The interaction of lactose-specific lectins with acid-treated and lactose-substituted sepharose

Cytotoxicity against human peripheral blood mononuclear cells and T cell lines mediated by anti-T cell immunotoxins in the absence of added potentiator

Several in vitro assays have indicated that anti-T cell immunotoxins (IT), composed of monoclonal antibodies (MoAbs) conjugated to ricin A chain (RTA), are maximally effective against T cells only in the presence of potentiators. It was thought that such IT might not be sufficiently cytotoxic to deplete T cells in vivo upon administration to patients. Therefore, we have re-evaluated the in vitro assays and report herein that even with a short exposure time (2 h), the two anti-T cell IT, H65-RTA (anti-CD5 MoAb coupled to RTA) and 4MRTA (anti-CD7 MoAb coupled to RTA30), were specifically cytotoxic for peripheral blood mononuclear cells (PBMC) in the absence of potentiators. Moreover, as has been reported for IT when tested against T cell lines, prolonging the exposure time of the IT with PBMC from 2 h to as long as 90 h, without added potentiators, enhanced their cytotoxicity from 2- to 40-fold. In contrast, most T cell lines were more sensitive to IT in the presence of potentiator, and IT cytotoxicity was much less enhanced by prolonging the exposure time. Thus, T cell lines may not serve as accurate models to determine the efficacy of IT against PBMC in vitro or in vivo. We conclude that IT-induced cytotoxicity of PBMC can be demonstrated in vitro at pharmacologically achievable concentrations in the absence of added potentiators.

Monoclonal antibodies to purified ricin A-chain: production and properties

Ricin A-chain was purified from native ricin using lactosyl-Sepharose. It was non-toxic to whole cells at a concentration of 1 microM yet nearly as effective as an equimolar concentration of ricin in blocking in vitro protein synthesis. Hybridomas secreting monoclonal antibodies to ricin A-chain were produced using the murine myeloma cell line NS-1. These anti-ricin A-chain antibodies cross-reacted with whole ricin but exhibited little cross-reactivity with purified ricin B-chain. Antibodies 2F2 and 2F5 both immunoprecipitated ricin A-chain. Both antibodies also precipitated ricin B-chain, as did the irrelevant control antibodies MOPC-21 and MPC-11. Pre-incubation of B-chain with 0.1 M galactose eliminated greater than 90% of precipitation by 2F2, MOPC-21 and MPC-11 but effected minimally precipitation by 2F5. Enzyme-linked immunosorbent assays using antibodies 2F2 and 2F5 to detect ricin A-chain in murine or human serum were linear between 40 and 800 ng ricin A-chain per ml. Anti-ricin A-chain antibodies 2F2 and 2F5 produced some inhibition of in vitro A-chain catalytic activity. Specific monoclonal antibodies to A-chain hemitoxin will be useful for characterization of functional hemitoxin domains, in in vitro assays for the stability of A-chain immunotoxins, and in characterizing the cellular internalization and processing of conjugates containing ricin or ricin A-chain.