Data from: Highway to the danger zone: exposure-dependent costs of immunity in a vertebrate ectotherm

Parasite exposure often causes innate immune activation, resulting in tradeoffs among physiological processes and strong selection on the parasite. Costs of immune activation vary widely among and within host populations though, likely dependent on the evolutionary history of host-parasite interactions and the environments in which they occur. For hosts, degree of exposure may drive the magnitude of costs incurred, and subsequently whether hosts resist or tolerate infections.

If costs increase concomitantly with exposure, a threshold may exist where the expense of parasite resistance becomes prohibitive and parasite tolerance becomes favorable. Here, we characterized exposure-dependent costs of an innate immune response in brown anoles (Anolis sagrei) by tracking allocation of an isotopically-labelled essential amino acid (13C-leucine), to the liver and gonads. To elicit immune responses, we used lipopolysaccharide (LPS), a strongly immunogenic molecule from Salmonella spp.

We found that both sexes paid dose-dependent costs of Salmonella LPS-induced immune activation, but costs were experienced differently by the sexes, likely due to differences in life history. Males allocated more leucine to their livers in response to higher LPS doses. In females, a tendency for increased costs in response to dose were only revealed when leucine allocation ratios between lymphoid and reproductive organs were considered. We also found that regardless of dose, males always allocated more leucine to their gonads than females. Lastly, and perhaps most interestingly, cost functions in both sexes were linear, but with shallow slopes, indicating modest costs of immune activation in response to Salmonella LPS in this species.

Altogether, our results demonstrate that costs of immunity are dose-dependent in this introduced lizard species, but sexes experience costs differently. Characterization of relationships between host exposure and cost of immune activation such as these can facilitate predictions about how parasites might circulate through communities.

寄生虫暴露（parasite exposure）常引发固有免疫激活（innate immune activation），进而造成生理过程间的权衡，并对寄生虫产生强选择压力。不过，免疫激活的成本在宿主种群之间及种群内部差异极大，这可能取决于宿主-寄生虫互作（host-parasite interactions）的进化历史与发生该互作的环境。对宿主而言，暴露程度可能决定了其承担的成本大小，并最终决定宿主是选择抵抗还是耐受感染。
若成本随暴露程度同步上升，则可能存在一个阈值：此时抵抗寄生虫的代价高到难以承受，而耐受寄生虫感染则成为更有利的策略。本研究以褐安乐蜥（Anolis sagrei）为对象，通过追踪同位素标记必需氨基酸（isotopically-labelled essential amino acid，13C-亮氨酸（13C-leucine））在肝脏与性腺中的分配情况，刻画了其固有免疫应答的暴露依赖性成本。为诱导免疫应答，我们使用了脂多糖（lipopolysaccharide, LPS）——一种源自沙门氏菌属（Salmonella spp.）的强免疫原性分子。
研究结果显示，雌雄个体均会承担沙门氏菌LPS诱导的免疫激活所带来的剂量依赖性成本，但两性承担的成本存在差异，这可能与它们不同的生活史策略（life history）有关。当LPS剂量升高时，雄性会将更多亮氨酸分配至肝脏。而对于雌性，仅在考虑淋巴器官与生殖器官之间的亮氨酸分配比例时，才能观察到其成本随剂量上升的趋势。此外，无论剂量高低，雄性性腺中的亮氨酸分配量始终高于雌性。最后，或许也是最值得关注的一点：两性的成本函数均呈线性，但斜率平缓，表明本物种在应对沙门氏菌LPS引发的免疫激活时，仅需付出适度的成本。
综上，本研究结果表明，这种引入的蜥蜴物种的免疫成本具有剂量依赖性，但两性承担的成本存在差异。此类关于宿主暴露程度与免疫激活成本之间关系的刻画，有助于预测寄生虫在群落中的传播循环模式。