Although the mechanism of LAG-3 function remains unclear, a conserved KIEELE motif in the cytoplasmic domain of LAG-3 is essential 2. In contrast to CD4, LAG-3 is only expressed on the cell surface of activated T cells 1, 7–10. LAG-3 surface expression is further regulated by two metalloproteases, ADAM10 and ADAM17, which cleave surface LAG-3, a proportion of which is both constitutive and TCR-ligation induced 11. Importantly, prevention of LAG-3 cleavage blocks T-cell proliferation

and cytokine secretion 11 suggesting that LAG-3 surface expression is under tight regulatory control. This observation raised the question of whether other mechanisms are used to control the expression and distribution of LAG-3. Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4), which is another inhibitory molecule for T-cell activation, SCH727965 is mainly stored in DAPT intracellular compartments such as the trans-Golgi network, endosomes and lysosomes 12–17. Surface expression is tightly regulated by controlled internalization and trafficking to the plasma membrane. This raised the possibility that LAG-3 surface expression might also be regulated by modulating its intracellular storage and trafficking. In this study, we addressed the following questions.

First, what is the extent of intracellular storage and localization of LAG-3 versus its relative CD4? Second, what is the sub-cellular localization of LAG-3 and CD4 in activated T cells? Third, what is the fate of intracellular LAG-3? In order to determine cellular distribution of CD4 and

LAG-3, we performed intracellular staining for CD4 or LAG-3 using flow cytometry. Freshly isolated naïve CD4+ T cells do not express LAG-3 10; so naïve T cells were first stimulated with plate-bound anti-CD3 and anti-CD28 for 72 h and then treated with pronase to remove cell surface CD4 and LAG-3 from activated CD4+ T cells. Pronase treatment removed most of the surface CD4 and LAG-3 on activated T cells (Fig. 1A). While intracellular staining revealed that a relatively small amount (23%) of CD4 is present inside cells, in Histamine H2 receptor contrast a greater amount (49%) of LAG-3 appears to be retained intracellularly (Fig. 1A and B). One might speculate that the slightly lower LAG-3 surface expression compared with CD4 following T-cell activation and the increased percentage of intracellular LAG-3 versus CD4 is due to its continuous cleavage by the metalloproteases ADAM10 and ADAM17 that limits surface LAG-3 expression 11, 18. However, when T cells were treated with the metalloproteinase inhibitor TAPI (Calbiochem), cell surface LAG-3 expression was only slightly increased (data not shown). While prevention of LAG-3 cleavage by TAPI slightly changed the ratio of surface and intracellular LAG-3, the effect was small and not sufficient to account for the differences observed between LAG-3 and CD4. The extent of intracellular LAG-3 storage was also examined by Western blot analysis.