Antigen-specific suppressor T lymphocytes in human lymphatic filariasis

Insights into the pathogenesis of disease in human lymphatic filariasis

Although two thirds of the 120 million people infected with lymph-dwelling filarial parasites have subclinical infections, ∼40 million have lymphedema and/or other pathologic manifestations including hydroceles (and other forms of urogenital disease), episodic adenolymphangitis, lymphedema, and (in its most severe form) elephantiasis. Adult filarial worms reside in the lymphatics and lymph nodes and induce lymphatic dilatation. Progressive lymphatic damage and pathology results primarily from the host inflammatory response to the parasites but also perhaps from the host inflammatory response to the parasite's Wolbachia endosymbiont and as a consequence of superimposed bacterial or fungal infections. This review will attempt to shed light on disease pathogenesis in lymphatic filariasis.

Responses ofMastomys coucha, that have been infected withBrugia malayiand treated with diethylcarbamazine or albendazole, to re-exposure to infection

The responses of Mastomys coucha to re-exposure to infection with homologous infective larvae (L(3)) of Brugia malayi were investigated, after initial infections with the nematode had been treated subcutaneously for 5 days with diethylcarbamazine (DEC; 150 mg citrate/kg. day) or albendazole (ALB; 50 mg/kg. day). The parasite burdens, serum concentrations of IgG reacting with a soluble somatic extract of adult B. malayi (BmAS), and cytokine and lymphocyte-proliferative responses to filarial antigen (BmAS) or mitogen (concanavilin A or lipopolysaccharide) were studied. The results demonstrated, for the first time, that re-infection with L(3) was only successful in the DEC-treated animals, not the ALB-treated ones. When the ALB-treated animals were re-exposed, interferon-gamma production decreased, lymphocyte-proliferative responses either remained the same (with concanavilin A) or decreased (with BmAS), and concentrations of specific IgG decreased. When the DEC-treated animals were re-exposed, microfilaraemias re-appeared and, although production of interferon-gamma decreased, there were no detectable lymphocyte proliferative responses, and concentrations of specific IgG remained unchanged. Taken together, the results indicate that, at least in the M. coucha model of human filariasis, ALB but not DEC treatment may help to prevent the development of re-infections.

Immunopathogenesis of lymphatic filarial disease

Although two thirds of the 120 million people infected with lymph-dwelling filarial parasites have subclinical infections, ~40 million have lymphedema and/or other pathologic manifestations including hydroceles (and other forms of urogenital disease), episodic adenolymphangitis, tropical pulmonary eosinophilia, lymphedema, and (in its most severe form) elephantiasis. Adult filarial worms reside in the lymphatics and lymph nodes and induce changes that result in dilatation of lymphatics and thickening of the lymphatic vessel walls. Progressive lymphatic damage and pathology results from the summation of the effect of tissue alterations induced by both living and nonliving adult parasites, the host inflammatory response to the parasites and their secreted antigens, the host inflammatory response to the endosymbiont Wolbachia, and those seen as a consequence of secondary bacterial or fungal infections. Inflammatory damage induced by filarial parasites appears to be multifactorial, with endogenous parasite products, Wolbachia, and host immunity all playing important roles. This review will initially examine the prototypical immune responses engendered by the parasite and delineate the regulatory mechanisms elicited to prevent immune-mediated pathology. This will be followed by a discussion of the proposed mechanisms underlying pathogenesis, with the central theme being that pathogenesis is a two-step process-the first initiated by the parasite and host innate immune system and the second propagated mainly by the host's adaptive immune system and by other factors (including secondary infections).

Helminth infections and host immune regulation

Helminth parasites infect almost one-third of the world's population, primarily in tropical regions. However, regions where helminth parasites are endemic record much lower prevalences of allergies and autoimmune diseases, suggesting that parasites may protect against immunopathological syndromes. Most helminth diseases are spectral in nature, with a large proportion of relatively asymptomatic cases and a subset of patients who develop severe pathologies. The maintenance of the asymptomatic state is now recognized as reflecting an immunoregulatory environment, which may be promoted by parasites, and involves multiple levels of host regulatory cells and cytokines; a breakdown of this regulation is observed in pathological disease. Currently, there is much interest in whether helminth-associated immune regulation may ameliorate allergy and autoimmunity, with investigations in both laboratory models and human trials. Understanding and exploiting the interactions between these parasites and the host regulatory network are therefore likely to highlight new strategies to control both infectious and immunological diseases.

Regulatory T cells in infection

Infectious agents have intimately co-evolved with the host immune system, acquiring a portfolio of highly sophisticated mechanisms to modulate immunity. Among the common strategies developed by viruses, bacteria, protozoa, helminths, and fungi is the manipulation of the regulatory T cell network in order to favor pathogen survival and transmission. Treg activity also benefits the host in many circumstances by controlling immunopathogenic reactions to infection. Interestingly, some pathogens are able to directly induce the conversion of naive T cells into suppressive Foxp3-expressing Tregs, while others activate pre-existing natural Tregs, in both cases repressing pathogen-specific effector responses. However, Tregs can also act to promote immunity in certain settings, such as in initial stages of infection when effector cells must access the site of infection, and subsequently in ensuring generation of effector memory. Notably, there is little current information on whether infections selectively drive pathogen-specific Tregs, and if so whether these cells are also reactive to self-antigens. Further analysis of specificity, together with a clearer picture of the relative dynamics of Treg subsets over the course of disease, should lead to rational strategies for immune intervention to optimize immunity and eliminate infection.

Regulation of immune responses in lymphatic filariasis

The nature and intensity of immune reactions to filarial antigens appear to be controlled by two broad mechanisms: immunoregulation and immune tolerance. Parasite molecules of high molecular weight activate suppressor T lymphocytes; suppressive parasite products are present in sera from microfilaraemic patients. Prenatal or perinatal exposure to soluble parasite antigens may influence a person's future ability to react to filarial antigens.

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.

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.

Transmission intensity determines lymphocyte responsiveness and cytokine bias in human lymphatic filariasis

Humans living in areas where filariasis is endemic vary greatly in their exposure to mosquito-borne infective third-stage larvae (L3) of these parasitic helminths. Because the intensity of exposure to Ags affects T cell differentiation and susceptibility to parasitic infections in murine models, we compared T cell and cytokine responses in 97 residents of two villages in Papua New Guinea, where transmission intensity of Wuchereria bancrofti differed by 63-fold (37 vs 2355 L3 per person per year). Residents of the high transmission village had 4- to 11-fold lower proliferation and IFN-gamma responses to filarial Ags, nonparasite Ag, and PHA by PBMC compared with the low transmission village (p < 0.01) even when subjects were matched for intensity of infection. In contrast, filarial Ag-driven IL-5 production was 5.5-fold greater (p < 0.001), and plasma IL-4 and TGF-beta levels were 4-fold and 34% higher, respectively, in residents of the high transmission village. IL-4 and IL-10 responses by PBMC differed little according to village, and increased production of the counterregulatory cytokines IL-10 or TGF-beta by PBMC did not correlate with weak proliferation and IFN-gamma responses. Plasma IL-5, IFN-gamma, and IL-10 levels were similar in the two villages. These data demonstrate that the intensity of exposure to L3 affects lymphocyte responsiveness and cytokine bias possibly by a mechanism that alters APC function.

Cellular immune response of Mastomys and gerbils in experimental filariasis

OBJECTIVE

To determine mitogenic and antigen-specific cellular immune responses of two species of rodents, viz. Meriones unguiculatus and Mastomys coucha to assess the usefulness of the A. viteae/Mastomys model for cellular immune studies in experimental filariasis.

METHODS

Lymphocyte blast transformation test (LTT) using spleen cells of normal and A. viteae infected animals.

RESULTS

The proliferative response of gerbils was much higher than that of Mastomys to both ConA and filarial antigens. Cells of both species of rodents did not respond to microfilarial (mf) antigen, however, their mitogenic response differed during infection. Some degree of nonspecific suppression was observed in gerbils during prepatent and patent stages of infection, while Mastomys revealed highest proliferation during patent microfilaraemia. Mastomys cells did not respond to adult or mf antigen, while adult-specific proliferation was detected in the case of gerbils.

CONCLUSION

The A. viteae/gerbil model shows more similarity to human filarial infection regarding cellular immune response. Markedly low responsiveness of a high percentage of Mastomys and wide variations in the cellular response to nonspecific mitogen limit the usefulness of Mastomys coucha in immunological studies, especially cellular immunity.

Regulation of parasite antigen-driven immune responses by interleukin-10 (IL-10) and IL-12 in lymphatic filariasis

We investigated the mechanisms by which interleukin-10 (IL-10) regulates antigen-specific hyporesponsiveness in asymptomatic microfilaremic (MF) individuals. Peripheral blood mononuclear cells from MF individuals (n = 11) were stimulated in vitro with Brugia malayi antigen (BMA) or mycobacterial purified protein derivative (PPD) in the presence of neutralizing anti-IL-10 or isotype control monoclonal antibodies. As expected, BMA stimulated little or no gamma interferon (IFN-gamma) secretion in MF individuals, whereas PPD stimulated IFN-gamma in all but one. Neutralization of endogenous BMA-driven IL-10 secretion led to augmentation of IFN-gamma in seven of nine MF individuals (1.5- to 10-fold) and did so in a BMA-specific manner (PPD-driven IFN-gamma was augmented in only two of eight MF individuals and only 1.5- to 2-fold), indicating that IL-10 downregulates type 1 responses in these individuals. Type 2 responses (IL-5 secretion) were unaffected by the IL-10 blockade. To assess whether IL-12 could reverse the type 1 downregulation observed, the effect of recombinant human IL-12 (rhIL-12) on BMA-driven IL-5 and IFN-gamma production was also evaluated. rhIL-12 augmented both BMA- and PPD-driven IFN-gamma production 5- to 10-fold in six of nine MF individuals. These data demonstrate that IL-10 downregulates BMA-driven type 1 responses and that IL-12 can overcome downregulation of Th1 responses associated with MF but does so in a non-antigen-specific manner.

Engineered human vaccines

The limitations of human vaccines in use at present and the design requirements for a new generation of human vaccines are discussed. The progress in engineering of human vaccines for bacteria, viruses, parasites, and cancer is reviewed, and the data from human studies with the engineered vaccines are discussed, especially for cancer and AIDS vaccines. The final section of the review deals with the possible future developments in the field of engineered human vaccines and the requirement for effective new human adjuvants.

Cytokine control of parasite-specific anergy in human lymphatic filariasis. Preferential induction of a regulatory T helper type 2 lymphocyte subset

The immunological mechanisms involved in maintenance of an asymptomatic microfilaremic state (MF) in patients with lymphatic filariasis remain undefined. MF patients have impaired filarial antigen (Ag)-specific lymphocyte proliferation and decreased frequencies (Fo) of Ag-specific T cells, and yet elevated serum IgE and antifilarial IgG4. To investigate the mechanism of Ag-specific anergy in MF patients in contrast to amicrofilaremic individuals with chronic lymphatic obstruction (CP), the Fo of Ag-specific lymphocytes from peripheral blood mononuclear cells secreting either IL-4 or IFN-gamma were assessed by filter spot enzyme-linked immunosorbent assay, and IL-10 and transforming growth factor-beta (TGF-beta) mRNA transcript levels were assessed by a semiquantitative reverse transcriptase polymerase chain reaction technique. The Fo of filaria-specific IL-4-secreting lymphocytes were equivalent in both MF (geometric mean [GM] = 1:11,700) and CP (GM = 1:29,300 P = 0.08), whereas the Fo of IFN-gamma-secreting lymphocytes were lower in MF (GM = 1:39,300) than in CP (GM = 1:4,200, P < 0.01). When the ratio of IL-4/IFN-gamma (T helper type 2 [Th2]/Th1)-secreting cells was examined, MF subjects showed a predominant Th2 response (8:1) compared with a Th1 response in CP individuals (1:4). mRNA transcript levels of IL-10 were also significantly elevated in MF compared with CP individuals (P < 0.01). Further, IL-10 and TGF-beta were shown to have a role in modulating the Ag-specific anergy among MF subjects, in that neutralizing anti-IL-10 or anti-TGF-beta significantly enhanced lymphocyte proliferation response (by 220-1,300%) to filarial Ags in MF individuals. These findings demonstrate that MF subjects respond to parasite antigen by producing a set of suppressive cytokines that may facilitate persistence of the parasite within humans while producing little clinical disease.

Experimental onchocerciasis in chimpanzees: cellular responses and antigen recognition after immunization and challenge with Onchocerca volvulus infective third-stage larvae

Immunization of chimpanzees with radiation-attenuated infective 3rd-stage larvae (L3) of Onchocerca volvulus did not induce strong protective immunity against a subsequent challenge infection; only 1 out of 4 immunized animals remained non-patent (i.e. microfilariae-negative) after challenge, and may have been protected. However, during immunization and before challenge, a broad range of adult O. volvulus-derived antigens (OvAg) and also uterus-derived OvAg were recognized by circulating antibodies; moreover, the repertory of antigens recognized increased further in subsequently patent animals after challenge, particularly in the range of M(r) 12-42 kDa. In the immunized and non-patent chimpanzee, by contrast, serological recognition of uterus-derived OvAg with M(r) 14 kDa and 105 kDa disappeared by 19 months post-challenge (p.c.). During immunization, Acanthocheilonema viteae L3 antigens of M(r) 11-12 kDa were strongly recognized only by the non-patent animal, suggesting that recognition of these antigens may have supported resistance to the subsequent challenge infection. In immunized chimpanzees, a substantial increase in the cellular reactivity to OvAg was induced; this, however, declined by 19 months p.c. to levels similar to those seen prior to immunization. At that time, 3 out of 4 immunized animals were patently infected. The effect of exogenous cytokines on in vitro-reactivity of PBMC to OvAg was examined. Addition of exogenous IL-2 alone, IFN-gamma alone, and IFN-gamma in combination with IL-2, did not augment net cellular responses to OvAg by PBMC from infected and control chimpanzees. In the presence of IL-4 alone, IL-6 alone, IL-2 with IL-4, IL-2 with IL-4 and IFN-gamma, or IL-2 with IL-4 and IL-6, the net cellular reactivity to OvAg increased significantly in patent chimpanzees and reached levels similar to non-patent animals. Thus, non-patent chimpanzees maintain high cellular reactivity to OvAg and in vitro cellular unresponsiveness to OvAg on the part of patent chimpanzees is reversible after addition of several cytokines which act individually or synergistically.

Cellular immune responsiveness in rabbits with Setaria digitata filarial antigen and TDM adjuvant

The purified surface antigens of the bovine filarial parasite Setaria digitata were used as an antigen to immunize rabbits. The aqueous suspensions of trehalose 6-6' dimycolate (TDM) has been successfully used as an effective immunomodulator in experimental studies on filariasis. The effectiveness of such an antigen-TDM combination was demonstrated by enhanced humoral and cellular immunity. Administration of antigen alone shows only humoral immunity. The detectable cellular immune responses further confirm the effect of filarial antigen-TDM combination. The cell-mediated immunity was expressed in vivo by delayed skin reaction and in vitro by leukocyte and macrophage migration inhibition tests.

Induction of murine T-helper-cell responses to the filarial nematode Brugia malayi

The purpose of this study was to examine the murine T-helper-cell (Th) cytokine response to the human filarial parasite Brugia malayi. In the first 14 days following intraperitoneal inoculation of live microfilariae into BALB/c mice, filarial antigen-driven splenic lymphoid cells produced gamma interferon (IFN-gamma) and little or no interleukin-5 (IL-5). After this time, IL-5 production increased (to 10 to 12 ng per 5 x 10(6) cells) coincident with a marked diminution in IFN-gamma generation. A single subcutaneous immunization with soluble microfilarial antigens also induced an IFN-gamma but no IL-5 response, whereas immunization three times elicited a predominant Th2-like reaction characterized by IL-4 and IL-5 production by CD4+ lymph node lymphocytes and a 10-fold increase in serum immunoglobulin E. The importance of IL-10 in establishing the balance between parasite-specific Th1 and Th2 responses was demonstrated by the ability of neutralizing monoclonal antibody to this cytokine to increase IFN-gamma production by splenic and lymph node cells from mice chronically exposed to live microfilariae or immunized multiple times with soluble filarial antigens.

Ivermectin-facilitated immunity in onchocerciasis. Reversal of lymphocytopenia, cellular anergy and deficient cytokine production after single treatment

A longitudinal investigation has been conducted into the cell-mediated immune responses of onchocerciasis patients after a single-dose treatment with ivermectin. Untreated patients tested for delayed cutaneous hypersensitivity (DCH) to seven recall antigens showed lower responses than infection-free control individuals (P less than 0.01), but 6 and 14 months after treatment DCH reactions increased to similar levels to those seen in the controls. The in vitro cellular reactivity to Onchocerca volvulus-derived antigen (OvAg) was reduced in untreated patients as compared with controls, and the lymphocyte blastogenic responses to OvAg and streptolysin-O clearly improved up to 14 months after treatment. Peripheral blood mononuclear cells (PBMC) from untreated patients produced IL-1 beta, tumour necrosis factor-alpha (TNF-alpha) and IL-6 in response to mitogenic stimulation with phytohaemagglutinin (PHA), only low levels of IL-1 beta, IL-2 and TNF-alpha in response to OvAg, but higher amounts of IL-4 and interferon-gamma (IFN-gamma) in response to OvAg than control individuals. After ivermectin treatment, the OvAg-induced production of IL-1 beta and TNF-alpha increased significantly 1 and 14 months after treatment. The PHA-induced production of IL-2 and IL-4 increased 1 month after treatment and remained significantly elevated until 14 months after treatment, whereas the OvAg-specific secretion of IL-2, IL-4 and IFN-gamma did not change after ivermectin treatment. Flow cytometric analysis of lymphocyte-subsets in the peripheral blood of untreated patients revealed a relative and absolute (P less than 0.01) diminution of CD4+ cells and a significantly smaller CD4+/CD8+ cell ratio as compared with controls. By 4 weeks after treatment and thereafter, CD4+ T cells increased relatively and absolutely (P less than 0.01); likewise there was an absolute increase in T-helper-inducer cells (CD4+CD45RO+) and a temporarily improved CD4+/CD8+ cell ratio (P = 0.001). The expression of the low-affinity receptor for IgE (CD23) on total lymphocytes decreased from 14% to 7% by 14 months after treatment. The CD8+ cells and CD3+TCR gamma delta + cells were higher in patients than in controls and both remained elevated until 14 months after treatment. These results suggest a distinctly improved cellular immunity in human onchocerciasis that was facilitated by ivermectin therapy.

Induction of protective immunity to Brugia pahangi in jirds by drug-abbreviated infection

Protective immunity of homologous challenge infection was examined in jirds after drug-abbreviated infection with Brugia pahangi. Mebendazole (MBZ) treatment at the early prepatent (5-7 weeks of post infection) or the late prepatent (7-9 weeks of post infection) period was highly effective in causing almost complete eradication of the primary infection. After challenge infection, the worm burden was significantly reduced 19% (31.1 in average) and 77% (9.5) to that of the controls (38.8 and 41.7), respectively. The magnitude of eosinophil response paralleled the degree of protection. No or only a few microfilariae were seen after challenge infection in jirds treated during the prepatent periods. They were also resistant to intravenous challenge with the microfilariae of B. pahangi. MBZ treatment at the patent period was, on the contrary, incomplete against primarily infected adult worms, and was not able to induce either significant protection (30.1 vs 33.1 in control) or eosinophil response to the challenge infection.

Immunologic tolerance in lymphatic filariasis. Diminished parasite-specific T and B lymphocyte precursor frequency in the microfilaremic state

To explore the mechanisms of antigen-specific immune unresponsiveness seen in microfilaremic patients with bancroftian filariasis, T and B cell precursor frequency analysis was performed using PBMC from individuals with either asymptomatic microfilaremia (MF, n = 7) or chronic lymphatic obstruction (CP, n = 20). Highly purified CD3+ cells were partially reconstituted with adherent cells and their proliferative response to parasite antigens determined in cultures of T cells by limiting dilution analysis. A filter immunoplaque assay also assessed the frequency of both total and parasite-specific Ig-producing B cells. While the lymphocyte proliferation to mitogens and to a nonparasite antigen (Streptolysin-O, [SLO]) were similar in all groups of patients, the frequency of parasite-specific CD3+ T cells was significantly lower (geometric mean [GM], 1/3,757) in MF patients when compared to that in CP patients (GM 1/1,513; P less than 0.001). Similarly, the proportion of lymphocytes producing parasite-specific IgE or IgG was significantly lower in MF patients (IgE mean, 0.2%; IgG mean, 0.33%) compared with CP patients (IgE mean, 3.2%; IgG mean, 1.76%; P less than 0.05 for both comparisons). These observations imply that low numbers of parasite-specific T and B lymphocytes may be partially responsible for the severely diminished capacity of lymphocytes from patients with MF to produce parasite-specific antibody and to proliferate to parasite antigen in vitro. Such differences in parasite-specific lymphocyte responses suggest that tolerance by clonal anergy may be a critical mechanism for maintaining the microfilaremic state.

Suppression of human lymphocyte responses to specific and non-specific stimuli in human onchocerciasis

Characterization of in vitro lymphocyte responsiveness was performed on selected groups of onchocerciasis patients from Sudan and Sierra Leone. These patients manifested a very broad range of clinical signs and showed widely divergent parasite infection intensities. Lymphocyte proliferative responses to soluble Onchocerca volvulus antigen (sAg) were poor in infected persons; mitogen and PPD responses were maintained in the normal range in one group of patients from southwestern Sudan, but were profoundly depressed in a group from N.E. Sudan. Proliferative responses and interferon-gamma (INF-gamma) secretion were very significantly depressed in the presence of live microfilariae of O. volvulus or secretions/excretions (S/E) from microfilariae (mf) or from female, but not male, adult parasites. Lymphocyte responses were maintained near normal when exogenous IL-2 was added to these cultures. The results indicate that O. volvulus infection and its clinical consequences are not consistently associated with systemic deficits in immune responsiveness. However, suppression of lymphocyte reactivity by mf and S/E in vitro suggests that direct parasite intervention in host cell responses could be taking place in vivo, perhaps at the local microenvironment level; mediated by effects on cytokine production.

Mechanisms of immunosuppression

Immunosuppresive disease is a major economic concern in domestic poultry production. Although many immunosuppressive agents have been described, mechanisms of how infectious and noninfectious agents compromise the immune system are poorly understood in avian species. Two categories, generalized and antigen-specific immunosuppression have been described in mammals. Generalized immunosuppression produces overall reduced responsiveness and increased susceptibility to a wide variety of infectious and neoplastic diseases. The best characterized immunosuppressive mechanisms are described in HIV-1 infections that lead to acquired immune deficiency syndrome (AIDS) in humans. In contrast, the antigen-specific suppression observed in human leprosy illustrates how an infecting agent selectively suppresses host responses against itself favoring bacterial spread. Both diseases have well-defined clinical staging classifications that correlate with specific immunological defects. An approach to studying immunosuppressive mechanisms in the avian suggests the need for relating pathogenesis with tests of immune responsiveness using a series of increasingly more specific immunological assays to pinpoint defects.

Regulation of the immune response in lymphatic filariasis and onchocerciasis

The persistence of microfilariae in the blood or skin accompanied by a prominent eosinophilia and elevated serum IgE levels are common features of human infection with filarial parasites. In this review Christopher King and Thomas Nutman discuss recent findings on the role of interleukin 4 (IL-4), IL-5 and gamma-interferon (IFN-gamma) in the induction of these immediate hypersensitivity responses. They discuss the role of hypersensitivity in immunity, the development of immune tolerance to filarial antigens and suggest that could explain the impaired immune response of some individuals to filarial infections and the persistence of the microfilaremic state.

Molecular parasitology: progress towards the development of vaccines for malaria, filariasis, and schistosomiasis

Advances in molecular biology have allowed for the identification of potential vaccine candidates against several parasitic diseases. Antigens from various life stages of Plasmodium and Schistosoma species and filarial worms have been cloned, sequenced and tested as vaccines. Results to date in animal models have been promising. Modest levels of protection against experimental human malaria have been obtained using both sporozoite and blood-stage antigens. However, a greater understanding of the mechanisms which lead to immunity against parasites is required before effective vaccines can be developed.

Diagnostic use of polyclonal antibodies raised in mouse ascitic fluid in bancroftian filariasis

Polyclonal antibodies were produced against Brugia malayi adult antigens (BmA (PBS) SAg and BmA (SDS) SAg) in mouse ascitic fluid by immunising Balb/c mice intraperitoneally with high ratio of adjuvant to immunogen. The diagnostic use of these antibodies in detecting circulating filarial antigen in bancroftian filariasis was studied by sandwich enzyme-linked immunosorbent assay (sandwich ELISA) using stick assay system. Both antibodies raised against PBS and SDS soluble antigens were found to be equally sensitive and relatively specific in detection of circulating filarial antigen. When anti BmA (PBS) SAg antibody was used in sandwich ELISA, 90% of microfilaraemic sera, 30-40% of acute and sub acute filarial sera, 20% of chronic filarial sera, 7% of endemic normal sera and none of 15 non-endemic normal sera were positive for filarial antigen. Using anti BmA (SDS) Sag antibody, 93% of microfilarial sera, 40% of acute and sub acute filarial sera, 20% of chronic filarial sera and none of 15 endemic and non-endemic normal sera showed the presence of filarial antigen. The filarial antigen detection using anti BmA S Ag antibodies produced in mouse ascitic fluid in sandwich ELISA may be useful in detection of active stage (microfilaraemia) of infection.

Suppression of T-lymphocyte responses to Entamoeba histolytica antigen by immune sera

Lymphocytes from patients cured of amebic liver abscesses proliferate and produce gamma interferon upon incubation with soluble Entamoeba histolytica antigen: however, amebic liver abscesses exhibit a relentless progression without treatment. To determine whether suppressive factors are present in sera, we studied T-lymphocyte responses to total soluble E. histolytica antigen by using cells from five patients treated for amebic liver abscesses in the presence of 15 different immune sera and 10 control sera. In the presence of immune sera, E. histolytica antigen-induced lymphocyte proliferation decreased by 63% and production of gamma interferon was reduced by 93.2% (P less than 0.01). Immune sera had no effect on the mitogenic responses of patient lymphocytes to phytohemagglutinin or on the proliferative responses of control lymphocytes to phytohemagglutinin or tetanus toxoid. The suppressive activity of immune sera diminished as the time between therapy for amebic liver abscesses and serum collection increased (P less than 0.05). Suppressive activity did not correlate with the titers of serum anti-amebic antibody and was not affected when serum was absorbed with viable amebic trophozoites. In conclusion, soluble factors present in the sera of amebic liver abscess patients suppressed in vitro lymphocyte responses to E. histolytica antigen and may have contributed to the lack of development of effective in vivo cell-mediated immune responses following the onset of amebic liver abscesses.

Non-specific immune suppression by CD8+ T cells in Brugia pahangi-infected rats

Non-specific suppression of the immune response was investigated in Brugia pahangi-infected Lewis rats. The proliferative response of peripheral blood lymphocytes or splenic non-adherent cells to mitogens was significantly reduced by B. pahangi infection. The degree of hyporesponsiveness of splenic non-adherent cells to mitogens was comparable between microfilaremic and non-microfilaremic animals. The suppressed proliferative response of splenic non-adherent cells was restored by blocking with anti-CD8 monoclonal antibody. After separation of T cells into CD4+ and CD8+ subpopulations, only CD8+ T cells from B. pahangi-infected rats suppressed the proliferative response of normal spleen cells to concanavalin A. CD8+ T cells from normal rats had no suppressive effect. On the other hand, the proliferative response of CD4+ T cells to concanavalin A was comparable between normal and infected rats. These results suggest that CD8+ T cells participate in the non-specific suppression of immune response in experimental filariasis.

Differential recognition of microfilarial antigens by sera from immigrants into an area endemic for brugian filariasis

Studies in animal models indicate that antibodies to surface antigens of microfilariae participate in the control of parasitaemia resulting from infections with lymphatic filarial nematodes. In an attempt to identify parasite antigens that elicit such 'protective' host responses, we compared the antigen recognition patterns of persons who remained amicrofilaraemic after 3-6 years of exposure to Brugia malayi with those of individuals who developed patent filariasis during the same period. IgG antibodies in sera from immigrants identified between 0 and 25 microfilarial antigens on Western blots. The highest degree of reactivity was observed with antigens in the 65-75 kD and 20-30 kD ranges, and with a group of high mol. wt antigens (greater than 180 kD). Sera from amicrofilaraemic donors preferentially reacted with 70/75 kD microfilarial antigens. A proportion of such sera inhibited the binding of monoclonal antibody MF1 to its target epitope; eight of nine inhibitory sera were from patients with active infections, evidenced by the presence of microfilariae or filarial antigens in the donors' blood, but who were amicrofilaraemic. These results indicate that some amicrofilaraemic residents of areas where brugian filariasis is endemic develop immune reactions to a microfilarial stage-specific antigen that was previously identified as a potentially 'protective' parasite antigen in animal models of lymphatic filariasis.

Microfilaraemia, serum antibody and development of clinical disease in microfilaraemic subjects infected with Wuchereria bancrofti and treated with diethylcarbamazine citrate

A seroepidemiological survey of bancroftian filariasis was carried out in 2 townships in Sri Lanka with the objectives of determining the microfilaraemia rates, dependence on age and sex, susceptibility to re-infection, effect of diethylcarbamazine therapy on serum antibodies to microfilarial surface antigens, and the predictive value of the indirect fluorescent antibody test. The mean microfilaraemia rate was 5.4%. Microfilaraemia was not sex-dependent but a marginally elevated incidence was seen in the 6-35 year age groups. In up to 58% of the microfilaraemic patients who had been treated for microfilaraemia previously, a second phase of microfilaraemia was seen 2-7 years after treatment. This was unlikely to have been due to incomplete parasite elimination. Antibodies to microfilarial surface were found in 24-35% of microfilaraemic patients and in 14-63% of amicrofilaraemic symptomatic subjects. Serum anti-microfilarial surface antibody levels did not alter with chemotherapy with diethylcarbamazine citrate. The findings of follow-up investigations of microfilaraemic subjects were compatible with the notion that microfilaraemia does not necessarily lead to clinical disease.

Clinical and laboratory aspects of filariasis

Human filarial infections afflict over 150 million persons worldwide and are major causes of morbidity in many developing countries. Onchocerca volvulus infection is a leading preventable cause of blindness, while bancroftian and brugian filariasis may produce lymphatic obstruction of the genitalia and extremities (elephantiasis). Definitive diagnosis of these helminthic infections currently depends on demonstration of microfilariae in host tissues, i.e., the skin in the case of O. volvulus and the bloodstream in the cases of Wuchereria bancrofti and Brugia malayi. Many investigations are now directed at developing specific and sensitive serum antigen assays that will allow diagnosis of active infection (i.e., presence of adult-stage parasites) in the absence of detectable microfilariae. With respect to the immunology of these parasitic infections, efforts are being directed at elucidating the role of T- and B-cell responses in the development of pathologic lesions and resistance to reinfection. These data as well as molecular biologic approaches to identify and study filarial molecules which are immunogenic are discussed. Finally, since treatment of filariases at present depends on antiparasitic drugs, the clinical indications and dosages of diethylcarbamazine and ivermectin are summarized.

Differential regulation of in vitro humoral and cellular immune responsiveness in Brugia pahangi-infected jirds

Patent filarial infection is associated with the downregulation of parasite-specific immune reactivity. In the present study, the relationship between in vitro parasite antigen-specific humoral and cellular immune responsiveness was investigated in Brugia pahangi-infected jirds and in jirds immunized with soluble antigens. Spleen cells from B. pahangi-immunized jirds or from jirds with prepatent infections mounted significant in vitro proliferative and antibody responses to B. pahangi. The antigen concentration which elicited optimal antibody production was 10- to 10,000-fold lower than that required to stimulate optimal blastogenesis. Lymph node cells from both immunized and infected jirds consistently produced lower levels of parasite-specific antibody than spleen cells, yet generated higher proliferative responses to filarial antigen. A dissociation between in vitro antibody production and proliferation was also observed in experiments with spleen cells from microfilaremic jirds; spleen cells from patent animals did not proliferate when stimulated with B. pahangi antigen, but did produce significant levels of parasite-specific antibody. Depletion of adherent or histamine receptor-bearing cells restored the proliferative reactivity of spleen cells from microfilaremic jirds, but had limited effects on antibody production. In admixture experiments, spleen cells from microfilaremic animals suppressed the proliferative responsiveness of cells from keyhole limpet hemocyanin (KLH)-immunized jirds to KLH by 38%, but had no effect on KLH-specific antibody production. The present results support the hypothesis that parasite-specific cellular and humoral reactivity are differentially regulated in experimental filariasis.

The effect of diethylcarbamazine treatment of Bancroftian filariasis on the immunological reactivity of microfilaraemic individuals

Patent filarial infection has been correlated with a profound suppression of humoral and cellular responses to filarial antigens. In the present study, the filarial antigen-specific humoral and cellular reactivity of 30 Haitian subjects with patent Wuchereria bancrofti infection was monitored before and after treatment with diethylcarbamazine. Microfilarial density was reduced from a pre-treatment mean of 1778/ml to 9/ml, with residual microfilaraemias detectable in 10 subjects. Peripheral blood mononuclear cells from 18 of the 30 patients responded to an extract of Brugia pahangi before treatment, and this number increased to 25 after treatment. There was no significant change in the mean level of response to B. pahangi in patients who were responsive to filarial antigen before treatment; however, the mean responsiveness to B. pahangi of individuals who were classified as nonresponders before treatment was significantly increased following treatment. Cellular reactivity to purified protein derivative and geometric mean titres to soluble B. pahangi, measured by enzyme-linked immunosorbent assay, were unaffected by treatment. Similarly, most post-treatment sera did not recognize new B. pahangi bands on Western blots, compared to pre-treatment controls. These observations imply that the relationship between microfilariae and immunosuppression is complex.

Tolerance in rats by transplacental transfer of Dipetalonema viteae microfilariae: recognition of putative tolerogen(s) by antibodies that inhibit antigen-specific lymphocyte proliferation

We have previously reported (Nature 1982. 299:361) that the transplacental transfer of Dipetalonema viteae microfilariae (mf) can induce an antigen-specific tolerance in rats. Rats thus tolerized have serum factor(s) which block(s) antigen-specific lymphocyte proliferation. The results of experiments involving fractionation of antisera from tolerant animals indicate that the inhibitory activity for antigen-specific blastogenesis resides in IgG antibodies. Absorption of IgG (eluted from protein A) with specific filarial antigens reduced the inhibition from 58% to 9% whereas a similar immunosorption of IgG size fraction (obtained by applying to AcA 34 Ultrogel) resulted in a decrease from 72% to 35%. This suggests that IgG size fraction might include factor(s) derived from mf and was partially blocking the blastogenic response. Since the tolerant animals harbor only mf, we have used radiolabeled mf surface antigens for immunoprecipitation by antisera from tolerant animals. Antibodies from tolerant animals have a different specificity for filarial antigens compared to those from immunocompetent and mf-resistant rats.

Brugia pahangi: granulomatous lesion development in jirds following single and multiple infections

The development of adult worm burdens and microfilaremias were determined in jirds which received 2, 3, or 4 subcutaneous inoculations of 50 Brugia pahangi infective larvae. Parasite burdens in multiply inoculated jirds were compared to those in four different groups of jirds which received single inoculations of 50 infective larvae. One of each of these singly inoculated groups was infected on the same day that one of the inoculations was given to the multiply infected jirds. Thus, the duration of the infections in the four groups of jirds receiving one inoculation was 54, 118, 189, and 254 days. The development of lymphatic lesions and granulomatous hypersensitivity to B. pahangi antigen was assessed in all jirds at necropsy. The percentage recoveries of adult worms and their locations did not differ in the singly inoculated jirds with infections of different durations. A protective resistance to reinfection, as measured by adult worm recovery in multiply infected jirds, did not occur. The lymphatic lesion scores and numbers of intralymphatic thrombi was greatest in singly inoculated jirds examined 54 days after infection. Pulmonary granuloma areas around adult filarial antigen coated beads embolized in the lungs of jirds 3 days prior to necropsy were also greatest in singly inoculated jirds examined 54 days after infection. Using criteria of lesion scores and lymph thrombi numbers to assess lymphatic lesion severity, a decrease in lesion severity as well as pulmonary granuloma size around antigen coupled beads was seen by 118 days after infection in singly inoculated jirds.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure of immunosuppressive mechanisms in human Schistosoma mansoni infection with hepatosplenomegaly

The basis for development of hepatosplenic disease and attendant morbidity in Schistosoma mansoni-infected individuals is uncertain but may relate to defective modulation of immunopathology. Individuals 14 to 30 years of age from a village in the Nile Delta in Egypt were selected for study: 32 were infected with S. mansoni but lacked hepatosplenomegaly (mean fecal egg excretion +/- standard error of the mean, 1,142 +/- 79 eggs per g), 9 had S. mansoni infection and hepatosplenomegaly (1,267 +/- 197 eggs per g), and 12 were uninfected. The ratio of OKT4 helper/OKT8 suppressor cells in peripheral blood mononuclear cells was reduced in infected subjects without hepatosplenomegaly to 1.4 +/- 0.1 compared with a ratio of 1.7 +/- 0.1 (P less than 0.05) in uninfected subjects. In contrast, this ratio was increased in the group with hepatosplenomegaly to 2.7 +/- 0.3 (P less than 0.01). Schistosome antigen-induced [3H]thymidine incorporation in peripheral blood mononuclear cells was comparable in infected subjects without (5,837 +/- 1,009 cpm) and with (3,329 +/- 738 cpm; P greater than 0.1) hepatosplenomegaly. Depletion of adherent suppressor cells significantly increased the responses in the group lacking organomegaly (14,028 +/- 1,683 cpm; P less than 0.001) but not in the hepatosplenomegaly group (5,046 +/- 1,830 cpm; P greater than 0.5); this difference in response of nonadherent cells to soluble worm antigenic preparation was statistically significant (P less than 0.02) and not explained by quantitative shifts in OKT8 suppressor cells. Thus, in S. mansoni infection, subjects with hepatosplenomegaly are distinctive in their lack of an immunosuppressive balance of T-lymphocyte subpopulations and in the absence of functional adherent suppressor cells. Defective immunoregulatory mechanisms could be important in the genesis of hepatosplenic disease and its morbid sequelae.

Parasite-specific anergy in human filariasis. Insights after analysis of parasite antigen-driven lymphokine production

The antigen-specific immune unresponsiveness seen in bancroftian filariasis was studied by examining lymphokine production in peripheral blood mononuclear cells (PBMC) or PBMC subpopulations from 10 patients with asymptomatic microfilaremia, 13 patients with elephantiasis and 6 normal North Americans. In each group of patients, the kinetics of the lymphokine response and the response to mitogens and nonparasite antigens did not differ significantly. In marked contrast, when antigen-induced lymphokine production was examined, most patients with microfilaremia were unable to produce either interleukin 2 (IL-2) or gamma-interferon (i.e., were nonresponders), and the few who could (hyporesponders, generally with quite low microfilaremia levels) did so at levels significantly less than those of patients with elephantiasis, all of whom showed strong responses to parasite antigen. Removal of neither adherent cells or T8+ cells affected the parasite-specific anergy seen in those with microfilaremia, suggesting a state of T cell tolerance to the parasite in patients with this most common clinical manifestation of bancroftian filariasis.

Immunochemical characterization and biosynthesis of major antigens of iodo-bead surface-labeled Brugia malayi microfilariae

The objectives of the present study were to identify and characterize biochemically the major antigens of Brugia malayi microfilariae, a filarial parasite that infects humans. IgG antibodies in sera of mice which had cleared parasites from the bloodstream reacted with 30, 55, 94 and 150 kDa molecules of living microfilariae radioiodinated by the Iodo-bead method. Sera of humans infected with the related filariae Wuchereria bancrofti, Loa loa or Onchocerca volvulus immunoprecipitated molecules of similar size as well as two additional proteins of 22 and 43 kDa. Sera of uninfected North Americans or mice infected with Trichinella spiralis or Schistosoma mansoni did not recognize these radioiodinated antigens. Experiments to examine the possible surface localization and metabolism of these antigens showed that they were removed from intact parasites exposed to chymotrypsin or trypsin and that immunogenic molecules of 30, 55, and 150 kDa were released into excretory-secretory products by viable microfilariae. [35S]Methionine biosynthetically labeled polypeptide antigens of 22, 30, 35 and 150 kDa were detected by antibody reacted with intact microfilariae and/or their excretion products. Antigens of 30, 55, and 150 kDa appear to be glycoproteins as they bound wheat germ agglutinin and were biosynthetically labeled with [14C]N-acetyl-D-glucosamine. These data suggest that the surface of B. malayi microfilariae is a dynamic structure which synthesizes and sheds antigens.

Dipetalonema viteae infection in three species of rodents: species specific patterns of the antibody response

Groups of jirds (Meriones unguiculatus), multimammate rats (Mastomys natalensis) and golden hamsters (Mesocricetus auratus) were infected with third stage larvae (L3) of Dipetaloneam viteae and the course of infection was followed until 20 weeks post infection. Worm growth was best and microfilaraemia was high and long lasting in jirds and in multimammate rats, whereas golden hamsters were poor hosts as measured by these parameters. The IgG and IgM antibody responses of the species were compared by immunoblotting and ELISA using proteins of D. viteae, separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The levels of antibodies against most proteins of high molecular weight declined during infection in jirds and in multimammate rats, whereas an increase was observed in golden hamsters. In contrast, several antigens of low molecular weight induced increasing antibody levels in all species. Species specific antigen recognition was observed for a number of protein bands of L3, microfilariae and female worms. The data suggest that susceptibility to immunesuppression and the species specific pattern of antigen recognition might determine the qualities of a rodent species as host for D. viteae.

Dipetalonema viteae: resistance in Meriones unguiculatus with multiple infections of stage-3 larvae

The jird, Meriones unguiculatus, infected with 80 normal infective larvae of Dipetalonema viteae, revealed a recovery rate of 27.9% 12 weeks after infection. A pretreatment by three injections of 50 normal larvae each and challenge by 80 larvae resulted in a recovery rate of 10.7%. The recovered worms were longer than those from the challenge control animals. When three times 50 irradiated larvae (35 krad) were inoculated, the recovery rate of the challenge decreased to 2.6%, representing a protection of 90.7%. The surviving adult worms were stunted and derived exclusively from the 80 normal larvae given for challenge, since absolutely no adult worms were recovered in eight animals inoculated three times with 50 irradiated larvae only. Sera of all pretreated jirds contained IgG and IgM antibodies which bound in immunoblotting experiments bound predominantly to three proteins of larvae with molecular masses of 68,140, and 165 kDa, respectively. Enzymatic surface iodination revealed that the three antigens were exposed on the larval surface. The coincidence of a partial resistance to a challenge infection and of an antibody response against surface proteins of infective larvae suggests an importance of these antigens for the rejection of D. viteae mediated by an acquired immunological resistance of M. unguiculatus.

A study on antigen recognition by onchocerciasis patients with different clinical forms of disease

Sera from 40 onchocerciasis patients from the Yemen Arab Republic with either mild localized forms of onchocerciasis, intermediate or severe localized forms of the disease or generalized forms of infection were studied with respect to their IgG and IgM response against Onchocerca volvulus antigens. Immunoblotting, performed with SDS-PAGE-separated proteins of female O. volvulus and quantified by densitometric scanning, revealed IgG and IgM antibodies against worm components in sera of all patients. Persons with intermediate or severe localized forms of onchocerciasis had a stronger IgG response against more proteins than individuals of the other groups. However, some antigens (Mr 21, 23, 30, 33 kDa) induced comparable quantities of IgG in all groups. The IgM response of patients with mild localized forms of onchocerciasis was more intensive and directed against more antigens than in the other groups. No antigens were detected that were recognized only by individuals with low levels of microfilaridermia. In all groups, varying concentrations of antibodies against cuticle, muscle/hypodermis layer and/or uterus of female O. volvulus were detected by the indirect immunofluorescence test using frozen worm sections as antigen. The highest mean antibody titres were found in patients with intermediate and severe localized forms of disease.

Differential recognition of a protective filarial antigen by antibodies from humans with bancroftian filariasis

The objectives of this study were to identify filarial antigens which induce enhanced clearance of circulating microfilariae and to establish if human antibody reactivity with these molecules correlates with the apparent parasite burdens of residents of an endemic area of Bancroftian filariasis. Mice immunized with an extract of Brugia malayi microfilariae develop IgG antibodies to four major filarial antigens with an apparent molecular weight (Mr) of approximately 112,000, 60,000, 45,000, and 25,000. Animals immunized with gel slices containing the approximately 25,000-Mr antigen are resistant to intravenous challenge with live microfilariae (78-98% reduction in parasitemia vs. controls, P less than 0.01). A group of 22 amicrofilaremic humans had a significantly higher (P less than 0.025) mean antibody titer to the Mr 25,000-Mr antigen (1: 424) than 16 microfilaremic individuals (1:95). There were no significant differences between the two groups in antibody titers to filarial antigens of Mr approximately 112,000, 60,000, and 45,000 Mr. These data suggest that a high degree of reactivity to the 25,000-Mr antigen in humans with lymphatic filariasis correlates with a parasitologic status that is least conducive to transmission of infection.

Surface components of Onchocerca volvulus

A technique employing Sephadex G25 gel filtration has been developed for the rapid isolation and purification of live microfilariae of Onchocerca volvulus from subcutaneous nodules and skin samples. Microfilariae, adult worms and L3 larvae have been surface radiolabelled using the Iodogen technique. Two proteins have been characterised on the surface of uterine microfilariae: these have apparent molecular weights of 14,800 and 15,000. A MW 15,000 protein was the only molecule labelled on the surface of skin microfilariae. Ten proteins were labelled on adult male worms: these have molecular weights of 15,000, 17,500, 20,000, 22,000, 24,000, 29,000, 32,000, 37,000, 42,000, and 50,000. Some, if not all, of these proteins were also identified on female worms. Seven proteins were labelled on the surface of L3 larvae: these have molecular weights of 17,500, 48,000, 50,000, 52,000, 54,000, 57,000, and 105,000. Three of the adult surface proteins were precipitated by selected human infection serum: these are the MW 17,500, 32,000 and 42,000 molecules. The microfilarial surface proteins were not precipitated by human infection serum. The antiserum used in these experiments was shown by Western blot analysis to contain high levels of antibody with specificity for microfilarial and adult antigens. Indirect immunofluorescent assays showed these sera to contain antibody which bound to the surface of adult worms and eggs but not microfilariae. The possibility that skin microfilariae absorb host serum albumin was investigated: Western blot analysis and surface immunofluorescence assays using a specific anti-human albumin serum gave negative results. Fluorescent lectin binding studies revealed the presence of stage-specific carbohydrate moieties exposed on the surface of adult worms and eggs. Microfilariae do not have surface carbohydrate determinants.

Protective immunity to Ascaris suum: analysis of swine peripheral blood cell subsets using monoclonal antibodies and flow cytometry

Outbred domestic swine or SLA inbred miniature swine were exposed to Ascaris suum either naturally on contaminated lots or by inoculation with UV-irradiated attenuated eggs. Both inbred and outbred swine developed virtually complete protection to a challenge of 10 000 eggs after natural exposure, but inbred swine were less resistant than outbred swine after UV-egg exposure. Flow cytometric analysis of peripheral blood mononuclear cells from these animals, performed to determine changes in cell subsets including helper T-cells, cytotoxic/suppressor T-cells, macrophages, and cells expressing class II major histocompatibility antigens, showed that both outbred and inbred swine had similar responses after parasite exposure. The levels of helper T-cells and cytotoxic/suppressor T-cells did not change after parasite exposure, while there was an appreciable but transient increase in macrophages only in those swine naturally exposed to A. suum. Swine exposed to A. suum, both naturally and by inoculation with UV-eggs, showed an increase in the amount of class II antigens detectable per cell. In a second set of experiments, outbred swine were exposed to A. suum naturally or by repeated experimental inoculation with different doses of normal eggs, and protective immunity and changes in blood cell subsets were determined. The greatest change in blood cell subsets was found at 3 and 5 weeks after initial parasite exposure, when macrophages were elevated moderately in a group of pigs inoculated every other day with 1000 eggs and markedly in a group that was naturally exposed; class II antigen expression was also increased during this period. These increases preceded peak serum antibody responses, which were lower in the naturally-exposed group relative to the experimentally-inoculated group. Both groups had high levels of protective immunity. This suggests than natural exposure to A. suum may activate cells and enhance specific immune responses to give high levels of protection.

Reduced lymphocyte response to mitogens in patients with Bancroftian filariasis

Peripheral blood lymphocytes from patients with acute and chronic Wuchereria bancrofti infections responded poorly to concanavalin A, phytohaemagglutinin and pokeweed mitogen when cultured in heat-inactivated pooled normal serum. The lymphocyte response to mitogens in carriers of microfilariae (mff) were normal. The suppression of transformation to mitogens was not reversible by the removal of plastic adherent cells. Incubation with mitogens and the adult filarial worm antigen (BmA) did not alter the mitogen response either in control subjects or in filarial patients. The possible mechanism of immunosuppression is discussed.

Suppression of immune response to Listeria monocytogenes: mechanism(s) of immune complex suppression

We have investigated possible mechanisms underlying immune complex suppression of resistance to Listeria monocytogenes. Inhibition of resistance was found when immune complexes were formed in vivo in immune mice or in nonimmune mice adoptively transferred with specific antibody. Suppression was also found when nonimmune mice were injected with immune complexes preformed in vitro. We investigated the role of complement by decomplementing mice with cobra venom factor purified by high-pressure liquid chromatography. Complete depletion of serum C3 did not eliminate immune complex suppression of resistance to L. monocytogenes, suggesting that complement activation is not required for immune complex suppression. Infection-induced changes in the surface phenotype and functional properties of macrophages from normal and immune complex-suppressed mice were also investigated. Macrophage expression of both H-2K and Ia molecules increased during the response of normal mice to L. monocytogenes. However, these changes were not found in immune complex-suppressed mice. In contrast, membrane interleukin 1 expression was increased in macrophages from suppressed mice compared with macrophages from normal mice. Macrophages from L. monocytogenes-infected normal and immune complex-suppressed mice expressed cytotoxicity against tumor cells in vitro. We conclude that immune complexes do not inhibit resistance to L. monocytogenes by activation of complement or decreasing macrophage cytotoxic activity. Rather, defects in Ia expression by macrophages from suppressed mice might be one component responsible for immune complex suppression of resistance to L. monocytogenes.