Figure 1. Suppressor monocytes secrete a number of molecules including inducible NOS and arginase I to promote allograft survival and suppress alloreactive T cells.

IL-34 is produced by monocytes and cells from a number of organs including the heart and binds to its receptor on monocytes. IL-34 is also expressed by CD45RC^lo CD4⁺ and CD8⁺ Tregs. IL-34 treatment of mixed lymphocyte cultures improves Treg-dependent suppression of effector T cell proliferation. The IL-34 receptor CD115 is not known to be expressed on Tregs; therefore, suppression of the alloresponse and FOXP3 upregulation may be mediated through a suppressor monocyte intermediate. iNOS, inducible NOS; Teff, T effector cell.