disrupted transferrin receptor allele (TfR-/-) die of anemia before embryonic day 12.5, although most other embryonic tissues appear to be developing normally. Here, we have investigated the importance of TfR postnatally, by using TfR-/- embryonic stem cells to produce chimeric animals. We find that TfR-/- embryonic stem cells give rise to most tissues and organs, but do not contribute to hematopoietic tissues on a wild-type C57BL/6J background, indicating that both adult erythropoiesis and lymphopoiesis require TfR. On an immunodeficient RAG2-/- background, TfR-/- B-cell development proceeds at least to the IgM+ stage, although significantly fewer IgM+ cells are present in peripheral lymphoid organs. Conversely, T cells lacking TfR are arrested very early in their development, at the CD4-8-3- stage. These results indicate that TfR is necessary for the normal maturation of thymocytes, but that B-cell development is less severely affected by the absence of TfR.

Iron status, immune capacity and resistance to infections

1. The importance of iron on immune functions is reviewed. 2. The consequences of iron deficiency upon resistance to infection in men (adults and children) and animals are controversial. 3. Cellular immunity is often altered in iron-deficient humans and in murine species. 4. Humoral immune responses seem far less affected in iron-deficient humans than is cellular immunity, but is impaired in iron-deficient animals. Results on complement are scarce and controversial. 5. There is almost no perturbation of phagocytosis but bactericidal activity is decreased in most studies on iron-deficient subjects. 6. Natural Killer activity is decreased in iron-deficient mice. Iron deficiency also affects lymphokine production in mice and rats.

Immunological and biochemical characterization of an epitope of the transferrin receptor involved in the production of interferon-gamma and B-cell growth factor

We have investigated the effect of a newly developed monoclonal antibody, MoAb NDA9, on human lymphocyte function. This MoAb inhibits the capacity of peripheral blood lymphocytes to display blastogenic responses and to produce immunoglobulins when stimulated in vitro with PWM or with soluble antigens. The inhibitory effect seems to result from the decreased ability of T lymphocytes to produce B cell growth factors (BCGF) in the presence of MoAb NDA9. This antibody also blocks the capacity of polyclonal or monoclonal populations of activated human T cells to produce immune interferon (gamma) but has no direct effect on B cell activation and growth in T-cell-independent systems. Immunochemical studies of the antigen recognized by MoAb NDA9 showed that it is an epitope of the transferrin receptor molecule which is distinct from that recognized by the MoAb OKT9.

Effect of iron and transferrin on pure oligodendrocytes in culture; characterization of a high-affinity transferrin receptor at different ages

Oligodendrocytes in pure culture can grow on relatively low iron concentrations (0.1-0.3 microM), in the absence of transferrin; with micromolar concentrations of iron, toxic effects can be seen after one week in culture. When transferrin is added, the toxic effect of iron is increased. These properties account for the mode of selection of oligodendrocytes for pure cultures. Each oligodendrocyte presents between 1100 and 3600 receptor molecules, with a dissociation constant of 0.2-0.6 nM corresponding to a high affinity transferrin-binding site; these constants vary little with age in culture. These receptors may function as autoreceptors regulating transferrin synthesis by oligodendrocytes.

Biological effect of transferrin on mast cell mediator release during the passive cutaneous anaphylaxis reaction: a possible inhibition mechanism involving iron

A purification procedure for passive cutaneous anaphylaxis inhibitory factor from BALB/c mouse serum has been previously described. In the present work, this inhibitory activity was found to be related to transferrin. No activity was obtained using iron-unsaturated transferrin, whereas iron-saturated transferrin appeared to be potent. The in vivo inhibition of IgE-dependent mediator release from rat mast cells was also obtained using free iron. This effect was observed when iron was injected prior to or simultaneously with mouse IgE in rat skin. Iron and iron-saturated transferrin could play a role in the mechanism of desensitization by modulating the responsiveness of mast cells.

Diltiazem inhibits transferrin receptor expression and causes G1 arrest in normal and neoplastic T cells

Transferrin receptor expression is essential for the proliferation of both normal and malignant T cells. While transferrin receptor expression in normal T cells is tightly coupled to interleukin-2 receptor expression, transferrin receptor expression in malignant cells is usually constitutive and is released from this constraint. Temporally, the appearance of these membrane receptors is preceded by changes in the expression of the proto-oncogenes c-myc and c-myb. In addition, although an increase in the level of intracellular free calcium occurs early in the sequence of T-cell activation, the activation events dependent on this calcium flux have not been resolved. In the present study we report that diltiazem, an ion channel-blocking agent that inhibits calcium influx, arrested the growth in vitro of both normal and malignant human T cells in the G1 phase of the cell cycle. However, diltiazem did not inhibit the expression of c-myc or interleukin-2 receptor mRNA and protein in normal mitogen-activated T cells or the constitutive expression of c-myc and c-myb mRNA in malignant T cells (T acute lymphoblastic leukemia cells). In contrast, diltiazem prevented the induction of transferrin receptor (mRNA and protein) in normal T cells and caused a progressive loss of transferrin receptor (mRNA and protein) in malignant T cells. These data demonstrate that diltiazem can dissociate several growth-related processes normally occurring in G1 and thereby disrupt the biochemical cascade leading to cell proliferation.

Increased expression of the B lymphocyte receptor for transferrin is stimulated by in vivo crosslinking of cell surface IgD

Expression of a receptor for the serum protein transferrin has been shown to be a characteristic of several cell lineages and increased transferrin receptor (TFR) expression to reflect cellular activation. In vitro studies of human B lymphocytes stimulated with antibodies to IgM have demonstrated that these cells have the ability to express TFR and that increased B-cell TFR expression is seen first sometime after these cells enter the G1 phase of the cell cycle. It also has been shown that TFR expression is necessary for proliferation to occur and may be regulated by a factor produced by mitogen-activated T lymphocytes. To examine expression of TFR by activated B lymphocytes in vivo, and to determine the kinetics of induction of TFR expression, we have studied the effects of injecting mice with an affinity-purified goat antibody to mouse IgD (GaM delta) on TFR expression. This antibody previously has been shown to activate polyclonally mouse splenic B cells in vivo in a T-independent fashion. Results show that there is a small but definite quantity of TFR on resting splenocytes, at 24 hr after injection nearly all B cells but not T cells express increased amounts of TFR, TFR is increased to nearly the same extent in congenitally athymic BALB/c nu/nu mice as in their normal nu/+ littermates and therefore GaM delta-induced increased B lymphocyte TFR expression is relatively T independent, TFR expression increases as early as 3 hr after injection of 800 micrograms of GaM delta and increases steadily until it peaks 24-48 hr later, and TFR expression in GaM delta-injected mice increases concomitantly with surface Ia antigen and cell size.

Diltiazem inhibits transferrin receptor expression and causes G1 arrest in normal and neoplastic T cells

Transferrin receptor expression is essential for the proliferation of both normal and malignant T cells. While transferrin receptor expression in normal T cells is tightly coupled to interleukin-2 receptor expression, transferrin receptor expression in malignant cells is usually constitutive and is released from this constraint. Temporally, the appearance of these membrane receptors is preceded by changes in the expression of the proto-oncogenes c-myc and c-myb. In addition, although an increase in the level of intracellular free calcium occurs early in the sequence of T-cell activation, the activation events dependent on this calcium flux have not been resolved. In the present study we report that diltiazem, an ion channel-blocking agent that inhibits calcium influx, arrested the growth in vitro of both normal and malignant human T cells in the G1 phase of the cell cycle. However, diltiazem did not inhibit the expression of c-myc or interleukin-2 receptor mRNA and protein in normal mitogen-activated T cells or the constitutive expression of c-myc and c-myb mRNA in malignant T cells (T acute lymphoblastic leukemia cells). In contrast, diltiazem prevented the induction of transferrin receptor (mRNA and protein) in normal T cells and caused a progressive loss of transferrin receptor (mRNA and protein) in malignant T cells. These data demonstrate that diltiazem can dissociate several growth-related processes normally occurring in G1 and thereby disrupt the biochemical cascade leading to cell proliferation.