Acts as a MHC class II receptor . When stimulated on its own, does not play a role in cytokine production or the release of cytotoxic granules by NK cells and cytotoxic CD8(+) T cells (, ). Does not act as an Fc receptor .
Subcellular locations: Cell membrane
Expressed by cytolytic cells including NK cells, effector and effector-memory CD8(+) T-cells, and a subset of NKT cells (at protein level) ( ). Also expressed in gamma delta T cells and in a rare subset of effector CD4(+) T-cells (at protein level) . Expressed in spleen, skin, peripheral blood leukocytes, liver, lung, bone marrow, small intestine and placenta . Expression among T-cells is greatly expanded in HIV-1 infected individuals, and includes not only effector and effector-memory CD8(+) T-cells but also populations of CD4(+) T-cells (, ). Expression among CD8(+) T-cells and NK cells is expanded in individuals with chronic lymphocytic leukemia (CLL) but is reduced in PBMCs from patients with acute (AML), chronic myeloid leukemia (CML) and non-Hodgkin's lymphoma (, ). Expression is higher in PBMCs and/or CD3(+) cells of patients with autoimmune diseases, such as rheumatoid arthritis (RA), systemic lupus erythematosus (SLE) and idiopathic thrombocytopenia purpura (ITP) . In contrast, expression in CD3(+) cells from patients with lupus anticoagulans (LA) is higher .