Immunocontraception: Filamentous Bacteriophage as a Platform for Vaccine Development.

BACKGROUND: Population control of domestic, wild, invasive, and captive animal species is a global issue of importance to public health, animal welfare and the economy. There is pressing need for effective, safe, and inexpensive contraceptive technologies to address this problem. Contraceptive vaccines, designed to stimulate the immune system in order to block critical reproductive events and suppress fertility, may provide a solution. Filamentous bacteriophages can be used as platforms for development of such vaccines. OBJECTIVE: In this review authors highlight structural and immunogenic properties of filamentous phages, and discuss applications of phage-peptide vaccines for advancement of immunocontraception technology in animals. RESULTS: Phages can be engineered to display fusion (non-phage) peptides as coat proteins. Such modifications can be accomplished via genetic manipulation of phage DNA, or by chemical conjugation of synthetic peptides to phage surface proteins. Phage fusions with antigenic determinants induce humoral as well as cell-mediated immune responses in animals, making them attractive as vaccines. Additional advantages of the phage platform include environmental stability, low cost, and safety for immunized animals and those administering the vaccines. CONCLUSION: Filamentous phages are viable platforms for vaccine development that can be engineered with molecular and organismal specificity. Phage-based vaccines can be produced in abundance at low cost, are environmentally stable, and are immunogenic when administered via multiple routes. These features are essential for a contraceptive vaccine to be operationally practical in animal applications. Adaptability of the phage platform also makes it attractive for design of human immunocontraceptive agents.

Impact of a killed Tritrichomonas foetus vaccine on clearance of the organism and subsequent fertility of heifers following experimental inoculation.

Tritrichomonas foetus is a sexually transmitted reproductive pathogen of cattle that causes transient infertility, early embryonic death, metritis, pyometra, and sporadic abortions. The objective of this research was to assess the impact on reproductive health of vaccinating naïve heifers with a killed T. foetus vaccine (TrichGuard) before experimental exposure followed by breeding. A total of 40 beef heifers were randomly assigned into two treatment groups. Heifers where then vaccinated with two doses of TrichGuard or sham vaccinated with 0.9% sterile saline according to their respective groups. Sixty days following vaccination or sham vaccination, heifers were intravaginally inoculated with 2 × 106 organisms of a cloned isolate of T. foetus of bovine origin (CDTf-4) during synchronized estrus. Three days following inoculation of T. foetus, bulls free of T. foetus were introduced for natural breeding. Three bulls were maintained with the 40 heifers (20 vaccinated; 20 sham vaccinated) for a 49-day breeding season. Cervical mucous samples were obtained from each heifer at Day 0 and at 29 additional time points throughout the study for T. foetus culture. Pregnancy assessments were performed routinely by using transrectal palpation and ultrasonography. Pregnancies were detected in 19/20 (95%) vaccinated heifers and 14/20 (70%) sham-vaccinated heifers (P = 0.046). Only 4/20 (20%) of the sham-vaccinated heifers gave birth to a live calf compared with 10/20 (50%) of the vaccinated heifers (P = 0.048). Thus, embryonic or fetal loss was detected in 9/19 (47%) vaccinated heifers and 10/14 (71%) sham-vaccinated heifers (P = 0.153). The interval of time between inoculations with T. foetus and conceptions of pregnancies that were maintained until birth did not differ significantly between groups (vaccinated = 18.7 days; sham-vaccinated = 17.3 days; P = 0.716). The infectious challenge in this study proved to be very rigorous as a positive culture was detected from all heifers. The culture-positive results on the last culture day did not differ significantly (P = 0.115) between vaccinated heifers (63.9 days) and sham-vaccinated heifers (79.2 days). All uterine culture samples collected from the 26 nonpregnant heifers on Day 207 postinoculation did not result in the detection of T. foetus. These findings indicate that the killed, whole cell vaccine used in this study (TrichGuard) was effective in improving reproductive health evidenced by significantly reducing losses associated with T. foetus infections.

Experimental infection of cats (Felis catus) with Tritrichomonas foetus isolated from cattle.

Tritrichomonas foetus is recognized as the causative agent of venereal trichomoniasis in cattle. It is characterized by embryonic and early fetal death and post-coital pyometra, and feline trichomoniasis, manifest as chronic, large bowel diarrhea. Many of the infected cats are less than 2 years old and specific routes of transmission remain unknown. We recently demonstrated that feline isolates of T. foetus can successfully infect heifers, resulting in pathologic changes similar, but not identical to those previously reported as representative of bovine trichomoniasis. In this study, we experimentally infected six cats less than 1 year of age with a bovine (D-1) isolate of T. foetus and one cat with a feline (AUTf-1) isolate of T. foetus. Within 2 weeks, the cat infected with the feline (AUTf-1) isolate was culture positive for trichomonads in weekly fecal samples. At the end of 5 weeks, only one cat infected with the bovine (D-1) isolate was fecal culture positive for trichomonads. At necropsy, the intestine of each cat was removed and divided into five sections (ileum, cecum, anterior, medial and posterior colon). Contents from each section were collected and cultured. The cat infected with the feline (AUTf-1) isolate was culture positive in the ileum, cecum, medial and posterior colon. Two cats infected with the bovine (D-1) isolate were culture positive in the cecum only. Additionally, each intestinal section was submitted to a pathologist for histopathological examination. The combined results indicate that there are demonstrable differences between the feline (AUTf-1) and bovine (D-1) isolates regarding their infectivity in cats.

Prevalence of Anaplasma phagocytophilum in domestic felines in the United States.

Anaplasma phagocytophilum is among the more common tick-borne disease agents in the United States. It is of veterinary and public health significance as dogs, cats, and human beings are known to be susceptible. A. phagocytophilum is transmitted trans-stadially by either nymphs or adults of either the black-legged tick (Ixodes scapularis) or the western black-legged tick (Ixodes pacificus). Little information is available regarding either the prevalence of this agent in cats or the dynamics of vector transmission. Four hundred and sixty feline blood samples from sites throughout the United States were assayed for antibodies to A. phagocytophilum using an indirect immunofluorescence assay (IFA). Results of the prevalence study showed that 20 samples (4.3%) were positive for A. phagocytophilum antibodies by IFA at a 1:50 dilution, however these results could not be confirmed by PCR analysis. PCR analysis for other cross-reacting Ehrlichia/Anaplasma spp. was also negative. These results demonstrate that natural infection of A. phagocytophilum in cats is uncommon.

Neospora caninum: adoptive transfer of immune lymphocytes precipitates disease in BALB/c mice.

Neospora caninum is a recently described apicomplexan parasite first isolated from a dog in 1988 and has subsequently been shown to infect a wide range of mammals. In mice, Neospora can cause primary pneumonia, myositis, encephalitis, radiculoneuritis, and pancreatitis. Whereas, certain aspects of the host immune response to Toxoplasma gondii have been well studied, not as much is known about the full immune response to Neospora. This paper examines whether or not immune splenocytes are able to adoptively transfer protection against N. caninum infection in BALB/c mice. Mice receiving immune enriched CD8+ cells had severe neurological signs by 19 days post infection. Mice receiving immune enriched CD4+ cells had mild neurological signs on day 22 post infection. It would appear that additional immune cells can precipitate disease in the presence of circulating lymphocytes.

Cytokine gene expression in response to SnSAG1 in horses with equine protozoal myeloencephalitis.

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on a Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. In a previous study, cell-mediated immune responses to SnSAG1 were shown to be statistically significantly reduced in horses with EPM in comparison to EPM-negative control horses. It therefore appears as though the parasite is able to induce immunosuppression towards parasite-derived antigens as parasite-specific responses are decreased. Isolated peripheral blood lymphocytes from 21 EPM (cerebrospinal fluid [CSF] Western blot)-negative horses with no clinical signs and 21 horses with clinical signs of EPM (CSF Western blot positive) were cocultured with SnSAG1 for 48 and 72 h, and the effect on cytokine production was investigated by means of reverse transcriptase PCR. Cytokines assayed include gamma interferon (IFN-gamma), tumor necrosis factor alpha, interleukin (IL)-2, IL-4, and IL-6. beta-Actin was used as the housekeeping gene. A Wilcoxon signed-rank test of the findings indicated that there was a statistically significant decrease in IFN-gamma production after 48 h in culture for samples from horses with clinical disease. There was also a statistically significant increase in IL-4 production after 72 h in culture for samples from horses with EPM. These results further support the notion that this parasite is able to subvert the immune system in horses with clinical disease.

Use of imidacloprid-permethrin to prevent transmission of Anaplasma phagocytophilum from naturally infected Ixodes scapularis ticks to dogs.

One group of eight beagles was treated with a combination of imidacloprid and permethrin 7 days before exposure to Ixodes scapularis ticks that were naturally infected with Anaplasma phagocytophilum. A second group of eight beagles was not treated and was also exposed to infected ticks. Seven of eight non-treated dogs--but none of the treated dogs--developed specific antibodies to A. phagocytophilum. Results of this study indicate that a combination of imidacloprid and permethrin can prevent transmission of A. phagocytophilum to dogs if administered before exposure to infected ticks.

Serosurvey of viral infections in free-ranging Namibian cheetahs (Acinonyx jubatus).

Cheetahs (Acinonyx jubatus) in captivity have unusually high morbidity and mortality from infectious diseases, a trait that could be an outcome of population homogeneity or the immunomodulating effects of chronic stress. Free-ranging Namibian cheetahs share ancestry with captive cheetahs, but their susceptibility to infectious diseases has not been investigated. The largest remaining population of free-ranging cheetahs resides on Namibian farmlands, where they share habitat with domestic dogs and cats known to carry viruses that affect cheetah health. To assess the extent to which free-ranging cheetahs are exposed to feline and canine viruses, sera from 81 free-ranging cheetahs sampled between 1992 and 1998 were evaluated for antibodies against canine distemper virus (CDV), feline coronavirus (feline infectious peritonitis virus; FCoV/ FIPV), feline herpesvirus 1 (FHV1), feline panleukopenia virus (FPV), feline immunodeficiency virus (FIV), and feline calicivirus (FCV) and for feline leukemia virus (FeLV) antigens. Antibodies against CDV, FCoV/FIPV, FHV1, FPV, and FCV were detected in 24, 29, 12, 48, and 65% of the free-ranging population, respectively, although no evidence of viral disease was present in any animal at the time of sample collection. Neither FIV antibodies nor FeLV antigens were present in any free-ranging cheetah tested. Temporal variation in FCoV/FIPV seroprevalence during the study period suggested that this virus is not endemic in the free-ranging population. Antibodies against CDV were detected in cheetahs of all ages sampled between 1995 and 1998, suggesting the occurrence of an epidemic in Namibia during the time when CDV swept through other parts of sub-Saharan Africa. This evidence in free-ranging Namibian cheetahs of exposure to viruses that cause severe disease in captive cheetahs should direct future guidelines for translocations, including quarantine of seropositive cheetahs and preventing contact between cheetahs and domestic pets.

Cell-mediated immune responses in horses with equine protozoal myeloencephalitis.

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Cell-mediated immune responses to mitogens have been shown to be reduced in horses with EPM, although it is not known whether the parasite causes this immunosuppression or if the immunosuppression is required for disease manifestation. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. Isolated peripheral blood lymphocytes from 43 EPM-negative horses and 28 horses with clinical EPM were cocultured with a mitogen or SnSAG1, and lymphocyte blastogenic responses to these antigens was measured by tritiated thymidine uptake. The ability of SnSAG1 to induce gamma-interferon (gammaIFN) production was also investigated with reverse transcriptase-polymerase chain reaction. There was no significant differences between EPM-positive and -negative horses in lymphocyte responses to ConcanavalinA. However, lymphocytes from EPM-negative horses responded significantly higher to SnSAG1 than lymphocytes from EPM-positive horses. GammaIFN production was detectable by 24 hr in culture in response to SnSAG1 in all EPM-negative horses. There was still no detectable gammaIFN production in EPM-positive horses after 72 hr in culture. It appears that the parasite is also able to induce an immunosuppression toward parasite-derived antigens as parasite-specific responses are decreased.

The use of CpG as an adjuvant to Toxoplasma gondii vaccination.

Unmethylated CpG dinucleotides have been found to stimulate general immune responses in mammals. CpG motifs have further been shown to be potent adjuvants when used in conjunction with vaccines for viral and bacterial organisms. It was necessary to determine whether these CpG motifs will also enhance immune responses in parasitic diseases. We therefore decided to test the effect of CpG adjuventation on immunization with a temperature-sensitive mutant (ts4) strain of Toxoplasma gondii. Mice were divided into groups receiving either ts4 only, CpG only, ts4 with CpG, or untreated controls. Mice were challenged with a lethal dose of T. gondii (RH strain) tachyzoites 28 days after vaccination. There were significant differences observed between the ts4-only group and the other three groups with regards to antibody isotype and survival, with the former group surviving lethal challenge. CpG adjuventation appeared to not enhance survival.

Seroprevalence of Neospora caninum and Toxoplasma gondii in captive and free-ranging nondomestic felids in the United States.

Seven serum samples of 101 samples from nondomestic, captive and free-ranging felids from the United States were indirect fluorescent antibody positive for antibodies to Neospora caninum, whereas 44 samples were positive for antibodies to T. gondii. Although none of the captive animals displayed clinical signs of disease, nondomestic felids in the United States have been exposed to, and are likely infected with, N. caninum and T. gondii. This may have serious implications for zoological gardens exhibiting susceptible animals, such as kangaroos, close to felids.