Data from: Avian malaria and bird humoral immune response

Background: Plasmodium parasites are known to impose fitness costs on their vertebrate hosts. Some of these costs are due to the activation of the immune response, which may divert resources away from self-maintenance. Plasmodium parasites may also immuno-deplete their hosts. Thus, infected individuals may be less able to mount an immune response to a new pathogen than uninfected ones. However, this has been poorly investigated.

Methods: The effect of Plasmodium infection on bird humoral immune response when encountering a novel antigen was tested. A laboratory experiment was conducted on canaries (Serinus canaria) experimentally infected with Plasmodium relictum (lineage SGS1) under controlled conditions. Birds were immune challenged with an intra-pectoral injection of a novel non-pathogenic antigen (keyhole limpet haemocyanin, KLH). One week later they were challenged again. The immune responses to the primary and to the secondary contacts were quantified as anti-KLH antibody production via enzyme-linked immunosorbent assay (ELISA).

Results: There was no significant difference in antibody production between uninfected and Plasmodium infected birds at both primary and secondary contact. However, Plasmodium parasite intensity in the blood increased after the primary contact with the antigen.

Conclusions: There was no effect of Plasmodium infection on the magnitude of the humoral immune response. However, there was a cost of mounting an immune response in infected individuals as parasitaemia increased after the immune challenge, suggesting a trade-off between current control of chronic Plasmodium infection and investment against a new immune challenge.

研究背景：已知疟原虫（Plasmodium）会对其脊椎动物宿主造成适合度代价。其中部分代价源于免疫应答的激活，这可能会将资源从自身维持过程中转移。此外，疟原虫还可能导致宿主免疫耗竭。因此，相较于未感染个体，受感染个体对新型病原体发起免疫应答的能力可能更弱。但目前针对该现象的研究仍较为匮乏。
研究方法：本研究检测了疟原虫感染对宿主遭遇新型抗原时体液免疫应答的影响。在受控实验条件下，对人工感染残疟原虫（Plasmodium relictum，谱系SGS1）的家金丝雀（Serinus canaria）开展了实验室实验。通过胸内注射新型非致病性抗原——钥孔戚血蓝蛋白（keyhole limpet haemocyanin, KLH）对金丝雀进行免疫刺激，并于一周后再次进行免疫刺激。采用酶联免疫吸附试验（enzyme-linked immunosorbent assay, ELISA）定量检测初次及再次免疫刺激后的抗KLH抗体水平，以评估免疫应答强度。
研究结果：在初次和再次免疫刺激后，未感染与疟原虫感染的金丝雀的抗体产生量均无显著差异。然而，初次免疫刺激后，血液中的疟原虫载量有所上升。
研究结论：疟原虫感染并未对体液免疫应答的强度产生显著影响。但在受感染个体中，免疫刺激后虫血症水平升高，提示发起免疫应答存在代价，这表明当前对慢性疟原虫感染的控制与针对新型免疫刺激的免疫投入之间存在权衡关系。