Preparation of antibody-toxin conjugates

Large scale preparation of immunotoxins constructed with the Fab' fragment of IgG1 murine monoclonal antibodies and chemically deglycosylated ricin A chain

In this report, we describe a method for the preparation of large amounts (grams) of immunotoxins (ITs) consisting of Fab' fragments of murine IgG1 monoclonal antibodies conjugated to chemically deglycosylated ricin A chain (dgA). The preparation of Fab' and dgA chain and the purification of the Fab'-dgA IT were accomplished by gel filtrations and affinity chromatography utilizing six Pharmacia Bioprocess columns (Sephadex G-25M, Sephacryl S-200HR and Blue Sepharose CL-4B) integrated into a semi-automatic chromatography system controlled by a Pharmacia C3-process controller. The final Fab'-dgA ITs were highly purified, potent, sterile and low in endotoxin concentration.

Protein A vectorized toxins--II. Preparation and "in vitro" cytotoxic effect of protein A-ricin A chain conjugate on antibody coated human tumour cells

Protein A of Staphylococcus aureus was covalently bound to reduced ricin A chain toxin by N-succinimidyl 3-(2-pyridyldithio)propionate. The conjugate consisting mainly of one molecule of protein A bound to two molecules of A chains (Mr 107,000) was purified by tandem affinity chromatography on ConA-Sepharose 4B and IgG-Sepharose 4B. The purified protein A-A chain conjugate was able to bind and kill human lymphoma cells coated either with monoclonal mouse IgG2a anti-kappa antibody or with polyclonal rabbit anti-kappa antibody. The cytotoxic activity of protein A-A chain conjugate in conjunction with either mouse or rabbit anti-kappa antibodies was 10 times higher than that of rabbit IgG anti-mouse IgG coupled with A chain on Daudi cells coated with mouse anti-kappa antibody and 100 times higher than that of rabbit anti-kappa antibody coupled with A chain on non-coated Daudi cells. The cytotoxic effect of protein A-A chain conjugate on antibody-coated Daudi cells (9 x 10(-12) M) was comparable with that of ricin toxin on non-coated Daudi cells (2 x 10(-12) M). The results recommend the use of protein A-ricin A chain toxin conjugate as a unique specific toxin for the "in vitro" killing of antibody-coated target cells.

Elimination of T cells from human peripheral blood and bone marrow using a cocktail of three anti-T cell immunotoxins

Four anti-T cell monoclonal antibodies were coupled to ricin-A and tested for their ability to kill T cells in peripheral blood and bone marrow using a clonogenic assay to quantify T cell survival. The immunotoxins (IT) prepared from RFT11 (CD2) and WT1 (CD7) antibodies were the most toxic to peripheral blood T cells. The immunotoxin prepared from RFT1 (CD5) was the next most efficient toxin and the immunotoxin prepared from RFT8 (CD8) was the least toxic. When these reagents were applied to peripheral blood cells at 3 x 10(-8) M the number of T cell colonies was reduced by an average of 95%, 94%, 84% and 50%, respectively. Peripheral blood T cells from different donors showed marked variability in their sensitivity to ITs. However, a cocktail of three ITs prepared from RFT11, WT1 and RFT1 gave superior and consistent killing (mean 99.9%; range 99.8-100%) of peripheral blood T cells from six donors. When this cocktail was applied to bone marrow cells from six donors, an average of 99.6% (range 99.5-99.8%) of the T cells were killed. Under the same conditions there was little or no reduction in the number of normal haematopoietic progenitors (CFU-GM, CFU-GEMM, CFU-Meg and BFU-E).

Comparison of inhibitory effect of galactose analogs on the binding and cytotoxicity of an anti-globotriaosylceramide monoclonal antibody coupled or not coupled to pokeweed antiviral protein

The results described here provided an example of a human IgM monoclonal antibody against a tumor-associated glycolipid and of the unusual properties of its corresponding immunotoxin (IT). The monoclonal antibody referred to as 38-13 has been previously described and reacted with the globotriaosylceramide [Gb3:Gal(alpha 1----4)-Gal(beta 1----4)-Glc(beta 1----1)ceramide] specifically expressed on surface membrane of Burkitt's lymphoma (BL) cells. An immunotoxin (38-13 IT) combined with the pokeweed antiviral protein (PAP) toxin via S-S bridges showed paradoxically a lower cytotoxic effect in BL Ramos cells than in non-BL cells such as leukemic mouse L1210 cells, while these cells appeared not to be involved by flow cytometric analysis and complement-dependent cytotoxicity. Consequently, the inhibitory effect of selective galactose analogs on the binding and the cytotoxicity of 38-13 antibody conjugated or not to PAP toxin was compared on BL and non-BL cells. Only the galactose blocked in alpha configuration provided a fine inhibition of 38-13 binding on BL Ramos cells and both alpha and beta-galactose allowed us to establish a clear distinction between the pathway entry of 38-13 IT in BL and non-BL cells; in close correlation with the 38-13 binding specificity the 38-13 IT cytotoxic effect in Ramos BL cells could also be prevented by alpha-Gal only, suggesting that this toxic action is probably mediated through the IT binding to Gb3 antigenic sites. In contrast, on apparently irrelevant L1210 cells, 38-13 IT showed a cytotoxic effect which was inhibited preferentially by lactose (Gal in beta configuration). It was discussed that IT binding alone to either antigenic sites which are inhibited by the hapten alpha-Gal, or nonspecific sites which can compete with the hapten beta-Gal is unable to induce efficient killing of cells. But cooperation of both bindings might give an attractive explanation of IT cytotoxic effect. It was concluded that the unexpected activity of 38-13 IT in non-BL cells probably could be mediated through an active macromolecular transport process which could implicate a beta-galactoside-binding protein (lectin).

Mannose receptor dependent uptake of a ricin A chain--antibody conjugate by rat liver non-parenchymal cells

Mannose receptor mediated uptake by the reticuloendothelial system has been suggested as an explanation for the rapid removal of ricin A chain antibody conjugates from the circulation after their administration. We have measured, in the rat, hepatic uptake of a ricin A chain antibody conjugate in vivo and its susceptibility to inhibition by a mannosylated protein and have measured uptake of the conjugate in vitro by rat parenchymal and non-parenchymal liver cells. The results indicate that rapid hepatic uptake of conjugate does occur in vivo; cultured non-parenchymal cells accumulate the conjugate to a much greater degree than cultured parenchymal cells and that mannose receptors appear to be involved in the process.

Fate of an antibody-ricin A chain conjugate administered to normal rats

The pharmacokinetics and catabolism of ricin A chain, a mouse monoclonal antibody (LICR-LOND-Fib 75) and a disulphide linked conjugate of the two have been studied following their intravenous administration to normal rats. Results indicate that the conjugate was removed from the circulation much more rapidly than the antibody but less quickly than the free ricin A chain. Disappearance of the conjugate from the circulation appeared to be biphasic with an early rapid initial phase followed by a much more rapidly than the antibody but less quickly than the free ricin A chain. Disappearance of the conjugate from the circulation appeared to be biphasic with an early rapid initial phase followed by a much slower phase. The fate of a conjugate with a 125I iodide label in the antibody component was compared with that of a conjugate similarly labelled but in the ricin A chain component. The results indicate that breakdown of the conjugate involves both cleavage of the disulphide linkage and complete catabolism of the whole conjugate molecule with the release of 125I iodide. Rapid cleavage of the disulphide bond in the vasculature does not appear to be responsible for the initial rapid disappearance of the conjugate from the circulation.