Model experimental systems—E. coli, yeast, Drosophila, C. elegans, mice—are a bedrock of modern experimental biology. Enormous investment has been made in these models, and they underpin large, international research efforts. Choosing to work with the right model can make or break an academic career.

One of us (Mark Viney) has used a simple laboratory model—infecting laboratory rats (Rattus norvegicus) with the nematode parasite Strongyloides ratti (1–4). Moreover, the model had a purity: in the wild S. ratti infects R. norvegicus, so the model was simply moving parasites from wild rats into laboratory rats. Experiments using this elegantly simple model had discovered how the parasite’s life cycle was controlled by a range of environmental effects, including the host immune response (1). This was a fascinating discovery, illuminating an association of such intimacy, where a parasite used the host to control its life cycle decisions.

But there was a gnawing problem. Each rat that was infected became immune to S. ratti after about 5 weeks, expelling the worms and becoming resistant to reinfection (5). Because of this, parasites were maintained by continually infecting parasite-naïve rats. In contrast, in the wild, almost two-thirds of rats were infected with S. ratti (6), suggesting that wild animals were not becoming immune to S. ratti, as their lab cousins so readily were. If the immunology of lab rats infected with S. ratti was so different to that of wild rats, then what had this lab model really revealed about how the host immune response actually controlled the parasite’s life cycle in the wild? Intuitively, we reasoned that laboratory and wild rats were immunologically different: wild animals were leading stressful, resource-limited lives, and so were making low-level, insufficient immune responses against parasites, and that was why S. ratti infections were so common in the wild.

Intuition is one thing; what did the literature say? The answer—remarkably little. We could find almost no studies of the immune systems of wild rodents, and the little evidence that existed did not obviously support the hypothesis that wild rodents’ immune responses were impaired or impoverished. There were rather more studies of laboratory animals, livestock, as well as some wild animals (mainly birds), generally supporting the idea that immune resources were energetically and resource costly (7), and so one could argue that there were likely to be some resource-based constraints on wild rodents’ immune responses. But, overall, there was not any clear information on what immune responses wild rodents were making or how this might explain why S. ratti infections were so much more common in the wild than laboratory studies predicted that they would be. This disconnect from the lab to the wild in this hitherto, elegantly simple model—and that so little was known about wild animal immunology in general—spurred our determined look into wild rodents’ immune responses.

Eco-immunology is the study of the immune responses of wild animals in ecologically relevant settings. The broader rationale for studying the immune responses of wild animals is that these responses contribute to wild animals’ evolutionary fitness (7). Immune systems respond to antigenic stimuli received by an animal and so different individuals within a population, different populations of a species, and different species, will each have qualitatively and quantitatively different exposure to antigens. Mammalian immune systems are adaptable and will respond, in one way or another, to any antigen they encounter. However, selection will act to optimize the form and nature of these immune responses to maximize fitness, in the context of other selection pressures to which animals are subject (8, 9). This leads to the first key concept of eco-immunology: immunoheterogeneity, so that different species, different populations within species, or different individuals within populations may differ in the immune responses that they make. These differences will be seen (i) in the resting status of the immune system, observed as the standing immune response, but also (ii) when animals are compared for their responses to a standard antigenic challenge, for example to vaccination. Animals will differ in these regards for both intrinsic reasons (e.g., genetically) and for extrinsic, contextual reasons, for example their different exposure to infection, and other challenges during their lives. It is appropriate that immunoheterogeneity is the first key concept of eco-immunology, because understanding both the ultimate and proximate causes of this heterogeneity, and its consequences, is arguably the central question in eco-immunology.

Making immune responses is just one aspect of an animal’s physiological demands. In addition, animals have to grow, seek, and compete for food and mates, and reproduce. All of these processes require energy. It is clear that immune responses are energetically demanding (7), as too is growing, foraging, and reproducing. Therefore, with the assumption that many wild animals are energy limited, then these limited resources have to be deployed among these competing physiological and life-history processes in such a way as to maximize evolutionary fitness. This means that the immune responses of wild animals may be sub-maximal because of energy limitation. This is the second key concept of eco-immunology: that individuals’ immune responses may be constrained by resource availability, and more generally that immune responses can be affected by an animal’s wider physiological state. Further, variation among individuals in the quantities of resource they have and in how they allocate these resources to immune function (or not) importantly contributes to immunoheterogeneity (key concept 1, above).

Animals are exposed to and infected with a myriad of organisms—viruses, bacteria, fungi, protozoa, and metazoa, both internally and externally (7). The vast majority of these are usually harmless commensals which the immune system has to learn to ignore, but a small proportion are potentially dangerous pathogens that need to be recognized and, if possible, eliminated. Being infected with these organisms is a normal part of animal life, and animals and their immune systems have evolved in their presence. All of these organisms are sources of antigenic stimulus for the immune system, though the nature and degree of this stimulation varies among different types of organisms, their number, and their location in, or on, their host. This is the third key concept of eco-immunology: that animals have a potentially very large and diverse antigenic load. This antigenic load will differ among individuals within populations, among populations within species, and among species, and so also contribute to immunoheterogeneity (key concept 1, above).

Applying these three key concepts to laboratory and wild animals reveals substantial differences between them. In laboratory animals, immunoheterogeneity (key concept 1) among individuals within a species is minimized, if not extinguished, while in wild animals it exists in abundance. Laboratory animals are rarely resource limited (key concept 2), suggesting that their immune responses are maximal, while in wild animals we presume that resource limitation is widespread and so that immune responses may be sub-maximal (though they may be optimal) (10). Laboratory animals are usually maintained with a much reduced burden of infection (key concept 3), while wild animals have infections in abundance. Therefore, fully understanding the immune responses of animals and how they contribute to their fitness absolutely requires that they are studied in wild populations—this is the raison d’eˆtre of eco-immunology. However, the enormously detailed understanding of mammalian immune systems that has come from the extensive study of laboratory animals will play a central role in this endeavour, giving eco-immunologists both the tools to probe the immune systems of wild animals and some founding concepts from which to interpret data from these studies.

There are notably few studies of the immune systems of wild rodents, whereas immunological studies of laboratory rodents abound. Specifically, a Web of Knowledge search for publications whose titles and abstracts mention “mouse/mice” and “immun-” finds 19,997 papers in 2016 alone (and 187,154 between 2006 and 2016, inclusive). By contrast, we are aware of only 26 published studies of the immunology of wild rodents. A predominance per se of studies of laboratory models over wild systems is not necessarily problematic but, as discussed above, extrapolation from laboratory models to wild animals is unlikely to be straightforward so that the underrepresentation of the immunological study of wild animals is worrying.

The published studies of wild rodent immunology are summarized in Table 1 (11–36). The criterion for inclusion was (i) that the animals had to directly originate from the wild (though we include studies that use pet shop-acquired animals too) and (ii) that some immunological parameter was measured. Here, we review these studies.

Taken together, a coherent picture of wild rodent immune responses begins to emerge from these 26 studies. Specifically, that the humoral, and adaptive and innate cellular compartments of their immune systems are highly active—and more so than those of laboratory animals—with this immune phenotype entirely consistent with high level and continuous antigenic challenge (especially compared with laboratory animals). But functional cellular responses of wild rodents—in particular, proliferative and cytokine responses to broad spectrum stimuli such as mitogens or TLR agonists—are lower than those observed in laboratory animals. There is therefore a disconnect between different aspects of the immune biology of wild animals when judged against the accepted paradigms derived from studies of laboratory animals. Specifically, in wild rodents, despite evidence of extensive prior and current exposure to immunological challenges (i.e., high antibody titres, highly differentiated lymphocyte populations expressing markers of recent or current activation and immune memory) their ability to respond appropriately to an additional immunological stimulus is lower than one might expect given data from laboratory animals. Although data are limited, there is some evidence that antigen-specific T cell responses of wild rodents may be similar to (or higher than) those of laboratory mice (16), such that lower-than-expected immune function may be limited to innate immune responses—but much more work is required to determine whether this is indeed true.

Notwithstanding this current unknown, an important question is: which of these situations is “normal” and which is “abnormal”? Are wild animal responses abnormally low, or are laboratory animal responses abnormally high? Perhaps the most parsimonious explanation to this paradox is that, in the face of persistent or repeated infectious challenge, high-level proliferative and cytokine responses can cause severe tissue pathology, and to avoid this, functional immune responses of wild rodents are moderated so that individuals are adequately protected from infection while avoiding immune-mediated damage. This explanation has to be understood with the context that wild animals are exposed to a very large and diverse antigenic load (key concept 3, above). By comparison, laboratory animals have a very low antigenic burden, and thus their resting level of immune activation is much lower, and they may, therefore, be able to sustain higher proliferative and cytokine responses to any individual stimulus without causing immunopathology. In support of this hypothesis, immune responses of laboratory mice share many features of the neonatal human immune response (15). If true, this idea implies that the constrained cellular immune responses of wild rodents are both mechanistically and evolutionarily appropriate. In contrast, immune responses of laboratory rodents are more typical of those of a naïve, antigen-inexperienced animal. If so, then the initial immune response of any animal (in the lab, animals entering an experiment; in the wild, newly born animals) are high, but that these then decline with continued antigenic exposure (which is the natural life course in wild animals, if they survive, but which rarely happens in laboratory experiments).

It is also important to remember that these functional immune responses are, at least in part, measures of the influence of immunoregulatory processes. There is evidence from studies in humans that immunoregulatory mechanisms accrue over the life course and in response to microbial exposure, for example, by the accumulation of regulatory T cells (37, 38), though the relevance of age-dependent effects in laboratory rodents to those of wild rodents which are typically short lived (16, 32) remains unclear. We therefore need to be mindful that while one can measure many immune parameters, these represent different, discrete aspects of the functioning immune system, ranging from frontline effector responses against a pathogen, to back-room regulatory processes. The immunoregulatory processes and the homeostatic state of wild rodents’ immune systems will be appropriate to the nature and degree of antigenic stimulation to which these animals are exposed (and so will vary among individuals within populations, among populations within species, and among species). In one sense, this is self evident—the immune system responds in a regulated way to antigenic stimulation. The immunoregulatory processes of laboratory animals may be qualitatively or quantitatively different, given the very different environments in which these animals live.

This analysis generates two important research priorities for future studies of wild rodent populations. First, to discover and quantify the totality of infections—and so antigenic exposure—to which wild animals are exposed. While we are confident in asserting that wild animals have very heavy, extensive, high-level infections, there is actually rather little substantive evidence of this. While certain infections (particularly protozoan and metazoan parasites, together with some named viruses) are quite well characterized, the microbiota and pathogen load of wild animals is poorly characterized (both qualitatively and quantitatively) (39), despite this likely being the dominant source of antigenic challenge for wild rodents. The second research priority is to understand the immunoregulatory processes of wild rodents. These processes are the proximate mechanism by which the magnitude of many effector immune responses is determined. Linking these two questions together makes it interesting to ask: what is the quantitative relationship between antigenic load, immunoregulation, and effector outputs? A quantitative understanding of the action and interaction of components of the mammalian immune system is, arguably, woefully lacking, but such quantitative understanding is likely to be necessary to understand the relationship between infection and immune responses in wild animals. Ultimately, this level of understanding is necessary to be able to make predictions about which immune responses will be induced in response to particular infections given the overall antigen load, something that is not generally possible even in laboratory animals.

Other important ideas arise from these 26 studies—that there can be strong seasonal effects on the immune system [a pattern that is commonly seen among animals more generally (40)], and that these can also be affected by aspects of animals’ state (for example, their age, reproductive status, etc.), and the infections that they have. Interpreting an immune measure of a wild animal critically depends on understanding how, and under what scenarios, the immune measures of interest change. Interpretation of single immune measures without a broader context is likely to be very difficult and unreliable. Moreover, given the immunoheterogeneity that exists among individuals, both substantial sample sizes and, ideally, longitudinal measures of immune parameters are needed to properly study the immunology of wild rodents.

The energetic costs of immune responses are well established in studies of laboratory animals and of some wild animals, particularly birds, though this has actually been rather little studied in wild rodents (7). Notably, many of the studies reviewed above have sought relationships between measures of body condition and immune responses, but very often fail to find them. As noted above, manipulation of the diet of captive wild-caught cotton rats (24, 25) affected their investment in lymphoid tissue and some immune reactions. We assume that many wild animals are resource limited, and so expect that immune responses may be submaximal. However, in truth, high levels of immune responsiveness are, in general, seen in wild rodents. There are two aspects to resolving this conundrum. The first is that resource availability will vary among individual wild animals, so that some will be well resourced, while others will be poorly resourced. The second is that animals’ immune responses will have evolved in the actual resource context of those animals, so that if animals are normally resource limited, then the optimal immune response that can be achieved under those conditions will be selected for. Showing that enhancing an animal’s diet can further increase measures of immune responses does not invalidate this view, but just shows that resource availability can alter immune responses. This, therefore, raises a third key question for future studies, which is to better understand the actual resource state of wild animals and its temporal variation, and so understand how this impacts, if at all, on animals’ immune responses.

The focus of most of these studies of wild rodents is on measures of aspects of the immune response. While this is perfectly reasonable, it is also important to bear in mind that the ultimate functional effect of these immune responses is what really matters. Understanding this is much harder, especially given the high level and heterogeneous infectious challenge that we presume wild animals are exposed to. There is substantial interest in understanding the factors that affect the zoonotic spill-over of pathogens from wild animals, including from rodents (41, 42). The immune responses that wild animals make against their infections and the effect that these have on those infections is likely to be a critical factor affecting the potential for such zoonotic spill-over. This, therefore, emphasizes the applied relevance of understanding the immune responses of wild rodents in the context of the infections that these animals harbour, especially since many of these species (Mus, Rattus) live commensally with humans, especially in population-dense urban areas.

The intuition—that wild rats with their stress-filled, resource-limited lives were making very poor immune responses against the nematode S. ratti and so making it more common in the wild, than would be predicted from the behaviour of infections in laboratory animals—is likely wrong. From the review of the available literature (Table 1), it seems clear that wild rats’ immune systems are probably making very high-level responses to S. ratti, as well as to their myriad other infections. Studies of laboratory rats show that while there are a number of different effector processes acting against S. ratti parasites (both infective stages migrating through the host toward the gut, and adult parasites in the gut), clearance of adult stages from the host gut is achieved by an intestinal mucosal mast cell response that depends on IL-3, among other cytokines (43–45). Wild animals’ cytokine responses to antigenic stimulation are often depressed, compared with those of laboratory animals, and it is this that may underlie the apparent failure of wild rats to eliminate S. ratti infections. To put it formally, we can hypothesize that in wild rats infected with S. ratti the resultant antigenic stimulation is insufficient to generate an IL-3 response (and response of other cytokines) that is able to drive an effective mast cell response in the rat gut mucosa, with the consequence that the S. ratti parasites survive. Under this hypothesis the non-response could be due to ineffective (i) presentation of S. ratti antigen and/or (ii) subsequent induction of cytokine responses in wild rats.

The immune system and its responses play a critical role in the lives of wild animals, but these responses are very often poorly understood. Immune processes are a key factor affecting the ecology of wild animals, and to fully understand the ecology of animals, these immune processes must be brought to the fore. The current small number of studies on wild rodents shows that such studies are possible, and the emerging picture of wild rodent immunology suggests some of the next research questions that the field should address.

The authors co-wrote the paper and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.