Development and Characterization of Alternatively Activated Macrophages in the Treatment of Colitis

Abstract

Inflammatory bowel disease (IBD) encompasses two forms of chronic colitis: Crohn’s disease and ulcerative colitis. The prevalence of IBD continues to increase, especially in Westernized countries, and there still remains a lack of a cure. Cell-based therapies have gained momentum as a promising therapy for IBD among other diseases. However there is a paucity of research exploring the use of macrophages as a therapeutic cell. Although macrophages are often considered pro-inflammatory, activation by IL-4 induces an anti-inflammatory phenotype known as an alternatively activated macrophage (AAM). This work shows for the first time that an adoptive transfer of IL-4-treated bone marrow-derived macrophages can attenuate mouse models of both Crohn’s disease and ulcerative colitis, the latter of which is of novel importance considering that AAMs are associated with Th2 environments and ulcerative colitis is considered a Th2-like disease. Currently used treatments can lose efficacy over time, however we have tested multiple AAM administrations to treat multiple bouts of colitis and found no loss in efficacy with no overt fibrosis as a potential adverse event when examined by a collagen assay, PCR, or Masson’s trichrome stain. Treatment with AAMs was effective when administered before or after the induction of colitis, while cryopreservation—-as therapeutic macrophages would be stored for convenience—-did not affect their ability block colitis. The anti-inflammatory activity was dependent on their expression of IL-10 and integrin β7 (of the gut homing receptor α4β7), but independent of recipient macrophages or lymphocytes, and CCR2 or CX3CR1 expression on AAMs. Transferred macrophages managed to retain their AAM phenotype as assessed by Arg1 and MRC1 expression, and were localised preferentially to the colon of mice with colitis. As an autologous transfer would be ideal, monocytes from patients with IBD were evaluated in their capacity to become alternatively activated in response to IL-4. Macrophages were found to have reduced expression of AAM markers compared to controls, which was not associated with differential IL-4Rα expression, and pre-application of an inflammatory cytomix partially reduced the AAM phenotype. Herein we propose that cultured AAMs can be used as a safe and effective therapy for all forms of IBD.