A: Intestinal transplantation (ITx) patients are disproportionally riddled with immune complications in comparison to less immunogeneic transplanted organs. Despite advances, severe acute cellular rejection (ACR) remains a critical source of morbidity and mortality. The gold standard treatment of severe ACR is with Thymoglobulin (Thy) T cell depletion therapy. The response rate to depletion is conventionally monitored by CD3 levels in peripheral blood. However, a significant number of ACR patients fail to clinically respond to Thy. This study aimed to determine the mechanisms underlying Thy resistant allograft rejection. B: A cohort of 21 out of a total 207 ITx patients, with histologically confirmed severe Grade III ACR was identified from the Georgetown Transplant Institute Tissue Bank Study. Immunomonitoring was accomplished by immunohistochemistry (IHC) and polychromatic flow cytometry (PFC). Clinical non-responders were defined as patients in rejection refractory to treatment or re-rejection within 90 days. C: Serial histological analysis of peripheral blood was performed on 12 clinical responders and 9 non-responders. Surprisingly, peripheral CD3 populations appeared uniformly depleted in both responders and non-responders (3.58% vs. 2.33%, p=0.488). This indicated that monitoring CD3 in peripheral blood is not reliable. We hypothesized that Thy-resistant allograft rejection is modulated by depletion resistant T cells in the allograft itself. Longitudinal IHC analysis of rejecting allografts confirmed that the degree of allograft T cell depletion correlated with clinical responsiveness. Specifically, 70% T cell depletion was found in responders to Thy treatment versus only 40% in non-responders (p<0.03). Next, to precisely characterize the depletion resistant cell phenotypes in the allograft, patients with ongoing severe ACR were prospectively immunomonitored with PFC. The vast majority of depletion resistant CD4+and CD8+ cell populations demonstrated more than 90% presence of CD45RO4+ effector memory T cells (TEM) phenotype, which have been implicated in other solid organ transplant rejections. D: This study demonstrates that T cell depletion immunomonitoring in the allograft is clinically superior to peripheral blood during treatment of ACR with Thy in human ITx patients. Phenotype analysis suggests that TEMs are critical effector cells in Thy depletion resistant ACR, which may have implications for the design of novel clinical treatment strategies. Karen Creswell, PhD. References: 1. Fishbein, Thomas M. "Intestinal Transplantation." New England Journal of Medicine 361.10 (2009): 998–1008. 2. "Organ Procurement and Transplantation Network." OPTN:. N.p., n.d. Web. 08 Mar. 2016. "This work was supported in part by Health Resources and Services Administration contract 234-2005-37011C. The content is the responsibility of the authors alone and does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government." 3. Smith JM, Skeans MA, Horslen SP et al. Intestine. Am J Transplant. 2016;16 Suppl 2:99–114. 4. Desai CS, Khan KM, Girlanda R et al. Intestinal transplantation: a review. Indian J Gastroenterol. 2012; 31(5):217-22. 5. De Greef E, Avitzur Y, Grant D et al. Infliximab as salvage therapy in paediatric intestinal transplant with steroid- and thymoglobulin-resistant late acute rejection. J Pediatr Gastroenterol Nutr. 2012;54(4):565-7. 6. Mueller TF. Mechanisms of Action of Thymoglobulin. Transplantation. 2007;84(Supplement):S5-S10