Expression and characterization of Ov-47, a dominant antigen of Onchocerca volvulus

Antibody responses against the vaccine antigens Ov-103 and Ov-RAL-2 are associated with protective immunity to Onchocerca volvulus infection in both mice and humans

Background

The current strategy for the elimination of onchocerciasis is based on annual or bi-annual mass drug administration with ivermectin. However, due to several limiting factors there is a growing concern that elimination of onchocerciasis cannot be achieved solely through the current strategy. Additional tools are critically needed including a prophylactic vaccine. Presently Ov-103 and Ov-RAL-2 are the most promising vaccine candidates against an Onchocerca volvulus infection.

Methodology/Principal findings

Protection induced by immunization of mice with the alum-adjuvanted Ov-103 or Ov-RAL-2 vaccines appeared to be antibody dependent since AID^-/- mice that could not mount antigen-specific IgG antibody responses were not protected from an Onchocerca volvulus challenge. To determine a possible association between antigen-specific antibody responses and anti-larvae protective immunity in humans, we analyzed the presence of anti-Ov-103 and anti-Ov-RAL-2 cytophilic antibody responses (IgG1 and IgG3) in individuals classified as putatively immune, and in infected individuals who developed concomitant immunity with age. It was determined that 86% of putatively immune individuals and 95% individuals with concomitant immunity had elevated IgG1 and IgG3 responses to Ov-103 and Ov-RAL-2. Based on the elevated chemokine levels associated with protection in the Ov-103 or Ov-RAL-2 immunized mice, the profile of these chemokines was also analyzed in putatively immune and infected individuals; both groups contained significantly higher levels of KC, IP-10, MCP-1 and MIP-1β in comparison to normal human sera. Moreover, human monospecific anti-Ov-103 antibodies but not anti-Ov-RAL-2 significantly inhibited the molting of third-stage larvae (L3) in vitro by 46% in the presence of naïve human neutrophils, while both anti-Ov-103 and anti-Ov-RAL-2 antibodies significantly inhibited the molting by 70–80% when cultured in the presence of naive human monocytes. Interestingly, inhibition of molting by Ov-103 antibodies and monocytes was only in part dependent on contact with the cells, while inhibition of molting with Ov-RAL-2 antibodies was completely dependent on contact with the monocytes. In comparison, significant levels of parasite killing in Ov-103 and Ov-RAL-2 vaccinated mice only occurred when cells enter the parasite microenvironment. Taken together, antibodies to Ov-103 and Ov-RAL-2 and cells are required for protection in mice as well as for the development of immunity in humans.

Conclusions/Significance

Alum-adjuvanted Ov-103 and Ov-RAL-2 vaccines have the potential of reducing infection and thus morbidity associated with onchocerciasis in humans. The development of cytophilic antibodies, that function in antibody-dependent cellular cytotoxicity, is essential for a successful prophylactic vaccine against this infection.

Onchocerca volvulus is the causative agent of river blindness that infects approximately 17 million people, mostly in Africa. The current strategy for elimination of O. volvulus focuses on controlling transmission through ivermectin-based mass drug administration programs. Due to potential ivermectin resistance, the lack of macrofilaricidal activity by ivermectin, and the prolonged time (>20 years) needed for successful interruption of transmission in endemic areas, additional tools are critically needed including a vaccine against onchocerciasis. Ov-103 and Ov-RAL-2 are presently the most promising vaccine candidates for a prophylactic vaccine. The mechanism of protective immunity induced in mice by the alum-adjuvanted Ov-103 or Ov-RAL-2 vaccines appear to be multifactorial with essential roles for antibodies, chemokines and the specific effector cells they recruit. In this study, we show for the first time that, anti-Ov-103 and anti-Ov-RAL-2 antibodies, chemokines and innate cells also appear to be associated with protective immunity against O. volvulus infection in humans, similar to the vaccine studies observed in the O. volvulus mouse model.

Prediction and validation of the structural features of Ov58GPCR, an immunogenic determinant of Onchocerca volvulus

Onchocerciasis is a severely debilitating yet neglected tropical disease (NTD) that creates social stigma, generates and perpetuates poverty, and leads ultimately in some cases to irreversible unilateral or bilateral blindness if untreated. Consequently, the disease is a major impediment to socioeconomic development. Many control programs have been launched for the disease with moderate successes achieved. This mitigated hit is partially due to the lingering need for reliable, non-invasive and easily applicable tools for mapping endemic regions and post-elimination surveillance. In this work, bioinformatics analyses combined with immunological assays were applied in a bid to develop potential tools for diagnosis and assessing the success of drug treatment programs. We report that (i) the O. volvulus antigen, Ov58GPCR is a G-protein coupled receptor (GPCR) conserved in related nematodes, (ii) synthetic peptides predicted to be in the extracellular domain (ECD) of Ov58GPCR are indeed immunogenic epitopes in actively-infected individuals, (iii) synthetic peptide cocktails discriminate between actively-infected individuals, treated individuals and healthy African controls, (iv) polyclonal antibodies against one of the peptides or against the bacterially-expressed ECD reacted specifically with the native antigen of O. volvulus total and surface extracts, (v) Ov58GPCR is transcribed in both larvae and adult parasite stages, (vi) IgG and IgE responses to the recombinant ECD decline with ivermectin treatment. All these findings suggest that the extracellular domain and synthetic peptides of Ov58GPCR, as well as the specific immune response generated could be harnessed in the context of disease diagnosis and surveillance.

The Onchocerca volvulus cysteine proteinase inhibitor, Ov-CPI-2, is a target of protective antibody response that increases with age

Background

Despite considerable efforts, a suitable vaccine against Onchocerca volvulus infection has remained elusive. Herein, we report on the use of molecular tools to identify and characterize O. volvulus antigens that are possibly associated with the development of concomitant immunity in onchocerciasis.

Methodology/Principal Findings

Third-stage larvae (L3) and molting L3 (mL3) O. volvulus stage-specific cDNA libraries were screened with a pool of sera from chronically infected patients who had likely developed such immunity. The 87 immunoreactive clones isolated were grouped into 20 distinct proteins of which 12 had already been cloned and/or characterized before and 4 had been proven to be protective in a small O. volvulus animal model. One of these, onchocystatin (Ov-CPI-2), a previously characterized O. volvulus cysteine proteinase inhibitor was, overall, the most abundant clone recognized by the immune sera in both the L3 and mL3 cDNA libraries. To further characterize its association with protective immunity, we measured the IgG subclass and IgE class specific responses to the antigen in putatively immune (PI) and infected (INF) individuals living in a hyperendemic area in Cameroon. It appeared that both groups had similar IgG3 and IgE responses to the antigen, but the INF had significantly higher IgG1 and IgG4 responses than the PI individuals (p<0.05). In the INF group, the IgG3 levels increased significantly with the age of the infected individuals (r = 0.241; p<0.01). The IgG1 responses in the INF were high regardless of age. Notably, culturing L3 in vitro in the presence of anti-Ov-CPI-2 monospecific human antibodies and naïve neutrophils resulted in almost complete inhibition of molting of L3 to L4 and to cytotoxicity to the larvae.

Conclusions/Significance

These results add to the knowledge of protective immunity in onchocerciasis and support the possible involvement of anti-Ov-CPI-2 IgG1 and/or IgG3 cytophilic antibodies in the development of protective immunity in the PI and the INF. The results further support the consideration of Ov-CPI-2 as a leading target for an anti-L3 vaccine.

Onchocerciasis is a chronic and highly debilitating disease of humans caused by a worm called Onchocerca volvulus. This worm can live in the human body for over 15 years. The disease affects mainly the skin and eyes and is the second leading infectious cause of blindness worldwide. There is currently no vaccine to prevent the infection. Available drugs can give short-term relief but cannot cure the infection. To prevent infection, a vaccine against the third-stage infective larva, L3, or the developing larva is required. These stages were shown to be the targets of protective immunity that develops in individuals who live in onchocerciasis endemic regions. One type of protective immunity has been shown to develop with age and is called concomitant immunity. In the present study, we have identified a number of larval antigens that may be associated with the development of such immunity. The most prominent of these antigens was Ov-CPI-2, also called onchocystatin, which had previously been shown to be a promising vaccine candidate. This antigen was further characterized and confirmed to be possibly also a target of immune protection that develops in the infected individuals with age and is referred to as concomitant immunity.

Isolation and expression of UB05, a Plasmodium falciparum antigen recognised by antibodies from semi-immune adults in a high transmission endemic area of the Cameroonian rainforest

BACKGROUND

Antibodies in adults living in malaria endemic areas that target specific parasite antigens are implicated in protective immunity to infection and disease. This study aimed to identify, isolate and characterise targets of protective immunity in malaria. A Plasmodium falciparum antigen termed UB05 (Genbank Accession Number DQ235690: PlasmoDB PF10_ 0372) that had been isolated by immunoscreening with semi-immune sera was studied.

METHODS

Polymerase chain reaction, sequencing and bioinformatics were used to analyse the UB05 gene. A specific mouse anti-UB05 antibody was used in parasite reinvasion growth/inhibition assays and in immunoflourescence to localise the antigen. In a cross-sectional study, enzyme linked immunosorbent assay was used to study immunoglobulin G (IgG) responses to the antigen.

RESULTS

The gene revealed significant homologies with gene sequences from Plasmodia and other apicomplexan parasites and had two alleles in the wild P. falciparum isolates. The antigen is expressed by schizonts and segmented merozoites. Mouse antibodies against it marginally inhibit in vitro invasion of erythrocytes by P. falciparum. The IgG responses to UB05 were found to be significantly lower (p<0.05) in the sera of children (2-5 years) compared with adults (>18 years), with or without parasitaemia. However, parasitaemia correlated inversely (r=0.7- 0.75) with serum anti-UB05 IgG concentrations. Furthermore, anti-UB05 IgG concentrations were lower in the sera of febrile patients (body temperature >37.5 degrees C) than their non-febrile counterparts regardless of parasitaemia status.

CONCLUSIONS

These results are compatible with a role for UB05 in the development of immunity and as a marker of protective immunity to malaria.