Tetrazolium salts have become some of the most widely used tools in cell biology for measuring the metabolic activity of cells ranging from mammalian to microbial origin. With mammalian cells, fractionation studies indicate that the reduced pyridine nucleotide cofactor, NADH, is responsible for most MTT reduction and this is supported by studies with whole cells. MTT reduction is associated not only with mitochondria, but also with the cytoplasm and with non-mitochondrial membranes including the endosome/lysosome compartment and the plasma membrane. The net positive charge on tetrazolium salts like MTT and NBT appears to be the predominant factor involved in their cellular uptake via the plasma membrane potential. However, second generation tetrazolium dyes that form water-soluble formazans and require an intermediate electron acceptor for reduction (XTT, WST-1 and to some extent, MTS), are characterised by a net negative charge and are therefore largely cell-impermeable. Considerable evidence indicates that their reduction occurs at the cell surface, or at the level of the plasma membrane via trans-plasma membrane electron transport. The implications of these new findings are discussed in terms of the use of tetrazolium dyes as indicators of cell metabolism and their applications in cell biology.

Tetrazolium dyes as tools in cell biology: new insights into their cellular reduction.

An understanding of the relationship between the transmission dynamics of infectious agents and herd immunity provides a template for the design of effective control programmes based on mass immunization. Mathematical models of the spread and persistence of infection provide important insights into the problem of how best to protect the community against disease.

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Vaccination and herd immunity to infectious diseases

A well recognized feature of the immune response to parasitic helminth infections, including schistosomiasis, is the production of large amounts of specific and nonspecific IgE1,2. Immunological pathways involving IgE can lead to damage to the developing schistosomulum and it has been suggested that responses involving IgE could have evolved as protection against helminth infections. There has been no epidemiological evidence to support this idea and the only significant IgE responses known in man are those involved in the pathogenesis of allergic disease. Here we measure serological response during reinfection with S. haematobium and demonstrate that IgE antibodies in man can be beneficial. Our results support the hypothesis that the slow build-up of IgE to high levels and the early production of IgG4 antibodies, which may block IgE pathways are responsible for delaying the development of protective immunity to S. haematobium.

Human IgE, IgG4 and resistance to reinfection with Schistosoma haematobium

When rodents are infected with the nematode Nippostrongylus brasiliensis, large numbers of eosinophils appear in their blood and lungs and their serum immunoglobulin E (IgE) is increased. Injection of a monoclonal antibody to interleukin-5 completely suppressed the blood eosinophilia and the infiltration of eosinophils in the lungs of parasitized mice but had no effect on serum IgE. In contrast, an antibody to interleukin-4 inhibited parasite-induced IgE but not the eosinophilia. These results show that interleukin-5 is important in eosinophil production in vivo and that IgE and eosinophil production are regulated by different cytokines produced by the TH2 subset of CD4-expressing T cells.

https://science.sciencemag.org/content/sci/245/4915/308.full.pdf

Antibody to interleukin-5 inhibits helminth-induced eosinophilia in mice.

Linkage between immunoglobulin E responses underlying asthma and rhinitis and chromosome 11q.

Nippostrongylus brasiliensis: A review of immunity and the host/parasite relationship in the rat

Most of the organisms of the phylum Nematoda are free living, but some are animal or plant parasites of major importance to man. During their life cycle all nematodes undergo a series of moults in which they shed an external cuticle, consisting of an outermost membrane-like layer of unknown composition and a series of fibrillar layers similar to collagens. Because of this structure, the cuticle has been viewed as an acellular exoskeleton with rather inert molecular components. However, observations have shown that it contains enzymes and sometimes haemoglobin, and that nutrients are absorbed through it in the infective larvae and adut stages of Brugia pahangi. It is bound by complement and antibody, resulting in the adherence of leukocytes, and antibody-dependent cell-mediated reactions damage the cuticle of newborn larvae of Trichinella spiralis and the microfilariae of Dipetalonema viteae and Litomosoides carinii. We report here that the surface of the cuticle of the parasitic nematode Trichinella spiralis expresses protein molecules which change qualitatively following the moulting process, and quantitatively during growth of the worms within one stage. Also, surface proteins are released in vitro at a rate which depends on the conditions of culture of the worms.

Changing proteins on the surface of a parasitic nematode

RECENT investigations here have shown that rats, monkeys and sheep infected with helminth parasites produce an antibody with many of the properties of human reagin. Naturally occurring persistent antibodies of this type have not been demonstrated previously in animals, although in rats an antibody similar in its biological properties but transitory in appearance can be stimulated when antigen is injected with Haemophilus pertussis killed organisms as adjuvant1,2. Like human reagins3, the antibodies stimulated by helminth infections become rapidly and apparently permanently attached to the skin of animals of the species in which they are stimulated, but they are incapable of sensitizing guineapig skin in passive cutaneous anaphylaxis.

Reagin-like Antibodies in Animals Immune to Helminth Parasites

The functional changes in splenic lymphoid populations from mice infected with T. brucei strain S42 were studied throughout the 3 weeks of infection. Within a week of infection, proliferation of B and T cells profoundly increased as shown by 3H-labelled thymidine incorporation and fluorescent staining of surface Ig; the spleen cells secreted high levels of both IgM and IgG immediately cells were put into culture; but with progressing infection this Ig production declined. The early effect on T cells was reflected by lack of responsiveness to PHA. B-cell potential was studied in low-density cultures treated with lipopolysaccharide (E. coli). Normal spleen cells proliferate extensively in these cultures with subsequent secretion of IgG as well as IgM. The ability to proliferate and produce Ig in response to LPS was severely depressed by day 7 and almost totally absent by day 12 of infection. Removal of T cells from the spleen cells obtained early in infection partly restored the response to LPS but as the infection neared its fatal end, B-cell potential appeared to become exhausted. Macrophages obtained from infected mice even early in infection profoundly depressed the ability of normal spleen cells to proliferate and secrete immunoglobulin in LPS cultures. The general immunodepressing effect of trypanosomes can be attributed to clonal exhaustion of B-cell potential caused by an undefined blastogenic stimulus from the parasites which may operate at least in part by the generation of suppressive T cells and macrophages.

Suppressor cells and loss of B-cell potential in mice infected with Trypanosoma brucei.

THE surface of parasites seems to be especially important in the intricate relationship which is known to exist between these organisms and their hosts. This is particularly clear in the case of African trypanosomes, which are able to vary the antigenic glycoproteins of their surface sequentially during infection1. It is also known that the surface of schistosomes changes during infection in parallel with alterations in the susceptibility of this trematode to the action of immunity2. By comparison with these and other parasites, little is known of the properties of the surface of nematodes nor of its role in the host-parasite interaction. Both free-living and parasitic nematodes are bounded by a non-cellular cuticular layer which is probably living because it may contain, for example, enzymes and haemoglobin3. The cuticle, which is shed and replaced as the nematodes pass through the various moults in their life cycle, consists of fibrous collagen-like proteins, thought unlikely by some authors to be highly immunogenic in the host4. However, we now report that the serum of animals infected with the nematodes Trichinella spiralis and Nippostrongylus brasiliensis contain antibodies to nematode cuticle, each specific to the surface of a particular stage in the life cycle of the parasites and capable of mediating attack by inflammatory cells against the nematode surface.

Bridget M. Ogilvie

Papers

Nippostrongylus brasiliensis: A review of immunity and the host/parasite relationship in the rat

Changing proteins on the surface of a parasitic nematode

Reagin-like Antibodies in Animals Immune to Helminth Parasites

Suppressor cells and loss of B-cell potential in mice infected with Trypanosoma brucei.

Immunological properties of the surface of parasitic nematodes.