Efficient induction of apoptosis in cultured rat glomerular mesangial cells by dimeric monoclonal IgA anti-Thy-1 antibodies.

Apoptosis of glomerular cells (GMC) has been observed in the early phase as well as the resolution phase of Thy-1 nephritis. Recently, we and others reported that IgG2a (ER4G) and IgG1 (OX7) monoclonal mouse anti-Thy-1 antibodies (anti-Thy-1 MoAb) are able to induce apoptosis of rat GMC in vivo. The purpose of this study was to investigate whether cross-linking of Thy-1 would influence the degree of apoptosis in cultured rat GMC using monomeric and dimeric IgA anti-Thy-1 MoAb. IgA anti-Thy-1 MoAb (ER4A) was generated by class switching of the IgG producing ER4 (ER4G) hybridoma. The ER4A clone spontaneously produces monomeric (m-ER4A) and dimeric IgA anti-Thy-1 MoAb *di-ER4A). Unaltered epitope specificity of ER4A was confirmed by blocking experiments of the binding of fluorescence labeled ER4G to cultured rat GMC with unlabeled ER4A on FACS. For the experiments of apoptosis, quiescent rat GMC were incubated for eight hours with medium alone or with medium in the presence of 10 micrograms/ml of m-ER4A, di-ER4A or control IgA MoAb of corresponding sizes. Apoptosis was assessed by morphological studies, agarose gel electrophoresis and quantitative FACS analyses using terminal deoxynucleotidyl transferase (TDT) method and the annexin V method. The TDT method detects specific-DNA nicking in apoptosis. The annexin V method detects early membrane changes during apoptosis. In morphological studies, cells incubated with m-ER4A and di-ER4A showed typical apoptotic features such as nuclear condensation and fragmentation. DNA isolated from the cells incubated with di-ER4A was cleaved into a distinctive ladder pattern compatible with apoptosis. In contrast, both medium alone and control IgA MoAb did not reveal detectable changes in morphological studies and agarose gel electrophoresis. In quantitative analyses by FACS using the TDT method and the annexin method, both m-ER4A and di-ER4A induced significantly higher percentages of apoptosis in rat GMC as compared to the controls. Furthermore, di-ER4A was considerably more efficient than m-ER4A in inducing apoptosis possibly through additional cross-linking of Thy-1 on the cell surface. This notion was confirmed by experiments, in which the addition of goat anti-mouse kappa antibodies enhanced apoptosis of rat GMC pre-sensitized with m-ER4A. Taken together, our results indicate that apoptosis of rat GMC by anti-Thy-1 antibodies is enhanced by cross-linking of Thy-1 on the cell surface. These studies are of importance for our understanding of mechanisms that may play a role in glomerular diseases.