The role of the spleen in malaria

The spleen is a complex organ that is perfectly adapted to selectively filtering and destroying senescent red blood cells (RBCs), infectious microorganisms and Plasmodium-parasitized RBCs. Infection by malaria is the most common cause of spleen rupture and splenomegaly, albeit variably, a landmark of malaria infection. Here, the role of the spleen in malaria is reviewed with special emphasis in lessons learned from human infections and mouse models.     

Imaging of the spleen in malaria

Splenomegaly, albeit variably, is a hallmark of malaria; yet, the role of the spleen in Plasmodium infections remains vastly unknown. The implementation of imaging to study the spleen is rapidly advancing our knowledge of this so-called “blackbox” of the abdominal cavity. Not only has ex vivo imaging revealed the complex functional compartmentalization of the organ and immune effector cells, but it has also allowed the observation of major structural remodeling during infections. In vivo imaging, on the other hand, has allowed quantitative measurements of the dynamic passage of the parasite at spatial and temporal resolution. Here, we review imaging techniques used for studying the malarious spleen, from optical microscopy to in vivo imaging, and discuss the bright perspectives of evolving technologies in our present understanding of the role of this organ in infections caused by Plasmodium.     

The importance of human FcgammaRI in mediating protection to malaria

The success of passive immunization suggests that antibody-based therapies will be effective at controlling malaria. We describe the development of fully human antibodies specific for Plasmodium falciparum by antibody repertoire cloning from phage display libraries generated from immune Gambian adults. Although these novel reagents bind with strong affinity to malaria parasites, it remains unclear if in vitro assays are predictive of functional immunity in humans, due to the lack of suitable animal models permissive for P. falciparum. A potentially useful solution described herein allows the antimalarial efficacy of human antibodies to be determined using rodent malaria parasites transgenic for P. falciparum antigens in mice also transgenic for human Fc-receptors. These human IgG1s cured animals of an otherwise lethal malaria infection, and protection was crucially dependent on human FcgammaRI. This important finding documents the capacity of FcgammaRI to mediate potent antimalaria immunity and supports the development of FcgammaRI-directed therapy for human malaria.     

Variant genes and the spleen in Plasmodium vivax malaria

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria, does not cytoadhere in the deep capillaries of inner organs and thus this malaria parasite must have evolved splenic evasion mechanism in addition to sequestration. The spleen is a uniquely adapted lymphoid organ whose central function is the selective clearance of cell and other particles from the blood, and microbes including malaria. Splenomegaly is a hallmark of malaria and no other disease seems to exacerbate this organ as this disease does. Besides this major selective clearance function however, the spleen is also an erythropoietic organ which, under stress conditions, can be responsible for close to 40% of the RBC populations. Data obtained in experimental infections of human patients with P. vivax showed that anaemia is associated with acute and chronic infections and it has been postulated that the continued parasitemia might have been sufficient to infect and destroy most circulating reticulocytes. We review here the basis of our current knowledge of variant genes in P. vivax and the structure and function of the spleen during malaria. Based on this data, we propose that P. vivax specifically adhere to barrier cells in the human spleen allowing the parasite to escape spleen-clearance while favouring the release of merozoites in an environment where reticulocytes, the predominant, if not exclusive, host cell of P. vivax, are stored before their release into circulation to compensate for the anaemia associated with vivax malaria.     

Variant genes and the spleen in Plasmodium vivax malaria

It is generally accepted that Plasmodium vivax, the most widely distributed human malaria, does not cytoadhere in the deep capillaries of inner organs and thus this malaria parasite must have evolved splenic evasion mechanism in addition to sequestration. The spleen is a uniquely adapted lymphoid organ whose central function is the selective clearance of cell and other particles from the blood, and microbes including malaria. Splenomegaly is a hallmark of malaria and no other disease seems to exacerbate this organ as this disease does. Besides this major selective clearance function however, the spleen is also an erythropoietic organ which, under stress conditions, can be responsible for close to 40% of the RBC populations. Data obtained in experimental infections of human patients with P. vivax showed that anaemia is associated with acute and chronic infections and it has been postulated that the continued parasitemia might have been sufficient to infect and destroy most circulating reticulocytes. We review here the basis of our current knowledge of variant genes in P. vivax and the structure and function of the spleen during malaria. Based on this data, we propose that P. vivax specifically adhere to barrier cells in the human spleen allowing the parasite to escape spleen-clearance while favouring the release of merozoites in an environment where reticulocytes, the predominant, if not exclusive, host cell of P. vivax, are stored before their release into circulation to compensate for the anaemia associated with vivax malaria.     

Wave expansion of CD34 progenitor cells in the spleen in rodent malaria

Defense against malaria depends upon amplification of the spleen structure and function for the clearance of parasitized red blood cells (pRBC). We studied the distribution and amount of CD34⁺ cells in the spleens of mice infected with rodent malaria. We sought to identify these cells in the spleen and determine their relationship to infection. C57BL/6J mice were infected with self-resolving, Plasmodium chabaudi CR, or one of the lethal rodent malaria strains, P. chabaudi AJ and P. berghei ANKA. We then recorded parasitemia, mortality, and the presence of CD34⁺ cells in spleen, as determined by immunohistochemistry and flow cytometry. In the non-lethal strain, the spleen structure was maintained during amplification, but disrupted in lethal models. The abundance of CD34⁺ cells increased in the red pulp on the 4th and 6th days p.i. in all models, and subsided on the 8th day p.i. Faint CD34⁺ staining on the 8th day p.i., was probably due to differentiation of committed cell lineages. In this work, increase of spleen CD34⁺ cells did not correlate with infection control.     

Plasmodium berghei: The spleen in sporozoite-induced immunity to mouse malaria

A/J mice were splenectomized (Splx) or sham-splenectomized (SSplx) prior to administration of a single injection of irradiated sporozoites. Following challenge 7 days after immunization, it was found that none of the splenectomized mice were protected whereas 50% of the sham-splenectomized and intact animals were resistant to challenge. In another series of experiments similar groups, along with mice splenectomized just prior to challenge, were injected with 1.5 × 10⁵ irradiated sporozoites over a 5 week period. This resulted in protection of (1) 60–100% of the animals splenectomized before immunization, and (2) 90–100% protection of the animals splenectomized prior to challenge, as well as the intact and sham-splenectomized mice. Serum levels of antisporozoite antibodies (CSP and SNA) increased during immunization of the intact animals. Only 15–20% of the animals splenectomized prior to immunization presented positive CSP reactions and little if any sporozoite neutralizing activity (SNA) was detected. Serum from intact animals immunized and found resistant to sporozoite challenge was used for passive transfer studies. Immune serum recipients were challenged with small numbers of sporozoites. Only one out of 18 recipients was protected against sporozoite challenge.     

Transmission and immunity: the importance of heterogeneity in the fight against malaria

The complex relationship between transmission and parasite prevalence in humans is an important issue. Using a large dataset matching estimates of malaria transmission and Plasmodium falciparum prevalence in African children, a stimulating study published in Nature provides evidence that heterogeneity in susceptibility crucially determines the prevalence of infection. Moreover, it suggests that children who clear infections are not immune to new infections, irrespective of the amount of transmission. It is important to question the relevance of such results based on mathematical models when discussing host-parasite interactions, especially their implications for public health interventions.     

The relative importance of the bone marrow and spleen in the production and dissemination of B lymphocytes.

The relative importance of the bone marrow and spleen in the production of B lymphocytes was investigated in guinea pigs by the combined use of [³H]TdR radio-autography and fluorescent microscopy after the staining of B cells by FITC-F(ab′)₂-goat-anti-guinea pig Ig. Large and small lymphoid cells possess sIg in the marrow and spleen but B cell turnover in the marrow exceeds that in the spleen. That newly generated bone marrow B cells are not derived from an extramyeloid bursa equivalent was demonstrated by the absence of [³H]TdR labeled B cells in tibial marrow 72 hr after [³H]TdR was administered systemically, while the circulation to the hind limbs was occluded. Pulse and chase studies with [³H]TdR showed that large marrow B cells are derived from sIg-negative, proliferating precursors resident in the bone marrow and not from the enlargement of activated small B lymphocytes. The acquisition of [³H]TdR by splenic B cells lagged behind that observed in the marrow. Three days after topical labeling of tibial and femoral bone marrow with [³H]TdR, a substantial proportion of splenic B cells were replaced by cells that had seeded there from the labeled marrow. The studies unequivocally identify the bone marrow as the organ of primary importance in B cell generation and indicate that in the guinea pig rapidly renewed B lymphocytes of the spleen are replaced by lymphocytes recently generated in bone marrow. The rate of replacement of B lymphocytes in the lymph node by cells newly generated in the bone marrow takes place at a slower tempo than in the spleen.     

The unexpected importance of mosquito oviposition behaviour for malaria: non-productive larval habitats can be sources for malaria transmission

Background

Mosquitoes commute between blood-meal hosts and water. Thus, heterogeneity in human biting reflects underlying spatial heterogeneity in the distribution and suitability of larval habitat as well as inherent differences in the attractiveness, suitability and distribution of blood-meal hosts. One of the possible strategies of malaria control is to identify local vector species and then attack water bodies that contain their larvae.

Methods

Biting and host seeking, not oviposition, have been the focus of most previous studies of mosquitoes and malaria transmission. This study presents a mathematical model that incorporates mosquito oviposition behaviour.

Results

The model demonstrates that oviposition is one potential factor explaining heterogeneous biting and vector distribution in a landscape with a heterogeneous distribution of larval habitat. Adult female mosquitoes tend to aggregate around places where they oviposit, thereby increasing the risk of malaria, regardless of the suitability of the habitat for larval development. Thus, a water body may be unsuitable for adult mosquito emergence, but simultaneously, be a source for human malaria.

Conclusion

Larval density may be a misleading indicator of a habitat's importance for malaria control. Even if mosquitoes could be lured to oviposit in sprayed larval habitats, this would not necessarily mitigate – and might aggravate – the risk of malaria transmission. Forcing mosquitoes to fly away from humans in search of larval habitat may be a more efficient way to reduce the risk of malaria than killing larvae. Thus, draining, fouling, or filling standing water where mosquitoes oviposit can be more effective than applying larvicide.     

The importance of importance

Failure to distinguish between 'importance' and 'intensity' of competition has hindered our ability to resolve key questions about the role interactions may play in plant communities. Here we examine how appropriate application of metrics of importance and intensity is integral to investigating key theories in plant community ecology and how ignoring this distinction has lead to confusion and possibly spurious conclusions. We re-explore the relationship between competition intensity and importance for individuals across gradients, and apply our review of concepts to published data to help clarify the debate. We demonstrate that competition importance and intensity need not be correlated and show how explicit application of the intensity and importance of competition may reconcile apparently incompatible paradigms.     

The spleen of the cowbird

A histological study of the spleen of the Brown-headed cowbird, Molothrus ater, is presented. One of the most striking differences from the mammalian spleen is the lack of trabeculae and of smooth muscle in the capsule which would suggest that the spleen is not an organ of storage or pumping of blood. Without trabeculae to foster the close association of the major arteries and veins, these vessels take separate courses. Their support is provided by elaboration of the collagenous and reticular fibres of the stroma. A peculiar ovoid structure, the ampulla, carries the blood from the terminal arterioles of the white pulp to both the sinusoids and the reticular cords of the red pulp so that both open and closed circulations are seen but the open circulation predominates. The ampulla has perforated walls consisting of a simple cuboidal endothelium surrounded by a dense reticular sleeve. Leucocytes were seen passing through the holes in the walls of the ampullae by diapedesis. It is suggested that the ampullae may be contractile and act as sphincters controlling the flow of blood through the spleen. The major functions of the spleen appear to be haemopoiesis, production of antibodies, and filtration of blood.     

An ecoregional classification for the state of Roraima, Brazil: the importance of landscape in malaria biology

Understanding the different background landscapes in which malaria transmission occurs is fundamental to understanding malaria epidemiology and to designing effective local malaria control programs. Geology, geomorphology, vegetation, climate, land use, and anopheline distribution were used as a basis for an ecological classification of the state of Roraima, Brazil, in the northern Amazon Basin, focused on the natural history of malaria and transmission. We used unsupervised maximum likelihood classification, principal components analysis, and weighted overlay with equal contribution analyses to fine-scale thematic maps that resulted in clustered regions. We used ecological niche modeling techniques to develop a fine-scale picture of malaria vector distributions in the state. Eight ecoregions were identified and malaria-related aspects are discussed based on this classification, including 5 types of dense tropical rain forest and 3 types of savannah. Ecoregions formed by dense tropical rain forest were named as montane (ecoregion I), submontane (II), plateau (III), lowland (IV), and alluvial (V). Ecoregions formed by savannah were divided into steppe (VI, campos de Roraima), savannah (VII, cerrado), and wetland (VIII, campinarana). Such ecoregional mappings are important tools in integrated malaria control programs that aim to identify specific characteristics of malaria transmission, classify transmission risk, and define priority areas and appropriate interventions. For some areas, extension of these approaches to still-finer resolutions will provide an improved picture of malaria transmission patterns.     

Testosterone-induced persistent susceptibility to Plasmodium chabaudi malaria: Long-term changes of lincRNA and mRNA expression in the spleen

Testosterone (T) is known to induce persistent susceptibility to blood-stage malaria of Plasmodium chabaudi in otherwise resistant female C57BL/6 mice, which is associated with permanent changes in mRNA expression of the liver. Here, we investigate the spleen as the major effector against blood-stage malaria for any possible T-induced long-term effects on lincRNA and mRNA expression. Female C57BL/6 mice were treated with T for 3 weeks, then T was withdrawn for 12 weeks before challenging with P. chabaudi. LincRNA and mRNA expression was examined after 12 weeks of T-withdrawal and after subsequent infections using Agilent whole mouse genome oligo microarrays. Our data show for the first time long-term effects of T on lincRNA expression evidenced directly as persistent changes after T-withdrawal for 12 weeks and indirectly as altered responsiveness of expression to P. chabaudi infections. There are 3 lincRNA-species upregulated and 10 lincRNAs downregulated by more than 2-fold (p < 0.01). In addition, 11 and 10 mRNAs are persistently up- and downregulated by T, respectively. These changes remain not sustained during infections at peak parasitemia, when 15 other lincRNAs and 9 other mRNAs exhibit an altered expression. The only exception is the Tnk1-mRNA encoding the non-receptor tyrosine kinase 1 that is persistently downregulated by 0.34-fold after T-withdrawal and that becomes upregulated by 5.9-fold upon infection at peak parasitemia, suggesting an involvement of tyrosine phosphorylation by Tnk1 in mediating long-term effects of T in the spleen. The T-induced changes in splenic mRNA expression are totally different to those previously observed in the liver. Collectively, our data support the view that T induces long-term organ-specific changes in both lincRNA and mRNA expression, that presumably contribute to organ-specific dysfunctions upon infection with blood-stage malaria of P. chabaudi.     

Fitness effects of endemic malaria infections in a wild bird population: the importance of ecological structure

1.?Parasites can have important effects on host populations influencing either fecundity or mortality, but understanding the magnitude of these effects in endemic host-parasite systems is challenging and requires an understanding of ecological processes affecting both host and parasite. 2.?Avian blood parasites (Haemoproteus and Plasmodium) have been much studied, but the effects of these parasites on hosts in areas where they are endemic remains poorly known. 3.?We used a multistate modelling framework to explore the effects of chronic infection with Plasmodium on survival and recapture probability in a large data set of breeding blue tits, involving 3424 individuals and 3118 infection diagnoses over nine years. 4.?We reveal strong associations between chronic malaria infection and both recapture and survival, effects that are dependent on the clade of parasite, on host traits and on the local risk of infection. 5.?Infection with Plasmodium relictum was associated with reduced recapture probability and increased survival, compared to P.?circumflexum, suggesting that these parasites have differing virulence and cause different types of selection on this host. 6.?Our results suggest a large potential survival cost of acute infections revealed by modelling host survival as a function of the local risk of infection. 7.?Our analyses suggest not only that endemic avian malaria may have multiple fitness effects on their hosts and that these effects are species dependent, but also that adding ecological structure (in this case parasite species and spatial variation in disease occurrence) to analyses of host-parasite interactions is an important step in understanding the ecology and evolution of these systems.