Paradoxical role of programmed death‐1 ligand 2 in Th2 immune responses in vitro and in a mouse asthma model in vivo

Programmed death‐1 ligand 2 (PD‐L2) is a ligand for programmed death‐1 (PD‐1), a receptor that plays an inhibitory role in T cell activation. Since previous studies have shown up‐regulation of PD‐L2 expression by Th2 cytokines, and asthma is driven by a Th2 response, we hypothesized that PD‐L2 might be involved in regulation of the immune response in this disease. We have found that lungs from asthmatic mice had sustained up‐regulation of PD‐1 and PD‐L2, with PD‐L2 primarily on dendritic cells. Although addition of PD‐L2‐Fc in vitro led to decreased T cell proliferation and cytokine production, administration of PD‐L2‐Fc in vivo in a mouse asthma model resulted in elevated serum IgE levels, increased eosinophilic and lymphocytic infiltration into bronchoalveolar lavage fluid, higher number of cells in the draining lymph nodes, and production of IL‐5 and IL‐13 from these cells. Although PD‐1 was expressed on regulatory T cells, PD‐L2‐Fc did not affect regulatory T cell activity in vitro. This study provides in vivo evidence of an exacerbated inflammatory response following PD‐L2‐Fc administration and indicates a potential role for this molecule in Th2‐mediated diseases such as asthma.