- Email: [email protected]
Does resistance to filarial reinfections become leaky over time?
Opinion
Does resistance to filarial reinfections become leaky over time? Hans P. Duerr1, Wolfgang H. Hoffmann2 and Martin Eichner1 1 2
Department of Medical Biometry, University of Tu¨bingen, Westbahnhofstrasse 55, 72070 Tu¨bingen, Germany Institute of Tropical Medicine, University of Tu¨bingen, Wilhelmstrasse 27, 72074 Tu¨bingen, Germany
Strategies for the control of human parasitic diseases such as onchocerciasis and lymphatic filariasis require an understanding of how the parasite successfully infects and persists in humans. Despite the fact that the vast majority of infective larvae are eliminated after infection, this ‘protection’ is far from being an all-ornothing response. The hypothesis presented here, which is based on epidemiological observations, suggests that the resistance against filarial parasites includes a timedependent component, probably caused by an early immune response with short-term memory. Validating this hypothesis requires experimental studies with a longitudinal component. Such experiments would help to clarify whether the infection can be controlled by vaccination strategies at all. The epidemiological relevance of the infection process Onchocerciasis and lymphatic filariasis are subject to WHO control programmes covering an at-risk population of over one billion people (see the TDR Disease Watch page at www.who.int/tdr/dw/default.html). Apart from implementation issues, these programmes’ prospects of success depend on the regulatory mechanisms of the host and the parasite, which simultaneously promote and limit the establishment and persistence of the parasite in the host population [1]. Understanding these mechanisms helps to assess the possibilities of and constraints on programmes to control these diseases. This is particularly relevant for the Global Programme to Eliminate Lymphatic Filariasis (GPELF) (www.who.int/lymphatic_filariasis/disease/en/), which was initiated in 2000 and will continue for the next few decades. Among the regulatory mechanisms the invasion and development of the infective larvae (L3s) play a key role [2] because thousands of L3s can be transmitted to a human host each year, whereas only a few of them will successfully develop into adult parasites. This raises the question: what turns a potential infection event into a successful one? Only a few parasites are successful Epidemiological considerations usually involve terms like ‘prevalence’ or ‘intensity of infection’, but the following considerations require thinking in terms of rates. Here, the annual transmission potential (ATP) and the parasite establishment rate (PER) – both of which are measured in Corresponding author: Duerr, H.P. ([email protected]).
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number of parasites per person per year – are relevant. The ATP is the average number of L3s that are transmitted per year to a host and, thus, is a measure of the infection pressure. In onchocerciasis, for example, it can be determined by so-called ‘fly-catching studies’ quantifying the daily number of L3s in flies trying to feed on humans. The PER is the number of adult parasites that establish successfully per year per human host and, thus, is a measure of the actual infection rate. It can be estimated roughly if the number and the lifespan of adult parasites in humans are known (see Box 1). The proportion of parasites successfully developing to the adult stage is the ratio of PER divided by ATP. In experimental studies this quantity is usually called the ‘parasite recovery rate’ [3,4]. Model-based analysis of epidemiological data suggests that the PER does not increase linearly with ATP [2] (Figure 1). Although proportionality between ATP and PER might exist for low values of the ATP (necessarily because the PER is zero when there is no transmission), this does not continue for higher ATPs: the infection rate remains almost constant despite increasing infection pressure. Villages where the ATPs differ by a factor of 100 can show comparable parasite burdens, implying that their average PERs are similar, too. This means that the successful establishment of parasites in hosts must be regulated somehow because in a nonregulated relationship the PER would be proportional to the ATP. Regulation is strict but leaky and independent of ATP at higher values Figure 1 reveals two properties of this infection process. Property 1: Only a tiny proportion of L3s successfully infect the host and develop into adult parasites. (A ‘proportion’ implies that in the absence of regulatory processes, the PER would increase linearly with the ATP.) Property 2: At higher ATPs, however, an almost constant number of parasites establish per year in a host; that is, the PER is limited and does not increase further with the ATP. For decades antifilarial immunity has been investigated as the factor responsible for eliminating a large proportion of the infection dose [5]. Such immunity, however, must somehow become downregulated with increasing infection pressure. If it did not, the infection would eliminate itself (in other words, increasing the infection pressure would cure hosts). The current point of view is that regulatory processes control the relationship between the host and the parasite [6,7]. Regulation here means that only a certain number of parasites can establish successfully in a given
1471-4922/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2008.04.005 Available online 19 June 2008
Opinion Box 1. The parasite establishment rate (PER) The PER is the average number of adult parasites that successfully establish per year per human host. It can be estimated roughly by dividing the average parasite burden of hosts by the life expectancy of the parasite [2]. An individual PER can be calculated in the same way, by using the parasite burden of a patient instead. If, for example, an adult patient harbours 40 parasites and each parasite has a life expectancy of 10 years, then, on average, four parasites have established per year in this patient. The life expectancy of adult filariae is 5–10 years for Wuchereria bancrofti [26] and 9–11 years for Onchocerca volvulus [27]. The PER has been estimated for onchocerciasis only, for which the number of adult parasites in patients can be extrapolated roughly from palpation or nodulectomy studies [2]. The burden of adult parasites in patients with lymphatic filariasis has been explored by using ultrasound techniques in case studies [28], but epidemiological data are not available. In chronic infections with ongoing reinfection, the PER should consider the age of hosts because the parasite burden is correlated with age. Crude estimates for the PER are systematically lower in children whose age is less than the life expectancy of the parasite. A population-based estimate for the PER tends to be small if many children contribute to the sample. To avoid these conflicts, the present considerations focus on the infection process in adults.
host each year, and all other parasites will be unsuccessful, for reasons that remain unknown. Regulation is not necessarily limited to immunemediated processes. Some parasites are capable of regulating their densities by using non-immune mechanisms (e.g. Taenia [8,9]). The overall strategy of most parasites is to establish and persist in hosts without killing them or affecting them too adversely. Despite having been transmitted to the human host, the vast majority of L3s do not successfully establish; most of the ATP does not result in new infections, and parasites establish only at a very low rate (i.e. the PER remains small). From a static to a dynamic point of view What determines whether a particular L3, transmitted at a certain moment, will be successful or eliminated? In a
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regulated host–parasite relationship, the outcome can be based on feedback mechanisms originating from the parasites in the host (i.e. the developing L3- and L4-stage larvae, the adult parasites and the microfilariae) and/or host immune factors [10–12]. It is important to note that if the difference between elimination and success were purely random, the PER would be proportional to the ATP, contradicting property 2 (the PER is almost constant at high ATPs). A model must be capable of explaining why the PER remains constant despite increasing infection pressure as measured by the ATP. The necessity of a dynamic point of view also was suggested in the context of age-dependent data on prevalence and intensity of infection with lymphatic filariasis [13,14]. A dynamic hypothesis Figure 2 illustrates a dynamic model that explains epidemiological observations in human filariasis based on available experimental and epidemiological findings: (i) Each successful infection provokes an early immune response [3,15,16], which efficiently protects from further infections, explaining property 1. (ii) This immune response fades over time because of an immunological short-term memory [17,18], and each infection induces only a short period of resistance, explaining ongoing reinfection. The annual number of successfully establishing L3s (PER) is limited, even for high ATPs [2], explaining ‘property 2’. A fading degree of resistance requires the assumption of a protective threshold below which infection can occur again. There are currently no findings for such a threshold, simply because the immunological correlates of resistance are not exactly known. In a concept that assumes regulation originates only from the parasite, the threshold would describe a state in which the parasite population in a host switches between promoting and suppressing the development of a new L3. The duration of the protective period is the reciprocal of the maximum PER. Consider the following example: in
Figure 1. The proportion of ‘successful’ parasites. Relationship between the annual transmission potential (ATP, i.e. the number of parasites inoculated per year by vectors into the human host) and the parasite establishment rate (PER, i.e. the number of adult parasites successfully establishing per year in adult human hosts). If all inoculated parasites were to establish in the human host, the dashed line in blue would hold. The vast majority of L3s, however, are neutralised (green) so that very few of them establish per year in the human host (red). It is still unclear why the PER approaches an almost constant value at higher ATPs.
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Figure 2. Hypothesis on the infection process with filariae. Unsuccessful and successful infection events are shown as grey and red arrows, respectively, below each time axis (1 year). Successful infection with an L3 causes an early immune response (green), which temporarily protects the host against reinfection. The immunological memory diminishes over weeks. Reinfection can occur when the degree of resistance falls below the protective threshold (blue dashed lines). (a) The annual transmission potential (ATP) is low, representing areas with hypoendemic transmission. Periods of susceptibility exist between infection events; thus, the parasite establishment rate (PER) does not reach its maximum. (b) The ATP has a borderline value such that super-infections occur as soon as the degree of resistance falls below the protective threshold. Periods of susceptibility do not exist; thus, the PER has reached its maximum of four parasites per year. (c) The ATP is high, representing areas with hyperendemic transmission. The PER is identical to the borderline case (b) because there are no additional periods of susceptibility. (d) Relationship between ATP and PER for the scenarios (a)–(c). Regulatory processes, the genetic background of hosts or heterogeneities in general might lengthen or shorten periods of resistance by modulating the threshold, the slope in the loss of resistance, or the peak height [17]. The latter case, which increases the PER and might be caused by immunosuppression, is represented in (e). Values were chosen arbitrarily for the purpose of illustration.
onchocerciasis the PER, averaged over the adult population, is between two and four adult female parasites per person per year under moderate-to-heavy transmission (Box 1). This corresponds to a protective period of 3–6 months. The PER can be estimated from epidemiological data, whereas the duration of the protective period could be experimentally determined. Experiments and studies required Further experiments are necessary to confirm or reject this hypothesis. To mimic the infection dynamics observed in human hosts, repeated inoculations with small doses of 352
infective larvae (trickle inoculations) should be performed in appropriate animal models over relevant periods of time [4,19]. Investigating immunological and genetic aspects requires well-established mouse models [20] or natural hosts, such as cotton rats or jirds. The natural hosts are particularly relevant for longitudinal studies because filariae in rats or jirds typically persist longer (>1 year) than in mouse models (weeks). The minimal interval between successful infections, which is related to a protective threshold (see Figure 2), can be determined by quantifying worm burdens after trickle infections at consecutive time points. In particular, the early fate of the invading L3s
Opinion (i.e. their development or elimination) should be monitored longitudinally. First approaches in this direction have been published recently [17,21]. Immunological correlates for host resistance are not precisely defined. The classical approach would be to transfer humoral and cellular host factors from infected to naive animals, which are infected after the transfer. Recent techniques, such as gene expression profiling with microarrays, enable more explorative approaches that do not restrict on classical immune markers but can identify a broader spectrum of potential correlates. Statistical methodologies, such as cluster analyses, can then point at associations between potential correlates. The search for factors associated with infection should not focus on host resistance alone but also should consider correlates for the hosts’ parasite burden. This is relevant for the long-living adult filariae in humans; the filariae are difficult to diagnose and quantify but determine the longterm intervention success or the risk of reinfection in areas of the GPELF and the African Programme for Onchocerciasis Control (http://www.apoc.bf/en/). Correlates for the infection intensity also would allow for migration studies, which have been published only for lymphatic filariasis using IgG4 as a correlate for infection intensity [22]. Extending aspects and vaccine development The dynamic hypothesis can provide a framework in which existing concepts (see Box 2) can be unified: concomitant immunity is not the presence of resistance and susceptibility at the same time but results from an alternation of gain and loss of resistance. Likewise, immunological tolerance or unresponsiveness is not a coexistence of resistance and susceptibility but results from an alternation between these two states in a host. Using these dynamic interpretations, the distinction between the terms ‘concomitant immunity’ and ‘immunological tolerance’ blurs. These concepts might emerge from a common dynamic principle. Heterogeneities in these concepts can originate from individual differences in the intensity of the immunological response (e.g. higher response levels extend the length of time until the protective threshold is crossed, and lower response levels increase the PER), the rate at which resistance fades (e.g. by a response that is correlated with age or coinfections; increasing this rate increases the PER) or the level of the protective threshold (e.g. by regulatory host– parasite interactions; raising the threshold increases the PER). From this point of view, immunosuppression might result not only from suppressed immune responses (Figure 2e) but also from an accelerated loss of resistance or from processes raising the threshold. Processes that operate in the opposite direction can explain the existence of ‘putative immunes’ or ‘endemic normals’. An immunological short-term memory might explain why filariasis research has faced enormous problems in developing a successful vaccine despite various options that seemed to be within reach [23–25]. If natural immunity against filarial parasites rapidly faded over time, vaccine-mediated immunity could inherit the same dynamics and require booster vaccinations as frequently as natural infections can occur (i.e. at intervals of months or even weeks). Thus, investigations into early immune
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Box 2. Existing hypotheses, concepts and open questions It still is not understood why only a tiny proportion of infective-stage larvae successfully infect the host. This phenomenon is formulated by the concept of concomitant immunity [29,30], describing the simultaneous presence of chronic infection with and resistance against newly incoming infective larvae [31,32]. Regarding the other parasite stages (adult parasites and microfilariae), the balanced presence of resistance and susceptibility is represented by the concept of immunological tolerance [6,33,34]. It describes a state in which immunological processes in the host control the parasite population while, at the same time, tolerating a certain number of parasites. Immunological tolerance involves immunoregulation, which seems to result from a balanced interaction of host factors such as lymphocytes – in particular T cells – and cytokines, as well as immunomodulatory parasite components [24,35,36]. For instance, immunosuppression protects hosts against self-destructive immune responses like inflammation and autoimmunity [11]. By contrast, it promotes the establishment and persistence of parasites in hosts [37]. Up to today, it cannot be ruled out that only the parasite regulates its density in the host, without any active control through host immunity or resistance. Resistance in human hosts is suggested by the existence of ‘putative immunes’ or ‘endemic normals’ – hosts who are free of demonstrable infection and/or disease despite living in endemic areas [38,39]. The concept of resistance receives support from the observation that immunological factors improve after chemotherapy of onchocerciasis with the microfilaricide Ivermectin (Ivermectin-facilitated immunity [10,40]). This observation might be specific to humans infected with O. volvulus; in cattle infected with O. ochengi, the cessation of treatment leads to an increased posttreatment susceptibility [41], which is difficult to bring in line with Ivermectin-facilitated immunity. The phrase ‘immunity against filariae’ is too general because it is a conglomerate of regulatory processes between the host, the parasite and its different life-cycle stages (e.g. resistance against L3s does not affect microfilaria or adult worms in the same host). Recent concepts address the role of early and unspecific immune responses [16]. The hypothesis presented in Figure 2 tries to explain infection with filariae by combining the concepts of concomitant immunity and early immune responses. The concept of a dynamically defined infection process offers a framework for unifying existing concepts like concomitant immunity, immunological tolerance and immunosuppression (e.g. see Figure 2e).
responses should be performed not only from a static perspective but also (more importantly) from a dynamic perspective that considers when resistance emerges and for how long it lasts. Acknowledgements We thank Chris Leary for helpful comments on this manuscript. This investigation was supported by the Deutsche Forschungsgemeinschaft (DFG DU1105/1–2).
References 1 Duerr, H.P. et al. (2005) Determinants of the eradicability of filarial infections: a conceptual approach. Trends Parasitol. 21, 88–96 2 Duerr, H.P. et al. (2006) High infection rates at low transmission potentials in West African onchocerciasis. Int. J. Parasitol. 36, 1367–1372 3 Le Goff, L. et al. (1997) Early reduction of the challenge recovery rate following immunization with irradiated infective larvae in a filaria mouse system. Trop. Med. Int. Health 2, 1170–1174 4 Barthold, E. and Wenk, P. (1992) Dose-dependent recovery of adult Acanthocheilonema viteae (Nematoda: Filarioidea) after single and trickle inoculations in jirds. Parasitol. Res. 78, 229–234 5 Maizels, R.M. et al. (1993) Immunological modulation and evasion by helminth parasites in human populations. Nature 365, 797–805 6 Maizels, R.M. et al. (2004) Helminth parasites – masters of regulation. Immunol. Rev. 201, 89–116 353
Opinion 7 Rajakumar, S. et al. (2006) Concomitant immunity in a rodent model of filariasis: the infection of Meriones unguiculatus with Acanthocheilonema viteae. J. Parasitol. 92, 41–45 8 Escobedo, G. et al. (2005) Parasite regulation by host hormones: an old mechanism of host exploitation? Trends Parasitol. 21, 588–593 9 Spolski, R.J. et al. (2000) Parasite-secreted products regulate the host response to larval Taenia crassiceps. Parasite Immunol. 22, 297–305 10 Schulz-Key, H. et al. (1992) Ivermectin-facilitated immunity. Parasitol. Today 8, 152–153 11 Ottesen, E.A. (1995) Immune responsiveness and the pathogenesis of human onchocerciasis. J. Infect. Dis. 171, 659–671 12 Hoffmann, W.H. et al. (2001) Determinants for resistance and susceptibility to microfilaraemia in Litomosoides sigmodontis filariasis. Parasitology 122, 641–649 13 Bundy, D.A. et al. (1991) Immunoepidemiology of lymphatic filariasis: the relationship between infection and disease. Immunol. Today 12, A71–A75 14 Day, K.P. et al. (1991) Age specific patterns of change in the dynamics of Wuchereria bancrofti infection in Papua New Guinea. Am. J. Trop. Med. Hyg. 44, 518–527 15 Babayan, S. et al. (2005) Increased early local immune responses and altered worm development in high-dose infections of mice susceptible to the filaria Litomosoides sigmodontis. Med. Microbiol. Immunol. 194, 151–162 16 Babu, S. and Nutman, T.B. (2003) Proinflammatory cytokines dominate the early immune response to filarial parasites. J. Immunol. 171, 6723–6732 17 Dash, Y. et al. (2007) Determinants of memory in experimental filarial infections in mice. Parasite Immunol. 29, 567–574 18 Paciorkowski, N. et al. (2000) B1 B lymphocytes play a critical role in host protection against lymphatic filarial parasites. J. Exp. Med. 191, 731–736 19 Kowalski, J.C. and Ash, L.R. (1975) Repeated infections of Brugia pahangi in the jird, Meriones unguiculatus. Southeast Asian J. Trop. Med. Public Health 6, 195–198 20 Allen, J.E. et al. Of mice cattle and humans: the immunology and treatment of river blindness. PLoS Negl. Trop. Dis. 2, e217. DOI:10.1371/journal.pntd.0000217 (http://www.plosntds.org) 21 Babayan, S.A. et al. (2006) Vaccination against filarial nematodes with irradiated larvae provides long-term protection against the third larval stage but not against subsequent life cycle stages. Int. J. Parasitol. 36, 903–914 22 Terhell, A.J. et al. (2001) Adults acquire filarial infection more rapidly than children: a study in Indonesian transmigrants. Parasitology 122, 633–640 23 Cook, J.A. et al. (2001) Towards a vaccine for onchocerciasis. Trends Parasitol. 17, 555–558 24 Hoerauf, A. and Brattig, N. (2002) Resistance and susceptibility in human onchocerciasis – beyond Th1 vs. Th2. Trends Parasitol. 18, 25–31
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Trends in Parasitology Vol.24 No.8 25 Lustigman, S. et al. (2002) Towards a recombinant antigen vaccine against Onchocerca volvulus. Trends Parasitol. 18, 135–141 26 Dreyer, G. et al. (2005) Does longevity of adult Wuchereria bancrofti increase with decreasing intensity of parasite transmission? Insights from clinical observations. Trans. R. Soc. Trop. Med. Hyg. 99, 883–892 27 Plaisier, A.P. et al. (1991) The reproductive lifespan of Onchocerca volvulus in West African savanna. Acta Trop. 48, 271–284 28 Mand, S. et al. (2004) Reliable and frequent detection of adult Wuchereria bancrofti in Ghanaian women by ultrasonography. Trop. Med. Int. Health 9, 1111–1114 29 Grenfell, B.T. et al. (1991) A model for the dynamics of human lymphatic filariasis. Parasitol. Today 7, 318–322 30 Brown, S.P. and Grenfell, B.T. (2001) An unlikely partnership: parasites, concomitant immunity and host defence. Proc. Biol. Sci. 268, 2543–2549 31 Devaney, E. and Osborne, J. (2000) The third-stage larva (L3) of Brugia: its role in immune modulation and protective immunity. Microbes Infect. 2, 1363–1371 32 Rajasekariah, G.R. et al. (1989) Protective immune responses with trickle infections of third-stage filarial larvae of Wuchereria bancrofti in mice. Clin. Exp. Immunol. 78, 292–298 33 Maizels, R.M. and Lawrence, R.A. (1991) Immunological tolerance: the key feature in human filariasis? Parasitol. Today 7, 271–276 34 King, C.L. and Nutman, T.B. (1991) Regulation of the immune response in lymphatic filariasis and onchocerciasis. Immunol. Today 12, A54–A58 35 Hoerauf, A. et al. (2005) Immunomodulation by filarial nematodes. Parasite Immunol. 27, 417–429 36 Zaccone, P. et al. (2008) Interplay of parasite-driven immune responses and autoimmunity. Trends Parasitol. 24, 35–42 37 Duerr, H.P. et al. (2003) Density-dependent parasite establishment suggests infection-associated immunosuppression as an important mechanism for parasite density regulation in onchocerciasis. Trans. R. Soc. Trop. Med. Hyg. 97, 242–250 38 Ravindran, B. et al. (2003) Protective immunity in human lymphatic filariasis: problems and prospects. Med. Microbiol. Immunol. 192, 41– 46 39 Turaga, P.S. et al. (2000) Immunity to onchocerciasis: cells from putatively immune individuals produce enhanced levels of interleukin-5, gamma interferon, and granulocyte-macrophage colony-stimulating factor in response to Onchocerca volvulus larval and male worm antigens. Infect. Immun. 68, 1905–1911 40 Soboslay, P.T. et al. (1992) Ivermectin-facilitated immunity in onchocerciasis. Reversal of lymphocytopenia, cellular anergy and deficient cytokine production after single treatment. Clin. Exp. Immunol. 89, 407–413 41 Njongmeta, L.M. et al. (2004) Cattle protected from onchocerciasis by ivermectin are highly susceptible to infection after drug withdrawal. Int. J. Parasitol. 34, 1069–1074
Does resistance to filarial reinfections become leaky over time? Hans P. Duerr1, Wolfgang H. Hoffmann2 and Martin Eichner1 1 2
Department of Medical Biometry, University of Tu¨bingen, Westbahnhofstrasse 55, 72070 Tu¨bingen, Germany Institute of Tropical Medicine, University of Tu¨bingen, Wilhelmstrasse 27, 72074 Tu¨bingen, Germany
Strategies for the control of human parasitic diseases such as onchocerciasis and lymphatic filariasis require an understanding of how the parasite successfully infects and persists in humans. Despite the fact that the vast majority of infective larvae are eliminated after infection, this ‘protection’ is far from being an all-ornothing response. The hypothesis presented here, which is based on epidemiological observations, suggests that the resistance against filarial parasites includes a timedependent component, probably caused by an early immune response with short-term memory. Validating this hypothesis requires experimental studies with a longitudinal component. Such experiments would help to clarify whether the infection can be controlled by vaccination strategies at all. The epidemiological relevance of the infection process Onchocerciasis and lymphatic filariasis are subject to WHO control programmes covering an at-risk population of over one billion people (see the TDR Disease Watch page at www.who.int/tdr/dw/default.html). Apart from implementation issues, these programmes’ prospects of success depend on the regulatory mechanisms of the host and the parasite, which simultaneously promote and limit the establishment and persistence of the parasite in the host population [1]. Understanding these mechanisms helps to assess the possibilities of and constraints on programmes to control these diseases. This is particularly relevant for the Global Programme to Eliminate Lymphatic Filariasis (GPELF) (www.who.int/lymphatic_filariasis/disease/en/), which was initiated in 2000 and will continue for the next few decades. Among the regulatory mechanisms the invasion and development of the infective larvae (L3s) play a key role [2] because thousands of L3s can be transmitted to a human host each year, whereas only a few of them will successfully develop into adult parasites. This raises the question: what turns a potential infection event into a successful one? Only a few parasites are successful Epidemiological considerations usually involve terms like ‘prevalence’ or ‘intensity of infection’, but the following considerations require thinking in terms of rates. Here, the annual transmission potential (ATP) and the parasite establishment rate (PER) – both of which are measured in Corresponding author: Duerr, H.P. ([email protected]).
350
number of parasites per person per year – are relevant. The ATP is the average number of L3s that are transmitted per year to a host and, thus, is a measure of the infection pressure. In onchocerciasis, for example, it can be determined by so-called ‘fly-catching studies’ quantifying the daily number of L3s in flies trying to feed on humans. The PER is the number of adult parasites that establish successfully per year per human host and, thus, is a measure of the actual infection rate. It can be estimated roughly if the number and the lifespan of adult parasites in humans are known (see Box 1). The proportion of parasites successfully developing to the adult stage is the ratio of PER divided by ATP. In experimental studies this quantity is usually called the ‘parasite recovery rate’ [3,4]. Model-based analysis of epidemiological data suggests that the PER does not increase linearly with ATP [2] (Figure 1). Although proportionality between ATP and PER might exist for low values of the ATP (necessarily because the PER is zero when there is no transmission), this does not continue for higher ATPs: the infection rate remains almost constant despite increasing infection pressure. Villages where the ATPs differ by a factor of 100 can show comparable parasite burdens, implying that their average PERs are similar, too. This means that the successful establishment of parasites in hosts must be regulated somehow because in a nonregulated relationship the PER would be proportional to the ATP. Regulation is strict but leaky and independent of ATP at higher values Figure 1 reveals two properties of this infection process. Property 1: Only a tiny proportion of L3s successfully infect the host and develop into adult parasites. (A ‘proportion’ implies that in the absence of regulatory processes, the PER would increase linearly with the ATP.) Property 2: At higher ATPs, however, an almost constant number of parasites establish per year in a host; that is, the PER is limited and does not increase further with the ATP. For decades antifilarial immunity has been investigated as the factor responsible for eliminating a large proportion of the infection dose [5]. Such immunity, however, must somehow become downregulated with increasing infection pressure. If it did not, the infection would eliminate itself (in other words, increasing the infection pressure would cure hosts). The current point of view is that regulatory processes control the relationship between the host and the parasite [6,7]. Regulation here means that only a certain number of parasites can establish successfully in a given
1471-4922/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2008.04.005 Available online 19 June 2008
Opinion Box 1. The parasite establishment rate (PER) The PER is the average number of adult parasites that successfully establish per year per human host. It can be estimated roughly by dividing the average parasite burden of hosts by the life expectancy of the parasite [2]. An individual PER can be calculated in the same way, by using the parasite burden of a patient instead. If, for example, an adult patient harbours 40 parasites and each parasite has a life expectancy of 10 years, then, on average, four parasites have established per year in this patient. The life expectancy of adult filariae is 5–10 years for Wuchereria bancrofti [26] and 9–11 years for Onchocerca volvulus [27]. The PER has been estimated for onchocerciasis only, for which the number of adult parasites in patients can be extrapolated roughly from palpation or nodulectomy studies [2]. The burden of adult parasites in patients with lymphatic filariasis has been explored by using ultrasound techniques in case studies [28], but epidemiological data are not available. In chronic infections with ongoing reinfection, the PER should consider the age of hosts because the parasite burden is correlated with age. Crude estimates for the PER are systematically lower in children whose age is less than the life expectancy of the parasite. A population-based estimate for the PER tends to be small if many children contribute to the sample. To avoid these conflicts, the present considerations focus on the infection process in adults.
host each year, and all other parasites will be unsuccessful, for reasons that remain unknown. Regulation is not necessarily limited to immunemediated processes. Some parasites are capable of regulating their densities by using non-immune mechanisms (e.g. Taenia [8,9]). The overall strategy of most parasites is to establish and persist in hosts without killing them or affecting them too adversely. Despite having been transmitted to the human host, the vast majority of L3s do not successfully establish; most of the ATP does not result in new infections, and parasites establish only at a very low rate (i.e. the PER remains small). From a static to a dynamic point of view What determines whether a particular L3, transmitted at a certain moment, will be successful or eliminated? In a
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regulated host–parasite relationship, the outcome can be based on feedback mechanisms originating from the parasites in the host (i.e. the developing L3- and L4-stage larvae, the adult parasites and the microfilariae) and/or host immune factors [10–12]. It is important to note that if the difference between elimination and success were purely random, the PER would be proportional to the ATP, contradicting property 2 (the PER is almost constant at high ATPs). A model must be capable of explaining why the PER remains constant despite increasing infection pressure as measured by the ATP. The necessity of a dynamic point of view also was suggested in the context of age-dependent data on prevalence and intensity of infection with lymphatic filariasis [13,14]. A dynamic hypothesis Figure 2 illustrates a dynamic model that explains epidemiological observations in human filariasis based on available experimental and epidemiological findings: (i) Each successful infection provokes an early immune response [3,15,16], which efficiently protects from further infections, explaining property 1. (ii) This immune response fades over time because of an immunological short-term memory [17,18], and each infection induces only a short period of resistance, explaining ongoing reinfection. The annual number of successfully establishing L3s (PER) is limited, even for high ATPs [2], explaining ‘property 2’. A fading degree of resistance requires the assumption of a protective threshold below which infection can occur again. There are currently no findings for such a threshold, simply because the immunological correlates of resistance are not exactly known. In a concept that assumes regulation originates only from the parasite, the threshold would describe a state in which the parasite population in a host switches between promoting and suppressing the development of a new L3. The duration of the protective period is the reciprocal of the maximum PER. Consider the following example: in
Figure 1. The proportion of ‘successful’ parasites. Relationship between the annual transmission potential (ATP, i.e. the number of parasites inoculated per year by vectors into the human host) and the parasite establishment rate (PER, i.e. the number of adult parasites successfully establishing per year in adult human hosts). If all inoculated parasites were to establish in the human host, the dashed line in blue would hold. The vast majority of L3s, however, are neutralised (green) so that very few of them establish per year in the human host (red). It is still unclear why the PER approaches an almost constant value at higher ATPs.
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Figure 2. Hypothesis on the infection process with filariae. Unsuccessful and successful infection events are shown as grey and red arrows, respectively, below each time axis (1 year). Successful infection with an L3 causes an early immune response (green), which temporarily protects the host against reinfection. The immunological memory diminishes over weeks. Reinfection can occur when the degree of resistance falls below the protective threshold (blue dashed lines). (a) The annual transmission potential (ATP) is low, representing areas with hypoendemic transmission. Periods of susceptibility exist between infection events; thus, the parasite establishment rate (PER) does not reach its maximum. (b) The ATP has a borderline value such that super-infections occur as soon as the degree of resistance falls below the protective threshold. Periods of susceptibility do not exist; thus, the PER has reached its maximum of four parasites per year. (c) The ATP is high, representing areas with hyperendemic transmission. The PER is identical to the borderline case (b) because there are no additional periods of susceptibility. (d) Relationship between ATP and PER for the scenarios (a)–(c). Regulatory processes, the genetic background of hosts or heterogeneities in general might lengthen or shorten periods of resistance by modulating the threshold, the slope in the loss of resistance, or the peak height [17]. The latter case, which increases the PER and might be caused by immunosuppression, is represented in (e). Values were chosen arbitrarily for the purpose of illustration.
onchocerciasis the PER, averaged over the adult population, is between two and four adult female parasites per person per year under moderate-to-heavy transmission (Box 1). This corresponds to a protective period of 3–6 months. The PER can be estimated from epidemiological data, whereas the duration of the protective period could be experimentally determined. Experiments and studies required Further experiments are necessary to confirm or reject this hypothesis. To mimic the infection dynamics observed in human hosts, repeated inoculations with small doses of 352
infective larvae (trickle inoculations) should be performed in appropriate animal models over relevant periods of time [4,19]. Investigating immunological and genetic aspects requires well-established mouse models [20] or natural hosts, such as cotton rats or jirds. The natural hosts are particularly relevant for longitudinal studies because filariae in rats or jirds typically persist longer (>1 year) than in mouse models (weeks). The minimal interval between successful infections, which is related to a protective threshold (see Figure 2), can be determined by quantifying worm burdens after trickle infections at consecutive time points. In particular, the early fate of the invading L3s
Opinion (i.e. their development or elimination) should be monitored longitudinally. First approaches in this direction have been published recently [17,21]. Immunological correlates for host resistance are not precisely defined. The classical approach would be to transfer humoral and cellular host factors from infected to naive animals, which are infected after the transfer. Recent techniques, such as gene expression profiling with microarrays, enable more explorative approaches that do not restrict on classical immune markers but can identify a broader spectrum of potential correlates. Statistical methodologies, such as cluster analyses, can then point at associations between potential correlates. The search for factors associated with infection should not focus on host resistance alone but also should consider correlates for the hosts’ parasite burden. This is relevant for the long-living adult filariae in humans; the filariae are difficult to diagnose and quantify but determine the longterm intervention success or the risk of reinfection in areas of the GPELF and the African Programme for Onchocerciasis Control (http://www.apoc.bf/en/). Correlates for the infection intensity also would allow for migration studies, which have been published only for lymphatic filariasis using IgG4 as a correlate for infection intensity [22]. Extending aspects and vaccine development The dynamic hypothesis can provide a framework in which existing concepts (see Box 2) can be unified: concomitant immunity is not the presence of resistance and susceptibility at the same time but results from an alternation of gain and loss of resistance. Likewise, immunological tolerance or unresponsiveness is not a coexistence of resistance and susceptibility but results from an alternation between these two states in a host. Using these dynamic interpretations, the distinction between the terms ‘concomitant immunity’ and ‘immunological tolerance’ blurs. These concepts might emerge from a common dynamic principle. Heterogeneities in these concepts can originate from individual differences in the intensity of the immunological response (e.g. higher response levels extend the length of time until the protective threshold is crossed, and lower response levels increase the PER), the rate at which resistance fades (e.g. by a response that is correlated with age or coinfections; increasing this rate increases the PER) or the level of the protective threshold (e.g. by regulatory host– parasite interactions; raising the threshold increases the PER). From this point of view, immunosuppression might result not only from suppressed immune responses (Figure 2e) but also from an accelerated loss of resistance or from processes raising the threshold. Processes that operate in the opposite direction can explain the existence of ‘putative immunes’ or ‘endemic normals’. An immunological short-term memory might explain why filariasis research has faced enormous problems in developing a successful vaccine despite various options that seemed to be within reach [23–25]. If natural immunity against filarial parasites rapidly faded over time, vaccine-mediated immunity could inherit the same dynamics and require booster vaccinations as frequently as natural infections can occur (i.e. at intervals of months or even weeks). Thus, investigations into early immune
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Box 2. Existing hypotheses, concepts and open questions It still is not understood why only a tiny proportion of infective-stage larvae successfully infect the host. This phenomenon is formulated by the concept of concomitant immunity [29,30], describing the simultaneous presence of chronic infection with and resistance against newly incoming infective larvae [31,32]. Regarding the other parasite stages (adult parasites and microfilariae), the balanced presence of resistance and susceptibility is represented by the concept of immunological tolerance [6,33,34]. It describes a state in which immunological processes in the host control the parasite population while, at the same time, tolerating a certain number of parasites. Immunological tolerance involves immunoregulation, which seems to result from a balanced interaction of host factors such as lymphocytes – in particular T cells – and cytokines, as well as immunomodulatory parasite components [24,35,36]. For instance, immunosuppression protects hosts against self-destructive immune responses like inflammation and autoimmunity [11]. By contrast, it promotes the establishment and persistence of parasites in hosts [37]. Up to today, it cannot be ruled out that only the parasite regulates its density in the host, without any active control through host immunity or resistance. Resistance in human hosts is suggested by the existence of ‘putative immunes’ or ‘endemic normals’ – hosts who are free of demonstrable infection and/or disease despite living in endemic areas [38,39]. The concept of resistance receives support from the observation that immunological factors improve after chemotherapy of onchocerciasis with the microfilaricide Ivermectin (Ivermectin-facilitated immunity [10,40]). This observation might be specific to humans infected with O. volvulus; in cattle infected with O. ochengi, the cessation of treatment leads to an increased posttreatment susceptibility [41], which is difficult to bring in line with Ivermectin-facilitated immunity. The phrase ‘immunity against filariae’ is too general because it is a conglomerate of regulatory processes between the host, the parasite and its different life-cycle stages (e.g. resistance against L3s does not affect microfilaria or adult worms in the same host). Recent concepts address the role of early and unspecific immune responses [16]. The hypothesis presented in Figure 2 tries to explain infection with filariae by combining the concepts of concomitant immunity and early immune responses. The concept of a dynamically defined infection process offers a framework for unifying existing concepts like concomitant immunity, immunological tolerance and immunosuppression (e.g. see Figure 2e).
responses should be performed not only from a static perspective but also (more importantly) from a dynamic perspective that considers when resistance emerges and for how long it lasts. Acknowledgements We thank Chris Leary for helpful comments on this manuscript. This investigation was supported by the Deutsche Forschungsgemeinschaft (DFG DU1105/1–2).
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