Siplizumab Induces NK Cell Fratricide Through Antibody-Dependent Cell-Mediated Cytotoxicity

The glycoprotein CD2 is expressed on T and NK cells and contributes to cell-cell conjugation, agonistic signaling and actin cytoskeleton rearrangement. CD2 has previously been shown to have an important function in natural NK cell cytotoxicity but to be expendable in antibody-mediated cytotoxicity. Siplizumab is a monoclonal anti-CD2 IgG1 antibody that is currently undergoing clinical trials in the field of transplantation. This study investigated the effect of CD2 binding and Fc γ receptor binding by siplizumab (Fc-active) and Fc-silent anti-CD2 monoclonal antibodies in allogeneic mixed lymphocyte reaction and autologous lymphocyte culture. Further, induction of NK cell fratricide and inhibition of natural cytotoxicity as well as antibody-dependent cytotoxicity by these agents were assessed. Blockade of CD2 via monoclonal antibodies in the absence of Fc γ receptor binding inhibited NK cell activation in allogeneic mixed lymphocyte reaction. In contrast, siplizumab increased NK cell activation in both mixed lymphocyte reaction and autologous lymphocyte culture due to FcγRIIIA binding. However, experiments using purified NK cells did not show an inhibitory effect of CD2 blockade on natural cytotoxicity or antibody-dependent cytotoxicity. Lastly, it was shown that siplizumab induces NK cell fratricide. Concluding, siplizumab is a promising biopharmaceutical drug candidate for depletion of T and NK cells with minimal off-target effects.

Siplizumab, an Anti-CD2 Monoclonal Antibody, Induces a Unique Set of Immune Modulatory Effects Compared to Alemtuzumab and Rabbit Anti-Thymocyte Globulin In Vitro

Antibodies are commonly used in organ transplant induction therapy and to treat autoimmune disorders. The effects of some biologics on the human immune system remain incompletely characterized and a deeper understanding of their mechanisms of action may provide useful insights for their clinical application. The goal of this study was to contrast the mechanistic properties of siplizumab with Alemtuzumab and rabbit Anti-Thymocyte Globulin (rATG). Mechanistic assay systems investigating antibody-dependent cell-mediated cytotoxicity, antibody-dependent cell phagocytosis and complement-dependent cytotoxicity were used to characterize siplizumab. Further, functional effects of siplizumab, Alemtuzumab, and rATG were investigated in allogeneic mixed lymphocyte reaction. Changes in T cell activation, T cell proliferation and frequency of naïve T cells, memory T cells and regulatory T cells induced by siplizumab, Alemtuzumab and rATG in allogeneic mixed lymphocyte reaction were assessed via flow cytometry. Siplizumab depleted T cells, decreased T cell activation, inhibited T cell proliferation and enriched naïve and bona fide regulatory T cells. Neither Alemtuzumab nor rATG induced the same combination of functional effects. The results presented in this study should be used for further in vitro and in vivo investigations that guide the clinical use of immune modulatory biologics.

Effects of STI571 on the Development of Dendritic Cells Derived from Bone Marrow Mononuclear Cells from Patients with Chronic Myeloid Leukemia.

Mature dendritc cells(DCs) are potent antigen-presenting cells that prime effective T-cell response aginst tumour antigens. However, in the tumour microenviroment the differentiation and maturation of DCs is always suppressed by many factors, such as VEGF, IL-10,TGF-βet al. Though DC derived from CML patients have been shown to process and present endogenous BCR-ABL to CD4+ T cells, the function of DC may be abnormal in CML, which have been shown differences in cytoskeletal organisation and impaired migration, and a reduced capacity to capture and present nonleukaemic antigen, and less effective than normal DC in eliciting an allogeneic mixed lymphocyte reaction. STI571(trade name Glivec or Gleevec) is a specific inhibitor of Abl tyrosine kinase and has been a first- line agent for the treatment of chronic myelogenous leukemia (CML). Previous studies have found that defective blood dendritic cells in chronic myeloid leukemia correlate with high plasmatic VEGF and can be partially normalized by STI571. We, therefore, explored whether treatment with STI571 may influence the CML -derived DC in vitro. Treatment bone marrow mononuclear cells(BMMNCs) of 11 patients with CML in chronic phase with STI571+rhGM-CSF+IL-4(STI571-DC) generated DCs with more mature phenotype properties expressing higher of CD80,CD86,HLA-DR,CD83 compared to the CML- BMMNCs treated with rhGM-CSF+IL-4(IL-4-DC). And in parallel with phenotypes, STI571-DC also showed more effective than IL-DC in eliciting an allogeneic mixed lymphocyte reaction by MTT assay. FISH confirmed the DCs of both groups were leukemic origin. To explore whether the positive effects of STI571 on CML-DC may attributed to negative factors of DC differentiation/maturation suppressed by STI571, media were tested for VEGF concentrations by using the sandwich enzyme-linked immunosorbent assay (ELISA). As a result, the concentration of VEGF was dramatically reduced in STI571-DC. These findings demonstrate that treatment with STI571 promotes maturation and enhances the antigen-presenting capabilities of CML- BMMNCs derived DCs, these studies also offer a more comprehensive understanding of the role of STI571 in the immunotherapy of CML.

IκB kinase 2 but not NF-κB–inducing kinase is essential for effective DC antigen presentation in the allogeneic mixed lymphocyte reaction

Although dendritic cells (DCs) are the most potent antigen-presenting cells involved in numerous physiologic and pathologic processes, little is known about the signaling pathways that regulate DC activation and antigen-presenting function. Recently, we demonstrated that nuclear factor (NF)-κB activation is central to that process, as overexpression of IκBα blocks the allogeneic mixed lymphocyte reaction (MLR), an in vitro model of T-cell activation. In this study, we investigated the role of 2 putative NF-κB–inducing components, NF-κB–inducing kinase (NIK), and IκB kinase 2 (IKK2). Using an adenoviral gene transfer method to efficiently express dominant-negative (dn) forms of these molecules in monocyte-derived DCs, we found that IKK2dn but not NIKdn inhibited the allogeneic MLR. When DCs were fixed, this inhibitory effect of IKK2dn was lost, suggesting that IKK2 is involved in T-cell–derived signals that enhance DC antigen presentation during the allogeneic MLR period and does not have an effect on viability or differentiation state of DCs prior to coculture with T cells. One such signal is likely to be CD40 ligand (CD40L), as IKK2dn blocked CD40L but not lipopolysaccharide (LPS)–induced NF-κB activation, cytokine production, and up-regulation of costimulatory molecules and HLA-DR in DCs. In summary, our results demonstrate that IKK2 is essential for DC activation induced by CD40L or contact with allogeneic T cells, but not by LPS, whereas NIK is not required for any of these signals. In addition, our results support IKK2 as a potential therapeutic target for the down-regulation of unwanted immune responses that may occur during transplantation or autoimmunity.