719. Neutralization of Free but Not Cell Associated Oncolytic Measles Vurses by Antibody and Complement
Authors: Ianko Iankov, Stephen J. Russell
Journal: Molecular Therapy (2005)

Attenuated measles viruses (MV) are potent oncolytic agents. However, the major obstacle to clinical application of oncolytic measles by systemic intravenous (i.v.) route is pre-existing antibody response in human plasma as result of immunization or natural infection. The surface viral glycoproteins, H and F, induce protective neutralizing humoral immunity. H specific antibodies can interfere with virus attachment to the host cells, while antibodies directed against F protein can block subsequent viral entry and cell-cell fusion. Complement activation by the classical pathway can enhance antibody-mediated neutralization of the viruses. Recently, F protein mediated activation of the alternative complement pathway and C3b deposition on the surface of infected cells has been demonstrated. However, the time kinetics of neutralization of different viral doses by the complement system and is still not well characterized. In our in vitro tests we used purified monoclonal antibodies (MAbs) to determine their ability to inactivate measles virus infectivity after exposure for different time intervals alone or in the presence of active complement proteins. In the absence of complement, the anti-H IgM antibody CL48 almost completely inhibited MV infectivity whereas the CL55 antibody (IgG2b) was only partially protective (60-85% protection) in virus neutralization (VN) assays. In contrast, the anti-F antibody Y-503 (IgG1) had no effect on virus infectivity but could inhibit syncytia formation in Vero cell monolayers. Both H specific MAbs completely neutralized MV in the presence of active guinea pig complement (GPC) system. In contrast, preincubation with IgG antibody against F protein and complement could not neutralize the virus. GPC alone reduced the virus titer by 60-70% in comparison to heat inactivated control serum, possibly due to F protein mediated alternative pathway activation. In kinetic studies the H specific IgM antibody and GPC rapidly neutralized a high dose (>5106 TCID50/ml) of MV after a short preincubation (from 30 s to 5 min) prior infection of human monocytic cells. Thus, the classical complement pathway is a very efficient mechanism of cell-free virus neutralization and could be a serious problem for oncolytic MV systemic administration. A higher dose of infectious virions could not overcome this inactivation. Since free MV is highly vulnerable for antibody and complement inactivation, we hypothesized that peripheral blood cells could be a suitable carrier of the virus. Infection of a human monocytic line only for 5 min before treatment with IgM MAb and GPC completely protected the attached virus from neutralization. The virus-infected cells survived treatment with antibodies and complement and subsequently transferred infection efficiently to overlaid tumor lines. These in vitro experiments show that oncolytic MV can escape from antibody and complement mediated neutralization within infected monocytes. We are therefore exploring the possibility that i.v. injection of infected cells (isolated monocytes or whole peripheral blood mononuclear preparation) rather than free viruses can successfully overcome the pre-existing antibody response in patients.