Immunologic responses to repeated ivermectin treatment in patients with onchocerciasis

Onchocerca volvulus-specific antibody and cellular responses in onchocerciasis patients treated annually with ivermectin for 30 years and exposed to parasite transmission in central Togo

BACKGROUND

Annual mass drug administrations (MDA) of ivermectin will strongly reduce Onchocerca volvulus microfilariae (mf) in the skin and in the onchocerciasis patients' eyes. Ivermectin treatment will also affect the expression of immunity in patients, such that activated immune defenses may help control and contribute to clearance of mf of O. volvulus. Longitudinal surveys are a prerequisite to determining the impact of ivermectin on the status of anti-parasite immunity, notably in risk zones where parasite transmission and active O. volvulus infections persist.

METHODOLOGY/PRINCIPAL FINDINGS

Onchocerciasis patients were treated annually with ivermectin and their Onchocerca volvulus antigen (OvAg) specific IgG and cellular responses were investigated before and at 30 years post initial ivermectin treatment (30yPT). Repeated annual ivermectin treatments eliminated persisting O. volvulus microfilariae (mf) from the skin of patients and abrogated patent infections. The OvAg-specific IgG1 and IgG4 responses were diminished at 30yPT to the levels observed in endemic controls. Prior to starting ivermectin treatment, OvAg-induced cellular productions of IL-10, IFN-γ, CCL13, CCL17 and CCL18 were low in patients, and at 30yPT, cellular cytokine and chemokine responses increased to the levels observed in endemic controls. In contrast, mitogen(PHA)- induced IL-10, IFN-γ, CCL17 and CCL18 cellular production was diminished. This divergent response profile thus revealed increased parasite antigen-specific but reduced polyclonal cellular responsiveness in patients. The transmission of O. volvulus continued at the patients' location in the Mô river basin in central Togo 2018 and 2019 when 0.58% and 0.45%, respectively, of Simulium damnosum s.l. vector blackflies carried O. volvulus infections.

CONCLUSIONS/SIGNIFICANCE

Repeated annual ivermectin treatment of onchocerciasis patients durably inhibited their patent O. volvulus infections despite ongoing low-level parasite transmission in the study area. Repeated MDA with ivermectin affects the expression of immunity in patients. O. volvulus parasite-specific antibody levels diminished to levels seen in infection-free endemic controls. With low antibody levels, antibody-dependent cellular cytotoxic responses against tissue-dwelling O. volvulus larvae will weaken. O. volvulus antigen inducible cytokine and chemokine production increased in treated mf-negative patients, while their innate responsiveness to mitogen declined. Such lower innate responsiveness in elderly patients could contribute to reduced adaptive immune responses to parasite infections and vaccines. On the other hand, increased specific cellular chemokine responses in mf-negative onchocerciasis patients could reflect effector cell activation against tissue invasive larval stages of O. volvulus. The annual Simulium damnosum s.l. biting rate observed in the Mô river basin was similar to levels prior to initiation of MDA with ivermectin, and the positive rtPCR results reported here confirm ongoing O. volvulus transmission.

Modelling exposure heterogeneity and density dependence in onchocerciasis using a novel individual-based transmission model, EPIONCHO-IBM: Implications for elimination and data needs

BACKGROUND

Density dependence in helminth establishment and heterogeneity in exposure to infection are known to drive resilience to interventions based on mass drug administration (MDA). However, the interaction between these processes is poorly understood. We developed a novel individual-based model for onchocerciasis transmission, EPIONCHO-IBM, which accounts for both processes. We fit the model to pre-intervention epidemiological data and explore parasite dynamics during MDA with ivermectin.

METHODOLOGY/PRINCIPAL FINDINGS

Density dependence and heterogeneity in exposure to blackfly (vector) bites were estimated by fitting the model to matched pre-intervention microfilarial prevalence, microfilarial intensity and vector biting rate data from savannah areas of Cameroon and Côte d'Ivoire/Burkina Faso using Latin hypercube sampling. Transmission dynamics during 25 years of annual and biannual ivermectin MDA were investigated. Density dependence in parasite establishment within humans was estimated for different levels of (fixed) exposure heterogeneity to understand how parametric uncertainty may influence treatment dynamics. Stronger overdispersion in exposure to blackfly bites results in the estimation of stronger density-dependent parasite establishment within humans, consequently increasing resilience to MDA. For all levels of exposure heterogeneity tested, the model predicts a departure from the functional forms for density dependence assumed in the deterministic version of the model.

CONCLUSIONS/SIGNIFICANCE

This is the first, stochastic model of onchocerciasis, that accounts for and estimates density-dependent parasite establishment in humans alongside exposure heterogeneity. Capturing the interaction between these processes is fundamental to our understanding of resilience to MDA interventions. Given that uncertainty in these processes results in very different treatment dynamics, collecting data on exposure heterogeneity would be essential for improving model predictions during MDA. We discuss possible ways in which such data may be collected as well as the importance of better understanding the effects of immunological responses on establishing parasites prior to and during ivermectin treatment.

Helminth-Tuberculosis Co-infection: An Immunologic Perspective

Over 2 billion people worldwide are infected with helminths (worms). Similarly, infection with Mycobacterium tuberculosis (Mtb) occurs in over a third of the world's population, often with a great degree of geographical overlap with helminth infection. Interestingly, the responses induced by the extracellular helminths and those induced by the intracellular Mtb are often mutually antagonistic and, as a consequence, can result in impaired (or cross-regulated) host responses to either of the infecting pathogens. In this review, we outline the nature of the immune responses induced by infections with helminths and tuberculosis (TB) and then provide data from both experimental models and human studies that illustrate how the immune response engendered by helminth parasites modulates Mtb-specific responses in helminth-TB coinfection.

Filarial infection modulates the immune response to Mycobacterium tuberculosis through expansion of CD4+ IL-4 memory T cells

Exaggerated CD4(+) T helper 2-specific cytokine producing memory T cell responses developing concomitantly with a T helper 1 response might have a detrimental role in immunity to infection caused by Mycobacterium tuberculosis. To assess the dynamics of Ag-specific memory T cell compartments in the context of filarial infection, we used multiparameter flow cytometry on PBMCs from 25 microfilaremic filarial-infected (Inf) and 14 filarial-uninfected (Uninf) subjects following stimulation with filarial Ag (BmA) or with the M. tuberculosis-specific Ag culture filtrate protein-10 (CFP-10). Our data demonstrated that the Inf group had a marked increase in BmA-specific CD4(+)IL-4(+) cells (median net frequency compared with baseline [Fo] = 0.09% versus 0.01%; p = 0.038) but also to CFP-10 (Fo = 0.16% versus 0.007%; p = 0.04) and staphylococcal enterotoxin B (Fo = 0.49% versus 0.26%; p = 0.04). The Inf subjects showed a BmA-specific expansion of CD4(+)CD45RO(+)IL-4(+) producing central memory (TCM, CD45RO(+)CCR7(+)CD27(+); Fo = 1.1% versus 0.5%; p = 0.04) as well as effector memory (TEM, CD45RO(+)CCR7(-)CD27(-); Fo = 1.5% versus 0.2%; p = 0.03) with a similar but nonsignificant response to CFP-10. In addition, there was expansion of CD4(+)IL-4(+)CD45RA(+)CCR7(+)CD27(+) (naive-like) in Inf individuals compared with Uninf subjects. Among Inf subjects with definitive latent tuberculosis, there were no differences in frequencies of IL-4-producing cells within any of the memory compartments compared with the Uninf group. Our data suggest that filarial infection induces Ag-specific, exaggerated IL-4 responses in distinct T cell memory compartments to M. tuberculosis-specific Ags, which are attenuated in subjects who are able to mount a delayed type hypersensitivity reaction to M. tuberculosis.

Impact of filarial infections on coincident intracellular pathogens: Mycobacterium tuberculosis and Plasmodium falciparum

PURPOSE OF REVIEW

To examine the consequences of the immune modulation seen in chronic filarial infection on responses to intracellular pathogens (and their antigens) that are often co-endemic with filarial infections, namely Plasmodium and Mycobacterium tuberculosis.

RECENT FINDINGS

Much of the recent data on filaria/mycobacteria or filaria/Plasmodium co-infection has focused on the modulation of mycobacteria-specific or malaria-specific responses by chronic filarial infection. As such, filarial infections very clearly alter the magnitude and quality of the mycobacteria-specific or malaria-specific cytokine responses, responses that have been typically associated with control of these intracellular pathogens.

SUMMARY

Although phylogenetically distinct, mycobacteria and Plasmodium spp. often share the same geographical niche with filarial infections. The complex interplay between filarial parasites that are associated with immunomodulation and those microbial pathogens that require a proinflammatory or unmodulated response for their control is easily demonstrable ex vivo, but whether this interplay affects disease outcome in tuberculosis or malaria remains an open question.

Elimination of human onchocerciasis: history of progress and current feasibility using ivermectin (Mectizan(®)) monotherapy

We review and analyze approaches over a 65 year period that have proven successful for onchocerciasis control in several different epidemiological settings. These include vector control with the goal of transmission interruption versus the use of mass drug administration using ivermectin (Mectizan(®)) monotherapy. Ivermectin has proven exceedingly effective because it is highly efficacious against Onchocerca volvulus microfilariae, the etiological agent of onchocercal skin and ocular disease and the infective stage for the vector. For these reasons, the drug was donated by the Merck Company for regional control programs in Africa and the Americas. Recurrent treatment with ivermectin at semi-annual intervals also impacts adult worms and result in loss of fecundity and increased mortality. Using a strategy of 6-monthly treatments with high coverage rates, the Onchocerciasis Elimination Program for the Americas has interrupted transmission in seven of the thirteen foci in the Americas and is on track to eliminate onchocerciasis in the region by 2015. Treatments given annually or semi-annually for 15-17 years in three hyperendemic onchocerciasis foci in Mali and Senegal also have resulted in a few infections in the human population with transmission levels below thresholds postulated for elimination. Follow-up evaluations did not detect any recrudescence of infection or transmission, suggesting that onchocerciasis elimination could be feasible with Mectizan(®) treatment in some endemic foci in Africa.

Expansion of Foxp3+ regulatory T cells in mice infected with the filarial parasite Brugia malayi

Many helminths, including Brugia malayi, are able to establish long-lived infections in immunocompetent hosts. Growing evidence suggests that the immune system's failure to eliminate parasites is at least partially due to the effects of regulatory T cells (Tregs). To test whether parasites may directly stimulate host regulatory activity, we infected mice with two key stages of B. malayi. Both mosquito-borne infective larvae and mature adults i.p. introduced were found to preferentially expand the proportion of CD25(+)Foxp3(+) cells within the CD4(+) T cell population. The induction of Foxp3 was accompanied by raised CD25, CD103, and CTLA-4 expression, and was shown to be an active process, which accompanied the introduction of live, but not dead parasites. CTLA-4 expression was also markedly higher on Foxp3(-) cells, suggesting anergized effector populations. Peritoneal lavage CD4(+)CD25(+) cells from infected mice showed similar suppressive activity in vitro to normal splenic "natural" Tregs. Both B. malayi larvae and adults were also able to induce Foxp3 expression in adoptively transferred DO11.10 T cells, demonstrating that filarial infection can influence the development of T cells specific to a third party Ag. In addition, we showed that induction was intact in IL-4R-deficient animals, in the absence of a Th2 or alternatively activated macrophage response. We conclude that filarial infections significantly skew the balance of the host immune system toward Treg expansion and activation, in a manner dependent on live parasites but independent of a concomitant Th2 response.

Onchocerca volvulus-specific antibody and cytokine responses in onchocerciasis patients after 16 years of repeated ivermectin therapy

The recommended control option against onchocerciasis is repeated ivermectin treatment, which will need to be implemented for decades, and it remains unknown how repeated ivermectin therapy might affect immunity against Onchocerca volvulus in the long term. O. volvulus-specific antibody reactivity and cellular cytokine production were investigated in onchocerciasis patients receiving ivermectin (150 microg/kg) annually for 16 years. In treated patients, the T helper type 2 (Th2) cytokine interleukin (IL)-5 and T regulatory IL-10 in response to O. volvulus antigen (OvAg) and bacteria-derived Streptolysin O (SL-O) diminished to levels found in infection-free endemic controls; also, cellular release of Th1-type interferon (IFN)-gamma at 16 years post initial ivermectin treatment (p.i.t.) approached control levels. In ivermectin-treated onchocerciasis patients, IL-5 production in responses to the mitogen phytohaemagglutinin (PHA) decreased, but IL-10 in response PHA increased, and neither attained the cytokine production levels of endemic controls. At 16 years p.i.t., O. volvulus-specific IgG1 and IgG4 subclass reactivity still persisted at higher levels in onchocerciasis patients than in O. volvulus exposed but microfilariae-free endemic controls. In addition, cytokine responses remained depressed in onchocerciasis patients infected concurrently with Mansonella perstans and Necator americanus or Entamoeba histolytica/dispar. Thus, long-term ivermectin therapy of onchocerciasis may not suffice to re-establish fully a balanced Th1 and Th2 immune responsiveness in O. volvulus microfilariae-negative individuals. Such deficient reconstitution of immune competence may be due to an as yet continuing and uncontrolled reinfection with O. volvulus, but parasite co-infections can also bias and may prevent the development of such immunity.

Antibody responses in onchocerciasis as a function of age and infection intensity

Onchocerciasis is caused by the filarial nematode Onchocerca volvulus and is a major public health problem in West and Central Africa. With only partial and long-term treatment currently available, there is a need to develop a suitable vaccine. We analysed the antibody response to infective L3 larvae because this stage is thought to be associated with host protective immunity. In addition, we have related our findings to the age, gender and current infection intensity of our participants: variables that may significantly influence antibody production. Interestingly, whilst 90% of our study group were seropositive for adult specific immunoglobulin (Ig)E, only 23% produced L3 specific IgE. This is in contrast to IgG4 where seropositivity was comparable at 96% and 92%, respectively. Furthermore, IgG levels were significantly affected by age and the intensity of infection but unaffected by host gender. This finding is independent for the IgG subclass (IgG1, IgG2, IgG3 and IgG4) and its specificity (L3 versus adult antigen). In summary, we show that L3 larvae induce little specific IgE and the antibody response shows a different isotype balance than that against adult antigens. Both host and parasite variables can influence antibody production in this disease.

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.

Influence of ivermectin on cellular and humoral immune responses of lambs

In view of the extensive use of anthelmintics in sheep and the fact that their activity may in part depend upon the immune system, we were interested to determine if ivermectin had any influence on aspects of the sheep immune response. Ten parasite-free 6-month-old lambs were drenched with ivermectin and 1 day later were given intravenously human erythrocytes and subcutaneously ovalbumin. Ten other lambs with injected antigens were not drenched and served as controls. Both groups were bled at intervals for cells and serum. The procedure was repeated on day 28. Lymphocytes from the drenched lambs, cultured in vitro in RPMI plus 50% autologous serum collected up to 7 and 14 days after the first and second antigen injections respectively, had decreased blastogenic activity compared with lymphocytes from control lambs. Similar results were obtained with lymphocytes cultured in RPMI 1640 supplemented with 50% autologous serum plus concanavalin A (Con A) or phytohaemagglutinin (PHA). When washed, lymphocytes were cultured in RPMI 1640 supplemented with 5% foetal calf serum (FCS) or 5% FCS plus Con A or PHA, decreased blastogenesis was observed but blastogenesis depression was not as marked as that observed with autologous serum. Similar antibody responses were seen for the drenched and control groups in response to the two injections of both antigens except that after the second injection, there was a significant reduction in antibody response to ovalbumin in the ivermectin-treated lambs. There were no differences in serum complement or serum nitric oxide levels between the two groups at any stage, but insulin-like growth factor-1 levels were significantly reduced in serum of the ivermectin-treated group, 4 days after each drench. Growth hormone levels were consistently significantly higher 22 days after both drenchings. There was no difference in mean body weight increase between the groups during the experiment.

The effect of ivermectin treatment on the antibody response to antigens of Onchocerca volvulus

The effect of the microfilaricidal drug ivermectin on the antibody response to a detergent extract of adult Onchocerca volvulus (OvAg) and a number of specific recombinant peptides was examined. Three of the peptides were combined in a serodiagnostic 'cocktail' and the effect of ivermectin on the diagnostic performance of this assay was assessed. Immunoglobulin (Ig) G1 serum levels in response to OvAg significantly decreased following ivermectin treatment. The antibody response to only one recombinant peptide (OvMBP29) was significantly affected, with IgG levels decreasing following treatment. Levels of total IgE increased following treatment. No correlation was observed between initial antibody level (or change in antibody level) and any adverse reaction to treatment. The serodiagnostic 'cocktail' was 100% sensitive before and after the use of ivermectin. A serodiagnostic assay using specific recombinant peptides can be used to evaluate infection in the absence of dermal microfilariae in areas where ivermectin is used.

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.

Long-term effect of prenatal exposure to maternal microfilaraemia on immune responsiveness to filarial parasite antigens

To identify long-term effects of prenatal exposure to maternal filarial-parasite infection, we assessed lymphocyte responses in 21 Polynesian children born 17-19 years previously to mothers diagnosed as being microfilaraemic or infection-free. All children lived on an island endemic for bancroftian filariasis but were free from infection at the time of study. While children (n = 10) of infection-free mothers responded vigorously to microfilarial antigen with lymphocyte proliferation, production of interleukin 2 (IL-2), IL-5, IL-10, granulocyte macrophage colony-stimulating factor (GM-CSF), and interferon gamma (IFN-gamma), cellular hyporesponsiveness was seen in children (n = 11) born to microfilaraemic mothers. The hyporesponsiveness appeared restricted to microfilarial antigens and did not extend to non-parasite antigens. These findings suggest that hyporesponsiveness resulted from in-utero acquisition of tolerance to microfilarial antigens in chronically-infected mothers.

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.

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.

Treatment of Onchocerciasis with ivermectin in Central America

The success o f ivermectin in controlling transmission of Onchocerca volvulus in Guatemala suggests a broader role for this drug than morbidity control alone. In this article Ed Cupp reviews recent findings from a three-year pilot study in that country which evaluated the effects of recurrent semiannual treatment on several important factors associated with the biology o f river blindness. These results illustrate the range o f beneficial effects that occur not only for this disease but for other parasitisms when this drug is administered at the community level.