Schistosoma mansoni: larval damage and role of effector cell(s) in the synergy between vaccine immunity and praziquantel treatment

Effect of levamisole on the immune response of patients with schistosomiasis after treatment with praziquantel

Successful therapy with praziquantel in schistosomiasis is dependent on a normal host immune system, i.e. the recognition of parasite antigen, to be effective in eradicating the parasite. Levamisole, a nonspecific immunomodulator, has been hypothesised to normalise deficient cell-mediated immunity. The aim of this study was to investigate the effect of levamisole on the immune response seen after presumed curative doses of praziquantel. After ethical approval, 29 male volunteers infected with Schistosoma haematobium were recruited and randomised to receive orally either (a) levamisole 5 mg/kg for 2 consecutive days, plus praziquantel 40 mg/kg as a single dose, (b) levamisole 5 mg/kg for 2 days, or (c) praziquantel 40 mg/kg as a single dose. Blood samples were collected on day 0 and again 3 months later for differential counts and specific IgG and IgE to whole-worm-antigen, as well as for the detection of eosinophil cationic protein (ECP). Levamisole treatment alone resulted in a significant drop in circulating eosinophil counts, i.e. a median (95% confidence interval) drop of 6.0 (1;7)%. Eosinophil counts did not change significantly after praziquantel therapy alone. ECP values were significantly reduced with levamisole alone and praziquantel alone, i.e. respective median (95% confidence interval) drops of 33.58 (18.94; 55.9) and 16.54 (3.6; 33.5) microg/L (p < 0.05). The group receiving combination therapy demonstrated significant increases in specific IgG values (p = 0.02) with no decline in ECP levels and eosinophil counts compared with baseline values. Levamisole alone had no effect on egg counts. In conclusion, levamisole inhibited circulating eosinophils and eosinophil activation when given alone, but caused immune stimulation when combined with praziquantel. The exact mechanism of action still needs to be determined.

Paramyosin is a major target of the human IgA response against Schistosoma japonicum

Human resistance and susceptibility to schistosomiasis is associated with age and specific antibody isotype responses against worm (SWAP) and egg (SEA) antigens. In a cross-sectional study of 176 individuals infected with Schistosoma japonicum in the Philippines, strikingly similar isotype response patterns against SWAP and SEA was observed when compared to other endemic areas. Interestingly, IgA titres to SWAP correlated with older age among S. japonicum-infected individuals (n = 176, P < 0.01), suggesting a role for this isotype in protective immunity. To identify the molecular targets of human IgA, 17 high-IgA/SWAP responders were identified from the said population. IgA antibodies from the majority (14/17) of these individuals recognized a band of 97 kDa (Sj97), comigrating in immunoblots with the myofibrillar protein paramyosin. The antigen was confirmed as paramyosin by expressed sequence tag (EST)-analysis of four clones obtained by screening an adult S. japonicum cDNA library with pooled IgA antisera and mouse antiparamyosin polyclonal antibodies. The identification of paramyosin as a major target of human IgA raises its potential as a vaccine candidate that targets mucosal immune responses. Since this antigen is exposed on the parasite surface only during the lung stages, we propose that human IgA contributes to parasite attrition during schistosome migration in the lungs.

Active immunization of mice with Schistosoma mansoni worm membrane antigens enhances efficacy of praziquantel

The efficacy of praziquantel treatment was significantly enhanced (P < 0.01) in CBA/Ca mice that had been immunized prior to Schistosoma mansoni infection with a crude extract of worm membrane antigens. In Western immunoblots sera from the worm antigen-immunized animals had a polyspecific antibody response, with a 25-27 kDa antigen being reacted against with particular intensity. A molecule of similar size was also recognized by rabbit antisera raised against an antigen with esterase activity that has been previously identified as a sensitive target for drug-antibody synergy. The increase in efficacy of subcurative doses of praziquantel in immunized animals is attributed to drug-induced tegumental damage causing antigens to become exposed on the worm surface. Thus, specific antigens, including the 25-27 kDa antigen, become accessible to circulating schistosomicidal antibodies. The role of antibodies that can synergize with praziquantel to kill schistosome worms is discussed.

Studies on host specificity in Paragonimus westermani: ultrastructural study of inflammatory reactions to the parasite in rat muscles

Inflammatory reactions to the lung fluke, Paragonimus westermani, in an abnormal host, the rat, were studied by electron microscopy. Inflammatory cells accumulated around the worms in rat muscles, but the reaction varied significantly in intensity as well as distribution of inflammatory cells from worm to worm in the same and different hosts. In an accumulation of inflammatory infiltrates around the worm, eosinophils were more commonly seen, and neutrophils and macrophages were fewer in number. Mast cells appeared rarely. Where inflammatory cells touched to the worms, the tegument of the worms was covered with moderately electron-dense flocculent material which included dense granules. This material, which probably consisted of immune complexes of host antibody and antigen secreted from tegumental granules, seemed to block the inflammatory cells from adhering to the worm surfaces. Inflammatory cells made direct contact with worm surfaces where flocculent material was absent. Dense secretory material was discharged from some eosinophils as a result of degranulation.

Immune-dependent chemotherapy of schistosomiasis

Host immune responses have been shown to enhance the efficacy of several schistosomicidal drugs. The evidence derives mainly from experiments on Schistosoma mansoni infections in the mouse with their immune status variously modulated; this review emphasises praziquantel (PZQ), which is now the main drug used for treatment of human schistosomiasis. Electron microscopy and indirect immunofluorescence indicate that PZQ disrupts the integrity of the surface membranes of S. mansoni, particularly those covering the dorsal tubercles of adult male worms, and this causes antigens which are the targets of antibody attack to be revealed. We review the evidence that two S. mansoni antigens in particular are implicated in the immune-dependent action of PZQ: a 200 kDa glycoprotein and a 27 kDa antigen with non-specific esterase activity. Consistent with the involvement of the latter antigen, increased non-specific esterase activity was demonstrated histochemically on the surface of intact PZQ-treated male worms, and we describe a chromogenic substrate assay for quantifying the amount of esterase activity that is exposed after drug treatment. The potential relevance of these observations for enhancing the efficacy of drugs currently used to treat human schistosomiasis, and for devising novel therapeutic strategies, is discussed.

Chemotherapy and immunity in opportunistic parasitic infections in AIDS

Parasitic diseases are endemic in parts of the tropics, but there is no convincing evidence that their prevalence or incidence is increasing due to the HIV epidemic. Available scientific data on parasitic infections in patients with the Acquired Immunodeficiency Syndrome (AIDS) suggests a predominance of Pneumocystis carinii, Toxoplasma gondii and Cryptosporidium spp. For reasons which are unclear, parasitic infections such as Plasmodium falciparum, Strongyloides stercoralis and Entamoeba histolytica, where cell-mediated immune responses are also thought to be significant, do not appear to be opportunists of importance. It is being increasingly recognized that chemotherapy for parasitic diseases has a host-dependent component, although scientific data on this subject remain scanty. The management of opportunistic parasitic infections in patients infected with HIV is dogged by failures and relapses, aptly illustrating the notion of the relationship between chemotherapy and the immune response. This review discusses the immunity and chemotherapy of opportunistic parasite infections in patients infected with the Human Immunodeficiency Virus (HIV).