Naive Human T Cells Are Activated and Proliferate in Response to the Heme Oxygenase-1 Inhibitor Tin Mesoporphyrin

Heme oxygenase-1 (HO-1) and its catabolic by-products have potent anti-inflammatory activity in many models of disease. It is not known, however, if HO-1 also plays a role in the homeostatic control of T cell activation and proliferation. We demonstrate here that the HO-1 inhibitor tin mesoporphyrin (SnMP) induces activation, proliferation, and maturation of naive CD4 + and CD8 + T cells via interactions with CD14 + monocytes in vitro. This response is dependent upon interactions of T cells with MHC class I and II on the surface of CD14 + monocytes. Furthermore, CD4 + CD25 + FoxP3 + regulatory T cells were able to suppress this proliferation, even though their suppressive activity was itself impaired by SnMP. Given the magnitude of the Ag-independent T cell response induced by SnMP, we speculate that HO-1 plays an important role in dampening nonspecific T cell activation. Based on these findings, we propose a potential role for HO-1 in the control of naive T cell homeostatic proliferation.