Identification of Chitinase as the Immunodominant Filarial Antigen Recognized by Sera of Vaccinated Rodents

Abstract

Acanthocheilonema viteae is a parasitic nematode of rodents. We identified the chitinase of A. viteae infective stage larvae (L3) as the main target of the humoral immune response of jirds, which were protected against challenge infection after vaccination with irradiation attenuated L3. The cDNA of the L3 chitinase has been sequenced, and the deduced amino acid sequence shows significant homologies to chitinases of Brugia malayi microfilariae, insects, yeast, bacteria, and Streptomyces sp. The protein has been characterized by monoclonal antibodies and substrate activity gels. The chitinase of L3 may contribute to degrading the nematode cuticle during molting and thus represents a target of protective immune responses in a phase where the parasite is highly vulnerable. In addition, it has been shown that a similar enzyme exists in uterine microfilariae, which probably has a role in casting the egg shell.Filarial parasites, such as Onchocerca volvulus, Brugia malayi, and Wuchereria bancrofti, the causative agents of river blindness and lymphatic filariases, affect more than 100 million people throughout the tropics (1). Infection of the human host occurs by a bite of an infected arthropod. There is evidence that some individuals develop protective immunity naturally, as they do not acquire filarial infections despite high levels of local transmission (2, 3). The humoral immune response of these patients differentially recognizes antigens of infective third stage larvae (L3) (4), and it is conceivable that vaccination with such antigens could prevent the infection. As the study of protective immunity in human filariae is hampered by the host specificity of the parasites, we adopted the approach to identify immunorelevant antigens of the rodent filaria Acanthocheilonema viteae, a parasite of the jird (Meriones unguiculatus).For animal models, the vaccination with irradiation-attenuated infective larvae (L3) has been demonstrated to be the most effective way to induce protective immunity (5). Immunization of jirds with irradiation-attenuated L3 of A. viteae induces more than 90% protection against a challenge infection (6). Sera of such vaccinated animals recognize few L3 proteins. We report here on the characterization and the molecular cloning of the immunodominant antigen recognized by the humoral immune response of challenge resistant jirds, a chitinase of A. viteae L3. Chitinases were recently described from microfilariae of B. malayi (7). It was supposed that the enzymes might have a role in casting the microfilarial sheath, a modified egg shell. This structure was believed to contain chitin on biochemical grounds (8, 9) and due to its analogy with the chitincontaining egg shell of other nematodes. Interestingly, the egg shell is the only structure of nematodes known to contain chitin. However, recent reports deny the presence of chitin in the microfilarial sheath (see Refs. 10 -12) and call the chitindegrading role of filarial chitinase into question. Our characterization of L3 chitinase (an active, chitin-d...

Genetic polymorphisms in molecules of innate immunity and susceptibility to infection with Wuchereria bancrofti in South India

“…30,31 The microfilarial chitinase is an immunodominant antigen and target of current strategies to develop candidate vaccines. [32][33][34] The functional contribution of the microfilarial chitinase to infectious pathophysiology is still not well understood. Still, it is notable that the microfilarial sheath contains chitin, a putative target for a human chitotriosidase, encoded by CHIT1.…”

Expression and Secretion of a Larval-specific Chitinase (Family 18 Glycosyl Hydrolase) by the Infective Stages of the Parasitic Nematode, Onchocerca volvulus

“…Several experiments have pointed to a relationship between chitinase and parasite transmission (10 -12). The evidence, however, in the filarial nematodes is more complex because of the existence of multiple genes and a spectrum of tissue and stage-specific expression patterns (8,9,13,14). The interest in filarial chitinases stems primarily from their potential as a drug or vaccination target.…”

Essential Role of Chitinase in the Development of the Filarial Nematode Acanthocheilonema viteae

“…Although chitinases of several filarial nematodes have been described at the molecular level (3,8,18,28), relatively little is known about the function of these proteins during the life cycle of the parasites. Several questions have yet to be answered, including the following.…”

“…Membranes were incubated with anti-DIG alkaline phosphataseconjugated antibody (Roche Diagnostics) as recommended. Blots were developed using disodium 3-(4-methoxyspiro[1,2-dioxetane-3,2Ј-(5Ј-chloro)tricyclo[3.3.1.1 (3,7)]decan]-4-yl) phenyl phosphate (CSPD) and then exposed to X-ray film. Following four rounds of hybridization, clonal plaques were used for the purification and subsequent analysis/manipulation of phage DNA.…”

“…Chitinases, in particular, have been proposed to have a role in remodeling processes during the molting of filariae and in the hatching of larvae from the eggshell (3,5). However, the existence of large families of chitinases in the free-living nematode Caenorhabditis elegans suggests that these enzymes also fulfill other functions (17).…”