The endemic normal in lymphatic filariasis: A static concept

Immune targeting of filarial glutaredoxin through a multi-epitope peptide-based vaccine: A reverse vaccinology approach

Despite the efforts of global programme to eliminate lymphatic filariasis (GPELF), the threat of lymphatic filariasis (LF) still looms over humanity in terms of long-term disabilities, and morbidities across the globe. In light of this situation, investigators have chosen to focus on the development of immunotherapeutics targeting the physiologically important filarial-specific proteins. Glutaredoxin (16.43 kDa) plays a pivotal role in filarial redox biology, serving as a vital contributor. In the context of the intra-host survival of filarial parasites, this antioxidant helps in mitigating the oxidative stress imposed by the host immune system. Given its significant contribution, the development of a vaccine targeting glutaredoxin holds promise as a new avenue for achieving a filaria-free world. Herein, multi-epitope-based vaccine was designed using advanced immunoinformatics approach. Initially, 4B-cell epitopes and 6 T-cell epitopes (4 MHC I and 2 MHC II) were identified from the 146 amino acid long sequence of glutaredoxin of the human filarid, Wuchereria bancrofti. Subsequent clustering of these epitopes with linker peptides finalized the vaccine structure. To boost TLR-mediated innate immunity, TLR-specific adjuvants were incorporated into the designed vaccine. After that, experimental analyses confirm the designed vaccine, Vac4 as anefficient ligand of human TLR5 to elicit protective innate immunity against filarial glutaredoxin. Immune simulation further demonstrated abundant levels of IgG and IgM as crucial contributors in triggering vaccine-induced adaptive responses in the recipients. Hence, to facilitate the validation of immunogenicity of the designed vaccine, Vac4 was cloned in silico in pET28a(+) expression vector for recombinant production. Taken together, our findings suggest that vaccine-mediated targeting of filarial glutaredoxin could be a future option for intervening LF on a global scale.

Pre-clinical development of a vaccine for human lymphatic filariasis

This study was conducted to optimize a fusion protein vaccine for translational development as a vaccine against the human tropical parasitic infection, lymphatic filariasis (LF). The vaccine candidate, His-tagged rBmHAXT was developed previously in our laboratory and was tested in various animal models including mouse, gerbils and Rhesus macaque where it exhibited significant levels of vaccine-induced protection. However, for commercial manufacturing and for regulatory approval for human use, there was a need to modify the vaccine antigen and its production and analytical release methods. Therefore, the major focus of this study was to develop a process for manufacturing an affinity tag-free rBmHAXT and evaluate its immunogenicity, potency and protective efficacy in both inbred and outbred mouse models, as well as in outbred gerbil models. Our results demonstrate that the tag-free rBmHAXT vaccine produced with a process suitable for cGMP production had protective properties equivalent to the original His-tagged rBmHAXT.

Advances in Vaccine Development for Human Lymphatic Filariasis

According to the World Health Organization, over 880 million people are currently at risk of acquiring lymphatic filariasis (LF) in over 52 countries worldwide. Current approaches to control LF by 2020 are short of the anticipated goal. Several studies suggest the existence of protective immunity against LF in humans. Thus, it is possible to develop a prophylactic vaccine against LF in humans. Several potential vaccine candidates were identified and tested for their potential against LF. To date, preclinical studies suggest that it is possible to develop a prophylactic vaccine against LF. Much work needs to be done, but it is clear that a prophylactic vaccine, combined with targeted chemotherapy, is critically required for eliminating LF worldwide.

Improving the efficacy of a prophylactic vaccine formulation against lymphatic filariasis

Mass drug administration (MDA) is the current strategy for interrupting the transmission of lymphatic filariasis (LF) infection and control of the disease in endemic areas. However, subject non-compliance has resulted in the presence of several "transmission hotspots" in the endemic regions threatening the reemergence of LF. This situation is further complicated by the fact that the drugs used in MDA are not effective against adult LF worms, a major concern for the control strategy. Thus, there is clearly a need for an effective and sustainable approach to control LF. Prophylactic vaccine combined with targeted treatment of infected patients and vector control is suggested as a more sustainable strategy to eliminate LF infection from endemic regions. A multivalent vaccine (rBmHAT) developed in our laboratory conferred about 90% protection in rodents. However, when we tested the rBmHAT vaccine along with alum in rhesus macaques, only about 40% protection was achieved and the immune response obtained was Th2 biased. In an attempt to improve the vaccine, in this study, we tested two vaccine antigens (rBmHAT and rBmHAX) along with two adjuvant formulations [alum + GLA (AL019) and mannosylated chitosan (MCA)] in a mouse model. Our results show that rBmHAT is a better vaccine antigen than rBmHAX. Combination of rBmHAT with AL019 or MCA adjuvants gave 94 and 88% protection, respectively, against challenge infections. Immunized animals developed antigen-specific memory T cells that secreted significant levels of IL-4, IFN-γ, and IL-17 suggesting the generation of a balanced Th1/Th2 responses following immunization. A major advantage of MCA adjuvant is that the vaccine booster doses can be administered orally. These studies thus showed that rBmHAT is a better vaccine antigen and can be given in combination with AL019 or MCA adjuvant to obtain excellent results.

Two decades of antifilarial drug discovery: a review

Filariasis is one of the oldest, most debilitating, disabling, and disfiguring neglected tropical diseases with various clinical manifestations and a low rate of mortality, but has a high morbidity rate, which results in social stigma.

Immunological evaluation of an rsmD-like rRNA methyltransferase from Wolbachia endosymbiont of Brugia malayi

Wolbachia is a wonderful anti-filarial target with many of its enzymes and surface proteins (WSPs) representing potential drug targets and vaccine candidates. Here we report on the immunologic response of a drug target, rsmD-like rRNA methyltransferase from Wolbachia endosymbiont of Brugia malayi. The recombinant protein generated both humoral and cell-mediated response in BALB/c mice but compromised its immunity. The humoral response was transient and endured barely for six months in mice with or without B. Malayi challenge. In splenocytes of mice, the key humoral immunity mediating cytokine IL4 was lowered (IL4↓) while IFNγ, the major cytokine mediating cellular immunity was decreased along with upregulation of IL10 cytokine (IFNγ↓, IL10↑). The finding here indicates that the enzyme has low immunogenicity and triggers lowering of cytokine level in BALB/c mice. Interestingly the overall immune profile can be summed up with equivalent response generated by WSP or whole Wolbachia.

Evaluation of a multivalent vaccine against lymphatic filariasis in rhesus macaque model

Lymphatic filariasis affects 120 million people worldwide and another 1.2 billion people are at risk of acquiring the infection. Chemotherapy with mass drug administration is substantially reducing the incidence of the infection. Nevertheless, an effective vaccine is needed to prevent the infection and eradicate the disease. Previously we reported that a multivalent fusion protein vaccine (rBmHAT) composed of small heat shock proteins 12.6 (HSP12.6), abundant larval transcript-2 (ALT-2) and large extracellular domain of tetraspanin (TSP LEL) could confer >95% protection against the challenge infection with Brugia malayi infective larvae (L3) in mouse and gerbil models. In this study we evaluated the immunogenicity and efficacy of rBmHAT fusion protein vaccine in a rhesus macaque model. Our results show that rBmHAT is highly immunogenic in rhesus macaques. All the vaccinated monkeys developed significant titers of antigen-specific IgG antibodies against each of the component antigens (16,000 for rBmHSP12.6), (24,000 for rBmALT-2) and (16,000 for rBmTSP-LEL). An in vitro antibody dependent cellular cytotoxicity (ADCC) assay performed using the sera samples from vaccinated monkeys showed that the anti-rBmHAT antibodies are functional with 35% killing of B. malayi L3s. Vaccinated monkeys also had antigen responding cells in the peripheral blood. Vaccine-induced protection was determined after challenging the monkeys with 500 B. malayi L3. Following challenge infection, 3 out of 5 vaccinated macaques failed to develop the infection. These three protected macaques had high titers of IgG1 antibodies and their PBMC secreted significantly high levels of IFN-γ in response to the vaccine antigens. The two vaccinated macaques that picked the infection had slightly low titers of antibodies and their PBMC secreted high levels of IL-10. Based on these findings we conclude that the rBmHAT vaccine is highly immunogenic and safe and can confer significant protection against challenge infections in rhesus macaques.

Single multivalent vaccination boosted by trickle larval infection confers protection against experimental lymphatic filariasis

The multivalent vaccine BmHAT, consisting of the Brugia malayi infective larval (L3) antigens heat shock protein12.6 (HSP12.6), abundant larval transcript-2 (ALT-2) and tetraspanin large extra cellular loop (TSP-LEL), was shown to be protective in rodent models from our laboratory. We hypothesize that since these antigens were identified using protective antibodies from immune endemic normal individuals, the multivalent vaccine can be augmented by natural L3 infections providing protection to the vaccinated host. This hypothesis was tested using single dose of DNA and protein or protein alone of the BmHAT vaccination in gerbils followed by live trickle L3 infection as booster dose. Vaccine-induced protection in gerbils was determined by worm establishment, micropore chamber assay and by antibody dependant cell cytotoxicity (ADCC) assay. Results were compared with the traditional prime-boost vaccination regimen. Gerbils vaccinated with BmHAT and boosted with L3 trickle infection were protected 51% (BmHAT DNA-protein) and 48% (BmHAT protein) respectively. BmHAT vaccination plus L3 trickle booster generated significant titer of antigen-specific IgG antibodies comparable to the traditional prime boost vaccination approach. BmHAT vaccination plus L3 trickle booster also generated antigen-specific cells in the spleen of vaccinated animals and these cells secreted predominantly IFN-γ and IL-4 in response to the vaccine antigens. These studies thus show that single dose of BmHAT multivalent vaccination followed by L3 trickle booster infection can confer significant protection against lymphatic filariasis.

Multivalent fusion protein vaccine for lymphatic filariasis

Lymphatic filariasis affects approximately 3% of the whole world population. Mass drug administration is currently the major control strategy to eradicate this infection from endemic regions by year 2020. Combination drug treatments are highly efficient in controlling the infection. However, there are no effective vaccines available for human or animal lymphatic filariasis despite the identification of several subunit vaccines. Lymphatic filariasis parasites are multicellular organisms and potentially use multiple mechanisms to survive in the host. Therefore, there is a need to combine two or more vaccine candidate antigens to achieve the desired effect. In this study we combined three well characterized vaccine antigens of Brugia malayi, heat shock protein 12.6 (HSP12.6), Abundant Larval transcript-2 (ALT-2) and tetraspanin large extra cellular loop (TSP-LEL) as a multivalent fusion vaccine. Putative immune individuals carry circulating antibodies against all three antigens. Depletion of these antigen specific antibodies from the sera samples removed the ability of the sera to participate in the killing of B. malayi L3 in an antibody dependent cellular cytotoxicity (ADCC) mechanism. Vaccination trials in mice with a bivalent [HSP12.6+ALT-2 (HA), HSP12.6+TSP-LEL (HT) or TSP-LEL+ALT-2 (TA)] or trivalent [HSP12.6+ALT-2+TSP-LEL (HAT)] vaccines using DNA, protein or heterologous prime boost regimen showed that trivalent HAT vaccine either as protein alone or as heterologous prime boost vaccine could confer significant protection (95%) against B. malayi L3 challenge. Immune correlates of protection suggest a Th1/Th2 bias. These finding suggests that the trivalent HAT fusion protein is a promising prophylactic vaccine against lymphatic filariasis infection in human.

Brugia malayi Wolbachia hsp60 IgG antibody and isotype reactivity in different clinical groups infected or exposed to human bancroftian lymphatic filariasis

Wolbachia, an endosymbiotic bacterium in filarial parasites, comes into contact with the host immune system upon parasite death. Here, we analyzed, total IgG and isotype antibody responses to Wolbachia hsp60 in individuals from an area endemic for Wuchereria bancrofti. Wolbachia derived hsp60 gene was cloned and the recombinant protein was used to determine the IgG and isotype reactivity by Western blotting and ELISA. All individuals from the endemic area generated antibody responses to Brugia malayi Wolbachia hsp60, which were elevated in the group with chronic pathology. Isotype analysis showed that, all clinical groups mounted IgG1-IgG4 responses with higher levels of B. malayi Wolbachia hsp60 specific IgG1 observed in the sera of patients with chronic pathology compared to microfilaraemics and endemic normals. These findings suggests that Wolbachia-derived hsp60 generates antibody responses in individuals infected or exposed to W. bancrofti and an elevated IgG and IgG1 reactivity is observed in people with filarial pathology.

T-cell activation and the balance of antibody isotypes in human lymphatic filariasis

Human filarial infection presents a spectrum of clinical states with two major poles: asymptomatic microfilaraemia and amicrofilaraemic chronic disease. Microfilaremia is associated with a Th1-type tolerance, and maximal IgG4 antibodies, while elephantiasis patients react across a broad range of immune parameters. In this review, Rick Maizels and his colleagues discuss recent advances in the immunology of human filariasis and present a summary of their latest studies in an endemic area of Indonesia.

Red blood cell antioxidant levels in Wuchereria bancrofti infection

The elimination of microfilariae of Wuchereria bancrofti is probably mediated by free radicals. Red cell catalase (C), glutathione peroxidase (GPX), and superoxide dismutase (SOD) activity levels were measured as an indirect method of assessing blood oxidant status in 29 asymptomatic microfilaraemics, 29 "endemic normals", and 29 controls living in a non-endemic area. Changes in the activity of these enzymes were also compared over a one month period in 22 asymptomatic microfilaraemics randomised to receive either single dose or 14 day treatment with diethyl carbamazine citrate (DEC). Red cell GPX activity levels were significantly higher in "endemic normals" when compared to mf positive cases and non-endemic controls. An early and significant increase in GPX activity (on days 3, 7 and 14 compared to pretreatment levels, p<0.01) was observed after DEC in both treatment groups. Increases in the activity of catalase and SOD became significant only on days 14 and 30 respectively. The percentage reduction in microfilaraemia correlated significantly with the percentage increase in GPX activity levels (R(2)=0.58, p=0.6 x 10(-5)). Our results may suggest a role for GPX related oxidant species in the elimination of microfilariae.

Infection with Schistosoma mansoni correlates with altered immune responses to Ascaris lumbricoides and hookworm

Studies were performed on humoral and cellular immune responses of patients from areas in Brazil endemic for hookworm and Ascaris lumbricoides, and either endemic or non-endemic for Schistosoma mansoni. Humoral and cellular responses were evaluated by enzyme-linked immunosorbant assay (ELISA) and peripheral blood mononuclear cell (PBMC) proliferation assays against larval hookworm antigens, A. lumbricoides egg antigens, and soluble egg antigens (SEA) or soluble whole adult antigenic preparation (SWAP) from S. mansoni. Patients from S. mansoni-endemic areas, who currently had only hookworm or Ascaris infections, expressed lower humoral and cellular responses to hookworm or Ascaris antigens, respectively, than did their counterparts from areas not endemic for S. mansoni. Individuals from S. mansoni endemic area, although without detectable S. mansoni infection, do mount humoral and cellular responses to SEA and SWAP. This group of individuals has been probably in contact with S. mansoni antigens, since the groups harboring A. lumbricoides or hookworm infections from non-S. mansoni endemic areas do not have detectable anti-S. mansoni responses. PBMC proliferative responses discriminated well between patients with active hookworm infections versus ascariasis, if they were from areas not endemic for S. mansoni.

Immunity to filarial nematodes

Mosquito-borne filarial nematodes cause the severe, debilitating disease of human lymphatic filariasis. In areas endemic for this disease, differential responses range from putative immunity through asymptomatic microfilaraemic infection to chronic pathology. Current research in mouse models of infection is elucidating the immunological mechanisms that can lead to immunity against this disease. In this review, the importance of different immunological pathways are discussed in relation to their role in human disease and in terms of their ability to kill separate developmental stages of the filarial parasite.

Host protective immunity and vaccine development studies in lymphatic filariasis

Lymphatic filariasis caused mainly by infection fromWuchereria bancrofti andBrugia malayi remains as the major cause of clinical morbidity in tropical and subtropical countries. Development of vaccine against filarial infection can act as additional measure to the existing therapeutic and vector control methods in the control of this disease. The main hurdles in the development of anti-filarial vaccine are the strict primate specificity ofWuchereria bancrofti, the paucity of parasite material, the diversity of clinical manifestations and their associated complex immune responses, lack of clear understanding on host-parasite interactions and the mechanisms involved in protective immunity. However in the past few years, the information generated in immuno-epidemiological studies, correlated with observations in experimental animals suggests that a filarial vaccine is feasible. Initially live irradiated infective larvae have been successfully used to induce high level of protective immunity in several animal models. Applying diverse strategies, variety of purified or recombinant filarial antigens have been explored for their ability to induce protection in different host-parasite systems. Some of these targeted filarial antigens induced high level of resistance in experimental animals against challenge infections. More focussed studies on thorough characterization of parasitological and immunological changes associated with resistance induced by such candidate protective antigens and on delivery mechanisms and safety aspects will be crucial in their selection for possible use in humans.

The abundant larval transcript-1 and -2 genes of Brugia malayi encode stage-specific candidate vaccine antigens for filariasis

Lymphatic filariasis is a major tropical disease caused by the mosquito-borne nematodes Brugia and Wuchereria. About 120 million people are infected and at risk of lymphatic pathology such as acute lymphangitis and elephantiasis. Vaccines against filariasis must generate immunity to the infective mosquito-derived third-stage larva (L3) without accentuating immunopathogenic responses to lymphatic-dwelling adult parasites. We have identified two highly expressed genes, designated abundant larval transcript-1 and -2 (alt-1 and alt-2), from each of which mRNAs account for >1% of L3 cDNAs. ALT-1 and ALT-2 share 79% amino acid identity across 125 residues, including a putative signal sequence and a prominent acidic tract. Expression of alt-1 and alt-2 is initiated midway through development in the mosquito, peaking in the infective larva and declining sharply following entry into the host. Humans exposed to Brugia malayi show a high frequency of immunoglobulin G1 (IgG1) and IgG3 antibodies to ALT-1 and -2, distinguishing them from adult-stage antigens, which are targeted by the IgG4 isotype. Immunization of susceptible rodents (jirds) with ALT-1 elicited a 76% reduction in parasite survival, the highest reported for a single antigen from any filarial parasite. ALT-1 and the closely related ALT-2 are therefore strong candidates for a future vaccine against human filariasis.

Reversal in microfilarial density and T cell responses in human lymphatic filariasis

This study reports reversals in microfilarial density and the accompanying changes in cellular immune responses to filarial antigens of 39 individuals (11 microfilaria-positives, 22 microfilaria-negatives and six converters) living in an area endemic for brugian filariasis. Microfilarial counts decreased from April, the end of the rainy season to July, middle of the dry season (g.m. 88 mf/ml and 38 mf/ml, respectively; P = 0.001) and subsequently increased in November, the beginning of the rainy season (P = 0.088). Whereas the proliferative responses remained low throughout the study period in microfilaraemic individuals, in amicrofilaraemics these responses changed in the opposite direction to that of microfilarial densities. In three converters, proliferation changed in the opposite direction to the presence or absence of microfilariae. Cytokine analysis in the converters revealed that interferon-gamma was most affected by the shifts in microfilarial densities. In contrast, interleukin-4 responses showed little correlation with changes in parasite densities.

Proposed panel of diagnostic criteria, including the use of ultrasound, to refine the concept of 'endemic normals' in lymphatic filariasis

Although living adult Wuchereria bancrofti worms can be detected by ultrasound examination of the scrotal area in approximately 80% of men infected with this filarial parasite, the location of the adult worms in the remaining 20% remains unclear. To determine this, 32 individuals who had W. bancrofti microfilaraemia but no adult worms detectable on ultrasound were treated with diethylcarbarmazine (DEC), either with a single 6 mg/kg dose (n = 13) or with a 12-day course of 6 mg/kg per day (n = 19). They were then monitored with serial physical and ultrasound examinations. Thirteen (41%) subjects developed small, single scrotal nodules 12 h to 7 days after treatment; this rate was unaffected by the dose of DEC. No nodules were detected outside the scrotal area. All 5 men with lymphangiectasia suspected on ultrasound before treatment developed scrotal nodules, compared to 8 (29.6%) of 27 men without ultrasonographic evidence (P = 0.006). Thus, using both ultrasound and 'provocative' treatment with DEC, adult W. bancrofti can be detected in the scrotal area of an estimated 88% of infected men. Because no single diagnostic test for W. bancrofti infection is completely sensitive, a panel of tests, including ultrasound, is proposed to identify with greater accuracy 'endemic normals' for immunological and epidemiological studies.

Brugia pahangi: quantitative analysis of infection in several inbred rat strains

We report a comprehensive study of the infectivity of Brugia pahangi in male and female rats of eight different inbred strains. A single infection of any inbred rat strain will produce rats that become microfilaremic, have occult infection, or clear the primary infection. The proportion belonging to any category is determined by the basic susceptibility level of that strain. Patency rates (blood microfilaria+) ranged from 24% (AO rats) to 73% (WKA rats). The period for which microfilaria were in the circulation was directly related to microfilarial burden, with rats carrying less than 50 mf/ml of blood patent for 11.8 weeks +/- 12.2; for 50-499 mf/ml it was 37.6 +/- 14.8 and for 500+ mf/ml it was 63.3 +/- 34.2 weeks. Suckling rats were resistant to infection (0 patent) and weanlings were intermediate in resistance between suckling and adult rats. Female rats were highly resistant to infection. Approximately half of amicrofilaremic rats have occult infections. A high proportion of patent infections involve the testes or testicular lymphatics. In the most susceptible rat strains, more than 95% of the administered L3 or developing L4 parasites were killed within 28 days. During the course of the first 6 months, the ratio of males to females fell significantly, suggesting a shorter life span in male worms. The features of the infectivity/patency patterns in rats are compared with recognized patterns obtaining in human populations. We conclude that rats provide a valuable and underutilized model for the experimental analysis of filarial infections.

Human antibody responses to Brugia malayi antigens in brugian filariasis

Human antibody responses to Brugia malayi antigens were studied with sera from a Brugia endemic area in South India. Patients with clinical filariasis had significantly higher IgE and lower IgG4 levels to adult worm antigens than people with asymptomatic microfilaraemia. Intermediate antibody levels were observed in endemic normals. A majority of sera from each clinical group contained IgG antibodies to surface antigens of infective larvae (L3) by IFAT. IgG immunoblot studies did not reveal group differences in L3 antigen recognition. IgE antibodies bound to a subset of antigens bound by IgG. IgE antibodies in sera from clinical filariasis patients preferentially bound to L3 antigens at 200, 97, 68 and 58 kDa compared with sera from microfilaria carriers. These results are consistent with prior studies of antibody responses in filariasis and add new information on the targets of IgG and IgE antibodies to L3 antigens in brugian filariasis.

Common features of T cell reactivity in persistent helminth infections: lymphatic filariasis and schistosomiasis

This review highlights striking similarities in the down-regulated inflammatory responses that are observed in chronic infectious diseases. It is clear that peripheral mononuclear cells show poor antigen-specific T cell proliferation in lymphatic filariasis as well as schistosomiasis. With respect to cytokines both Th1- and Th2-type products are down-regulated during chronic infection. Drug treatment restores responsiveness in both diseases indicating a link between infection and immune suppression. Interestingly, immunological responses in subjects carrying recent infections are characterized by strong proliferation to parasite antigen and IFN-gamma production. These heightened responses are gradually down-regulated with increased length of exposure to infection.

Brugia pahangi: differential induction and regulation of jird inflammatory responses by life-cycle stages

It has been hypothesized that different life-cycle stages of filarial nematodes induce different host responses. This concept was examined in the Brugia pahangi-jird model of lymphatic filariasis by measuring the kinetics of inflammatory responses to parasite antigens following intraperitoneal inoculation of different life-cycle stages. For this purpose, viable female or male worms, L3, L4, or microfilarial stage, were used. Dead worms served as controls. Worm and microfilarial burdens, pulmonary granulomatous inflammation (PGRN) to soluble adult worm antigen (SAWA)-coated beads, and peritoneal eosinophil and macrophage numbers were assessed at different days post-inoculation. All jirds inoculated with any of these life-cycle stages developed an early PGRN to SAWA which was later significantly reduced. Only viable worms induced down-regulation of the PGRN response. These results indicate that the hyporesponsive state is induced and maintained by all life-cycle stages. Also, the degree of granulomatous response was influenced by worm burden, with larger worm burdens inducing lower initial levels of PGRN to SAWA. Peritoneal inflammatory responses differed from the systemic response in that numbers of macrophages increased with time and microfilarial accumulation. No correlation was observed between peritoneal inflammatory responses measured by eosinophil and macrophage numbers and PGRN to SAWA.

Th1-like antifilarial immune responses predominate in antigen-negative persons

To characterize immune responses associated with the putatively immune state in bancroftian filariasis (that is, both microfilaria and antigen free), humoral and cellular responses were compared among antigen- and microfilaria-negative, antigen-positive and microfilaria-negative, and microfilaria-positive individuals. Antifilarial isotype levels were measured by enzyme-linked immunosorbent assay. Peripheral blood mononuclear cell responses were measured by proliferation, by bioassay for interleukin-2 (IL-2) and IL-10, and by reverse transcription-PCR for IL-4, IL-5, and gamma interferon. The absence of circulating filarial antigen was associated with Th1-like responses, including significantly higher proliferative (P < 0.001) and IL-2 (P = 0.008) responses and a higher prevalence of gamma interferon (0.02 < P < 0.1) responses. Significantly elevated antifilarial immunoglobulin G4 (IgG4) levels (P = 0.0035) were associated with antigenemia, whereas microfilaremia was associated with significantly decreased antifilarial IgG2 levels (P = 0.0014). IL-4 mRNA levels were not significantly different among the three groups; however, there was a subpopulation of microfilaremic individuals who did not make detectable levels of IL-4 mRNA and who produced low antifilarial IgG4 levels compared with those of individuals who had detectable levels of IL-4 mRNA. IL-5 mRNA levels also were not significantly different among groups; however, more microfilaremic individuals produced IL-5 mRNA in response to adult filarial antigens, and total parasite-specific IL-4 and IL-5 mRNA levels were significantly correlated (P = 0.05). Although longitudinal data are not currently available, the elevated Th1-like responses in antigen- and microfilaria-negative individuals are consistent with the hypothesis that these responses contribute to protection in putatively immune individuals.

Differential antibody isotype reactivity to specific antigens in human lymphatic filariasis: gp15/400 preferentially induces immunoglobulin E (IgE), IgG4, and IgG2

Lymphatic filarial infection in humans is associated with a strong skewing of the immune response towards the TH2 arm, with prominent interleukin 4-producing cells and elevated levels of immunoglobulin G4 (IgG4) and IgE antibodies in peripheral blood. To determine how such a generalized TH2 imbalance governs responses to individual parasite antigens, the profiles of isotypes of antibodies to two recombinant proteins of Brugia spp. were studied. One molecule was the C-terminal portion of the filarial heat shock protein 70 (Bpa-26), representative of a cytoplasmic protein, and the second antigen was a single unit of the tandem repeats of a Brugia polypeptide (BpL-4), a secreted product which is prominently exposed to the immune system. Serum samples from 146 individuals resident in areas in which brugian filariasis is endemic were used, and it was found that whereas the levels of IgG1 and IgG3 responses to both Bpa-26 and BpL-4 were high, IgG4 and IgE antibodies to only BpL-4, not to Bpa-26, were prominent. Thus, an antigen which is chronically exposed to the immune system elicited a TH2-dependent isotype switch, as manifested by increased IgG4 and IgE responses. Moreover, IgG4 and IgE responses to BpL-4 showed a strong negative association, suggesting that mediators other than interleukin 4 must be responsible for such differential regulation of these two isotypes. When the data were analyzed as a function of clinical status, a striking association between elevated levels of IgG3 antibodies to Bpa-26 and manifestation of chronic obstructive disease was found; elephantiasis patients showed significantly higher levels of IgG3 antibodies to Bpa-26 than microfilaremics and asymptomatic amicrofilaremics. This indicates that an imbalance of isotypes of antibodies to particular filarial antigens might play a role in the pathogenesis of chronic disease.

Vaccine research and development for the prevention of filarial nematode infections

The development of vaccines for the prevention of filarial nematode infections is in a state of relative infancy in comparison to vaccines for other parasitic diseases, such as schistosomiasis and malaria. There are many reasons for this slow start. Some of the principal problems are: (1) the lengthy and complex life cycle of these organisms with attendant complex immune responses, (2) the unique characteristics associated with a relatively large number of different pathogens, (3) the lack of suitable model systems for study of medically important infections, (4) the paucity of parasite material for antigen discovery and recombinant library construction, (5) the lack of substantial evidence suggesting the natural occurrence of protective immune responses, and (6) the limited data on mechanisms responsible for protective immunity. As technical hurdles are considered, it is also critical to focus on the characteristics of a vaccine necessary for its eventual utility. In the case of a vaccine for D. immitis a completely successful product will need to approach a 99+% efficacy. This is because of the 99+% efficacy of competitive chemotherapeutic products and the fact that microfilaremia observed on blood examination, resulting from as few as two worms, would present as a vaccine failure. Although very low worm burdens in large dogs could be perceived as success in the context of protection from clinical disease, because of the option of virtually complete chemoprophylactic protection, the typical veterinary practitioner would probably fail to appreciate less than complete vaccine protection. In contrast, a vaccine that produced a reduction in adult worm burdens without complete protection in either lymphatic filariasis or onchocerciasis would be very important. Highly effective chemoprophylactic agents are not widely available for prevention of the human filariases, and dramatically reduced clinical disease provided by less than a completely effective vaccine could occur as the result of fewer adult worms. The importance of developing these vaccines has outweighed the obstacles to this research. There has been a great deal of epidemiological and experimental evidence to suggest a vaccine is feasible and antigen discovery has progressed relatively rapidly within just the past few years. Efforts to generate appropriate larval cDNA libraries are beginning to yield dividends and a variety of fascinating vaccine candidates have been cloned. Additional antigen discovery, research on appropriate modalities for overexpression of genes from these parasites, and the complex tasks associated with vaccinology remain as significant research and development obstacles.(ABSTRACT TRUNCATED AT 400 WORDS)

Interferon-gamma production by peripheral blood mononuclear cells from residents of an area endemic for Schistosoma mansoni

During human schistosomiasis host responses to antigens of various parasite life-cycle stages may contribute to whether the severe, hepatosplenic state develops or the patient remains relatively asymptomatic throughout infection, and may play a role in resistance. This study evaluated production of interferon gamma (IFN-gamma) in vitro by schistosome antigen-stimulated peripheral blood mononuclear cells (PBMCs) from asymptomatic patients, and by PBMCs from apparently uninfected, untreated persons living in areas endemic for Schistosoma mansoni ('endemic normals'). IFN-gamma production parallels PBMC proliferation in that schistosomal egg antigens stimulate patent patients' cells poorly, but strongly stimulate PBMCs from 'endemic normals'. This is proportionally true for antigens from adult worms and cercariae. Although asymptomatic patent patients' cells produced little or no IFN-gamma in response to the 3 schistosomal antigenic extracts, their PBMCs, and PBMCs from 'endemic normals', produced expected amounts of IFN-gamma when exposed to phytohaemagglutinin. This implies that persons with patent infections have schistosome antigen-specific defects in their ability to respond to IFN-gamma production that are not exhibited by putatively resistant 'endemic normals'.

Ivermectin-facilitated immunity in onchocerciasis; activation of parasite-specific Th1-type responses with subclinical Onchocerca volvulus infection

The present study examined the quantitative and qualitative changes registered in the parasite-specific antibody response, cellular reactivity and cytokine production profile in onchocerciasis patients repeatedly treated with ivermectin over a period of 8 years. The densities of Onchocerca volvulus microfilariae (mf) in treated patients remained significantly reduced, whereas the number of permanently amicrofilaridermic patients (subclinical infection) increased with repeated treatments. In vitro cellular responses to O. volvulus antigen (OvAg) were highest (P < 0.01) in untreated control individuals exposed to infection, but negative for mf of O. volvulus (endemic normals). Cellular reactivity in repeatedly treated patients was higher at 84 than at 36 months post initial treatment (p.i.t); furthermore, the proliferative responses to OvAg, mycobacterial purified protein derivative (PPD) and streptococcal SL-O were greater (P < 0.05) at 84 months p.i.t. in amicrofilaridermic than in microfilaria-positive onchocerciasis patients. In amicrofilaridermic patients such reactivity approached the magnitude observed in endemic normals. Peripheral blood mononuclear cells (PBMC) from patients and endemic normals produced equivalent amounts of IL-2, IL-4 and interferon-gamma (IFN-gamma) in response to mitogenic stimulation with phytohaemagglutinin (PHA); in response to OvAg, however, significantly more IL-2 and IFN-gamma were produced by PBMC from subclinical amicrofilaridermic patients or endemic normals than by mf-positive patients. OvAg-specific production of IL-4 by PBMC from treated patients was lower at 84 than at 36 months p.i.t. At three months p.i.t. the titres of circulating OvAg-specific IgG1-3 had increased (P < 0.05), but they then continuously declined with repeated treatments. Only IgG1 and IgG4 bound to OvAg of mol. wt 2-12 kD at 1 month p.i.t., while recognition of OvAg of mol. wt 10-200 kD by IgG1, IgG2 and IgG4 reached a maximum intensity at 3-6 months p.i.t., with the overall intensity of binding to OvAg gradually weakening thereafter. These results suggest that onchocerciasis-associated immunosuppression is reversible following ivermectin-induced permanent clearance of microfilariae from the skin; and that a vigorous parasite-specific cellular reactivity and a sustained production of IL-2 and IFN-gamma in amicrofilaridermic individuals may contribute to controlling O. volvulus infection.

Elevated levels of T cell activation antigen CD27 and increased interleukin-4 production in human lymphatic filariasis

To assess the immunological changes occurring during filarial infection with or without elephantiasis, 145 patients in different clinical groups from an endemic area in Indonesia were compared with respect to plasma levels of both soluble CD25 (sCD25) and sCD27; interleukin-4 (IL-4) and interferon-gamma release by peripheral blood mononuclear cells was also measured in a smaller subset of individuals. Levels of sCD27 were significantly elevated in elephantiasis and microfilaremic patients compared with endemic normals (p < 0.002), whereas sCD25 levels remained low in microfilaremics and was only slightly elevated in elephantiasis patients compared with endemic normals (p < 0.02). As activated T cell populations release both sCD27 and sCD25, these findings imply that there is filarial-driven activation of a T cell subset that releases sCD27 rather than sCD25. The expansion of a particular T cell population by filarial parasites is further suggested by the enhancement in both IL-4-producing and CD4+CD27-T cells in PBMC from elephantiasis and microfilaremic patients compared with endemic normals. More detailed characterization and comparison of CD27-lymphocytes from these individuals may identify mechanisms involved in the pathogenesis of lymphatic filariasis.

Immunity and immunoregulation in helminth infections

Parasitic helminths continue to be a major cause of morbidity in human populations, particularly in the tropics and subtropics. The need for effective vaccines that minimize worm burdens, thus reducing associated pathology, is evident. With this goal in mind, an intense research effort is in progress to characterize immune responses to helminths, especially in the context of recent developments in our understanding of the cytokine network. The growing realization that the parasites can themselves subvert host immune responses to their own advantage makes the task of vaccine development that much harder.