TCRμ Recombination and Transcription Relative to the Conventional TCR during Postnatal Development in Opossums

Abstract

Marsupials are a distinct lineage of mammals notable for giving birth to highly altricial (relatively less developed) young. The recent discovery of a unique TCR chain in marsupials, TCRμ, raises questions about its possible role in early development. Here we compare the timing of V(D)J recombination and appearance of TCRμ transcripts relative to the conventional TCRα, β, γ, and δ mRNA during postnatal development in the opossum. There are two TCRμ transcript isoforms, TCRμ1.0 and TCRμ2.0. TCRμ1.0, which uses prejoined V(D)J segments, is detectable as early as day 1, when the thymus is primarily undifferentiated epithelium. The other isoform, TCRμ2.0, which requires V(D)J recombination and contains an unusual double V configuration, is not detectable until day 13 when the thymus is histologically mature. Surprisingly, we were able to detect TCRα, β, and δ mRNA transcribed from loci that had completed V(D)J recombination as early as day 1 as well. At this early age there is apparent evidence for preference in the V segments used in the TCRα and β genes. In the case of Vα this preference appears to be associated with position in the TCRα/δ locus. In Vβ, however, preference may be due to the use of microhomology in the V, D, and J segments. Mature TCRγ transcripts were not detected until day 8, suggesting that, in contrast to eutherian mammals, in the opossum αβ T cell development precedes γδ T cell development. The results support that there may be differences in T cell subset development between marsupials and placental mammals.

Comparative Phylogeny of the Mucosa-Associated Lymphoid Tissue

“…This is consistent with work on the opossum Monodelphis domestica, where αβ T cells can be detected at birth (Parra et al, 2009). By postnatal day 28, CD3 + cells were detectable in T. vulpecula in both the large and the small intestine, but few Ig + B cells were present at this time.…”

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

“…Analyses of lymphocyte ontogeny using reverse transcriptase PCR of B and T cell specific transcripts as proxies for developmental stages were consistent with the appearance of early committed cell lineages around the time of birth. Parra and colleagues (2009) demonstrated the presence of complete, functional TCR α and β transcripts on postnatal day 1, consistent with the initiation of T cell development in prenatal opossums.…”

“…Previous analyses of B and T cell receptor transcription in the gray short-tailed opossum, Monodelphis domestica , clarified many aspects of the postnatal progression of lymphocyte development (Parra et al 2009; Wang et al 2012b). Recombination and transcription of the immunoglobulin heavy chain ( IgH ) locus was evident within 24 hours following birth but not in prenatal embryos (Wang et al 2012b).…”

“…Clear cortical and medullary regions were not evident, however, until the end of the first postnatal week. Parra and colleagues (2009) reported that transcripts that could encode fully functional T cell receptor (TCR) α and β chains were present in the first 24 postnatal hours. These studies were performed using RNA extracted from the upper thoracic cavity, as a macroscopic thymus was not yet visible at this early stage.…”

“…These studies were performed using RNA extracted from the upper thoracic cavity, as a macroscopic thymus was not yet visible at this early stage. Unlike eutherian mammals, opossum αβT cell development precedes that of γδT cells (Parra et al 2009). The presence of functional α and β TCR transcripts at birth in M. domestica neonates is consistent with progression of T cell development prenatally.…”