Anti-transferrin receptor monoclonal antibody: a novel immunosuppressant

Iron deficiency after kidney transplantation

Iron deficiency (ID) is highly prevalent in kidney transplant recipients (KTRs) and has been independently associated with an excess mortality risk in this population. Several causes lead to ID in KTRs, including inflammation, medication and an increased iron need after transplantation. Although many studies in other populations indicate a pivotal role for iron as a regulator of the immune system, little is known about the impact of ID on the immune system in KTRs. Moreover, clinical trials in patients with chronic kidney disease or heart failure have shown that correction of ID, with or without anaemia, improves exercise capacity and quality of life, and may improve survival. ID could therefore be a modifiable risk factor to improve graft and patient outcomes in KTRs; prospective studies are warranted to substantiate this hypothesis.

Antigen-binding characteristics of AbCD71 and its inhibitory effect on PHA-induced lymphoproliferation

AIM

To investigate the anti-lymphoproliferative effect of the prepared recombinant chimeric human/murine anti-cluster of differentiation(CD) 71 monoclonal antibody (AbCD71), which is composed of mouse-derived, antigen-binding variable regions and human-derived constant regions.

METHODS

After plasmids construction and transfection, the expression of AbCD71 in the transfectoma supernatant was determined by the sandwich ELISA. Indirect immunofluorescence assay was used to measure the antigen-binding characteristic and the percent CD71-expressed peripheral blood mononuclear cells (PBMC). The antibodies were purified from the ascites via diethylaminoethyl(DEAE)-Sephadex A-50 chromatography and then identified by SDS-PAGE. At last, inhibitory effect of AbCD71 on PHA-induced PBMC proliferation was calculated by methyl thiazolyl tetrazolium(MTT) assay.

RESULTS

Constant domain of heavy chain (C(H)) and light chain (C(L)) of AbCD71 were in the human Cgamma family and human Ckappa family, respectively. AbCD71 could compete with its original murine mAb to bind with CD71-positive human leukemia cell line CEM cells. AbCD71 could inhibit the peripheral blood mononuclear cell proliferation induced by phytohemagglutinin(PHA) in vitro in a dose-dependent manner, especially at time-points 0 and 12 h after induction. There was no statistical difference when compared with original murine mAb.

CONCLUSION

The AbCD71 is a promising immunosuppressant. Our approach to blocking CD71 with the chimeric human/murine mAb provides a novel strategy for prolonging allograft survival.

Iron homeostasis

Iron is needed by all mammalian cells but is toxic in excess. Specialized transport mechanisms conduct iron across cellular membranes. These are regulated to ensure homeostasis both systemically in living organisms and within individual cells. Over the past decade, major advances have been made in identifying and characterizing the proteins involved in the transport, handling, and homeostatic regulation of iron. Molecular understanding of these processes has provided important insights into the pathophysiology of human iron disorders.

Effect of transfusional iron overload on immune response

Increased susceptibility to infectious disease is observed in persons with transfusion-dependent thalassemia and iron overload who experience increased exposure to pathogens and chronic immune stimulation. An abnormal low CD8(+) T (LT8) immune phenotype defines a subgroup of patients. The CD8(+) T cell immunophenotype is stable despite continued blood transfusion and is independent of age. CD8(+) T cells, but not CD4(+) T cells, were modulated during intravenous chelation with deferoxamine. Return to characteristic pretreatment levels of CD8 was observed in both the low and the normal groups, suggesting the possibility of a set point. Proliferative response to mitogens and antigens was increased by chelation. Because CD8(+) T cells are important in immune response to infectious disease, these studies suggest that intrinsic CD8(+) T cell subset differences may be a critical factor in determining susceptibility to infection independent of transfusional iron overload or alloantigen exposure.

Upregulation of interleukin-10 and inhibition of alloantigen responses by transferrin and transferrin-derived glycans

Previous studies have shown that critically timed administration of transferrin (Tf) facilitates induction of immunologic unresponsiveness. Here, we determined in mixed leukocyte culture (MLC) and in concanavalin A (ConA)-driven cultures the effect of exogenous Tf and Tf-derived glycans (Tf-Gly) on lymphocyte proliferation. In cultures of human blood lymphocytes, Tf inhibited selectively alloantigen-driven proliferation in MLC, but not ConA-stimulated lymphocyte proliferation. Deglycosylation of Tf abrogated the inhibitory effect of Tf on alloantigen-induced lymphocyte proliferation, and, consistent with a role for glycans, an effect qualitatively and quantitatively similar to Tf was exerted by purified Tf-Gly. Glycans isolated from other proteins, for example, immunoglobulin G (IgG) or fibrinogen, failed to inhibit alloantigen-induced proliferation selectively. Rather, they suppressed lymphocyte proliferation in a non-specific manner. Determination of cytokines in MLC supernatant showed a downregulation of interleukin-1beta (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), IL-2, and IL-12 (p40), along with an upregulation of IL-10, a pattern entirely consistent with the observed effects of Tf and Tf-Gly on alloantigen-induced lymphocyte proliferation. The effect of Tf on MLC was directly IL-10-dependent. IL-10 levels were inversely correlated with lymphocyte proliferation and CD86 expression. Neutralization of IL-10 by anti-IL-10 monoclonal antibody (mAb) blocked the effect of Tf. The MLC-modulating effect of Tf (or Tf-Gly) was not dependent upon the Tf receptor CD71 but appeared to be mediated by a Gly-responsive receptor. These data suggest a role of Tf, and, in particular, Tf-Gly, in allo-interactions that is independent from the role of Tf in iron metabolism, and appears to involve co-stimulatory signals.

Enhanced allograft survival via simultaneous blockade of transferrin receptor and interleukin-2 receptor

BACKGROUND

Transferrin receptor (TfR) expression follows the induction of interleukin 2 receptor (IL-2R) expression in a sequence that is necessary to initiate cell proliferation in quiescent T lymphocytes. Therefore, we tested the hypothesis that simultaneous blockade of TfR and IL-2R would be more effective in prolonging allograft survival and suppressing T-cell responses to alloantigen than single receptor blockade by modifying T-cell effectors to alloantigen.

METHODS

Neonatal C57BL/6 hearts were transplanted to CBA/J recipients in a heterotopic, nonvascularized cardiac allograft model. Anti-TfR and/or anti-IL-2R or isotype-matched control monoclonal antibodies (mAbs) were administered at 100 microg intravenously on days 0 and 1 of transplantation.

RESULTS

Anti-TfR mAb (25.7+/-0.9 days) significantly (P<0.01) prolonged cardiac allograft survival compared with anti-IL-2R mAb (12.5+/-0.9 days) or the isotype control (15.7+/-1.2 days, P<0.01, Wilcoxon rank-sum). Anti-TfR plus anti-IL-2R mAbs significantly (P<0.01) prolonged cardiac allograft survival to 50.7+/-2.0 days compared with the isotype control or either agent alone. These agents in combination down-regulated the intragraft T helper (Th)-1 cytokines, IL-2, interferon-gamma, and IL-15, while up-regulating the Th2 cytokine, IL-4, and completely abrogating the antigen-presenting cell IL-12p40 mRNA expression.

CONCLUSIONS

Anti-TfR and anti-IL-2R mAbs are potent immunosuppressants. Combined blockade of TfR and IL-2R at the time of antigen presentation seems to be the most effective by shifting the intragraft Th cytokine paradigm.

Differential effects of transferrin receptor blockade on the cellular mechanisms involved in graft rejection

Since transferrin receptor (TfR) appears on activated T cells following the interaction of the antigen-major histocompatibility complex (MHC) with the T cell receptor (TCR) and the appearance of interleukin (IL)-2R, we therefore hypothesize that in vivo blockade of TfR prolongs allograft survival by altering the cellular mechanisms involved in graft rejection. Previous results in our laboratory have demonstrated that anti-TfR monoclonal antibody (mAb) at 100 microg on days 0 and 1 of transplantation significantly prolonged allograft survival to 25.7 +/- 0.9 days in a murine heterotopic, nonvascularized cardiac allograft model. In the current studies, administration of anti-TfR mAb at the time of maximal TfR expression, on days 2 and 3 post-transplantation, failed to prolong allograft survival (13.0 +/- 0.0 days) compared to the isotype controls (10.5 +/- 0.5 and 10.7 +/- 0.4 days) (p < 0.01, Wilcoxon rank sum). A 4-day course of anti-TfR mAb significantly prolonged allograft survival compared to the isotype controls, but was no more effective than a 2-day course of the mAb. Anti-TfR mAb suppressed the mixed lymphocyte response to donor-specific and third-party alloantigen by 78.7% (p < 0.05) and 80.8% (p < 0.05), respectively, while stimulating the CTL response to donor-specific (16.3%, p < 0.05) and third party (49.3%, p < 0.01) alloantigen. Anti-TfR mAb suppressed IL-15 and increased IL-4 intragraft mRNA expression when compared to the isotype controls. Examination of cell surface receptors important during T cell activation revealed alterations in expression following anti-TfR mAb treatment. Anti-TfR mAb is an effective immunosuppressant prolonging allograft survival by altering cell-mediated immune responses and the intragraft cytokine micro-environment.

Recent advances in immunosuppressants

In recent years, a large number of structurally diverse immunosuppressants have been discovered that are effective for the treatment of organ transplantation. Some of them are undergoing clinical trials and may soon enter into routine clinical practice. These compounds are either chemical entities obtained from natural sources/synthetic means or biomaterials such as monoclonal antibodies/gene products/proteins. They have been found to interfere at different stages of T cell activation and proliferation, and can be identified as inhibitors of nucleotide synthesis, growth factor signal transduction and differentiation. Newer strategies involving combination of new agents with traditional immunosuppressants, monoclonal antibodies and gene therapy offer enormous potential, not only for the investigation of mechanisms pertaining to graft rejection, but also for its therapeutic prevention.