The Impact of Heat Stress on Immune Function

Twenty-first century immunology must embrace the reality of the climate crisis and its implications for global health. Heat waves are becoming increasingly common, intense and long. High ambient temperatures can not only shape pathogen dynamics but also induce a stress response in the host, which may compromise vaccine responsiveness and increase vulnerability to infection. Yet despite evident repercussions for entire ecosystems, the impact of heat stress on immune competence remains poorly understood. Indeed, both protective and detrimental consequences to immune function have been assigned to heat, including in the context of infection. This thesis combines in vivo models, flow cytometry and bioinformatics to investigate how heat stress influences haematopoiesis, immune function and susceptibility to viral infection. First, it was established that housing female mice at 36 °C was well tolerated over several weeks yet sufficient to induce heat stress, as indicated by weight loss, reduced food intake as well as elevated body temperature and circulating stress hormones. High-parameter spectral flow cytometry designed and optimised for this study revealed that acute heat stress severely disrupted B and T cell development in the bone marrow and thymus, triggering apoptosis particularly at the energy-intensive developmental checkpoints of positive selection. In parallel, heat stress promoted stress-driven myelopoiesis. These alterations were accompanied by persistent peripheral lymphopenia, initially due to increased cell death and later due to impaired replenishment of peripheral niches. The surviving leukocyte populations appeared primed and dysregulated, with spontaneous cytokine release and shifts toward regulatory and exhausted phenotypes. Given ongoing debates about optimal housing temperature for laboratory mice, key heat-related immune phenotypes were confirmed using controls housed at both room temperature (~22 °C) and thermoneutrality (30 °C), the latter referring to ambient temperatures at which mice do not need to expand additional energy for thermoregulation. No substantial differences in immune cell distribution or function were detected between room temperature and thermoneutral conditions. Under prolonged heat stress, animals displayed signs of allostasis, including normalisation of B cell production. This indicates that at least some aspects of the heat stress response depend on relative thermal differences rather than on a specific absolute temperature. However, stress levels remained high and peripheral leukocytes were persistently depleted and dysregulated under sustained exposure, characterised by immune priming and elevated production of the regulatory cytokine IL-10. Strikingly, even a single seven-day heat stress episode left long-lasting immune imprints. While most early and B-lineage progenitors recovered one month after stress cessation, haematopoietic stem cell (HSC) pools remained as depleted as under chronic stress. Single-cell RNA sequencing revealed that acute heat stress first metabolically activated then exhausted HSCs, indicating that forced HSC reactivation under both continued heat exposure and recovery conditions may compromise their long-term function and regenerative capacity. Together, this could have far-reaching implications for haematopoietic health, including impaired haematopoiesis, reduced immune competence, and diminished resilience to future immunological and environmental stressors. Next, the drivers of these immune impairments were investigated by comparing heat-stressed mice to animals subjected to dietary stress (40 % reduction in food intake) or pair feeding to heat-stressed animals (~33 % reduction in food intake). Diet stress but not pair feeding phenocopied key features of leukocyte suppression. This suggests, firstly, a shared stress mechanism involving glucocorticoids, and secondly, that heat-associated phenotypes are not primarily driven by reduced feeding under heat stress. Given emerging evidence for a role of the microbiome in responses to (heat) stress, metagenomic sequencing was performed in three longitudinal mouse cohorts. In adult mice, heat stress was associated with reduced microbial diversity and induced shifts in the relative abundance of species belonging to Firmicutes and Bacteroides. Ongoing functional analyses aim to clarify the predicted impacts on microbial metabolism, including the synthesis of immune-modulating metabolites such as short-chain fatty acids. Finally, the functional consequences of heat stress were assessed using influenza virus challenge in vivo. In line with previous reports, infection with the influenza A virus A/Puerto Rico/8/34 (PR8) resulted in dampened antiviral immune responses in hyperthermic conditions in vivo. However, PR8 displayed dampened replication at higher temperatures both in vitro and in vivo, suggesting that reduced immune responses to PR8 infection under heat stress could represent an appropriate response to lower viral loads rather than impaired immune competence. To test this, a temperature-resistant PR8 mutant virus was employed to isolate the impact of heat stress on the host immune system. Heat-stressed animals were still able to mount robust, and at times even heightened, inflammatory responses to this mutant PR8 virus, indicating that viral temperature sensitivity, rather than host immune impairment, was the primary determinant of reduced resposnes to wild-type PR8 infection. These findings underscore the need to account for viral temperature sensitivity when interpreting infection outcomes under heat stress and may help reconcile discrepancies in the literature by identifying viral temperature sensitivity as a major confounder in previous studies. The framework and tools developed here provide a foundation for disentangling the respective contributions of host physiology and pathogen biology to infection outcomes under heat stress. Collectively, these findings demonstrate how environmental conditions can profoundly reshape host physiology and immune function and highlight the need for systems-level approaches to evaluate infection risks in a warming world. Crucially, heat stress was shown to cause durable disruptions to haematopoiesis and immune function, even following a single exposure. As the climate crisis intensifies and extreme weather events become more frequent, understanding the mechanisms and lasting consequences of heat-induced immune dysfunction will be critical for designing effective vaccination, prevention and treatment strategies to safeguard immune resilience in the decades to come.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Jung, Johanna

Advisors dc:contributor.advisor

• Lyons, Paul
• Smith, Kenneth

• immune system
• heat stress
• stress
• climate change

Licence

• https://www.repository.cam.ac.uk/bitstreams/b120edbb-0b2c-44c3-a973-978201725fdd/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.122953

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/392148