Date Published: April 20, 2017
Publisher: Public Library of Science
Author(s): Sara Lustigman, Alexandra Grote, Elodie Ghedin, Judd L. Walson. http://doi.org/10.1371/journal.pntd.0005464
Partial Text: Lymphatic filariasis (LF) is a disease affecting approximately 120 million people in over 73 countries  and caused by infection with a group of filarial nematodes transmitted by mosquito vectors. Wuchereria bancrofti is responsible for approximately 90% of the disease worldwide, while the remaining cases are due to Brugia malayi and B. timori . These filarial nematodes have important social and economic impact causing high morbidity and serious illnesses resulting in social stigmatization, marginalization, and loss of work for the afflicted . In 2000, The Global Programme to Eliminate Lymphatic Filariasis (GPELF) was launched with the objective to eliminate this disease as a public health problem by 2020 . The eradication of lymphatic filariasis relies on mass drug administration (MDA) using the three drugs currently available for treatment: diethylcarbamazine (DEC), albendazole, and ivermectin. GPELF also has made significant progress in many countries, delivering, between the years 2000 and 2014, 5.6 billion treatments to more than 763 million people living in 61 countries. It was estimated that this directly prevented 36 million clinical cases and saved 175 million disability adjusted life years (DALYs) . However, it is unlikely that LF will be eliminated by the target year of 2020 as 55 of the 73 countries considered to be endemic for LF in 2015 still require MDA . Moreover, GPELF has lagged in Sub-Saharan Africa where only 2 of 35 LF-endemic countries have stopped MDA and started post-MDA surveillance. Notably, recent studies show that single-dose combination therapy with the three antifilarial drugs (ivermectin/albendazole/diethylcarbamazine, or IDA) appears to be superior to current regimens used in the elimination programs, which may help accelerate LF elimination in Africa. Although it has not yet been tested, IDA may also be useful for treating onchocerciasis [7,8]. However, reports of drug resistance to ivermectin and albendazole [9,10] as well as serious concerns about using DEC in Sub-Saharan Africa because of ocular adverse events after DEC treatment of onchocerciasis in the past, makes the discovery of novel drugs against onchocerciasis imperative .
In 2007, the same year PLoS NTD was launched, the first parasitic nematode genome was published with the draft genome of B. malayi [32,33]. In 2009, a review in PLoS NTD focused on helminth genomics and its implications for human health , predicting that new sequence information would revise what we knew of the host-parasite, vector-pathogen, and filaria-symbiont relationships. At that time, the genomes of B. malayi and its endosymbiont, Wolbachia (wBm)  were available along with expressed sequence tags or EST datasets of other filarial parasites, enabling the construction of a microarray containing 18,104 elements derived from B. malayi (15,412), O. volvulus (1,016), W. bancrofti (872) and Wolbachia (wBm, 804 genomic elements) genomic information. This microarray was used in many studies to analyze expression profiles during development and after drug treatments [36–42]. With new sequencing technologies, RNAseq has now become the more common tool to study stage-specific expression profiles of filarial worms [43,44] and the effects of known drugs on the worm’s transcriptome [45–47].
As current microfilaricidal drugs appear to be insufficient for the control and elimination of these parasitic infections, new drugs will be required. Present efforts focus on the screening of libraries of drugs, including FDA repurposed drugs, against adult Brugia and Onchocerca worms in vitro and the selection of those that are effective for additional pre-clinical development and testing in small animal models [65–67]. While the primary focus is development of a macrofilaricidal drug candidate for the treatment of onchocerciasis, it is expected that parallel screening of the closely related filarid, Brugia, will also yield drug candidates for the treatment of lymphatic filariasis. An example is the discovery of auranofin as a potent anti-filarial drug . Auranofin is an FDA-approved gold compound (2,3,4,6-tetra-O-acetyl-1-thio-beta-D-glucopyranosato-S (triethylphosphine) gold) that has been used to treat rheumatoid arthritis for over 25 years. In the study described by Bulman et al.  a library of over 2,000 FDA-approved compounds was screened first on B. malayi adult female worms and only auranofin was highly effective in inhibiting adult Brugia motility. It was then also shown to inhibit molting of O. volvulus and to kill adult O. ochengi worms. Additional studies will need to be conducted to determine efficacy with short treatment regimens in vivo using animal models and to obtain pharmacokinetic data before moving on to clinical development. Another approach is to screen repurposed and approved drugs from the human pharmacopoeia. This can be most easily done to target the Wolbachia endosymbionts of filarial worms, since macrofilaricidal effects have been observed when there is at least 90% reduction in the bacterial load [69,70], achievable with certain antibiotics.
There are two types of vaccines that would be necessary for the efficient control of onchocerciasis: (a) a prophylactic vaccine to be used in children <5 years old to block new infections and the accumulation of adult worms, thus reducing microfilarial densities in the skin, pathology and transmission; and (b) a therapeutic vaccine that would be used in older children and adults that already carry adult worms, to potentially impair the fertility of female parasites, suppress the release of nodular microfilariae from the female worm and/or kill them once released, reducing accumulation of skin microfilariae thus interrupting the transmission cycle . In both cases, the recipient of the vaccine benefits from a reduction in the only O. volvulus parasite stage that causes disease, the microfilaria. Importantly, the entire community also benefits since the microfilaria is the transmissible stage to insect vectors, further protecting areas from recurrence transmission where local elimination may have already been achieved. Vaccines may also lower the number of annual MDA with ivermectin, forestalling drug resistance, and ensuring the success of the existing MDA. Most of the present O. volvulus vaccine candidates were discovered by screening expression libraries with various antibody probes . Two O. volvulus vaccine proteins, Ov-103 and Ov-RAL-2, are promising candidates for prophylactic vaccine  and their homologues in B. malayi were shown to also induce protection against infection with B. malayi infective stage larvae . The disappointing results of clinical trials for several infectious diseases highlight the current limitations of vaccine candidate selection approaches that often fail to exclude at an early stage antigens with poor immunogenicity or low safety profiles in humans [76,77]. One approach for identifying novel vaccine candidates is immunomics [78–82], which allows high-throughput profiling of the host immune antibody responses to genome-wide candidate parasite antigens. Using this approach with putatively immune human sera and sera from infected individuals , six new potential vaccine antigens were identified by screening antibody responses (IgG1, IgG3 and IgE) against an O. volvulus recombinant protein array containing 362 proteins. Four of these antigens are highly expressed during the early stages of larval development in the human host and thus could be tested for efficacy in a prophylactic vaccine. The 2 other proteins are highly expressed by the microfilariae and are specifically recognized by sera from protected individuals who never developed a patent infection. This opens new possibilities for developing a safe anti-transmission or therapeutic vaccine. To the best of our knowledge, this is the first and only occasion in which genome-wide stage-specific expression data from O. volvulus have been exploited to discover novel vaccine candidates in an unbiased manner. Future studies using the diffusion chamber mouse model for O. volvulus will confirm whether these antigens do indeed protect against infection by L3s or against microfilariae . Gold standard diagnosis using blood films or skin snips has become less relevant as mass drug distribution programs for control of filarial infections have expanded. The spectrum of programmatic processes (mapping, mass drug interventions, monitoring and evaluation, and surveillance) require different approaches as different questions are asked at each stage . Infection intensity may refer to adult worm burden or microfilarial load in the skin of O. volvulus infected individuals. However, the relationship between microfilarial load, as assessed by quantification of microfilaridermia by skin snip in onchocerciasis, and the total adult parasite burden is at best semi-quantitative. Moreover, the current toolbox for diagnosis and surveillance of onchocerciasis, as well as other helminthic infections, is limited because many of the available tools suffer from lack of sensitivity and specificity and/or are cost-prohibitive [85,86]. Given the constraints of achieving elimination using MDA with ivermectin alone, and concerns about recrudescence in areas of previous onchocerciasis control, more efficient tools are needed for diagnosis and monitoring of current and future control measures using emerging technologies that are field-deployable or suitable for low-resource settings. The development of better diagnostic tools is greatly needed for post-treatment surveillance where transmission of infection has been brought under control, the certification phase, and for mapping prevalence in meso- and hypo-endemic areas that had heretofore been ivermectin-naive. Transcriptome and proteome data have helped in the discovery of new biomarkers for O. volvulus infection. Using immunomics and the O. volvulus protein array used for the discovery of vaccine candidates, we identified 7 previously unrecognized biomarkers of active patent infection (OVOC10469, OVOC10602, OVOC11950, OVOC3261, OVOC5127, OVOC8491, OVOC9988), based on IgG4 responses in infected individuals . Future assays, such as a luciferase immunoprecipitation system (LIPS) immunoassay, will help validate if such highly antigenic O. volvulus proteins can be used as specific and sensitive biomarkers of patent infection. The abundance of genomic, transcriptomic, and proteomic data has already provided novel biological insight into filarial nematodes, and led to the identification of novel drug targets, vaccine candidates and biomarkers of infection. We should expect that upcoming exploitation of these various novel datasets will further our understanding of these unique parasites and their interaction with the final host, ultimately helping us reach the goal envisioned by WHO to eliminate filarial infections for good . Source: http://doi.org/10.1371/journal.pntd.0005464