Introduction: Abdominal wall transplantation allows the closure of challenging abdominal defects, particularly after small bowel transplantation. It has been recognised that the abdominal wall transplant (AWTx) may be used for immunological monitoring of solid organ transplant, and as an easily accessible biopsy site. In this study we explored the tissue alloresponse in AWTx and small bowel biopsies, using spatial proteomic analysis of tissue biopsies.
Methods: This study examined 16 samples from 5 patients who underwent AWTx and small bowel transplantation in Oxford. Samples comprised five skin biopsies and eleven small bowel biopsies. Included are 5 paired samples of AWTx and small bowel biopsies taken simultaneously. The GeoMx Digital Spatial Profiler was used to compare an immune-focused panel of 44 proteins in regions of interest (ROIs) in the AWTx and small bowel biopsies. In the AWTx samples, ROIs were collected from perivascular and epidermal regions. In the small bowel samples, ROIs were collected from crypt regions and areas where dense CD45 staining was noted (called infiltrates hereafter). These regions were chosen as they form the basis for histological diagnosis of rejection. Use of CD45 as a morphology marker enabled further segmentation of these ROIs into CD45 positive and negative areas of interest (AOIs). In total 216 AOIs were collected for analysis.
Results: At rejection, perivascular ROIs expressed more monocyte markers and markers of T cell activation, in comparison to no rejection, when proteins such as CTLA4 and PD1 are more highly differentially expressed. This finding is mirrored in the epidermal ROIs. In perivascular CD45+ AOIs at rejection we noted increased levels of the dendritic cell marker CD11c along with IDO1. When comparing perivascular to epidermal ROIs, we found increased protein expression of monocyte, macrophage and T cell activation related proteins. Similar markers of T cell activation were also found in crypt ROIs. Of interest when comparing the CD45+ AOIs in crypt and infiltrate ROIs we noted enrichment of monocyte markers in the crypt AOIs and enrichment of B cell markers in the infiltrate AOIs. We then compared CD45+ AOIs in AWTx samples to those in small bowel samples at rejection and did not find any significant differences in protein expression.
Conclusion: Spatial proteomics offers a powerful way to investigate the alloresponse within tissues. We identified markers of monocytes, macrophages and T cell activation that were increased concurrently in both abdominal wall and small bowel transplants at rejection. Interestingly, CD45+ AOIs did not differ substantially between AWTx and small bowel biopsies, suggesting that the abdominal wall may provide a sufficient insight into the phenotype of the ongoing alloresponse within the bowel.