Thymocyte costimulating antigen is CD26 (dipeptidyl-peptidase IV). Costimulation of granulocyte, macrophage, and T lineage cell proliferation via CD26.

The rat thymocyte costimulating protein appears to be involved in the regulation of CD4-/CD8- thymocyte proliferation in vitro. We show that thymocyte costimulating protein is dipeptidyl peptidase IV (E.C.N.3.4.14.5) also known as CD26. Some bone marrow cells as well as CD4-/CD8- and, and to a lesser extent, more differentiated T cells respond by proliferating to a CD26 specific mAb-mediated costimulus that does not influence dipeptidyl dipeptidase IV enzyme activity. This suggests the existence of a CD26-linked regulatory mechanism of proliferation that is operational on granulocyte- and macrophage-lineage cells and throughout T cell development.