Pelegrin, M. are critical for MAb binding. One of them, D57, is not present in some other murine retroviral Env, which suggests a critical part for this residue in the selectivity of 667. MAb 667 weighty- and light-chain cDNAs were functionally characterized by transient transfection into Cos-7 cells. Enzyme-linked immunosorbent assays and Biacore studies showed the specificities as well as the antigen-binding thermodynamic and kinetic properties of the recombinant 667 MAb (r667) produced by Cos-7 cells and those of the parental hybridoma-produced MAb (h667) were similar. However, h667 was shown to contain contaminating retroviral and/or retrovirus-like particles which interfere with both viral binding and neutralization experiments. These pollutants could successfully become eliminated by a stringent purification protocol. Importantly, this purified 667 could completely prevent retrovirus binding to target cells and was as efficient as the r667 MAb produced by transfected Cos-7 cells in neutralization assays. In conclusion, this study demonstrates the primary mechanism of disease neutralization by MAb 667 is the blocking of the retroviral receptor binding website IACS-10759 Hydrochloride of CasBrE Env. In addition, the findings of this study constitute a warning against the direct use of hybridoma cell tradition supernatants for studying the initial events of retroviral cell illness as well in terms of carrying out in vivo neutralization experiments and suggest that either recombinant antibodies or highly purified antibodies are preferable for these purposes. CasBrE is a simple murine ecotropic retrovirus which causes a IACS-10759 Hydrochloride spongiform encephalopathy primarily affecting the engine center of the brain and the spinal cord. The disease was originally isolated from crazy mice (17), and its neurovirulence is determined by the envelope glycoprotein (Env) sequence (13, 46, 47). Due to the poor replication ability of the disease in the brain, long periods of incubation (3 to 6 months) after ACTB neonatal illness are required for the onset of neuropathology. However, when the Env of CasBrE is definitely introduced into the neuroinvasive but nonneuropathogenic Friend murine leukemia disease (MLV) strain FB29 in place of its natural Env, the producing disease, FrCasE, induces a rapidly progressing, noninflammatory spongiform neurodegenerative disease with an incubation period of only 2 weeks. Retroviral envelope glycoproteins are synthesized as precursors which are proteolytically processed into two parts. The producing heterodimer consists of a surface moiety (SU; also called gp70 in MLVs), responsible for disease binding to its cellular receptor, and a transmembrane part [TM; also called p15(E) in MLVs], anchoring Env in the viral envelope and transporting a latent fusion activity that is triggered upon the disease binding to its cognate receptor (22, 26). Monoclonal antibody (MAb) 667 is an IgG2a/ neutralizing mouse MAb that binds to the SU Env subunit of CasBrE but not to the subunits of additional ecotropic retroviruses IACS-10759 Hydrochloride (31). The neutralization effect in vitro of MAb 667, in the absence of complement, has been described for both the CasBrE retrovirus (31) and FrCasE (45). It has also been reported that, when mixed with retroviral particles prior to the illness of vulnerable mice, 667 prevents all manifestations of FrCasE-induced neurodegeneration, most probably because of in vitro neutralization of the viral inoculum (45). More recently, Pelegrin et al. have shown that IACS-10759 Hydrochloride intravenous injection of 667 can also protect mice from FrCasE, demonstrating that 667 can exert an antiviral effect in vivo, actually in animals with established infections (42). However, its mechanism(s) of action still remains to be elucidated. The quick onset, the obvious symptomatology, the highly predictable medical program, and the 100% incidence of the neurodegeneration induced by FrCasE make it a unique model for studying a virus-induced chronic neurological disease. Moreover, combined with the use of MAb 667, it is an invaluable tool for the optimization of antiviral antibody-based gene or cell therapy methods. The development of such fresh restorative methods is definitely justified by the fact that intravenous infusion of purified antibodies, despite its apparent simplicity, is not relevant for the long-term treatment of individuals suffering from severe or life-threatening diseases. The reasons for this (examined in referrals 41 and 43) include the high cost of treatments, side effects, and possible anti-idiotypic responses connected.