T-cell response to cottontail rabbit papillomavirus structural proteins in infected rabbits

Detection of L1, infectious virions and anti-L1 antibody in domestic rabbits infected with cottontail rabbit papillomavirus

Shope papillomavirus or cottontail rabbit papillomavirus (CRPV) is one of the first small DNA tumour viruses to be characterized. Although the natural host for CRPV is the cottontail rabbit (Sylvilagus floridanus), CRPV can infect domestic laboratory rabbits (Oryctolagus cuniculus) and induce tumour outgrowth and cancer development. In previous studies, investigators attempted to passage CRPV in domestic rabbits, but achieved very limited success, leading to the suggestion that CRPV infection in domestic rabbits was abortive. The persistence of specific anti-L1 antibody in sera from rabbits infected with either virus or viral DNA led us to revisit the questions as to whether L1 and infectious CRPV can be produced in domestic rabbit tissues. We detected various levels of L1 protein in most papillomas from CRPV-infected rabbits using recently developed monoclonal antibodies. Sensitive in vitro infectivity assays additionally confirmed that extracts from these papillomas were infectious. These studies demonstrated that the CRPV/New Zealand White rabbit model could be used as an in vivo model to study natural virus infection and viral life cycle of CRPV and not be limited to studies on abortive infections.

Immunisation with recombinant BCG expressing the cottontail rabbit papillomavirus (CRPV) L1 gene provides protection from CRPV challenge

Recombinant Bacille Calmette-Guerin (rBCG) could potentially be the vaccine vehicle of choice to deliver foreign antigens from multiple pathogens. In this study we have used the cottontail rabbit papillomavirus (CRPV) rabbit model to provide a "proof of concept" that immunisation with rBCG expressing the CRPV major capsid protein, L1 (rBCG/CRPVL1), will protect outbred New Zealand White rabbits against CRPV challenge. Rabbits immunised with rBCG/CRPVL1 (10(7) cfu/ml) were protected 5 weeks post-CRPV challenge. Rabbits immunised with rBCG/CRPVL1 (10(5) cfu/ml) had papillomas, which were smaller and took longer to appear than the control rabbits. None of the negative control rabbits vaccinated with rBCG expressing an irrelevant gene or PBS were protected from CRPV challenge. Sera from rabbits immunised with rBCG/CRPVL1 (10(7) cfu/ml) were able to neutralise 54.5% of CRPV at serum dilutions of 1:200. These results provide evidence that BCG could potentially be used as a vaccine delivery vehicle for human papillomavirus proteins as a possible prophylactic vaccine.

Therapeutic vaccination with papillomavirus E6 and E7 long peptides results in the control of both established virus-induced lesions and latently infected sites in a pre-clinical cottontail rabbit papillomavirus model

This study was performed to test the therapeutic efficacy of overlapping long E6 and E7 peptides, containing both CD4+ T-helper and CD8+ CTL epitopes, on CRPV-induced lesions, which is an appropriate pre-clinical model for HPV diseases, including recurrent respiratory papillomatosis (RRP). Therapeutic peptide vaccination was able to significantly control wart growth (p < 0.01) and abrogate latent CRPV infection (p = 0.0006) compared to controls. Vaccination was associated with a T(H)1 T cell response, as suggested by a strong DTH skin test, antigen-specific proliferation of PBMC and a minimal IgG antibody response. Thus, this study shows promise for treatment of RRP by vaccination with long peptides.

Amino acid residues in the carboxy-terminal region of cottontail rabbit papillomavirus E6 influence spontaneous regression of cutaneous papillomas

Previous studies have identified two different strains of cottontail rabbit papillomavirus (CRPV) that differ by approximately 5% in base pair sequence and that perform quite differently when used to challenge New Zealand White (NZW) rabbit skin. One strain caused persistent lesions (progressor strain), and the other induced papillomas that spontaneously regressed (regressor strain) at high frequencies (J. Salmon, M. Nonnenmacher, S. Caze, P. Flamant, O. Croissant, G. Orth, and F. Breitburd, J. Virol. 74:10766-10777, 2000; J. Salmon, N. Ramoz, P. Cassonnet, G. Orth, and F. Breitburd, Virology 235:228-234, 1997). We generated a panel of CRPV genomes that contained chimeric and mutant progressor and regressor strain E6 genes and assessed the outcome upon infection of both outbred and EIII/JC inbred NZW rabbits. The carboxy-terminal 77-amino-acid region of the regressor CRPV strain E6, which contained 15 amino acid residues that are different from those of the equivalent region of the persistent CRPV strain E6, played a dominant role in the conversion of the persistent CRPV strain to one showing high rates of spontaneous regressions. In addition, a single amino acid change (G252E) in the E6 protein of the CRPV progressor strain led to high frequencies of spontaneous regressions in inbred rabbits. These observations imply that small changes in the amino acid sequences of papillomavirus proteins can dramatically impact the outcome of natural host immune responses to these viral infections. The data imply that intrastrain differences between separate isolates of a single papillomavirus type (such as human papillomavirus type 16) may contribute to a collective variability in host immune responses in outbred human populations.

The immunology of animal papillomaviruses

Papillomaviruses are species- and tissue-specific double-stranded DNA viruses. These viruses cause epithelial tumours in many animals, including man. Typically, the benign warts undergo spontaneous, immune-mediated regression, most likely effected by T-cells (especially CD4, but also CD8 subsets), whereas humoral immunity can prevent new infections. Some papillomavirus infections fail to regress spontaneously and others progress to malignant epithelial tumours. Additionally, the impact of these lesions is greater in immunosuppressed individuals. Many therapies are ineffective, and there is much interest in the potential for immunological intervention in papillomavirus infections of man and animals. Vaccination can be achieved with 'live' virus, formalin-inactivated virus, synthetic virus-like particles, and DNA vaccination. There has been much recent progress in the development of such vaccines for papillomavirus infections in the rabbit, ox and dog. Success in these animal models suggests that similar approaches may prove useful for prophylactic or therapeutic vaccination against the important human papillomaviruses involved in the development of cutaneous and anogenital warts, laryngeal papillomatosis, and cervical cancer.

Recombinant Listeria monocytogenes vaccination eliminates papillomavirus-induced tumors and prevents papilloma formation from viral DNA

Listeria monocytogenes is a gram-positive, facultative intracellular bacterium that enters the cytoplasm of infected cells and spreads directly into neighboring cells without encountering the extracellular environment. Cytoplasmic L. monocytogenes efficiently presents secreted proteins to the major histocompatibility complex class I pathway which can stimulate protective T-cell-mediated immune responses. We have used a cottontail rabbit papillomavirus (CRPV) rabbit model to test the ability of recombinant L. monocytogenes strains secreting the viral E1 protein (E1-rLm) to protect outbred rabbits against CRPV- and CRPV DNA-induced tumors. CRPV infection of outbred rabbits serves as a model for oncogenic papillomaviruses since CRPV-induced papillomas progress with high frequency to malignant carcinoma. Rabbits were vaccinated with wild-type L. monocytogenes or E1-rLm and then challenged with CRPV or viral DNA. In contrast to 0% papilloma regression in control animals, 77% of E1-rLm-vaccinated rabbits generated protective immunity that controlled and induced complete regression of tumors induced by CRPV. Latent viral DNA was not detected at 71% of the papilloma regression sites examined 4.5 months postregression. E1-rLm responder rabbits were completely resistant to papilloma formation from viral DNA. In contrast to controls, peripheral blood mononuclear cells from E1-rLm responder rabbits were able to proliferate in response to in vitro E1 stimulation. These results indicate that E1-rLm immunization generated a systemic anti-CRPV E1 cell-mediated immune response which protected outbred rabbits from tumors induced by CRPV or CRPV DNA challenge.

Intracutaneous vaccination of rabbits with the cottontail rabbit papillomavirus (CRPV) L1 gene protects against virus challenge

A DNA vaccine encoding the major capsid protein L1 of cottontail rabbit papillomavirus (CRPV) was constructed and administered intracutaneously (i.c.) to rabbits as supercoiled plasmids bound to gold beads using a specialized delivery device ("gene gun"). L1 DNA-vaccinated rabbits developed cellular proliferative responses to CRPV virus-like particles and developed high titered antibodies with neutralizing activity to CRPV. Following experimental challenge with CRPV, all of the L1 DNA-vaccinated rabbits, vs none of the controls, were protected from papilloma formation. These results demonstrate that i.c. vaccination of rabbits with the L1 papillomavirus capsid gene can induce antibodies that protect against subsequent papillomavirus infection.

Immunology of papillomavirus infection

Studies of the immunology of papillomavirus infection have come of age. Synthetic virus-like particles have been validated as vaccines for several animal papillomaviruses, and have been used to map the sero-epidemiology of human papillomavirus infection and to define papillomavirus neutralizing antibodies. Induction of cell-mediated immunity to papillomavirus early proteins is poised to become a therapeutic approach to papillomavirus infection. Studies on the immune response to papillomavirus proteins in keratinocytes are shedding light on the immunological consequences of antigen presentation by epithelial cells.

Leukocyte proliferation in vitro against cottontail rabbit papillomavirus in rabbits with persisting papillomas/cancer or after regression

Leukocyte proliferation responses to cotton-tail rabbit papillomavirus (CRPV) were measured in vitro with fresh whole blood as well as with ammonium chloride lysis-separated leukocytes. The antigens used were (1) CRPV particles produced in the athymic( (nu/nu) mouse xenograft system and (2) purified bacterial fusion proteins of the CRPV major and minor capsid proteins L1 and L2. CRPV-infected domestic rabbits with persistent papillomas or after papilloma regression, as well as uninfected controls were studied. There was a clearcut difference between infected and uninfected animals. We demonstrated antigen-specific leukocyte proliferation to at least one CRPV antigen in 12 of 21 infected rabbits but there was no positivity in 9 control animals (P = 0.004). There was whole-blood reactivity preferentially to intact CRPV particles in regressors. Specific but weak leukocyte proliferation against CRPV particles was detected in 6 of 9 regressor rabbits (66%) but only in 1 of 12 progressors (8%; P = 0.0158). This trend of greater reactivity to intact CRPV particles in regressors as compared with progressors was not seen with peripheral blood leukocytes isolated by ammonium chloride lysis. We conclude that specific leukoproliferative responses against capsid CRPV proteins exist in rabbits experimentally infected with CRPV.