IDENTIFICATION OF CD8 AS A PEANUT AGGLUTININ (PNA) RECEPTOR MOLECULE ON IMMATURE THYMOCYTES

Differentiation of most T lymphocytes occurs within the thymus and is characterized by variable expression of CD4/CD8 coreceptor molecules, increased surface density of T cell antigen receptor (TCR) alpha beta proteins, and decreased expression of glycan chains recognized by the galactose-specific lectin peanut agglutinin (PNA). Although appreciate d for several decades that PNA agglutination is useful for the physica l separation of immature and mature thymocyte subpopulations, the iden tity of specific PNA-binding glycoproteins expressed on immature thymo cytes remains to be determined. In tile cut-rent report, we studied th e expression of PNA-specific glycans on immature and mature T cells an d used lectin affinity chromatography and immunoprecipitation techniqu es to characterize PNA-binding glycoproteins on thymocytes. Our data d emonstrate that PNA-specific glycans are localized on a relatively sma ll subset of thymocyte surface proteins, several of which were specifi cally identified, including CD43, CD45, and surprisingly, CD8 molecule s. CD8 alpha and CD8 alpha' proteins bound to PNA in the absence of CD 8 beta expression showing that O-glycans on CD8 beta glycoproteins are not necessary for PNA binding and that glycosylation of CD8 alpha and CD8 alpha' proteins proceeds effectively in the absence of CD8 beta. Finally, we demonstrate that PNA binding of CD8 is developmentally reg ulated by sialic acid addition as CD8 proteins from mature T cells bou nd to PNA only after sialidase treatment. These studies identify CD8 a s a PNA receptor molecule on immature thymocytes and show that PNA bin ding of CD8 on immature and mature T cells is developmentally regulate d by sialic acid modification.