Over the past decade, the emerging science of immunoecology has made substantial contributions to the fields of sexual selection (Zuk & Johnsen 1998, 2000); life-history evolution (Tella, Scheuerlein, & Ricklefs 2002); environmental toxicology (Smits & Williams 1999); and behaviour (Soler et al. 1999). Although several techniques have been used to measure immune function in natural contexts, the phytohemagglutinin (PHA) skin-swelling test has become one of the most popular because of its simplicity (Smits, Bortolotti, & Tella 1999), amenability to field work (Ardia 2005a; Martin et al. 2004), and history of use in domestic fowl (Stadecker et al. 1977; McCorkle, Olah, & Glick 1980). Phytohemagglutinin-induced immune responses are typically measured in vivo by injecting PHA subcutaneously (ideally dissolved in pyrogen-free saline) and quantifying concomitant swelling at the site of injection over time (Smits et al. 1999). In most immunoecological studies, swelling is measured 24 h post-injection (Zuk & Johnsen 1998; Soler et al. 1999). Skin swellings at this time are interpreted as indexes of cell-mediated immunocompetence largely based on four observations from domestic animals: (i) leukocytes infiltrate and/or proliferate in injected tissue after challenge (Stadecker et al. 1977; McCorkle et al. 1980); (ii) swelling responses are blunted in thymectomized birds (Goto et al. 1978); (iii) PHA predominantly stimulates T-lymphocytes but not B-lymphocytes in vitro (Elgert 1996); and (iv) delayed-type hypersensitivity responses, including PHA swelling, are sometimes correlated with an animal's capacity to control certain types of infection (most often viral: Turk 1967).
Although it is clear from these studies that skin-swelling responses to PHA represent immunological phenomena, it is not clear what swelling means in a functional sense, and thus how differences in swelling responses should be interpreted. Some studies on wild bird species reported that large swelling responses to PHA were predictive of adult survival probability (Gonzalez et al. 1999; Soler et al. 1999). For this reason, PHA-induced tissue swelling has been espoused as a reliable surrogate for generic disease resistance (e.g. immunocompetence). Still, many studies have found that PHA-swelling responses are labile and thus might be better characterized as indicators of the general health of individuals (Duffy & Ball 2002; Ardia 2005b) or surrogates for investments in immune activity at certain points in animals’ lives (Tella et al. 2002; Martin et al. 2000). Finally, some immunologists interpret large swelling responses as indications of allergy or unrestrained local inflammatory activity (Elgert 1996), suggesting that more swelling is not always better.
just what is pha?
Phytohemagglutinin is a compound generated by the red kidney bean (Phaseolus vulgaris) that is believed to serve as a defence against herbivory. In humans and other vertebrates, consumption of large volumes of raw kidney beans can cause inflammation of the intestinal tract. Phytohemagglutinin is a large molecule (molecular weight 138 000) with a long history of use in immunology, dating back to its original role as an agglutination agent for vertebrate erythrocytes (Naspitz & Richter 1968). Like other lectins, PHA is mitogenic to many vertebrate cell types including (but not limited to) T lymphocytes (Bonforte et al. 1972; Elgert 1996). However, stimulation of T-lymphocyte proliferation by PHA is different from most other antigens. Most importantly, PHA does not require antigen presentation or major histocompatibility complex costimulation by professional antigen-presenting cells (e.g. macrophages) to induce mitogenesis, like most antigens do; as many as 30% of T-cell lines are responsive to PHA, far more than a typical antigen (Elgert 1996). For these reasons, non-ecologists typically do not recognize the PHA-swelling response as solely a T-cell mediated hypersensitivity response. Instead, they typically call it cutaneous basophilic hypersensitivity (Stadecker et al. 1977; McCorkle et al. 1980).
This name is derived from the two distinct phases that occur during PHA skin swellings in domestic fowl. The first phase involves exudation of plasma from surrounding vascular tissue and edema in the injected region, which occurs within 6–12 h after injection and is driven by local innate cell populations (largely basophils and macrophages) activated by PHA-stimulated CD4+ T-cells (Elgert 1996). The second phase includes an infiltration of additional PHA-sensitive T-lymphocytes and occurs around 24 h post-injection (Stadecker et al. 1977; Goto et al. 1978; McCorkle et al. 1980). From these data, it is clear that activated T-cells are involved in the swelling response; they secrete some of the cytokines that recruit and activate effector cells. However, other leukocytes (particularly basophils, heterophils and macrophages) actually effect most of the vasodilation, edema and inflammation of tissue; moreover, these cells secrete additional cytokines, which can promote further infiltration by, or proliferation of, additional leukocytes (Stadecker et al. 1977; Goto et al. 1978; McCorkle et al. 1980; Elgert 1996). Even the most commonly cited study identifying the PHA-swelling response as a cell-mediated immune response supports a more complicated immunological cascade underlying swellings than is typically espoused. A few chickens thymectomized early in life (and hence possessing few if any circulating T cells) mounted measurable but reduced swelling responses to PHA relative to controls (Goto et al. 1978). Moreover, in vitro work indicates that other cell types in addition to T cells must be important mediators of swelling, as lymphocyte proliferation using PHA is often not related to in vivo tissue swelling in the same bird (Bayyari et al. 1997). Finally, it is clear from genetic studies that both innate and adaptive components of the immune system are involved in the swelling response, and each may be independently regulated (Taylor et al. 1987; Cheng & Lamont 1988).
To gain a better understanding of what PHA swellings represent in a functional sense in passerine birds, we investigated the cellular processes underlying the swelling response in the House Sparrow (Passer domesticus). Our goals were to (i) describe the cellular infiltration processes involved in the swelling response; and (ii) determine if and when cell infiltration was correlated to swelling. Although our approach cannot indicate whether PHA-induced swellings are predictive of disease resistance, it can provide a more detailed understanding of the currently most favoured technique in immunoecology. Indeed, although it is clear that measures of PHA swelling can be informative of the immunological state of animals in a coarse sense, we expect that more detailed characterizations of this response could provide greater insight into the functional significance of larger or smaller skin swellings.