Identification of tissue transglutaminase as a novel molecule involved in human CD8⁺ T cell transendothelial migration

During inflammation, T lymphocytes migrate out of the blood across the vascular endothelium in a multistep process. The receptors mediating T cell adhesion to endothelium are well characterized; however, the molecules involved in T cell transendothelial migration (TEM) subsequent to lymphocyte adhesion to the endothelium are less clear. To identify receptors mediating TEM, mAbs were produced against human blood T cells adhering to IFN-γ-activated HUVEC in mice and tested for inhibition of lymphocyte TEM across cytokine-activated HUVEC. Most of the mAbs were against β₁ and β₂ integrins, but one mAb, 6B9, significantly inhibited T cell TEM across IFN-γ, TNF-α, and IFN-γ plus TNF-α-stimulated HUVEC, and did not react with an integrin. 6B9 mAb did not inhibit T cell adhesion to HUVEC, suggesting that 6B9 blocked a novel pathway in T cell TEM. The 6B9 Ag was 80 kDa on SDS-PAGE, and was expressed by both blood leukocytes and HUVEC. Immunoaffinity purification and mass spectrometry identified this Ag as tissue transglutaminase (tTG), a molecule not known to mediate T cell TEM. Treatment of HUVEC with 6B9 was more effective than treatment of T cells. 6B9 blockade selectively inhibited CD4^-, but not CD4⁺, T cell TEM, suggesting a role for tTG in recruitment of CD8⁺ T lymphocytes. Thus, 6B9 is a new blocking mAb to human tTG, which demonstrates that tTG may have a novel role in mediating CD8⁺ T cell migration across cytokine-activated endothelium and infiltration of tissues during inflammation.