Expression of both, CD137 and CD137 ligand is regulated activation-dependently. CD137 ligand is expressed constitutively on many cells, and most if not all immune cells. Cell activation however may increase CD137 ligand expression. In contrast, expression of CD137 seems to be more tightly regulated. Except for dendritic cells (Wilcox et al., 2002) there is no constitutive expression of CD137 on primary cells known. Cell activation is more critical for CD137 expression. Therefore, the interaction of CD137 and CD137 ligand, and the initiation of signaling through both molecules is mainly regulated through CD137 expression.
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