CD137 ligand is expressed at low levels on murine and human dendritic cells which were derived in vitro from monocytes or hematopoietic progenitor cells or were isolated from tonsil or spleen. Its expression on dendritic cells is enhanced by proinflammatory stimuli, including IL-1, CD40 ligand, LPS, and double stranded RNA (Table 3.2).
Crosslinking of the CD137 ligand enhances the expression of CD11c, CD80, CD86 and MHC class II and induces cellular adherence and the release of IL-6 and IL-12 (Futagawa et al., 2002; Kim et al., 2002; Langstein et al., 1999). Different investigations come to different conclusions on whether the CD137 signal alone is sufficient to induce dendritic cell activation or whether it only works in combination with additional activating signals such as CD40 ligand. Interestingly, the CD137 ligand signal also upregulates CD137 ligand expression, pointing to a positive feed back loop (Kim et al., 2002).
These activities on dendritic cells are very similar to CD137 activities on monocytes. The physiological function of the CD137 ligand signal in dendritic cells is an induction or enhancement of their antigen-presenting capacity, and consequently an initiation or enhancement of immune responses.
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