Derek Wakelin

The Immune Response

Trichinella spiralis is a highly successful parasite in that it can establish, develop, and reproduce in a wide range of vertebrate hosts. This low host specificity implies that the organism has the ability to adapt to a variety of environmental conditions and hence is relatively unaffected by many of those aspects of natural immunity that impose a more rigid specificity on other species. However, in common with all metazoan parasites, T. spiralis presents the host with a complex antigenic stimulus and thus evokes a powerful immune response. The complexity of the stimulus arises not only from the diversity of antigens present at any one time, but also from the fact that antigens may show stage specificity, changing qualitatively and quantitatively throughout development, and may be released in different tissues of the body. Many components of the immunological response probably have little or no effect on either host or parasite, though they may prove useful in diagnosis; some are deleterious to the parasite and provide protective resistance against infection, yet others are actually or potentially harmful to the host, either from their involvement in immunopathological reactions or through their modulation of unrelated immune responses. This chapter will concentrate on those responses that confer resistance on the host; immune responses in the contexts of diagnosis and pathology are dealt

Enterochromaffin cell hyperplasia and decreased serotonin transporter in a mouse model of postinfectious bowel dysfunction.

Abstract  Patients with postinfective irritable bowel syndrome and Trichinella spiralis‐infected mice share many features including visceral hypersensitivity and disordered motility. We assessed enterochromaffin (EC) numbers and serotonin transporter (SERT) using National Institute of Health (NIH) female mice studied for up to 56 days post‐T. spiralis infection. The effects of steroid treatment and the T‐cell dependence of the observed responses were assessed by infection of hydrocortisone‐treated or T‐cell receptor knock out [TCR (β×δ) KO] animals. Enterochromaffin cell density in uninfected animals increased from duodenum 10.0 cells mm−2 (5.9–41.0) to colon 61.8. (46.3–162) cells mm−2P < 0.0001. Infection increased duodenal and jejunal counts which rose to 37.3 (22–57.7) cells mm−2 and 50.6 (7–110.8) cells mm−2, respectively, at day 14. Infection significantly reduced jejunal SERT expression, with luminance values falling from 61.0 (45.1–98.3) to a nadir of 11.6 (0–36.0) units at day 9, P < 0.001. Specific deficiencies in all T cells reduced EC hyperplasia and abrogated infection‐induced mastocytosis. Thus infection induced inflammation increases EC numbers, as has been reported in PI‐IBS, and reduces SERT. This may increase mucosal 5HT availability and contribute to the clinical presentation of PI‐IBS.

Genetic control of immunity to helminth infections.

Immune responses to infections with par~zsitic worms show well defined genetic control. In man, domestic animai~ and experimental rodents, the level and na:ture of these responses may differ markedly between individuals or be- tween breeds and strains, and be reflected in the overall outcome of the infection in terms of resis- tance, susceptibility and pathology. A basic tenet of evolutionary biology is that populations embrace a substantial degree of genetic diversity upon which natural selection can act to maximize the fitness of particular populations within particular en- vironments. An important element of such diversity is seen in the differing abilities of individuals to cope with infectious diseases, an element which par~,sitologists have rec- ognized much later th~xl their colleagues in virology, bacteriology and plant pathology. (Genetic diversity wiflfin parasite popula- tions probably has equal significance in the context of the evolulJLon and stability of host-parasite relationslfips but will not be considered here.) Although the existence of host variation in respo~Lse to helminth infec- tions has been recorded empirically for some time, the systematic exploration and exploitation of this phenomenon is com- paratively recent. It is now widely recog- nized that a substantial part of this variation has its origins in differential immune re- sponsiveness to infection and is genetically determined. Progress in understanding the genetic control of variation has gone hand- in-hand with analysis of the immunological mechanisms involved; indeed the two pro- vide reciprocally fruit_tiff fields of research 1. Inevitably attention has concentrated largely upon the genetic control of resis- tance elicited by infection or by vaccination, but immunosuppressive and im- munopathological phenomena have also been examined. Much of the work has been carried out in experimental systems using laboratory rodents and particularly the mouse, for which a wide variety of defmed strains is available (Table 1). Important studies have also been made in domestic ani- mals and in man. The extent of the variation of responses to infection that can occur can be illustrated by reference to the nematode Trichinella spiralis in the mouse, an experimental system which has been studied in greater detail than any other (Table 2). Variation exists in almost every parameter that has been moni- tored, but it is important to note that, as far as the infection itself is concerned, variation is essentially quantitative, not qualitative. All strains of mice so far tested (excluding mutant strains) terminate the intestinal phase of infection by means of an immune response, although some take considerably longer than others to do so and may, in con- sequence, acquire heavier burdens of larvae in the muscles. Qualitative variation has been recorded in other systems, for example individuals of certain mouse strains appear completely incapable of controlling infec- tion with Trichuris muris 3.

Immunopathology of intestinal helminth infection

The relationship between intestinal pathology and immune expulsion of gastrointestinal nematodes remains controversial. Parasite expulsion is associated with intestinal pathology in several model systems and both of these phenomena are T cell dependent. However, while immune expulsion of gastrointestinal helminth parasites is usually associated with Th2 responses, the effector mechanisms directly responsible for parasite loss have not been elucidated. In contrast, the intestinal pathology observed in many other disease models closely resembles that seen in helminth infections, but has been attributed to Th1 cytokines. We have used infection with the nematode Trichinella spiralis in mice defective for cytokines to demonstrate that although parasite expulsion is indeed IL‐4 dependent, contrary to expectations, the enteropathy is also regulated by IL‐4. Furthermore, abrogation of severe pathology in iNOS deficient and TNF receptor defective animals does not prevent parasite expulsion. TNF and iNOS are therefore involved in intestinal pathology in nematode infections, apparently under regulation by IL‐4 and Th2 mediated responses. Therefore, it appears that the IL‐4‐dependent protective response against the parasite operates by a mechanism other than merely the gross degradation of the parasites environment brought about by the immune enteropathy. However, it remains important to elucidate the protective mechanisms involved in parasite expulsion, which are still unclear.

Interactions involving intestinal nematodes of rodents: experimental and field studies.

Multiple species infections with parasitic helminths, including nematodes, are common in wild rodent populations. In this paper we first define different types of associations and review experimental evidence for different categories of interactions. We conclude that whilst laboratory experiments have demonstrated unequivocally that both synergistic and antagonistic interactions involving nematodes exist, field work utilizing wild rodents has generally led to the conclusion that interactions between nematode species play no, or at most a minor, role in shaping helminth component communities. Nevertheless, we emphasize that analysis of interactions between parasites in laboratory systems has been fruitful, has made a fundamental contribution to our understanding of the mechanisms underlying host-protective intestinal immune responses, and has provided a rationale for studies on polyparasitism in human beings and domestic animals. Finally, we consider the practical implications for transmission of zoonotic diseases to human communities and to their domestic animals, and we identify the questions that merit research priority.

Immune control of murine coccidiosis: CD4 + and CD8 + T lymphocytes contribute differentially in resistance to primary and secondary infections

The effect of treatment with monoclonal antibodies (Mabs) which deplete CD4+ or CD8+ T lymphocytes, on infections with Eimeria spp. was examined in NIH mice. Treatment with anti-CD4 Mab increased susceptibility to primary infections with E. vermiformis or E. pragensis and reduced the subsequent resistance of the mice to homologous challenge. Similar treatment of immune mice did not affect their resistance to re-infection but this was reduced in mice depleted of CD8+ T lymphocytes. In mice immunized with E. vermiformis the effect of CD8(+)-depletion was very slight, apparent only as the presence of small numbers of oocysts in the faeces of some mice; in mice immunized with E. pragensis there was a small, though significant, increase in oocyst production, compared with controls and anti-CD4-treated groups. These results confirm the importance of mechanisms involving the function of CD4+ T lymphocytes in the control of primary infections with Eimeria spp. and indicate that CD8+ cells play some part in the expression of resistance to reinfection. They also show that a major part of this resistance was not affected by either of the treatments given.

Functional correlations between mucosal mast cell activity and immunity to Trichinella spiralis in high and low responder mice

Summary Levels of intestinal mast cell protease (IMCP) were quantified in serum, gut tissue and in intestinal fluids taken from mice infected with Trichinella spiralis during primary and secondary infections. The ability to generate a mast cell response was dependent on the response phenotype of the mouse strain used. The mast cell response in rapid responder mice (NIH) occurred sooner and was more pronounced than in either intermediate (SWR) and low responder (BIO) mice. This pattern was also reflected in the concentration of IMCP found in various tissues examined. The correlations between IMCP concentrations in blood, and worm expulsion, are discussed.

Early cytokine responses during intestinal parasitic infections

Infections with gastro‐intestinal nematodes elicit immune and inflammatory responses mediated by cytokines released from T‐helper type‐2 (Th2) cells. In vitro assays of cells from the mesenteric lymph nodes (MLN) of experimentally infected rodents confirm that, after about 1 week, the dominant cytokine responses to mitogens and antigens are those associated with this Th‐cell subset. Polarization of the Th response in this way implies an initial local cytokine enviroment that favours Th2 development. However, experimental infections with Trichinella spiralis and Nippostrongylus brasiliensis show that, within 2 days of worms reaching the intestine, MLN cells (MLNC) respond with a Th1 rather than a Th2 response [i.e. there is an increase in mRNA for the type 1 cytokine interferon‐γ (IFN‐γ), and mitogen‐stimulated MLNC release IFN‐γ rather than interleukin‐5 (IL‐5)]. Antigen stimulation at this time does not elicit IFN‐γ release and the MLNC cannot adoptively transfer immunity. Within a few days the MLNC phenotype changes. There is a Th2 response (IL‐5 release) to both mitogen and antigen stimulation and MLNC can adoptively transfer immunity. Early release of IFN‐γ is T‐cell dependent, with CD4+ T cells playing the major role. The data are discussed in relation to factors regulating the mucosal response to invasion by parasites.

Absence of lysozyme (muramidase) in the intestinal Paneth cells of newborn infants with necrotising enterocolitis.

AIM: To determine immunocytochemically whether preterm and newborn infants with necrotising enterocolitis (NEC) show differences in numbers of lysozyme positive Paneth cells compared with normal controls, and to relate the findings to the possibility that lysozyme deficiency may facilitate the bacterial infections thought to be associated with this condition. METHODS: Tissues from 10 infants with NEC and from 11 matched controls were sectioned and stained immunocytochemically for lysozyme. Differences in the numbers of Paneth cells and degree of lysozyme positivity in the tissues were assessed. RESULTS: Tissues from NEC patients showed no, or very few, lysozyme positive Paneth cells, whereas controls showed strong positive staining. CONCLUSIONS: A deficiency or developmental defect in Paneth cells, resulting in an absence of lysozyme, may render the intestine more susceptible to bacterial infection, allowing organisms to adhere and translocate across the mucosa. Such enhancement of infection may contribute to the pathogenesis of NEC.

Distribution of intestinal mast cell proteinase in blood and tissues of normal and Trichinella-infected mice.

Summary A sensitive and specific enzyme‐linked immunosorbent assay (ELISA) was developed for mouse intestinal mast cell proteinase (IMCP). Specificity was demonstrated by the absence of immunoreactivity with extracts of isolated serosal mast cells (SMC), or with high concentrations (50 μg/ml) of the antigenically similar rat mast cell proteinases I or II. The small and large intestines in normal mice were the major sources of IMCP, there being little or no IMCP in non‐mucosal tissues. Concentrations of IMCP in normal (non‐parasitized) mice were low, but were increased 100–1000‐fold in intestines of mice infected 10 days earlier with Trichinella spiralis. The kinetic response of secreted IMCP into the blood of mice following infection with T. spiralis was also studied. Systemic release of IMCP coincided with the immune expulsion of adult worms from the intestine, and peak concentrations (9.45 μg/ml IMCP) occurred 9 days after infection. The tissue distribution of IMCP, its secretion into blood, and its enteric accumulation during parasite infection, are consistent with a mucosal mast cell (MMC) source for IMCP. The results are discussed in the context of similar findings for rat mast cell proteinase II.