In Vivo Efficacy of Anti-Glycoprotein 41, But Not Anti-Glycoprotein 120, Immunotoxins in a Mouse Model of HIV Infection

Abstract

Immunotoxins (ITs) targeting the HIV envelope protein are among the most efficacious antiviral therapies when tested in vitro. Yet a first-generation IT targeted to gp120, CD4-PE40 (chimeric immunotoxin using CD4 and the translocation and enzymatic domains of Pseudomonas exotoxin A), showed limited promise in initial clinical testing, highlighting the need for improved ITs. We have used a new mouse model of HIV infection to test the comparative efficacy of anti-HIV ITs targeted to gp120 or to gp41. Irradiated SCID/nonobese diabetic mice are injected with a tumor of human CD4+ cells susceptible to infection and at a separate site persistently HIV-infected cells. The spread of infection from infected to susceptible tumor is monitored by plasma p24 and the presence of HIV-infected cells in the spleen. Anti-gp41 ITs in combination with tetrameric CD4-human Ig fusion protein have pronounced anti-HIV effects. Little if any anti-HIV efficacy was found with either CD4-PE40 or an Ab-targeted anti-gp120 IT. These data support continued exploration of the utility of ITs for HIV infection, particularly the use of anti-gp41 ITs in combination with soluble CD4 derivatives.

Design andIn VivoCharacterization of Immunoconjugates Targeting HIV gp160

“…This was observed in mice (8) and by the relative lack of efficacy of HY constructs in macaques, despite their use at much higher doses. The greater in vivo effect of ITs using intact Ig is most likely due to the increased plasma residence time (days for Ig versus hours for scFv), as we have previously demonstrated (8).…”

“…Our efforts have primarily focused on comparing different anti-Env MAbs for their ability to target ITs to infected cells. We have consistently found that those targeted to the gp41 external helix-loop region (for example, MAb 7B2), when used with sCD4, were the most efficacious MAbs for delivering anti-HIV ITs to HIV-infected cells (4,8,18,19,41). To extend this observation, we tested ITs and ADCs based upon MAb 7B2 for safety and efficacy in both mice and SHIV-infected macaques.…”

“…Anti-Env ITs and radio-ICs have been shown to have potent in vitro anti-HIV activity at therapeutically obtainable concentrations when tested in a variety of different cell types, including primary T cells and macrophages, and tested against clinical virus isolates. Efficacy has also been demonstrated in murine models of active and persistent HIV infection (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). Expression of Env on the cells that constitute persistent reservoirs in patients in whom HIV is suppressed by antiretroviral therapy may be through ongoing low-level, anatomically localized transcription of viral RNA or may require pharmacologic activation of latent provirus utilizing so-called activate-and-purge protocols (14)(15)(16)(17).…”

“…The lack of activity of scFv HY-KDEL may be due to its structure, since an scFv has a shorter plasma residence time (2 to 3 h) than an intact Ig (2 to 3 days) and thus a shorter window of therapeutic opportunity (8). Targeting the CD4bs using ITs constructed from intact IgGs (HY-RAC and CD4-IgG2-RAC) was ineffective both in vitro and in vivo, confirming our observation that killing by ITs directed against the CD4bs is limited by size; i.e., scFv ITs function in vitro, whereas those based on intact IgGs do not Groups of 8 cyclophosphamide-treated animals were injected with 10 7 H9 cells (i.p.)…”

“…These ITs are especially effective when used in association with soluble CD4 (sCD4), which destabilizes the interaction between gp120 and gp41, causing increased exposure of this partially obscured epitope (4,8,18). Others propose targeting the CD4-binding site (CD4bs) with either MAbs or sCD4 itself (1, 5-7, 9-11, 13).…”