Elimination of microfilariae (Litomosoides carinii Filarioidea) in the patent and in the immunized cotton-rat

Differential tissular distribution of Litomosoides sigmodontis microfilariae between microfilaremic and amicrofilaremic mice following experimental infection

Filariases are caused by onchocercid nematodes that are transmitted by arthropod vectors. More than 180 million people are infected worldwide. Mass drug administration has been set up in many endemic areas to control the parasite burden. Although very successful in limiting microfilarial load, transmission has not been completely interrupted in such areas. A proportion of infected patients with lymphatic filariasis or loiasis are known to be amicrofilaremic, as they do not present microfilariae in their bloodstream despite the presence of adult worms. A mirror status also exists in CBA/Ca mice infected with Litomosoides sigmodontis, the well-established model of filariasis. Using this model, the goal of this study was to determine if the kinetics of blood clearance of microfilariae differed between amicrofilaremic CBA/Ca mice and microfilaremic BALB/c mice. For this purpose, a qPCR approach was devised to detect microfilariae in different tissues, after a controlled inoculation of microfilariae. We showed that the rapid clearance of microfilariae from the pleural cavity or from the bloodstream of CBA/Ca mice was associated with a massive accumulation of first stage larvae in the lungs, liver and spleen.

ST2 deficiency does not impair type 2 immune responses during chronic filarial infection but leads to an increased microfilaremia due to an impaired splenic microfilarial clearance

Background

Interactions of the Th2 cytokine IL-33 with its receptor ST2 lead to amplified Type 2 immune responses. As Type 2 immune responses are known to mediate protection against helminth infections we hypothesized that the lack of ST2 would lead to an increased susceptibility to filarial infections.

Methodology/Principal Finding

ST2 deficient and immunocompetent BALB/c mice were infected with the filarial nematode Litomosoides sigmodontis. At different time points after infection mice were analyzed for worm burden and their immune responses were examined within the thoracic cavity, the site of infection, and systemically using spleen cells and plasma. Absence of ST2 led to significantly increased levels of peripheral blood microfilariae, the filarial progeny, whereas L. sigmodontis adult worm burden was not affected. Development of local and systemic Type 2 immune responses were not impaired in ST2 deficient mice after the onset of microfilaremia, but L. sigmodontis infected ST2-ko mice had significantly reduced total numbers of cells within the thoracic cavity and spleen compared to infected immunocompetent controls. Pronounced microfilaremia in ST2-ko mice did not result from an increased microfilariae release by adult female worms, but an impaired splenic clearance of microfilariae.

Conclusions/Significance

Our findings suggest that the absence of ST2 does not impair the establishment of adult L. sigmodontis worms, but is important for the splenic clearance of microfilariae from peripheral blood. Thus, ST2 interactions may be important for therapies that intend to block the transmission of filarial disease.

A comprehensive, model-based review of vaccine and repeat infection trials for filariasis

SUMMARY Filarial worms cause highly morbid diseases such as elephantiasis and river blindness. Since the 1940s, researchers have conducted vaccine trials in 27 different animal models of filariasis. Although no vaccine trial in a permissive model of filariasis has provided sterilizing immunity, great strides have been made toward developing vaccines that could block transmission, decrease pathological sequelae, or decrease susceptibility to infection. In this review, we have organized, to the best of our ability, all published filaria vaccine trials and reviewed them in the context of the animal models used. Additionally, we provide information on the life cycle, disease phenotype, concomitant immunity, and natural immunity during primary and secondary infections for 24 different filaria models.

Immunization with L. sigmodontis microfilariae reduces peripheral microfilaraemia after challenge infection by inhibition of filarial embryogenesis

BACKGROUND

Lymphatic filariasis and onchocerciasis are two chronic diseases mediated by parasitic filarial worms causing long term disability and massive socioeconomic problems. Filariae are transmitted by blood-feeding mosquitoes that take up the first stage larvae from an infected host and deliver it after maturation into infective stage to a new host. After closure of vector control programs, disease control relies mainly on mass drug administration with drugs that are primarily effective against first stage larvae and require many years of annual/biannual administration. Therefore, there is an urgent need for alternative treatment ways, i.e. other effective drugs or vaccines.

METHODOLOGY/PRINCIPAL FINDINGS

Using the Litomosoides sigmodontis murine model of filariasis we demonstrate that immunization with microfilariae together with the adjuvant alum prevents mice from developing high microfilaraemia after challenge infection. Immunization achieved 70% to 100% protection in the peripheral blood and in the pleural space and furthermore strongly reduced the microfilarial load in mice that remained microfilaraemic. Protection was associated with the impairment of intrauterine filarial embryogenesis and with local and systemic microfilarial-specific host IgG, as well as IFN-γ secretion by host cells from the site of infection. Furthermore immunization significantly reduced adult worm burden.

CONCLUSIONS/SIGNIFICANCE

Our results present a tool to understand the immunological basis of vaccine induced protection in order to develop a microfilariae-based vaccine that reduces adult worm burden and prevents microfilaraemia, a powerful weapon to stop transmission of filariasis.

Dipetalonema viteae: resistance in Meriones unguiculatus with multiple infections of stage-3 larvae

The jird, Meriones unguiculatus, infected with 80 normal infective larvae of Dipetalonema viteae, revealed a recovery rate of 27.9% 12 weeks after infection. A pretreatment by three injections of 50 normal larvae each and challenge by 80 larvae resulted in a recovery rate of 10.7%. The recovered worms were longer than those from the challenge control animals. When three times 50 irradiated larvae (35 krad) were inoculated, the recovery rate of the challenge decreased to 2.6%, representing a protection of 90.7%. The surviving adult worms were stunted and derived exclusively from the 80 normal larvae given for challenge, since absolutely no adult worms were recovered in eight animals inoculated three times with 50 irradiated larvae only. Sera of all pretreated jirds contained IgG and IgM antibodies which bound in immunoblotting experiments bound predominantly to three proteins of larvae with molecular masses of 68,140, and 165 kDa, respectively. Enzymatic surface iodination revealed that the three antigens were exposed on the larval surface. The coincidence of a partial resistance to a challenge infection and of an antibody response against surface proteins of infective larvae suggests an importance of these antigens for the rejection of D. viteae mediated by an acquired immunological resistance of M. unguiculatus.

Immunity to Litomosoides carinii in Mastomys natalensis. II. Effects of chemotherapeutically abbreviated and postpatent primary infections on challenges with various stages of the parasite

Naive Mastomys natalensis, Litomosoides carinii-infected M. natalensis at a postpatent stage of the infection and L. carinii-infected M. natalensis treated chemotherapeutically with furazolidone (FUR), FUR and diethylcarbamazine (FUR/DEC) or amoscanate (AMOS) were challenged by either injection or implantation of 40 third stage larvae (L3, s.c.), 40 fourth stage larvae (L4, 16 days old, i.p.), 20 male and 20 female preadult worms (36 days old, i.p.), 12 adult female worms (i.p.) or 6 X 10(6) microfilariae/kg (i.v.). Microfilaraemia in animals challenged at a postpatent stage (independent of the kind of challenge), was either totally suppressed or at least greatly reduced. Necropsy of L3-challenged animals showed that neither the length of the worms nor their content of morphologically intact, intrauterine stages was affected. Infected, treated animals challenged with developing stages (L3, L4 and preadult worms) showed reduced levels of microfilaraemia (by up to 75%). Dissection of AMOS-treated, L3-challenged animals showed that both the developmental rate and the fertility of the worms were affected. Microfilaraemia was also reduced after implantation of adult worms into treated animals. This was independent of the interval between treatment and challenge (44-150 days) except in animals challenged 10 days after AMOS-treatment, which showed no difference from naive controls. However, infected, treated M. natalensis, cotton rats and gerbils did not develop immunity against intravenously injected blood microfilariae.

Immunity to Litomosoides carinii in Mastomys natalensis. I. Effect of immunization with microfilariae and existing primary infections on the parasitaemia after microfilariae injection and challenge infection

Subcutaneous injections of intrauterine stages of Litomosoides carinii into Mastomys natalensis induced strong immunity to i.v. injected blood microfilariae. Immunity, developed after boostering with an i.p. and an i.v. injection of microfilariae, did not totally suppress the parasitaemia of a challenge infection but reduced significantly the microfilaraemia level. No effect was found on number and size of the worms of the challenge infection, the number of microfilariae or the number of leucocytes in the pleural cavity. Delayed type hypersensitivity reactions in challenged animals were similar to those in non-immunized, infected controls. Sera of immunized animals agglutinated microfilariae and mediated cell attachment to microfilariae. Challenge infections did not change this until the end of the fourth week post infection but sera taken 32 days after challenge and later failed to induce such reactions. Challenge infections performed 120 or 240 days after a primary infection did not increase the parasitaemia of recipients. Dissections carried out 130 days after the challenge showed that (a) the developmental rate of the challenge infection was reduced by about 50%; (b) the size of the challenge parasites was reduced; and (c) that these worms produced significantly less embryonic stages in comparison to worms of primary infections, of which about 90% were abnormal.

Immunization of calves against the microfilariae of Onchocerca lienalis

Three Jersey bull calves were immunized by subcutaneous injections of sonicated Onchocerca lienalis microfilariae suspended in phosphate buffered saline, and three control animals were injected with medium only. All calves received booster injections 27 days later, and were challenged with live microfilariae 44 days after the booster. Following challenge, only the immunized animals developed an elevated level of circulating eosinophils. When necropsied ten days after challenge there was a 97% reduction in recoveries of microfilariae from immunized animals compared to challenge controls. In human onchocerciasis it is the microfilariae which are the principal cause of pathology, and we believe that studies on O. lienalis in both the natural bovine host and in inbred rodents provide a promising model to investigate immunity to Onchocerca microfilariae.

Studies on acquired resistance of the cotton rat against microfilariae of Litomosoides carinii. 2. Injection of microfilariae during prepatency

Cotton rats infected by infective third-stage larvae of Litomosoides carinii were treated at increasing time intervals by a threefold injection of living homologous microfilariae (mf) during the prepatent period. Starting with the first treatment 3, 4 or 5 weeks p.i. seven animals remained completely and two almost mf-negative (1 or 2 mf/mm3 each only once) until 16 weeks p.i. Starting 6, 7 or 8 weeks p.i. six animals developed a normal level of parasitaemia between 42 and 436 mf/mm3, two animals developed a continuous level of 1-2 mf/mm3. The number of fertile adult worms shedding great numbers of microfilariae in the pleural cavity was equal in all animals. However, in mf-negative animals the lung capillary blood showed, in the geometric mean, only 0.6% of the mf-concentration seen in mf-positive animals. The hypothesis is proposed that microfilariae accumulating primarily in the lung capillaries absorb all aggressive components specifically reacting with microfilarial antigens, i.e. neutralize the immune response against them to enable the development of the parasitaemia in the peripheral blood.