CD5-mediated inhibition of TCR signaling during intrathymic selection and development does not require the CD5 extracellular domain

CD5 functions as a negative regulator of TCR signaling during intrathymic T cell development, but it is not known if this negative regulatory function requires CD5 engagement of an extracellular ligand. The present study has specifically examined the role of the CD5 extracellular domain in T cell development by introducing into CD5^-/- mice a chimeric CD5 molecule in which the extracellular domain of CD5 is replaced with the extracellular domain of human IL-2R p55 (Tac) for which no ligand exists in the mouse. We now report that CD5 mediated down-regulation of TCR signaling during thymocyte development does not require the CD5 extracellular domain and, consequently, does not involve CD5 binding of an extracellular ligand in the thymus.