Anellovirus constraint from 2 to 6 months postpartum followed by betatorquevirus and gammatorquevirus dominance in serum and milk.

Anelloviruses are ubiquitous, diverse viruses in the human virome not causing disease. In adults, the anellovirus composition (anellome) is dominated by alphatorqueviruses. However, infants seem to acquire their first anellovirus infections beginning with beta- and gammatorquevirus, soon followed by alphatorquevirus. Transmission via human milk is suggested as a potential transmission route. Consequently, we hypothesize that milk may contain predominantly beta- and gammatorqueviruses. That would also imply that compartmentalization between milk and serum takes place as the anellome in adults is dominated by alphatorqueviruses. We therefore examined anellovirus distribution in paired serum and milk samples of lactating mothers in the first 15 months after delivery (n = 30). Unexpectedly, genus-specific qPCRs revealed that the majority of serum and milk were negative for anelloviruses 2-6 months postpartum (4 serum and 1 milk positives out of 13), whereas positive from 6 months postpartum onwards (11 serum and 8 milk positives out of 16). Importantly, a dominance of beta- and gammatorquevirus in both serum and milk from 6 months postpartum was found. We also found significantly higher anellovirus loads in serum compared to milk, with no obvious difference in anellovirus genera presence between compartments. Anellovirus variant analysis using Oxford Nanopore sequencing revealed that 31 out of 45 variants were shared with the corresponding serum of the mother. We conclude that anellovirus compartmentalization between the circulation and the mammary glands could not be detected. IMPORTANCE: Anelloviruses are omnipresent in the human population, yet their transmission routes, especially in early life, remain unclear. This study focused on compartmentalization between blood and the mammary gland to investigate the likelihood of virus transmission via milk. We show that serum and milk samples generally share anellovirus genera and lineages. Interestingly, serum as well as milk samples collected between 2 and 6 months postpartum tested mainly negative for anelloviruses, but those received thereafter were dominated by beta- and gammatorquevirus, reflecting the initial anellovirus colonizers observed in children.