On the prenatal initiation of T cell development in the opossum Monodelphis domestica

Thymus‐dependent lymphocytes (T cells) are a critical cell lineage in the adaptive immune system of all jawed vertebrates. In eutherian mammals the initiation of T cell development takes place prenatally and the offspring of many species are born relatively immuno‐competent. Marsupials, in contrast, are born in a comparatively altricial state and with a less well developed immune system. As such, marsupials are valuable models for studying the peri‐ and postnatal initiation of immune system development in mammals. Previous results supported a lack of prenatal T cell development in a variety of marsupial species. In the gray short‐tailed opossum, Monodelphis domestica , however, there was evidence that αβT cells were present on postnatal day 1 and likely initiated development prenatally. Demonstrated here is the presence of CD 3ε + lymphocytes in late‐stage embryos at a site in the upper thoracic cavity, the site of an early developing thymus. CD 3ε + cells were evident as early as 48 h prior to parturition. In day 14 embryos, where there is clear organogenesis, CD 3ε + cells were only found at the site of the early thymus, consistent with no extra‐thymic sites of T cell development in the opossum. These observations are the first evidence of prenatal T cell lineage commitment in any marsupial.