Characterisation of the immune compounds in koala milk using a combined transcriptomic and proteomic approach

Abstract

Production of milk is a key characteristic of mammals, but the features of lactation vary greatly between monotreme, marsupial and eutherian mammals. Marsupials have a short gestation followed by a long lactation period, and milk constituents vary greatly across lactation. Marsupials are born immunologically naïve and rely on their mother’s milk for immunological protection. Koalas (Phascolarctos cinereus) are an iconic Australian species that are increasingly threatened by disease. Here we use a mammary transcriptome, two milk proteomes and the koala genome to comprehensively characterise the protein components of koala milk across lactation, with a focus on immune constituents. The most abundant proteins were well-characterised milk proteins, including β-lactoglobulin and lactotransferrin. In the mammary transcriptome, 851 immune transcripts were expressed, including immunoglobulins and complement components. We identified many abundant antimicrobial peptides, as well as novel proteins with potential antimicrobial roles. We discovered that marsupial VELP is an ortholog of eutherian Glycam1, and likely has an antimicrobial function in milk. We also identified highly-abundant koala endogenous-retrovirus sequences, identifying a potential transmission route from mother to young. Characterising the immune components of milk is key to understanding protection of marsupial young, and the novel immune compounds identified may have applications in clinical research.

Infectious KoRV-related retroviruses circulating in Australian bats

“…This receptor is almost ubiquitously expressed throughout the mammalian body at variable levels (41)(42)(43)(44) and is highly expressed in many tissues, including the colon, breast, testes, bladder, placenta, and brain (41,42). KoRV and GALV have been detected in numerous tissues and body fluids, including blood, sperm, breast milk, feces, and urine (5,27,(45)(46)(47)(48)(49)(50). Given the wide distribution of PiT-1 expression and the detection of KoRV and GALV in body fluids including blood, urine, and feces, it is possible that interspecies transmission might occur along routes including blood during fighting/predation and contamination of food sources by feces and urine.…”

Adaptation and conservation insights from the koala genome

“…Analysis of the genome, in conjunction with a mammary transcriptome and a milk proteome, enabled us to characterise the main components of koala milk (Supplementary Fig. 8;) (Supplementary Table 16; Supplementary Note 3.9; and 63). The high-quality assembly of the genome enabled both identification of marsupial-specific genes, and determination of their evolutionary origins based on their genomic locations.…”

“…The long-read-based genome enabled the de novo assembly of complex, highly duplicated immune gene families and the most comprehensive annotation of immune gene clusters in any marsupial 63,67,68. These include the Major Histocompatibility Complex ( MHC ) 69, as well as T-Cell Receptors ( TCR ), immunoglobulin ( IG ) (Supplementary Fig.…”

Antimicrobial Protection of Marsupial Pouch Young

“…Transferrin, a family of iron-binding proteins responsible for iron storage and transport, has been identified in the milk of brushtail possums, tammar wallabies, and koalas ( Adamski and Demmer, 2000 ; Lefèvre et al, 2007 ; Morris et al, 2016 ). These multi-functional proteins are also considered a component of innate immunity.…”