CTLA-4 Blockade Enhances the Therapeutic Effect of an Attenuated Poxvirus Vaccine Targeting p53 in an Established Murine Tumor Model

Abstract

p53 is overexpressed by half of all cancers, and is an attractive target for a vaccine approach to immunotherapy. p53 overexpression is frequently the result of point mutations, which leaves the majority of the protein in its wild-type form. Therefore, the majority of p53 sequence is wild type, making it a self-protein for which tolerance plays a role in limiting immune responses. To overcome tolerance to p53, we have expressed wild-type murine p53 in the nonpathogenic attenuated poxvirus, modified vaccinia virus Ankara (recombinant modified vaccinia virus Ankara expressing wild-type murine p53 (rMVAp53)). Mice immunized with rMVAp53 vaccine developed vigorous p53-specific CTL responses. rMVAp53 vaccine was evaluated for its ability to inhibit the outgrowth of the syngeneic murine sarcoma Meth A, which overexpresses mutant p53. Mice were inoculated with a lethal dose (5 × 105 cells injected s.c.) of Meth A tumor cells and vaccinated by i.p. injection 3 days later with 5 × 107 PFU of rMVAp53. The majority of mice remained tumor free and resistant to rechallenge with Meth A tumor cells. We wished to determine whether rMVAp53 immunization could effect the rejection of an established, palpable Meth A tumor. In subsequent experiments, mice were injected with 106 Meth A tumor cells, and treated 6 days later with anti-CTLA-4 Ab (9H10) and rMVAp53. The majority of treated mice had complete tumor regression along with lasting tumor immunity. In vivo Ab depletion confirmed that the antitumor effect was primarily CD8 and to a lesser extent CD4 dependent. These experiments demonstrate the potential of a novel cell-free vaccine targeting p53 in malignancy.

TLR ligand suppression or enhancement of Treg cells? A double-edged sword in immunity to tumours

“…It has been shown in several animal models that CTLA-4 signalling can restrict antitumour immune responses (Leach et al, 1996;Kwon et al, 1997;Espenschied et al, 2003). Antibodies against CTLA-4 are currently being tested for their ability to treat human tumours.…”

Two Distinct Pathways of Immuno-Modulation Improve Potency of p53 Immunization in Rejecting Established Tumors

“…Prior studies from our laboratory have demonstrated the ability to generate p53-specific responses after immunization with modified vaccinia Ankara (MVA) expressing WT murine p53 (MVAp53; Ref. 14). Immunized mice developed vigorous p53-specific CTL responses and were able to reject small, established p53-overexpressing Meth A tumors.…”

“…MVA-expressing p53 and pp65 were generated by homologous recombination with WTMVA infection (obtained from Drs. Moss and Wyatt) and transfected pMCO3 containing an insert in BHK-21 cells (14). MVA were screened for gus marker expression and evaluated for purity and absence of WTMVA by PCR.…”

“…Construction of MVA-expressing p53 (14) and pp65 (28) was performed as described previously. In brief, the entire cDNA of murine WT p53 and CMV antigen pp65 was ligated into the cloning site of the MVA expression vector pMCO3 [a kind gift of Drs.…”

Improving Antitumor Immune Responses by Circumventing Immunoregulatory Cells and Mechanisms

“…Based on successful preclinical murine tumor therapy models (158,159), a number of ongoing and completed clinical trials have used anti-CTLA-4 to treat patients with cancer of different histologic types. The initial reports have been encouraging, with two groups documenting significant increases in antitumor immunity in vaccinated melanoma patients following anti-CTLA-4 treatment, including the induction of some objective tumor regressions (160 -162).…”