Protein kinase C-0 is required for efficient positive selection

Protein kinase C-theta (PKC theta) is critical for TCR-initiated signaling in mature T cells, but initial reports found no requirement for PKC theta in thymocyte development. Thymocytes and peripheral T cells utilize many of the same signaling components and, given the significant role of PKC theta in peripheral T cells, it was surprising that it was not involved at all in TCR signaling in thymocytes. We decided to re-evaluate the role of PKC theta(-/-) in thymocyte development using the well-characterized class II-restricted n3.L2 TCR-transgenic TCR model. Analysis of n3.L2 PKC theta(-/-) mice revealed a defect in thymocyte-positive selection, resulting in a 50% reduction in the generation of n3.L2 CD4 single-positive thymocytes and n3.L2 CD4 mature T cells. Competition between n3.L2 WT and n3.L2 PKC theta(-/-) thymocytes in bone marrow chimeras revealed a more dramatic defect, with a >80% reduction in generation of n3.L2 CD4 single-positive thymocytes derived from PKC theta(-/-) mice. Inefficient positive selection of n3.L2 PKC theta(-/-) CD4 single-positive cells resulted from weaker signaling through the TCR and correlated with diminished ERK activation. The defect in positive selection was not complete in the PKC theta(-/-) mice, most likely accounted for by compensation by other PKC isoforms not evident in peripheral cells. Similar decreased positive selection of both CD4 and CD8 single-positive thymocytes was also seen in nontransgenic PKC theta(-/-) mice. These findings now place PKC theta as a key signaling molecule in the positive selection of thymocytes as well as in the activation of mature T cells.