Molecular evidence for a thymus-independent partial T cell development in a FOXN1-/- athymic human fetus.

The thymus is the primary organ able to support T cell ontogeny, abrogated in FOXN1(-/-) human athymia. Although evidence indicates that in animal models T lymphocytes may differentiate at extrathymic sites, whether this process is really thymus-independent has still to be clarified. In an athymic FOXN1(-/-) fetus, in which we previously described a total blockage of CD4(+) and partial blockage of CD8(+) cell development, we investigated whether intestine could play a role as extrathymic site of T-lymphopoiesis in humans. We document the presence of few extrathymically developed T lymphocytes and the presence in the intestine of CD3(+) and CD8(+), but not of CD4(+) cells, a few of them exhibiting a CD45RA(+) naïve phenotype. The expression of CD3εεpTα, RAG1 and RAG2 transcripts in the intestine and TCR gene rearrangement was also documented, thus indicating that in humans the partial T cell ontogeny occurring at extrathymic sites is a thymus- and FOXN1-independent process.