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Immunity in Ecology and Evolution: 'Hidden' costs of disease, immune function and their consequences for Darwinian fitness

Periodic Reporting for period 2 - EcoImmuneCosts (Immunity in Ecology and Evolution: 'Hidden' costs of disease, immune function and their consequences for Darwinian fitness)

Reporting period: 2019-02-01 to 2020-07-31

Is there such a thing as a harmless parasite infection? Why does the immune system induce large costs and sometimes even attack our own body, leading to devastating outcomes? How can seemingly immune-deficient individuals persist in a population? These are fundamental questions in biology and medicine - and key aspects of this ERC project.

There is currently an immense interest in factors that mediate and shape disease resistance, immune genes and immune response-related diseases (i.e. immunopathology), as well as patterns of ageing and what limits lifespan. There is a need to better understand how the immune system has evolved to optimize protection and minimize immunopathology (incl. autoimmune) costs. For this, a whole-organism approach is necessary that integrates concepts and methods of evolutionary ecology, immunology and molecular biology (a so-called ‘immunoecology approach’) – this is the core approach in my ERC Advanced Grant project entitled ‘EcoImmuneCosts’. Hence, my ERC AdG project is by its nature, cross-disciplinary and I expect that the research results coming out of this project will be exciting not only to evolutionary, behavioral and functional ecologists, but also to researchers from many other disciplines such as medicine and gerontology, in conjunction with seeking to understand causes, consequences (and precautions) of infectious, autoimmune and degenerative diseases. Gaining a deeper understanding of the long-term consequences of mild infections on ageing (i.e. senescence), lifetime reproductive success and lifespan (so called, Darwinian fitness costs) is of great importance for humans and the society.

A primary target of my project is to study the long-term consequences of low-virulent pathogens causing mild infections, which for long have been considered harmless. Recent research suggests that the idea of mild, cost-free infections is false and that it instead may be so that seemingly harmless pathogens entail delayed (‘hidden’) Darwinian fitness costs. However, the mechanisms mediating these costs are still unknown. A suspected mechanism modulating this cost is degradation of telomeres (repetitive, non-coding DNA sequences that are essential for chromosome integrity and hence cell function and survival). In my ERC project, we experimentally test if accelerated telomere degradation is a causative mechanism through which small immune costs can accumulate and be translated into accelerated senescence and reduced Darwinian fitness. We investigate if early-life telomere length (eTL) predicts the likelihood of contracting and recovering from disease, as well as to what extent eTL predicts life history strategy. We also investigate the effect of retained infection vs. recovery from infection on the rate of telomere shortening. Finally, we investigate between-generation effects by studying how parental factors (such as age and eTL) determine the (early-life) telomere length of their offspring and how juvenile telomere length can predict immune capability, immune costs and physiological health at adult age. Individuals vary in their susceptibility to infection and ability to recover. Some of this variation may be due to general differences, e.g. between males and females, others due to individual differences in immune gene variation. Immune costs (i.e. reduced health and physiological deterioration) may be ‘hidden’ because of sexually antagonistic effects, and we therefore study how male and female differences may affect immune gene variation, immune costs and Darwinian fitness. These aspects are central for advancing our understanding of the evolution of disease resistance and immune function, including immune over-reactions (immunopathology including autoimmune reactions).
Research problem 1: Are correlations between telomere shortening and ageing processes merely a result of telomere length (TL) being a marker of biological age or does TL play a causal role in physiological deterioration and (immune-) senescence?

This large, long-term experiment (collaboration with Prof. G Valkiunas, Nature Research Center, Vilnius, Lithuania) will allow me to critically test the separate (and combined) effects of a chronic infection and telomere length/shortening on disease progress and physiological senescence. Treatment administration has worked very well and ~8000 samples (red blood cells, plasma and RNA from 160-180 canaries) have so far been collected and shipped to Lund for analysis. We have also repeatedly assessed physiological health parameters from 120 individuals and are now analyzing covariation with telomere shortening, and whether patterns differ between treatments.

Research problem 2: Is telomere length an important agent mediating transgenerational effects on immunity? To address this, we are conducting two different sub-projects:

(A) Longitudinal studies of wild great reed warblers (GRWs) to investigate if eTL predicts risk of contracting as well as likelihood of recovering from malaria, and the effects of disease recovery on subsequent telomere shortening and reproduction. We have conducted two fieldwork seasons and extracted >3000 DNA samples from GRWs now available to our analyses. Furthermore, we have meticulously worked out a qPCR protocol that gives us reliable measures of telomere length.
(B) Experiment on captive zebra finches to investigate how parental factors (eTL, age) determine eTL of their offspring, and how offspring eTL can predict immune capability, immune costs and physiological health at adult age. In this experiment, zebra finches will be paired based on their early life telomere length (eTL) to create offspring phenotypes with short or long eTLs. We have now raised a cohort of young zebra finches for which we have collected blood samples to measure eTL. We are currently in the process of calibrating the qPCR method for zebra finches and will soon measure eTL to set up TL-matched pairs for breeding. Data is expected to be collected over the next two years.

Research problem 3: Can sexually antagonistic selection explain patterns in immune gene diversity and risk of immunopathology/autoimmunity? Two sub-projects are conducted to address this issue:

(A) Investigate sex-specific effects of MHC allele diversity on Darwinian fitness traits, and to what extent sexually antagonistic selection has an impact on MHC gene diversity and immunopathology costs. We have found that immune gene diversity at the Major histocompatibility complex class I (MHC-I) in great reed warblers (GRW) is under antagonistic selection: there is a higher offspring survival of adult males with a higher number of different MHC-I alleles, whereas the opposite pattern is true for adult females (Roved et al. 2018 Proc Roy Soc B). More results are in preparation for publication, e.g.: we show that MHC-I alleles are inherited in haplotypes, that haplotypes of wild GRWs are more diverse than MHC-I alleles drawn randomly from the GRW population, and that MHC-I alleles of GRW cluster in five groups that differ in variation and evolutionary history; most probably signifying different functions.
(B) Sex-specific long-term effects of chronic mild disease on gene expression, immune responses and physiological senescence. In this malaria infection experiment on GRW (at Kalimok research station, Bulgaria; collaboration with Prof. P Zehtindjiev)), we have investigated >20 birds for 1.5 years. Also, 20 GRWs have been studied into the chronic infection phase (ca. 3 months). We anticipate to conduct the gene expression, immune measures and telomere length analyses before end of 2020.
The experiment on canaries will allow us to critically test the separate (and combined effects) of chronic infection and telomere length/shortening on disease progress and physiological senescence. This is a key experiment designed to test the contentious issue of the potentially causative role of TL as a currency mediating seemingly small (immune) costs into long-term costs, thus being a driving force in biological ageing. The results from this experiment have potential for paradigm-shifting insights in how mild disease and immune resistance connect to senescence.

Virtually nothing is known about how eTL influences offspring immune function, because studies of TL and disease/immune function have almost exclusively focused on adults. It is an intriguing possibility that the astonishing positive association between eTL and life span could be linked to immune performance. However, to the best of my knowledge, no experimental approaches have been attempted to address this aspect directly. Here, we directly test the significance of eTL for immune capacity and disease resistance in adult life. Given the currently budding interest in eTL as a predictor of organismic performance and health, I anticipate that the results will be of very high interest to a broad range of scientists.

The long-term study (34 years) of the GRWs at Lake Kvismaren, providing multi-generational DNA samples and data of breeding and life history parameters, gives us an unprecedented opportunity for in-depth studies of how immune gene diversity may affect Darwinian fitness under natural conditions and if sexually antagonistic selection plays a role. This project is built on a wild animal data set of extraordinary quality and exploits state-of-the-art genomic methods to score immune gene diversity (and antigenic repertoire). It will significantly advance our understanding of the fascinating evolution of the exceedingly variable MHC genes key to vertebrate immune defense. To the best of my knowledge, no such long-term study of a vertebrate subjected to natural selection in the wild has previously been conducted.

Using a songbird host (GRW) - malaria parasite (P. ashfordi) system to conduct a powerful infection experiment and comparing sex-specific MHC and other immune gene expression patterns before infection, at high parasite load (after 3 weeks), and low parasite load (repeatedly up to 2 years; chronic phase) will allow us to draw conclusions about how sex differences in immune responses may affect(immune) gene variation. This type of experimental study, aiming to reveal sexually antagonistic patterns in immune reactions and disease costs, is according to my knowledge novel - thus, I anticipate that it will open up new exciting research avenues!

Eco-immunology targets one of the great challenges in biology and medicine, namely the question of how the immune system has evolved to optimize protection and minimize immunopathology (incl. autoimmune) costs. Great strengths of this field are the cross-disciplinary nature (integrating concepts and methods of evolutionary ecology, immunology and molecular biology) and the whole-organism approach. The proposed project will continue to foster these aspects while at the same time venturing into hitherto little explored areas of research: the accumulating costs of seemingly mild diseases, and the importance of telomere dynamics and sexually antagonistic selection for disease resistance, immune development and performance. I therefore envision that the research results coming out of this proposal will be exciting not only to evolutionary ecologists, but also to researchers from many other disciplines such as medicine and gerontology, in conjunction with seeking to understand causes, consequences (and precautions) of infectious, autoimmune and degenerative diseases.
To catch great reed warblers, mist nets are set up in the reed beds.
Singing reat reed warbler male at Lake Kvismaren. All birds are marked with colour rings.
A great reed warbler is blood sampled.