Although T helper 2 (Th2) cytokines are known to be critical in the generation of protective immunity against intestinal helminths in mouse models, it is unclear whether they are important in natural immunity against gut helminthiases in humans. Therefore, we investigated cytokine production in ex vivo whole-blood cultures in response to Ascaris lumbricoides antigen and mitogen in a cross-section of a community where the parasite is hyperendemic. The intensity of A. lumbricoides infection was significantly reduced after age 11 years. Levels of cytokines associated with Th2 lymphocytes (interleukin [IL]-4, IL-9, IL-10, and IL-13) demonstrated an inverse relationship with intensity of A. lumbricoides infection only in individuals aged >11 years. Furthermore, the IL-9, IL-10, and IL-13 produced in response to parasite antigen were of primary importance in this relationship. These findings promote a role for Th2-mediated responses in the age-dependent reduction of intestinal helminth infections in humans.

A revolutionary advance in ecological immunology is that postgenomic technologies now allow molecular mediators defined in laboratory models to be measured at the mRNA level in field studies of many naturally occurring species. Here, we demonstrate the application of such an approach to generate meaningful immunological profiles for wild mammals. We sampled a natural field vole population across the year (n = 307) and developed a battery of cellular assays in which functionally different pro- and anti-inflammatory signalling responses (transcription factors and cytokines) were activated and quantified by Q-PCR. Temporal trends were the strongest feature in the expression data, although some life history stages (mating vs. nonmating males and pregnant females) were also associated with significant variation. There was a striking set of significant negative associations between inflammatory mediators and condition indices reflecting packed erythrocyte volume and relative liver size, spleen size and splenocyte count. Grouped (principal component) measures of inflammatory and anti-inflammatory expression were high in winter, with minima in the breeding season that occurred earlier for grouped anti-inflammatory responses than for grouped inflammatory responses. Some individual immunological mediators also showed patterns unrelated to the breeding season or annual periodic cues. For example, interferon regulatory factor 5 (IRF5) expression declined throughout the study period, indicating a systematic trend in antimicrobial defences. Pinpointing the causes and consequences of such variation may help identify underlying environmental drivers of individual fitness and demographic fluctuation.

SummaryAlthough the molecules and cells involved in triggering immune responses against parasitic worms (helminths) remain enigmatic, research has continued to implicate expansions of T-helper type 2 (Th2) cells and regulatory T-helper (T reg ) cells as a characteristic response to these organisms. An intimate association has also emerged between Th2 responses and wound-healing functions. As helminth infections in humans are associated with a strong Th2/T reg immunoregulatory footprint (often termed a 'modified Th2' response), plausible links have been made to increased susceptibility to microbial pathogens in helminth-infected populations in the tropics and to the breakdowns in immunological control (allergy and autoimmunity) that are increasing in frequency in helminth-free developed countries. Removal of helminths and their anti-inflammatory influence may also have hazards for populations exposed to infectious agents, such as malaria and influenza, whose worst effects are mediated by excessive inflammatory reactions. The patterns seen in the control of helminth immunity are discussed from an evolutionary perspective. Whilst an inability to correctly regulate the immune system in the absence of helminth infection might seem highly counter-adaptive, the very ancient and pervasive relationship between vertebrates and helminths supports a view that immunological control networks have been selected to function within the context of a modified Th2 environment. The absence of immunoregulatory stimuli from helminths may therefore uncover maladaptations that were not previously exposed to selection.

A large-scale field study in naturally occurring vole populations identified gene expression changes over time and demonstrates how wild mammals exhibit tolerance to chronic parasite infections.

Pathogens are believed to drive genetic diversity at host loci involved in immunity to infectious disease. To date, studies exploring the genetic basis of pathogen resistance in the wild have focussed almost exclusively on genes of the Major Histocompatibility Complex (MHC); the role of genetic variation elsewhere in the genome as a basis for variation in pathogen resistance has rarely been explored in natural populations. Cytokines are signalling molecules with a role in many immunological and physiological processes. Here we use a natural population of field voles (Microtus agrestis) to examine how genetic diversity at a suite of cytokine and other immune loci impacts the immune response phenotype and resistance to several endemic pathogen species. By using linear models to first control for a range of non-genetic factors, we demonstrate strong effects of genetic variation at cytokine loci both on host immunological parameters and on resistance to multiple pathogens. These effects were primarily localized to three cytokine genes (Interleukin 1 beta (Il1b), Il2, and Il12b), rather than to other cytokines tested, or to membrane-bound, non-cytokine immune loci. The observed genetic effects were as great as for other intrinsic factors such as sex and body weight. Our results demonstrate that genetic diversity at cytokine loci is a novel and important source of individual variation in immune function and pathogen resistance in natural populations. The products of these loci are therefore likely to affect interactions between pathogens and help determine survival and reproductive success in natural populations. Our study also highlights the utility of wild rodents as a model of ecological immunology, to better understand the causes and consequences of variation in immune function in natural populations including humans.

Background: Immunological analyses of wild populations can increase our understanding of how vertebrate immune systems respond to 'natural' levels of exposure to diverse infections. A major recent advance in immunology has been the recognition of the central role of phylogenetically conserved toll-like receptors in triggering innate immunity and the subsequent recruitment of adaptive response programmes. We studied the cross-sectional associations between individual levels of systemic toll-like receptor-mediated tumour necrosis factor alpha responsiveness and macro-and microparasite infections in a natural wood mouse (Apodemus sylvaticus) population.

The chronic nature of intestinal nematode infections suggests that these parasites have evolved sophisticated immunomodulatory strategies. The induction of regulatory responses during chronic helminth infections could be advantageous to the host by minimising damage incurred by these organisms. Regulation of the host immune response to infection could however be exploited by parasites as a survival strategy. We have explored both these aspects using the murine model of whipworm infection, Trichuris muris. Of the three laboratory isolates of T. muris in use, two (the E (Edinburgh) and J (Japan - sub-cultured from E) are readily expelled by C57BL/6 mice whereas the third, the S isolate (Sobreda - isolated from wild mice in Portugal) survives for much longer. The existence of the T. muris isolates thus presents a powerful tool to explore the mechanisms underlying chronic infection in a single strain of mouse. Here we show that S isolate infected mice have increased numbers of Foxp3+ T cells in the gut compared to mice infected with the E isolate. Treatment of mice infected with the S isolate with either anti-CD25 or anti-GITR exacerbated intestinal pathology, and, in addition, mice treated with anti-GITR were able to expel worms more rapidly, implying the release of local effector mechanisms from a regulatory influence. Thus our data show for the first time that T regulatory cells protect the host from worm driven intestinal pathology. In addition, our data reveal a subversion of this damage-limiting response by the S isolate to facilitate its own survival.

Carefully chosen immunological measurements, informed by recent advances in our understanding of the diversity and control of immune mechanisms, can add great interpretative value to ecological studies of infection. This is especially so for co-infection studies, where interactions between species are often mediated via the host's immune response. Here we consider how immunological measurements can strengthen inference in different types of co-infection analysis. In particular, we identify how measuring immune response variables in field studies can help reveal inter-species interactions otherwise obscured by confounding processes operating on count or prevalence data. Furthermore, we suggest that, due to the difficulty of quantifying microbial pathogen communities in field studies, innate responses against broad pathogen types (mediated by pattern response receptors) may be useful quantitative markers of exposure to bacteria and viruses. An ultimate goal of ecological co-infection studies may also be to understand how dynamics within host-parasite assemblages emerge from trade-offs involving different arms of the immune system. We reflect on the phenotypic measures that might best represent levels of responsiveness and bias in immune function. These include mediators associated with different T-helper cell subsets and innate responses controlled by pattern response receptors, such as the Toll-like receptors (TLRs).