Historically, researchers have described monotremes, marsupials, and bats as primitive mammals.
However, recent advances in immunology have revealed that monotremes and marsupials possess all the essential immune cells and tissues required to mount an effective immune response.
Current knowledge of bat immunology is limited, although research is expanding and results point towards a typical mammalian immune system similar to other eutherian mammals. However, their ability to co-exist with pathogenic viruses suggests novel immune strategies that are yet to be fully characterised. Our current understanding of monotreme, marsupial and bat immunology indicates a complex immune system involving innate and adaptive arms that evolved before the divergence of monotremes from Therian mammals (Belov et al. 2007).Monotremes and marsupials generally conform to the typical eutherian immune system, with similarities in gene density, complexity and organisation across major immune gene families such as the major histocompatibility complex (MHC) (Cheng et al. 2009), T-cell receptors (TCR) (Parra et al. 2008), toll-like receptors (TLR) (Cui et al. 2015a), immunoglobulins (Belov and Hellman 2003) and cytokines (Harrison and Wedlock 2000). However, the marsupial and monotreme immune systems have unique features that separate them from other mammals: immunological development after birth, presence of a fifth TCR chain (TCRμ) (Baker et al. 2005) and expansion of antimicrobial peptide families (Belov et al. 2007). Genome sequencing has advanced our understating of monotreme and marsupial immunomics in the past decade, enabling comparative analysis and highlighting the expansion and contraction of gene families during evolution (Johnson et al. 2018; Zhou et al. 2021; Peel et al. 2022).
Bats are potential reservoirs for numerous viral pathogens, several of which have been associated with deadly human and other animal diseases (e.g. Hendra, Nipah, Ebola viruses and SARS coronaviruses) (Wong et al. 2007).
A landmark comparative bat genome study suggested that bats’ adaptation to flight had resulted in positive selection of important DNA damage checkpoint and innate immune genes (Zhang et al. 2013). This may contribute to explanations surrounding bats’ longevity, low rates of tumorigenesis and ability to asymptomatically harbour deadly viruses. Studies are slowly emerging, looking at the characterisation of various aspects of the bat immune system, such as MHC (Ng et al. 2016; Ng et al. 2017), TLRs (Cowled et al. 2011), interferon genes (Zhou et al. 2011; Zhou et al. 2016) and immunoglobulins (Wynne et al. 2013). Significant gaps remain in our knowledge of bat immune tissues, the number of receptor families and antimicrobial peptides.This chapter will not cover passive immunity in PY or neonatal monotremes. Briefly, during development the young receive passive immunity in utero and via the milk. Passive transfer of maternal immunoglobulins in utero has only been documented in the tammar wallaby (Notamacropus eugenii) (Deane et al. 1990). Immune cells and immunoglobulins are secreted into the milk of numerous marsupial species and the short-beaked echidna (Tachyglossus aculeatus), where they are ingested by the young and absorbed across the intestinal wall throughout lactation (Griffiths 1978; McCracken 2008). Immune compounds within platypus (Ornithorhynchus anatinus) milk have not been studied and passive immune protection in platypus hatchlings remains unknown.
This chapter will describe the major immune tissues, cells and receptors involved in the innate and adaptive immune response in monotremes, marsupials and Australian bat species. Comparative analysis across the three mammalian groups highlights evolutionary differences and unique characteristics.
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