Qa-2 associated lipid rafts are indispensable in the final maturation of CD4⁺CD8⁻ thymocytes.

Lipid rafts have been shown to play significant roles in thymocyte development. However, the exact role of lipid rafts in single positive (SP) thymocyte differentiation is poorly characterized. We previously defined a developmental program (SP1→SP4) for CD4SP thymocytes. In this study, we found that lipid raft components were up-regulated during CD4SP maturation. Qa-2, a unique marker for the most mature SP4 subset, was localized to lipid rafts and heterogeneously expressed in SP4 cells. Functional assays showed that the proliferation capacity of SP4 cells correlated with the expression of Qa-2. Raft-disruption on both CD4SP and epithelial cells by cholesterol extraction or cholesterol oxidation in a medullary thymic epithelial cell (mTEC)-supported co-culture system impaired the transition from SP3 to SP4. This result was further confirmed in fetal thymic organ culture system. Collectively, these studies suggest that raft-associated signaling between mTECs and thymocytes drives the differentiation of CD4SP thymocytes and lipid rafts are involved in the final maturation of SP4 thymocytes.