IgG1 and IgG4 serum antibody responses in asymptomatic and clinically expressed cystic echinococcosis patients

Abstract

IgG1 and IgG4 subclass antibody responses were investigated in two clinically different groups of hepatic cystic echinococcosis (CE) patients. One group consisted of surgically proven CE cases (clinically expressed hospitalized cases) and a second group comprised asymptomatic CE patients (first time community detected cases) diagnosed by portable ultrasound and serology in four different endemic communities. Fifty eight sera from surgically proven CE patients and 133 sera from asymptomatic ultrasound diagnosed CE cases were screened by ELISA using purified human hydatid cyst fluid antigen B for total specific IgG, IgG1 and IgG4 antibodies. Fifty sera from healthy individuals from within endemic regions were used as control negatives. Compared to control negatives total IgG antibody levels were elevated in both surgically proven (85%) and asymptomatic CE cases (77%) but there was no significant difference between the two groups. IgG1 subclass antibody levels were also elevated in both surgically proven (55%) and asymptomatic cases (58%) compared to endemic controls and similarly the difference in this response was not significant between these two groups. In contrast the prevalence of IgG4 antibodies in surgically confirmed chronic CE patients was greater (71%) than the respective IgG1 antibody levels (55%). The greatest difference in specific antibody response, between advanced surgically confirmed CE patients and ultrasound detected asymptomatic CE cases, was observed for IgG4 antibody levels which were detected in 71% of symptomatic compared to only 23% of asymptomatic patients (P<0.0001). These observations confirm that IgG4 is an important diagnostic parameter for clinically expressed cystic echinococcosis in humans and suggest that a switch from a predominant IgG1 response to IgG4 might occur in CE patients as the disease progresses.

Introduction

Human cystic echinococcosis (CE) is caused by infection with the larval stage (hydatid) of Echinococcus granulosus. Humans acquire infection by accidental ingestion of Echinococcus eggs voided in the faeces of infected dogs. Human CE is one of the most important and widespread of the parasitic helminthic zoonoses (Schantz, 1991). Hydatid disease in humans usually manifests as space-occupying, discrete, fluid filled unilocular cyst(s) commonly in the liver and or lungs. Diagnosis relies predominantly on imaging techniques, i.e. computerized tomography scan, magnetic resonance imaging, ultrasound, and/or X-ray, however immunodiagnosis is also important and may provide specific confirmation of CE both in clinical and community screening studies (Craig, 1993). The production of IgG antibodies in CE may also depend on the number, size, location and condition of hydatid cysts, and only approximately 60–80% of confirmed CE patients are seropositive for antibodies (Schantz and Gottstein, 1986). Despite a large number of studies on antibody response to Echinococcus antigens (mainly derived from hydatid cyst fluid) in patients with CE, virtually nothing is known about the early antibody and cellular response in infected humans (Craig, 1993).

The natural history of human CE has in general only been characterised in the late symptomatic stages when significant pathology has already occurred (Pawlowski, 1993). Animal models of CE are usually confined to protoscolex passage in rodents and not oral egg infection, although a small number of studies have sought to measure serum antibody response in experimental oral egg derived infection in mice (Dempster et al., 1991), sheep (Yong et al., 1984) or non-human primates (Rogan et al., 1993). IgG antibodies appeared in serum within 4–12 weeks after oral infection of vervet monkeys however subclass responses were not measured in the study of Rogan et al. (1993).

Analysis of IgG subclass (G1, G2, G3 and G4) antibody responses in other parasitic helminth infections has shown increase in IgG4 levels, especially during the chronic phase of infection in schistosomiasis (Boctor and Peter, 1990) and filariasis (Kurniawan et al., 1993), and also in hospitalised neurocysticercosis patients (Short et al., 1990). Recent studies of IgG subclass antibody response in advanced human CE indicated a significant increase in both IgG1 and IgG4 (Aceti et al., 1993; Wen and Craig, 1994). Furthermore albendazole treated CE patients who exhibited a good therapeutic and clinical response to treatment had significantly lower levels of serum IgG4 antibodies, than poor or non-responders; while IgG1 antibody levels showed a reverse trend (Rigano et al., 1995).

In screening for human CE in endemic communities, studies using sensitive immunodiagnostic tests like ELISA or IHA have often resulted in a lower rate of test sensitivity than expected (based on previous standardisation of the test) when compared to imaging methods like ultrasound and X-ray (Craig, 1986; Nahmias et al., 1992). Our hypothesis is that CE patients detected in community based screening programmes, are in general terms a different clinical group to those CE patients who form part of hospital case studies, and therefore may have a different serologic profile for IgG subclass antibodies, especially IgG1 and IgG4 compared to hospitalized cases. These two broad groups should represent the asymptomatic and symptomatic ends (respectively) of the CE disease spectrum and therefore might reflect different serum antibody responses both quantitatively and qualitatively. The current study measured Echinococcus specific total IgG, IgG1 and IgG4 antibody responses in a large group of endemic community derived-asymptomatic CE patients and compared this to a group of hospital derived chronic and symptomatic CE cases.