Competitive adaptation of the cation exchange capacity of roots

Summary The capability of root C.E.C. in exerting a regulative influence on the relative amount of mono- and divalent cations absorbed is underlined by demonstrating an adaptivity of this characteristic.It is shown that root C.E.C. is raised in a medium where it must compete for cations, while it is somewhat lower in a medium where only anions are available. This adaptive variation was demonstrated on the roots of single plants of several species using split root-systems.     

Inside-out regulation of Fc alpha RI (CD89) depends on PP2A

To achieve a correct cellular immune response toward pathogens, interaction between FcR and their ligands must be regulated. The Fc receptor for IgA, FcalphaRI, is pivotal for the inflammatory responses against IgA-opsonized pathogens. Cytokine-induced inside-out signaling through the intracellular FcalphaRI tail is important for FcalphaRI-IgA binding. However, the underlying molecular mechanism governing this process is not well understood. In this study, we report that PP2A can act as a molecular switch in FcalphaRI activation. PP2A binds to the intracellular tail of FcalphaRI and, upon cytokine stimulation, PP2A becomes activated. Subsequently, FcalphaRI is dephosphorylated on intracellular Serine 263, which we could link to receptor activation. PP2A inhibition, in contrast, decreased FcalphaRI ligand binding capacity in transfected cells but also in eosinophils and monocytes. Interestingly, PP2A activity was found crucial for IgA-mediated binding and phagocytosis of Neisseria meningitidis. The present findings demonstrate PP2A involvement as a molecular mechanism for FcalphaRI ligand binding regulation, a key step in initiating an immune response.     

Targeting FcαRI on polymorphonuclear cells induces tumor cell killing through autophagy

Neutrophils are the most abundant circulating FcR-expressing WBCs with potent cytotoxic ability. Currently, they are recognized as promising effector cells for Ab-mediated immunotherapy of cancer, because their capacity to kill tumor cells is greatly enhanced by tumor Ag-specific mAbs. The FcαRI represents the most potent FcR on neutrophils for induction of Ab-mediated tumor cell killing. However, the mechanisms of cell death that are induced are poorly understood. Because these mechanisms can be used for modulation of anticancer treatment, we investigated the tumor cell death induced by neutrophil-mediated Ab-dependent killing via FcαRI. Human mammary carcinoma cells were efficiently killed when incubated with human neutrophils and tumor-specific FcαRI bispecific or IgA Abs. Interestingly, we observed characteristics of autophagy such as autophagic structures by electron microscopy and LC3B(+) autophagosomes in different human epithelial carcinoma cells, which resulted in tumor cell death. To a lesser extent, necrotic features, such as cellular membrane breakdown and spillage of intracellular content, were found. By contrast, apoptotic features including fragmented nuclei, Annexin V-positivity, and presence of cleaved caspase-3 were not observed. These findings indicate that neutrophils mainly facilitate autophagy to induce tumor cell death rather than the more commonly recognized apoptotic cell death mechanisms induced by NK cells or cytotoxic T cells. This knowledge not only reveals the type of tumor cell death induced in neutrophil-mediated, Ab-dependent cellular cytotoxicity, but importantly opens up additional perspectives for modulation of anticancer therapy in, for example, apoptosis-resistant tumor cells.     

Blocking fcα receptor I on granulocytes prevents tissue damage induced by IgA autoantibodies

IgA represents the most prominent Ab class at mucosal surfaces and the second most prevalent Ab in human blood after IgG. We recently demonstrated that cross-linking of the granulocyte IgA FcR (FcαRI) by IgA induces a chemotactic-driven positive-feedback migration loop, hereby amplifying recruitment of granulocytes to IgA deposits. Therefore, we postulated that aberrant IgA-Ag complexes, which can be found in tissues in IgA-mediated diseases, are responsible for tissue damage by inducing continuous granulocyte migration and activation. Using an IgA-dependent skin-blistering disease as a model system, we demonstrated colocalization of FcαRI-positive granulocyte infiltrates with IgA in cryosections of lesional skin of patients suffering from this disease. Furthermore, we showed granulocyte migration to IgA deposits injected in human skin explants and in murine skin of FcαRI transgenic mice in vivo. Importantly, ex vivo migration and tissue damage were inhibited by blocking FcαRI, indicating that these events are dependent on the interaction of IgA autoantibodies with FcαRI. Thus, interrupting the granulocyte migration loop by blocking FcαRI reduces tissue damage in diseases with aberrant IgA-immune complexes. As such, our results may lead to development of new therapies for IgA-mediated chronic inflammatory diseases, hereby decreasing severe morbidity and improving quality of life for these patients.     

Modulation of FcgammaRI (CD64) ligand binding by blocking peptides of periplakin

FcgammaRI requires both the intracellular domain of the alpha-chain and associated leukocyte Fc receptor (FcR) gamma-chains for its biological function. We recently found the C terminus of periplakin to selectively interact with the cytoplasmic domain of the FcgammaRI alpha-chain. It thereby enhances the capacity of FcgammaRI to bind, internalize, and present antigens on MHC class II. Here, we characterized the domains involved in FcgammaRI-periplakin interaction using truncated and alanine-substituted FcgammaRI mutants and randomly mutagenized periplakin. This allowed us to design TAT peptides that selectively interfered with endogenous FcgammaRI-periplakin interactions. The addition of these peptides to FcgammaRI-expressing cells modulated FcgammaRI ligand binding, as assessed by erythrocyte-antibody-rosetting. These data support a dominant-negative role of C-terminal periplakin for FcgammaRI biological activity and implicate periplakin as a novel regulator of FcgammaRI in immune cells.     

Immunoglobulin A: A next generation of therapeutic antibodies?

Although immunoglobulin (Ig) A is commonly recognized as the most prevalent antibody subclass at mucosal sites with an important role in mucosal defense, its potential as a therapeutic monoclonal antibody is less well known. However, IgA has multifaceted anti-, non- and pro-inflammatory functions that can be exploited for different immunotherapeutical strategies, which will be the focus of this review.Key words: IgA, SIgA, dIgA, CD89, FcαRI, therapy, pro-inflammatory, mucosal defence, tumor, ITAM     

Signaling through mutants of the IgA receptor CD89 and consequences for Fc receptor gamma-chain interaction

The prototypic receptor for IgA (FcalphaRI, CD89) is expressed on myeloid cells and can trigger phagocytosis, tumor cell lysis, and release of inflammatory mediators. The functions of FcalphaRI and activating receptors for IgG (FcgammaRI and FcgammaRIII) are dependent on the FcR gamma-chain dimer. This study increases our understanding of the molecular basis of the FcalphaRI-FcR gamma-chain transmembrane interaction, which is distinct from that of other activatory FcRs. FcalphaRI is unique in its interaction with the common FcR gamma-chain, because it is based on a positively charged residue at position 209, which associates with a negatively charged amino acid of FcR gamma-chain. We explored the importance of the position of this positive charge within human FcalphaRI for FcR gamma-chain association and FcalphaRI functioning with the use of site-directed mutagenesis. In an FcalphaRI R209L/A213H mutant, which represents a vertical relocation of the positive charge, proximal and distal FcR gamma-chain-dependent functions, such as calcium flux, MAPK phosphorylation, and IL-2 release, were similar to those of wild-type FcalphaRI. A lateral transfer of the positive charge, however, completely abrogated FcR gamma-chain-dependent functions in an FcalphaRI R209L/M210R mutant. By coimmunoprecipitation, we have demonstrated the loss of a physical interaction between FcR gamma-chain and FcalphaRI M210R mutant, thus explaining the loss of FcR gamma-chain-dependent functions. In conclusion, not only the presence of a basic residue in the transmembrane region of FcalphaRI, but also the orientation of FcalphaRI toward the FcR gamma-chain dimer is essential for FcR gamma-chain association. This suggests the involvement of additional amino acids in the FcalphaRI-FcR gamma-chain interaction.