Immunoregulation in onchocerciasis. Functional and phenotypic abnormalities of lymphocyte subsets and changes with therapy

Wuchereria bancrofti infection is linked to systemic activation of CD4 and CD8 T cells

Background

Susceptibility to HIV has been linked to systemic CD4+ T cell activation in cohorts of seronegative individuals with high HIV-exposure risk. We recently described an increased risk of HIV transmission in individuals infected with Wuchereria bancrofti, the causative agent for lymphatic filariasis, in a prospective cohort study. However, the reason for this phenomenon needs further investigation.

Methodology/Principal findings

Two-hundred and thirty-five HIV negative adults were tested using Trop Bio ELISA for detection of W. bancrofti infection and Kato Katz urine filtration and stool based RT-PCR for detection of soil transmitted helminths and schistosomiasis. FACS analysis of the fresh peripheral whole blood was used to measure T cell activation markers (HLA-DR, CD38), differentiation markers (CD45, CD27), markers for regulatory T cells (FoxP3, CD25) and the HIV entry receptor CCR5. Frequencies of activated HLA-DR^pos CD4 T cells were significantly increased in subjects with W. bancrofti infection (n = 33 median: 10.71%) compared to subjects without any helminth infection (n = 42, median 6.97%, p = 0.011) or those with other helminths (Schistosoma haematobium, S. mansoni, Trichuris trichiura, Ascaris lumbricoides, hookworm) (n = 151, median 7.38%, p = 0.009). Similarly, a significant increase in HLA-DR^posCD38^pos CD4 T cells and effector memory cells CD4 T cells (CD45RO^posCD27^neg) was observed in filarial infected participants. Multivariable analyses further confirmed a link between W. bancrofti infection and systemic activation of CD4 T cells independent of age, fever, gender or other helminth infections.

Conclusions/Significance

W. bancrofti infection is linked to systemic CD4 T cell activation, which may contribute to the increased susceptibility of W. bancrofti infected individuals to HIV infection.

The importance of CD4 T cell activation for HIV susceptibility has been emphasized in several studies focusing on HIV transmission and prevention. Particularly, activated HLA-DR+ CD4 T cells may play a major role in HIV susceptibility. In this analysis we describe systemic activation of CD4 T cells in individuals infected with W. bancrofti the causative agent of lymphatic filariasis. This helminth disease leads to debilitating pathology in some of the individuals; however, the majority of infected persons remain asymptomatic. We recently described an increased HIV incidence in subjects infected with W. bancrofti compared to uninfected individuals from the same area. To decipher underlying reasons for this phenomenon, we measured immune activation parameters in CD4 and CD8 T cells. The increased percentage of HLADR positive and HLADR/CD38 positive CD4 T cells and also effector memory CD4 T cells that we describe here could be a possible mechanism to explain our previous findings of increased HIV incidence in individuals infected with this filarial nematode.

Parasites and immune responses: memory illusion?

Immunological memory responses to intracellular protozoa and extracellular helminths govern host resistance and susceptibility to reinfection. Humans and livestock living in parasitic disease endemic regions face continuous exposure from a very early age that often leads to asymptomatic chronic infection over their entire lifespan. Fundamental immunological studies suggest that the generation of T-cell memory is driven by tightly coordinated innate and adaptive cellular immune responses rapidly triggered following initial host infection. A key distinguishing feature of immune memory maintenance between the majority of parasitic diseases and most bacterial or viral diseases is long-term antigen persistence. Consequently, functional parasite immune memory is in a continuous, dynamic flux between activation and deactivation producing functional parasite killing or functional memory cell death. In this sense, T-cell immune memory can be regarded as "memory illusion." Furthermore, due to the finite capacity of memory lymphocytes to proliferate, continuous parasite antigen stimulation may exceed a threshold level at some point in the chronically infected host. This may result in suboptimal effector immune memory leading to host susceptibility to reinfection, or immune dysregulation yielding disease reactivation or immune pathology. The goal of this review is to highlight, through numerous examples, what is currently known about T-cell immune memory to parasites and to provide compelling hypotheses on the survival and maintenance of parasite "memory illusion." These novel concepts are discussed in the context of rationale parasite vaccine design strategies.

Down-regulated lymphoproliferation coincides with parasite maturation and with the collapse of both gamma interferon and interleukin-4 responses in a bovine model of onchocerciasis

Onchocerciasis is a debilitating parasitic infection caused by the filarial nematode Onchocerca volvulus. Infections are chronic, and persistence of the parasites for several years argues for highly adapted mechanisms of immune evasion. Due to the restricted host repertoire of O. volvulus, we have used the cattle parasite Onchocerca ochengi to investigate the nature of immunomodulation underpinning these long-term infections. Cattle were infected with a single inoculation of 350 infective-stage larvae under laboratory conditions (n = 6). Intradermal nodules containing immature adult worms were detected from 110 days postinfection, and microfilariae in skin were detected from day 280 postinfection. Parasite-specific responses during early infection were nonpolarized with respect to the major Th cytokines (interleukin-4 [IL-4], IL-2, and gamma interferon [IFN-gamma]) produced by antigen-stimulated peripheral blood mononuclear cells (PBMC) or serum antibody isotypes. Antigen-induced proliferation of PBMC peaked shortly after exposure and remained high during the prepatent infection. As the parasites matured and animals developed patent infections, there was a profound down-regulation of lymphoproliferation, accompanied by sharp falls in the expression of both IL-4 and IFN-gamma and a gradual decline in IL-2. Levels of immunoglobulin G2 (IgG2) fell, while those of IgG1 remained high. We conclude that neither a classical Th2 response nor a simple Th1-to-Th2 switch is sufficient to explain the immunomodulation associated with patent Onchocerca infections. Instead, there is an initial Th0 response, which matures into a response with some, but not all of the features of a Th2 response. The natural host-parasite relationship of O. ochengi in cattle may be useful as both a descriptive and predictive tool to test more refined models of immunomodulation in onchocerciasis.

Chronic immune activation associated with intestinal helminth infections results in impaired signal transduction and anergy

Helminthic parasites cause widespread, persistent infections in humans. The immigration of Ethiopians to Israel (a group denoted here by "Eth."), many of them infested with helminths and in a chronic immune-activation state, enabled us to investigate the effects of such immune activation on immune responses. We studied the immune profile and immune functions of 190 Eth. and Israeli non-Eth. (Isr.) highly, partially, or non-immune-activated individuals. Immune cells from highly immune-activated individuals were defective in several signaling responses, all of which were restored gradually following anti-helminthic treatment. These cells showed poor transmembrane signaling, as seen by the phosphorylation of various tyrosine kinases and of the MAPK kinases, ERK1/2 and p38; deficient degradation of phosphorylated IkappaBalpha; increased expression of cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), which appears to block proliferative responses in these cells; decreased beta-chemokine secretion by CD8(+) cells after stimulation; and reduced proliferation to recall antigen stimulation. Highly immune-activated individuals also showed decreased delayed-type skin hypersensitivity responses to recall antigen before deworming. These findings support the notion that chronic helminthic infections cause persistent immune activation that results in hyporesponsiveness and anergy. Such impaired immune functions may diminish the capacity of these individuals to cope with infections and to generate cellular protective immunity after vaccination.

Decreased CD4 and increased CD8 counts with T cell activation is associated with chronic helminth infection

We have previously reported the presence of marked immune dysregulation with a dominant Th2 profile, in a population of Ethiopian immigrants (ETH) in Israel heavily infected with helminths. In order to characterize better this immune dysregulation we studied by flow cytometry the expression of several activation markers on peripheral T cell populations, and lymphocyte apoptosis, in blood samples obtained from 63 'new' ETH (recently arrived), 18 'old' ETH (> 5 years since immigration) and 34 non-Ethiopian Israelis. The main findings in the 'new' ETH group in comparison with the non-Ethiopian controls were: (i) decreased CD4 and increased CD8 lymphocyte counts; (ii) elevated levels of activated T cells (CD3, CD4 and CD8) expressing HLA-DR; (iii) decreased levels of 'naive' CD4+ cells (CD45RA+), with increased levels of 'memory' CD4+ cells (CD45RO+); (iv) decreased numbers of CD28+ CD8+ lymphocytes; (v) marked increase in lymphocyte apoptosis. These T cell alterations and activation profile remained unchanged in 10 'new' ETH in whom the helminth infections persisted for 6-11 months. In contrast, in 18 'old' ETH, without helminth infections, the T cell activation profile was within the normal range. These findings suggest that chronic helminth infections may have a profound effect on the immune system of the host that disappears after eradication of these infections and adjustment to the new environment. It should therefore be taken into consideration for every immunomodulation therapy and especially in vaccine design and trials, in regions endemic for helminth infections.

T cell responses in coinfection with Onchocerca volvulus and the human immunodeficiency virus type 1

Onchocerca volvulus and the human immunodeficiency virus (HIV) are two immunocompromising infectious agents of major public health concern in Uganda. To examine the effect of coinfection with O. volvulus and HIV on cellular immune responses, lymphocyte proliferative responses and cytokine production of peripheral blood mononuclear cells (PBMC) from persons infected with O. volvulus with and without HIV type 1 infection were compared. Proliferation of PBMC to PHA and tuberculin (PPD) in coinfection was less (P = 0.08, P < 0.01) than in O. volvulus infection. O. volvulus extract stimulated lymphocyte proliferation in microfilaria-negative and HIV-negative O. volvulus infection while only an inconspicuous response was observed in microfilaria-negative coinfection. After stimulation of PBMC with PPD, the production of interferon-gamma (IFN-gamma), interleukin (IL)-4 and IL-5-demonstrated in O. volvulus infection-were reduced in coinfection with HIV (P < 0.01). While both groups failed to produce IFN-gamma in response to O. volvulus extract, only O. volvulus infected persons generated pronounced IL-5 and low IL-4 levels (0.01 > P = 0.02). The cellular immune responses in coinfection suggested an HIV-related lack of specific reactivity to O. volvulus antigen and impairment of IL-4 and IL-5 production in addition to the lack of IFN-gamma response on antigenic stimulation.

Microfilarial clearance in loiasis involves elevation of Th1 and Th2 products and emergence of a specific pattern of T-cell populations

Diethylcarbamazine (DEC) induced clearance of microfilaraemia in loiasis is associated with severe posttreatment reactions. To define the switch from hypo- to hyper-responsiveness associated with DEC treatment, phenotypic alterations of T-lymphocytes, characterized by flow cytometry, and cytokines, determined by enzyme linked immunosorbent assay, were monitored in a microfilaraemic patient. In contrast to reports on onchocerciasis and lymphatic filariases, no elevation of interleukin (IL)-6 and tumour necrosis factor (TNF)-alpha was observed. The most severe side effects coincided with an elevation of interferon (IFN)-gamma on day 3, followed by IL-10, transforming growth factor (TGF)-beta 2 and macrophage inflammatory protein-1 alpha (MIP-1 alpha) peaking on day 5. Phenotypically, T-cell activation markers CD38, CD54 and CD25 were significantly expressed before treatment, with high CD38 expression still existing one year after clearance of microfilaraemia. Treatment-related increases were observed with anti-CD122, anti-HLA-DR and anti-CD69. CD28 was expressed before treatment on almost 100% of CD4+ and CD8+ T cells and dropped to 20% by day 5, reaching again baseline levels on day 21. Furthermore, there emerged 20% TCR alpha beta+/CD3+ T cells and 10% anti-beta V5(c)+ T cells, altogether indicating a specific pattern of T-helper (Th) 1 and Th2 cytokines as well as expansion of certain pauciclonal T-cell populations in response to microfilarial clearance.

Interleukin-5 is the predominant cytokine produced by peripheral blood mononuclear cells in alveolar echinococcosis

An involvement of cellular immunity in alveolar echinococcosis is strongly suggested by the intense granulomatous infiltrations observed around the hepatic parasite lesions. However, the basis of cellular immunoregulation in patient with alveolar echinococcosis is poorly understood. The present report shows a comparative analysis of lymphoid cell function in peripheral blood mononuclear cells (PBMC) of 16 patients with alveolar echinococcosis and of healthy individuals. Our in vitro restimulation studies with crude Echinococcus multilocularis antigen demonstrated that PBMC from patients with alveolar echinococcosis were responsive to challenge with parasitic antigen as measured by lymphoid cell proliferation. In this system, we also evaluated cytokine expression at the gene and protein levels after stimulation with E. multilocularis antigen. Analysis of cytokine mRNA expression revealed distinct patterns of cytokine expression in patients and normal donors. By using reverse transcriptase PCR, we could demonstrate that the TH1 cytokine transcripts interleukin-2 (IL-2) and gamma interferon (IFN-gamma) are present in PBMC from patients with alveolar echinococcosis. Moreover, it was found that stimulation with E. multilocularis antigen induced or enhanced the expression of the TH2 cytokine IL-3, IL-4, IL-10, and especially IL-5 mRNAs in PBMC from 13 of 16 patients with alveolar echinococcosis. Two patients who were examined after radical surgery, as well as another patient with a stable course of the disease under continuous chemotherapy, were not able to generate the same pattern of cytokine response and had no evidence of IL-5 mRNA synthesis. In contrast to the frequent expression of TH2 cytokine mRNAs observed in patients with alveolar echinococcosis, PBMC cultures from normal donors showed prominent IL-2 and IFN-gamma mRNA expression but weak IL-3, IL-4, and IL-10 mRNA expression. Most interestingly, IL-5 mRNA was substantially absent in PBMC from healthy individuals. In accordance with the mRNA studies, it was found that E. multilocularis antigen induced the secretion of large amounts of IL-5 and intermediate amounts of IFN-gamma in patients with alveolar echinococcosis, whereas large amounts of IFN-gamma and no or threshold amounts of IL-5 were detected in supernatants from healthy individuals. Collectively, the present study provides the first evidence that a TH2 immune response is gradually activated during the course of E. multilocularis infection, indicating a critical role for IL-5 in the manifestation of human alveolar echinococcosis.

Evidence for an abnormal profile of interleukin-4 (IL-4), IL-5, and gamma-interferon (gamma-IFN) in peripheral blood T cells from patients with allergic eosinophilic gastroenteritis

Allergic eosinophilic gastroenteritis is characterized by elevated total IgE, specific IgE to food antigens, and eosinophilia of tissue and blood. Because the lymphokines IL-4, IL-5, and gamma-interferon, regulate IgE synthesis, and eosinophilopoiesis in vitro, we examined whether there is an imbalance in their production in allergic eosinophilic gastroenteritis. To explore this hypothesis, three adult patients with allergic eosinophilic gastroenteritis were studied. Flow cytometric studies of peripheral blood mononuclear cells from these patients did not reveal evidence of T cell activation or disturbance of T cell numbers or subsets. T cells were capable of normal mitogenic activation in vitro. IL-4 and IL-5 production were markedly elevated with mitogenic stimulation. Most IL-4 and IL-5 production was by CD4+ T cells. Synthesis of IL-5 by CD4+ T lymphocytes in three patients and CD8+ T lymphocytes in two patients occurred in the absence of mitogen. Mitogen-stimulated GM-CSF and gamma-interferon synthesis by CD4+ T cells was normal. Lymphokine mRNA in total cellular RNA derived from endoscopic biopsies was examined by reverse transcription/polymerase chain reaction. Mucosal biopsies from control subjects and most biopsies from allergic eosinophilic gastroenteritis patients contained less than 10(-8) micrograms IL-5 mRNA/1 microgram total cellular mRNA. gamma-Interferon mRNA was not detected by reverse transcription/polymerase chain reaction in biopsies from patients with allergic eosinophilic gastroenteritis but was present in controls. These lymphokine abnormalities are consistent with the elevated IgE and eosinophilia seen in allergic eosinophilic gastroenteritis and suggest that strategies targeting T lymphocytes may be efficacious in treatment of this disease.

Epitopes of the Onchocerca volvulus RAL1 antigen, a member of the calreticulin family of proteins, recognized by sera from patients with onchocerciasis

RAL1 is an antigen (Ag) encoded by the filarial nematode Onchocerca volvulus, the parasite causing onchocerciasis (river blindness). RAL1 shares 64.4% identity with the autoantigen calreticulin. The striking similarity of the parasite Ag and the human autoantigen has led to the hypothesis that RAL1 may induce a cross-reactive immune response to calreticulin, which in turn may be involved in the pathogenesis of onchocerciasis. To test this hypothesis, we explored the immune response to RAL1 recombinant Ag (RAL1 rAg) and human calreticulin in patients with O. volvulus infection. A total of 86% of the O. volvulus-infected individuals produced antibodies recognizing RAL1 rAg. Antibody reactivity to RAL1 rAg in patient sera was confined primarily to the central and carboxyl-terminal parts of the molecule. No significant correlations were found to associate recognition of RAL1 rAg, or any particular portion thereof, with a particular disease state. Antibodies against RAL1 thus appear to be produced as a general immune reaction to O. volvulus infection and do not necessarily lead to a cross-reacting response with the host protein. In contrast, 33% of the patient sera tested bound recombinant human calreticulin. All of these sera also recognized a polypeptide encompassing the carboxyl-terminal portion of the RAL1 rAg. These results suggest that recognition of an epitope encoded in the carboxyl-terminal portion of RAL1 is at least in part responsible for inducing a cross-reacting immune response to the host protein.

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

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

Type 2 cytokines and negative immune regulation in human infections

Recent studies indicate that the human immune response to infection is regulated by the balance between the T helper type 1 cytokines, interleukin-2 and interferon-gamma, and the T helper type 2 cytokines, interleukin-4, interleukin-5 and interleukin-10. Interleukin-4 and interleukin-10 can facilitate antibody production but can also suppress cell-mediated immune responses. The net effect of these negative immunoregulatory cytokines is to favor progression of infection.

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.

Changes in interleukin-2 and interleukin-4 production in asymptomatic, human immunodeficiency virus-seropositive individuals

Infection with HIV results in an incremental loss of T helper cell (TH) function, which can occur years before CD4 cell numbers are critically reduced and AIDS is diagnosed. All TH function is not affected, however, because B cell activation and hypergammaglobulinema are also characteristic of this period. Recently, in a murine model of AIDS an early loss in production of the CD4 cytokines IL-2 and IFN-gamma was correlated with an increase in the B cell stimulatory cytokines IL-4, IL-5, and IL-10. We therefore assessed the production of IL-4 generated by PBL from HIV-seropositive (HIV+) individuals who did not have AIDS, yet who exhibited different TH functional categories based on their IL-2 production profiles. We observed that the decreases in recall antigen-stimulated IL-2 production were accompanied by an increase in IL-4 production. The loss of recall antigen-stimulated responses in HIV+ individuals could be reversed in vitro by anti-IL-4 antibody. Our results suggest that the TH functions assessed by IL-4 production replace the normally dominant TH function of antigen-stimulated IL-2 production in the progression toward AIDS, and raise the possibility of cytokine cross-regulation in AIDS therapy.

A TH1-->TH2 switch is a critical step in the etiology of HIV infection

This viewpoint proposes that an imbalance in the TH1-type and TH2-type responses contributes to the immune dysregulation associated with HIV infection, and that resistance to HIV infection and/or progression to AIDS is dependent on a TH1-->TH2 dominance. This hypothesis is based on the authors' findings that: (1) progression to AIDS is characterized by loss of IL-2- and IFN-gamma production concomitant with increases in IL-4 and IL-10; and (2) many seronegative, HIV-exposed individuals generate strong TH1-type responses to HIV 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.