LAG-3 regulates plasmacytoid dendritic cell homeostasis

Lymphocyte activation gene 3 (LAG-3) is a CD4-related, activation-induced cell surface molecule expressed by various lymphoid cell types and binds to MHC class II with high affinity. We have previously shown that LAG-3 negatively regulates the expansion of activated T cells and T cell homeostasis, and is required for maximal regulatory T cell function. In this study, we demonstrate for the first time that LAG-3 is also expressed on CD11^clow/B220 ⁺/PDCA-1⁺ plasmacytoid dendritic cells (pDCs). Lag3 expression, as determined by real time PCR, was ∼10-fold greater in pDCs than in either regulatory T cells or activated T effector cells. Activated pDCs also generate ∼5 times more sLAG-3 than activated T cells. LAG-3-deficient pDCs proliferate and expand more than wild-type pDCs in vivo in response to the TLR9 ligand, CpG. However, the effect of LAG-3 appears to be selective as there was no effect of LAG-3 on the expression of MHC class II, TLR9, and chemokine receptors, or on cytokine production. Lastly, adoptive transfer of either Lag3^+/+ or Lag3^-/- T cells plus or minus Lag3 ^+/+ or Lag3^-/- pDCs defined a role for LAG-3 in controlling pDC homeostasis as well as highlighting the consequences of deregulated Lag3^-/- pDCs on T cell homeostasis. This raised the possibility of homeostatic reciprocity between T cells and pDCs. Collectively, our data suggests that LAG-3 plays an important but selective cell intrinsic and cell extrinsic role in pDC biology, and may serve as a key functional marker for their study.