The invention relates to an isolated interleukin-34 (IL-34) polypeptide for use in preventing or treating graft rejection, autoimmune disease, unwanted immune response against therapeutic proteins and allergy. The invention also provides an in vitro method for determining whether a patient is at risk of transplant rejection, autoimmune diseases, unwanted immune response against therapeutic proteins or allergies, comprising a step of determining the expression level of IL-34 in a biological sample obtained from said patient, wherein the presence of IL-34 is indicative of a reduced risk of transplant rejection, autoimmune diseases, unwanted immune response against therapeutic proteins or allergies.
