International Journal of Molecular Zoology 2024, Vol.14, No.4, 197-210 http://animalscipublisher.com/index.php/ijmz 202 4.3 Immune adaptations in Arctic vertebrates 4.3.1 Immune response to extreme cold Arctic vertebrates, such as the Antarctic notothenioid fish, have evolved unique immune responses to survive in extreme cold environments. Transcriptome analyses of the Antarctic bullhead notothen Notothenia coriiceps revealed that exposure to bacterial and viral pathogens induces distinct immune responses, with antigen presentation mechanisms being up-regulated in response to bacterial infections and TNF-mediated apoptosis being prominent in viral infections (Ahn et al., 2016). These adaptations are crucial for maintaining immune function in the cold and thermally stable Antarctic sea. 4.3.2 Pathogen resistance in polar environments Pathogen resistance in polar environments is facilitated by specific immune adaptations. For example, the Cryonotothenioidea, a group of Antarctic fish, exhibit unique genetic and molecular adaptations that enhance their immune responses. Comparative transcriptomics of these fish have identified several up-regulated genes involved in immune function, which are shared among different species within the group. These genetic adaptations likely contribute to their evolutionary success in the harsh Antarctic environment (Ansaloni et al., 2021). Additionally, the study of immune responses in Notothenia coriiceps provides insights into the specific defense strategies employed by Antarctic fish, such as the use of antigen presentation against bacterial infections and TNF-mediated apoptosis against viral infections. In summary, the immune function of vertebrates is intricately linked to their ability to adapt to diverse and extreme environments. Genetic adaptations in immune genes, such as TLRs, and specific immune responses to environmental stressors, play a crucial role in ensuring survival and promoting longevity in these organisms. 5 Case Study: The Naked Mole Rat 5.1 Unique immune characteristics of the naked mole rat The naked mole rat (Heterocephalus glaber) exhibits several unique immune characteristics that distinguish it from other mammals. Notably, its immune system features a higher myeloid-to-lymphoid cell ratio and lacks natural killer cells, which are typically present in other mammals (Figure 3) (Hilton et al., 2019). Additionally, naked mole rats have a novel subset of neutrophils that are highly responsive to lipopolysaccharides and express several antimicrobial peptides (Lin and Buffenstein, 2021). These immune traits suggest an atypical mode of immunosurveillance and a greater reliance on myeloid-biased innate immunity. Furthermore, the naked mole rat's immune system shows higher pro-inflammatory cytokine production in macrophages, which may contribute to its cancer resistance and longevity. The study of Hilton et al. (2019) showcases a comparative single-cell RNA sequencing (scRNA-seq) analysis between mouse and naked mole-rat (NM-R) spleens, revealing significant differences in immune cell populations. The UMAP projections highlight the distinct clustering of immune cells in both species, with notable variations in the proportions and gene expression profiles of T cells, NK cells, and other immune cell types. The heatmaps provide a detailed comparison of gene expression levels across different cell clusters, emphasizing the unique immune cell landscape in the NM-R spleen compared to the mouse. This comparison underscores the potential evolutionary adaptations in immune function between these species. 5.2 Correlation between immune function and longevity The unique immune characteristics of the naked mole rat are closely linked to its extraordinary longevity. Unlike other mammals, naked mole rats do not exhibit an age-related increase in mortality and maintain physiological functions well into old age (Oka et al., 2022). Their immune system's unusual features, such as the absence of natural killer cells and the presence of a novel neutrophil subset, may play a role in their resistance to cancer and other age-related diseases. Additionally, the naked mole rat's immune system appears to be adapted to its subterranean environment, which is low in oxygen and high in carbon dioxide, further contributing to its longevity (Kim et al., 2011). These adaptations may have evolved to protect the naked mole rat from the harsh conditions of its habitat, thereby enhancing its lifespan (Fang et al., 2014).
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