During embryogenesis, colonization of the thymic rudiment by hemopoiet
ic progenitor cells depends on the adhesion of these cells to the jugu
lar endothelium. Previously, we showed that progenitor T cells (pro-T
cells) interact with alpha(6) integrins present on vascular endotheliu
m. Here, we demonstrate that anti-alpha(6) integrin antibodies reduced
the number of thymocytes up to 80% in a congenic mouse model for thym
us colonization by pro-T cells. In organotypic thymus cultures, the an
ti-alpha(6) integrin antibodies did not influence T cell development a
nd proliferation. From this, we conclude that alpha(6) integrin partic
ipates in thymus homing. During mouse thymus ontogeny, alpha(6) integr
in mRNA and protein expres sion was found as early as day 10 of develo
pment; at day 11, perithymic endothelial cells were alpha(6), integrin
positive. Two alpha(6) integrin mRNA exist which are produced by alte
rnative exon usage. The longer form, alpha(6) integrin, predominates d
uring early embryonic stages, while the shorter alpha(6) form was pres
ent later during development. Although alpha(6) integrins can be displ
ayed by immature thymocytes, strongest expression was found on intra-
and perithymic vascular endothelium. These data suggest that alpha(6)
integrins are involved in the homing of pro-T cells to the developing
thymus by mediating adhesion of pro-T cells to the vascular endotheliu
m.