Evidence for protective immunity to bancroftian filariasis in the Cook Islands

Ligand binding properties of two Brugia malayi fatty acid and retinol (FAR) binding proteins and their vaccine efficacies against challenge infection in gerbils

Parasitic nematodes produce an unusual class of fatty acid and retinol (FAR)-binding proteins that may scavenge host fatty acids and retinoids. Two FARs from Brugia malayi (Bm-FAR-1 and Bm-FAR-2) were expressed as recombinant proteins, and their ligand binding, structural characteristics, and immunogenicities examined. Circular dichroism showed that rBm-FAR-1 and rBm-FAR-2 are similarly rich in α-helix structure. Unexpectedly, however, their lipid binding activities were found to be readily differentiated. Both FARs bound retinol and cis-parinaric acid similarly, but, while rBm-FAR-1 induced a dramatic increase in fluorescence emission and blue shift in peak emission by the fluorophore-tagged fatty acid (dansyl-undecanoic acid), rBm-FAR-2 did not. Recombinant forms of the related proteins from Onchocerca volvulus, rOv-FAR-1 and rOv-FAR-2, were found to be similarly distinguishable. This is the first FAR-2 protein from parasitic nematodes that is being characterized. The relative protein abundance of Bm-FAR-1 was higher than Bm-FAR-2 in the lysates of different developmental stages of B. malayi. Both FAR proteins were targets of strong IgG1, IgG3 and IgE antibody in infected individuals and individuals who were classified as endemic normal or putatively immune. In a B. malayi infection model in gerbils, immunization with rBm-FAR-1 and rBm-FAR-2 formulated in a water-in-oil-emulsion (®Montanide-720) or alum elicited high titers of antigen-specific IgG, but only gerbils immunized with rBm-FAR-1 formulated with the former produced a statistically significant reduction in adult worms (68%) following challenge with B. malayi infective larvae. These results suggest that FAR proteins may play important roles in the survival of filarial nematodes in the host, and represent potential candidates for vaccine development against lymphatic filariasis and related filarial infections.

Elimination of lymphatic filariasis as a public health problem from the Cook Islands

Background

The Cook Islands has a long history of high-endemicity lymphatic filariasis (LF) transmitted by Aedes vector mosquitoes. Though the infection prevalence had declined between 1975 and 1999 following episodic treatment activities, still infection was widespread with pockets of persistent infection. Beginning in 1999, the Cook Islands embarked on a national program, in partnership with Pacific Programme to Eliminate LF (PacELF), to eliminate LF as a public health problem.

Methods

All 12 inhabited islands were identified as endemic, and six rounds of mass drug administration (MDA) with once-yearly, single-dose albendazole plus diethylcarbamazine (DEC) were implemented during 2000–2006 to interrupt transmission of LF. Surveys carried out at the baseline, mid-term, stop-MDA, and post-MDA periods assessed LF antigen (Ag) prevalence in children and adults. Historical data, health workers’ observations, and hospital records were used to assess the trend and burden of chronic disease.

Results

The baseline Ag prevalence (1999) ranged from 2.0% in Manihiki to > 18.0% in Aitutaki, Mitiaro, and Pukapuka, and the national average Ag prevalence was 8.6%. MDA, carried out with a national treatment coverage over six annual rounds of MDA ranging from 63.5 to 96.7% in different years, was stopped in 2007. By then, the national Ag prevalence had declined to 0.27%. The post-MDA surveillance survey results (2013–2014) showed that Ag prevalence had fallen to 0% in 11/12 islands, and the national prevalence was only 0.03%. Chronic filarial disease had almost entirely disappeared.

Conclusion

The Cook Islands met all the criteria required for the World Health Organization (WHO) to acknowledge elimination of LF as a public health problem, as it did officially in 2016. This success also confirms that LF, even when transmitted by Aedes mosquitoes that are recognized to be more efficient than other vector species, can be eliminated as a public health problem by six rounds of MDA.

Human Helminths and Allergic Disease: The Hygiene Hypothesis and Beyond

There is much debate about the interaction between helminths and allergic disease. The "Hygiene Hypothesis," a very popular concept among scientists and the lay public, states that infections, especially during childhood, can protect against allergic diseases. Indeed, helminth infections are known to induce regulatory responses in the host that can help the control of inflammation (including allergic inflammation). However, these infections also induce type-2-associated immune responses including helminth-specific IgE that can cross-react against environmental allergens and mediate IgE-driven effector responses. Thus, it is the delicate balance between the parasites' anti- and pro-allergenic effects that define the helminth/allergy interface.

Parasite-antigen driven expansion of IL-5(-) and IL-5(+) Th2 human subpopulations in lymphatic filariasis and their differential dependence on IL-10 and TGFβ

BACKGROUND

Two different Th2 subsets have been defined recently on the basis of IL-5 expression - an IL-5(+)Th2 subset and an IL-5(-)Th2 subset in the setting of allergy. However, the role of these newly described CD4(+) T cells subpopulations has not been explored in other contexts.

METHODS

To study the role of the Th2 subpopulation in a chronic, tissue invasive parasitic infection (lymphatic filariasis), we examined the frequency of IL-5(+)IL-4(+)IL-13(+) CD4(+) T cells and IL-5(-)IL-4 IL-13(+) CD4(+) T cells in asymptomatic, infected individuals (INF) and compared them to frequencies (Fo) in filarial-uninfected (UN) individuals and to those with filarial lymphedema (CP).

RESULTS

INF individuals exhibited a significant increase in the spontaneously expressed and antigen-induced Fo of both Th2 subpopulations compared to the UN and CP. Interestingly, there was a positive correlation between the Fo of IL-5(+)Th2 cells and the absolute eosinophil and neutrophil counts; in addition there was a positive correlation between the frequency of the CD4(+)IL-5(-)Th2 subpopulation and the levels of parasite antigen - specific IgE and IgG4 in INF individuals. Moreover, blockade of IL-10 and/or TGFβ demonstrated that each of these 2 regulatory cytokines exert opposite effects on the different Th2 subsets. Finally, in those INF individuals cured of infection by anti-filarial therapy, there was a significantly decreased Fo of both Th2 subsets.

CONCLUSIONS

Our findings suggest that both IL-5(+) and IL-5(-)Th2 cells play an important role in the regulation of immune responses in filarial infection and that these two Th2 subpopulations may be regulated by different cytokine-receptor mediated processes.

The Larval Specific Lymphatic Filarial ALT-2: Induction of Protection Using Protein or DNA Vaccination

Genes from the infective stage of lymphatic filarial parasites expressed at the time of host invasion have been identified as potential vaccine candidates. By screening an L3 cDNA library with sera from uninfected longstanding residents of an area endemic for onchocerciasis, so-called "endemic normals" (EN), we have cloned and characterized one such gene termed the abundant larval transcript two (ALT-2). The stage specificity of ALT-2 gene transcription and protein synthesis was confirmed by PCR using genespecific primers, and by western blot analysis of protein extracts from various stages of the parasite life cycle using specific antisera. Significant differences in antibody response to the recombinant ALT-2 were observed in endemic populations with differing clinical manifestations of lymphatic filariasis with an antibody response present in sera from 18 of 25 (72%) EN subjects compared to 9 of 25 (36%) with subclinical microfilaracmia (MF) and 14 of 25 (52%) of those with chronic lymphatic obstruction (CP) (P=0.01 for comparison of EN to CP or to MF). This differential responsiveness suggests that the protective immunity postulated to account for their uninfected status might be associated with a response to this protein. When the utility of ALT-2 as a vaccine candidate was tested in a murine model using either recombinant protein or a DNA vaccine construct, statistically significant protection was observed when compared to a control filarial gene product expressed across all stages of the parasite lifecycle (SXP-1; P=0.02 for protein and P=0.01 for the DNA vaccine) or compared to adjuvant alone. This level of protection indicates that this vaccine is a promising candidate for further development.

Large extracellular loop of tetraspanin as a potential vaccine candidate for filariasis

Lymphatic filariasis affects nearly 120 million people worldwide and mass preventive chemotherapy is currently used as a strategy to control this infection. This has substantially reduced the incidence of the infection in several parts of the world. However, a prophylactic vaccine would be more effective in preventing future infections and will supplement the mass chemotherapy efforts. Unfortunately, there is no licensed vaccine available currently to prevent this infection. Molecules expressed on the surface of the parasite are potential candidates for vaccine development as they are exposed to the host immune system. In this study we show that the large extracellular loop of tetraspanin (TSP LEL), a protein expressed on the cuticle of Brugia malayi and Wuchereria bancrofti is a potential vaccine candidate. Our results showed that BmTSP LEL is expressed on the surface of B. malayi infective third stage larvae (L3) and sera from human subjects who are putatively immune to lymphatic filariasis carry high titer of IgG1 and IgG3 antibodies against BmTSP LEL and WbTSP LEL. We also showed that these antibodies in the sera of human subjects can participate in the killing of B. malayi L3 in an antibody dependent cell-mediated cytotoxicity mechanism. Vaccination trials in mice showed that close to 64% protection were achieved against challenge infections with B. malayi L3. Immunized animals showed high titer of anti-WbTSP LEL IgG1, IgG2a and IgG2b antibodies in the sera and IFN-γ secreting cells in the spleen. Onchocerca volvulus another filarial parasite also expresses TSP LEL. Cross-reactivity studies showed that IgG1 antibody in the sera of endemic normal subjects, recognize OvTSP LEL. Similarly, anti-OvTSP LEL antibodies in the sera of subjects who are immune to O. volvulus were also shown to cross-react with rWbTSP LEL and rBmTSP LEL. These findings thus suggested that rTSP LEL can be developed as a potential vaccine candidate against multiple filarial infections.

Antibody to the filarial antigen Wb123 reflects reduced transmission and decreased exposure in children born following single mass drug administration (MDA)

Background

Antibody (Ab) to the Wuchereria bancrofti (Wb) infective larval (L3) antigen Wb123, using a Luciferase Immunoprecipitation System (LIPS) assay, has been shown to be a species-specific, early marker of infection developed for potential use as a surveillance tool following transmission interruption post mass drug administration. To examine its usefulness in a single filarial-endemic island assessed at two time points with markedly different levels of transmission, Ab to Wb123 was measured in sera collected from subjects from Mauke, Cook Islands in 1975 (no previous treatment) and 1992 (5 years after a one time island-wide treatment with diethylcarbamazine [DEC]).

Findings

Between 1975 and 1992, Wb transmission decreased dramatically as evidenced by reduced prevalences of microfilariae (31% vs. 5%) and circulating Ag (CAg, 49% vs. 16%). Age specific prevalence analysis showed a dramatic reduction in Wb123 Ab positivity from 54% (25/46) in 1975 to 8% (3/38) in 1992 in children 1–5 years (p<0.0001), reflecting the effects of single-dose treatment five years earlier. By 1992, Wb123 Ab prevalence in children 6–10 years had fallen from 75% (42/56) in 1975 to 42% (33/79) consistent with a lower cumulative transmission potential. In the whole population, Wb123 seropositivity decreased from 86% to 60% between 1975 and 1992. In CAg+ subjects the levels of Wb123 Ab were indistinguishable between the 2 time points but differed in those who were CAg− (p<0.0001). In paired sample analysis, individuals who were CAg+ in 1975 but became CAg− in 1992 had significantly lower Ab levels in 1992 (p<0.0001), with 9/40 (23%) becoming seronegative for Wb123.

Conclusions

The relationship between reduction in Wb123 Ab prevalence and the reduction of transmission, seen most clearly in young children, strongly advocates for the continuing assessment and rapid development of Wb123 as a surveillance tool to detect potential transmission of bancroftian filariasis in treated endemic areas.

Lymphatic filariasis (LF) causes an enormous disease burden throughout the tropics and subtropics. The Global Programme to Eliminate Lymphatic Filariasis was begun in 2000 following the advent of large donations from drug companies for treating LF and the development of a rapid antigen assay for detection of infection. As more countries undergo mass drug administration (MDA), the driving need is for development of a highly sensitive and specific antibody assay for detecting ongoing exposure to vector-borne filaria following MDA. The target group for such surveillance is children born during or following MDA. Current assays, while sensitive, are not specific enough where non-LF filaria species are co-endemic. Recently, we developed an antibody assay based upon the highly specific larval antigen Wb123 using the Luciferase Immunoprecipitation System (LIPS). In the current study, we determined that the Wb123 LIPS assay detects a reduction in LF transmission on an endemic island following a one-time island wide MDA with diethylcarbamazine, with the most pronounced reduction in prevalence of antibody to Wb123 occurring in young children born just prior to and following this MDA. We propose that Wb123 can be an extremely useful surveillance tool following MDA and should be developed into a rapid test format.

Brugia malayi thioredoxin peroxidase as a potential vaccine candidate antigen for lymphatic filariasis

Attempts were made to evaluate the protective efficacy of Brugia malayi thioredoxin peroxidase (BmTPX) in a mouse model. Mice immunized with a protein vaccine containing rBmTPX developed higher titres (1:5,000/1:10,000) of anti-BmTPX antibodies, compared with the mice immunized with the alum control. There was a higher level of cellular proliferative response in mice immunized with BmTPX compared with the alum control (p < 0.05), which was associated with a Th2-type of response. In order to compare the prophylactic efficacy of BmTPX in natural infection, we evaluated the human immune responses to these antigens in endemic normals (EN) and infected individuals (microfilaraemic and chronic pathology). Results showed that EN subjects carry BmTPX-specific IgG1 and IgG3 circulating antibodies against natural exposure to filariasis. Peripheral blood mononuclear cells from EN subjects responded strongly to rBmTPX by proliferating, as well as by secreting interferon (IFN)-γ (Th1) and IL-5 (Th2), a mixed type of response to rBmTPX. In the case of infected individuals, there was no IFN-γ or IL-5 response. Thus, there was a clear dichotomy in the cytokine production by infected versus EN individuals. Our findings suggest that BmTPX may be a suitable antigen candidate for lymphatic filariasis, but a further study is still required.

Biochemical characterization and evaluation of a Brugia malayi small heat shock protein as a vaccine against lymphatic filariasis

Filarial nematodes enjoy one of the longest life spans of any human pathogen due to effective immune evasion strategies developed by the parasite. Among the various immune evasion strategies exhibited by the parasite, Interleukin 10 (IL-10) productions and IL-10 mediated immune suppression has significant negative impact on the host immune system. Recently, we identified a small heat shock protein expressed by Brugia malayi (BmHsp12.6) that can bind to soluble human IL-10 receptor alpha (IL-10R) and activate IL-10 mediated effects in cell lines. In this study we show that the IL-10R binding region of BmHsp12.6 is localized to its N-terminal region. This region has significant sequence similarity to the receptor binding region of human IL-10. In vitro studies confirm that the N-terminal region of BmHsp12.6 (N-BmHsp12.6) has IL-10 like activity and the region containing the alpha crystalline domain and C-terminus of BmHsp12.6 (BmHsp12.6αc) has no IL-10 like activity. However, BmHsp12.6αc contains B cell, T cell and CTL epitopes. Members of the sHSP families are excellent vaccine candidates. Evaluation of sera samples from putatively immune endemic normal (EN) subjects showed IgG1 and IgG3 antibodies against BmHsp12.6αc and these antibodies were involved in the ADCC mediated protection. Subsequent vaccination trials with BmHsp12.6αc in a mouse model using a heterologous prime boost approach showed that 83% protection can be achieved against B. malayi L3 challenge. Results presented in this study thus show that the N-BmHsp12.6 subunit of BmHsp12.6 has immunoregulatory function, whereas, the BmHsp12.6αc subunit of BmHsp12.6 has significant vaccine potential.

Regulation of global gene expression in human Loa loa infection is a function of chronicity

Background

Human filarial infection is characterized by downregulated parasite-antigen specific T cell responses but distinct differences exist between patients with longstanding infection (endemics) and those who acquired infection through temporary residency or visits to filarial-endemic regions (expatriates).

Methods and Findings

To characterize mechanisms underlying differences in T cells, analysis of global gene expression using human spotted microarrays was conducted on CD4⁺ and CD8⁺ T cells from microfilaremic Loa loa-infected endemic and expatriate patients. Assessment of unstimulated cells showed overexpression of genes linked to inflammation and caspase-associated cell death, particularly in endemics, and enrichment of the Th1/Th2 canonical pathway in endemic CD4⁺ cells. However, pathways within CD8⁺ unstimulated cells were most significantly enriched in both patient groups. Antigen (Ag)-driven gene expression was assessed to microfilarial Ag (MfAg) and to the nonparasite Ag streptolysin O (SLO). For MfAg-driven cells, the number of genes differing significantly from unstimulated cells was greater in endemics compared to expatriates (p<0.0001). Functional analysis showed a differential increase in genes associated with NFkB (both groups) and caspase activation (endemics). While the expatriate response to MfAg was primarily a CD4⁺ pro-inflammatory one, the endemic response included CD4⁺ and CD8⁺ cells and was linked to insulin signaling, histone complexes, and ubiquitination. Unlike the enrichment of canonical pathways in CD8⁺ unstimulated cells, both groups showed pathway enrichment in CD4⁺ cells to MfAg. Contrasting with the divergent responses to MfAg seen between endemics and expatriates, the CD4⁺ response to SLO was similar; however, CD8⁺ cells differed strongly in the nature and numbers (156 [endemics] vs 36 [expatriates]) of genes with differential expression.

Conclusions

These data suggest several important pathways are responsible for the different outcomes seen among filarial-infected patients with varying levels of chronicity and imply an important role for CD8⁺ cells in some of the global changes seen with lifelong exposure.

Infection with the filarial parasite Loa loa causes a parasite-specific downregulation of T cell responses. However, differences exist (clinical and immunologic) between patients born and living in filarial endemic regions (endemics) and those who become infected during travel or short-term residency (expatriates). T cell responses are more depressed in endemics while expatriates have more clinical “allergic-type” symptoms. In this study, we showed that these differences reflect transcriptional differences within the T cell compartment. Using microarrays, we examined global gene expression in both CD4⁺ and CD8⁺ T cells of microfilaremic endemic and expatriate patients and found differences not only ex vivo, but also to parasite and, for CD8⁺ cells, to nonparasite antigens. Functional analysis showed that endemic patients expressed genes linked to inflammatory disease and caspase associated cell death at homeostasis while expatriates tended to have a more activation-induced gene profile at homeostasis and a CD4⁺ inflammatory response to parasite antigen. Patient groups were similar in their CD4⁺ response to nonparasite antigen but strongly differed in their CD8⁺ responses, demonstrating the potential global ramifications of chronic, longstanding infection. Our study describes potential transcriptional mechanisms for the variability seen in patients with different levels of exposure to and chronicity of filarial infection.

Altered T cell memory and effector cell development in chronic lymphatic filarial infection that is independent of persistent parasite antigen

Chronic lymphatic filarial (LF) infection is associated with suppression of parasite-specific T cell responses that persist even following elimination of infection. While several mechanisms have been implicated in mediating this T cell specific downregulation, a role for alterations in the homeostasis of T effector and memory cell populations has not been explored. Using multiparameter flow cytometry, we investigated the role of persistent filarial infection on the maintenance of T cell memory in patients from the filarial-endemic Cook Islands. Compared to filarial-uninfected endemic normals (EN), microfilaria (mf) positive infected patients (Inf) had a reduced CD4 central memory (T_CM) compartment. In addition, Inf patients tended to have more effector memory cells (T_EM) and fewer effector cells (T_EFF) than did ENs giving significantly smaller T_EFF ∶ T_EM ratios. These contracted T_CM and T_EFF populations were still evident in patients previously mf+ who had cleared their infection (CLInf). Moreover, the density of IL-7Rα, necessary for T memory cell maintenance (but decreased in T effector cells), was significantly higher on memory cells of Inf and CLInf patients, although there was no evidence for decreased IL-7 or increased soluble IL7-Rα, both possible mechanisms for signaling defects in memory cells. However, effector cells that were present in Inf and CLInf patients had lower percentages of HLA-DR suggesting impaired function. These changes in T cell populations appear to reflect chronicity of infection, as filarial-infected children, despite the presence of active infection, did not show alterations in the frequencies of these T cell phenotypes. These data indicate that filarial-infected patients have contracted T_CM compartments and a defect in effector cell development, defects that persist even following clearance of infection. The fact that these global changes in memory and effector cell compartments do not yet occur in infected children makes early treatment of LF even more crucial.

Impairment of tetanus-specific cellular and humoral responses following tetanus vaccination in human lymphatic filariasis

To investigate the consequences of the impaired parasite-specific immune response in lymphatic filariasis, the effect of concurrent Wuchereria bancrofti infection on the immune response to tetanus toxoid (TT) following tetanus vaccination was studied in 20 asymptomatic microfilaremic (MF) patients, 20 patients with chronic lymphatic obstruction/elephantiasis (chronic pathology [CP]), and 10 endemic normal (EN) control individuals at baseline and at 3 and 6 months after TT vaccination. Peripheral blood mononuclear cell (PBMC) proliferative responses to TT before vaccination were not significantly different between the EN control and CP groups, but the MF group showed significantly lower baseline proliferative responses to TT compared with either the EN or CP group. Six months following vaccination, the change in proliferative response to TT was significantly greater in the EN and CP groups than in the MF group. This difference in proliferative response was reiterated in the gamma interferon (IFN-gamma) response in the EN group, in that they increased IFN-gamma production by 400% at 6 months, in contrast to that seen in the filaria-infected groups. In contrast to the IFN-gamma responses, PBMCs from the MF group produced significantly increased levels of TT-specific IL-10 compared with PBMCs from the EN group. Although there was significantly greater TT-specific immunoglobulin G (IgG) production at baseline between the EN and MF groups, postvaccination IgG (and IgG1 isotype) responses did not differ among the groups, whereas TT-specific IgG2, IgG3, and IgG4 were all increased in the EN group compared with the filaria-infected groups. These studies indicate that concurrent infection with W. bancrofti can diminish the immune response to an unrelated antigen by a mechanism that is likely to involve IL-10.

Onchocerca ochengi acquisition in zebu Gudali cattle exposed to natural transmission: parasite population dynamics and IgG antibody subclass responses to Ov10/Ov11 recombinant antigens

Ngaoundere Gudali zebu cattle naturally exposed to Simulium damnosum s.l. and Culicoides spp. bites were examined during 4 years for O. ochengi adult worm acquisition, Onchocerca ochengi and Onchocerca gutturosa skin microfilaria dynamics, and IgG1 and IgG2 antibody subclass responses. Eleven animals acquired a total of 465 O. ochengi nodules (average of 17 per female and 72 per male). The O. ochengi nodule load was highly variable in individual animals and exacerbated in mature male cattle. Three patterns of acquisition of O. ochengi (resistant to new infestation, early susceptibility and late susceptibility), not associated with Simulium biting intensity (P > 0.05), were distinguished. The minimum prepatent periods for O. ochengi nodules, O. ochengi microfilariae and O. gutturosa microfilariae were 10, 20 and 21 months, respectively. The O. ochengi microfilaria density significantly (P < 0.001) increased with age, was higher in young mature bulls than female animals (P < 0.001) and finally reached highest levels (P < 0.005) during the dry season. Antibody responses to Ov10/Ov11 recombinant O. volvulus antigens were predominantly of the IgG1 subclass. High levels of this subclass (not IgG2) observed in new born calves declined to almost zero levels at the age of 5-8 months but IgG1 levels significantly increased (P < 0.05) with age subsequently during patency. Put together the acquisition and accumulation of O. ochengi parasites in zebu cattle, apart from being season, sex (gender) and host age associated, may also suggest a density-dependent regulation of parasite establishment in a proportion of the exposed population.

CTLA-4 in filarial infections: implications for a role in diminished T cell reactivity

To determine the role that CTLA-4 might play in mediating the diminished parasite Ag-specific T cell responsiveness that is characteristically seen in filaria-infected patients, several study populations and methods were used. First, quantitative assessment of mRNA expression determined that PBMC from uninfected adolescents exposed in utero to microfilarial (Mf) Ag demonstrated a strong up-regulation of CTLA-4 to the Mf stage of the parasite in contrast to that observed in cells from children born of uninfected mothers (p = 0.005). Next, the frequency of CTLA-4 expression was examined using flow cytometry in cells from filaria-infected and -uninfected individuals ex vivo. Individuals born in filarial endemic regions of the world (with long-standing infections) had greater percentages of CD4(+)CTLA-4(+) cells than did expatriate infected or uninfected individuals (p = 0.005 and 0.05, respectively); in addition, Mf(+) patients demonstrated higher frequencies of CD4(+)CTLA-4(+) and CD8(+)CTLA-4(+) cells (p = 0.027 and 0.037, respectively) than did Mf(-) infected individuals. Of interest, the greatest intensity of CTLA-4 expression occurred in CD4(+)CD25(+) cells, a population purported to include suppressor cells. Finally, in vitro blocking of CTLA-4 expression in PBMC from filaria-infected individuals induced a mean increase of 44% in IL-5 production to Mf Ag, whereas there was a concurrent mean decrease of 42% in IFN-gamma production, suggesting that CTLA-4 also acts to alter the Th1/Th2 balance in filaria-infected individuals. Together, these data indicate a significant role for CTLA-4 in regulating the host response to filarial infections and that factors such as length of exposure and patency are important codeterminants.

Lymphatic filariasis-specific immune responses in relation to lymphoedema grade and infection status. I. Cellular responses

The filariasis-specific cellular responsiveness was assessed in 109 adult individuals from a Wuchereria bancrofti-endemic area in north-east Tanzania. There were 9 study groups. Five groups of individuals were negative for microfilariae (mf) and specific circulating filarial antigen (CFA) and had leg lymphoedema of varying severity ranging from early to more advanced grades (pathology groups 1-5). Another group comprised individuals with mixed grades of lymphoedema and positive for mf and/or CFA (mixed pathology group). Three asymptomatic groups consisted of individuals without leg pathology but with different infection status: (i) CFA- and mf-negative individuals, (ii) CFA-positive but mf-negative individuals, and (iii) CFA- and mf-positive individuals. Peripheral blood mononuclear cells were collected and proliferative responsiveness and secretion of interleukin (IL)-4, IL-10 and gamma interferon (IFN gamma) were measured upon stimulation with a Brugia pahangi antigen. No distinct differences in responses were observed between the 5 uninfected pathology groups. Instead, responses were associated with infection status, with generally higher proliferative activity and higher levels of IL-4 and IFN gamma in uninfected as compared to infected individuals. High levels of IL-10 were observed in asymptomatic individuals without infection and in asymptomatic CFA-positive but mf-negative individuals. Asymptomatic individuals with mf had relatively low IL-10 levels. Groups presenting with chronic pathology generally had low levels of IL-10 independently of infection status. The findings thus give no immediate indication that the measured immunological parameters are related to progression of leg pathology. However, alternative interpretations are presented which suggest a possible role of immunological reactions in development of pathology in lymphatic filariasis.

A simple and quick method for enhanced detection of specific IgE in serum from lymphatic filariasis patients

A new simple and quick technique, using a suspension of protein A agarose beads to absorb IgG4 from sera prior to determination of filarial-specific IgE in ELISA, is presented. The optimal ratio between serum and absorbant was determined by absorbing fixed volumes of sera from individuals from a Wuchereria bancrofti endemic area with different volumes of the protein A agarose bead suspension and testing supernatants for filaria-specific IgG4 and IgE. The effect of absorption on measured IgG4 and IgE intensities in sera from various categories of individuals from the endemic area was thereafter examined. Overall, absorption resulted in a 96.5% decrease in mean ELISA OD values for IgG4 and a 41.6% increase in mean ELISA OD values for IgE. Higher increases in IgE measurements were seen with sera from circulating filarial antigen (CFA) negative individuals (64.7%), microfilaria (mf) negative individuals (56.1%) and individuals with chronic filarial disease (62.7%) than with sera from individuals who were CFA positive (23.4%), mf positive (10.0%), or without chronic disease (36.5%). These differences indicate that the degree to which IgE detection in unabsorbed serum is blocked by IgG4 varies with infection and disease status. Absorption of IgG4 from serum with a protein A agarose bead suspension prior to measurement of specific IgE is a useful alternative to conventional gel column absorption methods, particularly when processing many samples.

Lymphatic filariasis: an infection of childhood

Lymphatic filariasis (LF), already recognized as a widespread, seriously handicapping disease of adults, was generally thought to occur only sporadically in children. New, highly sensitive diagnostic tests (antigen detection, ultrasound examination) now reveal, however, that LF is first acquired in childhood, often with as many as one-third of children infected before age 5. Initial damage to the lymphatic system by the parasites generally remains subclinical for years or gives rise only to non-specific presentations of adenitis/adenopathy; however, especially after puberty the characteristic clinical features of the adult disease syndromes (lymphoedema, hydrocoele) manifest themselves. Recognizing that LF disease starts its development in childhood has immediate practical implications both for management and prevention of the disease in individual patients and for the broader public health efforts to overcome all childhood illnesses. For the new World Health Organization (WHO)-supported, public-/private-sector collaboration (Global Alliance) to eliminate LF through once-yearly drug treatment, this recognition means that children will be not only the principal beneficiaries of LF elimination but also a population particularly important to target in order for the programme to achieve its twin goals of interrupting transmission and preventing disease.

Regulation of the immune response in lymphatic filariasis: perspectives on acute and chronic infection with Wuchereria bancrofti in South India

Delineating the immune responses in lymphatic filariasis has been complicated not only by the rapidly expanding knowledge of new immunological mediators and effortors, but also by new methodologies (in particular, circulating filarial antigen detection) for defining and categorizing filarial-infected individuals. By using assays for circulating antigen in the sera collected as part of the many immunological studies performed on individuals in a Wuchereria bancrofti-endemic region of South India, we have attempted to explore the influence of patency on the antigen-driven proliferative and cytokine responses seen in peripheral blood mononuclear cells of individuals with varying clinical manifestations of lymphatic filarial infection. Moreover, we have provided perspectives on the differences between acute and chronic infection with W. bancrofti and suggested mechanisms that may underly the modulation of the immune response as patency occurs.

Calgranulin C has filariacidal and filariastatic activity

The calgranulins are a family of calcium- and zinc-binding proteins produced by neutrophils, monocytes, and other cells. Calgranulins are released during inflammatory responses and have antimicrobial activity. Recently, one of the calgranulins, human calgranulin C (CaGC), has been implicated as an important component of the host responses that limit the parasite burden during filarial nematode infections. The goal of this work was to test the hypothesis that human CaGC has biologic activity against filarial parasites. Brugia malayi microfilariae and adults were exposed in vitro to 0.75 to 100 nM recombinant human CaGC. Recombinant CaGC affected adult and larval parasites in a dose-dependent fashion. Microfilariae were more sensitive to the action of CaGC than were adult parasites. At high levels, CaGC was both macrofilariacidal and microfilariacidal. At lower levels, the percentage of parasites killed was dependent on the level of CaGC in the culture system. The larvae not killed had limited motility. The filariastatic effect of low-level CaGC was reversed when the CaGC was removed from the culture system. Immunohistochemical analysis demonstrated that human CaGC accumulated in the cells of the hypodermis-lateral chord of adult and larval parasites. The antifilarial activity of CaGC was not due to the sequestration of zinc. Thus, the cellular and molecular mechanisms that result in the production and release of CaGC in humans may play a key role in the regulation of filarial parasite numbers.

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.

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.

Filarial nematode parasites secrete a homologue of the human cytokine macrophage migration inhibitory factor

Filarial nematode parasites establish long-term chronic infections in the context of an antiparasite immunity that is strongly biased toward a Th2 response. The mechanisms that lead to this Th2 bias toward filarial antigens are not clear, but one possibility is that the parasites produce molecules that have the capacity to proactively modify their immunological environment. Here we report that filarial parasites of humans secrete a homologue of the human proinflammatory cytokine macrophage migration inhibitory factor (MIF) that has the capability of modifying the activity of human monocytes/macrophages. A cDNA clone isolated from a Brugia malayi infective-stage larva expression library encoded a 12.5-kDa protein product (Bm-MIF) with 42% identity to human and murine MIF. MIF homologues were also found to be expressed in the related filarial species Wuchereria bancrofti and Onchocerca volvulus. Bm-mif was transcribed by adult and larval parasites, and the protein product was found in somatic extracts and in the parasite's excretory-secretory products. Immunohistocytochemistry revealed that Bm-MIF was localized to cells of the hypodermis/lateral chord, the uterine wall, and larvae developing in utero. Unexpectedly, the activities of recombinant Bm-MIF and human MIF on human monocytes/macrophages were found to be similar. When placed with monocytes/macrophages in a cell migration assay, Bm-MIF inhibited random migration. When placed away from cells, Bm-MIF induced an increase in monocyte/macrophage migration that was specifically inhibited by neutralizing anti-Bm-MIF antibodies. Bm-MIF is the first demonstration that helminth parasites produce cytokine homologues that have the potential to modify host immune responses to promote parasite survival.

The diverse expression of immunity in humans at distinct states of Onchocerca volvulus infection

This study examined the development and persistence of immunity in humans presenting defined states of Onchocerca volvulus infection, i.e. in exposed endemic control individuals without microfilaridermia and clinical disease, in patients with patent or post-patent onchocerciasis, and in patients concurrently infected with Mansonella perstans. Onchocerca volvulus antigen (OvAg)-specific cellular reactivity was significantly diminished in microfilariae (mf)-positive patients, while the highest reactivity was measured in exposed but mf-negative endemic controls, those being free of any clinical signs of onchocercal disease. In patients who became post-patent, responses to OvAg were significantly augmented, but did not approach entirely the magnitude observed in endemic controls. In onchocerciasis patients with concurrent mansonelliasis, cellular unresponsiveness to OvAg persisted, even when mf of O. volvulus were eliminated permanently by repeated ivermectin therapy. Cells from mf-positive onchocerciasis patients produced significantly less interferon-gamma (IFN-gamma) (P < 0.01) and interleukin-5 (IL-5) (P < 0.05) in response to OvAg than those taken from endemic controls or post-patent individuals in whom IFN-gamma and IL-5 production was similarly high. In contrast, both OvAg-driven as well as spontaneous IL-10 secretion was higher in mf-positive patients than in endemic controls or post-patent cases. In all individuals examined, serological recognition of OvAg by immunoglobulins was dominated by IgG4; in mf-positive patients OvAg of 205,000-12,000 molecular weight (MW) were strongly bound. In post-patent individuals, and similarly in endemic controls. OvAg recognition by IgG4 varied from intense (with numerous antigens being recognized) to weak or absent antigen binding. Significantly elevated OvAg-specific IgG isotypes were measured in mf-positive onchocerciasis patients in comparison with endemic controls or post-patent individuals (with the exception of IgG3). IgG1, IgG2 and IgE were higher, but IgG4 was lower in endemic controls compared with post-patent onchocerciasis patients. The ratios of IgG4/IgG1 differed (P < 0.001) between endemic controls and mf-positive or post-patent onchocerciasis patients, with IgG4/IgG1 ratios of R < 3.0 being characteristic for endemic controls and post-patent O. volvulus infection. In conclusion, this cross-sectional immunoepidemiological investigation showed that distinct states of O. volvulus infection correlate with a particular cellular and humoral immune response. The mf-free condition appeared to be associated with a vigorous parasite-specific cellular reactivity and a particular cytokine production profile, while concurrent M. perstans infection depressed OvAg-specific cellular responsiveness. Antibody responses, in all likelihood, reflected the intensity and state of infection, and not the degree of acquired immunity protective against parasite aggregation.