Regulatory role of mouse epidermal growth factor-like protein 8 in thymic epithelial cells

Unlike epidermal growth factor-like protein 7 (EGFL7), which is a secreted protein implicated in the regulation of blood vessel formation and cell migration, little is known about the physiological function of EGFL8. Thymic epithelial cells (TECs) play a pivotal role in T-cell development by regulating cellular interactions and expression of growth factors, cytokines, and chemokines. In order to investigate the functional role of EGFL8 in TECs, we transfected TECs with an EGFL8-expressing vector to overexpress EGFL8 protein and with an EGFL8 siRNA to knockdown EGFL8 expression. EGFL8-silenced TECs showed significant increase in the number of adherent thymocytes by enhancing the expression of intercellular adhesion molecule-1 (ICAM-1), while the overexpression of EGFL8 inhibited the adherence of TECs to thymocytes by suppressing ICAM-1 expression. Furthermore, in vitro co-culture study revealed that knockdown of EGFL8 facilitated the maturation of thymocytes to CD4(+) and CD8(+) single-positive populations. These regulatory effects of EGFL8 in T-cell development were further confirmed by the results that knockdown of EGFL8 enhanced the expression of genes involved in thymopoiesis, such as interleukin-7 (IL-7), granulocyte/macrophage-colony stimulating factor (GM-CSF), and thymus-expressed chemokine (TECK). Our data show that EGFL8 exerts inhibitory effects on TECs and thymocytes, suggesting that EGFL8 acts as a negative regulatory molecule in the development of T cells in the mouse thymus.