Objective  To investigate the changes of macrophage polarization during acute rejection (AR) after intestinal transplantation.

  Methods  Six Brown Norway (BN) rats and 24 Lewis rats were divided into the sham operation group (6 Lewis rats), syngeneic transplantation group (Lewis→Lewis, 6 donors and 6 recipients) and allogeneic transplantation group (BN→Lewis, 6 donors and 6 recipients). At postoperative 7 d, the intestinal graft tissues in all groups were collected for hematoxylin-eosin (HE) staining and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay. Pathological manifestations and cell apoptosis were observed. The expression levels of serum cytokines related to M1 and M2 macrophage polarization were determined by enzyme-linked immunosorbent assay (ELISA). Surface markers of M1 and M2 macrophages of intestinal graft tissues in each group were co-localized and counted by immunofluorescence staining.

  Results  HE staining and TUNEL assay showed that the intestinal epithelial morphology and structure were normal and no evident apoptotic bodies were found in the sham operation and syngeneic transplantation groups. At 7 d after transplantation, the epithelial villi structure of intestinal graft tissues was severely damaged, the number of crypts was decreased, the number of apoptotic bodies was increased, and inflammatory cells infiltrated into the whole intestinal wall, manifested with moderate to severe AR in the allogeneic transplantation group. ELISA revealed that the expression levels of serum cytokines related to M1 macrophage polarization, such as tumor necrosis factor (TNF)-α, interferon (IFN)-γ and interleukin (IL)-12, of the recipient rats in the allogeneic transplantation group were higher than those in the sham operation and syngeneic transplantation groups. The expression levels of serum cytokines related to M2 macrophage polarization, such as IL-10 and transforming growth factor (TGF)-β, in the syngeneic transplantation group were higher compared with those in the sham operation and allogeneic transplantation group, and the differences were statistically significant (all P<0.05). Immunofluorescence staining showed that the number of M1 macrophages in the allogeneic transplantation group was higher than those in the sham operation and syngeneic transplantation groups, and the number of M2 macrophages in the syngeneic transplantation group was higher than those in the sham operation and allogeneic transplantation groups, and the differences were statistically significant (all P<0.05).

  Conclusions  Among the allografts with AR after intestinal transplantation, a large number of macrophages, mainly M1 macrophages secreting a large number of pro-inflammatory cytokines, infiltrate into the whole intestinal wall. Regulating the direction of macrophage polarization is a potential treatment for AR after intestinal transplantation.